|Editor's note: Estimating shrinkage remains one of the biggest mysteries in the molding industry. The shrinkage rate determines how tools will be cut and how the part will be processed, yet data sheet information, by its very nature, cannot tell the whole story. Three researchers from Dow Chemical derived some concrete shrinkage results from experiments with polypropylene, and share them in this excerpted paper, winner of the Best Injection Molding Paper at Antec '99, presented by IMM and SPE's Injection Molding Division. The authors-Patrick Gipson, Peter Grelle, and Brent Salamon-selected PP for its high shrinkage rate and growing popularity among automotive and durable goods designers.|
Finding out how much a specific part will shrink is not a straightforward matter. In order to cut molds to the correct dimensions, the expected so-called shrinkage number must be known. However, resin suppliers cannot provide a number for a specific polymer grade that will be accurate for different thicknesses of parts and different molding conditions. The shrinkage rate published on a data sheet applies only to the specific molding conditions and thickness of test plaques.
We can start, however, with several given facts. Semicrystalline materials generally have higher shrinkage rates because polymer chains can arrange themselves into crystalline regions where the chains are more densely packed than in amorphous materials. We know that the degree of crystallinity is affected not only by the chemical structure, but also by the rate of cooling during processing. It's logical to assume that cooling variables such as mold temperature, melt temperature, and part thickness play a major role in determining final part dimensions.
When it comes to polypropylene, determining shrinkage can be complicated by the material's high shrinkage characteristics. For this reason, the molding conditions used to fabricate PP parts are critical. To determine some meaningful relationships, experiments using the following eight PP grades were conducted:
- RG-1: reactor grade, nonnucleated, 35 MFR (melt flow rate).
- RG-2: reactor grade, nucleated, 35 MFR.
- RG-3: reactor grade, nonnucleated, 20 MFR.
- CP-1 to CP-5: compounded grades containing varying amounts of reactor grade materials, HDPE, talc, and elastomers. MFRs range from eight to 14.
For each of these materials, we studied the effects of variables such as part thickness, hold pressure, mold temperature, melt temperature, injection speed, and flow length.
|Figure 1. Small mold: shrinkage vs. part thickness.
Two types of plaque molds were used for these experiments-a small mold measuring 6.67 by 6.67 cm (2 5/8 by 2 5/8 inches) and a large mold of 10.1 by 30.5 cm (4 by 12 inches).
Small plaques were molded on a 15-metric-ton Arburg that had a 25-mm screw diameter, and a shot size of 1.1 oz. Part thickness was varied by using different shims behind the scribed insert. The oil-cooled mold contained a full width fan gate, producing one-dimensional flow down the part length.
For the large plaque, a 300-metric-ton Demag machine was used with a 60-mm screw and a 20-oz shot size. The mold cavity, textured on one side, was 2.5 mm thick. This water-cooled mold had a 1-inch