How do I enlarge or shrink a casting?

Article courtesy of Smooth-On, Inc.

You can accomplish this by either shrinking or expanding a rubber mold using solvent. Once the rubber mold has expanded or shrank, you can cast a Smooth-On plastic into the mold. Here are examples using two different mold making rubbers for these purposes.


To enlarge or reduce a piece, it is recommended that Mold Max™ 30 with Fast Cat™ 30 as catalyst or Body Double™ be used as a mold material. Mold Max 30 with Fast Cat accepts solvent readily, coming to a full cure even with a large amount of solvent added. Body Double has a high level of curatives, which allow it to perform well in a solvent-heavy application. Smooth-Cast™ 300Q is recommended for a casting resin, as the speed of the cure does not allow the solvent to affect the surface of the casting. duoMatrix™ NEO may also be used.

Enlarging a Casting

To enlarge a piece, a mold of the original is made using Mold Max 30 with Fast Cat 30 or Body Double. The mold is enlarged instead of enlarging a casting because a solvent filled silicone casting is quite difficult to handle and nearly impossible to make a second mold of.

The mold rubber should be vacuumed or pressure-molded if possible to reduce air bubbles. The mold is then soaked in a sealed container in a bath of solvent. NOVOCS™ Silicone Solvent works the best. Toluene, xylene or naptha are also suitable. Observe all safety precautions when handling combustible or flammable solvents. The solvent will become absorbed into the silicone rubber, enlarging the mold. Over time, an expansion of up to 200% can be reached. A mold soaked in toluene for 7 days expands approximately to 130% of the original size. Make sure the solvent bath completely covers the mold or distortion may occur.

Once the mold has expanded, the mold should be removed from the solvent bath, and the surface should be thoroughly dried. The solvent will begin evaporating immediately, so you must work quickly. Once the surface is dried, a batch of Smooth Cast 300Q or duoMatrix NEO can be mixed and poured into the mold. A light mist coating of Ease Release™ 200, while optional, may help to allow for an easier demold.

Further expansion can be achieved by repeating this process. Please note, an expanded mold will be more fragile and may require extra care in handling.

The following chart may assist in determining the expansion rates you may experience:

 Expansion After 3 Days (approx)Expansion After 7 Days (approx)
Body Double Standard Set 110% 120%
Mold Max 30 with Fast Cat 30 115% 130%

Reducing a Casting

Reducing a piece is somewhat more difficult than enlarging a piece. To reduce a piece, make a mold of your original using Mold Max 30 with Fast Cat 30 as catalyst or Body Double silicone with NOVOCS solvent mixed into the catalyzed silicone mold rubber. The mold rubber should be vacuumed or pressure-molded to reduce air bubbles when using this method.

The original must be made of a solvent-resistant material, or coated with a solvent resistant spray. It may be necessary to first mold the piece, then pour a casting out of Smooth Cast resin to use as an original, as Smooth Cast resins are somewhat solvent resistant.

The mold will take longer than normal to cure, dependent on the amount of solvent added to the material. After the mold has cured to a non-tacky finish, the piece should be demolded quickly. Care should be taken when demolding, as the mold will be more fragile than a standard mold.

Over time the NOVOCS solvent will evaporate and the mold will shrink, and a casting can be poured into the shrunken silicone mold. The amount of shrinkage is dependent on the amount of solvent added.

A 1:1 mix by volume of catalyzed Mold Max 30 to toluene, demolded, then allowed to shrink for 3 days will produce a casting which is approximately 85% of the original size. Allowing the mold to shrink for 7 days produces a piece which will be 80% of the original size.

Further reduction in size can be achieved by repeating this process multiple times.

 Silicone Ratio (by volume)Solvent Ratio (by volume)Shrinkage After 3 Days (approx)Shrinkage After 7 Days (approx)
Body Double Standard Set1 part1 part 15% 25%
2 parts1 part 6% 8%
4 parts1 part 2% 4%
Mold Max 30 with FastCat 301 part1 part 15% 20%
2 parts1 part 9% 10%
4 parts1 part 5% 7%


Shrinking or expanding a piece using NOVOCS solvent in the mold making process can produce very good results, but it may require some experimentation to achieve a result which is appropriate for your application. Every shape expands or shrinks differently, so it is recommended to do a small scale test before attempting larger projects.


This FAQ article is offered as a guideline and offers possible solutions to problems encountered during mold making and casting. No warranty is implied and it is up to the end user to determine suitability for any specific application. Always refer to the provided Technical Bulletins (TB) & Safety Data Sheets (SDS) before using any material. A small scale test is suggested to determine suitability of any recommendation before trying on a larger scale for any application.