What will you make?

You will make a two-part silicone mold of an intact light lens cover and cast an exact copy with it in clear, transparently colored epoxy resin. The mold consists of two halves that you cast one after the other, separated by a layer of plasticine with keys (registration locks). Once the mold is ready, you can cast copies with it again and again.

Required materials

Preparation

Before you make the mold, it is important that the original is clean. Remove dirt, dust, and unwanted imperfections. The mold picks up every small detail: glossy parts return glossy, matte parts stay matte. Because the silicone does not adhere to the cover in this case, you do not need a release agent on the original.

Step-by-step guide

Assess and clean the original

Look at the cover you want to replicate. If the defective specimen is beyond repair, use an intact cover from the other side (fitting mirrored), a repaired specimen, or a borrowed cover as a model. Clean the model thoroughly.

Lay out the cleaned model

Lay out the cleaned, intact cover as a model. Check that the surface is free of dust and grease, so that the mold gives a perfect impression later.

Roll out plasticine

Knead a quantity of plasticine firmly until it becomes a bit softer. Roll the clay out to a pancake of ± 0.5–1 cm thick and cut the edges straight into a rectangle. This plasticine layer will later form the separation between the two mold halves.

Seal screw holes and apply a venting channel

Plug the screw holes with a thin layer of clay. Apply venting channels if necessary so that the mold can fill well later and air can escape.

💡 Tip: A venting channel between two highest points prevents air from being trapped in hard-to-reach spots in the mold.

Press the cover into the plasticine and make keys

Press the cover into the plasticine, with the side where the pouring opening will later be exactly flush with the edge of the plasticine. Do not press too deep, but far enough that no silicone can run underneath. Then apply keys (registration locks) around the model. The back of a pen or pencil is a handy tool for this.

Place the formwork

Place the formwork around the plasticine and make sure all cracks and seams are sealed. Silicone is very fluid: the smallest holes are big enough for it to flow away, so seal everything well. Make sure the top of the cover fits well against the formwork and that the formwork extends above the model.

Mix and pour the first half

Weigh out the A-component and add the B-component in the correct ratio. Mix very well with a stirring stick and go along all edges and corners so that everything is evenly mixed. Pour the silicone with a thin stream from one point, so that air bubbles can escape during the fall and the air is pushed ahead of the silicone.

💡 Tip: First pour the mixed compound into a second container and stir again. A vacuum pump is not a requirement but improves quality. For a bubble-free contact layer, you can first brush the model with a brush and only then fill up.

⏱️ Mind the potlife: Wacker Elastosil Vario 15 Normal has a potlife of ± 150 minutes and a curing time of ± 6 hours at room temperature. Process the silicone within the potlife.

Turn over and remove plasticine

After the first half has cured, turn the whole thing over and remove the plasticine. The edges of the mold may contain skins and films; cut these away neatly with a sharp knife.

Apply release paste and pour the second half

Coat the silicone of the first mold half well with release paste (or Vaseline), so that the second half does not adhere to it later. Make sure every part is coated. Place the formwork back around the first mold half, again watch for sealed seams, and pour the second half in exactly the same way as the first. Then leave the silicone alone for ± 20 minutes to cure further.

⚠️ Don't forget: Without release paste, the two mold halves adhere to each other and you will not be able to open the mold later. Coat everywhere, including in the crevices.

Open the mold and cast the copy

Open up the mold and remove the original. The mold is now ready. Close the mold again, put two elastic bands around it, and place it level. Fill the mold with clear epoxy resin, optionally with a transparent dye as desired. For a fog light you can choose a lighter color, but you can also closely match the original color.

Additional photos

End result: the cast cover, mounted and as good as new

Frequently asked questions

Why a two-part mold and not a one-part one?

A light lens cover has a hollow, three-dimensional shape that you must be able to demold from two sides. A two-part mold encloses the model completely and can be opened up after curing, so that you remove the original undamaged and then cast new copies again and again.

What are the keys in the plasticine for?

The keys (registration locks) ensure that the two mold halves fit together exactly later. Without these recesses, the halves can shift, causing a seam or misalignment in your copy.

Why must I use release paste between the halves?

Silicone adheres to silicone. Without release paste (or Vaseline) on the first mold half, the second cast half adheres to it and you will not be able to open the mold. Therefore coat every part carefully.

Which color of epoxy do I choose?

That depends on the cover. For a red taillight you approach the original color with a red transparent dye; for a fog light you can choose a lighter shade. Always use a transparent dye so that the light shines through it well.