When it comes to producing accurate and exact duplicates of a product, GRP moulds are a common choice. Manufactured from hard-wearing material, it is used to produce a great number of products including PPE, Piping, Anti-Slip surfaces and Electronic enclosure. Our comprehensive guide covers everything you need to know about this versatile material, allowing you to discover whether it is ideally suited to your project.

What is GRP?

So, let’s start with the basics. What exactly is GRP? Well, the acronym stands for Glass Reinforced Plastic and it is commonly known as fibreglass or composite plastic. Similar to other conventional thermoplastics, it is a versatile material chosen for a variety of reasons. GRP is made from strands of glass which are also known as fibres. These are woven together, creating a ‘fabric’. GRP moulds are produced when fibres are placed in or on the shape of a pattern or tool to produce an exact replica.

It is made from two layers – a hard-wearing gel coat and a reinforced glass fibre material (either strand matting or glass fabric). We tend to use 4 layers of woven mat for the production of our moulds and tend to favour vinyl ester resin as it is stronger than polyester. Both the gel coat and resin for the mat are mixed with a catalyst which starts the chemical reaction and the curing process.

Why is GRP such a popular material?

There are a number of key benefits that explain why GRP moulds are an ever-increasingly popular choice. They are robust and extremely durable meaning that many units can be cast from the same mould. They have a high strength-to-weight ratio and are resistant to both corrosion and chemical exposure. GRP is easy to shape and can be used to produce a wide variety of products and it does not burn like other plastics. When moulded, it is extremely durable and offers brilliant value for money. The strong surface won’t wear and, therefore, minimises the need for costly repairs or replacements.

It can also be used across various applications, making it a flexible option to suit your industry.

What are GRP moulds used for?

As we have mentioned above, the uses for GRP are endless. It can be used to form part of a mould or the entire piece, depending on the qualities needed. Some of the most popular uses for GRP moulds include:

  • Ornamental precast.
  • Spheres for pier caps.
  • Bespoke designer furniture.
  • Precast concrete street furniture.
  • Pizza ovens (commercial and residential).
  • Wine Storage Bins.
  • Separators for use with large diameter drainage pipes.
  • Commercial railway components.
  • Railway platform copings.
  • Boats.
  • Shower Trays.
  • Bathtubs.

This list is in no way exhaustive. GRP is constantly being trialled and used for various industries, allowing it’s key benefits to be manipulated to suit any need.

How are GRP moulds produced?

There are a number of ways that GRP products are produced, depending on the equipment available and the client’s requirements. We will cover a range of them here to give an overview.


This process is used to manufacture tubes, rods and other items such as piping. It allows continuous lengths of the same structural shape to be produced, with the option to add in cross-sections. The fibres are pulled through a pultrusion machine and left to harden through dies. The structural shapes produced are made from densely packed fibreglass which gives an exceptional strength across the entire product.

Hand lay-up

Here, glass fibre mats are ‘impregnated’ with liquid polyester. It is an entirely hands-on process, where layers are built up through a hand lay-up application. Some manufacturers will also offer a spray system offering, where layers are sprayed on. From all of the different methods outlined, hand lay-up is the method of choice for our business.

Compression Moulding

One of the most common techniques, compression moulding is not a continuous process. Blends or compounds are placed into a mould and heated, under pressure. The combination of heat and pressure causes a reaction, allowing it to form the final product. This process is much slower than others and is normally used for large sheet mouldings instead of individual products.

Injection Moulding

Probably the term you’ve heard of the most, injection moulding is used widely to produce electrical components. The process is very similar to the ones used to produce thermoplastics, however, the fibres are not allowed to cool within the machine. The heated material is injected into the mould quickly, to ensure the mould accurate copies.

There are other options too, including filament winding, reaction injection moulding, compression moulding and resin transfer moulding too.

What materials are required?

Depending on the process of GRP chosen, the equipment will also vary. Most GRP laminates can work with a brush, paddle roller, mixing bucket, mixing sticks, sandpaper and a catalyse measuring tool.

Here at 3D Pattern and Mouldmakers, we have produced GRP Mouldings for a wide variety of clients across many industries. Generally, we use a gel coat and light tissue before applying 4 layers of fibre matting and resin. Finally, to reinforce the product, we use plywood. In addition to this, we work with a team of highly talented pattern makers who manufacture the vast majority of our tooling in-house, allowing us to fit bespoke needs exactly.

Founded in 2000, our business has continued to grow as more and more businesses discover the benefits of GRP Moulds. Our team is always on hand to provide advice and recommendations to better your product development needs. Get in contact today if you have a project you would like to discuss.