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Vacuum Injection (Overmolding) Materials: Temperature Resistance and Post-Processing Baking Guide

Introduction

The heat resistance of prototypes produced by vacuum injection molding (overmolding) is a key indicator of their functionality. Furthermore, if the prototype requires post-processing such as painting, understanding the safe upper baking temperature limit is crucial for ensuring cosmetic quality and preventing part deformation. This article will explain the thermal performance limitations of commonly used overmolding materials.

Post-Processing Paint Baking Limits

First, it is important to note that the structural stability of most PU (polyurethane) resin materials used for overmolding is limited by temperature. To ensure that the paint coating dries and cures without causing softening, deformation, or dimensional loss due to high temperatures, the recommended upper baking temperature limit is 60°C.

For silicone parts, due to their extremely low surface energy and poor adhesion, they require the use of special primers and treatment agents. Conventional paint baking is generally not recommended.

Detailed Explanation of the Temperature Resistance Characteristics of Various Molding Materials

Conventional hard/transparent/tough materials (similar to ABS, PC, PMMA, PP, PA)
These most commonly used molding materials have a short-term heat resistance temperature (HDT) range of approximately 70°C–90°C. Tough materials like PP/PE have slightly lower temperature resistance, ranging from approximately 60°C–75°C.

Soft rubber materials (similar to TPU/TPE)
Soft PU materials used to simulate rubber textures have the lowest temperature resistance, with a short-term heat resistance temperature range of approximately 50°C–70°C.

Special Performance Materials

High-Heat-Resistant-Grade PU: For higher temperature resistance requirements, specially formulated high-heat-resistant-grade PU materials can be used, with short-term heat resistance temperatures of 100°C–120°C.

Fiber/Mineral Filling: Adding glass fiber or mineral powder to PU resin can improve rigidity and increase short-term heat resistance to 80°C–100°C.

Key Concepts

Short-Term Heat Deflection Temperature (HDT): Also known as heat deflection temperature, this refers to the temperature at which a sample undergoes a specified deformation when placed under a specific load and heated at a constant rate. It is an indicator of a material’s short-term rigidity and heat resistance, not its safe long-term use temperature. Generally, the temperature for long-term continuous use is approximately 60%–80% of its HDT value.

Conclusion

Vacuum-molded parts are useful for functional verification, but their temperature resistance is generally lower than that of engineering plastics used in production molds. The short-term heat resistance of conventional PU overmolded parts typically falls between 70°C and 90°C. For special requirements, heat-resistant grades of PU can be selected to increase this value to 100°C–120°C. When performing any heating or post-processing steps, 60°C should be considered a safe upper bake temperature limit to ensure the integrity and dimensional accuracy of the prototype.