Flexible components are often discussed as though softness alone chooses the process. It does not. Liquid silicone rubber molding starts with a two-part material that is mixed and heat-cured into an elastomer, while common thermoplastic elastomer routes melt a pelletised material and let it solidify by cooling.[1] [2] That difference affects what the buyer should ask about the mold, material handling, part release, overmolding sequence and validation. The appropriate choice remains part-specific: a soft touch surface, a compression seal, a valve feature and an insert-bearing component can place different demands on the same nominal hardness range.
The process choice is not a hardness lookup: confirm the material grade, geometry, mating interface, use conditions and acceptance method together.
01
Start with the material’s route to a finished elastomer
The first practical distinction is irreversible cure versus reversible softening. Dow describes LSR as a two-part dispensable silicone material that is mixed and rapidly heat-cured; liquid injection molding is one of the stated fabrication methods.[1] A thermoplastic elastomer is a family term rather than one chemistry. In a conventional injection process, TPE pellets enter a heated barrel, become a melt, fill the closed mold and are removed after cooling.[2] A name on a drawing therefore does not establish the relevant flow, shrinkage, drying, temperature or adhesion behaviour. The proposed grade and its current technical data should do that work.
For procurement, this changes the first comparison. Ask whether the design needs a cured silicone network or a remeltable thermoplastic route, then document the service conditions that make that distinction meaningful. Consider the actual contact media, temperature history, deformation mode, appearance, assembly and expected life; do not infer suitability from a generic material family label. A data sheet can guide an initial shortlist, but functional performance belongs to the specified grade in the finished part and to the product owner’s validation plan. If a requirement is uncertain, preserve it as an open decision rather than converting it into an unsupported material claim.
02
Let geometry and handling define the mold conversation
A flexible part changes the ejection and handling questions. Thin lips, long seals, internal profiles, fine shut-offs, undercuts and cosmetic surfaces should be reviewed as a set because they can alter fill, venting, release and part support. For styrenic TPEs, Avient notes that flow behaviour is shear-dependent and that wall transitions, gate location, knit-line location, undercuts and shrinkage all need part-design attention.[3] Its guidance also notes that shrinkage is a range affected by the grade, wall thickness, temperatures, injection speed and time before measurement.[3] That is why a shrinkage value from a different shape is not a dimensional commitment for the present component.
Machine orientation is a separate question from material family. The terms vertical injection molding and horizontal injection molding should prompt the buyer to describe insert loading, operator or automation access, part orientation during molding, and the desired transfer sequence—not to assume that one layout is inherently better for every flexible part. A vertical arrangement may suit one insert-loading concept; a horizontal arrangement may suit another production flow. The decision should follow the actual mold layout, safety concept, handling method and part stability after release. Request a proposed sequence or layout sketch when orientation affects the quotation.
| Decision area | LSR route to examine | TPE route to examine | Question that keeps the decision part-specific |
|---|---|---|---|
| Material state | Two components are metered, mixed and heat-cured.[1] | Pellets are melted, injected and cooled to solidify.[2] | Does service performance call for one route, based on the nominated grade? |
| Tooling focus | Review metering/mixing, cure, venting and controlled release. | Review resin preparation, melt flow, cooling, venting and ejection. | Which geometry and cosmetic requirements create the highest process risk? |
| Dimensional planning | Treat cure behaviour and post-mold condition as validation inputs. | Account for grade- and process-dependent shrinkage and orientation.[3] | Which dimensions and interfaces must be measured, and when? |
| Two-material interface | Define the substrate, retention geometry and required interface evidence. | Confirm the grade/substrate pairing and any melt-adhesion evidence.[5] | Is the interface mechanical, material-led, or both? |
| Machine layout | Check the proposed handling and insert sequence. | Check the proposed handling and insert sequence. | Does the real loading and release method favour a particular orientation? |
03
Select against the function, not the word ‘soft’
A useful specification begins with what the flexible feature must do. A seal may be assessed for compression and recovery in its assembled condition. A grip may need a particular feel, colour stability or bond to a rigid body. A diaphragm, valve or protective cover may need to survive repeated movement, fluid exposure or a defined cleaning regime. Those are different product questions, even if each component is described informally as rubber-like. State the mating surface, load direction, contact media, temperature profile, visible faces and prohibited residues or additives where they matter. Then ask the material producer or qualified technical owner for grade-specific evidence relevant to those conditions.
Do not turn broad material associations into approval language. For example, a material supplier’s processing page says TPU can be processed on standard thermoplastic equipment and lists injection molding among its common techniques; that does not establish that every TPU grade is appropriate for an individual component.[4] Similarly, a silicone family listing does not prove sealing, chemical, food-contact, medical or regulatory suitability for a finished product. Define the acceptance criteria before the tooling decision: what will be inspected or tested, who owns the method, what conditioning applies, and what result constitutes acceptance. This prevents the process choice from outrunning the product requirement.
04
Treat overmolding as an interface design problem
Flexible material is frequently intended to sit on a rigid substrate, but ‘overmolded’ is not enough direction for a quote. Show the substrate grade, its supplier if controlled, the interface area, allowable gate locations, required coverage, cosmetic boundary and any mechanical retention features. HEXPOL explains that melt adhesion in TPE overmolding relies on compatible materials and that adhesion can vary with the engineering-plastic supplier; it specifically recommends testing adhesion on the relevant materials.[5] This is a reason to define the actual pairing, rather than assuming that an acronym such as PP, PC or PA predicts the finished interface.
Separate three questions that are often combined: can the tool form the geometry, can the two materials form the required interface, and can the assembled part meet its use requirement? A retaining groove or undercut may supply mechanical retention even where material-led adhesion is uncertain. Conversely, a clean cosmetic boundary may limit available retention geometry. The right balance depends on the part and should be visible in the design record. If an insert or substrate moves between stations, also define handling, location datum, contamination controls and the condition of the surface at the second shot. These are scope inputs, not details to leave to a material name.
05
Build a comparison package that produces a decision
When comparing process proposals, normalise the scope before comparing price. Provide the current CAD and drawing, target material grade or allowable alternatives, colour and surface expectations, critical dimensions, annual demand, sample phase, interfaces and any inserts or substrate. Ask each respondent to identify its assumptions about material preparation, mold concept, cavity count, gate and parting-line location, handling, cure or cooling, post-molding operations and inspection. A short marked-up response is more useful than a generic assurance because it connects each assumption to the part. It also reveals where the program is relying on buyer decisions that have not yet been made.
Finally, agree on the evidence plan for the selected route. Dimensional inspection alone may not answer an interface, sealing or repeated-flexing question. Conversely, a functional test without a defined conditioning method may not be comparable across samples. Record the material identification, revision, sample condition, test or inspection method, acceptance criteria and change-control path. This does not prescribe a universal qualification protocol; the needed checks depend on the component, its end use and the product owner’s obligations. It does create a clearer handoff from early process choice to a controlled, part-specific review.
Practical takeaway
Before you move
to the next review.
- Name the exact flexible-material grade, colour and permitted alternatives; do not specify only a family acronym or hardness range.
- Map the feature’s real job: deformation, mating surface, contact media, temperature history, appearance and assembly sequence.
- Share CAD and a drawing that mark critical dimensions, cosmetic zones, undercuts, thin sections, inserts and interface boundaries.
- Ask for the proposed mold, gate, release, handling and machine-orientation assumptions to be shown against the actual part.
- Define sample conditioning, inspection or functional tests, acceptance criteria and change-control responsibility before release.
FAQ
Common questions
Is LSR automatically the better choice for a seal?
No. A seal decision is part-specific. The assembly geometry, compression behaviour, contact media, temperature exposure, mating surfaces and required validation method all matter. Start with those requirements and compare nominated grades and process proposals against them.
Can a TPE overmold be assumed to bond to any rigid plastic?
No. Material compatibility, the particular grades, the substrate condition, interface geometry and processing conditions can affect the result. The relevant material pairing and the required interface test should be agreed for the intended part rather than inferred from generic resin names.[5]
Should I choose a vertical or horizontal machine before requesting quotations?
Usually not as a standalone decision. Describe inserts, loading sequence, automation or operator access, part orientation and handling constraints. The proposed mold and production flow can then show whether a particular machine layout is appropriate for the component.
References
Source material
- [1] Dow, “Silicone Rubber: Liquid Silicone Rubber (LSR).”
- [2] HEXPOL TPE, “TPE Injection Moulding Guide.”
- [3] Avient, “Injection Molding: Part Design.”
- [4] Covestro, “Processing of Thermoplastic Polyurethanes.”
- [5] HEXPOL TPE, “TPE Overmoulding Guide.”
This guide is general information for early procurement and engineering discussion. It does not replace product-specific DFM, qualification, validation, regulatory assessment or legal advice.