7 Factors Affecting Post-Mold Shrinkage and Warpage Most part shrinkage takes place within a very short time after the part is molded, typically within sixteen to forty-eight hours after demolding The reduction in volume during this initial time period is a result of solidification and thermal contraction as the molded part cools to room temperature This rapid size change is influenced by the variables discussed in Chs 2–6: material properties, part geometry, the runner and gate systems, melt temperature, mold temperature, injection pressure, holding pressure, and so on The same variables affect post-mold shrinkage, occurring more than forty-eight hours after demolding Especially important phenomena in post-mold shrinkage are temperature and moisture conditions during molding, along with in-service exposure after manufacturing This chapter reviews the factors of greatest influence on post-mold shrinkage.[39] 7.1 Effects of Temperature on Dimensions Time and temperature conspire to allow molded-in stress relaxation and some slight additional crystallization in semicrystalline materials after the molded part is ejected Some semicrystalline materials such as acetal, PBT, and PB can shrink as much as 0.5% after molding The longer the time and the higher the ambient temperature, the greater the tendency for the molded part to shrink after molding Plastics, by their very nature, have more thermal expansion and contraction than metals When plastics are constrained by being attached to a metal part, they may crack or totally fail if exposed to widely varying temperatures This type of failure is due to the frequent change in stress from tension to compression and back again under the influence of the temperature variations In molding operations, the plastic material is cooled from the outer surface Solidification occurs against the mold surface and the solidification front proceeds from that surface toward the center of the thickness of the plastic part Several factors affect the rate of heat transfer from the plastic to the mold The mold temperature is the most significant factor and most subject to the control of the molder The higher the mold temperature, the slower the plastic will cool because © Plastics Design Library the temperature gradient between the molten plastic and the mold wall is lower Higher mold temperatures slow the cycle and increase the in-mold shrinkage, but reduce long-term or post-mold shrinkage The net result is that the parts molded in a hot mold need little or no annealing and exhibit little or no post-mold shrinkage For example, in molding Delrin® at moderate temperatures, good stability can be obtained with a mold temperature of 90°C (194°F) For more severe conditions, the mold temperature for Delrin may need to be as high as 120°C (250°F).[33] The cooling efficiency of the mold contributes to the cooling rate of the plastic part For example, if cooling channels in the mold are placed very near the molding surface, the heat transfer into the cooling water is quite rapid near the water channels but somewhat slower between water channels This results in a variation of the temperature of the surface of the mold from a minimum immediately over the water channel to a maximum half-way between the channels The variation in mold temperature across a large, flat surface that results from cooling channels placed too near the surface may cause a visible “ripple” on the surface of the part Placing the cooling channels at a greater distance from the molding surface results in a more uniform surface temperature At one time it was advocated that cooling channels not be placed in the inserts but instead be placed in the holder blocks or the plates immediately behind the mold inserts This resulted in very uniform temperatures on the mold surfaces initially, but the continuous, very slow heat-transfer ultimately caused a rise in the mold surface temperature This “uniformity” theory actually can result in a reduction of mold-temperature consistency If there are mold details that are difficult to cool, remote cooling lines increase that difficulty and increase the mold surface-temperature variations In addition, if there are mold cycle-time variations, as there frequently are with manually operated molding machines, the mold surface temperature drops more during any delays (such as when the operator sprays the mold surface, smokes a cigarette, drops a part, extracts a stuck part, etc.) After a delay such as this, the next few parts are molded in a cooler mold than those molded during a consistent cycle Ch 7: Factors Affecting Post-Mold Shrinkage and Warpage 98 In some cases, it is impossible to maintain absolutely uniform mold surface temperature Very small and long core-pins cannot be effectively cooled throughout their length Usually, most of the cooling around such core pins is from the outside surface of the part around the cored hole, with little of the heat transferred through the core pin A similar problem exists in the vicinity of sharp, inside corners of a molded part This type of uneven cooling shifts the neutral axis toward the hot side of the part and increases the tendency toward warpage As the plastic part cools, it pulls away from the mold surface due to volumetric shrinkage The lower the packing pressure, the sooner the separation occurs As the plastic pulls away from the mold wall, there is a sharp reduction in heat transfer from the plastic to the mold wall This happens because dead air space is an excellent insulator A vacuum is an even more effective insulator and a vacuum is often present as the plastic shrinks away from the cavity wall because there is no source for air until the mold opens Inadequate packing pressure can cause significant variations in the cooling rates thus cooling inconsistency across the surface of a molded part as a result of this type of separation In summary, higher mold or melt temperature results in less post-mold shrinkage However, higher mold temperatures are often localized because of inefficient cooling Localized hot spots cause shrinkage variation and warpage Post-mold annealing can accelerate the post-mold shrinkage and minimize later size change Parts molded in cooler molds can be annealed (stress relieved) to achieve better mechanical properties and stability in the final part Fixturing may be required to stabilize parts during the annealing process Fixturing is a complex process and should only be used when molded parts require very tight tolerances and exposure to high temperatures for prolonged periods while in use Attempts to reach good dimensional stability by annealing parts molded in a cold mold are likely to lead to high post-molding shrinkage and may introduce stresses causing uncontrolled deformation This is especially true for semicrystalline materials such as acetal or nylon Post-mold shrinkage of acetal parts molded at a variety of mold temperatures when exposed to different temperatures for 1000 hours are shown in Fig 7.1 The annealing procedures for the parts showing the least shrinkage in the charts in Fig 7.1 were subject to the following guidelines: • Parts should be exposed to air or an inert mineral oil at 160 ±3°C for 30 minutes plus minutes per mm of wall thickness Ch 7: Factors Affecting Post-Mold Shrinkage and Warpage • Overheating and hot spots should be avoided • Parts should neither contact each other nor the walls of the container • Parts should be left in the container to cool slowly until 80°C is reached • Stacking or piling, which may deform the parts while they are hot, should be delayed until the parts are cool to the touch • Annealing can also be used to test molded parts to determine their long-term stability and size change Annealed parts closely resemble the ultimate size of the parts after long-term use For maximum in-service stability of the molded part, mold temperatures should be near the high end of the plastic supplier’s recommendations For example, post-mold shrinkage can be estimated for Delrin® acetal from Fig 7.1.[33] 7.2 Effects of Moisture on Dimensions Post-mold size change also can come about as a result of absorption or loss of fluids such as water or plasticizers The loss of plasticizers causes a plastic part to become more brittle and to shrink How many automobile dashboards have you seen that have lost color or cracked? This type of failure is caused by the loss of plasticizers Some materials are hygroscopic; that is, they attempt to absorb moisture from the environment As they absorb moisture, the material properties change Sometimes the materials become tougher, usually there is dimensional change Figure 7.2 shows the change in size due to moisture absorption of Zytel® 101.[9] Size changes for Delrin® 100 and 500 are shown in Fig 7.3.[33] Other moisture absorption curves can be found in the material-specific data section (Ch.11 of this book) Nylons are strong materials with good chemical resistance, but they absorb large amounts of water if immersed It is not generally considered a good application for nylon if the part is to be immersed in or continually exposed to water unless full consideration is given to the amount of post-mold growth that nylon can experience in water Applications using nylon have failed because the nylon parts that were immersed in water swelled so much that they did not allow the moving parts to move freely Some nylons can absorb mois- © Plastics Design Library 99 ture to such an extent that the totally saturated nylon part is larger than the cavity in which it was molded Figure 7.2 shows the dimensional change of nylon as it absorbs moisture The change shown here is not necessarily equal in flow and cross flow The measurement direction is not specified but is probably in the flow direction.[13] Figure 7.3 implies that the molded part was probably a tensile test (dog-bone) specimen and that the measurements were along the long or flow-direction axis There is no indication that the cross-flow changes are the same The presence of moisture during molding inhibits a glossy surface Moisture usually causes surface splaying (which normally manifests itself as silvery streaks parallel to the flow direction of the plastic, sometimes as irregularly shaped silver spots) or other imperfections because it inhibits close contact with the cavity wall and can cause foaming or voids within the molded part Moisture in the plastic pellets as they enter the heating section of the molding machine often cause plastic-property degradation because of chemical reactions between the plastic and superheated steam Table 7.1 shows the equilibrium water absorption percentages for several polyamides.[9] Nylons must be molded dry to avoid material degradation, but in the dry condition, they tend to be brittle When they have absorbed moisture, they become tougher Figure 7.1 Post-molding shrinkage of Delrin ® acetal resins.[33] (Courtesy of DuPont.) © Plastics Design Library Figure 7.2 Size change of Zytel ® 101 vs moisture absorption.[9] (Courtesy of DuPont.) Ch 7: Factors Affecting Post-Mold Shrinkage and Warpage 100 The 24-hour absorption levels of water by nylon compared to the equilibrium levels of water in nylon in an environment where the relative humidity is less than about 25% are as follows: Type of nylon Figure 7.3 The effect of temperature and moisture content on the dimensions of Delrin® 100 and Delrin ® 500 [33] (Courtesy of DuPont.) Table 7.1 Water Absorption of Nylons in Air and Water Nylon 66 24 hours in water 1.2 Equilibrium % of water content 9.0 Nylon 610 Nylon 11 0.4 0.3 3.5 2.0 Figure 7.4 shows longer-term water absorption for Nylon 11 and two other grades.[13] Note that Nylon absorbs significantly more water than the other grades In most cases, it is a good idea to condition nylon parts in hot water before placing them in service to stabilize the moisture absorption and increase the toughness of the nylon Dry nylon as molded is relatively brittle Suppose a flat part is exposed to water on one side and a dry environment on the other The bow-shaped warpage as shown in Fig 7.5 could take place The same sort of warpage can take place if one side of a part is coated with an impermeable layer and the other side is left uncoated Plastics will absorb all kinds of fluids to a measurable level Inspection of the chemical compatibility of the plastic in question will give a good indication of likely absorption of a particular fluid If a supplier states that a plastic is compatible with a particular fluid or is resistant to that fluid, it can be assumed that after two weeks of immersion, the plastic will absorb an amount of fluid that is less than 1% of the weight of the part.[13] Absorption Polyamides In Water at 20°C (%) In Air at 50% RH, 23°C (%) 8.5 2.8 66 7.5 2.5 6/66 7.5 2.5 6/12 3.0 1.3 6/10 3.0 1.2 Amorph 5.8 2.8 Figure 7.4 The percentage of water absorbed by some grades of nylon over long periods of time Ch 7: Factors Affecting Post-Mold Shrinkage and Warpage © Plastics Design Library 101 Figure 7.5 Potential warpage (exaggerated) due to nonuniform exposure to moisture Many plastics contain mobile fluids such as plasticizers, antistatic agents, lubricating oils, dyes, etc Most users are aware of the problem of plasticizer migration and that plasticizer loss will cause significant changes in dimensions (shrinkage) The migration of mobile fluids is accelerated by contact with a wide range of organic fluids which, having greater affinity for the plasticizer than the molded plastic, may cause rapid shrinkage.[13] Some materials contain plasticizers without this being explicitly stated Flexible grades of cellulosics and nylons (particularly Nylon 11 and Nylon 12) are quite common, and these will be prone to migration-induced shrinkage, just as will any plastic containing mobile fluids Figure 7.6 shows the moisture absorption as a percentage of the weight of the part of certain glass-fiber plastics immersed in water.[40] This figure does not differentiate between hygroscopic and non-hygroscopic materials, but rather suggests at least some moisture migration along the glass fibers into the plastic part From Fig 7.7 it is obvious that nylon is hygroscopic and its level of water is strongly affected by the environment.[35] The more water that is available, the more nylon absorbs to reach equilibrium The time that is required for a plastic part to reach an equilibrium condition, for any given moisture concentration, is affected by the environmental temperature and thickness of the plastic part The thicker the part, the longer it takes for the moisture to migrate through the plastic and uniformly permeate the part Figure 7.8 shows how thicker walls of Zytel® 101 take longer to reach equilibrium.[35] © Plastics Design Library The equilibrium condition for this material is the same, about 2% to more than 5% moisture, no matter how thick the walls are This graph indicates that a 1.5-mm thick wall reaches equilibrium in about months, but the thicker walls may not reach equilibrium in a year Figure 7.9 shows another nylon resin that has not reached equilibrium in thicker sections in a year.[35] When immersed in water, these same two resins approach equilibrium more rapidly than at 50% RH in air See Fig 7.10.[35] Figure 7.11 shows the time required to condition Zytel® 101 to 3% moisture and to saturation for various wall thicknesses.[35] Figure 7.12 shows that nylon can increase in size as a result of moisture absorption as much or more than it can shrink out of the mold (as much as 0.025 inches per inch).[35] We have dealt here primarily with size change of nylon due to absorption of water The wrong chemical can affect any plastic While water is probably the most common environmental fluid that is likely to be absorbed by a plastic, and some plastics react more strongly to its presence than others, many plastics react adversely to hydrocarbons that are quite common in the petroleum and automotive industry Check the plastic’s reaction to known or suspected chemicals that are likely to be present in the expected environment Figure 7.6 The percentage of moisture absorption (but not the size change) of a variety of plastics as a result of immersion in water.[40] (Courtesy of Hoechst Celanese.) Ch 7: Factors Affecting Post-Mold Shrinkage and Warpage 102 Figure 7.7 The equilibrium conditions of moisture content vs relative humidity for a variety of Zytel® nylon resins.[35] (Courtesy of DuPont.) Figure 7.8 Moisture content vs time for Zytel® 101F exposed to 50% RH air at 23ºC.[35] (Courtesy of DuPont.) Figure 7.9 Moisture content of Zytel® 151 as time passes when the Zytel is exposed to air at 50% RH at 23°C Three different thicknesses are shown.[35] (Courtesy of DuPont.) Figure 7.10 Moisture content vs time for Zytel® 101 and Zytel® 151 when immersed in water at 23°C.[35] (Courtesy of DuPont.) Ch 7: Factors Affecting Post-Mold Shrinkage and Warpage © Plastics Design Library 103 Figure 7.11 Boiling times to condition Zytel® 101.[35] (Courtesy of DuPont.) Figure 7.12 The size change of Zytel® 101 in the stressfree (annealed) condition as it absorbs moisture [35] (Courtesy of DuPont.) 7.3 of any significant part of the tensile strength of the material, long-term measurement of deflection (six months minimum exposure) should be conducted The test should be conducted at the highest expected stress and at the highest expected environmental temperature Any significant deflection over time would indicate the need for additional structural support It does happen that product suppliers introduce new resins that have had only short-term testing A few years ago, a company introduced a new large product line in which the thermoplastic was expected to carry significant structural loads The initial short-term testing of the product yielded outstanding results However, after six months to a year in the field, the product sagged to the point that it became unacceptable for the intended purpose This ultimately led to bankruptcy of the company Had the long-term creep characteristics of the thermoplastics been recognized, other structural elements could have been included in the design that would have produced an excellent product Unfortunately, the failure to recognize the creep characteristics of the plastic led to the company failure and added another black mark to consumers’ concepts of plastic Creep While it is not strictly a shrink or warp phenomenon, if a plastic part is loaded to a significant fraction of its tensile strength, it can be subject to creep failure For most practical purposes, plastic can be thought of as molasses in January in Alaska Fiber fillers increase the stiffness of plastics but they not eliminate the tendency to creep As a general rule, it is unwise to use thermoplastics as load-bearing structures without huge safety factors or extensive, long-term, elevated-temperature testing For this type of application, the creep data for the plastic is much more significant than the tensile or compressive strength Creep is a phenomenon that is foreign to most designers Most thermoplastics are subject to at least some creep Amorphous thermoplastics are similar to glass; the slow rate of creep has no limit Semicrystalline materials are somewhat more rigid and the creep rates tend to diminish over time The physical property data for a given plastic is for short-term loading Long-term deflection versus stress is rarely published Before marketing a product that is exposed to long-term stress © Plastics Design Library Ch 7: Factors Affecting Post-Mold Shrinkage and Warpage