Understanding Shrinkage in Injection Molding: Causes, Effects, and Design Rules, Slides of Mechanics of Materials

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Shrinkage Chapter 8 Docsity.com Mold Layout e Summary — Source: http://www.paralleldesign.com/moldability_101/ LOCATING RING ah ala UL ead ee INE pels & a Rens eT be) CaN a a Aaeals)§ Seay pr EJECTOR PIN EJECTOR BOX EJECTOR PLATE GUIDED EJECTION SUPPORT PILLAR Docsity.com Shrinkage • Theory • Practice and Applications – Injection variables and shrinkage – Basic shrinkage formula – Responsibility for shrinkage – Effect of shrinkage on mold opening – Approximate shrinkage values for some materials – Factors affecting shrinkage – SPI standard molding tolerance • References Docsity.com Shrinkage • Shrinkage 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. Docsity.com Shrinkage • Shrinkage varies with wall thickness also. The material supplier will usually give a range such as 0.005-0.007/in/in. with a 0.100 inch wall. If your wall thickness is 0.100 you would go right in the middle with 0.006. • The 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. Docsity.com Shrinkage • 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. Docsity.com Shrinkage • The 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 shrinkage • Amorphous 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 pressure – short pack-hold time or cooling time – high melt temperature – high mold temperature – low holding pressure. Docsity.com Shrinkage and Warpage • Warpage 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. Docsity.com Shrinkage and Warpage • These causes are described more fully below. – Non-uniform mold cooling across the part thickness or over the part – Cooling rates that differ because of Part thickness variation – Part geometry asymmetry or curvature Docsity.com Design 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 parts – Wall thickness: Avoid non-uniform wall thickness or design a transition length of three times the thickness of the thinner region Docsity.com Design Rules for Shrinkage • The following design rules provide some guidelines for developing low-shrinkage, warp-free parts – Thick 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. Docsity.com Residual 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. Docsity.com Residual 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. Docsity.com Residual 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 use • higher melt temperature • higher mold-wall temperature • longer fill time • lower melt velocity • decreased packing pressure • shorter flow path. Docsity.com Thermal Induced Residual Stress • Unbalanced Cooling – Variation 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. Docsity.com Effects of Shrinkage on Containers • Shape – Cylindrical containers • More 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 8.9 – Conical containers • Wall near top will pull in due to same reasons as before. – Fig 8.11 • Beads, lips and stacking shoulders can introduce even more toe-in. and other distortions in these products. – Fig 8.12 Docsity.com