PLET 323
EXTRUSION BLOW MOLDED PITCHFORK TO REDUCE COST OF WOODEN AND METAL PITCHFORKS
BOBBI DOUBET AND JOSHUA STANFORD Penn State Erie, The Behrend College
December 18, 2019
Introduction
The product that will be created is a five-tined pitchfork. This fork will not be made into multiple pieces. This fork will be molded as a single piece. The fork will compose of a long, thin handle so that the user can easily grab the handle while using it. Instead of the plastic part ending at the bottom of the fork where the metal tines would start, the fork will morph into the fork portion. All five tines of the pitchfork will be made from plastic as well. As in any other extrusion blow molded part, this part will be hollow. For this part to be hollow, a small hole will be located on the top of the handle, so that the pressurized air will be allowed to blow the plastic so that it will conform to the mold surface. For the entire part to be hollow and to have a constant wall thickness throughout the part, the tines of the fork may have a thicker diameter so it will not be a solid little rod of plastic going all the way down the length of the tine. Even though the tines will be a larger relative diameter, the tips of the tines will still be sharp enough so that the fork can stab into the hay or into the dirt. A different design will be used when creating the tines. Old designs of forks show the tines being small in diameter and the tines very far apart. Stuff can fall through those gaps. In this design, the tines will be morphed together in a web-like structure to add more surface area to the part. This surface area will decrease the chance of substance falling off the fork. There will still be tines at the bottom of the fork part, but the top of the fork will look like a shovel. This will then morph into the thinner tines. This design will allow the fork to hold more material without it falling through the gaps, but also allows it to be used as a shovel, while still being able to scrape off manure behind a farm animal or being able to dig it into a pile of hay. Plus, having the webbing at the top of the fork part will also help in the design aspect as well, by creating a way for the part to be hollow and to create a passageway for the pressurized air to go down the tines of the fork. Figure 1 shows the modern pitchfork with a wooden handle and five metal tines and Figure 2 shows the preliminary design sketches of the hollow pitchfork [1].
Figure 1: Modern Pitchfork Figure 2: Detailed Designs of Pitchfork
Pitchforks were first conceived around the time of the Middle Ages. These forks were used to aid the farmers in haying their animals and bedding their animals with hay and straw. These pitchforks were only made from wood, which lacked the metal tines of later pitchforks [2]. In the European Middle Ages, these pitchforks were sometimes used as improvised weapons by people who were poor enough to not afford higher end weapons, such as swords or guns [2]. When a man had to fight in the army, sometimes the pitchfork was the only weapon he had to defend himself. The pitchfork also became the weapon of choice for gangs of peasants and farmer and angry mobs [2]. There are multiple variations of forks. The number of tines a fork can have as few as two tines and can reach up to ten tines [3]. Eventually, metal was used to forge the tines of the fork. As a molten material, the tines would be shaped and cooled. Then the connector of the tines was forged to hook onto the wooden handle. These would eventually be screwed in by washers and bolts for easy changing of handles when the wooden handles eventually broke. In 1907, a patent was created by a man by the name of John C. Frees that allowed the pitchfork to be used in loading manure [4]. He designed the handle to be sturdier at the bottom of the fork so the load of manure or dirt could be applied. The tines also became curved to act like a shovel to pick up manure from the straw [4]. In the same year, another man named Arthur Holmes Knox patented an invention of the fork that allowed the material to slide off the tines of the fork without having clinging material left over [5].
Figure 1: Patent of Pitchfork by Frees [4]
Figure 2: Patent of Pitchfork by Knox [5]
Pitchforks today are still made with wood, such as pine, birch or ash, and made with metals and alloys such as steel and iron. Plastic is being used to make either the handles or the tines of forks. The following image is variation of the pitchfork that acts as a scooper [6]. The scooper is made of plastic, but the handle is made of metal. These products can also have over molded parts. Forks currently made with wooden handles and metal tines use these materials because they are made to be used for heavy-duty use. They are built to last a long time until the handle breaks off. Handles can be replaced. Plastic is being incorporated into the making of pitchforks to make them more lightweight but strong and to decrease the cost of a pitchfork.
Figure 3: Scooper Pitchfork [6]
Currently, extrusion blow molding is not used to create the whole pitchfork. Pitchforks with plastic still have either wooden or metal handles that are either machined or forged. The plastic parts are either made by injection molding and connecting the tines and the handle through secondary operations or are over molded onto the handles. One example that was found of a fork made entirely out of plastic was still being assembled through secondary operations after the pieces were created. The fork being designed in this project will be one hollow continuous piece.
Application
Extrusion blow molding will work very well with this new design of a pitchfork, because it is hollow. One of the main design features of this pitchfork is that it will be hollow. A hollow part will reduce the overall weight of the part. Plastic weighs a lot less than wood or metal, but the right material will still give it the strength it needs to perform its tasks. Producing the whole part out of plastic will reduce any costs associated with secondary operations, over molding, and assembling multiple parts together. The cost of molding multiple parts to be assembled into one working product will be the material cost used in both parts, the tooling cost of the two molds to make the plastic handle and the plastic tines, and the cost it takes to assemble the whole part, in other words, the entire time cost. Within the time cost parameter, there will be added time to assemble the two parts together with nuts, bolts and washers, adding an extra cost to the time cost. With extrusion blow molding, the secondary operation costs with two parts molded to be put together will not exist because the whole part will be created in one process. The only secondary operation that will occur on this part is to trim any excess plastic off from the fork. Extrusion blow molding will also reduce the cost of the wood and metal being used. When you buy a wooden handled fork with metal tines, the price of the fork ranges from as low as $20 to around $60, depending on what the fork is made out of, the length of the handle, and where the fork is being sold at. Sometimes it might be around $20 just for the wooden handle. The plastic-based scoopers cost considerably less, ranging from around $14 to $35. Just substituting plastic for wood and metal reduces the price, making it more affordable to the consumers and farmers. Extrusion blow molding, however, will reduce costs even further than that. For reducing the over molding costs associated with over molding a set of tines to a metal handle, extrusion blow molding will eliminate the tooling cost of the mold used in over molding the part. Any material costs used to make the over molded part will be eliminated as well. The time and assembly cost to make the over molded component and to assemble the two parts together will not exist in blow molding because the whole part is created in one shot. Plus, extrusion blow molding will reduce the amount of internal stresses the polymer and part will witness if the fork was being over molded. When over molding a component overtop of an existing part, the existing part will undergo another injection molding process, which can create internal stresses that can affect how the product will perform in its lifetime of use. Extrusion blow molding is also used to create large parts with ease. The fork is not very large when it comes to the width and depth of the part, but the fork will be a long part, possibly over five feet long in total length. Compared to injection molding, both the injection mold and the extrusion blow mold will be approximately the same size. However, with the injection mold, a runner system must be created and added to the part, so the polymer can fill the part evenly. Extrusion blow molding does not need a runner connected to the fork to create it. The parison will extend past the mold before the mold closes to ensure that the plastic will cover the entire part without the fear of short shot or a partially unfilled cavity, as typically seen with the injection molding process of long products. Compared to extrusion blow molding, injection molding cannot make hollow parts as easily. To create the hollow fork out of plastic, the part must be split into two halves. Two different injection molds will have to be created to create the two halves of the part. Since the will at least double the width of the mold, the machine to hold a mold that tall will have to have a large amount of clamp tonnage and the size of the platens needs to be large enough to fit the mold. Not a lot of molds are that size, so it would be easier to create two separate molds. Creating two injection molds for one part will increase the cost of the fork. It will also be hard to place a gate on that part to ensure even polymer flow throughout the part to minimize any defects like warpage, shrinkage, short shots, weld lines and other defects. When assembling the fork, the fork pieces will have to be welded together, adding assembly cost to the part. Extrusion blow molding will eliminate all of those problems, because only one mold needs to be created to produce the part, no gate needs to be placed to assure proper melt flow and the part will not need to be welded once the part is produced, since blow molding will create the entire hollow part.
Part Specifications
The five-tine pitchfork needs to be able to pick up a load and hold it for a very short period until the load is released. It also needs to be able to withstand a pulling force across multiple surfaces. The material used to make the fork needs to be scratch resistant to multiple surfaces while being subjected to a pulling force, so the tips of the tines do not break. This fork also needs to function within a large range of temperatures and must be able to withstand multiple repetitive forces throughout its life cycle. There are around 42,000 dairy farms located in the United States and about 97% of those farms are small family owned farms [12, 13]. There are a variety of ways that small dairy farms dispose of their manure from their cows. One way is through a stable cleaner. A stable cleaner has a trench about a foot deep where the manure can be put into or scraped into with a pitchfork. There is a chain with paddles on it that transports the manure out of the barn and into a manure spreader when the stable cleaner is turned on [14]. The technology of the stable cleaner has evolved, but most farms do not make enough money to afford to buy newer technology [14]. Figure 1 shows what the primitive stable cleaners look like. Another way that farmers dispose of their manure is through a skid steer. If the trench behind the animals is large enough, the skid loader will fit in the trench and simply push the manure out of the barn. If the trench is not wide enough for the skid steer to fit into it, the manure will be pitched into the bucket of the skid loader and then dumped into a manmade pit, in which the manure will be dealt with later [15].
Figure 6: Stable Cleaner System [14]
To figure what kind of loads that the pitchfork will go through, the ways that farmers clean their barns needed to be analyzed. With the stable cleaner method, the fork will be used to clean off the paddles as the chain moves. This means that the fork will withstand a pulling force on the tips of the tines scraping against the steel surface of the paddles. The tips should not break when scraping the steel surface. With the skid loader method, the fork will have to pitch manure into the bucket. The fork will need to withstand the load that will be put upon it without the tines deflecting too much. Both types of load will be repetitive loads, so repetition will be analyzed as well. To figure out an appropriate load for the pitchfork, information on how much a large farm animal excretes in one day needs to be found. Two of the larger sized farm animals are the horse and the cow. Cows can be either dairy cows or beef cows, so both are included here. The average horse excretes a total of 51 pounds per day [14]. The amount of bedding also needs to be looked at as well. The average horse uses around 8 to 15 pounds of bedding per day, which makes the total waste produced by one horse a day is around 60 to 70 pounds per day [14]. Comparing that to what a dairy cow produces in waste, the average dairy cow produces around 82 pounds of excretion per day and uses around 1.3 cubic feet of bedding per day per 1000 pounds of cow weight [16]. The manure of cows will be used in this study, since they produce more waste per animal. Approximately, both cows and horses use the dame amount of bedding, so the factor of 8 to 15 pounds of bedding can be assumed, so the total amount of waste that one cow produces is approximately 90 to 100 pounds per day [14, 16]. The average dairy farm in the United States has around 234 cows [17]. To make the math easier, 250 cows will be used. The amount of waste produced by cattle will be used to determine the load size of the plastic pitchfork. However, this number needs to be divided by the number of times a cow excretes per day. Assuming the number of times a cow excretes a day is around ten times, the load that the fork will receive directly onto the fork’s surface is approximately 10 pounds. A safety factor of 2 will be applied, just in case the original estimation will be exceeded. The total amount of times that the load will be applied throughout the span of one year will be the estimated load on the fork at one time, multiplied by how many times one animal defecates in a day, multiplied by how many cows are on an average farm then multiplied by the days in one year. This will be multiplied once more by the safety factor. Other specifications that need to be analyzed is how much force can the pitchfork’s tines withstand before possible failure. The fork will need to withstand a pulling force of a human scraping the manure off steel and cement. This means that the material of the fork needs to be a material that can withstand the hardness of the steel paddles and the concrete base of the stalls the cows are in. The hardness of SAE AISI 4140 Steel with a tensile strength of 1075 MPa is 311 HB [18]. This value will be used as a baseline to test the scrape resistance of the material of the fork to see if the material will be resistant to the steel without the tips of the tines breaking off. The fork also needs to be resistant to multiple temperatures, since the pitchfork will be used year-round. Our range for testing the manure fork will be as low as –40 ℉ and as high as 120 ℉. These results are based off the highest and lowest extreme temperatures witnessed across the United States [19, 20]. The temperature extremes of all time will not be used to analyze the manure fork while operating because those temperatures are harder to produce in the lab. The loads that will be applied to the fork will be repetitive loads. This load will also be a repetitive load. In a stable cleaner chain, there is one paddle per foot of chain. With an approximate stall length of five feet for a dairy cow, the number of times the load will occur over the span of one year will be the load size multiplied by the number of paddles per feet for the length of one stall, multiplied by the number of cows on an average dairy farm, multiplied by the number of days of the year and the safety factor will be accounted for as well. The span of one year will be tested to make the calculations of both loads easier to calculate and analyze. The pulling force on the fork while scraping will be calculated before analyzing the loads of that analysis. These loads will be tested using ANSYS, a software program used to analyze loads and conditions acting on a part or assembly at a given time, place and condition. For the first loading condition, a load of 20 pounds will be put on the area of the fork where the tines are located, while the fork is positioned 45 degrees from the vertical position. This counts for the safety factor of 2 put onto the fork. This is to simulate the using position of the fork while pitching manure into the skid loader. First, a linear analysis will be used to see if the load on the fork can be carried once. Then, a nonlinear analysis will be used to see if the fork can handle one year’s worth of use for the average size of a dairy farm. A deflection of the tines will be used to analyze the success of the fork. The deflection of the tines will tell if the tines can successfully carry the load without the load falling off onto the ground. For the second loading condition, the same load size will be used to see if the tines can handle the scraping across the surfaces of steel and concrete multiple times. A pulling force will be put on top of the fork handle to simulate a person pulling back manure on the stable cleaner chain paddles and the concrete. A linear analysis will be used to see if the fork can withstand one use. Then a nonlinear analysis will be used to simulate a year’s worth of use. The deflection of the tines, Von Mises stress and Normal Stress plots will be used to analyze the successfulness of the plastic pitchfork. The Von Mises and Normal Stress plots will show the stress where the tines will fail if the tips of the tines break off while scraping for an extended period. The deflection plot will also show this but will show how much the tines will deflect before the possible point of failure.
Material Selection
Material selection for the blow molded pitchfork is a very important step for the development of the part. The right material needs to be assigned to allow the best performance possible to the part. For this to occur, a weighted material matrix was created based off the specifications that were set upon the pitchfork to select the material family that would be the best fit for the applications of the pitchfork. Five material families were tested based upon the creep resistance of material, the impact resistance of the material due to scraping, the operating temperature ranges of each material, the cost of the material and the chemical resistance of the material. In the end, polycarbonate was chosen to be the best fit for the material. The table below shows the weighted material matrix created for the pitchfork.
Weight Polyethylene Polypropylene Polycarbonate PVC PET, (HDPE) PBT, etc. Creep 30 1 30 2 60 4 120 5 150 4 120 Resistance Temperatures 20 3 60 1 20 5 100 3 60 3 60 Impact 30 4 120 4 120 5 150 2 60 4 120 (Scraping) Cost of 10 5 50 5 50 3 30 3 30 3 30 Material Chemical 10 5 50 5 50 2 20 4 40 4 40 Resistance Total Points 100 310 300 420 340 370
The criteria chosen for the matrix were based on the specifications that were created earlier for the pitchfork, plus a couple properties that have a certain level of importance. They were creep resistance of the material, range of operating temperatures, impact resistance mainly by scraping, the cost of the material per pound and the chemical resistance of the material. Creep resistance was chosen as one of the specifications for the pitchfork because one of the specifications was that it had to pick up a certain load many times during the pitchfork’s use over one year. This is specifically for the tines. The tines need to be able to hold the load multiple times without the tines bending in the opposite direction, so the manure does not slip off the tines of the fork when picking up the manure. It is ranked at 30 points, the largest amount of points given to the specifications, because this is the main analysis of the part that will be conducted using ANSYS and it contributes to one of the main uses of the fork. The material families that were chosen are polyethylene, specifically HDPE, polypropylene, polycarbonate, rigid PVC and the polyester family. In general, these were good materials that could be used for the fork because each have good impact resistance and relatively fair to excellent chemical resistance. Plus, all these families were relatively cheap in price. The polymers will be scored on a scale from one to five, one being the worst and five being the best. For creep resistance, the flexural moduli of the materials were compared because the load that will be applied to the fork will be a flexural load. Therefore, the creep will be due to the flexural load on the fork. Rigid PVC is ranked first, followed by a tie between PET and PC, followed by PP and then followed by HDPE. Rigid PVC had the highest flexural modulus compared to the other materials at 481,000 psi [21]. PET has a flexural modulus of 400,000 psi and PC had a flexural modulus of 345,000 psi [21]. However, there was a PC that was 20% glass-filled that had a stiffness of 800,000 psi [21]. They are tied because PET has a higher flexural modulus than PC but filling it with glass fibers increases it significantly. Next was PP, with a flexural modulus of 225,000 psi and HDPE came up in the rear with 200,000 psi [21]. PP is a bit stiffer than HDPE because PP has the methyl group on one of its branches in its chain, making the material stiffer than HDPE [22, 23]. The impact resistance of the material was chosen for this matrix because the fork must go through intense scraping on steel and cement. The fork tines need to be able to withstand numerous scrapes against steel so that the tips of the tines do not shear off. Unlike metal tines, the tines on this pitchfork are going to be hollow, so it is very important that the material can withstand so much abuse at the tips of the tines. It is also ranked at 30 points because scraping on metal is another main application of the manure fork in the agricultural industry. Polycarbonate scored the highest because it has the right balance of ductility and stiffness that is needed for this application. However, polycarbonate is somewhat susceptible to scratches. This means that the material eventually will get scratched up enough that the hollow tines might break off. However, scratch resistant coatings are put onto safety glasses made from polycarbonate that help prevent scratches from occurring [24]. A coating can be put onto the fork so that the material has an extra layer of protection on the surface of the part. Polycarbonate is also susceptible to stress concentrations. In Notch Izod testing, a stress concentration is put onto the test bar in the form of the notch. However, with the notch, polycarbonate still has notched- Izod readings around 12 to 16 ft lbs./in [21]. Polyethylene, polypropylene and PET were rated equally on the material matrix. Both polyethylene and polypropylene are very ductile materials. Their notched-Izod values are higher than a lot of polymers because due to their lower glass transition temperatures, they act like a rubber at room temperature. The chains in both these polymers have lots of space in between the chains, allowing more room for the chains to move and absorb energy when receiving an impact force [22, 23]. The range of operating temperatures was chosen for the matrix because the material of the fork needs to withstand a wide range of temperatures, since the fork will be used throughout the course of a year. Dairy farming is a year-round operation. Cleaning the barn is also a year- round operation, so the fork needs to be designed to be used all year. Farming occurs during the hottest parts of the year and the coldest parts of the year. Plus, the weather conditions differ from state to state. It is ranked at 20 points because the creep resistance and the impact resistance of the material will be tested at each extreme of the temperature spectrum that was previously set. However, the temperature of the environment will not prove if the fork fails in application. The tests will be if the fork will fail at the temperature. Polycarbonate is ranked first in this category. Polycarbonate has very high heat resistance. It can withstand a temperature range of -215 F to around 260 F without losing any of its properties nor its stability [25]. It rivals the heat resistance of PEEK. PET and HDPE follow polycarbonate. HDPE has a glass transition temperature of around -180 F, so it can still maintain its material properties at the lower extremes of -40 F listed in the specifications [22]. It will not become a brittle material at normal atmospheric temperatures. However, it has a lower melt temperature than most materials, but it would still be stable enough for the higher extreme temperature stated in the specifications. PET has poor to fair thermal properties. However, it will be able to withstand most of the temperatures as described in the specifications. PVC will start degrading at temperatures around 170 F and would give off hydrochloride gas once it starts degrading, however that temperature exceeds even the highest temperatures ever recorded throughout the history of the United States. Polypropylene is last because it cannot maintain its properties below its glass transition temperature. Its glass transition temperature is approximately 15 F [22]. This means that polypropylene will act more brittle at temperatures lower than 15 F. This is not acceptable because the temperatures that the United States encounters over the winter months are lower than 15 F. The fork would be brittle and would fail quicker at the lower temperatures and would become useless. The material cost was chosen for the matrix because one of the issues with forks made from wood and metal is that they are too expensive. These forks can cost up to $45 [26], the main cost coming from the wooden handle. The forks that have parts of plastic on them are considerably cheaper, but still cost around $30 because of the wooden handle. Plus, these forks have multiple components that need to be assembled in a separate process. This is where the design of the fork to be one continuous part will cut down on cost. It is ranked at 10 points because it will eliminate the higher costs of wooden handles and metal tines, however some of the cost of the fork will reduced because the design of the pitchfork allows the fork to be created in one process with one mold, eliminating the assembly cost and other secondary operations of the part. Polyethylene is the cheapest material out there, averaging around $0.50 - $0.65 per pound of material. Polypropylene is also a low-cost material, averaging around $0.60 per pound [27]. The other three materials have a price range around $1.25 - $2.50, depending on the specific grade of material, the brand of material and the volume at which it is sold at [25, 28, 29]. Polycarbonate and PET will be more expensive than the commodity plastics because they are engineering polymers with more enhanced properties [24]. PVC can be a little cheaper since it is a commodity plastic. Finally, the chemical resistance of the material is a criterion for this matrix because it should withstand the chemicals inside of manure, dirt and sawdust. The fork must not corrode when being in contact with the manure. However, the chemicals inside manure are not that corrosive to plastics. This is ranked at 10 points because it should be resistant to the chemicals inside manure, but the chemicals are not that corrosive to begin with. Both polyethylene and polypropylene have very high chemical resistances, so they were tied. PET also has very good chemical resistance to most chemicals out there, so it was third. These three scored high on chemical resistance because these materials are semi-crystalline materials. Part of the chemical structure of semi-crystalline materials are folded up into crystalline structures. These are dispersed within amorphous regions of the material. These crystals make it more difficult for chemicals to break down the polymer chains, therefore making it harder to break down the polymer. PVC’s large chlorine atom in its chemical structure makes it chemically resistant to alcohols, fats, alkalis, vegetable oils and other chemicals, but it is susceptible to other chemicals like hydrocarbons and aldehydes. Polycarbonate is resistant to alcohols and dilute and concentrated acids but is susceptible to alkali solutions and very strong acids [26]. However, the chemicals in cow manure do not go that extensive. The composition of cow manure consists of grass, the minerals in their grain, like copper, selenium, bicarbonates, sodium, magnesium, nitrogen and phosphorus. Polycarbonate is the best material for the fork according to the specifications created for the pitchfork. It has the right balance of impact strength, temperature resistance, creep resistance, chemical resistance and cost of material. The specific grade of material that will be used is Lexan Resin 153R created by Sabic [30]. It is a blow molding grade of polycarbonate that has a high impact resistance, a high melt strength and is available as a clear material. Colorant will be used, so that the fork is not clear. Sabic includes UV stabilizers in the 153R grade of polycarbonate.
Manufacturing Details
The specific type of extrusion blow molding that will be used to produce the pitchfork will be intermittent extrusion blow molding. Intermittent extrusion blow molding is a variation of extrusion blow molding where the material is pushed through an accumulator head, which is a chamber that collects an amount of molten plastic and a pair of cylinders that expel the resin into a shot and control the shape of the plastic as it leaves the accumulator [31]. When the volume of resin pushed into the accumulator reaches the capacity for the part, the polymer is then pushed through an extrusion die around a core which creates a tube that determines the inner diameter inside of a ring that creates the outer diameter of the part [31]. The wall thickness of the parison can be changed as it forms through moveable dies [31]. The intermittent method of extrusion blow molding will work for the plastic pitchfork because it is a very large part, about five feet long in total length, and it is a complex part in terms of its shape. The intermittent process is used to create playground equipment and toys, 55- gallon drums, carrying cases for tools and musical instruments, baseball bats, hospital beds and so many more parts whereas the continuous process is primarily used for making bottles and small drums [31]. The biggest reason that continuous blow molding will not work for the pitchfork because if the fork was made for continuous blow molding, the parison would be very long. At the very least, the parison would be five feet long. It will be longer to account for the entire height of the mold. With a parison so long, the parison would become very heavy and would be subject to the effects of gravity. Parison sag would stretch the parison out so much that the wall thickness of the parison would change. With intermittent blow molding, the parison will not be as long. Since the material will be continuously fed into the machine, the parison will be shorter, reducing the chance of extreme wall thickness variation due to parison sag. The fork will be molded using only one material, so it will go through single layer blow molding. Single layer blow molding is the simplest and most common variation of blow molding [32]. It only consists of a single material and can be used in both continuous and intermittent blow molding. Multi-layer blow molding will not work for the pitchfork because the fork will only be made from one specific grade of polycarbonate and does not need another material to be blended with the polycarbonate. Similarly, this is the reason why sequential, flashless extrusion blow molding and 3-D blow molding will not be used for the process of making the fork. They all involve the process of extrusion of multiple materials to create a blend to be used to create the parison [32, 33]. The specific grade of polycarbonate that was chosen is enough to satisfy and meet the part specifications of the part, so there is no need for any variation of blow molding that extrudes using two materials. Injection blow molding and press blow molding will also not be acceptable in producing the pitchfork. Injection blow molding needs a preform that is created using injection molding to create the shape of the part. The parison is then inserted into the preform and air is blown into it to conform to the shape of the preform. Press blow molding combines extrusion blow molding and injection blow molding to create hollow parts [32]. It provides high control for product dimensions and wall thickness with fast cycles with little scrap or tool changeover [32]. It will be very difficult to create a preform of the pitchfork with injection molding, since the fork is designed to be a hollow part and injection molding cannot create large hollow parts with ease without using secondary operations and two separate molds to do so, increasing tooling cost The fork is simply too large for an injection molding machine to mold the preform for the fork because this part would need a machine that can hold the mold between the tie bars and enough clamp tonnage to support the mold while molding. Even if the preform is made, the complex shape of the fork might hinder the blowing portion of the process, since the material will be blown inside the preform. It might be difficult for the material to reach the tines of the fork and conform to the shape of the preform. The material tube might rip when it reaches the tines. Press blow molding would increase the tooling cost of the fork, since it uses injection molding with a combination of injection blow molding and extrusion blow molding. A head type also needed to be chosen. For the pitchfork, the accumulator head type was chosen. It was chosen because this head is used specifically for the intermittent method of extrusion blow molding. This is the combination of an extrusion head with a first in and out tubular ram-melt accumulator [32]. Figure 7 shows what an accumulator head looks like.
Figure 7: Accumulator Head
The accumulator head is specifically used for long and heavier parts and is designed to avoid parison sag [32]. It can also be customized for multi-layer and sequential blow molding [32]. Since the pitchfork is a very long part that will be subject to parison sag if being produced with continuous blow molding, the accumulator was chosen as the best head type for the pitchfork, since the fork will be produced with intermittent, or accumulated, blow molding. The axial flow head, or the spider head type, would also work with the pitchfork, since the spider head works well with materials that require precision of temperature control and flow behavior, such as PVC and polycarbonate [34, 35]. However, with the intermittent method of blow molding, the material needs to be fed into the screw sporadically so the parison is not so large, since the fork is a very long part. Plus, the accumulator head type is specifically designed for the intermittent blow molding process. The radial flow head design is not applicable for molding the pitchfork because the material enters from the side. With such a long part, the material would need to be entered closer to the top of the part. The machine that will be used to create the blow molded pitchfork using intermittent extrusion blow molding will be the 35 lb. Cincinnati Milacron Single Accumulator Head Blow Molding Machine Model T-1100 [36]. This machine has the capabilities to use the required accumulator heads and the platens are large enough to encompass the entire part. The total part length for the fork is around 56 inches. It has a platen space large enough for the mold of the part to fit onto when molding. Since this is a very long and narrow part with only a width of 12 inches, it is harder to find a machine that can take the pitchfork mold. This platen size was the best platen size that was found. It is a little big for the width of the mold, however it is a blow molding machine that specializes in intermittent extrusion blow molding and comes with an accumulator head type with it. One mold had a platen size of 110 inches X 74 inches. That machine is larger than what the pitchfork mold needs to fit on. There is enough clamp tonnage to support the pitchfork mold with a platen just big enough to hold the mold in place without the platen size being larger than what it needed to be. Figures 2 and 3 show a picture of the machine and some of the specs on the website’s data sheet [36].
Figure 8: Cincinnati Milacron Accumulator Machine
Figure 9: Spec sheet of Milacron Machine
Preliminary Calculations
Calculations need to be performed to find the thickness of the pitchfork that will be able to hold the load of material onto the fork, the area moment of inertia of the cross section of the handle of the pitchfork and the maximum vertical deflection of the fork with the load on the tines of the fork. Calculations also need to be performed to find out on average how many times this load will be applied on the pitchfork on an average dairy farm over the course of the year. The fork will be analyzed as a cantilever beam with a single load at the free end of the beam. The attached end of the beam represents the farmer holding the fork at the worst position, using both hands to hold the fork at the very end of the handle. The maximum deflection at this load condition should be no greater than 1 inch. These calculations will be used to analyze the deflection of the part at the first loading condition of the fork explained in the specifications for the ANSYS study. The fork is best modeled by a cantilever beam because the pitchfork is a very long part that has one single load acting at the end of the part. Figure 10 shows what a cantilever beam looks like [37]. In this case, the handle will be represented as the beam. The portion of the fork with the tines is represented by the single load at the free end. This simplifies the overall model to solve for the wall thickness of the fork. The confined end of the beam will represent a person holding the fork at the very end of the handle with both hands. This accounts for the worse-case scenario of someone holding the fork since the fork is not held this way to pick up material from the ground. With both hands spread out across the handle of the fork, the deflection of the part will be less that the deflection of the part with the hands at the end of the handle. The load in this case will be 20 pounds. The load that was calculated in the part specifications is too light of a load for the fork to handle, so the load was doubled to account for more weight. The length of the beam in this case is 44 inches, which is the length of the fork handle.
Figure 10: Figure of Cantilever beam [37].
Multiple spreadsheets were created to produce these calculations. To figure out the wall thickness of the part that will comply with the maximum deflection of the handle, a recommended wall thickness range for polycarbonate was needed. The recommended wall thickness range of polycarbonate ranges from 0.040 inches to 0.150 inches [38]. The first wall thickness that was tested was at 0.125 inches. The original outer diameter of the handle was 1.625 inches. For the wall thickness to be 0.125 inches, the inner diameter of the handle will be 1.375 inches. The calculation for the thickness is shown in Figure 11. Once this was known, the area moment of inertia needs to be calculated. The cross-sectional area of the fork is a hollow circle. The area moment of inertia equation for this cross-section is shown in Figure 12 [39]. For the outer and inner diameters that were chosen, the moment of inertia is 0.167 inches4. Finally, the deflection of the handle needed to be calculated to see if the fork can withstand the deflection of the part. The material modulus needs to be found before this calculation can occur. According to the material data sheet for the Lexan Resin 153 R PC, the material chosen in the material matrix, the flexural modulus of the material is 2340 MPa or around 339,400 psi. This calculation is also shown in the spreadsheet below. The equation for this load setting for the cantilever beam is shown in Figure 13 [40]. The final calculation for the deflection of the fork handle was around 10 inches, which is ten times higher than the recommended maximum deflection of the handle.
Outer Diameter of Handle 1.625 inches Inner Diameter of Handle 1.375 inches Length of Handle 44 inches Flex Modulus of PC Lexan 153R 2340 Mpa Force 20 pounds psi in 1 Mpa 145.038 psi
Flex Modulus in psi Modulus Mpa * psi/1 Mpa 339388.92 psi
Moment of Inertia pi*(OD^4-ID^4)/64 0.167 inches^4
Thickness OD/2-ID/2 0.125 inches
Deflection of handle (Force * Length^3)/(3*Modulus*Inertia) 10.030 inches
Figure 11: Spreadsheet 1
Figure 12: Equation of Moment of Inertia of Hollow Circle [39].
Figure 13: Equation of Deflection of Cantilever with Load at Free End [40].
Since the deflection of the handle is way too large for the current design, modifications needed to be made to the dimensions of the part. First, the thickness of the part was increased from 0.125 inches to 0.150 inches, which is the maximum wall thickness for polycarbonate. This raised the moment of inertia of the cross-sectional area of the handle to 0.191 inches4 and lowered the inner diameter to 1.325 inches. However, this change only lowered the maximum deflection of the handle to around 8.75 inches. These calculations are shown in the spreadsheet below.
Outer Diameter of Handle 1.625 inches Inner Diameter of Handle 1.325 inches Length of Handle 44 inches Flex Modulus of PC Lexan 153R 2340 Mpa Force 20 pounds psi in 1 Mpa 145.038 psi
Flex Modulus in psi Modulus Mpa * psi/1 Mpa 339388.92 psi
Moment of Inertia pi*(OD^4-ID^4)/64 0.191 inches^4
Thickness OD/2-ID/2 0.15 inches
Deflection of handle (Force * Length^3)/(3*Modulus*Inertia) 8.761 inches
Figure 14: Spreadsheet 2
The next change to the design that was made was changing the diameters of the fork handle. The original design of the handle had a dimension of 1.625 inches. This would allow a person to easily grab the handle without dropping it or losing grip of the handle. Increasing both diameters of the fork handle will increase the moment of inertia which will decrease the deflection of the handle. The outer diameter of the part was changed to 2 inches and the inner diameter was changed to 1.7 inches to account for the wall thickness of 0.150 inches. With this change, the moment of inertia increased to 0.375 inches4 and decreased the deflection to 4.457 inches. This is still too large, but it is heading in the right direction. The calculations are shown in Figure 15. The diameters of the handle were increased in 0.25-inch increments until the outer diameter was 3 inches. The outer diameter should not exceed 3 inches because this diameter is starting to get too large for a person’s hand to grab onto the handle. At an outer diameter of 3 inches, the moment of inertia increased to 1.367 inches4 and the deflection decreased to 1.224 inches. These calculations are shown in Figure 16. Outer Diameter of Handle 2 inches Inner Diameter of Handle 1.7 inches Length of Handle 44 inches Flex Modulus of PC Lexan 153R 2340 Mpa Force 20 pounds psi in 1 Mpa 145.038 psi
Flex Modulus in psi Modulus Mpa * psi/1 Mpa 339388.92 psi
Moment of Inertia pi*(OD^4-ID^4)/64 0.375 inches^4
Thickness OD/2-ID/2 0.15 inches
Deflection of handle (Force * Length^3)/(3*Modulus*Inertia) 4.457 inches
Figure 15: Spreadsheet 3
Outer Diameter of Handle 3 inches Inner Diameter of Handle 2.7 inches Length of Handle 44 inches Flex Modulus of PC Lexan 153R 2340 Mpa Force 20 pounds psi in 1 Mpa 145.038 psi
Flex Modulus in psi Modulus Mpa * psi/1 Mpa 339388.92 psi
Moment of Inertia pi*(OD^4-ID^4)/64 1.367 inches^4
Thickness OD/2-ID/2 0.15 inches
Deflection of handle (Force * Length^3)/(3*Modulus*Inertia) 1.224 inches
Figure 16: Spreadsheet 4 Finally, the deflection is getting close to the recommended deflection, yet it is still too large. The only other thing that can be changed is the length of the handle. According to the formula in Figure 3, decreasing the length of the beam will decrease the deflection of the beam. The length of the handle was decreased by 1-inch increments until the deflection was below 1 inch. In the end, the final fork handle length is 40 inches and the maximum deflection is 0.919 inches. These calculations are shown in Figure 17.
Outer Diameter of Handle 3 inches Inner Diameter of Handle 2.7 inches Length of Handle 40 inches Flex Modulus of PC Lexan 153R 2340 Mpa Force 20 pounds psi in 1 Mpa 145.038 psi
Flex Modulus in psi Modulus Mpa * psi/1 Mpa 339388.92 psi
Moment of Inertia pi*(OD^4-ID^4)/64 1.367 inches^4
Thickness OD/2-ID/2 0.15 inches
Deflection of handle (Force * Length^3)/(3*Modulus*Inertia) 0.919 inches
Figure 17: Spreadsheet 5
The previous spreadsheets have shown the calculations for the linear analysis portion of the ANSYS study of the fork, however there is a nonlinear side to the ANSYS analysis. Since the first loading condition is only a short-term load, a possible maximum time of five seconds when pitching the dirt or other material off the fork, a nonlinear analysis of the fork needs to be conducted to see if the fork would fail anytime during the year. The number of load steps or sub-steps needs to be calculated to determine how many times the fork will approximately be used in one year on an average dairy farm. This will be calculated by multiplying the amount of piles per day by the number of cows in an average dairy barn multiplied by the number of days in a year. This turned out to be 912,500 repetitions per year for an average farm. Spreadsheet 6 shows these calculations. Since this number is extremely large and would take a very long time to run the analysis, this number might be cut down by a significant amount, possibly analyzing the use of the fork over one month for one cow, which would add up to 300 sub-steps. number of cows in average barn 250 cows number of piles per day 10 piles number of days in a year 365 days number of repetitions per year # cows*# piles* days in a year 912500 piles Figure 18: Spreadsheet 6 Part Design 1
Designing the pitchfork to be made from extrusion blow molding was a challenging task. Multiple factors had to be considered. These included where the parting line of the part was going to be, what kind of structural components would be needed to hold a specified load, what the blow ratio of the parison was going to be, what type of radii or chamfers were going to be incorporated into the design and what draft angles would be included. These design factors would also tell how the part was going to be produced and what, if any, secondary operations would be needed to produce the part. The pitchfork needed to be one continuous part to eliminate the costs of secondary operations due to assembly. The tines could not be very long and narrow like they are in a wooden fork with metal tines. The tines needed to account for the wall thickness of the entire part while providing space for the pressurized air to enter the tines and push the material to the mold surface. This all needed to be done while maintaining the original look and feel of a
Figure 49: Original Hand Sketches of the Design wooden pitchfork while making it more advanced at the same time. Figure 19 shows the original design sketches of the fork and Figure 20 shows the final CAD model of the fork.
Figure 50: Final CAD Model
Structural Requirements – Ribs, Tack-Offs and Gussets
Extrusion blow molding allows the production of very large parts to be made with ease. However, some parts produced might need additional structure added to withstand any forces and loads that the part might encounter. In the case of the pitchfork, this is no exception. The decision needed to be made if the fork needed any additional support added to the design. A lot of factors came into play to determine if the fork needed more support. Wall thickness needed to be considered because the larger the wall thickness of the part, the stiffer the part would be, and the part would in return have a larger moment of inertia to withstand loads better. The wall thickness could not be too thick, however, to prevent the moldability of the part to be inhibited. The diameter and length of the handle needed to be considered as well, because increasing the diameter would decrease deflection but making it too large would make it difficult for a person to hold the fork. Decreasing the handle length would also decrease the deflection, but the handle cannot be too short to inhibit the usage potential of the fork. The shorter the handle, the harder it is to use and the more someone would have to bend over to use the part. Two structural additions were inspected. One idea was to incorporate ribs and tack-offs into the structure of the fork [41]. Adding ribs would increase the rigidity of the part which in turn would decrease the overall deflection of the part. However, it is very difficult to add ribs inside the handle without losing any moldability. Plus, the part is a hollow round cylinder and would be difficult to model those in a CAD modeling program. The next idea was adding tack- offs to the back of the tines. The tack-offs would also provide more structure and added rigidity to the part. These would be easier to mold and design, but the addition of tack-offs would decrease the overall appearance of the fork, not making it look sleeker than the original design while making it less like the original design. This idea was still not discarded. The other idea was to add a structural foam to the inside of the part [41]. This would increase the area moment of inertia into the part which would increase the stiffness of the part while maintaining the look of the part. However, this would add a secondary operation to the part, increasing the cost of the part, which was something to be avoided. Another material cost would also be added to the total part cost of the fork. There would also be another surface defect put onto the part which would be the hole that would have to be cut out if the foam were to be injected into the part [41]. To see if the fork needed any more additional structural support, the preliminary calculations had to be referenced. In one of the earlier designs of the part, the handle length was 44 inches, the outer diameter was 1.625 inches and the wall thickness was 0.125 inches. These dimensions allowed the fork to be picked up by a farmer with ease. However, the deflection calculated of the fork was over 10 inches. This was ten times the recommended deflection of the fork, which was 1 inch. Figure 21 shows the earlier design of the part with those dimensions.
Figure 21: Earlier Updated Design of the Fork To try and lower that deflection to 1 inch, one variable had to be changed at a time until the deflection met the specifications. First, the wall thickness was increased to 0.15 inches. The wall thickness could not get any higher than this because this was the highest recommended wall thickness for the grade of the polycarbonate that was chosen. The deflection did decrease, but only to 8.75 inches. Figures 22 and 23 show the change in wall thickness that was produced.
Figure 22: Wall Thickness at 0.125 inches
Figure 23: Wall thickness at 0.15 inches The next value that was changed was the outer diameter of the handle. Increasing the diameter would decrease the deflection of the part. The diameter was increased from 1.625 inches to 2 inches. This cut down the deflection to around 4.5 inches which was still not low enough. Figure 24 shows the increase of the diameter of the handle. The diameter was increased again to 3 inches where the deflection decreased to 1.25 inches. Figure 25 shows the increase of the diameter to 3 inches. Modeling this, however, needed a change in the depth of the tines to allow the diameter to be 3 inches. This depth was increased from 2.5 inches to 3.5 inches as shown in Figure 26. Figure 26: Depth of Tines at 3.5 inches
Figure 24: Diameter at 2 inches Figure 25: Diameter at 3 inches Finally, the length of the handle was decreased to around 40 inches. This decreased the deflection to less than 1 inch which was the recommended deflection set for the pitchfork. Figure 27 shows the shortening of the fork handle.
Figure 27: Shortening of Handle These modifications to the original design allowed the maximum deflection to be met. However, this caused for the handle to be too large for a person’s hands. It would be very difficult for anyone to grab on to this fork and use it successfully. It looked like that the fork needed to have additional structural support. However, there was one more set of calculations that needed to be calculated. The calculations that were explained above were based off the idea that the fork acted like a cantilever beam when in use. A cantilever beam is where one end of the beam is fixed, and the other end has a load pushing down on the beam. With this model, it can be illustrated that the fork was being held with both hands at the end of the handle, which is the worst way to hold the handle. No one can hold anything at a length of almost five feet long and be able to use it successfully. There was a better way to model the fork and how it was being used. An overhang beam with a simple support and a rolling support was used to see if the design could be changed again. Figure 10 shows what the beam looks like and the equation for the calculations [42]. The two supports represent the hands of the person holding the fork handle, with one reaction force acting upwards on the handle and the other reaction force acting down onto the handle with the load acting on the free end of the part. To produce these calculations, the assumptions were made that the average spread of a person holding on to a fork measured from 24 to 30 inches, making this the “L” dimension and the remainder of the length of the handle to range from 14 inches to 20 inches, which is the “a” dimension. The following spreadsheets shows the calculations of the deflections as well as the reaction forces acting on the fork handle at each case calculated.
Figure 28: Diagram and Deflection Equation for Overhang Beam Case G [42]. Table 1: Calculations at Original Wall Thickness, Length and Diameter at 24 and 20 inches
Outer Diameter 1.625 inches Inner Diameter 1.375 inches Wall Thickness 0.125 inches Length 44 inches Flex Modulus of Material 339389 psi Force 20 pounds L 24 inches a 20 inches
Moment of Inertia pi*(OD^4-ID^4)64 0.167 inches^4
Deflection at the end ((-Pa^2)/(3EI))/(L+a) 2.07 inches Table 1: Reaction Forces at 24 inches and 20 inches at Original Design Dimensions
Force 20 pounds Sum of the Moments will be found analyzed at Reaction A Upward Forces will be positive CCW Moments will be Positive
Ʃma = 0 L 24 inches Overall Length 44 inches
Force B (Distance of Load from A* Load) /(Distance of B From A) 36.667 pounds
Force A Force B - Load 16.67 pounds
Table 3: Calculations at 0.15 inches WT, OD 2 inches and 24 inches and 20 inches
Outer Diameter 2 inches Inner Diameter 1.7 inches Wall Thickness 0.15 inches Length 44 inches Flex Modulus of Material 339389 psi
Force 20 pounds L 24 inches a 20 inches
Moment of Inertia pi*(OD^4-ID^4)64 0.375 inches^4
Deflection at the end ((-Pa^2)/(3EI))/(L+a) 0.92 inches
Table 4: Calculations of Deflection at OD 2 inches at 30 inches and 14 inches
Outer Diameter 2 inches Inner Diameter 1.7 inches Wall Thickness 0.15 inches Length 44 inches Flex Modulus of Material 339389 psi Force 20 pounds L 30 inches a 14 inches
Moment of Inertia pi*(OD^4-ID^4)64 0.375 inches^4
Deflection at the end ((-Pa^2)/(3EI))/(L+a) 0.45 inches Table 5: Reaction Forces at 30 and 14 inches at recommended design
Force 20 pounds Sum of the Moments will be found analyzed at Reaction A Upward Forces will be positive CCW Moments will be Positive
Ʃma = 0 L 30 inches Overall Length 44 inches
Force B (Distance of Load from A* Load) /(Distance of B From A) 29.333 pounds
Force A Force B - Load 9.33 pounds
With these calculations, it is shown that the overhang beam is a more accurate model of the fork handle. It also shows that the recommended design of the fork handle will have a length of 44 inches, an outer diameter of 2 inches and a wall thickness of 0.15 inches with a deflection of 0.92 inches with the hands 24 inches apart and a deflection of less than 0.5 inches when the hands are 30 inches apart. This means that the fork can still be easily used by a person without compensating for moldability and function. This means that there will not be any ribs, gussets, tack-offs or any material being injected into the part for any structural support. However, we can analyze the latter in ANSYS to see if the deflection decreases from the values above.
Parting Line All blow molding tools open and close, so a parting line needs to be created for the pitchfork to allow the mold to close and to make the plastic form to the mold surface. In bottles and tanks, the parting line runs through the middle of the part, splitting the part into two identical pieces. The parting line for the bottles is in the middle because the bottle has at least one axis of symmetry. In fact, bottles have multiple axes of symmetry so the parting line for a bottle can be right through the middle of the part [41]. For the pitchfork, the parting line will go right through the middle of the part. Since this part is a longer part with one axis of symmetry, the part and the mold will be split into two equal parts, making it easier to design the mold for the part. Figure 29 shows where the parting line
will be on the part. The parting line will cut through the part horizontally, because the seam needs to be on the side of the fork. Irregular geometry on both halves of the mold will make it harder to design the mold halves. Toward the bottom of the mold, the plate thickness will change due to the curved nature of the part.
Figure 29: Parting Line Placement
Blow Ratio When the parison has reached the desired length for the mold, the mold closes and the parison is blown out from the center by air pressure. As the parison expands, the parison wall thickness decreases because the surface area is increasing while the volume of material remains constant. The ratio between the starting parison thickness and the final part wall thickness is called the blow ratio. [41] The blow ratio for this part was calculated using the widest section of the part, the fork head. The shape was also simplified into a rectangle of 12” by 2.5” with a depth of 12”. The desired wall thickness for this part is 0.150”. Using this thickness and the total surface area of the cavity, the volume can be calculated. The total cavity volume is 56.7 cubic inches. That means the same volume of material must be in the parison. To achieve this, the parison diameter must be 9.8” and the parison thickness must be 0.750”, resulting in the two volumes being equal. With the starting parison thickness being 0.750” and the final wall thickness being 0.150”, the blow ratio is equal to 5. Radii In extrusion blow molding, as in any other plastics processes, it is impossible to create a part with very sharp corners. These sharp corners will create stress concentrations in the part, leading to failures at those sharp corners, such as cracks. These cracks can allow for more cracks to form, eventually leading to the part chipping off or breaking into huge chunks. For the fork, rounds were included into this structure to allow for even wall thicknesses and to create a new and sleeker design of the pitchfork, while keeping some of the original features of the pitchfork Some of the rounds put onto this part were part of the actual design. These rounds came from the original design of the fork. The round at the top of the handle is set to equal half the diameter of the handle. Most forks have a circular round on top of the handle when manufacturing the handle out of wood. In this case, the radius at the top of the handle is set at 1 inch, so that it has that circular effect. This was added before the fork was shelled because the shell will create an even wall thickness for the top of the handle without adding another round feature. This round is shown in Figure 30.
Figure 30: Round at Top of Handle
The rounds on the side of the fork were also based on the original design of the fork. The tines on a regular fork are rounded on the sides. This was done to the plastic pitchfork to keep the aesthetic of the fork like the wooden forks. These rounds were designed to have a radius of 4 inches. This was the radius that looked most aesthetically like a wooden fork. Other radii made the fork look too wide or too narrow. A 4-inch round had the balance of making the tines the right size for the application while making it easier to blend the tines into the handle. Figure 31 shows this design decision.
Figure 31: Side Round of Tines One of the design criteria for the fork was that the handle had to be blended into the tines. This would allow the fork to be molded as one part, decreasing the tooling cost associated to the part cost of the fork. To achieve this, the fork handle needed either a chamfer or a round at the base of the handle. Either one of these would allow the fork to hold the loads put upon it easier. At first, the fork did not have a chamfer or a huge radius as shown in Figure 32. However, this did not allow for a smooth transition from the handle to the tines.
Figure 32: Fork with No Chamfer or Radius - Original Design A chamfer was added to see how the transition between the two features would look. The chamfer or radius should be at least a quarter of the way up the handle. In the original design, the handle length was 44 inches, so the chamfer or radius should be 11 inches tall. When creating the
Figure 33: Addition of Chamfer chamfer, to attain a height of 11 inches, the width of the chamfer would only be a little over 1 inch. Increasing the width of the chamfer would make the feature fail. Figure 33 shows what the chamfer looked like in the model. It provided more of a transition, but more was needed. A radius of 11 inches was added to the part. It provided a much better transition between the parts than the chamfer did, so the radius was chosen for the end of the handle. Even though chamfers are better than radii than blow molded parts, because chamfers reduce the stretching of the parison better than radii, the radius was chosen because the transition between the handle and
Figure 34: Fork with the 11-inch Radius the tines was smoother and would allow the pressurized air to reach the tines better than the chamfer would have [1]. Figure 34 shows how the radius at the end of the handle looks like. The other radii were the rounds in each of the sharp edges and corners of the part. These allowed the wall thickness of the part to be even while eliminating sharp corners and stress concentrations on the part. For these, the inner radii were one half the size of the wall thickness and the outer radii were one and a half times larger than the wall thickness of the part. Since the wall thickness that was chosen for the part was 0.15 inches based off the hand calculations and the recommended wall thickness for the material, the inner radii had a radius of 0.075 inches and the outer radii had a radius of 0.225 inches. Draft After the part has been formed inside the mold, it cools and shrinks. The shrinkage can be an issue because the material will shrink onto core features and be difficult to remove from the mold. To avoid this, draft was added to all non-round features to assist with the removal of the part from the mold. To minimize the amount of draft visible to the consumer, angles of 0.5° were used. Mold and Tooling Details The pitchfork mold will be made through casting methods. Since the mold is over five feet in length and almost two feet in width, it is too large for a CNC machine to machine the parts of the mold. The mold will be made from aluminum, because aluminum has a high thermal conductivity and get the heat out of the mold more efficiently. It is also cheaper than other metals. A shrink factor of 5% was considered to account for the shrink rate of the material [30]. Die, Mandrel, and Parison Programming
Figure 35: Drawing of Die and Mandrel for Pitchfork Mold The parison diameter needed to fill this mold is large, at least 6 inches in diameter. With a parison diameter of that size, a diverging die is required. This is shown above in Figure 35.
Figure 36: Rough Sketch of Parison Programming A profile of the parison program is shown above. Starting from the bottom, the parison thickness will be very close to the desired wall thickness because the bottom of the parison does not have to expand very far in order to fill the tines. The bottom of the parison also does not have much weight below it, so it will not sag or thin due to stretching. Just above the tines is the area of this pitchfork that will require the parison to expand the farthest, stretching and thinning the most. To combat the thinning due to stretching in this section, the parison program will move the mandrel to increase the width of the land between the die and mandrel, allowing more material to flow out, resulting in an increase in parison thickness. The rest of the parison above those sections will form the handle of the pitchfork. The parison does not need to expand far to form the handle, so the parison thickness will be very close to the desired part wall thickness. This section, however, will experience parison sag due to the weight of all the material below it. The parison program can slowly increase the parison thickness towards the top of the handle to help combat the effects of parison sag, resulting in a more uniform wall thickness. Draft Analysis of Mold The draft analyses for each side of the mold are shown in Figures 37 and 38. The draft analysis for each half shows that no undercuts were produced in the creation of the tool. This means that the part should eject with ease.
Figure 67: Draft Analysis of Back Half of Mold
Figure 38: Draft Analysis of Front Half of Mold
Overall Mold and Waterline Details Pictured in Figure 39 is the pitchfork mold. This mold was modeled in Creo Parametric 5.0 and was modeled with a shrinkage factor of 5% [30]. For this mold, two mold plates needed to be modeled with waterlines, pinch-off details, flash pocket details, venting and blow pin details for each half of the mold. These mold drawings are in metric. When creating the mold, the template used to create each half was a metric template because an English template was not created. Therefore, the drawings are automatically metric, and the units could not be changed.
Figure 39: Overall Views of Mold Figure 40 shows the waterline details on the front half of the mold. Each of the seven waterlines that were designed were single channel waterlines. Each waterline provides its own inlet and outlet. Since the mold is over five feet long, it would be difficult to drill a hole from the top of the mold to the bottom of the mold to create a series circuit. Since the width of the mold is approximately 18 inches across, it is easier for a machinist to machine a hole 18 inches long rather than a hole 60 inches long. Since the parting line is not a straight line toward the bottom of the mold, there is no series circuit at the bottom of the mold, because it would have been difficult to create a cooling line circuit near the bottom of the mold while simultaneously being steel safe, since the amount of steel near the bottom is thinner than at the top of the mold half. Each waterline has a diameter of 1.5 inches and are located 3 inches from the flat surface of the mold plate. The distances between each waterline vary from 4 inches to 13 inches. The waterlines closest to the tines are closer to each other because it allows the tines to cool more evenly with respect to the cooling of the handle. This will also reduce the chance of the tines warping due to insufficient cooling of the mold. Warpage to the tines will hinder the design of the part by hindering the design of the tines. The tines might shrink differently from tine to tine, allowing different tine lengths to develop. These waterlines are 4 inches apart. The waterlines cooling the handle are 9 inches apart. The distance between the fourth and fifth waterline from the left of the mold half is 13 inches apart.
Figure 40: Front Half Waterline Details
The waterline system for the back half of the mold is more complex. Shown in Figure 41, there is a series circuit near the top of the mold, a series circuit near the bottom of the mold, and some single channels running through the middle of the mold. The curved parting line near the bottom allows a series circuit to be created to cool the tines of the mold while being steel safe at the same time, since the bottom of the mold is thicker than the rest of the mold. Another series circuit was designed at the top of the mold half to make the cooling near the top of the mold equal to the bottom of the mold half. These waterlines are 1.5 inches in diameter and the center of the waterlines are 3 inches from the flat side of the mold plate. The bottom of the mold needed more waterlines to cool that part of the mold. Since there is more steel near the bottom of the mold half, the bottom of the mold needed more waterlines to make sure that the steel near the bottom gets cooled. To solve this problem, a water channel running straight through the width of the mold was created. This waterline is also 1.5 inches in diameter, 3 inches from the bottom of the mold half and approximately 5 inches from the vertical side of the mold half.
Figure 41: Back Half Waterline Details Figure 42 shows the inlets, outlets, and plugged areas of the waterlines created in the back half of the mold. Like the front half of the mold, the channels running straight through the mold serve as both inlets and outlets, because it does not matter which side of the mold half serves as the inlet side or the outlet side. The waterlines not completely drilled through the steel will be over drilled to assure that the waterlines will be drilled to the required length. Figure 42: Inlet and Outlet Details of Back Half Individual Mold Half Dimensions and Venting Figure 43 shows the overall dimensions of the front half of the mold. The front half is approximately 65.55 inches tall, 18 inches wide and 9.75 inches in depth. The mold depth narrows at the bottom of the mold to take in account of the curved shape of the tines. At the thinnest part, the depth of the mold is approximately 4.6 inches. There are six relief cuts along the sides of the mold half to help with the ventilation of the back half of the mold. Each contact area measures 3 inches tall by 3 inches wide and are 0.15 inches deep. These vents are deeper than most vents seen in other molds. Since the mold is so large, there is a lot more heat being stored in the metal. More heat will escape the mold with deeper draws. Each vent was placed in areas of least concern. The two bottom vents are placed in the bottom corners of the plate. These were placed to help air escape from the tine portion of the plate. Two vents were placed in the middle of the mold plate approximately 36.7 inches from the top of the mold plate. These were placed to help air escape from the base of the handle. More material is used to create the base of the handle, since the handle merges with the tines, so vents needed to be put near that area to release hot air from the mold and the material. The other vents were put near the top of the mold plate and are placed 2 inches from the top. These two vents allow air to escape from the handle portion of the pitchfork.
Figure 43: Overall Front Half Dimensions with Venting
Figure 44 shows the overall dimensions of the front half of the mold. The front half is approximately 65.55 inches tall, 18 inches wide and 6 inches in depth. The mold depth widens toward the bottom of the mold to take in account of the curved shape of the tines. At the thickest part, the depth of the mold is approximately 11.2 inches. There are six relief cuts along the sides of the mold half to help with the ventilation of the back half of the mold. Each contact area measures 3 inches tall by 3 inches wide and are 0.3 inches deep. These vents are deeper than most vents seen in other molds. Since the mold is so large, there is a lot more heat being stored in the metal. More heat will escape the mold with deeper draws. These vents are placed at the same spots as the vents on the front plate.
Figure 44: Overall Back Half Dimensions with Venting
Flash Pocket and Pinch Off Details Figure 45 shows the flash pocket details on the front half mold plate. This is a very large pocket because the diameter of the parison is very large, approximately 10 inches in diameter. A large parison is needed to make sure that the tine region of the fork will have the same wall thickness as the handle. Since the diameter of the parison is larger compared to the diameter of the handle, a flash pocket needed to be created. This means that there will be a lot of flash to trim off both sides of the handle. Each pocket on both sides of the part is 4 inches in length and 0.15 inches deep. The flash pocket was designed to be the same depth as the wall thickness of the part. A pinch off was created to allow for the easy removal of the extra material. The pinch off land is 0.015 inches in length and the flash angle is set at 30⁰. A recommended flash pocket angle is between 30⁰ and 45⁰ [32]. The shallower angle was used to possibly allow plastic to push into the parting line and creating a stronger seal in the process. The flash pocket stretches from the top of the mold plate to the tine portion of the fork. Figure 46 shows the flash pocket details on the back half of the mold. The flash pocket is designed very similar to the flash pocket in the front half of the mold. According to the drawing, the depth of the flash pocket is 0.12 inches. It should be 0.15 inches.
Figure 45: Front Half Flash Pocket Details
Figure 46: Back Half Flash Pocket Details
One of the most important design choices was to design the pinch off for the pitchfork mold. This was also one of the most difficult parts to model. The pinch off designed for the pitchfork mold was a single angle pinch off. This type of pinch off was chosen because it is one of the simplest pinch offs to model. For a complex part such as the pitchfork, a simpler pinch off design worked best. The pinch off runs around the tines of the fork, starting approximately 3 inches from the tip of outer tines. The land length for the pinch off on both mold halves was 0.015 inches, which is the recommended length for the land [32]. A 30⁰ was used to design the pinch off for both mold halves because it allows the plastic to create a better seal for the part. This is essential for the pitchfork, since the tines are an important feature of the pitchfork. The pitchfork should not come apart at the seal near the tines. According to the drawings, the pinch off depths are different from each other. Each was designed to be 0.15 inches deep. However, it is not the case. When modeling the pinch offs near the tines, a sweep feature needed to be created. However, multiple sweeps and extrudes were used to create the pinch off feature. Creo Parametric 5.0 would not allow the sweep of multiple edges to occur at once. Each sweep was created one by one. To speed up the modeling process, most of the sweeps and extrudes were created using references of previous sweeps. Since the geometry of the mold halves change toward the bottom of the mold, the dimensions of each sweep varied a little bit. Therefore, the depths of each pinch off design are not equal to 0.15 inches or to each other. The length of each pinch off is approximately 0.4 inches. These details can be seen in Figures 47 and 48.
Figure 47: Back Half Pinch Off Details
Figure 48: Front Half Pinch Off Details Blow Pin Details A single blow pin will be used to blow the pressurized air into the parison when forming the pitchfork. The blow pin hole should be designed to be 1.5 times the diameter of the pin [32]. A 1-inch diameter blow pin will be used. Therefore, the blow pin hole diameter was designed to be 1.5 inches. This hole is split between both mold halves. The hole stretches from the top of the mold halves to where it encounters the part. This can be shown in Figure 49.
Figure 49: Blow Pin Dimensions Detail Design 2 ANSYS Analysis ANSYS Workbench 19.2 was utilized to find out if the deflection of the pitchfork would be less or more than the maximum recommended deflection when a force is applied to the pitchfork. The ANSYS Workbench software is an essential tool for designing new parts or existing parts with new materials. It can accurately predict the mechanical properties of a part when loading and boundary conditions are applied to the part. It can also show how accurate the simulation was. Multiple analyses were conducted to see if anything needed to be added to the design of the fork. A load of 20 pounds was put on the tines of the fork. The maximum recommended deflection of the pitchfork should not exceed 1 inch. 1st Analysis – Simulation of Pitchfork as Hollow Cantilever Beam Before the analysis was conducted, material data needed to be entered. The grade of polycarbonate was not in the Workbench material database, so the data needed to be entered manually. The material properties that were entered were the density of the material, Poisson’s Ratio, and the modulus used for the material. Since this load is a flexural load, the flexural modulus was added in the Young’s Modulus box. The material data entered for the polycarbonate is shown in Figure 50. The temperature was also set to 23 ℃ or 73.5 ℉ to simulate the usage of the pitchfork on a warm summer day.
Figure 50: Material Properties of Polycarbonate The next step in the analysis was to establish a mesh to the part. A tighter mesh can lead to more accurate results. However, too many nodes and elements will make the analysis take a longer time to run. The model was meshed using the Global mesh size of 0.5 inches. The mesh contained almost 36,000 nodes and over 18,000 elements. The mesh can be shown in Figure 51.
Figure 51: Mesh of Pitchfork
Once the mesh was created, constraints were added to the model. These constraints must be added to accurately simulate what can and will occur to the pitchfork. For this analysis, a fixed support was added to the very end of the handle. This simulates the worst-case scenario of holding the pitchfork carrying the applied load. Then the 20-pound force was added to the base region of the tines. This analysis simulates the pitchfork as a cantilever beam, which was shown in the preliminary calculations. The conditions can be shown in Figure 52.
Figure 52: Constraints Added to the Pitchfork Once all the conditions were set, the analysis was simulated. With the 20-pound force added to the fork, a maximum deflection of over 7 inches in the negative vertical direction, in this case, the negative Z direction. This makes sense because since the part was hollow with a small moment of inertia and the part is very long, the deflection should be large. The von-Mises stress was also found. The maximum von-Mises stress was found to be almost 2500 psi toward the top of the handle. This makes sense because since the handle is being constrained at the top of the handle, the maximum stress will be right below the handle. Both these diagrams can be shown in Figures 53 and 54. Since this deflection is very large, either the geometry of the part needs to be modified or the loading conditions and constraints need to be changed.
Figure 53: Deflection Plot of Pitchfork
Figure 74: Von-Mises Stress Plot of Pitchfork
2nd Analysis – Simulation of Pitchfork as Simply Supported with Proper Grip The first modification that was made to the model was to add forces to the model to simulate the proper usage of the pitchfork. The fork was modeled as a simply supported beam with two supports, one at the end of the pitchfork and one about 30 inches from the top of the handle. A fixed displacement was also put upon the top of the handle to constrain the model. Figure 55 shows the constraints and loads put upon the pitchfork.
Figure 55: Constraints and Loads on Pitchfork for Second Analysis
Once these new constraints were put onto the model, the analysis was solved. The deflection of the part decreased by over 2 inches, but it was still too high. The deflection of the fork after the new constraints and loads were added was 4.5 inches in the negative vertical direction. The stress acting on the part also decreased. The maximum stress acting on the part decreased to around 1250 psi. The maximum stress on the part also covers a larger area, meaning that the handle will less likely break at a specific point. The hand calculations performed earlier did predict that the deflection would decrease, however the decrease was not enough. More modifications to the design needed to be conducted. Figures 56 and 57 show the deflection plot and the equivalent stress plot respectively.
Figure 56: Deflection of Pitchfork at Second Analysis
Figure 57: Von-Mises Stress of Pitchfork During Second Analysis
3rd Analysis – Solid Cantilever Beam of Polycarbonate One way that the design of the fork could be enhanced would be to add ribs or tack offs to the design of the pitchfork. Tack offs could be added to the inside the tines of the pitchfork to increase the rigidity and stiffness of the part, since the tines are rectangular in shape. However, it would be very difficult to add tack offs to the handle of the fork since it is a cylindrical part. Adding ribs to the inside structure would increase the stiffness of the part as well, but it would be difficult to mold those ribs using a parison. It was discussed that the part could be filled with a structural foam after the polycarbonate fork was produced. This would increase the moment of inertia of the hollow handle of the part and would increase the flexural modulus of the part, which in theory would decrease the deflection and overall stress acting on the part. The third analysis of the part was to see if the solid pitchfork would yield a smaller deflection using the chosen polycarbonate as the injected material. This analysis was used as a test to see if the deflection would decrease. If it did, a fourth analysis would commence to resemble the part with a structural foam injected into the polycarbonate shell Before the simulation commenced, a solid geometry version of the fork was uploaded into ANSYS. The material properties of the chosen material were still the same. The model was again treated as a cantilever beam to resemble the worst-case scenario of holding the pitchfork with the desired load. The deflection of the part did decrease by over 4 inches from the original analysis and by 1.5 inches from the second analysis. The equivalent stress also decreased. The part deflected less than 3 inches in the negative vertical direction. This still was not enough, but another analysis could be conducted to see if a structural foam could be injected into the hollow polycarbonate shell. Figure 58 shows the deflection plot of the fork during this analysis.
Figure 58: Deflection of 3rd Analysis Pitchfork 4th Analysis – Solid Cantilever Beam with Structural Foam A final analysis was done to see if using a structural foam that has a higher flexural modulus than polycarbonate could be used to stiffen the part. This means that the flexural modulus had to be changed in the Engineering Material Data section. Once this was changed, the analysis commenced. The model was constrained as a cantilever beam once more. This time, the deflection was under the maximum recommended deflection of 1 inch. The deflection of the pitchfork using a structural foam did decreased by almost 2 inches, resulting in a deflection just under 1 inch. Figure 59 shows this plot.
Figure 59: Deflection of 4th Analysis Pitchfork
For the final design of the pitchfork, structural will be injected into the polycarbonate shell once the main part cools. Unfortunately, this will add a secondary operation to the production of the part. Avoiding the addition of secondary operations was the goal of this design project, so that the production cost of the part could be minimized, therefore producing a cheaper, but functional part. This operation needs to be conducted to ensure that the pitchfork will not fail after many uses. However, this will still be cheaper than making a pitchfork with the current production methods, since only one tool will be used, and the cost of the materials are cheaper than metal and wood. The deflection of the part will also decrease since the force put on the end of the fork is supposed to be a distributed load. A single point load was used in simulation to ensure that the fork would handle a load at a specified point and not just over a specified area and to see how much stress the part could handle. Since loading can change throughout the life of the fork, loading conditions can vary. The deflection would further decrease by simulating the pitchfork with the structural foam and by constraining the part to resemble the fork being held by two hands, as shown in the second analysis. Polyflow Analysis A Polyflow analysis was conducted on the pitchfork to see how the parison would act when forming the part. This was done by modeling an assembly of a simplified version of the pitchfork mold with the parison in between the mold halves. Normally, a polyflow analysis uses one side of a mold and half of a parison to simulate the formation of a part. Since the parting of the mold is not a straight line, two mold halves and a parison with a full diameter needed to be modeled. Figure 60 shows the geometry of the polyflow mold and parison assembly.
Figure 60: Polyflow Geometry and Molds
Once the geometry was imported, a mesh was created for both molds and the parison. A global mesh control was used to create the mesh. The number of nodes and elements created were both approximately 41,000. Figure 61 shows the mesh of the assembly.
Figure 61: Polyflow Mesh
Once the mesh was created, the polyflow was set up for the mold close phase. This was done to ensure that the mold would close successfully with the blown parison inside the mold. This can be shown in Figure 62.
Figure 62: Polyflow Mold Close Phase Figure 63 shows the thickness of the parison inside the mold when the mold is closed. It shows that the parison has an overall thickness of approximately 0.16 inches, shown by the dark blue color. The parison should be a little bit thicker than the part thickness of the part to allow for the parison to stretch and fill the different regions of the part. The parison thickness was set to be constant and was set to 0.2 inches to compensate for the stretching of the parison and to make sure that the thickness of the part results to 0.15 inches.
Figure 63: Thickness of Parison at 0.25 seconds
Figure 64 shows the wall thickness of the parison when the parison is blown for 3 seconds. The results for this plot are not very good. At this stage, the entire part should be shown blown up as air pressure is applied to the parison. There is something strange happening toward the bottom of the part near the tines. A lot of pixels are shown at the area and the features of the tines themselves are not shown and are not defined. This could have occurred due to the complexity of the part geometry. It might be difficult for Polyflow to determine the wall thickness for a part with complex geometry and an assembly so complex. Overall, it does not give much information. However, there is some information to grasp. The overall wall thickness does seem to be constant throughout most of the part, especially the handle. This makes sense because the handle will undergo less parison stretch than the tines will. Since the handle is a narrow part compared to the diameter of the parison, smaller geometry allows the handle to be stretched less. Near the tine region, the thickness of the parison does seem to be smaller than the thickness of the fork handle, indicated by the number of blue pixels near that region. This makes sense because the parison will need to stretch more to make sure that each of the tines are blown to equal thicknesses. Since the parison must stretch more, the wall thickness near the part will be smaller than the thickness at the handle.
Figure 64: Thickness of Parison at 3 seconds
Figure 65 shows the blow ratio of the pitchfork when the parison has been blown for 3 seconds. Again, the chart shows similar problems as the previous thickness plot. However, conclusions can be made about the blow ratios of the parison. The blow ratio is approximately 1 at the handle of the pitchfork. This makes sense because since the parison is much larger than the diameter of the handle, the parison will not have to be blown very much to completely fill the handle of the part in the mold. Toward the tines, there is some parison stretch occurring. It is impossible to say how much, but there is stretching of the parison. This makes sense because the parison is smaller in diameter than the length of the tines. Some stretching will have to occur to make sure that the parison fills the mold completely. To make sure that the part has an even wall thickness, parison programming will be used to make the parison thicker near the tine region of the fork. The thicker parison will allow more material to fill that section of the part to compensate for the stretching near that part
Figure 65: Blow Ratio of Parison
Detailed Drawings Table of Contents Part Drawings ...... 66 Overall Part Dimensions...... 66 Section View of Pitchfork ...... 67 Tine Details...... 68 Mold Drawings ...... 69 Views of Mold Halves ...... 69 ...... 69 Front Half Waterlines ...... 70 Back Half Waterlines ...... 71 Back Half Inlets and Outlets ...... 72 Overall Front Half Dimensions and Venting ...... 73 Overall Back Half Dimensions and Venting ...... 74 Front Half Flash Pocket ...... 75 Back Half Flash Pocket ...... 76 Back Half Pinch Off Details ...... 77 Front Half Pinch Off Details ...... 78 Blow Pin Dimensions ...... 79
Note: The mold drawings are in metric units and the part drawings are in English Units (See Mold and Tooling Details Section).
Part Drawings Overall Part Dimensions
Section View of Pitchfork
Tine Details
Mold Drawings Views of Mold Halves
Front Half Waterlines
Back Half Waterlines
Back Half Inlets and Outlets
Overall Front Half Dimensions and Venting
Overall Back Half Dimensions and Venting
Front Half Flash Pocket
Back Half Flash Pocket
Back Half Pinch Off Details
Front Half Pinch Off Details
Blow Pin Dimensions
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Appendix
The following documents show the hand calculations used to prove that the spreadsheets were done correctly.
Figure 66: Original Loading and Dimensional Conditions of Handle
Figure 67: Hand Calcs for Spreadsheet 1 Figure 68: Hand Calcs for Spreadsheet 2
Figure 69: Hand Calcs for Spreadsheet 3
Figure 70: Hand Calcs for Spreadsheet 4
Figure 71: Hand Calcs for Spreadsheets 5 and 6
Figure 72: Hand Calculations of Simply Supported Beam Forces for T = 0.125 in.
Figure 73: Hand Calculations for Supports for 0.15 inches