204 3 Plasticizing Materials of Construction formula assumes pumping against low pres- sure, giving no consideration to melt quality Different materials of construction are and leakage flow of worn screws. used to meet the requirements of the dif- With all these and other limitations, the for- ferent plastics being processed. As an exam- mula can still provide guidance as follows: ple, bimetallic barrels offer extensive durabil- ity when processing abrasive materials such 1. It can serve as a general guide to the as glass- and mineral-filled plastics, certain output of the screw. engineering plastics, and granulatedhecycled 2. If the actual output of the screw is signifi- plastics. In contrast, when processing unfilled cantly greater than calculated, it is caused by nylon continuously, the probability is that you high compression ratios that overpump the will have to replace a worn out screw about metering section. Sometimes this is desirable, every six months. These questions are dis- but it can lead to surging and rapid screw wear cussed below under the subheading Screw if it is excessive. Wear Protection. 3. If the output is a lot less, it usually indi- cates a feed problem or a worn screw or bar- rel. The latter can be determined by measure- Screw Outputs ment. A feed problem can, on occasion, be corrected by changes in barrel temperature The rate of output (throughput), or the settings. More often, the problem is caused by speed at which plastic is moved through other items, such as screw design, shape and the plasticator, has been pushed continu- bulk density of the feedstock, surface condi- ally higher as a result of design advances in tion of the screw root and barrel ID in the screws, IMM equipment, and plastic materi- feed area, feed-throat design, or screw tem- als. Output rates generally range from a few perature. kilograms to tons per hour on single-screw machines. (With twin-screw extruders using large diameters, output rates range from a Influence of Screw and Barrel Wear few kilograms to at least 30 tons per hour.) on Output A rough estimate for output rate (OR) in lb/h can be calculated by using the barrel’s There are two types of wear. One is me- ID in inches and using the following equation: chanical, such as adhesive and abrasive wear. OR = 16 ID2; for kg/h multiply by 0.4536. The other is corrosion, which produces pitted The output of a screw is fairly predictable, surfaces. Adhesive wear is caused by contact provided that the melt is under control and between the flight and the barrel. The screw reasonably repeatable. With a square-pitch and barrel are engineered to minimize such screw (a conventional screw where the dis- contact, but some is unavoidable. The plastic tance from flight to flight is equal to the di- material being processed can significantly in- ameter), a simplified formula for output is fluence the abrasive and/or corrosive actions. R = 2.3D2hgN, where R is the rate or out- Wear does not occur suddenly but builds put in lb/h (kg/h), D is the screw diameter in up over months of machine operation. It fi- in. (mm), h is the depth in the metering sec- nally shows itself in one of several ways. The tion in in. (mm) (for a two-stage screw use examples to be discussed concern recipro- the depth of the first metering section), g is cating screw machines, since they have the the specific gravity of the melt, and N is the major wear problems; but wear also occurs screw rotation speed (rpm). in two-stage IMMs, including the breaking This formula does not take into account of nonreturn valve rings. There can be loss back flow and leakage flow over the flights. of shot control or consistency, requiring in- These flows are not usually a significant factor creased feed to make up for melt slippage unless the plastic has a very low viscosity dur- back over the valve and screw. Screw recov- ing processing or the screw is worn out. The ery time can increase. There can be a decrease 3 Plasticizing 205 in product quality. The cycle time increases that of the screw’s root is usually about 2 : 1. due to higher-temperature melt. The barrel ID enlarges at twice the rate of the Mechanical wear is usually in the part of decrease of the root of the screw. This differ- the screw where the feed section ends and the ence occurs because the heat source comes transition starts. It is usually caused by the use from the outside of the barrel. The screw root of high back pressure, an improper heat pro- is not as hot as the barrel’s interior surface. file, or a worn nonreturn valve that restricts When a nonreturn valve is new, it will fit flow. the barrel closely, preventing leakage of melt Often the plastic is filled with talc, glass, during injection. When the barrel ID changes or other materials that do not melt but form due to wear, leakage begins to occur during slugs and can cause scrubbing of the screw injection. If the barrel wear reaches 0.010 to and barrel, particularly the roots of screw 0.012 in. (0.025 to 0.030 cm), the ring on cer- flights. This action continues until the flight tain nonreturn valves can break. Then the is worn away. The barrel is likewise worn in IMM does not operate efficiently. the corresponding area. A change in the heat profile in the rear and center zones of the bar- rel will usually eliminate this problem. The Screw Wear plastic must start to melt as it reaches the end of the feed zone in order to move easily into The wear in screw plasticators generally the transition zone. If the screw returns in an causes an increase in the clearance between erratic manner, the plastic does not have the screw flight and barrel (Fig. 3-37). It often oc- required temperature, and screw and barrel curs toward the end of the compression sec- wear result. tion. This type of wear is more likely to oc- Another complication can be that the shot cur when the screw has a high compression size and the cycle time do not allow sufficient ratio. Regardless of where it occurs, the plas- residence time for the plastic to melt properly ticator’s melting capacity is reduced. If the as it passes through the barrel. A usual guide wear is serious enough, it will cause the prod- is that if the residence time is less than 1 min, ucts to exit at a slower rate or (more likely) there may be a cold condition. to have lower quality. In addition to adhesive wear (caused by metal to metal contact under high stress), abrasive wear (galling), and cor- Influence of the Material on Wear rosion wear (chemical reaction/mechanical attack on the sliding surfaces), screws are sub- Corrosive wear due to plastic materials ject to laminar wear (affecting thin outer lay- usually occurs in the front of the screw and ers of metal at interfaces) and surface-fatigue barrel. The major wear is in the metering sec- wear (micro- or macroscopic separation from tion of the screw, at times extending a little the surfaces). into the transition section. Most of the wear problem is with the barrel core and the screw root. It leads to darkening of the screw and Production Variations pitting of its surface. Note that certain plastics and fillers (nylon, phenolic, etc.) when heated As screw flights and the insides if bar- degrade, giving off corrosive gases and/or rels wear, the pumping ability of the screw liquids. is diminished. Some materials and some ad- This wear usually occurs during startup and ditives will cause higher wear than others; shutdown when the melt is not moving on for example, linear low-density polyethylene cycle and sits in the barrel under heat. The (LLDPE) will cause more wear than conven- long soaking time of the barrel on startup and tional LDPE or polypropylene. Many fillers, shutdown can cause degradation of the plas- such as titanium dioxide (used for white tic when it is in contact with the steel screw coloring) and reinforcing fibers, also create and barrel. The ratio of wear of the barrel to high-wear situations. Under some conditions, 206 3 Plasticizing ,- - -.- -. .. .- -_ T' F Fig. 3-37 Example of severe screw wear. screw/barrel wear can lead to instability of the extent of deterioration. The wear pattern machine output, but typically the main effect then can be plotted to show the screw and is output reduction. At some point, wear will barrel life for the given production case. It create an unacceptable situation that necessi- has been estimated that a 10% reduction in tates rebuilding or replacement of machinery screw surface thickness can cause as much as parts, such as the screws, barrel, and feed sec- a 25% reduction in the output of an IMM. tions. Changes in the ability to increase the In plastics processing, there are two major screw speed and still produce an acceptable types of wear with which to contend: abra- melt contribute to the decision about when sive and adhesive. Abrasive wear is a func- wear has passed acceptable limits. tion of the type of material being processed Variations involved in production oper- and the volume of material pumped through ations, including the materials run, screw the machine. There are two types of abrasive speeds used, melt pressures, barrel set tem- wear: two-body and three-body.