Posted by Alumilite on Sep 2nd 2021


Need to replace a tail light but lack the tools?

This How To process will walk you through step by step the procedure for making a two piece mold of a
tail light lens without the assistance of vacuum.

Products Used


Step 1 Choose a material

Once you've chosen a lens to reproduce, choose a material to make your mold base and box out of. Here you see us using some inexpensive corrugated plastic. It is easy to cut with an Excel knife and can be used for the base as well as the sides if you so choose.

Step 2 Make a base

Using Alumilite's Modeling Clay, make a base for the lens to sit on.

Step 3 Trim

Trim the base of clay approximately a half inch around the lens.

Step 4 Smooth the clay

Use Alumilite's sculpting tools to smooth out the surface of the clay and especially the clay that touches the lens. The clay that touches the part will determine the parting line so it is highly recommended to spend the time to make it is as clean as possible to minimize the amount of flash your cast piece will have.

Step 5 Mold box

We chose to use a plastic container that leaves 1/4" clearance between the container and the lens as our mold box. In this picture you can see that we cut the bottom of the container off so we can pour through that hole. Keep your mold box (whatever you choose to use to construct it) tight to your piece to decrease the amount of rubber needed to make the mold.

Step 6 Press into clay

Once we have cut the container's bottom off, we flip the container over and press it into the clay around the lens leaving a 1/4" gap between the lens and the mold box.

Step 7 Trim excess

Push the container down into the clay as far as you can. Then trim the excess clay around the mold
container and continue to press the container down to the piece of corrugated plastic (mold base).

Step 8 Clean

Before pouring the rubber, make sure to clean any fingerprints, smudges, or clay off of the lens.

Step 9 Calculate measurements

Next you will need to calculate how much rubber you need to fill the mold box minus the volume the lens. To calculate the volume of a round mold box you need to know the radius (which is half the diameter or the length across the box). Visit the Material Calculator page on this site to calculate the amount of silicone and resin you will need for your part.

Step 10 Calculate measurements pt.2

In this case, the diameter is 3.4". So the radius is half of 3.4" which is 1.7". Multiply 1.7 x 1.7 (radius
squared) x 3.14 (pie) x 2 (height). This will give you the total volume of the mold box. You then need to estimate and subtract the volume of the lense itself. The above equation equals approx 18 cubic inches. Before pouring the rubber, make sure to clean any fingerprints, smudges, or clay off of the lens. There are 21 cubic inches in 1 lb of rubber. So once we subtract the area of the lens sticking up, we estimate needing approximately 1/2 lb of rubber to make the first half of the mold.

Step 11 Calculate measurements pt.3

We've decided to use Alumilite High Strength 2 silicone mold making rubber because of its phenominal physical properties (extremely high tear strength) as well as its low viscosity which doesn't require a vacuum to achieve a perfectly good bubble free mold.

Step 12 Calculate measurements pt.4

The instructions tell us to measure 8 cups of base and 4 scoops of catalyst to equal the amount of rubber we require.

Step 13 Remember to work in a easy to clean space

We recommend working over an area that is easy to clean up. Most of the mold making and casting
materials stain fabric and carpet so be careful not to splash or spill the materials.

Step 14 Ready to mix

Once you've added the catalyst you are now ready to mix. The work time of the High Strength 2 is approx 45 minutes and the demold time is 12-18 hours.

Step 15 Try to be neat

Taking the time to clean up the sides of the cup before mixing will save you even more clean up later. Try to be as neat as possible.

Step 16 Mix thoroughly

Mix the material thoroughly until absolutely no swirls are remaining. Once you think you have it mixed enough, spend an extra minute or two to mix it one more time.

Step 17 Let cure

Allow the rubber to cure overnight at room temperature before beginning to work on the second half of your lens mold.

Step 18 Trim the bottom

Using the rubber as a guide, trim the bottom of the mold box so it sits nice and flat.

Step 19 Remove the clay

Begin removing all of the clay WITHOUT removing the lens or the mold from the mold box. Simply
remove the clay and nothing else.

Step 20 Trim rubber

Once you have removed every single spec of clay, trim off the rubber that may have flowed underneath the lens with a sharp knife.

Step 21 Cut locators

Using an Excel knife, cut some locators on the outside of the silicone rubber to the mold box. Put your
knife at an angle and cut a wedge shape locator approximatley an inch long. Do not allow the knife to get to the original part. You can see one that is already cut at approximately the 11:00 position. It is slightly more shady and will allow the second half of the rubber mold to flow in that channel and locate the two halves.

Step 22 Use mold release

Once you have cut locators in the first half of the rubber mold, use Alumilite's Rubber to Rubber mold
release or Alumilite's UMR to make sure the second layer of rubber will not bond to the first. The mold release only needs to be applied to the rubber and not to the part to prevent witness marks on the lens itself.

Step 23 Wait between coats

If you are using Alumilite's Rubber to Rubber Mold Release, we recommend waiting 5-10 minutes
between coats and putting at least 2 if not 3 individual coats on to make sure the rubber does not stick. Also, make sure to shake the bottle well before using

Step 24 Calculate measurements again

After calculating the volume required for the second half of the mold the same way we did for the first we realize we need approximately the same amount of rubber for the second half of the mold. Since we already used a 1/2 lb out of the 1 lb kit, we can simply dump the remaining catalyst in the base container of the 1 lb High Strength 2 kit.

Step 25 Mix thoroughly

Mix the rubber thoroughly. Be sure to scrape the sides and bottom extremely well. This is very important when mixing in a container with a lip. The lip will limit your access to the side walls so make sure to scrape them vertically as well as horizontally.

Step 26 Paint on the rubber

Once the rubber has been mixed well, use a disposable paint or acetone brush to paint on a thin layer of rubber onto the original lens to make sure the rubber is forced into the detailed cracks and crevices of the inside of the lens. This is a great thing to do when you do not have the ability to vacuum the rubber.


Step 27 Pour remaining

Once you've painted in a skin coat, simply pour the remaining rubber into the mold.

Step 28 Remove from mold box

Allow the rubber to cure overnight and then remove the mold from the mold box.

Step 29 Begin separating

Once you've removed the mold box, find the parting line (area where the two halves of the mold meet) and begin separating the two halves. Do not use a knife. The mold halves should separate using just your fingers.

Step 30 Admire your work

This picture shows you the two halves of the rubber mold with the lens still inside. If you look closely on the outside of the mold edge, you can see the locators that will line up your mold halves in the correct position.

Finished Project

Remove the original lens and you are now ready to start pouring parts in your high tear strength silicone rubber mold that will produce exact replicas of your original.