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    Making fibreglass moulds

    Making fibreglass moulds

    Moulds can be classified into two main groups: male and female moulds. In the case of male moulds, glassfibre is laminated on their outer surface, while for female moulds, it is applied on the inner surface. The desired surface finish, whether smooth or textured, will be on the side closest to the mould.

    Female moulds are ideal for producing items like car body parts and boat hulls, where the finished surface needs to be on the outside. On the other hand, male moulds are used for products such as baths and shower trays, where the finished surface needs to be on the inside.

    Before proceeding with the mould-making process, it is crucial to ensure optimal workshop conditions. The temperature in the workshop should ideally be maintained between 18ΒΊC and 22ΒΊC. It is essential to avoid direct sunlight, and any workshop heating should be directed away from the work area to prevent drafts, whether hot or cold.

    Additionally, maintaining normal and constant humidity levels is essential for achieving successful results in the mould-making process. Creating a controlled environment will contribute to the overall quality and precision of your work.

    The plug/ master mouldΒ 

    To create a mould, you'll need a pattern or former known as a "plug," which serves as an exact replica of the finished item. While existing items like motorbike panels or canoes can be used as plugs, it's essential to avoid copyright issues. In most cases, you'll have to build the plug from scratch. The material for the plug can vary, but it must be rigid, accurate, dimensionally stable, and securely placed on a solid foundation.

    To ensure easy removal of the mould, it's necessary to incorporate a slight taper on the plug's side walls. For large moulds, a rigid wood frame covered with hardboard, plywood, MDF, clay, or plaster can be used. When working as toolmakers, we usually opt for wood. Before applying the primer, ensure you fill in any grain, holes, dents, and joints, as these defects will be visible on the finished mould. Any nails should be hammered down using a punch and countersunk, and screws or nails should be covered using polyester body filler. The plug's surface must be smooth and free from blemishes.

    To further prepare the plug, seal the wood, especially when using MDF, with a hard varnish like sand and sealer or shellac varnish. Sand it down with 60-grit sandpaper for a smooth finish. Next, apply several coats of Durabuild surface primer, allowing each coat to cure before reapplying. For the best results, Durabuild is typically applied using a spray gun, and you may need to add Durabuild thinners to achieve the right consistency for your gun (not exceeding 10% by volume). Add 2% catalyst and ensure a thorough mix before applying the Durabuild. Alternatively, you can use a brush to apply it without thinning, but this method may leave brush marks that will require sanding.

    Proper maintenance of your spray gun after using Durabuild surface primer is crucial for its optimal performance and longevity. Follow these steps to ensure your gun stays in top shape:

    1. Flush with Acetone: Immediately after using Durabuild, thoroughly flush the spray gun with acetone before the primer has a chance to cure. It's advisable to flush the gun through twice to ensure all traces of the primer are removed. Additionally, for small parts of the gun, you can disassemble them and soak them in acetone to clean thoroughly.

    2. Flush with Warm Water: After the acetone flush, follow up by flushing the spray gun with warm water. This step is essential to prevent any residue or uncured primer from hardening and potentially clogging up your gun.

    Tip: For small projects where time is a concern, brushing on Durabuild can be a practical alternative to spraying. This approach helps minimize the preparation and cleaning time required for the spray gun, making it a convenient choice for certain applications.

    By adhering to these maintenance practices, you'll ensure your spray gun remains in excellent condition, allowing you to achieve consistent and professional results in your projects.

    To achieve a flawless result in your mould making process, follow these steps:

    1. Smoothing the Durabuild: Once the Durabuild has cured overnight, use fine grades of wet and dry sandpaper to smooth the surface. Polish it to a high sheen, as shown in (pic 2). To ensure no residue remains from the polish, wash the plug with warm soapy water. Next, treat it with the necessary release agents. We recommend using 12 coats of a solid carnauba-based wax, applying each coat at minimum one-hour intervals.

    2. Important Note: Before using Durabuild, carefully read and follow the provided instructions to ensure optimal results.

    3. Preparing Plugs: If your plugs are made from porous materials like plaster or wood, it is essential to seal them properly before applying release agents. Apply 2 - 3 coats of shellac to create a smooth surface for the lay-up process.

    The GRP Mould:

    1. Preparing the Mould: Once the plug is finished and treated with release agents or wax, you can begin applying the laminate layers. Use the standard method for any other fiberglass lay-up. However, keep in mind that the mould should be considerably thicker and more rigid than the finished item. A general rule of thumb is to make the mould three times the thickness of the finished part.

    2. Applying Gelcoat: Start by applying a layer of catalyzed polyester gelcoat in one continuous film, approximately 0.6mm thick (Pic 3). You may choose to add a second layer of gelcoat if any further smoothing is needed.

    3. Mirror the Plug Finish: Remember, the finish you achieve on the plug will be mirrored on your mould. So, if needed, you can rework the mould surface using wet and dry paper and then polish it to a high sheen. Aim for the best possible finish on the plug, as this will reduce the amount of rework required on the finished mould. Ideally, you want a high gloss finish on your plug to minimize additional polishing. Using porous materials, regardless of how well they are sealed, may result in a substandard mould finish, even after polishing.

    By carefully following these steps and paying attention to the surface preparation of your plug and mould, you can achieve excellent results in your mould making process and ensure a smooth and high-quality finish on your final products.

    The GRP Mould:

    After finishing and treating the plug with release agents or wax, you can start applying the laminate layers using the standard method as with any other fiberglass lay-up. The key difference is that the mould must be considerably thicker and more rigid than the finished item. As a rule of thumb, making the mould three times the thickness of the finished part is a good indication.

    Begin by applying a layer of catalyzed polyester gelcoat in one continuous film, approximately 0.6mm thick (Pic 3). Applying a second layer of gelcoat is an option if any further smoothing is needed.

    When crafting the plug, keep in mind that the finish you achieve will be mirrored on your mould. If necessary, you can rework the mould surface using wet and dry paper and polish it to a high sheen. However, always aim for the best finish on the plug, as less rubbing down on the finished mould will lead to better performance. An ideal high gloss finish on your plug will ensure minimal rework. It's important to avoid using porous materials, even if they are well-sealed, as they may result in a substandard mould finish, even after polishing.


    For instances where you have a poor surface on your plug, let the gelcoat cure to finger tackiness (check the deepest part of the mould) after approximately 2 – 3 hours. Then apply one layer of fibreglass matting, preferably a light mat like 300g csm, as it's easier to roll out any air pockets. Thoroughly consolidate this layer with a metal roller (Pic 4).

    Once the first layer has cured, sand off any nibs and wipe it lightly with acetone on a clean, dye-free cotton cloth. You can then apply the rest of the layers, whether it's 450g or 600g csm. Avoid applying more than four layers at a time to prevent generating too much heat and causing shrinkage and other issues. While standard Polyester laminating resin can be used, for large projects and best results, opt for a low shrinkage tooling resin like Optimold. If using Optimold, read all technical data and seek advice on how to use the product. Optimold is an excellent choice for moulds producing products such as automotive panels, where distortion or shrinkage of the mould is not acceptable.

    On larger projects, you can add reinforcements such as angle iron, aluminium, chipboard, or plywood to the mould for extra strength. Bond these reinforcements particularly on flanges where constant stress might arise from levering the finished items from the mould. Adding ribs to the main body by laminating over a former of cardboard, wood, or polyurethane foam can provide additional rigidity. However, make sure the mould has cured first, as bonding ribs can cause sink marks in the mould surface, which are difficult to remove.

    If the plug has any undercuts, they will be challenging (if not impossible) to remove unless the mould is made in two halves. For example, on a boat hull, a split can be made along the keel line. These sections should have flanges that are at least 80mm wide and 50% thicker than the rest of the mould, allowing these flanges to be bolted together.

    Leave the mould to cure for up to two weeks at room temperature, and post-curing is desirable but sometimes not practical, especially if using low shrinkage tooling resin (2 – 3 days). Releasing a mould too early can cause distortion. When the mould is released from the plug, any imperfections can be filled or smoothed down. If the plug was properly finished, very little work should be needed.

    Treat the mould with no less than 12 coats of TR-102 wax, with 1-hour intervals between each application. Before making any parts, it is advisable to break in the tool first. To do this:

    1. Coat the mould with PVA release agent, applying it lightly by hand with a sponge is usually sufficient.

    2. Once dry, apply a layer of gel coat with 3% catalyst to ensure quick gelling.

    3. Once the gelcoat has cured, apply one layer of 450csm with resin. To achieve quick curing, add 2 – 3 % catalyst.

    4. After 3 – 4 hours, peel off this layer as soon as it cures. Failing to break in the mould this way may cause the styrene from the fiberglass laminate to attack the waxed surface of the mould, resulting in the gelcoat shearing off or the part sticking to the mould. Wash off the PVA release agent with warm soapy water, then apply an additional 5 waxes, leaving at least 1 hour between each application. After this treatment, the mould is ready to use.

    Remember, it's always better to be safe than sorry! Taking meticulous care in each step will lead to successful and high-quality moulds for your fiberglass products.