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Neither epoxy nor urethane is the universal choice for mold making. For a flexible mold, look for a polyurethane elastomer formulated specifically for mold making; for a rigid industrial tool, an epoxy tooling system may be a better fit. If you mean casting resin into a mold, that is a separate decision: the mold material and release agent must be compatible with the casting resin.
First, clarify what “mold making” means
The phrase can describe two different jobs: making the mold or tool itself, or pouring resin into a mold to make a part. A material suitable for one job may not suit the other. “Urethane resin” is especially broad: products include rigid casting resins, flexible elastomers, and tooling compounds.
Sika’s casting-systems overview lists mold-making silicones, fast-cast urethanes, and epoxies, as well as polyurethane casting resins and epoxy tooling systems. Treat the product’s stated purpose and technical data sheet—not the family name—as the basis for selection.
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- Rapid Curing Casting Resin: Our 2-part polyurethane resin can quickly cure from liquid to a durable, white, shore 75D plastic within 10 minutes. It has only 2 minutes working time so you need to work very fast, saving you more time to make more resin artwork. Compared to epoxy resin, polyurethane casting resin has a much quick curing time and opaque white finish while epoxy resin is crystal clear after curing.
- Ultra-Low Odor& Easy Mixing: LET'S RESIN liquid polyurethane resin features super low viscosity and low odor, with a simple 1:1 mix ratio by volume or weight. It's perfect for making beautiful and bubble-free white cast replicas with minimal shrinkage, No need for a vacuum chamber or pressure pot for de-gassing. This kit contains 30oz of part A and 30oz of part B. Due to the different densities of part A and part B, they may appear to have different capacities, but rest assured that both part A and part B are definitely no less than 30oz.
- Versatile Application: Our polyurethane casting resin is designed for pouring into rubber molds and enabling you to create accurate casts. It's perfect for making prototype models, sculptures, figurines, special effects props, and decorative jewelry. This makes it be a perfect gift choice for occasions such as Christmas, Valentine's Day, Mother's Day, birthdays, and more. The quick de-mold times also make it ideal for high-volume and fast-casting projects
- Colorable & Machinable: LET'S RESIN polyurethane resin effortlessly fills the tiniest crevices and cracks in your projects, thanks to its low viscosity. Once fully cured, the castings are lightweight, tough, machinable, and paintable. They are also waterproof and resistant to mild solvents
- Warm Tips: The resin has a shelf life of 1 year. If it is damaged during transportation, for example, in case of leakage or partial loss, you can contact us at any time. We will handle the issue immediately
Epoxy and urethane for making the mold or tool
| Need | What to investigate | Why the label alone is not enough |
|---|---|---|
| Flexible mold | A polyurethane elastomer specifically described for mold making; check hardness, elongation, intended casting material, release, and cure schedule. | Not every urethane resin is flexible or intended to become a mold. |
| Rigid industrial tooling | An epoxy tooling system rated for the required process, casting thickness, and service temperature. | Epoxy products vary in pot life, hardness, heat limits, and permitted casting thickness. |
| Rigid fast-cast model or part | A casting resin whose product data and mold compatibility suit the job. | A fast-setting urethane casting resin is not automatically a mold-making material. |
Flexible polyurethane mold material: a named example
Sika describes Sikabiresin UR303/UR402 as a two-component polyurethane elastomeric casting resin designed for flexible molds for ceramic and gypsum processing, as well as elastic components. Its December 2025 product data sheet lists Shore A hardness of 81A, elongation at break of 400%, linear shrinkage of 0.1% under the specified test and cure conditions, and a 100:35 mixing ratio by weight. These are formulation-specific values, not typical values for urethanes generally. See the UR303/UR402 product information.
Epoxy tooling: a rigid-tool option
Sika identifies epoxy formulations for stretch-form and hydroform dies, vacuum-form molds, prototype injection molding, compression molding, and other tooling. The right formulation depends on the process and its demands; consult the epoxy casting systems information for product-specific limits rather than assuming all epoxy systems behave alike.
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- FAST-CURING BRIGHT WHITE CASTING RESIN – Smooth-Cast 300 is an ultra-low viscosity, two-part liquid polyurethane plastic that produces bright white castings with excellent detail. Its free-flowing formula fills intricate molds with ease for professional-quality results.
- VIRTUALLY BUBBLE-FREE CASTINGS – The ultra-low viscosity formula allows trapped air to escape naturally, eliminating the need for vacuum degassing in most applications. Create smooth, high-quality castings with minimal bubbles and outstanding surface finish.
- RAPID SET & DEMOLD TIMES – Designed for high productivity with a 3-minute pot life and 10-minute cure time, allowing you to make multiple castings in a short period. Ideal for prototyping, production runs, cosplay props, model making, and special effects.
- STRONG, RIGID & MACHINABLE – Cures to a durable Shore 70D hardness, producing rigid castings that can be drilled, sanded, carved, machined, and painted. The bright white finish also provides an excellent base for custom colors and decorative finishes.
- EASY 1:1 MIXING FOR PROFESSIONAL RESULTS – Mix equal parts of Part A and Part B by volume—no specialized equipment required. Compatible with most silicone rubber molds and trusted by artists, designers, manufacturers, prop makers, and hobbyists for detailed, reliable castings every time.
If you are casting resin into a mold
Choose the casting resin and the mold as a compatible pair. Sika lists silicone mold products for casting both epoxies and fast-cast urethanes, but that does not establish compatibility for every silicone or resin formulation. Confirm the mold maker’s and resin maker’s instructions, including whether a release agent is required.
Compatibility can be product-specific even within one resin family. Smooth-On says not to cast TASK 13 or TASK 14 into platinum-cure (addition-cure) silicone molds. Its bulletin also gives release-agent guidance for polyurethane, latex, or metal molds. Follow the instructions for the exact mold and resin rather than relying on a general rule about epoxy, urethane, or silicone.
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- Trusted Resin Formula - Enjoy the same exceptional quality and performance with our Amazing Casting Resin, now with a fresh identity. This resin kit maintains the same trusted formula you've always loved, perfect for all your crafting and art project.
- Detailed Resin Castings - Capture intricate details with our low viscosity, two-part resin kit. Achieve a beautiful opaque white appearance in just 2 minutes and demold after 5-10 minutes. Ideal for casting mold such as handles and knobs
- Colorful Craft Creations - Make your crafts stand out by adding colorful accents! Our casting resins can be dyed or pigmented using Alumilite dyes, mica powder. Paint it while the resin is still curing to achieve amazing colors.
- Easy Mixing Process - Our resin kit is beginner-friendly. Follow the 1:1 mix by weight or volume, pour the liquid cast materials together, and mix vigorously for about 20-30 seconds. Simple enough for any hand to handle.
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Compare the product data that affects the result
- Intended use: Is the product for a flexible mold, rigid tooling, or a cast part?
- Mechanical properties: Check hardness and, where relevant, elongation, tear strength, or impact performance. Do not infer flexibility from “urethane.”
- Heat and process limits: Match the rated service temperature and permitted casting thickness to the job. Thick pours can have different limits from thin sections.
- Mixing instructions: Confirm whether the ratio is by weight or volume, then use that exact ratio and method. Ratios are not transferable between products.
- Working and cure times: Distinguish pot life from demold time and full cure. Temperature, batch size, and mold configuration can affect practical timing.
- Moisture instructions: Check requirements for both uncured handling and cured performance; they are not interchangeable.
- Mold compatibility and release: Check the mold substrate, surface preparation, and approved release agent under the conditions you will use.
- Safety: Review the current safety data sheet (SDS) for every component before mixing.
Mixing and timing examples are product-specific
Fast-setting urethanes can leave little time to mix and pour. Smooth-On’s TASK 13/14 technical bulletin (document identifier 122223DH; year not stated) gives these product-specific figures:
| Product | Mix ratio | Pot life | Demold time |
|---|---|---|---|
| TASK 13 | 100 parts A to 120 parts B by weight | 3 minutes | 20 minutes |
| TASK 14 | 100 parts A to 120 parts B by weight | 10 minutes | 45 minutes |
Smooth-On notes that timing depends on mass and mold configuration and requires an accurate digital gram scale for the by-weight ratio. The bulletin recommends clean, dry tools and containers, controlled temperature around 23 °C, and thorough scraping while mixing. It describes pressure casting as a way to reduce bubbles, while noting it is unnecessary for most applications. These TASK timings and instructions should not be applied to other urethane products.
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- Professional and Beginner Friendly - Model-Pro is a high-quality polyurethane resin kit, perfect for both professional and beginner model makers. Ideal for casting models, prototypes, figurines, and making reproductions, it's a reliable tool for your creative projects.
- User-Friendly - This 2-part epoxy resin kit is designed for ease of use, with a simple 1:1 mix ratio. No special equipment needed, just mix the two parts together and start crafting your masterpiece.
- Quick Curing Time - With Model-Pro, you can complete your projects in just 15 minutes! Its quick curing time makes it compatible with room-temperature-vulcanized or RTV silicone resin molds.
- Strong and Bubble-Free - Model Pro offers high strength and impact resistance for sturdy model production. Its low viscosity ensures easy mixing and minimal bubbles, making it a solid choice for your casting and repair needs.
- Detailed and Paintable - Our resin captures intricate details, making it perfect for art, crafts, or model-making. You can easily paint your creations, bringing them to life. Follow our blog for painting instructions.
For Sika UR303/UR402, the December 2025 product data sheet lists a 25-minute pot life at 23 °C, 16-hour demolding time, and seven-day cure time at 23 °C. It specifies keeping the material, process, and mold or master between 18–25 °C. The manufacturer says the mold or master should be clean, dry, and free of dust and grease; porous surfaces should be sealed before applying release agent. It recommends wax-based release agents and compatibility testing with coatings and release agents under actual conditions. These values and instructions apply to that formulation, not to polyurethane resin as a whole.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Moisture and safety need formulation-specific care
Uncured moisture behavior can differ from claims about a cured material. Smooth-On describes TASK 13/14 liquid urethanes as moisture-sensitive and recommends use below 50% relative humidity. Sika, by contrast, describes cured UR303/UR402 as insensitive to moisture. Those statements address different products and stages; neither supports a blanket claim about all urethanes.
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- Trusted Resin Formula - Enjoy the same exceptional quality and performance with our Amazing Casting Resin, now with a fresh identity. This resin kit maintains the same trusted formula you've always loved, perfect for all your crafting and art project.
- Detailed Resin Castings - Capture intricate details with our low viscosity, two-part resin kit. Achieve a beautiful opaque white appearance in just 2 minutes and demold after 5-10 minutes. Ideal for casting mold such as handles and knobs
- Colorful Craft Creations - Make your crafts stand out by adding colorful accents! Our casting resins can be dyed or pigmented using Alumilite dyes, mica powder. Paint it while the resin is still curing to achieve amazing colors.
- Easy Mixing Process - Our resin kit is beginner-friendly. Follow the 1:1 mix by weight or volume, pour the liquid cast materials together, and mix vigorously for about 20-30 seconds. Simple enough for any hand to handle.
- Versatile Resin Applications - Let your imagination run wild with our 2-part Amazing Casting Resin White! Use our white resin for a wide range of DIY crafting projects, from customized pens to ornamental features and more.
Smooth-On’s TASK bulletin recommends ventilation, safety glasses, long sleeves, and rubber gloves. Read the current SDS for both components of the particular resin you plan to use and follow its handling, protective-equipment, and disposal instructions. Do not assume a product is harmless because it is used for hobby casting or cures to a solid.
Quick Recap
A practical selection sequence
- Name the operation. Decide whether you are making a flexible mold, making a rigid tool, or casting a part inside an existing mold.
- Filter by intended use. Select only products whose manufacturer data explicitly covers that operation and, for casting, the intended substrate or mold material.
- Check performance and process limits. Compare hardness, flexibility, heat limits, casting thickness, pot life, demold time, and full-cure time for the actual application.
- Plan mold preparation and release. Confirm surface sealing, mold cleanliness, release-agent type, and compatibility with any coatings or mold substrate.
- Verify ratio and safety requirements. Use the stated mixing basis and equipment, then read the current SDS and prepare the workspace before opening the components.
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