Surface Roughness Effects in Mold Cavity Filling

RPN20080421P014.qxp 4/16/2008 1:44 PM Page 1

14 Rubber & Plastics News ● April 21, 2008 www.rubbernews.com Technical Surface roughness effects in mold cavity filling

By Miroslav Manas, Michal Stanek, fully during the injection molding David Manas process. The ability of cavity filling is Tomas Bata University in Zlin, Czech Republic Executive summary affected by the polymer properties (es- The main aim of this work is to de- Injection molding is one of the most extended plastic processing technologies, pecially melt temperature) and the scribe the influence of surface roughness and is widely used with thermoplastic elastomers and rubber. Delivery of poly- properties of cavity walls (mold tem- of mold cavity and runners on the flow mer melts into the mold cavity is the most important stage of the injection perature, quality of cavity or runner ability of polymer melts. molding process. In the mold cavity the melt freezes on cures and the product surfaces). To be able to study this problem, spe- gains the final shape and form. Injection molding is also a well-estab- cial molds for both thermoplastics and This paper shows the influence of cavity surface roughness and technological lished fabrication process in the rubber elastomers were designed and ma- parameters on the flow length into mold cavity. industry. Injection molding has more chined. Testing plates machined by vari- The fluidity of polymers is affected by many parameters (mold design, melt advantages than compression or trans- ous cutting methods and various types temperature, injection rate and pressures) and by the flow properties of poly- fer molding. of polymers were tested. mers. These advantages comprise reduced The results of the tests were statisti- Evaluation of the data obtained by experiments where the testing conditions labor costs, shorter cure times, better cally calculated and evaluated. were widely changed shows that quality of the cavity surface does not affect the dimensional control and more consis- length of flow. Thermoplastics, TPEs and rubbers have been tested. tent mechanical properties of the prod- Introduction uct. Injection molding represents a way of During injection molding, rubber high quality and precision. a carpet. compounds are subjected to more se- TECHNICAL NOTEBOOK The injection molding cycle time is The flow front therefore is like a foun- vere processing conditions than during Edited by Harold Herzlichh relatively short and the process can be tain in which the center most material compression or transfer molding. Tem- widely automated. In case of cold run- flows out and deposits itself on the cavi- peratures, pressures, and shear stress- polymer processing that requires injec- ner mold the sprues and runners can ty wall. This type of laminar flow is es are higher, though cure times are tion of polymer melt into the mold cavi- be reused. Plastic material does not usually described as a “fountain flow” shorter. ty in a very high injection rate. This slide on the mold surface, but it is (Fig. 1). The control over process variables can technology facilitates the production of rather rolled over much like rolling out It is necessary to fill the mold cavity be more precise. The cycle time can be minimized by independently controlling Table I. Tooled surfaces of testing plates. Fig. 1. Flow of polymer melt into mold barrel temperature, screw speed, mold cavity. temperature, cure time and injection pressure. Compounds with widely differing flow and cure characteristics can be molded into a variety of complex shapes.

Experiment The main aim of our experiment is to describe the influence of surface roughness of mold cavity (runners) on the fluidity of polymer melt. Fig. 3. Channel cross-section. To be able to study this behavior, special injection molds both for thermoplastic and elastomers have been

Fig. 4. Sprue puller insert.

Fig. 2. Injection mold for thermoplastics. Left image: 1. Cavity plate; 2. plate; 3,4. ejector plates; 5. sprue puller insert; 6. ejector; 7. oil connection; 8. ejector rod. Right image: 1. Testing plate; 2. clamping plate; 3. sprue bushing; 4. sensor.

Table II. Injected rubber compounds. RPN20080421P015.qxp 4/16/2008 1:48 PM Page 1

www.rubbernews.com Rubber & Plastics News ● April 21, 2008 15 Technical

designed and machined. Then poly- ables the exchange of differently sized injection mold for elastomers is given in The shape of injected rubber samples mers with different flow properties, gates. The size of the gate could be 1, 2, Fig. 6, surface roughness of the testing is spiral (Fig. 7). Length of testing sam- commonly used in industry, have been 4 or 6 mm (Fig. 4 and Fig. 5). plates is given in Table I. See Mold, page 16 chosen. The length of testing samples has Injection mold for thermoplastics been measured by a measuring jig. After Fig. 6. Injection mold for rubber. The mold (Fig. 2) is equipped with that the experimental data were statis- pressure and temperature sensors in or- tically processed and evaluated. der to be able to measure the actual val- Injection mold for elastomers ue of pressure and temperature in the Obviously, the major design consider- mold cavity during the injection molding ation for the mold is the shape and size process. of the required molding. The design of the injection mold en- However, within this restriction, ables easy change of the testing plates. there are a number of points that have For machining of functional surfaces to be considered in determining the op- of testing plates, various tooling tech- timum mold design for a particular nologies were used. The surface rough- product. ness (Ra) is given in the Table I. To be able to study the influence of The injected sample is spiral shaped. surface quality of runners and cavities The cross-section of spiral is given in on the flow of elastomers, a special injec- Fig. 3 and Fig. 5. Its length is 2000 tion mold has been designed and ma- mm. A special sprue puller insert en- chined. The design and description of he

Fig. 5. Injected sample.

Fig. 7. Injected sample.

N-234 RSS#2 VISCOSITY VS MIX NUMBER

70 NO PREMASTICATION Ctrl-RSS#2 60 QDI RSS#2 Ctrl-PM-RSS QDI with PM-RSS Learning 50 about a new MS (1+4) 40 product is one thing. Witnessing PREMASTICATED 30 dramatic results from it is

20 quite another. QDI is something 02341 MIX NUMBER more than new. It's dramatically better.

VISCOSITY VS SHEAR HISTORY HIGH TEMP MIXES Consider the opportunity to eliminate 1-2 passes OXIDATIVE MIXING CONDITIONS in a multi-pass operation. Mix NR compounds up to 25% faster. 60 Achieve better viscosity reduction than controlled compounds, 55 Ctrl High-T and compounds mixed with peptizers. Reduce heat build up by PEPTIZED 50 QDI 5-10%. Reduce mixing costs up to 25%. 45

MS (1+4) We're going to change the way you look 40 at traditional mixing. Call Dr. Fred 35 (330-668-8346) and challenge us to 30 prove QDI in your mixing application. 0250 500 750 1000 1250 1500 Mix faster, smarter, better. With QDI TURNS OF THE MIXER . Flexsys America L.P. • QDI Challenge • Dr. Fred • 330-668-8346 • Fax 330-668-8345 • www.flexsys.com RPN20080421P016.qxp 4/16/2008 1:49 PM Page 1

16 Rubber & Plastics News ● April 21, 2008 www.rubbernews.com Technical

tistically processed and evaluated. The pounds with different flow properties 60, MFI = 20 maximum length of an injected sample were tested: ● thermoplastic polyurethane (TPU) Mold can be 2,000 mm. The width of the sam- ● polypropylene (PP) Mosten GB 003, Ellastolan C 78 A, MFI = 6,1 ple is 5,6 mm and its thickness is 2 mm. MFI = 3,3 ● acrylonitrile-butadiene-styrene (ABS) Continued from page 15 ● polyethylene (LDPE) Bralen VA 20- See Mold, page 17 ples has been measured by a measuring Used polymers jig. Selected types of thermoplastics, ther- Fig. 10. Dependence of flow length on surface quality and injection rate (Taboren, The experimental data have been sta- moplastic elastomers and rubber com- gate 6 mm, injection pressure 6 MPa). Fig. 8. Length of flow (injection pressure 10 MPa). a) Simulation of injection molding by Moldflow 6. b) Real injected sample.

Fig. 9. Dependence of flow length on surface quality and type of polymer (injection pressure 8 MPa).

Fig. 11. Dependence of flow length on surface quality and injection pressure (Keltan, gate 6 mm, injection rate 30 mm.s-1).

Fig. 12. Dependence of flow length on surface quality and type of thermoplastic elastomers.

The authors Miroslav Manas is head of the Department of Production Engineering, Faculty of Technology, Tomas Bata University in Zlin, Czech Republic. His main area of inter- est is tools and machines for polymer processing. He is a member or leader in many industry organizations, including the International Rubber Conference Organiza- tion and vice chairman of the Rubber Division Zlin of the Czech Society of Industri- al Chemistry. He is an author or co-author of 253 papers, including 16 patents. Michal Stanek, Department of Production Engineering, works with tools and machines for polymer processing and simulations of polymer processing. He is a member of the Society of Plastics Engineering, the Rubber Division Zlin of the Czech Society of Industrial Chemistry, the Czech Society of Manufacturing Technology and the Czech Toolmakers Association. He is the author or co-author of more than 70 papers. David Manas of the Department of Production Engineering is a member of the SPE, the Rubber Division Zlin of the Czech Society of Industrial Chemistry and the Czech Society of Manufacturing Technology. His work mainly involves material science and material testing. He is the author and co-author of more than 35 papers. RPN20080421P017.qxp 4/17/2008 1:21 PM Page 1

www.rubbernews.com Rubber & Plastics News ● April 21, 2008 17 Tire tread depth debate continues New sponsor Michelin is blunt about finding con- tion is whether the data justify a Industry mixed on siderable environmental and fuel change in the statute.” economy costs in the proposal to raise The BFS spokesman said his com- revives ACS Michelin’s stance to minimum tread depths to 4/32, with pany is considering the tread depth keep current standard no measurable benefits to offset them. question. David Stafford, chief operating of- Hankook Tire America Corp., how- 5K run/walk By Miles Moore ficer of Michelin Americas Research ever, has no plans to take a stand, Rubber & Plastics News Staff Co., said increasing minimum tread according to Hankook Marketing Di- AKRON—H.M. Royal Inc. has stepped in WASHINGTON—Different sectors depth would send an extra 65 million rector Bill Bainbridge. “It’s one of to sponsor the 5K Run/Walk at the ACS of the tire industry are reacting to tires annually to the scrap heap, in- those issues that, no matter how you Rubber Division spring and fall meetings. Michelin North America Inc.’s oppo- crease fuel consumption by 770 mil- look at it, it’s a 50-50 trade both The company agreed to sponsor the event sition to raising the allow- ways,” Bainbridge this year at the spring meeting in Dearborn, able minimum tread depth said. Mich., and future division meetings. No 5K for tires, and those reac- “It’s one of those issues that, Goodyear said it en- Run/Walk was held at last fall’s meeting. tions range from noncom- courages consumers “The 5K was such a tradition at past mittal to curious to hostile. no matter how you look at it, to replace tires at or meetings that we are proud to bring it back Continental Tire North above a 2/32-inch and assure the tradition will continue for America Inc. endorses re- it’s a 50-50 trade both ways.” tread depth. “Many many meetings to come,” said Joe Royal, moving all-season and drivers elect to re- president of H.M. Royal. high-performance tires Bill Bainbridge place their tires be- The event at the division’s spring meeting from service at 4/32-inch fore the tread reaches will take place April 29, beginning at 6:30 tread depth and winter tires at 5/32- lion gallons per year and raise annu- this minimum level as they take into a.m. Participants will be transferred from inch. The current allowable tread al carbon dioxide emissions by 7.5 account seasonal and regional condi- the lobby of the Hyatt Regency hotel to the depth is 2/32-inch. million tons. tions,” the company said. University of Michigan-Dearborn campus, “If Michelin thinks their tires can The safety compensation would be Besides citing its own research, the to run or walk on a flat, well-marked bike stop as well at 4/32 as at 9/32, good negligible, according to Stafford. Conti spokesman spoke highly of the path overlooking the Henry Ford estate. luck to them,” a Conti spokesman Raising minimum tread depths un- testing done by TireRack.com, the on- Prizes will be given to the top male and fe- said. doubtedly would reduce braking dis- line tire retailer, which describes its male finishers, and all participants will re- Other organizations, such as the tance, he said, but other factors such own test results on its Web site. ceive a long-sleeved T-shirt, Royal said. Rubber Manufacturers Association, as road surfaces, driver behavior, Tire Rack’s tests showed that a tire People who register for the meeting can do so take no stand on the issue of tread tire types and vehicle types make with 4/32-inch tread depth split the dif- for the event at the same time. Royal said any- depth, and still others, such as the braking distances a poor indicator of ference in stopping distances between a one who shows up on race day can sign up, too. Tire Industry Association, are study- the likelihood of an accident. 2/32-inch tread and a completely new Plans are under way for the 5K event to ing it but have reached no conclusions. Generally speaking, tire manufac- tread. The company recommends re- be held in Louisville, Ky., at the fall division “The question has been in the in- turers other than Michelin and Conti placing tires at 4/32, particularly in ar- meeting, Oct. 14-16. dustry for decades,” said Paul Fiore, haven’t yet taken strong stands on eas where wet roads are a concern, and Also helping sponsor the event are Akzo TIA director of government and busi- tread depth, though they agree the 6/32 if snow is a factor. Nobel Polymer Chemicals, Columbian ness affairs. “It’s in the government issue is worth considering. “Since water can’t be compressed, Chemicals Co., Horsehead Corp., Imerys, affairs committee right now. I would- “The whole idea that more tread is you need enough tread depth to al- Rohm and Haas Chemicals L.L.C., SI Group n’t say it’s high priority or a fast-mov- better is well-taken,” said a spokesman low rain to escape through the tire’s Inc. and Solvay Solexis Inc. ing item, but the topic is out there.” for Bridgestone/Firestone. “The ques- grooves,” Tire Rack said. Technical

pected. Fig.13. Dependence of flow length on surface quality and injection pressure (rub- New and very important results rise ber compound A, B, C). Mold from the experiments, which analyzed the influence of surface quality on injec- Continued from page 16 tion mold filling. Polylac PA 757, MFI = 2,4 It could be generally said that the sur- ● polypropylen filled by 20 percent face quality of flow pathway does not of chalk Taboren PH 89 T20, MFI = have a significant influence on the 14,4 length of flow. It was found that better ● polypropylen filled by 10 percent of quality of wall surface worsened the chalk Keltan TP 7603, MFI = 16,9 flow condition. ● TPU Desmopan DP 1485A The length of the injected sample spi- ● TPU Desmopan DP 8060SGN ral was shorter or had approximately ● TPU Desmopan DP 372 the same length of flow. Similar results have been found by in- Injection molding machines jection molding of thermoplastic elas- For thermoplastics and thermoplastic tomers (Fig. 12) and elastomers (Fig. elastomers: Arburg Allrounder 420C, 13). Advanced. For rubber samples: REP This finding has a very important ef- V27/Y125. fect for tool producers. It is not necessary to use high precision cutting opera- Simulation of injection molding tions, and it would be possible to exclude process some very costly final operations as for Simulation of the process has been example grinding or polishing. performed using simulation SW Mold- flow Plastic Inside 6. Conclusion The length of injected spiral was Measurement shows that surface searched. Results of this simulation are quality doesn’t have substantial influ- very similar to practically injected sam- ence on the length of flow. ples (Fig. 8). Samples which were injected into the spiral (cavity) with the worst surface Results quality have approximately the same The filling of mold cavity depends on length of flow. These findings are very material properties, technological condi- important for the usability and produc- tions and surface quality. tion points of view. The lower the viscosity of polymer (measured by Melt Flow Index) the bet- Acknowledgement ter cavity filling has been achieved This article is financially supported (Fig. 9). by the Czech Ministry of Education, Rising injection rate and filling Youth and Sports in the R&D project References 3. Beaumont, J.P., “Runner and gating design hand- pressure result in better mold cavity under the title “Modelling and Control 1. Johnson, P.S., “Rubber Processing,” Hanser Pub- book,” Hanser Publishers, Munich 2004. 4. Dick, J.S., “Rubber Compounds,” Hanser Publish- filling (Figs. 10 and 11). The above of Processing Procedures of Natural and lishers, Munich 2001. 2. Agassant, J.F.; Avenas, P.; Sergent, J.Ph. & ers, Munich 2004. mentioned results of polymer melt be- Synthetic Polymers,” No. MSM Careau, P.J., “Polymer processing,” Hanser Pub- 5. Manas, M.; Vlcek, J.; Aplikovana reologie. Zlin, havior during mold filling were ex- 7088352102. lishers, Munich 1991. UTB 2001.