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Injection and packing pressures should be adequate to permit easy fill but should not be set excessively high in order to allow some of the molded-in stresses to relax before the part sets.
Increasing the injection rate will decrease the injection time, which will allow the mold to fill faster before the extremities can cool too much. This gives the entire molding a chance to cool at a more uniform rate, which reduces the warpage.
Some of these remedies, such as high melt temperature or low injection pressure can increase the cycle time. Switching to a higher MFR/MI resin can offset the increase in time. Higher flow resins will allow lower injection pressure, which can shorten the molding cycle. In addition, higher flow resins typically exhibit less “elastic memory” which can also reduce warpage. Lower density resins (for PE) are only slightly less susceptible to warpage than higher density resins.
Differences between flow and transflow shrinkage can result in warpage. HDPE is known to have a large difference between these two while PP is more balanced between the flow and transflow shrinkage.
Because both shrinkage and warpage are strongly influenced by the mold cooling patterns and part geometry (uniformity of thicknesses and flow patterns), it is very important that these be considered in the early stages of part and mold design.
Table 5. Some ways to reduce shrinkage and warpage in polyolefin injection moldings
Color dispersion and air entrapment
An effective means of improving dispersion and preventing air bubbles from getting into the mold with the melt is to use a breaker plate at the end of the barrel between the screw tip and the nozzle (Figure 41). Backpressure on the melt, in most cases, squeezes all the air out between the melting pellets and produces bubble-free moldings. The breaker plate may be 1/4 inch (6.5 mm) thick and must be large enough to fit into the opening of the nozzle. The plate is drilled with 20- to-40, small diameter (1/32 inch or 0.8 mm) holes. Another option is to increase the back pressure on the screw making sure that it is not set too high so that the screw cannot recover in time for the next cycle. Increasing the backpressure and/or adding a breaker plate can also aid in color dispersion and melt temperature homogeneity. When running at high processing temperatures, backpressure should be kept to a minimum to reduce degradation of the resin.
Part ejection and mold release
Mold release is affected by a number of factors. Some polyolefin molding resins exhibit better mold release than others do. It has been found that these resins have accompanying disadvantages, such as less gloss. Resins that develop a grainy or frosty surface release better than smooth, high-gloss moldings, such as those made from high MI grades or clarified polypropylene. However, even polyolefins of the same MI may vary in their mold-release properties. Resins sometimes are compounded with a mold-release additive.
Changing the mold design or one or more molding conditions without affecting the end properties of the molding usually can alleviate mold release problems. Mold release may be difficult if the mold is packed too tightly in an effort to reduce shrinkage. Frequently, a molded article sticks to the mold if the cycle time is too long and the molding shrinks to a core. In such cases, shortening the cooling time may improve mold release. On the other hand, the same problem can occur if the cycle is too short to allow the molding to shrink away from the cavity walls. In such cases, lengthening the cycle time may improve mold release. Draw stoning of the surfaces in the direction of mold opening may also alleviate this problem.
Mold release greatly depends on the degree of polish on the inside of the mold. Proper surface finishes inside a mold for a deep-draw item determines whether the part can be ejected easily or will stick to the cavity or the mold core. Ejection pins may be used to move the molding from either the cavity or the core first.