Shrinkage Prevention Design for Automotive Exterior Molded Parts

Automotive exterior components (such as bumpers, grilles, fenders, and door guards) are essential parts of a vehicle’s appearance. They not only influence visual appeal but also directly affect brand image and aerodynamic performance. However, during the injection molding process, sink marks are one of the most common defects, often appearing in thick-walled areas, rib intersections, or regions with uneven wall thickness. This article explores key strategies for shrinkage prevention in automotive exterior molded parts design, helping manufacturers improve product quality and reduce defect rates.

Product Structure Optimization – The Foundation for Shrinkage Prevention

Uniform Wall Thickness Design

• Recommended wall thickness range for exterior parts: 2.5–3.5 mm
• Avoid sudden changes in local thickness to prevent uneven cooling and shrinkage differences.

Rib and Reinforcement Design

• Rib thickness should be 50–70% of the main wall thickness to avoid excessively thick intersections.
• Use smooth transitions to reduce stress concentration and volumetric shrinkage.

Gate and Runner System Design

Gate Position and Type

• Place gates near thick-walled areas to shorten the melt flow path.
• Common types: submarine gate, fan gate, edge gate.

Hot Runner and Balance Control

• Hot runner systems are recommended to maintain uniform melt temperature and avoid premature cooling-induced shrinkage.
• For large exterior parts (e.g., bumpers), ensure proper runner balance.

Mold Cooling System Optimization

Uniform Cooling

• Cooling channels should cover thick-walled areas and rib surroundings to avoid localized overheating.
• Use rapid heat exchange steel (e.g., 1.2738HH) to improve cooling efficiency.

Localized Cooling and Variotherm Molding

For areas prone to shrinkage, consider using insert cooling or variotherm temperature control systems (e.g., RHCM technology).

Material Selection and Process Parameter Control

Material Characteristics

• Common automotive exterior molded parts materials: PP+EPDM, ABS, PC/ABS — their shrinkage rates vary significantly.
• For example: PP shrinkage rate 1.2–1.8%, PC/ABS shrinkage rate 0.5–0.7%.

Injection Process Optimization

• Injection Pressure: Moderately increase holding pressure to reduce shrinkage voids.
• Holding Time: Extend holding phase according to the cooling rate of thick-walled areas.
• Melt & Mold Temperature: Control within recommended ranges to avoid uneven melt cooling.

Mold Design Details of Automotive Exterior Molded Parts and Preventive Measures

• Use insert structures for easy local optimization and maintenance.
• Add venting channels and optimize rib transitions on the backside of thick-walled areas.
• For appearance-critical parts, consider gas-assisted injection molding (GIT) or foaming injection (MuCell) to reduce shrinkage risk.

Post-Molding Treatment and Inspection

• Use coordinate measuring machines (CMM) to monitor critical dimensions and flatness.
• For high-demand appearance parts, apply laser polishing or local mold repair to eliminate residual sink marks.

Shrinkage prevention design for automotive exterior molds is a systematic engineering process that requires collaboration among product structure optimization, mold cooling layout, material selection, and injection process control. By implementing uniform wall thickness, hot runner systems, optimized holding parameters, and localized cooling techniques, manufacturers can significantly reduce sink marks, improving both appearance quality and assembly precision.

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