Thermal Resistance Basics in Textured Plastic Injection Molds
Textured mold cavity thermal resistance stems from trapped micro-air gaps, reducing interfacial heat transfer coefficients down to 800 W/m²K and extending cooling cycle duration.
Wall Thickness Bowing
Meaning
Curvature or deflection in a flat surface of a moulded part results from internal stresses or uneven cooling across the section. Wall thickness bowing is a common defect that compromises both the appearance and the function of a component. This issue is often seen in large, flat parts like panels or housings.
Stress Distribution
Alignment of the polymer molecules and the resulting shrinkage forces create a pulling effect within the part. Wall thickness bowing occurs when the shrinkage on one side of a wall is greater than on the other. This imbalance can be caused by a difference in the cooling rate or by a variation in the actual thickness of the wall itself.
Tool Alignment
Accuracy of the core and cavity positioning is essential for maintaining a uniform wall across the part. Wall thickness bowing can happen if the tool plates shift slightly, making the wall thicker on one side than the intended design. This non-uniformity causes the thicker section to cool more slowly and shrink more, pulling the part into a curve.
Quality Control
Measurement of the flatness of a part is performed using coordinate measuring machines to ensure it meets the specification. Wall thickness bowing can often be corrected by adjusting the cooling water temperatures or by changing the gate location to alter the flow path. If the bowing is severe, the part may not fit into its assembly or may fail to seal against a gasket.
Engineers often include ribs or other structural features to stiffen flat walls and resist this tendency to bend. Achieving a perfectly flat wall is a significant challenge in the moulding of high-shrinkage materials like nylon or polypropylene.