CASE STUDIES: Color Contaminants & Knit Lines

PELabs was hired by the molder for a major manufacturer of children's products to find the cause for cracking of field-parts from a large production lot of car-seat shells. Upon examining samples, our engineers first observed that the cracks were occurring on a principal weld-line in the part (See photo in Figure 1). They listed poor molding conditions, possible material deficiencies, contamination, degradation and too much regrind as possible causes. So, they formulated a list of questions for the molder and laid out a testing program to expose the culprit. The material was prime polypropylene (PP), and the molder related that the problem was unique to this one lot that was molded under the same conditions and percentage of regrind as previous good lots, but a new lot of resin had been used.

Figure 1. Weld-line Crack in Injection-molded, Car-seat Shell

A DSC (Differential Scanning Calorimetry) of three samples of resin — one from the virgin material in the silo, one from a part out of the questionable lot that had not cracked, and the other from a cracked field sample. All showed two melting peaks, one in the vicinity of 150-160°C, the melting point of the specified PP, and another around 130-140°C, about where most high-density polyethylenes melt. The presence of PE was confirmed with infrared spectroscopy (IR). Comparison of the energy spent in melting the two species indicated PE contents of 5-10% for the silo sample and the un-cracked sample, while it was nearly 50% for the returned field sample.. A thorough review by the molder uncovered that a railcar of PP had inadvertently been loaded into a silo with a residual of PE in it.

Even small amounts of PE in PP can result in weak weld lines. Consider the tensile data of Table I collected at PELabs for different levels of a 53-melt* PE-carrier colorant in a 16-melt* PP. The tensile bars were molded with gates at both ends, creating a weld line across each bar in the gauge length. (See the photo in Figure 2.) Each Pure-PP specimen necked at the weld-line, elongated a large distance and broke in the necked region, not necessarily at the weld line. All specimens in every sample group having the PE colorant, on the other hand, broke at the weld-line without necking, before the yield point was reached. A photograph of typical specimens after testing is shown in Figure 3.

The data in Table 1 show the weld-line strength of PP to be reduced appreciably (~25%) by the presence of even 2% of a colorant with a PE carrier. For further results on this subject, see the article Effect of Colorant Carriers on Strength of Weld Lines in Polypropylene in the TecH RESOURCES section of this Website.

Figure 2. Double-Gated Tensile Bar Mold and Shot

Figure 3. From the Top, Tested Tensile Bars with No Colorant, 4% PP- and 4% PE -colorant.

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