An overmolded TPE layer on a polycarbonate housing can add grip, impact protection, environmental sealing, or tactile differentiation — but only if the material pairing and process execution are correct. PC imposes requirements that do not apply to ABS overmolding: tighter moisture control, higher mold temperatures, chemical stress cracking risk from incompatible additives, and a narrower window of compatible TPE sub-classes. Engineers who bring ABS overmolding experience directly to PC applications without adjusting their approach encounter delamination, CSC-induced crazing, and dimensional problems that appear well after initial production validation.

Material Pairing: Which TPE to Specify on PC

The TPE family is not uniformly compatible with polycarbonate. Sub-class selection is the first and most consequential decision.

COPE (Copolyester Elastomer) — Recommended
COPE is the most appropriate TPE family for PC overmolding. The ester groups in COPE’s polyester backbone interact with the carbonate linkages in PC through compatible ester chemistry, enabling genuine chemical adhesion without adhesion promoters. In optimized overmolding applications, COPE on PC achieves cohesive failure — the elastomer tears before the bond separates. COPE is available in Shore hardness ranges appropriate for grip surfaces, flexible seals, and protective bumpers.

COPE provides higher service temperature capability than equivalent-hardness SEBS or SBS, which is relevant in electronics and automotive applications where component temperatures exceed 80°C during use.

SEBS with Adhesion Promoter — Conditional
SEBS-based TPEs do not bond consistently to PC without adhesion promotion. SEBS’s styrenic end-blocks have good compatibility with ABS’s styrene phase but limited affinity for PC’s ester-dominated surface. Where SEBS is preferred for cost or processing reasons, a silane-based coupling agent applied to the PC substrate before overmolding, or a COPE tie-layer molded as the first elastomeric layer, can bridge the adhesion gap. These approaches add process steps and require validation.

TPV, SBS, PEBA — Not Recommended for PC Without Intervention
TPV bonds poorly to PC without surface plasma treatment or tie-layer materials. SBS has inadequate UV and thermal stability for most PC applications regardless of adhesion. PEBA bonds well to polyamide substrates but not to PC.

Managing Chemical Stress Cracking Risk

Chemical stress cracking (CSC) is the defining complication in TPE-on-PC overmolding. PC under mechanical stress is vulnerable to surface crazing when contacted by chemical agents — including plasticizers, processing oils, and residual solvents in TPE compound formulations. CSC can develop slowly, appearing as whitening or cracking at the bond line weeks after the part has passed initial inspection.

Risk reduction practices:
– Request full additive formulation disclosure from TPE compound suppliers before evaluation
– Avoid compounds with aromatic processing oils or aggressive plasticizers
– Anneal PC inserts at 120°C for two hours before overmolding to relieve residual molding stress — stressed PC is significantly more susceptible to CSC
– Validate under sustained mechanical load, not just immediate peel testing
– Do not clean PC surfaces with ketones, aromatic solvents, or chlorinated cleaners before overmolding — use IPA only

For formulation review and CSC risk evaluation for your specific material combination, Email Us.

Process Best Practices for TPE on PC

Drying the PC substrate. PC is highly hygroscopic and must be dried at 120°C for four to six hours in a desiccant dryer before processing. Even brief ambient exposure after drying introduces enough moisture to cause surface defects and reduce bond area quality. Transfer dried PC inserts to the overmold tool immediately.

Drying the TPE compound. COPE requires drying at 100–110°C for four to six hours. SEBS compounds are generally less moisture-sensitive but should be dried per the supplier’s recommendation before processing.

Mold temperature. Maintain mold temperature at 75–95°C for COPE on PC. Lower mold temperatures reduce interfacial temperature below the threshold for adequate ester-ester interaction, producing weaker bonds. Higher mold temperatures extend cycle time without proportional bond strength improvement. Verify temperature consistency across all cavities.

Insert preheat for insert molding. Pre-formed PC substrates loaded at room temperature consistently produce weaker COPE bonds than those loaded immediately from the drying oven. Preheat PC inserts to 80–90°C immediately before loading to compensate for the substrate temperature loss inherent in insert molding.

Gate placement. Direct the TPE gate to flow across the bond surface rather than along it. Parallel flow to the interface generates weld lines at the bond zone — the weakest locations in the overmold. Position gates for perpendicular fill of major bond surfaces.

Pack pressure and hold time. Adequate pack pressure maintains COPE contact with the PC surface during solidification. Insufficient packing allows the elastomer to pull away from the substrate as it contracts, creating micro-gaps that reduce peel strength. Optimize pack pressure to just below the flash threshold.

Tooling Design Considerations

Mechanical interlocks. Through-holes, undercuts, and channels in the PC substrate provide mechanical anchoring for the TPE layer in addition to chemical adhesion. These features are essential for high-peel-load zones and for applications where thermal cycling stress concentrates at bond edges. Mechanical interlocks also provide redundant retention if the chemical bond is partially compromised.

Venting. Inadequate venting in the TPE cavity traps air at the bond surface, creating contact voids that appear as weak spots in peel testing. Position vents at all last-fill locations in the cavity. PC surfaces with mold release residues also trap gas more readily — surface cleanliness and venting work together.

Parting line location. Position the TPE parting line at design break lines or recessed grooves in the PC substrate. Edge parting lines concentrate peel stress and are the most common locations for delamination initiation in field-returned parts.

Validation Testing for TPE-PC Overmolds

Cohesive failure in peel testing is the acceptance criterion, but is not sufficient alone for PC applications. A complete validation sequence includes:

• 90-degree peel testing (ASTM D1876) at nominal process conditions — confirm cohesive failure
• Peel testing at process extremes (low mold temperature, maximum transfer time) to characterize the process window
• Sustained load CSC testing at 50–75% of PC tensile strength for 168 hours — observe for crazing at bond line
• Thermal cycling from -30°C to 85°C for 100 cycles — confirm no delamination or dimensional change beyond specification
• Chemical resistance exposure to any fluids the part will contact in use

Incure’s specialty coating and adhesive formulations support TPE-on-PC overmolding applications, including adhesion-promotion systems for SEBS on PC and bonding agents for difficult elastomer-substrate combinations. For technical support and material qualification assistance, Contact Our Team.

Visit www.incurelab.com for more information.