TPE and TPU Injection Molding: Hardness, Bonding and Overmolding

Soft parts are a different process, not a different material

Thermoplastic elastomers — TPE, TPU, TPV, TPS — are specified for one of two reasons: the part must feel soft, or the part must seal. Both are legitimate and both change how the part is designed and moulded, in ways that are not obvious from a hardness number.

The common mistake is to treat a TPE as "flexible ABS". It is not. A TPE has low modulus and high elongation, which means it needs different gates, different ejection, different draft and a completely different approach to overmoulding onto a rigid substrate. A tool designed for a rigid plastic and used for a TPE will produce flash, tearing at ejection, and a part that does not bond.

The families, and how to tell them apart

Table 1 — TPE families. The "bonds to" column is the one that decides most projects.

The bonding column is the practical starting point. If the substrate is polypropylene, TPS or TPV is the natural choice because they are PP-compatible by chemistry. If the substrate is ABS or PC, TPU is the answer. Trying to bond a TPS to ABS requires a tie layer or a mechanical interlock, and the result is usually a part that peels in the field.

Hardness: what the number means

Table 2 — Shore A hardness and what it means in practice. Below 40A the tool design changes fundamentally.

Two notes on hardness. First, Shore A and Shore D overlap and are not interchangeable — 55D is roughly 95A, and quoting "60" without the scale is meaningless. Second, hardness is temperature dependent: a 50A TPE at 23 °C is noticeably harder at 0 °C and noticeably softer at 50 °C. For a seal that must work across a temperature range, specify the hardness at the service temperature, not at room temperature.

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Table 3 — Processing windows. TPU must be dried; TPS usually does not.

The single biggest processing difference is ejection. A soft TPE stretches rather than releasing, so a part with rigid-plastic draft and ejector pins will tear or distort. The standard solutions are:

• Stripper plate or sleeve ejection — pushes the whole part off the core evenly.
• Air assist — a burst of air between the part and the core breaks the vacuum and the grip, and the part falls free.
• Generous draft — 2 – 3° instead of 1°, and more on textured surfaces.
• Textured cores — a light bead-blast on the core actually helps release, because the part grips less uniformly.

Overmoulding: where most TPE projects fail

Most TPE parts are overmoulded onto a rigid substrate — a grip on an ABS handle, a seal on a PA connector, a boot on a PBT housing. The bond is the critical feature, and it depends on four things.

Table 4 — The four factors in overmould bond strength. Incompatibility is not fixable by processing.

Substrate temperature is the one that gets missed. The TPE must melt the surface of the substrate to form a bond — that is what "chemical bonding" means in practice. If the insert is at room temperature when the TPE arrives, the interface cools too fast and the bond is mechanical only. The insert should be at 60 – 80 °C for most substrate pairs, and the mould should be warm enough not to chill it.

Table 5 — Substrate compatibility. The POM row is the one that ends projects.

Design rules

Table 6 — TPE design rules. Draft and ejection are where rigid-plastic habits cause failure.

Sealing: the other half of the market

For seals and gaskets, hardness is not the governing property — compression set is.

Table 7 — Compression set. Lower is better; TPS is poor and should not be used for a loaded seal.

Compression set is the percentage of deformation that does not recover after the load is removed. A TPS with 50 % set loses half its sealing force after the first thermal cycle and will leak. A TPV at 25 % retains most of it and seals for years. If the part is a seal under sustained compression, TPS is the wrong choice no matter how well it processes.

Defects and causes

Table 8 — TPE defects. Tearing and flash together usually mean the tool was designed for a rigid plastic.

Two-shot or insert overmoulding?

There are two ways to put a soft layer on a rigid substrate, and the choice is made on volume and geometry rather than on cost per part alone.

Table 9 — Two-shot versus insert overmoulding. The bond-quality row is the one that decides quality-critical parts.

Two-shot wins on bond quality because the substrate is still molten or just-solid when the TPE arrives, so the interface is a true melt bond rather than a re-melt. It also removes the handling labour, which on a 30-second cycle is often the largest cost. Insert moulding wins on flexibility: it can overmould metal, it can use a substrate that cannot be moulded in the same tool, and the tooling is far cheaper.

A middle route exists and is underused: overmoulding onto an insert that was moulded in the same cell, robot-transferred while still hot. It gets most of the bond benefit of two-shot at insert-tooling cost, and it is the standard solution for medium volumes.

Hardness, feel and surface

Two TPEs at the same Shore hardness can feel completely different, and the difference is not captured by the hardness number.

Table 10 — What makes a soft part feel right. Only the first row is normally on a drawing.

The practical advice is to approve a physical sample, not a specification. Soft-touch feel is a perception, and two materials that meet the same Shore A and the same density can feel different in the hand because of surface friction and rebound. Every overmoulded grip project we have run has been approved on a sample, and every one that was approved on a datasheet came back for a change.

TPE versus silicone versus rubber

Table 11 — TPE versus the thermoset alternatives. The bonding row is why TPE dominates overmoulded grips.

Silicone wins where temperature or biocompatibility matters — above 150 °C, or for long-term implant contact. Thermoset rubber wins on compression set in aggressive environments and on cost for very simple high-volume parts. TPE wins on everything else, and specifically on the ability to be overmoulded onto a rigid plastic in a single automated cycle, which is the requirement behind most soft-touch products.

Frequently asked questions