An overmold that peels off the rigid part is usually a chemistry or surface problem before it is a process problem. TPE bonds to a substrate only where the two melt together at the interface, and that happens when the TPE grade was made for that substrate and meets a clean, dry, warm surface. Check the grade first, then the substrate surface (cold, wet, mold release, handling), then melt temperature, TPE thickness and the time between shots. Mechanical interlocks are the fallback when chemistry alone cannot hold the load.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| The TPE peels off cleanly everywhere and the substrate underneath is shiny, with no residue. It is a new program or a new substrate. | The TPE grade was not made for this substrate, for example a PP-bonding TPE or a standard TPV molded onto PC, ABS or nylon. | The data sheet: which substrates the grade is listed to bond to |
| The bond is good on some insert-molded parts and poor on others. Inserts that sat around longest are worst. | The inserts are cold, or picked up moisture, dust or oil from storage and handling. | How old the inserts are and how they are stored; insert temperature at loading; gloves |
| The bond is strong near the TPE gate and weak at the far end of fill. | The TPE melt front has cooled before it reaches that area, because of a long flow, a thin section or slow fill. | Melt temperature; injection speed; TPE thickness where it fails |
| The bond dropped after mold release was sprayed, or after mold maintenance. | Release agent or rust preventive in the substrate cavity has transferred to the substrate surface. | Stop the spray; clean both cavities and run 20 shots |
| Nylon, PC or PC/ABS substrate, and the bond changes with the weather or from shift to shift. | Moisture in the substrate, either molded wet or picked up while waiting, or moisture in the TPE. | Substrate dryer settings; moisture at the feed throat; how long substrates wait before overmolding |
| Two-shot tool. The bond fell after a cycle change or a slower rotation. | The first shot cooled too far before the TPE reached it. | Time between shots; temperature on the substrate side of the tool |
| The edges lift but the middle holds. Lifting gets worse over days or after flexing. | The TPE shrinks more than the substrate and pulls at thin edges that end on a flat surface with nothing to hold them. | TPE thickness at the edge; whether the edge ends in a groove or wraps around |
Checks in order, cheapest first
- Peel a part and look at the failure. Pull the TPE back by hand or with a simple 90-degree peel. If the TPE tears and leaves a film on the substrate, the interface is fine; look at TPE thickness and design. If it comes off clean, the interface is the problem.
- Confirm the grade matches the substrate. Read the substrate list on the TPE data sheet, including glass fill or flame retardant in the substrate, which can weaken the bond. A grade made only for PP will usually peel from ABS or PC however you run it. See the overmold grades on our TPE / TPV page.
- Clean the substrate surface. Stop spraying release on the substrate tool. Handle inserts with clean gloves and keep them covered. A trial with freshly molded substrates shows whether storage is the problem.
- Dry both materials. Dry hygroscopic substrates (nylon, PC, PC/ABS, ABS) before molding them, and overmold soon after, or keep the substrates dry while they wait. Dry TPV and any overmold grade the data sheet says to dry. See the resin drying chart.
- Warm the substrate. Preheat inserts if you can, and load them promptly. In two-shot work, shorten the time between shots and run the substrate side of the tool as warm as the cycle allows. A warm surface gives the TPE longer to bond.
- Raise TPE melt temperature and fill speed, inside the range. A hotter, faster melt front arrives at the interface with more heat. Stay inside the supplier’s range and the residence limits; overheated TPE gets weak and tacky.
- Check TPE thickness and flow length. Thin TPE freezes against the substrate before it can bond, most often at the end of fill. Gate the TPE into a thicker area and let it flow across the substrate, not around a long path.
- Add mechanical interlocks last. Through-holes, undercuts, grooves and wrap-around edges hold the TPE when chemistry cannot. This needs tool work, so design it in from the start for TPV on anything other than PP.
The numbers
Heat and moisture at the interface are the settings you can measure. Typical starting points, consistent with our processing chart and drying chart:
| Item | Typical starting point | Why it matters for the bond |
|---|---|---|
| Styrenic TPE (SEBS) melt temperature | 190–245°C / 374–473°F | The upper part of the range usually bonds better. Keep residence under about 10 minutes. |
| TPV melt temperature | 175–230°C / 347–446°F | Same logic. Purge if the press idles longer than about 20 minutes. |
| Mold temperature, TPE / TPV | 15–60°C / 59–140°F (TPE); 10–66°C / 50–151°F (TPV) | A warmer tool usually improves the bond, at the cost of a longer cycle. |
| TPV drying | 80°C / 176°F, 3 h virgin, 4 h with regrind, to ≤0.08% | Steam at the interface keeps the layers apart. |
| Nylon 6 / 66 substrate drying | About 80°C / 176°F, 2–6 h depending on grade | Nylon is one of the harder substrates to bond and picks up moisture while it waits. |
| PC substrate drying | 120°C / 248°F, 2–4 h, to ≤0.02% | A wet PC substrate can splay at the surface the TPE has to bond to. |
| PC/ABS and ABS substrate drying | 100–110°C / 212–230°F (PC/ABS); 80–95°C / 176–203°F (ABS); 2–4 h | Same reason as PC. |
| TPE wall over the substrate | Often about 1–3 mm / 0.04–0.12 in in design guides | Thinner walls freeze before they bond, especially at the end of a long flow. |
Typical values. Overmold grades differ, so confirm melt temperature, drying and wall guidance against the data sheet for your grade.
Why overmolds peel
- The bond is a melt bond. Where the hot TPE meets the substrate, the surface softens and polymer chains from both sides mix over a very thin layer. That only works when the two are compatible and the interface stays hot long enough.
- Like bonds to like. PP-based TPV and PP-bonding styrenic TPE contain polypropylene, which fuses to a PP substrate. PC, ABS, PC/ABS and nylon are polar and do not fuse with that polypropylene, so they need TPE grades modified for that substrate. TPV generally bonds well only to PP; on other substrates it needs a special adhesion grade or mechanical interlocks.
- Anything at the interface blocks it. Mold release, oil from hands, dust and steam from moisture all sit between the two materials and stop the chains from mixing.
- The heat window is short. The TPE melt front cools as soon as it touches the substrate and the steel. A cold insert, a thin wall, a long flow or a slow fill closes the window before the bond forms.
- Shrinkage pulls at the edges. Soft TPV can shrink about 4.1% along the flow at 49A, far more than a rigid substrate. That difference leaves the edges under peel stress, and thin edges with nothing to anchor them lift over time.
Material changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| An overmold grade made for your substrate, including any glass fill | It bonds chemically to that substrate, so the process window gets wider. | A higher price than a general-purpose TPE, fewer hardness and color choices, and a new grade for your customer to approve. |
| A styrenic overmold TPE in place of TPV on a non-PP substrate | Bonding grades for PC, ABS and nylon are more widely available in styrenic TPE. | Lower heat resistance and more compression set. Not the right trade if the part has to seal at temperature. See TPV vs. SEBS TPE. |
| A PP substrate, where the design allows | PP-based TPV and TPE bond to it readily. | Lower stiffness and heat resistance than PC/ABS or nylon, a different shrinkage, new steel and a full requalification. |
| Mechanical interlocks in the substrate | They hold the TPE in place even when the chemical bond is weak. | Tool changes, a more complex substrate and possible marks on the show side. An interlock alone does not seal the joint against water. |
| A higher-flow TPE grade | Reaches the end of fill hotter and fills thin walls more easily. | Some high-flow grades give up tear strength or heat resistance. Check hardness and bond on your parts. |
Bond values depend on the test. Peel strengths on data sheets come from each producer’s own test piece and method, so compare grades by peeling your own parts side by side.
Often confused with
- Delamination inside the TPE. The TPE itself splits into thin layers, often from contamination with a resin that does not mix. See delamination.
- Tearing on ejection. Soft TPE that sticks to the core can tear or lift at ejection and look like a bond failure. See parts sticking in the mold.
- Oil at the interface. Oil bleeding from a very soft TPE can weaken a bond that was good when the part was new. See sticky or oily TPE surface.
- A short shot at the edge. TPE that never reached the edge leaves a gap that looks like a lifted edge. See short shots.
Quick answers
Why won’t my TPE stick to polycarbonate?
Most likely the grade was made for PP, not PC. PC, ABS and PC/ABS need a TPE modified for that substrate. Once the grade is right, keep the PC dry and clean, and keep the substrate and the TPE melt as warm as the process allows.
Does TPV bond to ABS or nylon?
Standard EPDM/PP TPV bonds well to polypropylene but generally not to ABS, PC or nylon. For those substrates use a TPV or TPE grade made to bond to them, or design in mechanical interlocks.
How do I test overmold bond strength?
Peel a strip of TPE from the substrate at 90 degrees and look at how it fails. If the TPE tears and leaves material behind, the bond is stronger than the TPE. A clean peel means the interface is weak.
Should I preheat inserts for overmolding?
Usually it helps. A warm, dry insert gives the TPE more time to bond, especially on nylon and PC. Load inserts promptly after preheating, handle them with clean gloves, and follow the TPE data sheet for any preheat guidance.
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