5Shrinkage Theory Practice and Applications References Injection variables and shrinkageBasic shrinkage formulaResponsibility for shrinkageEffect of shrinkage on mold openingApproximate shrinkage values for some materialsFactors affecting shrinkageSPI standard molding toleranceReferences
6ShrinkageShrinkage is the contraction of the molded part as it cools after injection.Most of the part shrinkage occurs in the mold while cooling.A small amount of shrinkage occurs after ejection as the part continues to cool, especially for Delrin or POM.After that the part may continue to shrink very slightly for several hours or even days until the temperature and moisture content stabilize. Thus dimensional inspection should wait at least a day.Part shrinkage units are expressed as thousandths of an inch per linear inch. ( 0.00X /in/in )Typical shrink rates vary between 0.001/in/in and .020/in/in with the most common being around 0.006/in/in.When calculating shrinkage for your part the tooling engineer simply scales your part by 1.00X. In pre-CAD days the engineer would expand your part by simply multiplying every number on the drawing by 1.00X.
7ShrinkageShrinkage varies with wall thickness also. The material supplier will usually give a range such as /in/in. with a inch wall. If your wall thickness is you would go right in the middle withThe molder can fine tune the shrinkage of the parts by adjusting the density of the material i.e. how hard he packs it out, and how long he holds it to cool in the tool.If your part is large, tolerances are critical, or you are using a new or unusual material. Then it is a good idea to do some test shots.Many molders have a huge rack of obsolete tools. Find one of these that makes a part somewhat similar in size,shape and wall thickness to your part.Then pay your molder to shoot your resin into it and use the parts to calculate a precise shrinkage for your material.The cost of doing this is small compared to that of reworking or scraping a tool.
8Shrinkage A note on asymmetrical shrink: A few plastics shrink differently in one direction than in the other.For example polymers filled with long glass fibers will shrink more in the cross (transverse) direction than the longitudinal (flow) direction.This poses an interesting dilemma for the mold designer. The material supplier will tell you that you have to use a different shrink in the X axis than in the Y. This is fine if you are making popsicle sticks or rulers. But if your part is complex with holes and flow fronts meeting at different angles and running different directions at different places in the part it is impossible to do, and would be outrageously expensive if you could.Even round holes would now become elliptical in the tool so that standard components such as core pins can not be used.What winds up happening is that the average between longitudinal and cross shrinkage is assumed, everybody buys off on it and then critical features are then altered or added after first shots.Bottom line… Do not use asymmetrical shrinkage resins if close tolerances matter.
10Shrinkage Shrinkage is inherent in the injection molding process. Shrinkage occurs because the density of polymer varies from the processing temperature to the ambient temperature (see Specific volume (pvT diagram)).During injection molding, the variation in shrinkage both globally and through the cross section of a part creates internal stresses or residual stresses act on a part with effects similar to externally applied stresses.If the residual stresses induced during molding are high enough to overcome the structural integrity of the part, the part will warp upon ejection from the mold or crack with external service load.
11ShrinkageThe shrinkage of molded plastic parts can be as much as 20 percent by volume, when measured at the processing temperature and the ambient temperature.Crystalline and semi-crystalline materials are particularly prone to thermal shrinkageAmorphous materials tend to shrink less.Excessive shrinkage can be caused by the following factors.The relationship of shrinkage to several processing parameters and part thickness is schematically plotted.low injection pressureshort pack-hold time or cooling timehigh melt temperaturehigh mold temperaturelow holding pressure.
12Shrinkage and WarpageWarpage is a distortion where the surfaces of the molded part do not follow the intended shape of the design.Part warpage results from molded-in residual stresses, which, in turn, is caused by differential shrinkage of material in the molded part.If the shrinkage throughout the part is uniform, the molding will not deform or warp, it simply becomes smaller.However, achieving low and uniform shrinkage is a complicated task due to the presence and interaction of many factors such as molecular and fiber orientations, mold cooling, part and mold designs, and process conditions.
13Shrinkage and WarpageWarpage in molded parts results from differential shrinkage.Variation in shrinkage can be caused by molecular and fiber orientation, temperature variations within the molded part, and by variable packing, such as over-packing at gates and under-packing at remote locations, or different pressure levels as material solidifies across the part thickness.These causes are described more fully below.Differences in filled and unfilled materials
14Shrinkage and Warpage These causes are described more fully below. Non-uniform mold cooling across the part thickness or over the partCooling rates that differ because of Part thickness variationPart geometry asymmetry or curvature
15Shrinkage and Warpage These causes are described more fully below. Non-uniform mold cooling across the part thickness or over the partCooling rates that differ because of Part thickness variationPart geometry asymmetry or curvature
16Design Rules for Shrinkage You can reduce or control shrinkage and warpage by properly designing the part, mold, and process, as well as through careful material selection.The following design rules provide some guidelines for developing low-shrinkage, warp-free partsWall thickness: Avoid non-uniform wall thickness or design a transition length of three times the thickness of the thinner region
17Design Rules for Shrinkage The following design rules provide some guidelines for developing low-shrinkage, warp-free partsThick Sections: Alter the design to replace thick sections that cause significant shrinkage and lead to sink marks or internal voids.A thin, uniform wall with ribs provides for uniform shrinkage, strength to weight ratio, and cost effectiveness.
18Design Rules for Shrinkage The following design rules provide some guidelines for developing low-shrinkage, warp-free partsBalanced FillingDesign the delivery system to provide a balanced filling pattern with a constant melt-front velocity.Packing PressureWhile high packing pressure can help reduce the shrinkage, it also potentially increases the level of residual stress and the clamp force requirement.For a better process design, use a proper packing pressure level, allow sufficient packing time, and remove the pressure after the gate freezes off.The packing pressure should be able to deliver additional material to compensate for the volumetric shrinkage in the part.
19Design Rules for Shrinkage Cooling SystemDesign the cooling system to apply uniform, balanced cooling, both across the thickness and throughout the part.Residual StressReduce residual stress and molecular or fiber orientation byincreasing the melt temperature, mold-wall temperature, fill time, and cavity thickness, or bydecreasingthe packing pressure andflow path.
20Residual Stress Culprit in shrinkage and warpage problems Residual stress is a process-induced stress, frozen in a molded part.It can be either flow-induced or thermal-induced. Residual stresses affect a part similarly to externally applied stresses.If they are strong enough to overcome the structural integrity of the part, the part will warp upon ejection, or later crack, when external service load is applied.Residual stresses are the main cause of part shrinkage and warpage.Coonditions that promote sufficient packing and uniform mold cooling will reduce thermal-induced residual stress.
21Residual Stress Flow induced stress Unstressed, long-chain polymer molecules tend to conform to a random-coil state of equilibrium at temperatures higher than the melt temperature (i.e., in a molten state).During processing the molecules orient in the direction of flow, as the polymer is sheared and elongated.If solidification occurs before the polymer molecules are fully relaxed to their state of equilibrium, molecular orientation is locked within the molded part.This type of frozen-in stressed state is often referred to as flow-induced residual stress.Because of the stretched molecular orientation in the direction of flow, it introduces anisotropic, non-uniform shrinkage and mechanical properties in the directions parallel and perpendicular to the direction of flow.
22Residual Stress Flow induced stress Process conditions can reduce the shear stress in the melt and reduce the level of flow-induced residual stresses.In general, flow-induced residual stress is one order of magnitude smaller than the thermal-induced residual stress.To reduce flow induced stresses usehigher melt temperaturehigher mold-wall temperaturelonger fill timelower melt velocitydecreased packing pressureshorter flow path.
23Thermal Induced Residual Stress Occurs due to the following reasons:Material shrinks as the temperature drops from the process settings to the ambient conditions reached when the process is complete.The material elements experience different thermal-mechanical histories (e.g., different cooling rates and packing pressures) as the material solidifies from the mold wall to the center.Changing pressure, temperature, and molecular and fiber orientation result in variable density and mechanical properties.Certain mold constraints prevent the molded part from shrinking in the planar directions.
24Thermal Induced Residual Stress Unbalanced CoolingVariation in the cooling rate from the mold wall to its center can cause thermal-induced residual stress.Furthermore, asymmetrical thermal-induced residual stress can occur if the cooling rate of the two surfaces is unbalanced.Such unbalanced cooling will result in an asymmetric tension-compression pattern across the part, causing a bending moment that tends to cause part warpage. (Figure below)Consequently, parts with non-uniform thickness or poorly cooled areas are prone to unbalanced cooling, and thus to residual thermal stresses.For moderately complex parts, the thermal-induced residual stress distribution is further complicated by non-uniform wall thickness, mold cooling, and mold constraints to free contraction.
25Thermal Induced Residual Stress Unbalanced CoolingVariation in the cooling rate from the mold wall to its center can cause thermal-induced residual stress.Furthermore, asymmetrical thermal-induced residual stress can occur if the cooling rate of the two surfaces is unbalanced.Such unbalanced cooling will result in an asymmetric tension-compression pattern across the part, causing a bending moment that tends to cause part warpage. (Figure below)Consequently, parts with non-uniform thickness or poorly cooled areas are prone to unbalanced cooling, and thus to residual thermal stresses.For moderately complex parts, the thermal-induced residual stress distribution is further complicated by non-uniform wall thickness, mold cooling, and mold constraints to free contraction.
26Effects of Shrinkage on Containers ShapeCylindrical containersMore shrinkage near the open end will cause the container to toe in.Due to bottom of the container being better packed than the rim.Fig 8.9Conical containersWall near top will pull in due to same reasons as before.Fig 8.11Beads, lips and stacking shoulders can introduce even more toe-in. and other distortions in these products.Fig 8.12