Mold making with fiber composites

 Polyester resin and fiberglass mat  

With a little craftsmanship, making a mold is not that difficult. A mold should be built under optimal conditions, i.e., the material and room temperature should be around 20°C. A dust-free environment and possibly an extraction system should be available. You will also need the necessary materials. The following is required for a GRP mold:

Materials:

Viapal Mold Building Gelcoat UP 935 B
Polyester resin DIEPAL 2140
Glass fiber mat 150g/m²
Glass fiber mat 300g/m²
Release wax
Film release agent PVA varnish
Acetone

Tools and aids:

Digital scale
Brush and deaerating roller
Mixing cup and stirring rods
Knife and scissors
Dosing syringe
Disposable gloves
Respiratory protection

 Mold types  

The negative mold is for smooth outer surfaces, e.g., visible parts such as model making or car bodies (hoods).
The positive mold is for smooth inner surfaces, e.g., containers or pools.

 Master model  

In these instructions, we assume that a master model is available.

You can also make a master model out of wood, plaster, modeling clay, aluminum, or fiberglass. If you are using a master model made of absorbent material, such as plaster, you can create a smooth surface by coating or sealing the mold with a topcoat resin or varnish. It is important that the master model is made very cleanly and accurately, as this saves a lot of time and rework on the finished parts at the end.

 Separation board joints / gap covers  

You can work with filler or plasticine.
Roll the plasticine into small beads, press them into the gap and then carefully remove the excess with a blade or spatula.
Mix the filler with a little hardener and apply it to the gap.
Using less hardener gives you time to cut off the filler with a blade or spatula before it hardens.

For an optimal separating layer, we recommend the following structure:

Layer 1:
Apply standard separating wax with a brush, cotton cloth, or spray gun (spray pressure of 2.5-3.5 bar and a nozzle of 1.2-1.5 mm), leave to work for 2 minutes, and polish. For hard-to-reach areas, we recommend release spray wax.

Layer 2:
The PVA varnish can be applied with a fine-pored sponge, brush, or spray gun (spray pressure of approx. 3–3.5 bar and a nozzle of 1–1.5 mm). The drying time is approx. 20 min. (at 20°C), approx. 10 min. with the spray gun. High humidity can prolong the drying time.

 Laminating  

You should have all materials ready and cut or tear the fiberglass mats to size.
To calculate how much gelcoat is needed, you can weigh a piece of fiberglass mat cut to size for the mold.

Example: The cut piece of fiberglass mat 300g/m² weighs 200 grams.
Calculate the surface area: 200g : 300g/m² = 0.67m²
Recommended application quantity of mold building gelcoat 500-850g/m².
Calculate the amount of resin: 850g/m² x 0.67m² = 569.5g
With a hardener addition of 2%, this would be (rounded down):
550g mold building gelcoat and 11g hardener

Stir the mold building gelcoat UP 935 B with the hardener and transfer it to another container. This prevents resin that has not been thoroughly mixed from remaining in the first container at the edges and bottom.

Apply the mixture to the original mold, always stroking in one direction if possible to avoid air pockets. Allow the resin to gel until the surface is still slightly sticky but no longer stringy. It is best to test this with your finger (wear gloves!). To achieve an even surface, it is advisable to apply two coats, allowing the first coat to gel until it passes the finger test.

After the second coat has gelled, continue laminating wet on wet! Mix DIEPAL polyester resin with 1.5% hardener. For the amount of resin, you can use the rule of thumb: glass weight (total) x 2.5 = amount of resin. Apply a layer of resin to the mold, insert the first layer of 150 g/m² fiberglass mat, and wet the fabric. Use a brush to smooth and dab the air out of the laminate, working from the center to the edges.

If the fiberglass layer appears transparent, it is optimal; if white spots are visible, some resin should be applied with a brush. Alternatively, a de-airing roller can also be used. Continue working with a 300 g/m² fiberglass mat and repeat the steps until the penultimate layer. Approximately 4-6 layers of 300g/m² fiberglass mat should be sufficient for a smaller mold. The number of layers depends on the subsequent requirements for the mold. A mold should always be laminated thicker than the final part so that the drying part cannot warp the mold due to shrinkage.
Alternatively, you can replace 300 g/m² fiberglass mats with 450 g/m² or 600 g/m² mats to achieve a certain material thickness more quickly.
To quickly produce thick laminates with high mechanical values, you can also work with complex fabrics, biaxial fabrics, roving fabrics, etc. Always alternate between roving fabrics and glass fiber mats to avoid glass pockets.
Glass fiber mats should always be impregnated from the bottom up. Make sure that there is always enough resin on the surface and that the mat is impregnated after the bottom layer has been inserted. The last layer should be impregnated with little resin and have a matte sheen after drying. If the surface shines, too much resin has been applied. You can counteract this with a tear-off fabric. After removing the tear-off fabric, the component is completely dust-free, clean, and has an even, rough surface structure, providing an optimal substrate for further laminating, sealing, or bonding. You save yourself the hassle of sanding and there is no need to remove sanding dust. No tear-off fabric was used for this mold.

The mold should be left to harden for at least 24 hours at 25°C.
The mold should be able to be removed from the original mold with the aid of plastic or wooden wedges.

After demolding, you can trim the mold and polish it to a high gloss if necessary. Silicone-free polishing agents should be used for this.
Small imperfections in the surface can be filled with a little mold building gelcoat and smoothed with a strip of adhesive tape.

Other defects can also be repaired with sandpaper (600 to 2000 grit).
When sanding and polishing, take care not to generate localized heat.

 Storage  

Seal opened containers tightly with the original lid.
Store in a cool, dry place. Can be stored for at least 6 months under optimal conditions.

 Cleaning of work equipment  

Cured resins can only be cleaned mechanically, for example by sanding!   Acetone can be used to remove uncured resin residues from tools, which must then be allowed to air sufficiently to prevent the cleaner from entering new containers!

Instructions in PDF format for downloading or printing can be found here: Mold making with polyester resin

In addition, you can quickly and easily find information about frequently asked questions here: Frequently asked questions

The information contained in these instructions is based on careful research. It is provided for informational purposes only and does not exempt the user from conducting their own tests for the intended purposes and from checking for any infringement of third-party property rights. The information is non-binding and does not constitute a guarantee of properties within the meaning of any laws. No liability is assumed for the accuracy and completeness of the information.