Views: 0 Author: Site Editor Publish Time: 2025-10-24 Origin: Site
This is the non-negotiable starting point. The wrong material choice can lead to catastrophic failure, both in function and regulatory compliance.
Biocompatibility is King: All materials—the rigid substrate, the soft TPE, colorants, and any additives—must have proven biocompatibility according to relevant standards like ISO 10993 and USP Class VI. This is not a recommendation; it's a mandatory requirement for regulatory approval.
Material Compatibility: The rigid and soft materials must form a strong, durable bond. This requires chemical compatibility at a molecular level. Common successful pairs include PC/ABS over TPU and PP over TPE/TPV.
Performance Under Stress: Materials must resist degradation from repeated exposure to harsh sterilization methods (autoclaving, gamma radiation, ETO, chemical disinfectants) without becoming sticky, cracking, or losing their mechanical properties.
The mold is the heart of the process. Its design dictates the quality, consistency, and manufacturability of the final part.
Bonding Mechanism: Relying solely on chemical adhesion is risky. Intelligent mold design incorporates mechanical interlocking features—such as undercuts, through-holes, and micro-textures—on the rigid substrate to give the soft material something to grip onto.
Mold Steel & Finish: Corrosion-resistant, high-quality mold steels (e.g., S136, 420 Stainless) are standard. Cavities are often highly polished or mirror-finished to achieve a easy-clean, low-particulate surface that meets stringent medical cleanliness standards.
Venting is Critical: Trapped air between the substrate and the overmold material is a primary cause of burn marks, short shots, and poor bonding. Precision venting channels (typically 0.01-0.03 mm deep) at the end of fill and along parting lines are essential to allow air to escape.
This is where the theory is put into practice. Consistency and control are everything.
Substrate Pre-conditioning: The temperature and cleanliness of the rigid substrate are arguably the most critical process parameters.
Cleanliness: Any dust, oil, or mold release agent will ruin the bond. In-mold or online cleaning with Isopropyl Alcohol (IPA) is often required immediately before overmolding.
Temperature: The substrate must be heated to an optimal "weld window." Too cold, and the TPE cannot fuse properly; too hot, and the substrate can deform. This is often achieved using infrared heaters or oven pre-heating.
Optimized Injection Parameters:
Mold Temperature: A high and stable mold temperature promotes better TPE flow, reduces internal stress, and enhances the bond strength.
Injection Speed: A medium-to-fast fill speed helps the TPE cover the substrate before it cools down, preventing flow lines and ensuring a uniform bond.
Pack & Hold Pressure: Sufficient pressure is needed to pack out the part, but excessive pressure can lead to flash and stress. Settings must be meticulously validated.
For medical devices, the "how" and "where" are as important as the "what."
Cleanroom Manufacturing: Production typically occurs in a controlled cleanroom environment (e.g., ISO Class 7 or 8) to minimize particulate and microbial contamination.
Full Traceability: A robust Quality Management System (QMS) like ISO 13485 must be in place, ensuring full traceability from raw material lot to finished device.
Process Validation: You must prove your process is capable and reproducible. This is done through a rigorous Installation Qualification (IQ), Operational Qualification (OQ), and Performance Qualification (PQ) protocol.
Stringent In-Process Testing:
100% Visual Inspection: For defects like shorts, flash, or contamination.
Destructive Bond Testing: Regular peel tests (90° or 180°) to quantitatively verify that the bond strength meets the validated specification.
Medical-grade overmolding is not a single step but a symphony of interdependent factors. It demands a holistic approach where Material, Mold, Process, and Environment are all in perfect harmony. Cutting corners in any one area risks product failure, patient safety, and regulatory non-compliance.
By mastering these key points, manufacturers can leverage overmolding to create medical devices that are not only safe and reliable but also a pleasure to use—ultimately contributing to better patient outcomes and clinical experiences.
