---
product_id: 458820556
title: "Smooth-Cast Onyx Fast - Ultra-Black Urethane Resin (Gallon Unit)"
brand: "smooth-on"
price: "2303 Lei"
currency: RON
in_stock: true
reviews_count: 12
category: "Smooth On"
url: https://www.desertcart.ro/products/458820556-smooth-cast-onyx-fast-ultra-black-urethane-resin-gallon-unit
store_origin: RO
region: Romania
---

# Fast 10-15 min cure time Higher heat resistance Ultra-deep black, no pigment needed Smooth-Cast Onyx Fast - Ultra-Black Urethane Resin (Gallon Unit)

**Brand:** smooth-on
**Price:** 2303 Lei
**Availability:** ✅ In Stock

## Summary

> 🖤 Cast in Black, Cure in a Flash — Own the Onyx Edge!

## Quick Answers

- **What is this?** Smooth-Cast Onyx Fast - Ultra-Black Urethane Resin (Gallon Unit) by smooth-on
- **How much does it cost?** 2303 Lei with free shipping
- **Is it available?** Yes, in stock and ready to ship
- **Where can I buy it?** [www.desertcart.ro](https://www.desertcart.ro/products/458820556-smooth-cast-onyx-fast-ultra-black-urethane-resin-gallon-unit)

## Best For

- smooth-on enthusiasts

## Why This Product

- Trusted smooth-on brand quality
- Free international shipping included
- Worldwide delivery with tracking
- 15-day hassle-free returns

## Key Features

- • **Mercury-Free & Safe:** Work confidently with a non-toxic formula that’s mercury-free, prioritizing your health and workspace safety.
- • **Lightning-Fast Curing:** Get from mix to solid in just 10-15 minutes—perfect for rapid prototyping and tight deadlines.
- • **Pure Onyx Black Finish:** Achieve a sleek, ultra-black cast without adding pigments, polished to a high-gloss shine effortlessly.
- • **Superior Heat Resistance:** Outperforms competitors with enhanced heat tolerance, ensuring durability under demanding conditions.
- • **User-Friendly 1:1 Mix Ratio:** Simplify your workflow with an easy, equal-part mix and low viscosity for smooth pouring every time.

## Overview

Smooth-Cast Onyx Fast is a premium, mercury-free urethane resin that cures rapidly at room temperature to a deep, ultra-black solid with Shore 80D hardness. Its 1:1 mix ratio and low viscosity make it easy to use, while offering superior heat resistance and a high-gloss finish without added pigments. Ideal for professionals demanding speed, quality, and safety in black resin casting.

## Description

Smooth-Cast ONYX is a mercury-free urethane resin that cures quickly at room temperature to a deep black, solid plastic. ONYX offers the convenience of a 1A:1B by volume mix ratio and has a very low viscosity, so it is easy to mix and pour. POT LIFE: 2.5 mins; CURE TIME: 10-15 mins. , SHORE HARDNESS: 80D and offers higher physical properties and higher heat resistance vs. other general purpose resins. Consult the Technical Bulletin for more information. Read the MSDS sheet before using. Visit us at: www.smooth-on.com

Review: Really good - There is no shortage of different PU based casting resins... this is the best I have tried so far. You have to work very fast... you got 2 minutes only. The material gets very hard, yet not overly brittle. Unfortunately it is one of the most expensive resin of its kind. Here a few tips: Use rubber gloves, this is a messy procedure... Measure into two separate cups. If you use one cup only you need more time, which you do not have. Pour cup 1 into 2, stirr 5 time, then pour 2 back into one and stirr 5 times, then pour into your mold and get it into the pressure vessel ASAP. Pouring from one cup into another reduces the chance that you have unmixed resin in your cup.
Review: Solid stuff - demands respect - You really have to pre-mix the ingredients well or you wont get a good solidification. You have to be skilled at working quickly and mixing for the FULL amt of time as described by the manufacturer. Do not start with your final project. Start with tests. Plan sacrifice at least 2x 50mL castings toward testing and getting to know how the material behaves. People complain about it expiring quickly -- smoothon sells a little can of heavy/dry inert gas that you can spray over the bottles and store them (probably a mix of argon or something). I cant comment too much but used the gas and no problems so far.

## Features

- Convenient if you desire a black casting, adding color pigment is not necessary (can be polished and buffed to a high gloss shine)
- Unlike other black resins, ONYX does not contain mercury
- ONYX offers higher heat resistance vs. competitive black resins
- ONYX is less expensive vs. other black resins
- Shore 80D

## Technical Specifications

| Specification | Value |
|---------------|-------|
| ASIN | B0B2YMG461 |
| Best Sellers Rank | #392,957 in Health & Household ( See Top 100 in Health & Household ) #2,647 in Trash Bags |
| Customer Reviews | 4.4 4.4 out of 5 stars (322) |
| Date First Available | February 24, 2014 |
| Is Discontinued By Manufacturer | No |
| Manufacturer | Smooth-On, Inc. |

## Images

![Smooth-Cast Onyx Fast - Ultra-Black Urethane Resin (Gallon Unit) - Image 1](https://m.media-amazon.com/images/I/41jWWhVGtGL.jpg)

## Available Options

This product comes in different **Size** options.

## Questions & Answers

**Q: Is this product and the 80D hardness suitable for casting automotive / tractor steering suspension bushings?**
A: for suspension bushings, I think you would be looking at something much more like a 60-80 Shore A, instead of 80 Shore D. Here's a chart: https://goo.gl/images/Yo0hnp

**Q: how much is there?**
A: 1 lb each

**Q: Can this be used to make  longboard wheels?**
A: If a proper mold is used, this material will harden into any desired shape. Although the shrinkage is minimal, I would suspect shrinkage should be taken into account when molding something as precise as a wheel. Whether the material holds up to the rigors of longboarding is questionable. The material is quite hard when fully cured. I haven't looked at the spec sheet in quite some time, but the cured product definitely feels harder than skateboarding wheels, and considerably more brittle. I would suspect the finished wheel would slide quite a bit. Hope this helps.

**Q: Is the final mold heavy? (Ex: mold size 11x6x2)**
A: If the final casting is 132 cubic inches - as you indicate - the piece would weigh 4.76 pounds

## Customer Reviews

### ⭐⭐⭐⭐⭐ Really good
*by D***L on January 7, 2025*

There is no shortage of different PU based casting resins... this is the best I have tried so far. You have to work very fast... you got 2 minutes only. The material gets very hard, yet not overly brittle. Unfortunately it is one of the most expensive resin of its kind. Here a few tips: Use rubber gloves, this is a messy procedure... Measure into two separate cups. If you use one cup only you need more time, which you do not have. Pour cup 1 into 2, stirr 5 time, then pour 2 back into one and stirr 5 times, then pour into your mold and get it into the pressure vessel ASAP. Pouring from one cup into another reduces the chance that you have unmixed resin in your cup.

### ⭐⭐⭐⭐⭐ Solid stuff - demands respect
*by A***R on February 1, 2020*

You really have to pre-mix the ingredients well or you wont get a good solidification. You have to be skilled at working quickly and mixing for the FULL amt of time as described by the manufacturer. Do not start with your final project. Start with tests. Plan sacrifice at least 2x 50mL castings toward testing and getting to know how the material behaves. People complain about it expiring quickly -- smoothon sells a little can of heavy/dry inert gas that you can spray over the bottles and store them (probably a mix of argon or something). I cant comment too much but used the gas and no problems so far.

### ⭐⭐⭐⭐⭐ Great product
*by B***K on June 10, 2019*

Works very well, I have ordered several times and haven't run into to many problems with this product, sets very fast so getting it to evenly coat has been a challenge, but working with smaller batches seems to help with that problem. Overall great product!

## Frequently Bought Together

- Smooth-Cast Onyx Fast - Ultra-Black Urethane Resin (Gallon Unit)
- Smooth-Cast 300 - Bright White, Ultra Low Viscosity Liquid Plastic - Gallon Unit
- Rebound 25 - Brushable Platinum Silicone Rubber - Gallon Unit

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*Product available on Desertcart Romania*
*Store origin: RO*
*Last updated: 2026-03-10*