Material selection, drying, process parameters, mold design and overmolding — organized in the order a real project actually moves through, with usable decision criteria and starting parameter ranges at every step.
QUICK ANSWER
TPE injection molding is the process of melting a thermoplastic elastomer and injecting it into a mold to make soft, flexible parts. It runs on a standard injection molding machine, needs no curing step, and is used for grips, seals, gaskets and soft-touch overmolded parts. The molding itself is not hard. The hard parts are sourcing the right grade and redesigning the mold for a soft material.
KEY TAKEAWAYS
- Check supply before you check hardness. Ultra-soft Shore 00 grades carry high minimum order quantities.
- SEBS grades run about 190–245°C, SBS grades about 170–200°C. TPU grades follow the TDS and must be dried below 0.02% moisture.
- Design for soft material: even wall thickness, 0.25° draft for soft TPE, large-area ejection, and short, thick runners.
- Soft TPE can eject over undercuts of up to about 2.0 mm, which often removes the need for side actions.
- Prototype resin is not production resin. Redo the fill study and the hold window after you switch.
- Never judge overmold bond strength by eye. Test it with peel, pull and aging.
Are you about to run TPE injection molding for the first time and not sure where to start? If yes, this guide walks you through the whole job: picking a grade you can actually buy, drying it, setting up the machine, designing the mold, and checking parts before you commit to production.
TPE gives you the soft, grippy feel of rubber with the speed of a thermoplastic. You can run it on a normal injection molding press. There is no curing step and no special equipment. That sounds easy, and the molding itself usually is.
The trouble sits somewhere else. Soft material behaves differently in the mold. It sticks in runners, it deforms if you eject it too early, and it tears at sharp corners. Add the sourcing problem for ultra-soft grades and the gap between a prototype resin and a production resin, and a simple project can stall fast.
This guide follows the order a real project moves in. Each section gives you decision criteria and starting numbers you can take to the shop floor. Let’s get straight to it.
01. What is TPE injection molding, and is it right for your project?
TPE injection molding suits parts that need to be soft, grippy, elastic or sealing, and that can be made on a standard thermoplastic press. It is a poor fit for parts that must stay rigid, hold tight tolerances, survive sustained high heat, or carry a high-gloss finish.
1.1 What TPE is, and how it differs from rigid plastics and silicone
TPE (thermoplastic elastomer) is a family of thermoplastic materials that can generally be reprocessed and can be run on a standard injection molding machine. No dedicated thermoset equipment is needed, and there is no curing or vulcanization step. The biggest difference from rigid plastics is simple: TPE is softer, more elastic, and has more surface grip.
Compared with silicone, TPE is thermoplastic. It can be re-melted, injected and cooled much like PP or ABS. Silicone is usually thermoset, so once it cures it cannot be re-melted. In practice, how well TPE regrind performs still depends on the grade, its thermal history, and how well contamination is controlled.
1.2 TPE vs silicone vs rigid plastic at a glance
| Factor | TPE | Silicone (LSR) | Rigid plastic (ABS/PP) |
|---|---|---|---|
| Material type | Thermoplastic | Thermoset | Thermoplastic |
| Can be re-melted | Yes | No | Yes |
| Equipment | Standard press | Dedicated LSR press | Standard press |
| Curing step | None | Required | None |
| Feel | Soft, high grip | Soft, high grip | Hard, low grip |
| Heat resistance | Limited by grade | High | Grade dependent |
| Surface finish | Usually lower gloss | Good | Best of the three |
| Regrind | Possible, controlled | Not possible | Common |
1.3 Good-fit applications for TPE molding
- Soft-touch parts: handles, grips, buttons, tool overmolds.
- Sealing: gaskets, seal strips, waterproof structures.
- Overmolding: a rigid plastic substrate with a soft TPE outer layer.
- Elastic or functional parts: anything that needs cushioning, grip, or shock absorption.
1.4 Cases that need caution, or aren’t a good fit
- Sustained high heat: TPE’s heat resistance is usually limited. Some grades sit around the 120°C class, so always check the TDS.
- High surface-finish requirements: TPE’s surface texture and gloss are typically weaker than ABS or PC.
- High tear-resistance requirements: softer TPE has lower tear strength and tears easily at sharp features.
- High-precision, high-stiffness structures: TPE is soft by nature. It holds dimensions less well and is less rigid than engineering plastics, so it does not suit load-bearing structures.
TPE isn’t hard to injection mold. What’s hard is redesigning for its softness and elasticity — ejection, gating, and the process window all need to change.
When the application needs thermoset behavior, very high-temperature sealing, or a different long-term compression profile, silicone injection molding may be a better fit than TPE. Compare the service conditions before choosing the material.
02. TPE material selection: check availability and bondability before hardness
Pick a TPE grade in this order: substrate and bonding need, service conditions, hardness, then supply. Hardness is the easiest number to specify and the easiest one to get stuck on, because the softest grades are often the hardest to buy in small volumes.
2.1 Common TPE families and how to filter through them
The common TPE families are SBC (which includes SBS and SEBS), TPV, TPU, TPO and COPE. Selection should weigh hardness, heat resistance, chemical resistance, tensile and tear strength, overmolding bond strength, color, certifications and cost together — not hardness alone.
| Family | Typical strengths | Watch-outs | Drying and sourcing note |
|---|---|---|---|
| SBC / SEBS | Soft touch, easy coloring, broad hardness range and efficient processing | Heat, oil and solvent resistance vary; creep can matter in loaded parts | Usually no strict drying; many grades are readily available, but ultra-soft grades can have high MOQs |
| TPU | Abrasion, tear and flex-fatigue resistance | Moisture-sensitive, higher cost and more shear/temperature sensitive | Dry to the grade TDS; confirm moisture before molding and ask for the actual production grade |
| TPV | Weathering, heat resistance and compression-set performance | Feel, flow and substrate adhesion are grade-specific | Usually less moisture-sensitive than TPU; follow the TDS and verify overmold compatibility |
| TPO | Low cost, impact performance and outdoor/weathering options | Often less soft or grippy; adhesion and surface finish depend strongly on the grade | Typically simple to process; confirm availability, hardness and the substrate pairing |
| COPE / TPEE | High fatigue, tear and temperature performance | Usually less ultra-soft and more expensive than SBC grades | Follow the TDS for moisture control; confirm the required hardness and supply lead time |
| TPE family | Common starting substrates | What must be verified before tooling |
|---|---|---|
| SBC / SEBS | PP/PE and selected ABS or PC grades | Grade-specific chemical compatibility, substrate temperature, surface cleanliness and peel/pull strength |
| TPU | Selected PC, ABS and PA grades | Drying state, hydrolysis/heat exposure, processing window and bond retention after aging |
| TPV | PP and other olefin-based substrates | Substrate polarity, adhesion package, compression set and weathering requirements |
| TPO | PP/olefin substrates | Surface energy, flow balance, required softness and whether mechanical retention is needed |
| COPE / TPEE | Engineering substrates with supplier-approved grades | Temperature window, fatigue/tear targets, substrate compatibility and production availability |
Screening rule: this table narrows the candidate list; it does not guarantee adhesion. The supplier’s grade-specific TDS, compatibility data and an actual molded-part bond test decide whether a pairing is production-ready.
The compatibility table is an initial screening aid compiled from common material-family behavior; the resin supplier’s current TDS and compatibility data control for a specific grade.
A faster filtering order is:
- What is the substrate? Does it need to bond through overmolding?
- What is the service temperature and chemical exposure? Is the part used indoors or outdoors?
- Shore A or Shore 00? Ultra-soft or semi-rigid?
- Does the supplier have stock? What is the minimum order quantity (MOQ)?
Only after working through these four questions should you lock in a specific grade — not the other way around.
2.2 The reality of low-volume sourcing
Ultra-soft Shore 00 TPE is genuinely hard to source in small quantities, because supplier MOQs tend to be high. The point where prototype and low-volume buyers actually get stuck is usually not “what temperature do I set.” It is “where do I even buy this material.”
Confirm a supplier’s minimum order quantity before locking in a hardness range. Distributors, compounders and suppliers that offer small-pack sizes are worth checking first. For a small run, include the target quantity, acceptable substitute grades and delivery window in the first sourcing request.
2.3 Prototype resin is not production resin
Prototyping often uses cast resin, 3D-printed material, or resin made for RTV silicone tooling. Production means switching to a real injection-grade TPE, and that switch means revalidating:
- Flow behavior
- Shrinkage
- Packing and hold window
- Overmold bond strength
- Appearance and cycle time
After switching resin, you must redo the fill study and the hold-pressure window. Do not carry over prototype-stage parameters.
2.4 Drying and storage
TPU-based TPE almost always needs drying, with a target moisture level below 0.02%, as stated in the supplier’s TDS. SEBS-based TPE usually does not need strict drying, but moisture, clumping and contamination still have to be managed.
Drying temperature and time should follow the supplier’s TDS, commonly in the 80–100°C range for 2–4 hours. Hopper sealing, packaging and shop-floor humidity all affect production consistency. It is not just about the dryer itself.
2.5 Regrind and contamination control
TPE is sensitive to contamination, so regrind content needs to be controlled. Start validation at a low ratio and watch for changes in appearance, hardness, tensile strength and bond strength.
Color changeovers, black specks, impurities and material degradation all show up directly in the finished part. It is worth running dedicated purging, a dedicated hopper, and clear labeling for regrind.
Material availability is part of the engineering decision: ask for the exact production grade, current stock or MOQ, TDS, color options and any approved substitute before you freeze the design.
03. TPE injection molding process parameters: can a standard machine run it?
Yes. A standard thermoplastic injection molding machine can run TPE, as long as the screw, nozzle and temperature control can support it. Barrel temperatures usually sit between 170°C and 245°C depending on the family, mold temperature between 15°C and 60°C, and hold pressure starts at 40–60% of transfer pressure.
3.1 Standard machine and older-machine compatibility
Before the first shot, confirm these components can support TPE:
- Screw: three-zone design, compression ratio of at least 2:1, L/D of at least 20.
- Nozzle: an open nozzle works, but a check-valve (non-return) nozzle is more consistent.
- Hot runner volume: should not exceed 1–3× the shot size.
On older equipment (a Van Dorn-type machine, for example), check temperature control accuracy, screw condition, nozzle type, back pressure setting and shot-to-shot repeatability. On an older machine, run a short-shot and fill study first rather than carrying over existing parameters.
3.2 TPE injection molding parameters: reference ranges
The ranges below are starting points, not universal settings. Final values must be confirmed against the resin supplier’s TDS, part geometry, mold design, machine behavior and molding-trial data. The supplier TDS always takes priority.
| Parameter | Reference range | Notes |
|---|---|---|
| Barrel temp · SEBS-based | 190–245°C | Larger parts may run higher; follow the TDS |
| Barrel temp · SBS-based | 170–200°C | SBS is a member of the SBC family; generally not above 220°C |
| Barrel temp · TPU-based | Per TDS | Watch moisture state and shear heating |
| Mold temp | 15–60°C | Depends on grade and finish requirement |
| Runner temp | Near mid-low barrel zone | Avoid cold slugs and runner sticking |
| Injection speed | Material- and geometry-dependent | Usually high for SEBS and thin-wall parts; moderate speed may be preferable for some SBS grades |
| Speed profile | Start with 2–3 zones | Use a faster fill through thin sections, slow near the gate or end of fill if jetting/flash appears, then validate with short shots |
| Hold pressure | 40–60% of transfer pressure | Starting point; use only enough to compensate shrinkage |
| Back pressure / screw speed | Low-to-moderate starting point | Increase only as needed for melt quality; avoid excessive shear heating, foaming or material degradation |
| Cooling time | Determine by ejection stability | Increase until the soft part ejects without distortion; validate with dimensional and warpage checks |
| Gate seal / freeze point | Increase hold time stepwise | Stop when part weight no longer increases |
| Cushion | About 10–15% of screw diameter | Starting target; confirm pressure transfer on the machine |
| Shrinkage | 1.0–2.5% | Affected by hardness, wall thickness, hold pressure, filler |
| Vent depth | 0.01–0.02 mm | Prevents burning and short shots |
3.3 How to run a TPE fill study: seven start-up steps
- Start with medium speed and no hold pressure.
- Increase shot size in 10% increments until fill reaches 95–98%.
- Record short-shot locations, flow balance and venting behavior.
- Turn on hold pressure, starting at 40–60% of transfer pressure.
- Increase hold time step by step, weighing the part until the weight stops rising. That is your gate seal or freeze point.
- Adjust the cushion to 10–15% of the screw diameter.
- Record the full process window as your production parameter sheet.
3.4 Common TPE molding defects and where to adjust
| Defect | Common causes |
|---|---|
| Short shot | Insufficient shot size, injection speed too low, poor venting, melt too cold |
| Warpage | Uneven cooling, excessive hold pressure, uneven wall thickness, ejecting too early |
| Ejector-pin marks | Ejector contact area too small, insufficient cooling, ejection speed too fast |
| Sink marks / vacuum voids | Uneven wall thickness, insufficient hold pressure, insufficient cooling |
| Runner sticking | Runner material too soft, poor release, poor runner design |
| Flash | Insufficient clamp force, melt too hot, vent too deep |
Ejecting soft TPE before it has cooled enough deforms it easily. Deep-texture surfaces are also harder to release and generally need a larger draft angle. For one common symptom of poor filling or venting, see the short-shot troubleshooting guide.
3.5 Resin changeover, purging and color switching
- Use a dedicated purging compound, or transition through PP or PE, as the supplier recommends.
- Pay close attention to dead spots and hot-runner residue during color changes.
- After a resin change, revalidate flow, shrinkage, bond strength and appearance.
04. TPE part and mold design: where it differs from rigid plastic
The main design changes for TPE are smaller draft angles, larger ejection surfaces, shorter and thicker runners, and more generous undercut allowances. Soft material releases and deforms differently, so a mold designed for ABS rarely transfers over unchanged.
4.1 Wall thickness, draft and radii
- Keep wall thickness as even as possible, typically in the 0.5–6 mm range.
- Draft angle: 0.25° for soft TPE, 0.5° for hard TPE.
- Radii of at least 0.5 mm, to avoid air traps and local burning.
- Transitions between thick and thin sections should be gradual, to avoid sink, warpage and vacuum voids.
4.2 Ribs, bosses and undercuts
- Rib and boss thickness should be roughly 50% of the nominal wall thickness.
- Undercuts: about 0.7 mm for hard TPE. Soft TPE can go up to 2.0 mm and eject directly.
- Soft TPE tolerates larger undercuts, which can cut out complex parting-line design.
- Undercuts still need to be checked for ejection deformation and tear risk, not just the numeric limit.
4.3 Ejection and runners
- Prefer a stripper plate for soft TPE, or large-area ejector pins.
- Runner material is soft and can stick during ejection.
- Deep-texture surfaces are harder to release and need a larger draft angle.
- Keep runners short, thick and smooth-flowing to limit pressure loss.
4.4 Gating and venting
- Common gate types: diaphragm gate, fan gate, submarine (tunnel) gate, multi-point valve gate.
- Gate location directly affects overmold bond strength, appearance and flow balance.
- Vent depth should be around 0.01–0.02 mm, to avoid burning and short shots.
- Parting lines, inserts and ejector-pin locations are all part of the venting design.
4.5 TPE overmolding essentials
- When soft TPE overmolds a rigid substrate, avoid deforming the soft section during molding.
- The TPE must be chemically compatible with the substrate for the bond to work.
- Substrate surface cleanliness, temperature and gate location all affect bond quality.
- Common pairings include ABS + TPE, PC + TPE and PA + TPE, but each pairing still needs grade-level validation.
Bond validation can’t rely on visual inspection alone. Run peel, pull and aging tests.
For multi-material parts, compare 2K molding and overmolding against the same bond, tooling, cycle-time and volume requirements; the right route depends on the substrate and production quantity.
4.6 Shrinkage and dimensional control
- Shrinkage is driven by hardness, wall thickness, hold pressure, mold temperature and filler content together.
- Mold design should build in shrinkage compensation from the start.
- Validate dimensions with the actual production resin before production. Do not cut a hard tool based on prototype-resin data. Use the injection molding tolerances guide to set realistic drawing requirements.
05. No injection machine, tooling too expensive? Prototyping and pilot runs
A supporting decision — this section helps you validate feasibility at lower cost before committing to a production tool.
For runs of roughly 20 to 1,000 parts, start with soft or aluminum tooling before cutting a production mold. RTV silicone tooling, 3D-printed tooling and polyurethane casting can confirm shape and feel cheaply, but none of them reproduce production properties.
5.1 Low-cost validation paths
- RTV silicone tooling with simple heating: validates shape and hardness, but not production-level strength, surface finish or cycle time.
- 3D-printed tooling: limited surface quality and tool life, so it suits very small quantities only.
- Polyurethane casting: can approximate the look of an injection-molded part, but the material properties differ from real TPE.
These approaches suit early-stage concept validation. They should not be used on their own as the basis for production decisions.
5.2 Choosing prototype tooling
3D-printed tooling, aluminum tooling and steel tooling each carry different cost, lead-time and quality trade-offs. For runs of roughly 20 to 1,000 parts, start with soft or aluminum tooling before deciding whether to cut a production mold. The separate injection molding tooling guide covers tool materials, components and cost trade-offs. If you need to validate overmolding, test with a TPE grade as close as possible to the material planned for production.
5.3 Checklist for switching from prototype resin to production resin
- Is flow behavior consistent?
- Is shrinkage consistent?
- Is overmold bond strength consistent?
- Are appearance, feel and hardness consistent?
- Are cycle time, ejection and runner behavior stable?
06. From pilot run to production: the easiest pitfalls to fall into
Most TPE production problems trace back to six things: an unvalidated resin change, too much regrind, skipped drying, an unchecked older machine, untested overmold bonds, and vague inspection standards.
6.1 Resin-change validation
Switching from prototype resin to production resin needs revalidation. Do not carry prototype-stage parameters straight into production.
6.2 Regrind ratio and contamination
Regrind content needs tight control. TPE is contamination-sensitive, and black specks, impurities and degradation all affect appearance and performance.
6.3 Drying, color changeovers and mold venting
TPU-based material must be dried. SEBS-based material should be judged case by case. Color changes need thorough purging, and poor venting leads to burning, short shots and surface defects.
6.4 Running TPE on an older machine
Start with a short-shot and fill study, and check temperature control, screw condition, nozzle, back pressure and shot-to-shot repeatability.
6.5 Overmold bonding
Bond strength has to be tested, not judged by appearance. Run peel, pull, heat-aging and humid-heat testing.
6.6 Quality inspection
Standard checks include hardness, tensile strength, tear strength, bond strength, dimensions and appearance. Add compression set, chemical resistance and heat resistance testing where relevant. Agree the method, specimen geometry, conditioning and acceptance limit before the first production run. Ezra’s quality assurance process is the relevant service page for inspection planning.
| Property | Common reference | Define before testing |
|---|---|---|
| Hardness | ASTM D2240; ISO 48-4 or ISO 868, as applicable | Scale (Shore A, D or 00), dwell time, specimen thickness and conditioning |
| Tensile and elongation | ASTM D412 or ISO 37 for elastomeric grades; ISO 527 where specified | Die/specimen type, crosshead speed, temperature and reporting points |
| Tear strength | ASTM D624 or ISO 34-1 | Method, specimen type, notch direction and conditioning |
| Compression set | ASTM D395 or ISO 815-1 | Method, temperature, duration and recovery time |
| Overmold adhesion | ISO 19095 where applicable; otherwise a documented customer-specific peel or pull method | Substrate and grade, bond area, loading direction, speed, conditioning and pass/fail limit |
| Heat or humid aging | Use the applicable ASTM/ISO aging method or a documented customer protocol | Temperature, time, humidity, exposure sequence and property-retention limit |
There is no single universal pass/fail value for TPE. The resin TDS, customer drawing, regulatory requirements and end-use environment determine the acceptance criteria.
6.7 How to prepare a TPE injection molding RFQ
Give a supplier enough information to quote the material, tooling and validation work together. A drawing alone usually creates avoidable assumptions.
- Part data: send a STEP/IGES model, 2D drawing, critical dimensions, tolerances, appearance zones and target wall thickness.
- Material target: state hardness scale and range, color, required TPE family or approved grades, and whether a specific supplier is required.
- Application conditions: list service temperature, chemicals, UV/outdoor exposure, compression or flexing, contact requirements and certifications.
- Overmolding details: identify the substrate resin and grade, insert geometry, surface condition, bond area and required peel/pull or aging test.
- Volume: separate prototype quantity, pilot quantity, annual volume, forecast and expected order lot size. Include the target launch date.
- Tooling and quality: request the recommended cavity count, tool material and life, runner/gate concept, sampling plan, inspection report and process-validation plan.
Ask the supplier to return: proposed grade and TDS, drying recommendation, estimated cycle time, tooling quotation, sample timing, assumptions, quality tests and any risks that could change the price or lead time. When your files are ready, you can request a quote from Ezra.
07. TPE injection molding launch checklist
Work through these twelve checks before you release a TPE part to production.
- Can the material be sourced in low volume? What is the MOQ?
- Are hardness, heat resistance, chemical resistance and bond requirements clearly defined?
- Can the machine’s screw, nozzle and temperature control actually run TPE?
- Have drying conditions been confirmed? Is TPU-based moisture below 0.02%?
- Have wall thickness, draft, ejection and undercut design been adjusted for TPE?
- Are gating, venting and runner design sound?
- Has shrinkage been validated? Is mold compensation enough?
- Was the right prototyping approach chosen?
- Has the switch to production resin been validated?
- Have regrind ratio, color changeover and purging procedures been confirmed?
- Has overmold bond testing been completed?
- Have quality inspection standards been defined?
08. TPE injection molding FAQ
What temperature should I use for TPE injection molding?
SEBS-based grades run roughly 190–245°C, SBS-based grades roughly 170–200°C, and TPU-based grades should follow the supplier’s TDS. These are starting ranges only. Always validate the supplier’s recommended parameters on the actual machine and mold.
Does TPE need to be dried before molding?
TPU-based TPE usually does, with a target moisture level below 0.02%. SEBS-based TPE typically does not need strict drying, but moisture, clumping and contamination still need to be managed.
How should I set TPE injection speed?
Do not use one speed for every TPE. SEBS and thin-wall parts commonly need a faster fill to prevent premature freeze, while some SBS grades or geometries benefit from a more moderate speed. Start with a two- or three-zone profile, run short shots, and adjust for jetting, burn marks, flash, weld lines and fill balance.
Can any TPE be overmolded onto ABS, PC or PA?
No. Compatibility is grade-specific. Screen the TPE family and substrate first, then confirm the supplier’s compatibility data and run peel or pull testing on molded samples before cutting production tooling. Surface cleanliness, insert temperature, gate location and aging conditions can change the result.
What is the typical shrinkage rate for TPE?
Commonly in the 1.0%–2.5% range, influenced by hardness, wall thickness, hold pressure and filler content. Always verify with actual measurements for the specific grade and part.
Can an older injection molding machine run TPE?
Yes, but first check the screw, nozzle, temperature control, back pressure and shot-to-shot repeatability, then run a short-shot and fill study before committing to production.
Why is ultra-soft Shore 00 TPE hard to source?
Minimum order quantities for small batches tend to be high, there are fewer suppliers, and lead times run longer. Confirm sourcing feasibility before locking in a hardness spec.
09. Before you release the TPE mold
TPE injection molding is rarely limited by the press itself. Projects usually slip when material availability, process development, or soft-part mold design is left until after tooling has started.
Before releasing the tool, confirm three things: the production grade is available at the required volume; a fill study has established the injection, hold, cooling, and ejection window; and the mold accounts for soft material through balanced flow, adequate venting, and large-area ejection.
The rule that matters most is simple: prototype data is not production data. When the resin, substrate, or tool changes, repeat validation with the production material and document the approved process window.
Need help with a TPE part?
Send us your drawing, target hardness and yearly volume, and our engineers will come back with a grade recommendation, a tooling approach and a quote.
Sources
These references support the material-family and process-range guidance in this article. They are reference sources, not substitutes for the current TDS of the specific resin grade.
- HEXPOL TPE Injection Moulding Guide — reference settings for SEBS/SBS, gates, venting, mold temperature and shrinkage.
- Protolabs TPE Material Guide — comparison of major TPE families and TPU drying considerations.
