As someone who’s spent years troubleshooting material issues in injection molding plants, I’ve seen my fair share of quirky defects that can throw a wrench into production. One issue that pops up frequently with Thermoplastic Elastomers (TPE) is peeling or skinning at the sprue, where the material forms a rough, flaky surface at the gate or runner. It’s frustrating—nobody wants defective parts or wasted material, especially when TPE’s flexibility and aesthetics are key to the product. Whether you’re molding soft grips, seals, or medical components, this defect can halt production and raise costs. In this article, I’ll share my insights on why TPE peels at the sprue, break down the root causes, and offer practical solutions based on my hands-on experience to help you get smooth, defect-free parts.
What Is Peeling at the Sprue, and Why Does It Matter?
Before diving into causes, let’s clarify what peeling at the sprue looks like. When TPE is injected into a mold, it flows through the sprue (the entry point) and runners into the mold cavity. After cooling and demolding, the sprue or gate area should be clean and smooth. But with peeling, the surface at the sprue appears rough, flaky, or layered, like skin peeling off. In severe cases, it can affect the part’s surface quality or cause weak spots near the gate.
This defect isn’t just cosmetic—it can lead to:
Rejected parts: Peeling at the sprue often signals inconsistent material flow, which may affect part quality.
Production delays: Troubleshooting and reworking molds eat up time.
Material waste: Defective sprues or parts increase scrap rates.
I recall a project where a client molding TPE grips for tools noticed peeling at the sprue, leading to 10% of parts being scrapped. Solving the issue not only saved material but also boosted their production efficiency. Let’s explore why this happens and how to fix it.
Root Causes of Peeling at the Sprue in TPE Molding
Peeling at the sprue is typically caused by a combination of material, process, mold design, or equipment issues. TPE’s unique properties—its blend of rubber-like elasticity and plastic processability—make it sensitive to certain molding conditions. Below, I’ll break down the main culprits based on my experience troubleshooting this defect.
1. Material Degradation
TPE, especially SEBS or TPU-based grades, can degrade if exposed to excessive heat or shear during molding. Degradation breaks down the polymer chains, causing the material to separate into layers at the sprue, where shear and heat are highest. Common triggers include:
Overheating: Melt temperatures above the recommended range (e.g., >220°C for SEBS-based TPE).
Long residence time: Material sitting too long in the barrel, especially in large machines with small shot sizes.
Regrind overuse: Excessive recycled TPE (regrind) can introduce degraded material, worsening peeling.
In one case, a client molding Shore 50A TPE seals was running the barrel at 230°C, well above the supplier’s 190–210°C recommendation. The sprue showed peeling, and parts had a burnt smell. Lowering the temperature solved the issue.
2. High Shear Stress
TPE is shear-sensitive, meaning excessive flow rates or narrow gates can generate high shear stress, tearing the material’s molecular structure. This shear damage manifests as peeling at the sprue or gate, where the material experiences the most turbulence. Causes include:
Fast injection speeds: Pushing TPE too quickly through the sprue or gate.
Small gate size: Narrow gates increase shear, especially for soft TPEs with high viscosity.
Sharp gate edges: Abrupt transitions at the gate can amplify shear.
I once helped a factory molding TPE gaskets where peeling occurred at a 0.5mm gate. Widening the gate to 1mm and slowing the injection speed eliminated the defect.
3. Moisture in the Material
TPE, particularly TPU-based grades, is hygroscopic, meaning it absorbs moisture from the air. Moisture in the melt can vaporize during molding, creating bubbles or splay that contribute to peeling at the sprue. Signs of moisture include:
Bubbling or sizzling sounds during molding.
Splay marks or rough surfaces near the gate.
Inconsistent part quality.
In a medical tubing project, we noticed peeling and splay at the sprue. Testing revealed the TPE pellets had 0.3% moisture content. After drying the material at 80°C for 4 hours, the issue disappeared.
4. Mold Design Issues
The mold’s sprue and gate design plays a big role in TPE flow. Poorly designed sprues or gates can cause turbulent flow, leading to peeling. Common mold-related issues include:
Cold sprue or runner: If the sprue or runner isn’t heated properly, TPE can cool prematurely, creating a skin that peels during demolding.
Improper gate location: Gates placed in high-stress areas (e.g., near thin walls) can increase shear.
Rough sprue surface: A poorly polished sprue can cause material to stick and peel.
In a project for TPE shoe soles, peeling was traced to a cold sprue bushing. Adding a hot runner system kept the sprue molten, resolving the defect.
5. Inadequate Cooling or Demolding
TPE requires consistent cooling to solidify properly. If cooling is uneven or demolding is too aggressive, the sprue can peel as it’s pulled from the mold. Causes include:
Short cooling time: Ejecting parts before they’re fully set can stretch or tear the sprue.
Uneven cooling: Hot spots in the mold cause inconsistent solidification.
High ejection force: Aggressive ejector pins can damage the sprue.
I worked with a client molding TPE handles where peeling occurred due to a 10-second cooling time. Extending it to 15 seconds allowed the sprue to solidify, eliminating the issue.
6. Material Incompatibility
When overmolding TPE onto another material (e.g., PP or PC), incompatibility between the TPE and substrate can cause peeling at the sprue, especially if the TPE doesn’t bond well. This is often mistaken for a molding defect but stems from material selection. Signs include:
Delamination at the gate or part interface.
Weak adhesion between TPE and substrate.
In an overmolding project for a tool grip, peeling at the sprue was linked to a TPE grade incompatible with the PP substrate. Switching to a compatible TPE grade fixed the issue.
Table: Common Causes and Solutions for TPE Peeling at the Sprue
To summarize, here’s a table outlining the main causes, signs, and fixes:
| Cause | Signs | Solutions | Prevention Tips |
|---|---|---|---|
| Material Degradation | Burnt smell, discolored sprue, peeling layers | Lower melt temperature, reduce residence time, limit regrind | Follow supplier’s temperature guidelines, use proper shot size |
| High Shear Stress | Peeling at gate, rough sprue surface | Slow injection speed, widen gate, smooth gate edges | Optimize gate size (1–2mm for TPE), use gradual transitions |
| Moisture | Splay, bubbles, rough sprue | Dry TPE at 70–80°C for 2–4 hours | Store pellets in sealed bags, use desiccant dryers |
| Mold Design | Peeling at cold sprue, turbulent flow | Use hot runner, polish sprue, reposition gate | Design sprue for smooth flow, ensure proper heating |
Step-by-Step Troubleshooting Guide
If you’re facing peeling at the sprue, here’s a systematic approach I’ve used to diagnose and fix the issue:
Check Material Condition:
Verify the TPE grade matches the supplier’s specs (e.g., Shore hardness, base polymer).
Test for moisture using a moisture analyzer; dry pellets if >0.1% moisture.
Reduce regrind ratio (start with <20%) and check for degradation signs (e.g., yellowing).
Review Process Parameters:
Compare melt temperature to the supplier’s recommended range (e.g., 180–210°C for SEBS).
Slow injection speed by 10–20% to reduce shear.
Extend cooling time by 2–5 seconds to ensure proper solidification.
Inspect Mold Design:
Check sprue and gate size (aim for 1–2mm for TPE).
Ensure sprue and runners are polished and free of burrs.
Consider a hot sprue or runner if the sprue is cooling prematurely.
Evaluate Equipment:
Confirm the injection molding machine’s screw is suitable for TPE (low-compression screw preferred).
Check barrel cleanliness to rule out contamination from previous materials.
Calibrate temperature controllers for accuracy.
Test Compatibility (for Overmolding):
Confirm TPE is compatible with the substrate via adhesion tests.
Consult the TPE supplier for recommended grades if delamination occurs.
Run Trials:
Make one change at a time (e.g., lower temperature) and mold a few parts.
Inspect sprues and parts for peeling, documenting results.
Repeat until the defect is eliminated.
In a factory molding TPE seals, we followed this process and found that a combination of high injection speed and moisture was causing peeling. Slowing the speed and drying the material resolved it in one afternoon.
Real-World Case Studies
Let me share two projects where I tackled peeling at the sprue to show how these solutions play out.
Case Study 1: TPE Medical Tubing
A medical device manufacturer was molding Shore 30A TPE tubing and noticed peeling at the sprue, along with splay marks. The parts were failing quality checks, costing them thousands in scrap. Initial checks showed the TPE was being molded at 220°C, above the supplier’s 200°C limit, and pellets were stored in a humid warehouse. We:
Lowered the melt temperature to 195°C.
Dried the TPE at 80°C for 4 hours, reducing moisture to <0.05%.
Slowed the injection speed by 15%.
The peeling disappeared, and part quality improved, saving 8% in material costs. This highlighted the importance of material handling and process optimization.
Case Study 2: TPE Overmolded Grips
A tool manufacturer was overmolding Shore 60A TPE onto PP handles but saw peeling at the gate and weak adhesion. The TPE grade was chosen for cost, not compatibility. We:
Switched to a TPE grade recommended by the supplier for PP adhesion.
Widened the gate from 0.6mm to 1.2mm to reduce shear.
Polished the sprue bushing to ensure smooth flow.
The new TPE bonded perfectly, and peeling stopped, improving part durability and reducing rejects by 12%. This showed how material selection and mold design are critical.
Preventing Peeling in Future TPE Molding
To avoid peeling at the sprue long-term, here are proactive steps I’ve found effective:
Work with Suppliers: Get detailed processing guidelines from your TPE supplier, including temperature, shear, and drying specs.
Train Operators: Ensure molding technicians understand TPE’s sensitivity to heat, shear, and moisture.
Maintain Equipment: Regularly clean barrels and screws to prevent contamination, and check temperature controllers for accuracy.
Optimize Mold Design: Design sprues and gates with TPE in mind (e.g., 1–2mm gates, polished surfaces, hot runners for large parts).
Store TPE Properly: Keep pellets in sealed bags with desiccant in a dry, cool environment to prevent moisture absorption.
Document Settings: Record successful process parameters (e.g., temperature, speed) for each TPE grade to streamline future runs.
In one factory, we implemented a TPE-specific training program and standardized drying protocols, reducing sprue-related defects by 20% across their product line.
When to Consult Experts
If peeling persists despite troubleshooting, it’s time to bring in help. Contact your TPE supplier or a molding consultant for:
Material analysis to check for degradation or contamination.
Mold flow simulation to identify shear or flow issues.
On-site process audits to pinpoint equipment or operator errors.
I’ve worked with suppliers like Kraiburg TPE, who provided free technical support to diagnose a stubborn peeling issue, saving a client weeks of trial and error.
Cost Implications of Peeling
Peeling at the sprue can hit your bottom line hard. Based on my experience:
Scrap Costs: 5–15% of parts may be rejected, increasing material expenses ($2.50–$6.50/kg for TPE).
Downtime: Troubleshooting can halt production for hours or days, costing $500–$2000/day in labor and overhead.
Rework: Modifying molds or gates may cost $1000–$5000, depending on complexity.
Solving peeling early, as we did in the medical tubing case, can save thousands and keep projects on schedule.
Future Trends in TPE Molding
Looking ahead, advancements in TPE and molding technology may reduce peeling issues:
Low-Shear TPEs: New formulations are less sensitive to shear, minimizing peeling even at high speeds.
Smart Molding Machines: Sensors and AI can monitor shear, temperature, and moisture in real-time, adjusting parameters automatically.
Improved Drying Systems: Automated dryers ensure consistent moisture control, especially for TPU-based TPEs.
I’ve seen early adopters of smart molding systems reduce defects like peeling by 15%, a trend worth watching.
Conclusion: Tackle TPE Peeling with Knowledge and Precision
From my years of wrestling with TPE molding challenges, I can tell you that peeling at the sprue is a solvable problem with the right approach. Whether it’s degradation from overheating, high shear from a narrow gate, or moisture in the material, identifying the root cause is half the battle. By optimizing process parameters, mold design, and material handling, you can achieve smooth sprues and high-quality parts. My advice? Start with small tweaks (like drying or slowing injection), document your results, and don’t hesitate to lean on suppliers for support.
If you’re dealing with peeling at the sprue and need guidance, share your project details, and I’ll offer tailored tips based on my experience to get your TPE molding back on track!
Frequently Asked Questions
1. How do I know if moisture is causing TPE peeling at the sprue?
Look for splay marks, bubbles, or a sizzling sound during molding. Test pellets with a moisture analyzer; if >0.1%, dry at 70–80°C for 2–4 hours.
2. Can I use the same mold for TPE and other plastics without peeling issues?
Not always. TPE requires larger gates (1–2mm) and polished sprues to reduce shear. If the mold was designed for high-flow plastics like PP, modify the gate or sprue for TPE.
3. Does regrind TPE cause peeling at the sprue?
Yes, excessive regrind (>20%) can introduce degraded material, leading to peeling. Limit regrind and test virgin material to confirm if it’s the cause.
4. What’s the ideal gate size for TPE to avoid peeling?
Aim for 1–2mm, depending on part size and TPE viscosity. Larger gates reduce shear, especially for soft TPEs (Shore 0A–40A).
5. Should I switch TPE grades if peeling persists?
If process and mold optimizations don’t work, consult your supplier. A different TPE grade (e.g., lower viscosity or shear-resistant) may resolve the issue.
