ME-542 Manufacturing with Non-metallic Materials

SILICONE MOLDS FOR THERMOSET PLASTIC INJECTION:
A Step by Step Methodology

List of Wrong Procedures:

1. Mix an excessive amount of silicone all at once
2. Leaving respectively big bubbles in the silicone
3. Taking out the molds before it completely cures
4. Leaving the part dirty
5. Put any fillers near the lens as the improper impression will occur

List of Proper Procedures:

1. Wear Gloves
2. Use bigger cup to mix in more easily
3. Use stronger tape to seal corners of box
4. Have about 1 inch of silicone around the part

Developing the Outer Casing

Aluminum sheet metal is used for the outer casing of the product mold as it is inexpensive. Our product mold was a rectangular car light lens, thus the outer casing should be a rectangular frame. We roughly estimated the volume to have 1 inch of space around the lens. The sheet metal was cut and folded into the shape of a rectangular box. The corners were sealed using regular masking tape.

Type of Silicone Used

The Silicone and Catalyst used is part of the MCP Vacuum Casting System VC 60T developed by MCP Systems Inc in Fairfield, Connecticut.

Measure the amount of silicone to be used. A catalyst is added to the silicone so that the mixture will dry and harden. 10 parts silicone to 1 part catalyst is the approximate ratio. Use a calibrated mixing container to measure by volume or scales to measure by weight. All measurements are approximate. Do not stir too vigorously as air might be introduced to the mixture, but instead, fold the mixture in -- like kneading dough. Make sure to stir completely so that all the silicone will catalyze and dry.

Preparation of Plastic Car Lens

Wooden sticks were attached to the car lens using super glue (be sure not to mar the lens). These sticks will support the lens and prevent it from sinking once it is placed into the silicone mixture. The sticks will also leave a porthole in the dried silicone mold once taken out. Future use of these holes would be as inlet tubes for the injection of resin into the mold. Scotch tape was used on the lens edges for a partition line. Vacuum formed plastic may also be used. Note that if vacuum forming is used, big bubbles will not be able to rise to the top but instead remain underneath the vacuum formed plastic. Use of a permanent marker was needed to mark the edge of the tape so that the partition line can be easily visible. Also make sure that the lens are free of dirt and grime.

Use of Fillers

Platinum based silicone is expensive so fillers, which are by far less expensive, were used to substitute for the silicone. Also, use colored fillers so that they can be easily seen through the translucent silicone. For this project, Polyvinyl Chloride (PVC) pellets were used as fillers.

Pouring in the Mixture

Determine where the partition line will be inside the outer casing by placing the lens (with it's wooden stick supports) inside the empty casing and marking it with a permanent marker. Take out the lens. Begin pouring a small portion of the mixture into the aluminum casing to cover the base. Add filler to the outer edges and corners of the mold while being careful not to get any in the center. Add more silicone to slightly below the partition line mark. Make sure there are no fillers across the partition plane. This will give unobstructed cutting through the partition line later on. Place the lens into the center of the mix and push slightly until the wooden supports rests on top of the outercasing. A release agent was not used since silicone does not stick to anything but silicone itself. Also make sure that no big bubbles are trapped around the partition tape. Drop the whole casing on a table several times to help the bubbles rise to the surface, then pop the big bubbles. Bubbles act as voids in the substrate so popping them is necessary. Continue pouring more silicone until silicone covers the top of the lens. Once covered, more filler can be added without risk of having any fillers on the lens.

Pressure Pot

Stop pouring once there is about 1 inch of silicone/filler over the lens. This will give a rigid frame for the lens impression. Place the unit in a pressure pot container at 30 to 40 psi. Leave it on for about 24 hours. That is the curing time for the silicone. The function of the pressure pot itself is to rid the silicone mixture of the small bubbles.

The Next Day

Once taken out of the pressure pot. Peel apart the masking tape from the corners of the outer casing, pry the sides of the casing apart and take out the dried block of silicone. Cut it apart at the partition line. An impression of the lens will be seen in the silicone mold.