A silicone overmold can look acceptable on the outside while remaining soft or gummy where it touches an insert. When that happens, increasing the mold temperature is not necessarily the right answer.
At DX, we recommend separating three questions before changing the process: did the silicone receive enough heat, were its components correctly metered and mixed, and did something at the contact surface interfere with curing?
That distinction matters when developing custom silicone parts with plastic inserts, metal components, primers or surface treatments. A drawing identifies the substrate’s shape and material, but it rarely tells us everything that has touched its surface.
This guide explains how we would structure a compatibility review before committing a material combination to production. The screening steps below are project-planning recommendations, not a universal test standard or a substitute for the selected silicone supplier’s instructions.
Platinum-catalyzed liquid silicone rubber cures through a crosslinking reaction. Certain substances can interfere with that reaction, leaving the material incompletely cured.
Dow identifies potential inhibitors including particular nitrogen-, sulfur-, tin- and phosphorus-containing compounds. Its guidance also flags some residual solvents, monomers and primer ingredients. These are screening categories, not a rule that every material containing one of those elements is incompatible. The exact chemistry and exposure matter. [1]
There is also an important distinction between unwanted contamination and intentionally controlled cure chemistry.
Research published in Polymer Degradation and Stability, Volume 240, October 2025, investigated modified platinum catalyst systems that deliberately adjust inhibition to improve LSR shelf stability. That is formulation research—not a production remedy for contaminated inserts. We are not claiming to use the experimental catalyst system. The practical lesson is that catalyst activity depends on chemical surroundings as well as temperature. [2]
For a custom molding project, our priority is therefore to identify the approved combination of silicone, substrate and surface preparation, rather than assuming that any clean-looking insert will behave the same way.
A sticky surface is a symptom, not a diagnosis.
Dow’s injection-molding troubleshooting guide lists insufficient vulcanization time, low tool temperature, incorrect mixing proportion and cure inhibition as separate causes of incompletely cured LSR. [3]
We suggest using the location and pattern of the problem to decide what to investigate first:
| Observation | First questions to investigate |
|---|---|
| Most of the part remains soft | Were dosing, mixing, material identity and actual thermal conditions correct? |
| The problem appears mainly against an insert | Did the insert introduce incompatible surface chemistry, or change local heat transfer? |
| Only certain cavities or production periods are affected | What differs in temperature, material delivery, insert handling or maintenance history? |
| Silicone appears cured but separates from the substrate | Is this an adhesion problem rather than a cure problem? |
| A failure begins after changing a primer or cleaner | Does the new material or preparation sequence affect cure, adhesion or both? |
These observations help narrow the investigation; they do not establish the cause on their own.
In particular, a fully cured silicone layer can still have poor adhesion. Conversely, a gummy interface should not be treated simply as insufficient bond strength. Adding another primer before understanding the failure may introduce another variable.
For an overmolding project, we would request more than “stainless steel” or “polycarbonate” on the drawing.
The useful question is: what is the actual surface presented to the silicone when molding begins?
Record the exact substrate grade, supplier, color and relevant surface finish. For metal inserts, ask about machining fluids, protective coatings, polishing compounds and cleaning. For plastic inserts, ask about molding aids, coatings and any subsequent treatment.
We recommend requesting representative production inserts, not only specially prepared development samples. Otherwise, an early trial may demonstrate compatibility with a preparation route that will not be used for normal orders.
A primer should be evaluated as part of a defined silicone–substrate system. Its name alone does not describe application thickness, drying conditions, storage time or what happens after handling.
Dow’s inhibitor guidance includes examples involving primer ingredients, which is why we would not assume that a product marketed as an adhesion aid is automatically compatible with every platinum-cure silicone. [1]
For a trial, identify the exact product and follow its supplier’s application instructions. Record the preparation sequence so that a successful result can be reproduced.
We recommend including cleaners, wipes, gloves, temporary tapes and storage-contact materials in the review wherever they can transfer material onto the molding surface.
This is a screening list, not a blanket prohibition. A glove or cleaner should be assessed by its actual formulation and use conditions, not rejected solely because of its broad product category.
A surface can look dry and still require further investigation. Equally, changing to a stronger solvent without checking compatibility and handling requirements can damage the substrate or create a new process problem.
When a new insert or preparation method is proposed, we suggest agreeing on a small compatibility-screening plan with the silicone supplier and molding team.
The aim is to compare contact conditions while keeping the silicone preparation and thermal exposure as consistent as practical.
Prepare a control using the selected silicone and an established compatible contact surface. Test the candidate surface alongside it under an appropriate, documented cure condition.
If the control also fails, the candidate insert cannot yet be identified as the cause. Material preparation, mixing or curing conditions need attention first.
If the control cures normally while the candidate interface remains gummy, contact compatibility becomes a stronger possibility. Differences in heat transfer still need to be considered.
Rather than testing only the final combination, a useful project-specific matrix could include:
| Sample | Contact condition | Purpose |
|---|---|---|
| Control | Known-compatible surface | Check that the baseline cure is working |
| A | Insert as received | Assess the incoming condition |
| B | Insert after proposed cleaning | Evaluate the cleaning step |
| C | Cleaned insert with proposed primer | Evaluate the complete preparation route |
Use separate, identified samples and avoid carrying residue between them. Agree on replicate numbers and observations before testing; this table does not prescribe a qualification sample size.
This comparison is particularly useful when a preparation step was intended to improve adhesion but coincided with a new cure problem.
We recommend documenting where softness, residue transfer or incomplete cure appears, including photographs and sample identification.
Do not rely on a single hardness reading from a convenient outer surface to approve a hidden interface. Where cure state remains uncertain, ask the material supplier or a qualified laboratory which analytical method is appropriate.
A screening pass also does not prove bond durability. Cure compatibility, adhesion and finished-part performance need their own acceptance criteria.
A longer cure or higher temperature may help when thermal exposure is insufficient. It does not establish that an incompatible contact material has become suitable.
Dow’s grade-specific literature illustrates this nuance: its SILASTIC LC 1000 guidance notes that low levels of inhibition may reduce adhesion without obvious stickiness, and that further heating can recover some cases. That observation belongs to that product and application; it is not a universal repair instruction. [4]
For a new custom part, we would review any proposed temperature increase against the substrate’s limitations and the intended molding process. A condition that improves the silicone surface but distorts the insert is not a successful solution.
We also recommend keeping suspect parts segregated until their disposition is agreed. Removing visible residue or making the surface feel dry is not evidence that the entire component meets its requirements.
Once a material combination passes screening and representative mold trials, the purchasing specification should preserve more than the base material name.
We suggest recording:
For example, replacing a cleaner with a locally available alternative should not be treated as an administrative change if its residue can contact uncured silicone. The same applies to a new insert coating or primer formulation.
The aim is not to make every change difficult. It is to ensure that the team knows when the original compatibility evidence may no longer apply.
At DX, we support custom silicone overmolding onto suitable substrates, alongside tooling and silicone molding services. We can discuss mechanical retention and bonding options as part of the component design. The appropriate route depends on the selected materials and the application. [5]
For a productive review, send us the silicone part drawing and insert drawing together. Include the substrate grade, any coating or primer, cleaning information, operating conditions and expected quantities.
If you are investigating an existing problem, also describe where it occurs and what changed before it appeared. “Gummy only against the insert after a primer change” gives us a much clearer starting point than “the silicone does not stick.”
Developing a custom silicone seal, protective component or plastic–silicone or metal–silicone assembly? Share your drawings, dimensions, insert materials, surface-preparation details, application conditions and quantity requirements with us.
We can review the proposed material combination, tooling approach and sample requirements with you before production. Contact DX to discuss your custom silicone product and tell us whether your priority is cure consistency, reliable bonding or both.
DX provides you with all-around silicone product customization services for valued customers like you.
