Casting resin is a synthetic resin used as a casting compound or for embedding objects. It can be clear or non-clear and is processed transparently, colored, opaque or with fillers depending on the application.

Typical casting resins have viscosity, reactivity and de-aeration properties adjusted for casting. Unlike laminating resins, the focus is not on wetting reinforcing fibers, but on achieving an even, low-bubble and dimensionally stable cast.

Casting resins are mainly based on epoxy resin, polyester resin and polyurethane resin. The right choice depends on whether clear appearance, fast demolding, mechanical durability, UV stability, price or fillability is the priority.

More about resin selection can be found in the comparison Epoxy Resin or Polyester Resin – Which Resin is Right for Your Project?.

The difference between clear casting resin and classic laminating resins based on polyester resin or epoxy resin lies mainly in the application. Casting resin is usually not processed with reinforcing fibers, but poured as a mass into a mold, cavity or around an object.

Laminating resin, on the other hand, is processed with glass fiber mats, glass filament fabric or other reinforcing fibers. It must wet out fibers well and transfer loads into the fibers. Casting resin, however, must de-aerate well, cure in a controlled way and remain optically clean in clear applications.

If you want to laminate GRP, the guide Repairing GRP – Step-by-Step Guide is helpful.

When processing casting resin, maximum casting height and maximum casting volume are decisive. The casting height influences whether the resin can de-aerate safely and whether air bubbles can rise out of the casting.

The maximum casting volume is important to ensure that heat development during curing remains within the system limits. If too much resin is poured at once, the reaction can become too hot.

During curing, reactive resin systems generate heat. This heat development is called exotherm. If the exotherm becomes too high, the casting resin can yellow, become cloudy, crack or deform.

Excessive exothermic reaction can result from too large casting volumes, too high layer thicknesses, too high processing temperatures or incorrect hardener addition. Excessive hardener addition can also cause yellowing and defects in suitable systems.

If epoxy resin does not cure properly, the guide Epoxy Resin Does Not Harden – Causes, Rescue and Prevention can help.

For clear casting resins, optical quality is especially important. The resin should be clear, low-bubble, UV-stabilized and suitable for the planned casting height. Especially with large casting volumes and high casting heights, the resin must be adjusted so that it remains crystal clear and does not develop yellowing or clouding.

Epoxy casting resins are often used for high-quality clear castings, for example river tables, wood casting, embeddings and decorative objects. A long open time, low reactivity, good de-aeration and suitable UV stabilization are decisive.

For table projects, the guide Building a River Table with Epoxy Resin is also helpful.

Another important casting resin is polyurethane casting resin. Polyurethane fast casting resins play an important role when components need to be molded or cast quickly. They are preferably processed in silicone molds.

Polyurethane fast casting resins are often used for bodies and components where a crystal-clear appearance is not the main priority. Examples include climbing holds, figures, technical molded parts or sample parts.

Polyurethane resins can usually be filled well with fillers such as quartz sand. This can reduce material costs and influence certain component properties.

For good results, resin and hardener should be dosed exactly according to the manufacturer’s instructions and mixed thoroughly. With epoxy and polyurethane casting resins, the mixing ratio must be strictly followed. With polyester casting resin, the hardener quantity depends on the system and must also be dosed accurately.

Step-by-Step Processing

1. Clean, dry and prepare the mold or casting area.
2. Apply release agent if necessary or fix the object in place.
3. Measure resin and hardener exactly according to the manufacturer’s instructions.
4. Mix slowly and thoroughly without introducing unnecessary air.
5. For clear casts, pour slowly and allow air bubbles to rise.
6. Observe maximum casting height and casting volume.
7. Allow the casting to cure protected from dust and at a stable temperature.

Casting resins are used in creative, decorative and technical areas. Depending on resin base and formulation, they are suitable for clear design objects, fast moldings, embeddings or durable technical casts.

The most common problems with casting resin are air bubbles, yellowing, clouding, cracks, sticky areas or overheating. They are usually caused by incorrect casting height, too large resin volume, wrong mixing ratio or unsuitable temperature.

© 2026 - SEO, structure and content revision - www.fiberglas-discount.de