STMPRO X50 Mould Making Silicone Instructions

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Material Safety Data Sheet

Please read these instructions carefully even if you are an experienced user, as the mix ratio may vary between batches.


Temperature has an impact on how liquid the silicone is while mixing. The optimum room temperature for storing unmixed silicone (before use) is 21OC. Storage at lower temperatures will cause the silicone to thicken and will be more difficult to pour. Because the unmixed silicone is a dense liquid, it may take several days to come up to room temperature.

Safety and Precautions

Skin and eye contact, ingestion and breathing of dusts and vapours should be avoided. It is recommended you use gloves, dust masks, and eye protection. Use with good ventilation. Material Safety Data Sheet is available on request. Supplies needed: disposable gloves, mixing containers, stirring sticks, a gram scale and a mould box*. Optional vacuum chamber** for de-gassing the mixed silicone to remove air bubbles.

  • * There are several web sites which can explain how to build an actual mould box. A brief summary of common mould boxes is listed in the section entitled “Types of moulds”.
  • ** Anyone wanting the optimum results with silicone moulding materials or using large volumes should investigate making or purchasing a vacuum chamber. There are several web sites which explain how to build a vacuum chamber. Further in these instructions there is information on alternative methods should a vacuum chamber not be available.

Technical Specifications (at 25OC Humidity 60%)

  • Cure type: Condensation
  • Mix Ratio (base:catalyst): 100:2
  • Shrinkage: Less than .03%
  • Viscosity: 30,000 – 32,000 CS
  • Hardness: 30-32 AO
  • Tensile strength: 36 - 38 kgf/cm2
  • Tear strength: 28 - 30 kgf/cm
  • Elongation break: 480 – 510%
  • Density: 1.07 – 1.09 g/cm
  • Working time: 20-25 minutes
  • De-mould time: After 12 hours (depending on mould thickness)
  • Full cure: After 24 hours (depending on mould thickness)
  • Shelf Life of unused X50: 12 months in unopened container

Pot Life & De-mould Time cannot be heat accelerated

Types of Silicone Moulds

There are four common types of moulds:

  • Block mould: one piece with no negative drafts or undercuts. This is probably the easiest mould to produce. The original part should be placed in the box so the silicone is able to flow around it. If the part is simple, it can be placed in a plastic cup, a disposable plastic container or a wooden box (pine wood not recommended). If you choose a wooden box make sure to seal it properly if it appears to be porous.
  • Block mould-multiple piece: This can be made exactly like a block mould and simply cut in half once it cures to remove the part. If you prefer not to cut the mould, partially fill the container and allow it to cure. Then apply a thin layer of release agent, and finish pouring the mould. When the second layer is cured, the mould should easily separate.
  • Glove mould brushed onto the pattern: This requires a thixotropic agent which allows the silicone to be brushed on an original piece without running off the surface. The thixotropic agent can be supplied in the catalyst or as a separate additive. This type of silicone is ideal for building restoration. The silicone can be brushed on an original piece of crown moulding, allowed to cure and then be removed. The parts can then be cast to produce replications of the original moulding. This type of moulding is also becoming popular to cast rocks or stones to create moulds of fake rocks, rock climbing walls, ground surfaces and props.
  • Cast Glove mould: This is made by creating a void around the original part in a consistent thickness. It is usually done by forming modelling clay around the pattern and then forming a hard shell around the clay. The shell can be made by using fibreglass, urethane or plaster. After the shell has cured, the clay is removed; creating a void which can be filled by silicone.

Preparing the Model

Porous surfaces, such as wood, should be dried and sealed. Also applying a release agent, such as petroleum jelly will aid in easy release of a silicone mould. Because silicone can sometimes bond to surfaces which contain silica, such as glass, cements and natural stone; it is recommended to do a spot test for possible adhesion.

Mixing and Curing

Stirring individual components before use is recommended. Make sure to weigh and measure accurately. Close containers tightly after use. Products should be stored in their original, sealed containers in an environment that does not exceed 32oC (90oF). Under these conditions, the product will achieve the expected shelf life.


We now package the silicone with more catalyst than needed for your kit size as you can adapt the mix by adding more catalyst if required (not recommended). We recommend a mix of 100:2 for standard pouring use, but mixing additional catalyst at a ratio of up to 100:3 will result in a thicker mix (not suitable for pouring) and will begin setting within minutes.

For standard use shake “Part B” thoroughly just before using, then add 100 parts by weight of “Part A” and 2 parts by weight of “Part B” (use a small digital scale for most accurate results) into a clean metal or plastic container (or use the entire contents of Part A and 2/3 of Part B as they are bottled by weight 100:3 however it is critical that you are accurate). When using a vacuum chamber, the volume of the container should be 3-4 times the volume of the material to be mixed. This allows for expansion of the silicone material as it de-gasses.

Starting slowly at first to avoid any catalyst splashing out of the mixing container, mix thoroughly by hand or with mixing equipment while minimizing air entrapment until a homogeneous mixture is obtained. This will occur when the material takes on a uniform blue colour with no streaks of white or dark blue. Scrape the sides and bottom of the container thoroughly several times while mixing. Once mixing is complete it is recommended that the material be de-aired 2-3 times by intermittent evacuation in a vacuum chamber for a few minutes to minimize any imperfections due to bubbles in the cured material. Typically after releasing the vacuum 2-3 times, the mass will collapse on itself, at which time the vacuum should be left on only 2-4 minutes longer.

Should a vacuum chamber not be available, firmly tap the container of mixed silicone on the workbench to attempt to force the air bubbles to the surface. Pour the silicone slowly in to the mould from a good height, this will also help remove air from becoming trapped on the surface of the mould. After pouring the silicone in to the mould, if the mould situation allows, tap the mould firmly to try and dislodge any trapped air bubbles from the surface of the item being cast.

Pouring the Mould

To pour the mould, begin by pouring a thin stream of silicone into the lowest point of the mould box very slowly. Allow the liquid to push the air out ahead of it, to avoid trapping air in the mould. If you have a highly detailed mould, some mould makers recommend brushing mixed silicone directly onto the detailed area before pouring the remainder of the mould.


After the silicone to set, you can demould. Remember to be careful at this point, as silicone will not reach it's full cure for approximately 24 hours.

Storing the Mould

It is best for the mould if it can be stored in the original mould box or on a surface that will stress it the least. If left in a distorted shape for extended periods of time, the mould may not be able to return to the original state. Keep in a cool, dry area, away from sunlight.