Moulding small plastic components Part 1

In which we make worms from Plasticene and indulge in some microwave abuse.

When I was restoring my Giulietta Spider there were a couple of items I needed which I couldn’t locate anywhere so I had to figure out a way of making them. The trickiest was the cam cover bolts which had cast onto the heads a black plastic knurled surround with  Alfa Romeo script on the top face, and the other was the number plate light.

What was left of the number plate light was demolished when the car rolled off the trailer and hit the garage wall, but I was fortunate in being able to borrow an original one to copy.

 To make moulds of the parts, I found in my local craft warehouse some new material which looks like rubber but is actually PVC. This is blue in colour and can be melted by putting it in the microwave oven for a couple of minutes, when it takes on the consistency of thick cream.

I tackled the number plate light lens first and started by gluing it down onto a piece of stiff card board. I then constructed a sort of dam round it by folding a 2” wide strip of card into a box and Selotaping it to the base. After heating the PVC I then poured it over the lens to a depth of about 1” and left it for a few minutes to cool. I then pealed off the card but left the lens embedded in the mould.

The next stage was to create a mould of the inside face of the lens, so the block of PVC complete with original lens was inverted and another strip of card was folded around the block of rubber and secured to it with Selotape. I cut it wide enough to leave the top edge standing above the face by about 1”.

To create a path for the resin to enter the mould and the air to get out, I rolled up a small amount of Plasticine into a worm about 1/8” diameter and cut it into four pieces, which I then very carefully stood on the four corners of the inverted lens.

I then coated the surface of the mould with some release agent which I had, but if you don’t have any, grease the surface lightly with Vaseline so you can separate the two halves. Next job was to melt some more PVC and pour it into the mould. When it was set, I separated the two halves, removed the lens and the Plasticene plugs, and was then in possession of a finished mould.

From the same place I bought the PVC, I bought a small can of casting resin and a small bottle of hardener. This stuff is completely clear and is fairly slow setting so you don’t get a problem with heat build up and discolouration.

To use the mould, I first Selotaped the two halves together making sure they were properly lined up. I then mixed a small amount of resin and a few drops of hardener in a small dish, being careful not to introduce any air bubbles.

 I then poured the resin down one of the holes left by the Plasticene plugs, just a bit at a time and tilting the mould back and forth to make sure the resin completely filled the mould and that all the air was excluded. I finally poured the last of the resin in until it came back up all the other holes.

After leaving for a few hours to set I split the mould to reveal a perfect reproduction of the original lens even down to the “Hela” name and serial numbers. The four extra branches created by the pouring holes were carefully cut off and all the edges given a final fettling and it was ready to use.

Casting the cam cover bolt surrounds was a bit more complicated and I will cover that next time.

Re-making these broken cam cover bolt

mouldings will be covered next time