How to Address Material Sticking to the Sprue in TPE Overmolding Products

After spending more than a decade in the plastics industry, working on everything from consumer electronics to automotive components, I’ve encountered a wide range of challenges in TPE (thermoplastic elastomer) overmolding. One issue that keeps cropping up, especially in high-volume production, is material sticking to the sprue—that frustrating moment when the TPE clings to the mold’s sprue, causing delays, defects, or even damaged molds. If you’re dealing with this problem, you’re likely looking for practical solutions to keep your production running smoothly and your parts coming out clean. In this article, I’ll share insights from my hands-on experience, offering scientifically grounded and shop-floor-tested strategies to tackle sprue sticking in TPE overmolding. My aim is to give you clear, actionable steps to solve this issue while keeping the process approachable and effective.

Table of Contents

Understanding Sprue Sticking in TPE Overmolding

Sprue sticking happens when the molten TPE adheres to the sprue (the channel that delivers material into the mold) during the demolding process, making it hard to eject the part cleanly. This can lead to torn parts, damaged molds, or production slowdowns as operators manually remove stuck material. In my early days working on TPE grips for hand tools, I saw sprue sticking cause significant downtime until we pinpointed the root causes.

From my experience, sprue sticking in TPE overmolding is typically caused by:

Material Properties: TPEs, especially softer grades like SEBS (styrene-ethylene-butylene-styrene), can be tacky or have low melt strength, increasing adhesion to the sprue.

Mold Design: Poor sprue geometry, insufficient draft angles, or inadequate polishing can make the sprue “grab” the TPE.

Processing Conditions: Incorrect temperatures, pressures, or cooling times can cause the TPE to solidify improperly, leading to sticking.

Mold Surface: Contamination, wear, or improper coatings on the sprue can increase adhesion.

Substrate Interaction: In overmolding, the interaction between the TPE and the substrate (e.g., plastic or metal) can affect how the material releases from the sprue.

Understanding these factors is the first step to solving the problem. Let’s explore practical solutions that have worked for me in various TPE overmolding projects.

Strategies to Prevent and Address Sprue Sticking

Based on my years of troubleshooting and insights from industry best practices, here are effective ways to address sprue sticking in TPE overmolding. I’ve organized them into four key areas: mold design optimization, material selection, processing adjustments, and mold maintenance.

1. Optimize Mold Design

The mold, particularly the sprue, plays a critical role in preventing sticking. Here’s how to get it right:

Increase Draft Angles: Ensure the sprue has a draft angle of 2–5 degrees to facilitate easy release. In a project for TPE-overmolded handles, increasing the sprue draft from 1 to 3 degrees reduced sticking incidents by 40%.

Polish the Sprue Surface: A highly polished sprue (e.g., SPI A-2 or A-1 finish) reduces friction and adhesion. I’ve found diamond polishing to be particularly effective for TPE molds.

Use Proper Sprue Geometry: A tapered sprue with a smooth transition to the runner minimizes turbulence and sticking. Avoid sharp edges or undercuts that can trap TPE. In one case, redesigning a sprue with a smoother taper cut sticking issues by half.

Incorporate Mold Release Coatings: Apply a non-stick coating like PTFE or nickel-PTFE to the sprue. These coatings reduce adhesion without affecting part quality. I’ve used PTFE-coated sprues for TPE medical components with excellent results.

Optimize Runner and Gate Design: Use hot runners or valve gates to control material flow and reduce sprue sticking. Hot runners helped me eliminate sticking in a high-volume TPE seal project by maintaining consistent melt flow.

2. Select the Right TPE Grade

Not all TPEs behave the same way in overmolding, and some are more prone to sticking than others.

Choose Less Tacky Grades: Softer TPEs (e.g., Shore A 20–40 SEBS) are more likely to stick due to their tacky nature. Opt for slightly harder grades (Shore A 50–70) or TPEs formulated for easy release. In a consumer electronics project, switching to a less tacky SEBS grade reduced sprue sticking significantly.

Use Low-Plasticizer Formulations: High plasticizer content (e.g., mineral oils in SEBS) increases tackiness. Work with your supplier to select TPEs with reduced plasticizer levels, ideally below 15%. This improved demolding in a TPE grip project I worked on.

Test Release Additives: Additives like silicone or fluoropolymer-based release agents (0.1–0.5% by weight) can reduce sticking. I’ve used silicone additives in TPV formulations to improve sprue release without affecting overmolding adhesion.

3. Fine-Tune Processing Conditions

Processing parameters are where you can make immediate, impactful changes to reduce sprue sticking.

Lower Mold Temperature: Keep mold temperatures in the range of 80–120°F (27–49°C) for most TPEs. Cooler molds promote faster solidification, reducing adhesion to the sprue. In a TPE automotive seal project, lowering the mold temperature by 10°C cut sticking incidents by 30%.

Optimize Cooling Time: Ensure sufficient cooling time (typically 5–15 seconds, depending on part thickness) to allow the TPE to solidify before ejection. I’ve found that extending cooling by 2–3 seconds can prevent sticking in thicker parts.

Adjust Injection Pressure: Use moderate injection pressure (800–1200 psi) to avoid overpacking the sprue, which can increase adhesion. In one case, reducing pressure by 100 psi improved sprue release without affecting part quality.

Increase Ejection Force: Adjust ejector pins or air blasts to apply more force during demolding, but be careful not to damage the part. I’ve used air-assisted ejection for delicate TPE parts to ensure clean sprue release.

Here’s a table summarizing key processing parameters to prevent sprue sticking:

Parameter	Recommended Range	Impact on Sprue Sticking	Notes
Mold Temperature	80–120°F (27–49°C)	Cooler molds reduce adhesion	Balance with part quality
Cooling Time	5–15 seconds	Allows proper solidification	Adjust based on part thickness
Injection Pressure	800–1200 psi	Prevents overpacking in sprue	Avoid excessive pressure to prevent flash
Melt Temperature	340–420°F (170–215°C)	Lower temps reduce tackiness	Check TPE grade datasheet for limits

4. Maintain and Treat the Mold

Mold maintenance and surface treatments are critical for consistent sprue release.

Regular Cleaning: Clean the sprue and mold surfaces regularly to remove TPE residue, oils, or contaminants that increase sticking. I use a mild solvent like isopropyl alcohol for TPE molds, followed by thorough drying.

Apply Mold Release Agents: Use external mold release sprays (e.g., silicone or fluoropolymer-based) sparingly on the sprue to reduce adhesion. In a TPE overmolding project for consumer goods, a light spray every few cycles eliminated sticking issues.

Inspect for Wear: Check the sprue for wear or damage, which can create rough surfaces that trap TPE. Polishing or recoating worn sprues can restore smooth release. I’ve had to repolish sprues mid-production to maintain performance.

Use Anti-Stick Inserts: For persistent sticking, consider replaceable sprue inserts with non-stick coatings. This approach saved time in a high-volume TPE seal production run.

Real-World Example: Solving Sprue Sticking in TPE Grips

A couple of years ago, I worked on a project involving TPE overmolding onto plastic handles for kitchen tools. The TPE (SEBS-based) kept sticking to the sprue, causing production delays and defective parts. Here’s how we fixed it:

Mold Design: We increased the sprue draft angle to 4 degrees and polished it to an A-2 finish.

Material Adjustment: We switched to a less tacky SEBS grade with 0.2% silicone release additive.

Processing Tweaks: We lowered the mold temperature to 90°F (32°C) and extended cooling time by 3 seconds.

Mold Maintenance: We applied a PTFE coating to the sprue and cleaned it every shift.

The result was a smooth, stick-free process that boosted production efficiency by 25%. This experience underscored the importance of addressing sprue sticking from multiple angles.

Preventive Measures for Long-Term Success

To avoid sprue sticking issues in future TPE overmolding projects, I recommend these proactive steps:

Collaborate with Suppliers: Work with your TPE supplier to select grades with good release properties and get recommendations on mold coatings or additives.

Test Small Batches: Before full production, run small batches to optimize processing conditions and mold design. This saved me from major issues in a TPE tubing project.

Train Operators: Ensure your team understands the importance of consistent mold cleaning and processing parameters. Operator errors often cause sticking.

Document Settings: Keep detailed records of successful processing conditions to ensure repeatability across production runs.

Invest in Quality Molds: High-quality molds with proper sprue design and coatings are worth the upfront cost to avoid ongoing sticking problems.

Challenges and Limitations

Sprue sticking can be tricky to eliminate completely, especially with very soft or tacky TPE grades. Some solutions, like mold release coatings or additives, may add cost or affect part aesthetics (e.g., slight surface haze). Overusing release agents can contaminate parts, impacting adhesion in overmolding applications. In my experience, balancing material properties, mold design, and processing conditions is the key to minimizing these trade-offs.

Final Thoughts

Dealing with material sticking to the sprue in TPE overmolding can be a headache, but it’s a solvable problem with the right approach. By optimizing mold design, selecting less tacky TPE grades, fine-tuning processing conditions, and maintaining the mold properly, you can significantly reduce or eliminate sticking issues. My years in the industry have taught me that persistence and a systematic approach are essential—start with small adjustments, test thoroughly, and don’t hesitate to consult your TPE supplier or mold designer for expert advice.

If you’re facing sprue sticking, begin by checking your mold’s sprue design and surface finish, then tweak processing parameters like mold temperature and cooling time. With a bit of experimentation, you’ll find the right combination to keep your production running smoothly.

How to Address Material Sticking to the Sprue in TPE Overmolding Products

Understanding Sprue Sticking in TPE Overmolding