How Two-Color Injection Molds Solve the Problem of TPE Soft Materials Sticking to the Mold

With soft touch, excellent elasticity, anti-slip and shock-absorbing properties as well as environmental safety, TPE soft elastomers are widely adopted in two-shot overmolded products, such as handle sleeves, electronic buttons, daily anti-slip accessories and tool grips. Hard plastic substrate + secondary TPE overmolding is a mature combination for two-shot injection molding. Nevertheless, TPE sticking to mold remains one of the most frequent and troublesome defects encountered on production floors. Common symptoms include tearing and deformation of the soft TPE section, parts adhering to the mold cavity, cracking during ejection, stringing, short shots and ejection scuffing. Minor sticking causes frequent machine stops for mold cleaning and lower yield rates; severe cases may scratch the cavity surface directly and delay mass delivery.

When facing sticking problems, many engineers tend to apply more release agent as the first solution. However, excessive release agent reduces bonding strength of overmolding and leaves oil marks on the surface, which further leads to defects in subsequent painting or silk-screen printing. It is not a sustainable fix for mass production. TPE mold sticking stems from multiple interrelated factors including material formulation, mold structure, surface treatment, molding parameters, cooling design and venting. Drawing on practical experience in two-shot mold development and mass production, this article systematically identifies the root causes of TPE sticking and provides actionable mold design and process improvement solutions to stabilize two-shot overmolding production.

I. Classify Common TPE Sticking Phenomena

• Full cavity adhesion: After mold opening, the TPE layer sticks entirely to the cavity side instead of staying attached to the hard substrate on the core side;
• Local sticking: Stringing and tearing occur on ribs, thin wall sections, corners and tiny raised features;
• Whitening and tensile deformation upon ejection: High elasticity of TPE makes it prone to elongation when pushed by ejector pins;
• Sticking accompanied by gas marks and burns: Poor venting causes thermal decomposition by-products to deposit on the cavity surface and increase adhesion.

The biggest difference between two-shot molds and single-shot TPE molds: secondary TPE is overmolded onto pre-cooled rigid substrates. The processing window for mold temperature differential and packing pressure is much narrower, resulting in significantly higher sticking risks compared with single-shot TPE molding.

II. Core Mold-Oriented Improvements (Fundamental Solutions, Priority Execution)

1. Mold Surface Treatment Selection to Reduce TPE Adhesion

This is the most widely used and reliable method for two-shot TPE overmolding. Different surface treatments suit different application scenarios:

• DLC (Diamond-Like Carbon) Coating: Preferred option for mass production. It features low surface energy and high wear resistance, resists TPE adhesion and does not impair overmolding bonding. It will not contaminate parts like release agents, ideal for cosmetic parts requiring silk-screen or painting. The downside is relatively high cost; recoating is needed if the coating is chipped.
• Nitriding + Polishing (Mirror / Fine Texture): Suitable for non-cosmetic and general overmolded parts. Important note: higher gloss does not equal better release. Ultra-high mirror finishes tend to attract TPE. For TPE cavities, fine matte or EDM texture is recommended; moderate texture actually facilitates demolding.
• PTFE Coating: Offers excellent release performance yet suffers from poor abrasion resistance and easy peeling. Suitable only for sampling and small-batch trials, not continuous mass production.

⚠️ Pitfall warning: Do not blindly polish TPE cavities to mirror finish. Numerous on-site cases prove high-gloss cavities are more likely to trap soft elastomers.

2. Ejection Structure Optimization for Soft Elastomer Characteristics

TPE is soft. Concentrated ejection force from small single pins easily causes penetration, stringing and sticking:

• Increase ejection contact area: Prioritize ejector blocks, stripper plates and ring ejection; replace small pins with flat ejectors and sleeve pins for tiny features;
• Enlarge draft angle: Draft angle for TPE overmold zones ≥3°, increased to 4°~5° for deep ribs and thin sections, larger than standard draft for hard plastics;
• Eliminate sharp undercuts and right-angle corners: Apply radius transitions on all TPE corners. Sharp corners create stress concentration and trigger tearing and sticking during mold opening;
• Verify draft direction for two-shot molds: Ensure the TPE overmold layer remains attached to the hard substrate on the core side when the mold opens. Minimize TPE coverage on the cavity side. Large TPE areas on the cavity side greatly increase sticking risk.

3. Optimize Venting, Gating and Runner Design

TPE generates high shear heat. Thermal degradation produces low-molecular substances that deposit on cavity surfaces and form sticky residues, repeatedly inducing sticking:

• Widen and deepen vents for secondary TPE cavities. Vent depth is adjusted according to TPE grades, generally 0.015~0.03 mm. Arrange distributed vents separately at rib ends and weld lines;
• Prefer fan gates and film gates to lower shear heat. Avoid tiny pinpoint gates which cause intensive shear and TPE thermal degradation;
• Enlarge cold slug wells to prevent overheated degraded material from entering the cavity.

4. Cooling Channel Design for Stable Mold Temperature Control

TPE is highly sensitive to mold temperature: excessive mold temperature leads to insufficient cooling, soft texture and high tackiness; overly low mold temperature weakens overmold bonding. Two-shot molds face greater challenges in managing temperature difference between two sets of cavities:

• Design independent cooling circuits for the TPE cavity, separated from the cooling circuit of the hard substrate;
• Achieve uniform TPE cooling. Route cooling channels close to thin and local thick sections to avoid heat accumulation;
• Deploy separate mold temperature controllers where feasible to stabilize mold temperature and eliminate intermittent sticking caused by temperature fluctuation.

III. Material & Process Adjustments (Main Optimization After Mold Completion)

1. TPE Material Selection and Drying

• Under the same hardness, high-flow grades with low oil bleed have lower sticking risk. Highly oil-extended soft TPE (0A~30A) is most prone to sticking;
• Confirm compatibility between TPE and rigid substrate (PP / ABS / PC etc.). Select dedicated overmolding-grade TPE. Conventional TPE delivers poor bonding and higher sticking tendency;
• Dry hygroscopic TPE following material supplier specifications. Moisture causes bubbles and decomposition and worsens adhesion.

2. Injection Molding Parameter Tuning

• Reduce injection speed and pressure to cut shear heating and avoid TPE thermal degradation;
• Shorten packing time and lower packing pressure: excessive packing presses TPE tightly against the cavity and sharply raises demolding adhesion;
• Appropriately extend cooling time to ensure full TPE solidification. Insufficient cooling leaving soft, tacky elastomer is a very common cause;
• Do not exceed recommended barrel temperature limits. Overheating-induced TPE degradation is a hidden root cause of sticking.

3. Proper Use of Release Agent (Auxiliary Only, Not for Long-Term Mass Production)

Dry release agents may be used for temporary trial runs and small batches as emergency measures:

• Choose dry-type release agents instead of oil-based ones;
• Apply thinly in multiple light coats; heavy spraying is forbidden;
• Avoid release agent entirely for parts requiring subsequent printing or bonding; prioritize mold coating solutions.

IV. Routine Production Maintenance to Prevent Recurring Sticking

• Periodic cavity cleaning: Bleed oil and low-molecular residues from TPE gradually accumulate on mold surfaces and aggravate sticking. Wipe cavities and vents regularly with dedicated mold cleaner;
• Inspect coating integrity: Scratches and local wear on DLC or PTFE coatings become sticking hotspots. Send molds for recoating in time;
• Regular inspection of springs and ejection systems: Unbalanced or seized ejection leads to local tensile sticking;
• Mold protection during long shutdowns: Clean residual material from cavities before prolonged stops to prevent cured TPE deposits.

V. Quick Reference for Improvement Priorities

Conclusion

TPE sticking in two-shot overmolding is seldom a single-factor defect. It is usually the combined result of adhesion force, cooling status, shear degradation and ejection design. Release agents only treat symptoms rather than the root cause. For stable long-run mass production, the recommended workflow is: enlarge draft angles, optimize venting and cooling circuits in the early mold design phase, pre-plan cavity coating according to product requirements, and pair it with compatible TPE material and stabilized molding process. Proactive sticking mitigation cuts trial molding cycles, improves yield of two-shot molds and reduces downtime and repair costs in mass production.