Today I had another go at making acrylic sensor domes. I've given up on vacforming as acrylic is not malleable enough to form properly this way with the equipment I have.
Instead, I decided to try press forming. This process involves heating the acrylic to make it soft, then pushing a tool into the sheet to make the desired shape.
As a test I made a simple cylinder out of two layers of 12mm MDF that I cut out with a holesaw. I fastened them both with a woodscrew to a base:
I fastened an a6 sheet of 2mm acrylic in the clamping plate I made a few weeks ago, and heated it under the grill. This was my first error, the grill is too fierce and the acrylic started to bubble. I tried it anyway as it had gone soft.
The plan was to use a piece of MDF left over from cutting the cylinder. This is about 2mm larger diameter than the cylinder and I used it to force the soft acrylic sheet over the tool.
Here's a picture after the operation. The MDF sheet used to force the acrylic is on the right.
It worked quite well, but as I had not put a draft taper on the tool, I broke the acrylic trying to remove it:
It occurred to me that the metal frame was not really necessary for this method of molding, so I made tried again with a new sheet of acrylic. This time heating it in the oven (gas mark 9, 5 minutes at the top) placed on the MDF sheet used to force it over the tool. I also smeared some vaseline as a release agent onto the cylinder.
Here's me waiting for the acrylic to harden:
With the forcing plate removed:
It was still difficult to remove the formed piece afterwards, but it did come off:
I then used the same sized holesaw to cut the piece out from underneath:
Much better than my vacforming attempt. Here's what I learned:
1. Draft angle is essential.
2. Use oven at high heat and time the heating process.
3. Don't use a grill!
4. Forcing plate makes all the difference.
I'm going to try again soon with a better tool. I think that will result in a dome that I can use.