Polypropylene Copolymer versus Homopolymer Impact Resistance
Impact copolymers deliver superior notched and subzero impact resistance over homopolymers by incorporating a dispersed ethylene-propylene rubber phase.
Weld Line Retention
Meaning
Measurement of the structural integrity at the interface where converging melt streams fuse during the injection moulding process determines the strength of a part. Calculating weld line retention involves comparing the breaking force of a sample containing a join to that of a solid piece of the same material. The resulting value is a percentage that shows how much of the original polymer strength is preserved at the seam.
This metric is critical for safety parts where a failure at the join could lead to a collapse. Moulders use this value to assess the impact of processing variables on the final quality of the component. The measurement is taken using standard tensile or flexural tests.
Flow Front
Temperature and pressure at the point of contact dictate the quality of the molecular entanglement across the interface. Since weld line retention is highly sensitive to the heat of the incoming melt, increasing the tool temperature often improves the bond. Rapid cooling prevents the chains from diffusing across the boundary.
Proper venting ensures that trapped air does not create a physical gap in the seam.
Strategic Positioning
Placement of the injection gates ensures that the meeting points of the melt are located in low stress areas of the geometry. If weld line retention is expected to be low for a specific resin, the designer may use a different gate configuration to move the join. Simulation software helps predict where these lines will form before the steel is cut.
High quality moulds minimize the visual and structural impact of these features.
Integrity Testing
Evaluation of the finished part under mechanical load confirms that the joins can withstand the required service stresses. Achieving high weld line retention ensures the part behaves as a single piece. Failure to manage these interfaces leads to cracking.
Durable parts are the result of careful material selection.