Practical guide

Cure inhibition in platinum-cure silicone: causes and solutions

If you have ever poured silicone and discovered areas that remain sticky or soft while the rest of the mold is fully cured, you have probably encountered cure inhibition.

On this page we explain, briefly and practically, why inhibition occurs, which materials can cause it, and how you can avoid it when working with SiliCreate platinum-cure silicone (RTV2).

In short: inhibition does not mean the silicone is “bad”, but that the platinum-based catalyst has been blocked (poisoned) by contact with certain incompatible substances or materials.

What is inhibition, exactly?

Platinum-cure silicone (platinum-cure) cures through a chemical reaction between two components (usually A and B) in the presence of a platinum-based catalyst. Under normal conditions, after mixing and the specified curing time, the material becomes elastic and fully dry.

Inhibition occurs when certain substances interact with the platinum catalyst and locally “shut it down”. The result is:

areas that remain sticky or gel-like on the surface;
corners/detailed areas that do not cure at all;
clear differences between fully cured areas and soft or dull areas;

Usually the problem occurs at the interface between the model/master and the silicone, not in the bulk of the material.

A bit of chemistry (explained simply)

The curing reaction

Platinum-cure silicone uses an addition reaction:

the liquid silicone chains bond with each other;
the platinum-based catalyst accelerates the process;
no by-products are released (almost zero shrinkage).

What “catalyst poisoning” means

Certain chemical compounds bind to the platinum and block the catalyst. In the affected area:

the reaction stops or slows down drastically;
the silicone remains soft or sticky;
the problem can be local (it does not affect the entire volume).

Typical substances that cause inhibition

Sulfur compounds Amines Tin-based catalysts Certain plasticizers Some adhesives & paints

These substances can be present in plastics, latex, clays, adhesives, coatings, 3D prints, etc.

Materials that can cause inhibition

Below you will find a list of materials that frequently create problems, and recommended alternatives.

Gloves

To avoid: natural latex gloves. Latex contains sulfur compounds that can inhibit silicone.
Recommended: nitrile or vinyl gloves.

Even if you do not touch the silicone directly, traces of latex on the model or in the container can be enough to cause inhibition.

Mixing bowls and containers

Recommended: PP (polypropylene), PE, silicone, glass, stainless steel.
To avoid (or test beforehand): PVC, some types of ABS, polycarbonate, unknown or very soft plastics (they may contain plasticizers/reactive additives).

If you are not sure what a plastic container is made of, it is safer to use PP/PE, glass or a silicone container.

Modeling clays and plastilines

To avoid: sulfur-based clays (“sulfur-based oil clay”). They are a classic cause of inhibition.
Recommended: sulfur-free clays and plastilines specifically labeled as compatible with platinum-cure silicone.

Tin-cure silicone

Molds or masters made from tin-cure silicone (condensation) may contain traces of tin-based catalyst or additives that inhibit platinum-cure silicone.

do not pour platinum-cure silicone directly over an unknown tin-cure silicone;
always make a small test in a hidden area.

Paints, coatings, adhesives

Epoxy, polyurethane or cyanoacrylate (super glue) adhesives can cause inhibition if they are not fully cured.
Some paints contain amines or active solvents that affect platinum.

General rule: let paints/adhesives cure completely (ideally 24 h or as indicated in the technical data sheet) and, ideally, test on a small area.

3D prints and resins

3D prints made from resin (SLA/DLP) can release monomers or inhibitors if they are not washed and post-cured properly.
Some filaments (e.g. certain types of TPU or modified ABS) may contain problematic additives.

Wash the model thoroughly, post-cure it according to the recommendations and, ideally, apply a coat of compatible acrylic varnish and let it dry completely before casting.

How to recognize inhibition

The most common signs that something is wrong with the cure are:

the mold surface remains sticky even after the normal curing time;
there are soft, gel-like areas, especially in corners or fine details;
clear differences between fully cured areas and areas that look “wet” or glossy;
the mold tears very easily in certain spots, while the rest seems OK.

Attention: extending the curing time (for example from 12 h to 24–48 h) does not fix severe inhibition. If the catalyst is blocked, that area simply will not cure properly.

Practical tips to avoid inhibition

1. Choose compatible materials for mixing and casting

To be as safe as possible, work with materials that are known to be compatible with platinum-cure silicone:

Mixing containers: PP, PE, glass, stainless steel, silicone.
Mixing tools: clean wooden sticks, PP or stainless steel spatulas.
Gloves: nitrile or vinyl, not latex.

2. Clean and prepare the model properly

Any dirt, grease or residue of adhesive/paint can contribute to inhibition.

clean the surface of the model with water and mild detergent, then rinse well;
if needed, degrease with isopropyl alcohol and let it evaporate completely;
avoid unknown sprays or release agents based on unknown silicones.

If you use a release agent, choose one specifically labeled as compatible with platinum-cure silicone.

3. Follow the proper mixing ratio and mix thoroughly

Although inhibition is related to compatibility, an incorrect mix can create the impression of “partial cure”.

respect the specified ratio (e.g. 1:1 by weight);
mix long enough, scraping the sides and bottom of the container;
then pour into a second clean container and mix again (“double mix”).

4. Always make a small test if you are unsure

If you are working with a new material (a certain plastic, a paint, a clay, etc.), the safest method is:

pour a small amount of silicone on/in a hidden area;
let it cure for the normal curing time;
check if the surface is perfectly dry and elastic.

If the test is OK, you can cast the full mold with more confidence.

5. Pay attention to your working environment

In general, platinum-cure silicone is quite tolerant, but:

work in a clean space, without excessive dust;
avoid mixing silicone next to aerosol sprays or strong solvents;
respect the temperature range recommended by the manufacturer.

Frequently asked questions about inhibition

Does inhibition make the silicone dangerous?

No, inhibition does not turn the material into a “toxic” product, but the mold will no longer be usable as it should be: it will have poor mechanical strength, may tear easily and may contaminate future castings.

Can I save a mold that has suffered inhibition?

Unfortunately, if the affected areas have not cured after a longer period (e.g. 24–48 h), it is very hard to “repair” that mold. Usually the solution is to identify the cause (incompatible material, release agent, paint, latex, etc.) and remake the mold using a properly prepared model.

Is platinum-cure silicone more sensitive than tin-cure?

Yes, platinum systems are generally more sensitive to contaminants, but they offer important advantages (dimensional stability, minimal shrinkage, skin safety and, in some cases, food contact suitability). With a bit of attention to the materials you use, they can be just as easy to work with.