11 Data This chapter gives samples of data—tables, charts, and figures—that are available within the plastics industry We have attempted to compile samples representative of a wide variety of plastic materials and molding conditions If the information you need is not in this book, you may be able to find it on the internet or from the plastic-material supplier The most readily available shrinkage data is that found in plastic-resin suppliers’ property sheets or in publications like the Modern Plastics Encyclopedia While these data are useful and valuable, they are far from complete and often misunderstood The data most often offered are the average shrinkage of the length of end-gated test bars about six inches long by one-eighth inch thick This is shrinkage in the flow direction Little or nothing is said about the shrink rate varying from end to end of the test bar Rarely is it suggested that if a part has a flow length of less than six inches the shrink rate will be less, or if the flow length is greater than six inches the shrink rate will be greater Cross-flow shrinkage is not as readily available and, depending on type of material and filler, may be greater than or less than flow-direction shrinkage Longterm shrinkage data and chemical-absorption sizechange data are rarely included in typical data sheets Creep under long-term load is emphasized far less than short-term strength and rigidity These data are usually available on request in publications that can be obtained from plastic-resin suppliers or from research papers © Plastics Design Library The processing conditions for molding test bars to determine shrinkage are somewhat vague ASTM D955-89 states that the molding conditions are to be those agreeable to the plastic manufacturer and the purchaser of the material I don’t know of many purchasers that have input on this test procedure; therefore we may assume that the molding conditions for the test bars are as recommended by the plastic manufacturer Those exact conditions are rarely specified (for example, in Modern Plastics Encyclopedia) The ASTM standard states that “The molding machine used should be such that it is operated without exceeding one-half to three-fourths of its rated shot capacity The temperature of the heating cylinder should be maintained at a point which will, on a cycle selected, produce a melt at a temperature within the range recommended by the manufacturer of the material.” Most materials have a fairly broad acceptable temperature range About all that can be stated with a reasonable degree of certainty is that the shrinkage in a part that has a flow length of about five inches and a thickness of one-eighth inch, with a gate that has a minimum dimension of one-eighth inch, will almost certainly match the published shrink rates at some acceptable molding condition Parts of other sizes, wall thicknesses, and gate designs will almost surely shrink at different rates This book attempts to give you guidance as to the magnitude and direction of these variations Ch 11: Data 158 11.1 Unfilled Materials Figure 11.1 Flow and cross-flow shrinkage of LNP D 1000 (unfilled polycarbonate) and R 1000 (unfilled Nylon 6/6), in a 2-mm thick plaque.[46] (Courtesy of SPE.) Figure 11.2 Measured shrinkage in the thickness of a 3-mm thick tensile test bar for polypropylene (PP), polyethylene (PE), and polystyrene (PS).[45] (Courtesy of SPE.) Note: The shrinkage in the thickness of a part is usually ignored because the change is so minor Only if the thickness of the finished product is critical is the change of thickness important Figure 11.2 shows the change in thickness of a 3-mm thick tensile test bar when molding polypropylene, polyethylene, and polystyrene.[45] In this figure (Fig 11.2), time zero is when the material separated from the mold surface while cooling and shrinking Before time zero, the material was already cooling and shrinking to overcome the compression of the plastic by the holding pressure The amount of compression depends on holding pressure and gate size Ch 11: Data © Plastics Design Library 159 11.2 Effects of Fillers Figure 11.3 Particulate filler and fiber effects on flow and cross-flow shrinkage of fiber-filled nylons in a 2-mm thick plaque.[46] (Courtesy of SPE.) Figure 11.5 The effects of glass fiber on shrinkage of Nylon 6/6 and polycarbonate in a 2-mm thick plaque.[46] (Courtesy of SPE.) Figure 11.4 The shrinkage in the flow direction and crossflow or transverse direction in a 4-in diameter by 0.062-in thick Nylon 6/6 disk, edge-gated at one point Each sample point represents a 40% by weight loading of glass beads or fibers.[47] (Reprinted by Permission of Hanser-Gardner.) Figure 11.6 Filler effects on flow and cross-flow shrinkage of polycarbonate in a 2-mm thick plaque.[46] (Courtesy of SPE.) Note: From Fig 11.3, it is obvious that flow-direction shrinkage is higher than cross-flow shrinkage until the glassfiber loading exceeds 10% by weight.[46] In Fig 11.4, it can be seen that cross-flow shrinkage is affected more by the total loading than by the type of loading The left ordinate represents unfilled Nylon 6/6 Each of the other points represents a different combination of glass beads with respect to glass fibers However, the fiber content strongly affects the flow-direction shrinkage because the glass fibers tend to orient in the flow direction and restrict shrinkage in that direction [47] Figure 11.5 shows much the same effect by glass fibers on both amorphous (PC) and semicrystalline (PA) materials The crossover point between more shrinkage in flow and cross-flow directions occurs at about 12–15% glass fiber by weight.[46] Figure 11.6 shows, once again, the crossover in maximum shrink rate in the flow direction to the cross-flow direction occurs with the addition of glass fibers.[46] In this case, all the materials are polycarbonate The stainless fibers, being much more flexible than glass, act more like particulate filler © Plastics Design Library Ch 11: Data 160 Figure 11.7 A comparison of the effects of glass fiber and carbon fiber on flow and cross-flow shrinkage of polycarbonate in 2-mm thick plaques.[46] (Courtesy of SPE.) Figure 11.8 The effects of fillers on flow and cross-flow shrinkage of Nylon 6/6 in 3-mm thick plaques.[46] (Courtesy of SPE.) Note: Figure 11.7 shows the difference in shrinkage of polycarbonate when filled with carbon fiber versus glass fiCarbon fibers are 30% lighter than glass and they are smaller in diameter This means that there are significantly more than 30% carbon fibers in a sample of polycarbonate when compared to a similar sample filled with the same weight of glass fibers With this information, it should be obvious why a sample with 10% carbon fibers shrinks more in the transverse and less in the flow direction, while the same material filled with 10% glass fibers shrinks more in the flow direction and less in the transverse direction It should also be evident why the sample with 10% carbon fibers shrinks significantly less in both directions than a similar sample filled with 10% glass fibers In Fig 11.8, [46] notice that the 15% PTFE fibers reduce the cross-flow shrink rate somewhat, but the flowdirection shrinkage increases slightly It may be that the PTFE, acting as particulate filler to reduce the cross flow, causes an increase in flow direction due to preservation of volumetric shrinkage ber.[46] Ch 11: Data © Plastics Design Library 161 Figure 11.9 A range of mold shrinkage for reinforced and unreinforced Nylon 6, PBT, and polycarbonate referenced against wall thickness Figure 11.10 How glass reinforcement affects mold shrinkage for Nylon 6/6, PBT, and polycarbonate Note: A careful examination of the graphs in Fig 11.9 shows that the amount of shrinkage is reduced by the filler, but the amount of variation (vertical range at a particular wall thickness) of shrinkage is virtually unchanged by the addition of filler.[10] Note that this graph gives no indication of the orientation or flow direction in relation to the shrinkage measurements As mentioned in Ch and elsewhere, orientation can have a significant anisotropic effect on shrinkages and tolerances Figure 11.10 shows the variation in shrinkage for the same three polymers as shown in Fig 11.9, at different reinforcement levels.[10] Since no wall thickness is specified, this figure is only beneficial to show the effect of different filler amounts rather than actual shrinkage at a specific wall thickness © Plastics Design Library Ch 11: Data 162 Figure 11.11 The effect of glass fiber and carbon fiber on shrinkage of Nylon 6/6 in a 2-mm thick plaque.[46] (Courtesy of SPE.) Figure 11.12 Flow and cross-flow shrinkage of 30% glassreinforced resins in a 2-mm thick plaque.[46] (Courtesy of SPE.) Note: Figure 11.11 compares the effects of carbon fiber and glass fiber on Nylon 6/6.[46] The effects are basically the same as they were on polycarbonate The shrinkage for 10% carbon fiber and 30% glass fiber were virtually the same for flow-direction shrinkage of Nylon 6/6 The cross-flow shrinkage becomes greater than flow shrinkage at about 15% glass-fiber fill and slightly less than 10% carbon-fiber fill The number of fibers rather than the loading by weight has the greater effect on flow-direction shrinkage Ch 11: Data © Plastics Design Library 163 Figure 11.13 Shrink rates of long glass-fiber vs short glass-fiber filled materials The long glass fibers are about 3/8 in (9 mm) long while the standard glass fiber length is no more than about 1/8 in (3 mm).[46] (Courtesy of SPE.) Figure 11.14 Plaque warpage comparing long glass-fiber reinforcement vs short glass-fiber reinforcement.[46] (Courtesy of SPE.) © Plastics Design Library Ch 11: Data 164 11.3 Shrinkage vs Various Parameters Figure 11.15 The general relationship between shrinkage and a variety of molding parameters.[48] 11.3.1 Shrinkage vs Injection Temperature Injection temperature is closely related to melt temperature As injection temperature on the machine’s temperature gauges rise, so does the melt temperature However, increasing back pressure during the time that material is conveyed forward along the injection screw also raises the melt temperature Longer cycle times also raise the melt temperature because there is more time for the plastic to soak and absorb heat Actual measurement of the melt temperature by injecting an air-shot into a paper cup and using a probe to measure the actual melt temperature is much more accurate than the machine’s temperature gauges Ch 11: Data © Plastics Design Library 165 11.3.2 Shrinkage vs Melt Temperature Figures 11.16, 11.17, and 11.18 serve to demonstrate how melt temperature affects shrinkage.[3] Together they show how pressure change affects size change at several different melt temperatures Figure 11.16 The variation in size of a molded lid of PE as the melt temperature changes while holding the injection pressure constant at 1000 psi on the molding machine gauge The actual pressure on the plastic is probably ten times that, or 10,000 psi.[3] (Reprinted with permission of Voridian, Division of Eastman Chemical Company.) Figure 11.17 The variation in size of a molded lid of PE as the melt temperature changes while holding the injection pressure constant at 1400 psi.[3] (Reprinted with permission of Voridian, Division of Eastman Chemical Company.) Figure 11.18 How the shrinkage of a PE lid changes as the melt temperature changes, and as the injection pressure remains constant.[3] (Reprinted with permission of Voridian, Division of Eastman Chemical Company.) © Plastics Design Library Ch 11: Data 166 11.3.3 Shrinkage vs Mold Temperature Figure 11.19 The effect of mold temperature on flow-direction shrinkage for two grades of Zenite® aromatic polyester thermoplastic resin.[49] (Courtesy of DuPont.) Note: In Fig 11.19, note the negative mold shrinkage, meaning that the part grows out of the mold to dimensions greater than the cavity dimensions.[49] Zenite ® is a liquid-crystal polymer (LCP) It is an exception to the general rule that higher mold temperatures result in higher shrinkage In this case, the higher mold temperatures allow more time for molecular organization thus reducing shrinkage in LCP materials 11.3.4 Shrinkage vs Density (Polyethylene) Polyethylene Relative Molecular Weight Mold Shrinkage Unit/Unit Melt Index Density 0.1–25 0.91–0.925 Low 0.010–0.050 0.1–25 0.926–0.94 Medium 0.010–0.050 0.1–25 0.941–0.96 High 0.010–0.090 Note: Injection molding grades of polyethylene such as those shown in this chart have molecular weights that are probably less than 1,000,000 Ultra high molecular weight polyethylene with a molecular weight greater than 3,000,000 is normally processed by extrusion and has a higher melting point and greater stiffness Ch 11: Data © Plastics Design Library 167 11.3.5 Shrinkage vs Holding Pressure Injection pressure and holding pressure are sometimes used interchangeably, however they are not the same Injection pressure is that pressure under which the mold fills; sometimes this is called the first-stage pressure The holding pressure is the pressure that is maintained on the melt after the mold is filled and until the gate freezes or the pressure is removed by cycle timer-control In most cases, it is the holding pressure that determines the shrinkage rather than the initial injection pressure; this holding pressure is the pressure normally referred to in these figures, whether it is labeled injection or holding pressure Figure 11.20 Shrinkage vs pressure for a Zytel® 101L part 3-mm thick with a mold temperature of 70°C.[9] (Courtesy of DuPont.) Figure 11.21 Shrinkage and part weight as a function of hold-pressure time for Zytel® 101L, for a part 3-mm thick.[9] (Courtesy of DuPont.) Notes: Increasing holding pressure reduces the shrink rate for most materials The curve shown in Fig 11.20 is typical.[9] Figure 11.21 shows that increasing the holding-pressure time reduces shrinkage until the holding-pressure time exceeds the time required for the gate to freeze.[9] Longer holding-pressure time also increases the weight of the molded part © Plastics Design Library Ch 11: Data 168 Figure 11.22 The shrinkage of a variety of Novolen® (PP) grades after days at 23°C [50] The melt temperature was 250°C and the mold temperature was 30°C The 1100 grades increase in flow ability from left to right through the 1148RC grade Novolen® 2300 L is a medium flow, 2340 P is relatively high flow, and 2500 H is a relatively low flow, about the same as 1100 N These measurements were on sprue-gated, molded, constrained boxes Note: Figure 11.22 shows the shrinkage data for various grades of Novolen at various holding pressures in a mold with constraints to inhibit shrinkage.[50] Pressures are molding-machine pressures which, in most cases, reflect about one-tenth of the actual pressure on the plastic material Ch 11: Data © Plastics Design Library 169 11.3.6 Shrinkage vs Thickness Table 11.1 Change in Shrinkage as a Result of Change in Thickness [51] Shrinkage in percent ABS mm thick mm thick 0.4 0.7 ABS (30% GF) 0.1 0.15 Acetal 1.7 2.1 Acetal (30% GF) 0.3 0.4 Nylon 1.3 1.6 Nylon (30% GF) 0.35 0.45 Nylon 66 1.6 2.2 Nylon 66 (30% GF) 0.5 0.55 PC 0.5 0.7 PC (30% GF) 0.1 0.2 PES 0.6 0.7 PES (30% GF) 0.2 0.3 Noryl 0.5 0.8 Noryl (30% GF) 0.1 0.2 PP 1.5 2.5 PP (30% GF) 0.35 0.4 HDPE 1.5 3.0 PEI 0.5 0.7 PEI (30% GF) 0.2 0.4 Polystyrene 0.4 0.6 © Plastics Design Library Ch 11: Data 170 Figure 11.23 This is an example of the change in shrinkage rate as thickness varies The material is Basell’s Pro-fax ® polypropylene.[62] Note: The shrinkage values in Fig 11.23 are the maximum that might be encountered under nominal molding conditions Thin parts (0.5 mm, 0.020 inches) can have higher shrinkage, perhaps as high as 0.020 inch per inch because of the difficulty of packing the part in such a thin section Optimized molding conditions can cause shrinkage to be lower than shown The variation in shrinkage shown here versus thickness illustrates the need for constant wall thickness in a molded part If close tolerances are required, a test cavity should be used Figure 11.24 The effect of thickness on shrinkage for RF 1002.[46] (Courtesy of SPE.) Ch 11: Data Figure 11.25 The effect of thickness of 30% glass-fiber filler on flow shrinkage for several materials.[46] (Courtesy of SPE.) © Plastics Design Library