11.9 Material-Specific Shrinkage Characteristics

198 11.9 Material-Specific Shrinkage Characteristics 11.9.1 Polybutylene Terephthalate (PBT) (Crastin®) (DuPont) Figure 11.54 The effects of holding pressure, holding time, and mold temperature on the shrinkage of unreinforced (S699) Crastin® Measured on a flat quadrant with a 100mm radius and 4-mm thickness.[20] (Courtesy of DuPont.) Figure 11.55 The effects of holding pressure, holding time, and mold temperature on the shrinkage of glass-fiberreinforced Crastin® (SK605, SK645 FR) Measured on a flat quadrant with a 100-mm radius and 4-mm wall thickness.[20] (Courtesy of DuPont.) Figure 11.56 Shrinkage of unreinforced S600 and S0655 Crastin® as a function of wall thickness measured on plates 100 × 100 mm with a fan gate (Not valid for ST820.)[20] (Courtesy of DuPont.) Figure 11.57 Shrinkage of glass-fiber-reinforced Crastin® types with a glass content of 15% by weight (e.g., SK642 FR) as a function of wall thickness Measured on plates 100 × 100 mm with a fan gate.[20] (Courtesy of DuPont.) Ch 11: Data © Plastics Design Library 199 Figure 11.58 Shrinkage of glass-fiber-reinforced Crastin® with glass content of 30% by weight (SK605 and SK645 FR) as a function of wall thickness when molded in plates 100 mm × 100 mm, with a fan gate.[52] (Courtesy of DuPont.) Figure 11.59 Post-mold shrinkage of unreinforced (S600 or S0655) Crastin® with wall thicknesses of mm and mm and mold temperatures of 80°C and 100°C, respectively, as a function of conditioning temperature.[20] (Courtesy of DuPont.) © Plastics Design Library Figure 11.60 Post-mold shrinkage of glass-fiber-reinforced Crastin® with a glass content of 30% by weight (SK605, SK645 FR), with wall thicknesses of mm and mm and mold temperatures of 80°C and 100°C, respectively, as a function of conditioning temperature.[20] (Courtesy of DuPont.) Ch 11: Data 200 11.9.2 Polyoxymethylene: Acetal (POM) (Delrin® ) (DuPont) Figure 11.61 Post-molding shrinkage of Delrin® acetal when exposed to various temperatures for 1000 hours All parts were measured at room temperature Notice that the post-mold shrinkage of thicker sections is less than the post-mold shrinkage of thinner sections This is probably because thicker sections cool more slowly in the mold, which gives more time for in-mold shrinkage and stress relaxation.[17] (Courtesy of DuPont.) Ch 11: Data © Plastics Design Library 201 Figure 11.62 A simple nomograph for estimating mold shrinkage for Delrin® acetal resin (SI units).[17] (Courtesy of DuPont.) Figure 11.63 A simple nomograph for estimating mold shrinkage for Delrin® acetal resin (English units).[17] (Courtesy of DuPont.) © Plastics Design Library Ch 11: Data 202 Note: The Delrin® estimated shrinkage is based on a minimum gate thickness of one-half of the wall thickness, screw forward until the gate freezes, mold temperature of 93°C (200°F), injection pressure of 112 MPa (16,000 psi), and a melt temperature of 210°C (410°F) Mold shrinkage will increase about 0.001 units/unit for each increase of 14°C (25°F) and vice-versa A decrease of 0.0004 units/unit will be seen for each MPa (1,000 psi) increase in holding pressure and vice-versa If screw-forward time is less than that required for the gate to freeze, then the shrinkage will be nearer the typical value than the optimum value The increase in shrinkage from optimum can range from 0.001 to 0.010 units/unit depending on the amount of time the screw-forward time is reduced from that required for gate freeze The time required for the gate to freeze is shown in Fig 11.64.[17] The importance of gate freeze is indicated in Fig 11.65 where the mold shrinkage for a 3-mm (0.12-in.) thick part ranges from about to 3% depending on screw-forward time alone.[17] Figure 11.64 Time for gate to freeze vs part thickness for Delrin ® where gate thickness is 50% of the part thickness.[17] (Courtesy of DuPont.) Figure 11.65 Screw-forward time vs Delrin® part weight and shrinkage.[17] (Courtesy of DuPont.) Note: Figure 11.64 shows why it is of supreme importance to be sure the Delrin® or any other plastic part is fully packed with adequate screw-forward time before taking other action to correct shrinkage or warp Other materials exhibit similar changes in weight and shrinkage versus screw-forward time The thinner the gate, the shorter the amount of time to affect the shrinkage and weight of the molded part before the gate freezes Ch 11: Data © Plastics Design Library 203 11.9.3 Polycarbonate (PC) Figure 11.66 Flow vs transverse flow shrinkage for various concentrations of glass fiber in polycarbonate [47] (Reprinted by permission of Hanser-Gardner.) Figure 11.67 Warpage across polycarbonate plaques [46] (Courtesy of SPE.) © Plastics Design Library Figure 11.68 Warpage across 30% glass-fiber-filled plaques.[46] (Courtesy of SPE.) Ch 11: Data 204 11.9.4 Polyphenylene Sulfide (Fortron®) (PPS) Fortron®, being a semicrystalline material, tends to shrink more in cross-flow, especially in heavier sections, due to longer heat exposure (see Table 11.10) When the mold temperatures are 275°F or higher, the material fully crystallizes, therefore no secondary annealing or heat-treating is required As with all fiber-filled and semicrystalline materials, anisotropic shrinkage can cause warpage and out-of-roundness Even though PPS is a semicrystalline material, its shrinkage factors are relatively low Unfilled PPS is relatively brittle and, as a result, is almost always reinforced When reinforced and fully crystallized, the material will ring when struck as if it were a metal like steel or aluminum Table 11.10 Differences in Shrinkage Rates in Flow and Transverse Directions Indicate Warpage Material ® ® Fortron Fortron Shrinkage Rate (in/in) Flow Transverse 40% Glass 0.001–0.003 0.005–0.007 65% Mineral/GF 0.001–0.002 0.003–0.005 Figure 11.69 Part thickness vs shrinkage of Fortron® PPS.[40] (Courtesy of Hoechst Celanese.) Note: Figure 11.69 shows the effect of part thickness on shrinkage in Fortron® PPS [40] Thicker parts result in slower cooling and a higher degree of crystallization When molded with a mold temperature of 275°F (130°C) or higher, PPS fully crystallizes and very little additional size change occurs by annealing The curves shown here were obtained using a cylinder temperature of 608°F (320°C) and a mold temperature of 320°F (150°C) The plaques were 80 × 80 × mm with a single side-gate × mm in size Ch 11: Data © Plastics Design Library 205 Figure 11.70 How injection pressure affects flow and transverse shrinkage of PPS with 40% glass-fiber filler.[40] (Courtesy of Hoechst Celanese.) Figure 11.71 How injection pressure affects flow and crossflow shrinkage of PPS with 65% mineral/glass filler.[40] (Courtesy of Hoechst Celanese.) Figure 11.72 Effect of filler level on shrinkage of Fortron® PPS.[40] (Courtesy of Hoechst Celanese.) Note: Figures 11.70 and 11.71 show the effects of injection pressure on shrinkage in PPS at two different filler-concentration levels [40] Figure 11.70 shows the transverse and flow shrinkage for 40% glass-filled PPS Figure 11.71 shows the transverse and flow shrinkage for 65% mineral/glass-filled PPS While not all of the filler concentrations shown are available, Fig 11.72 shows the effect of different filler levels on transverse shrinkage at two different thicknesses of a test plaque [40] © Plastics Design Library Ch 11: Data 206 11.9.5 Thermoplastic Elastomer Ether Ester Block Copolymer (TEEE) (Hytrel®) (DuPont) Figure 11.73 The influence of mold temperature on change in shrinkage of Hytrel® The absolute shrink rate ranges from about 0.8% to 1.8% depending on the compound.[34] (Courtesy of DuPont.) Figure 11.74 The effect of injection pressure on change in shrinkage of Hytrel®.[34] (Courtesy of DuPont.) Figure 11.75 The effect of part thickness on the change in shrinkage of Hytrel® [34] (Courtesy of DuPont.) Note: The recommended mold temperature for Hytrel® is 45°C The recommended melt temperature varies somewhat from compound to compound The recommended injection pressure is 70 MPa, and the optimum screw-forward time (SFT) is sufficient to allow the gate to freeze before pressure is removed Ch 11: Data © Plastics Design Library 207 nucleated The other five materials (CP-1 to CP-5) were compounded grades The compounded grades started with RG-3 and/or RG-4 To that mix were added other materials such as elastomers, talc, and HDPE The makeup and melt flow of each of the grades tested are shown in Table 11.11.[54] Table 11.12[54] shows the variation of the conditions during this data gathering The conditions that varied were part thickness, hold pressure, mold temperature, melt temperature, injection speed, and flow length These conditions ranged over the commonly used molding conditions for the materials tested The baseline conditions are shown in bold print in Table 11.12 Only one variable at a time was changed while holding the other variables constant at the baseline condition The injection pressure for the small plaque was 16.5 MPa (2400 psi) The holding and the cooling time were each set at 17 seconds For the large plaque, the injection pressure was set at 13.8 MPa (2000 psi) The holding time was 20 seconds and the cooling time was 17 seconds The extremes were the highest and lowest that would produce acceptable parts Different materials required different high and low conditions The values shown in parentheses (Table 11.12) are the variable conditions for the lower melt-flow-rate materials In addition, conditions calculated to produce the maximum and the minimum shrinkage were tested It should be noted that the actual conditions in the cavity varied from material to material for the same molding conditions because of the different melt-flow-rate conditions The cavity pressures shown were obtained using a cavity pressure sensor The results of this trial are shown in Table 11.13.[54] Each of the data shown are averages of four measurements of four plaques molded under the same molding conditions 11.9.6 Polypropylene (PP) Polypropylene has seen increasing use over the last few years in the automotive industry as a replacement for more expensive “engineering” grades of plastics Polypropylene is less expensive, even in the glass-fiber-filled grades where the physical characteristics of polypropylene match well with the more expensive grades of plastic However, the molding conditions for molding accurate parts of semicrystalline plastics like polypropylene are more critical than for the amorphous engineering grades of plastic This is due in part to the higher shrinkage rates of all semicrystalline materials Also contributing to the problem is that the cross-flow shrinkage can differ considerably compared to the flowdirection shrinkage, especially when the plastic is reinforced with glass fibers As mentioned elsewhere in this book (for example, Sec 4.1.2), the higher shrinkage rate is due in large part to the much more dense structure of the crystals that form compared to the amorphous condition that exists during the molten state The percentage of the plastic that forms crystals is affected by melt temperature, mold temperature, cooling rate, wall thickness, packing pressure, flow length, injection speed, and cooling efficiency of the mold To test polypropylene shrinkage characteristics, data were gathered using two different molds (See Tables 11.11 through 11.13.) The small mold was 66.7 mm square with a full-width fan gate This represents a short flow length The thickness of the part could be varied The large mold was 101 mm wide by 305 mm long with a 25 mm wide fan gate in the center of one narrow end This represents a longer flow length With the exception of the corners near the gate, both molds produced one-dimensional flow Three of the materials tested (RG-1 to RG-3) were pure reactor-grade materials Of these, RG-2 was Table 11.11 The Percentage of Various Compounds and the Melt Flow of Plastics Tested (See Table 11.12 for Conditions, and Table 11.13 for Test Results)[54] Content RG-1 RG-1 RG-2 RG-3 CP-1 CP-2 CP-3 100 77.5 30.0 31.0 15.0 30.0 32.0 10.0 100 RG-3 RG-4 HDPE 10.5 60.0 52.5 14.5 15.0 23.5 17.5 25.0 13.5 11 11 5.0 Talc Elastomer 3.75 15.0 Elastomer 3.75 15.0 14 11 © Plastics Design Library CP-5 100 RG-2 MFR CP-4 35 35 20 Ch 11: Data 208 Table 11.12 Molding Conditions Used for Evaluation (See Table 11.11 for Materials, and Table 11.13 for Test Results)[54] Variable Conditions Small Mold: Mold Temperature (°F) 80, 100, 120, 140 Melt Temperature (°F) 390, 408, 425, 443, 460 Hold Pressure (bar) 75, 125, 175, 225 , 275 (125, 175, 225, 275, 325) Injection Speed (cc/s) 10 (fill ~ 2.2 s), 20 (fill ~ 0.9 s), 30 (fill ~ 0.45 s) Part Thickness (mm) 2.0, 2.5, 3.0, 3.5 Large Mold: Mold Temperature (°F) 80, 100, 120, 140 Melt Temperature (°F) 390, 408, 425, 443, 460 Hold Pressure (steps of 100 or % of setpoint) 10, 15, 20 , 25 (15, 20, 25, 30) Injection Speed (steps) (fill ~ 6.5 s), 10 (fill ~ 3.5 s), 20 (fill ~ 1.6 s) Part Thickness (mm) 2.5 Table 11.13 Summary of Shrinkage Results (See Table 11.11 for Materials, and Table 11.12 for Conditions)[54] Material Small Mold: RG-1 RG-2 RG-3 CP-1 CP-2 CP-3 CP-4 CP-5 Large Mold: RG-1 RG-3 CP-3 CP-5 Shrinkage at Baseline Conditions Cross-flow Flow Direction Direction (cm/cm) (cm/cm) Flow Direction Shrinkage Minimum (cm/cm) Cross-flow Direction Shrinkage Maximum (cm/cm) Minimum (cm/cm) Maximum (cm/cm) 0.0149 0.0152 0.0148 0.0152 0.0063 0.0077 0.0070 0.0104 0.0159 0.0175 0.0161 0.0158 0.0060 0.0086 0.0070 0.0117 0.0130 0.0143 0.0130 0.0142 0.0049 0.0067 0.0054 0.0085 0.0214 0.0216 0.0193 0.0193 0.0087 0.0108 0.0096 0.0142 0.0130 0.0160 0.0135 0.0143 0.0050 0.0073 0.0057 0.0093 0.0229 0.0258 0.0210 0.0203 0.0088 0.0112 0.0096 0.0146 0.0145 0.0146 0.0064 0.0095 0.0132 0.0126 0.0050 0.0082 0.0124 0.0127 0.0050 0.0074 0.0160 0.0165 0.0068 0.0095 0.0123 0.0125 0.0041 0.0069 0.0167 0.0175 0.0094 0.0118 Note: In most cases, the shrink rate of the compounded materials is about half that of the reactor-grade materials All of these materials show similar relationships, therefore, the following curves are representative for all the materials, but only RG-1 is graphed Ch 11: Data © Plastics Design Library 209 Figure 11.76 Shrinkage vs time for RG-1.[54] (Courtesy of SPE.) Figure 11.77 Small mold, shrinkage vs pressure for RG-1.[54] (Courtesy of SPE.) Note: The curve in Fig 11.76 demonstrates that all significant size change after molding occurs in the first two days.[54] Therefore all measurements were made after a minimum waiting period of one week Until measured, the parts were stored in a lab at room temperature Figure 11.77 shows cross-flow shrinkage near the gate versus flow-direction shrinkage.[54] The bold line indicates the curve 0.017 - 1×106 × PAVG units/unit where PAVG is the average cavity pressure in psig (psi above atmospheric pressure) As the curves indicate, the formula closely approximates the shrinkage measured The cross-flow at the end of the part opposite the gate shows similar shrinkage The intercept value varies with the variable settings other than holding pressure The cross-flow shrinkage seems to change more rapidly at low pressure than the flow-direction shrinkage At higher cavity pressures, the difference between cross-flow and flow-direction shrinkage diminishes © Plastics Design Library Ch 11: Data 210 Figure 11.78 Small mold, shrinkage vs part thickness for RG-1.[54] (Courtesy of SPE.) Figure 11.79 Small mold, shrinkage vs mold temperature for RG-1.[54] (Courtesy of SPE.) Note: Figure 11.78 shows the variation in shrinkage when compared with part thickness.[54] Within the thicknesses shown, the shrinkage increases by 0.0025 mm/mm for each mm increase in thickness (0.0025 units/unit/unit increase in thickness) The minimum shrinkage varies depending on the particular formulation The thicker parts cool more slowly allowing more time for the molecules to build larger crystals As the part thickness increases above a certain point, the flow and cross-flow shrinkage diminish because the outer skin has enough strength to resist the molten core shrinkage and, as a result, voids form in the center instead of increasing the linear shrinkage yet more Figure 11.79 shows the plastic shrinkage versus mold-surface temperature and versus the coolant temperature.[54] The surface of the mold is warmer than the coolant temperature, therefore, the shrinkage is higher for a given coolant temperature than it is with a given mold-surface temperature The shrinkage increases by × 10-5 in/in/°F or 5.4 × 10-5 cm/cm/°C (The shrinkage increase would be × 10-5 cm/cm/°F, and 1°C = 1.8°F Since a degree Celsius is larger than a degree Fahrenheit, the shrinkage increase would be greater.) Ch 11: Data © Plastics Design Library 211 Figure 11.80 Small mold, shrinkage vs melt temperature for RG-1.[54] (Courtesy of SPE.) Figure 11.81 Small mold, part mass vs melt temperature for RG-1.[54] (Courtesy of SPE.) Note: Figure 11.80 shows that as melt temperature increases, the cooling time increases, allowing more time for crystals to form resulting in more and larger crystals.[54] The higher melt temperature allows more holding time and higher packing pressure before the gate freezes The shrink rate (Y) is  (melt temperature in °F) ( X )  (Y ) = 0.0249 –   10   Other studies have indicated that at very high melt temperatures the shrink rate starts increasing again Figure 11.81 shows that the weight (mass) of the part increases with an increase in melt temperature.[54] It is theorized that the higher melt temperature allows more time for crystals to build before the gate freezes, thus increasing the density of the final part Also, the higher melt temperature reduces the viscosity, which results in a lower pressure drop from the sprue to the last areas to fill This results in higher cavity pressure and greater part mass © Plastics Design Library Ch 11: Data 212 Figure 11.82 Large mold, shrinkage vs pressure for RG-1.[54] (Courtesy of SPE.) Note: Figure 11.82 shows the shrinkage in the direction of flow and the shrinkage in the cross-flow direction at three different locations on the larger plaque.[54] The heavy line represents the equation (Y ) = 0.0178 – (X ) 10 which approximates the average shrinkage shown in this part and closely approximates the flow-direction shrinkage down the length of the part The cross-flow near the gate is the lowest and the cross-flow far from the gate is considerably higher The flow at the gate is not strictly cross-flow Because of the radial nature of the flow at the gate, the flow near the gate is a combination of cross-flow direction and flow direction Also contributing to the low shrinkage at the gate is the higher pressure that exists in the cavity near the gate compared to the pressure further from the gate The shrinkage in the cross-flow direction in the center of the large part is somewhat erratic in that it is less than the end-of-flow shrinkage at 1500 psi cavity pressure but is higher than any other shrinkage at higher pressures The higher shrinkage is due to the lack of restraint in the center of the length of the part with the result that the plaque assumes an hourglass shape This phenomenon is discussed in more detail in Sec 8.5.3 Ch 11: Data © Plastics Design Library 213 Figure 11.83 Large mold, shrinkage vs injection speed for RG-1.[54] (Courtesy of SPE.) Figure 11.84 Large mold, shrinkage vs mold temperature for RG-1.[54] (Courtesy of SPE.) Note: In Fig 11.83,[54] see how the shrinkage tends to decrease as the injection speed increases throughout the range tested The shrink rate in this figure is represented by the equation (Y ) = 0.0149 – (X ) 10 At extremely high injection speeds, friction heating at the gate would likely give similar results to the extreme increased melt temperature shown in Sec 6.3, i.e., the shrinkage would start to increase again The shrinkage in the large mold as a result of mold-temperature increase is as shown in Fig 11.84[54] and the following equation: (Y ) = 0.0133 + (X ) 10 This correlates well with the observations in the small mold © Plastics Design Library Ch 11: Data 214 Figure 11.85 Large mold, shrinkage vs melt temperature for RG-1.[54] (Courtesy of SPE.) Figure 11.86 Mold comparison of shrinkage vs pressure for RG-1.[54] (Courtesy of SPE.) Note: In Fig 11.85,[54] the graph indicates the shrink rate versus melt temperature This correlates well with the small mold The large-mold melt-rate curve is: (Y ) = 0.0208 – (X ) 10 while the small-mold curve temperature minus shrink rate is: (Y ) = 0.0249 – (X ) 10 The reduction in shrinkage per degree increase in temperature is the same (2X/105) but the whole curve is slightly higher (by about 0.004 units/unit/oF) This differential may be wholly due to the increased injection pressure that reportedly was required to fill the larger mold Figure 11.86 shows the relationship of cavity pressure in both the large and small test cavities used in this series of experiments.[54] There appears to be no more than 0.001 units/unit difference in shrinkage between the large mold and the small mold at any given cavity pressure It must be noted, however, that the cavity pressure does not precisely follow the machine pressure due to differences in runner, gate, and cavity shapes Ch 11: Data © Plastics Design Library 215 11.9.7 Polyethylene Terephthalate (PET) (Rynite®) (DuPont) Table 11.14 The Effect of Mold Surface Temperature on Shrinkage of Two Grades of Rynite® [26] Mold Surface Temperature Rynite® 530 Rynite® 545 (°C) Flow Direction Transverse Direction Flow Direction Transverse Direction 50 0.09 0.35 0.07 0.29 95 0.15 0.75 0.13 0.75 105 0.16 0.88 0.14 0.77 11.9.8 Polyetherimide (PEI) (Ultem®) (GE Plastics) Figure 11.87 How the shrinkage of Ultem® is affected by mold temperature, injection rate, and wall thickness.[55] © Plastics Design Library Ch 11: Data 216 11.9.9 Liquid Crystal Polymer (LCP) (Zenite®) (DuPont) Figure 11.88 The shrink rates for Zenite® 6130 and 7130 as a function of mold temperature are reversed from that of most other thermoplastics.[49] The negative shrinkage values indicate that the part grows outside of the mold to dimensions greater than the cavity dimensions.[49] (Courtesy of DuPont.) Figure 11.89 Zenite ® 6130 grows during the annealing process (260°C for 30 minutes), but at a lower rate with increasing mold temperature.[49] (Courtesy of DuPont.) Note: Some materials actually grow after molding The usual condition where plastic shrinkage increases with rising mold temperature is reversed for aromatic polyester resins The negative values in Fig 11.88 indicate an increase in size greater than the size of the mold cavity This occurs at higher temperatures The increased mold temperature also reduces post-mold growth during an annealing process (see Fig 11.89) Ch 11: Data © Plastics Design Library 217 11.9.10 Polyamide: Nylon (PA) (Zytel®) (DuPont) Figure 11.90 Transverse-direction shrinkage vs flowdirection shrinkage for nylon 6/6 and polyacetal as glassfiber content increases.[47] (Reprinted by permission of Hanser-Gardner.) Figure 11.91 Warpage across nylon 6/6 plaques as thickness varies between mm and mm.[46] (Courtesy of SPE.) Figure 11.92 Shrinkage vs mold temperature for a 3-mm thick part of Zytel® 101 L.[9] (Courtesy of DuPont.) Figure 11.93 Shrinkage during annealing vs mold temperature for Zytel® 101 NC10.[35] (Courtesy of DuPont.) Note: Figure 11.93 shows that there is less annealing size change for material molded in a hot mold than there is for a part molded in a cold mold (See also Figs 11.61 and 11.94.) The quality of the part molded in a hot mold is generally higher and the part has less molded-in stress As a result, the part is tougher and more stable © Plastics Design Library Ch 11: Data 218 Figure 11.94 The total shrinkage for Zytel® 101 NC 10, including annealing shrinkage, for a 50 mm × 50 mm molded plaque of three different thicknesses.[35] The gate thickness is one-half of the part thickness The plaques are annealed at 160°C for one hour The total shrinkage is the mold shrinkage plus the annealing shrinkage.[35] (Courtesy of DuPont.) Figure 11.95 Size change for plaques, 76 mm × 127 mm, molded of Zytel® 101 caused by exposure to moisture.[35] (Courtesy of DuPont.) Note: At zero humidity, the parts shrink from to 2.1% As the relative humidity increases, the Zytel® parts grow until they are significantly larger than the mold cavity The mold temperature is 66°C, the injection pressure is 76 MPa, the gate thickness is one-half of the molded-part thickness If the mold temperature is not 66°C, add (if lower) or subtract (if higher) 0.03% per °C If the injection pressure is not 76 MPa, then add (if higher) or subtract (if lower) 0.007% for each MPa Ch 11: Data © Plastics Design Library 219 Figure 11.96 Shrinkage of Zytel ® 101 L as a function of part thickness for a mold temperature of 70oC and a hold pressure of 90 MPa.[9] The gate thickness was almost surely one-half of the part thickness (Courtesy of DuPont.) Figure 11.97 Dimensional changes due to moisture absorption of plaques of Zytel® 408.[35] The plaques are 76 × 127 × 1.6 mm The mold temperature was 66°C The injection pressure was 76 MPa The gate was one-half the part width and one-half the part thickness (Courtesy of DuPont.) Note: From this data it would appear that Zytel® 408 would almost return to the size of the mold cavity after it absorbs moisture in a 90% relative humidity environment Nylon has a high affinity for moisture and swells significantly as a result of its moisture absorption © Plastics Design Library Ch 11: Data 220 Figure 11.98 Dimensional changes due to moisture of plaques molded of Zytel® 151.[35] The plaques are 76 × 127 × 1.6 mm The mold temperature was 66°C The injection pressure was 76 MPa The gate was one-half the part width and one-half the part thickness (Courtesy of DuPont.) Figure 11.99 Dimensional changes due to moisture of plaques molded of Zytel® 70G30 HSL.[35] The plaques are 76 × 127 × 1.6 mm The mold temperature was 66°C The injection pressure was 76 MPa The gate was one-half the part width and one-half the part thickness (Courtesy of DuPont.) Figure 11.100 Dimensional changes due to moisture of plaques molded of Zytel® 77G33L.[35] The plaques are 76 × 127 × mm The mold temperature was 66°C The injection pressure was 76 MPa The gate was one-half the part width and one-half the part thickness (Courtesy of DuPont.) Figure 11.101 Time to absorb 2.8% moisture for Zytel ® 101 at 120°C in potassium acetate solution (125 g per 100 ml water).[35] (Courtesy of DuPont.) Ch 11: Data © Plastics Design Library