SikaBiresin F38 -Modeling resin-

SikaBiresin F38 is a low-viscosity rigid polyurethane resin developed specifically for the scale modelling and prototyping industry. Its fast cure, minimal shrinkage and ability to reproduce fine details make it a well-established reference for mould casting throughout Europe.

Technical specifications

What it is used for

SikaBiresin F38 is designed for mould casting where dimensional accuracy, surface hardness and a fast production cycle are required. Its low viscosity allows the material to flow and fill every mould detail without mechanical assistance, and its 1:1 mix ratio by weight simplifies batch work.

• Manufacture of miniatures and collectible figures
• Industrial prototypes and scale models
• Static and scale modelling parts
• High-detail master reproduction
• Block casting for large moulds or batch production
• Parts for special effects and character work
• Decorative replicas and set dressing elements

How to use it

1. Prepare your materials and workspace. Work at an ambient temperature of 18 °C or above. Both the mould and the components must be completely dry: any trace of moisture can cause bubbles or prevent proper curing.
2. Homogenise part A (polyol). Before measuring, stir component A thoroughly until the colour and appearance are uniform. This step is essential to ensure the final cured properties.
3. Apply release agent. If the mould is not self-releasing silicone, apply a suitable release agent for polyurethane to all mould surfaces and allow the solvent to evaporate.
4. Weigh the components. Use a precision scale and mix at a ratio of 100A : 100B by weight. Avoid measuring by volume whenever possible, as the density of both parts differs (volumetric ratio 100A : 85B).
5. Mix. Add part B to part A and stir vigorously for 20-30 seconds, scraping the sides and bottom of the container. You have approximately 2 minutes of pot life: be quick and precise.
6. Cast into the mould. Pour the mixture in a continuous, thin stream to minimise air entrapment. If the mould has complex areas, tilt it to aid filling.
7. Demould. After 20-25 minutes the part will be ready to demould. Full cure is completed in the following hours, at which point the part reaches its final hardness and mechanical strength.
8. Post-processing. The cured part can be sanded, painted or primed with standard polyurethane finishing products.

Usage tips

Always weigh — do not measure by volume

The 100A:100B ratio by weight is straightforward and eliminates errors, but the volumetric ratio (100A:85B) is less intuitive and more prone to deviations. Use a precision scale from the very first use: a 5% ratio error can result in soft, tacky or incompletely cured parts.

Control the temperature of the material and the mould

Below 18 °C the pot life appears to extend, but curing becomes uneven and the final mechanical properties are compromised. If you work in a cold workshop, warm the components in a warm water bath (no more than 30 °C) before mixing. Likewise, a pre-warmed mould reduces initial viscosity and improves fine-detail filling.

Batch production and block casting

SikaBiresin F38 is particularly efficient for large batches thanks to its low viscosity and fast cycle time. If producing in series, prepare all moulds before starting to mix. For large-volume block castings, bear in mind that the reaction is exothermic: masses above 500 g can generate enough heat to deform foam moulds or heat-sensitive materials. In those cases, split the pour into layers or use silicone or metal moulds.

Frequently asked questions