Overmolding TPE onto Rigid Plastic: Bonding, Substrate Choice and Tool Design

Why overmould at all

Overmoulding puts a soft elastomer where the user touches the part and a rigid plastic where the part has to hold a screw, a clip or a dimension. It replaces glue, gaskets, two-part assemblies and post-moulding operations with one cycle.

The functional reasons are grip, sealing, damping, soft touch, colour contrast and the elimination of a secondary assembly. The commercial reason is that one moulded part replaces three components and an assembly step, and the failure mode of a glued gasket disappears.

The engineering reason it works is that the TPE is injected onto a substrate that is still hot or is reheated at the interface, so the two materials interpenetrate at a molecular level before the TPE solidifies. That interpenetration is the bond, and everything in this article is about protecting it.

Chemical bonding depends on the substrate, not the TPE

A TPE bonds to a substrate for one of two reasons. Chemical adhesion happens when the two polymers are thermodynamically compatible and the TPE chains diffuse into the substrate surface. Mechanical adhesion happens when the TPE flows into undercuts, textures or a porous surface and locks in place.

Chemical adhesion is a property of the pair. ABS, PC, PC/ABS, PA6 and PMMA all have polar groups that a TPE with a compatible mid-block will wet and diffuse into. PP, PE and PA6-GF30 do not, and no amount of process tuning will make a standard TPE-S bond to them.

Table 1 - Peel strength for a 60 Shore A TPE-S on six substrates, moulded at a 100 C substrate surface temperature. Two newtons per millimetre is the usual minimum for a seal.

The glass-filled nylon result surprises people who expect glass to help. It does the opposite: the surface is rich in glass fibre, the polymer fraction is lower, and there is less material for the TPE to bond to. A compatibilised grade or a mechanical feature is the answer, not a higher melt temperature.

Surface temperature: a narrow window

The single process variable that matters most is the temperature of the substrate surface at the moment the TPE arrives. Too cold and the TPE freezes before it can diffuse; too hot and the substrate surface degrades, warps or sinks.

The window is roughly 80 to 120 degrees for most amorphous substrates, with a peak around 100. At 40 degrees the peel strength is 1.6 N/mm; at 100 it is 3.3; at 140 it falls back to 2.4 because the substrate has started to distort and the interface is no longer flat.

• Two-shot moulding: the substrate is still in the tool and still hot from its own cycle. The interface temperature is whatever the substrate was at when the second shot arrives, typically 90 to 110 degrees, and no reheating is needed.
• Insert overmoulding: a pre-moulded substrate is loaded into a second tool. It must be preheated, by hot air, infrared or the tool itself, to the same window.
• The tool temperature also matters: a cold second-shot cavity pulls heat out of the interface faster than the TPE can diffuse into it.

Measuring the surface temperature is not optional. A non-contact infrared thermometer on the substrate as the tool closes gives the number, and it is the only way to know whether the window is being held on every cycle rather than on the cycle that was set up.

Tool design: shut-offs, gates and wash-out

An overmould tool has one problem that a normal tool does not: it must contain a low-viscosity melt at high pressure inside a cavity bounded partly by a soft or a thin substrate.

• Shut-off: the TPE must be stopped exactly at the edge of the substrate. A steel-on-steel shut-off at the parting line is standard, with 0.02 to 0.05 mm of preload.
• Wash-out: if the TPE flows over the substrate edge, it lifts it or flows behind it. The fix is a mechanical stop, not a lower pressure.
• Gate position: gate so the TPE flows across the bond area rather than around it, and avoid impinging directly on a thin substrate wall.
• Venting: the bond area needs venting, or trapped air becomes a void that reads as a failed bond.
• Draft on the TPE side: 2 to 3 degrees, and a radius at the TPE-to-substrate transition, because that is where the part is handled.

Two-shot tooling is more expensive and dramatically more reliable, because the substrate never leaves the machine and never cools. Insert overmoulding is cheaper and depends entirely on the preheat discipline. The choice is usually volume: above about 100,000 parts a year the two-shot tool pays for itself.

Process settings that protect the bond

• Melt temperature at the top of the TPE range, not the middle: a hotter melt has more time before it freezes and diffuses further.
• High injection speed into the bond area, so the melt reaches the substrate before it loses heat. This is the opposite of the instinct to fill slowly to avoid washing out the substrate.
• Packing pressure low. Over-packing the TPE distorts the substrate and puts the bond in peel before the part is even used.
• Cooling time long enough for the TPE to set, but not so long that the substrate and the TPE shrink at different rates and load the interface.
• Dry the substrate. PA and PC absorb moisture and a wet substrate produces silver streaks and a weak bond at the same time.

Table 2 - Process window for overmoulding a 60 Shore A TPE-S onto ABS. The two temperature rows are the ones that decide the bond.

The counter-intuitive setting is injection speed. Most moulders slow the fill to protect the insert, and the result is a TPE that has cooled below its bonding window before it reaches the far edge of the bond area. Fast fill, low pack is the pattern that works.

Testing the bond, not the part

A peel test is the only measurement that correlates with field performance, and it must be done on a sample with the same geometry and the same process as the part.

• 90 degree peel on a 25 mm wide strip, at 50 to 100 mm per minute, recording the average force over the steady region and not the peak.
• Failure location: adhesive failure at the interface, cohesive failure inside the TPE, or substrate failure. A cohesive failure is the good result; an adhesive failure at 1 N/mm is the problem.
• Ageing: 72 hours at 85 degrees and 85 percent relative humidity, then peel again. A bond that loses 40 percent of its strength in damp heat will fail in a bathroom.
• Hot water immersion at 70 degrees for 7 days for a part that sees washing or a wet environment.
• Cycle testing for a grip or a seal: 10,000 flex cycles, then peel.

The ageing test is the one that separates a good project from a warranty claim. A fresh peel test on a dry part in a lab at 23 degrees will pass on a bond that fails in the first month of service, because moisture attacks the interface before it attacks either material.