In TPE (Thermoplastic Elastomer) injection molding, the processing of the gate (also known as the sprue) directly impacts the product’s appearance, performance, and production efficiency. Many practitioners encounter issues such as burrs, gate residue, color differences, or insufficient strength at the gate area. Improper handling not only affects aesthetics but may also lead to customer complaints or returns. As an expert in the TPE material industry, I combine years of experience and industry case studies to deeply analyze the key aspects of TPE gate processing and provide practical solutions to help achieve “near-perfect” results.
1. Why Is TPE Gate Processing a Challenge?
TPE, with its unique blend of rubber-like elasticity and plastic processability, is widely used in electronics, medical devices, automotive parts, and more. However, its properties also pose challenges for gate processing:
Flowability and Viscosity: TPE melts have good flowability but high viscosity, making them prone to burrs or mold sticking at the gate.
Uneven Cooling: The gate area is typically the last to cool, often resulting in shrinkage marks or uneven surfaces.
Mold Design Constraints: The gate’s position, size, and type (e.g., pinpoint gate, submarine gate) directly affect processing difficulty.
Material Formulation Variations: Different TPE formulations (e.g., SEBS-based, TPU-based) vary in hardness, elasticity, and processing characteristics, requiring tailored approaches.
These factors make the gate a critical “weak point” for the appearance and quality of injection-molded parts. Below, we analyze how to tackle this challenge from three dimensions: mold design, process optimization, and post-processing.
2. Mold Design: Addressing Gate Issues at the Source
Mold design is the foundation of TPE gate processing. A well-designed mold can significantly reduce subsequent processing challenges. Here are key considerations:
2.1. Choosing the Right Gate Type
Submarine Gate: Ideal for products with high aesthetic requirements, as the gate is hidden inside the product, minimizing surface marks. However, submarine gates require higher injection pressure and precise mold machining.
Pinpoint Gate: Suitable for small parts, with a small gate that’s easy to remove but may leave slight protrusions requiring post-processing.
Fan Gate: Appropriate for large, thin-walled parts, ensuring even material flow but leaving larger gate marks that require post-processing.
Recommendation: Select the gate type based on aesthetic requirements and production volume. Submarine gates are preferred for high-precision products, while pinpoint gates are more cost-effective for budget-conscious projects.
2.2. Optimizing Gate Position
Place the gate away from visible surfaces, ideally in hidden areas (e.g., the product’s inner side or edge).
Avoid positioning the gate in thin-walled areas to prevent deformation or strength issues due to pressure concentration.
Use mold flow analysis software (e.g., Moldflow) to simulate melt flow paths and ensure the gate position enables uniform filling.
2.3. Mold Surface Treatment and Venting
Polish the gate area to a mirror finish (Ra0.2 or below) to reduce TPE sticking.
Incorporate proper venting channels (depth 0.01-0.03mm) to prevent burn marks or burrs caused by trapped gases at the gate.
Case Study: A medical device client required a TPE handle with no visible marks. By adopting a submarine gate and optimizing the venting system, we hid the gate marks entirely on the handle’s inner side, achieving a 99-point aesthetic score during client inspection.
3. Process Optimization: Fine-Tuning Injection Parameters
With mold design finalized, optimizing the injection molding process is critical to achieving flawless gate results. Here are key parameter adjustments:
3.1. Injection Pressure and Speed
Pressure: Excessive pressure can cause overflow at the gate. Keep it between 80-120MPa, adjusted based on TPE hardness and mold structure.
Speed: Use segmented injection—start fast to ensure melt flowability, then slow down to reduce impact at the gate.
3.2. Mold Temperature
Maintain mold temperature between 30-50°C during TPE injection. Too high a temperature can cause insufficient cooling at the gate, leading to burrs; too low may result in incomplete filling.
For high-hardness TPE (e.g., Shore 80A or above), slightly increase mold temperature to improve surface finish.
3.3. Holding Time and Cooling Time
Shorten holding time (typically 2-4 seconds) to avoid material buildup at the gate.
Extend cooling time (5-10 seconds) to ensure complete solidification at the gate, reducing shrinkage marks.
3.4. Material Pre-Treatment
TPE is hygroscopic, so dry the material (80°C, 2-3 hours) before injection to remove moisture, minimizing bubbles or burn marks at the gate.
For TPE with high recycled content, strictly control impurities to avoid color differences or strength issues at the gate.
Tip: During debugging, adjust injection speed and holding time combinations while observing gate appearance changes. Record each adjustment to build a process parameter database for consistent production.
4. Post-Processing: Making the Gate “Invisible”
Even with optimized mold design and process, some cases require post-processing to further enhance gate quality. Here are common methods:
4.1. Mechanical Trimming
Use precision cutters or automated trimming equipment to remove gate residue, suitable for pinpoint or fan gates.
Ensure tools are sharp to avoid damaging the TPE surface during trimming.
4.2. Heat Treatment
For minor burrs, use a hot air gun (temperature controlled at 150-200°C) to lightly heat and shrink the burrs.
Limit heating time (under 2 seconds) to prevent material discoloration.
4.3. Sanding and Polishing
For slight protrusions at the gate, use fine sandpaper (2000 grit or higher) for light sanding, followed by polishing paste to restore surface gloss.
Test on a small sample first to ensure sanding doesn’t compromise TPE elasticity or surface texture.
4.4. Secondary Overmolding
For products with stringent aesthetic requirements, apply secondary overmolding at the gate area to cover original gate marks. This method is costly and suited for high-end products.
Note: Perform post-processing in a clean environment to avoid dust or oil contamination. For medical-grade TPE, re-clean and test the product after post-processing to ensure biocompatibility compliance.
5. Common Issues and Solutions
Issue 1: Severe Burrs at the Gate
Cause: Excessive injection pressure or poor mold clamping.
Solution: Reduce injection pressure and check mold clamping surfaces for flatness, recalibrating the mold if necessary.
Issue 2: Shrinkage Marks at the Gate
Cause: Insufficient cooling time or holding pressure.
Solution: Extend cooling time, increase holding pressure slightly, and verify mold cooling channel functionality.
Issue 3: Color Differences at the Gate
Cause: Material contamination or excessive gate temperature.
Solution: Ensure consistent material batches, lower mold temperature at the gate, and add dispersants for color masterbatch if needed.
6. Conclusion: A Systematic Approach to Perfection
Achieving perfect TPE gate processing isn’t about excelling in one area but requires seamless integration of mold design, process optimization, and post-processing. Optimize mold design to minimize gate marks from the outset; fine-tune process parameters for consistent quality; and use post-processing to address minor imperfections. Only through close coordination of these three aspects can gate processing evolve from “acceptable” to “perfect.”
For TPE injection molding beginners, start with small-batch trials to build experience. For seasoned professionals, consider adopting mold flow analysis or automated post-processing equipment to boost efficiency and quality. I hope this article provides valuable insights for your TPE injection molding projects. Feel free to share your experiences or questions in the comments!
