This is part of a project to make and improved enclosure for my all sky meteor camera and domes for other instruments that look at the sky. Looking around on the WEB I found some YouTube videos showing making acrylic domes. Most heated just the acrylic sheet and then placed it in the blowing fixture. I did not want to invest in some rapid clamping system so I decided to heat the sheet already mounted in the fixture. This also has the advantage that you don’t have to quite as speedy when inflating the dome.
The base of the fixture is a 12 inch square steel plate 1/4 inch thick. It is fitted with a 1/4 inch NPT pipe and air hose coupler. On the top is and adjustable gauge to control the height of the dome. This should be set to the radius of the dome. The dome is inflated until it almost touches the gauge.
To seal the acrylic sheet so that it can be inflated a silicone mat is used. These mats are used by crafters as a work surface because glue and paint do not stick and they are heat resistant.
I made the clamp ring out of 1/2 inch Baltic birch plywood. I cut a circular hole the size of the dome. The clamp ring also serves as a drilling template to drill the mounting holes in the flange of the dome.
The inflation hose attaches to my shop air supply and has a valve to control the air going into the dome. There is no easy way to let air out so you have to be careful to not over inflate the dome.
The recommended temperature to heat the plastic is 325F. Oven mitts or welding gloves are needed to handle the fixture when taking it out of the oven. I have a kitchen stove in my shop to use for projects such as this. After removing the heated fixture from the oven the clamps need to be retightened because the plastic softens and everything expands when heated. The dome is slowly inflated until it almost touches the height gauge. If there is a small leak the air has to be carefully adjusted until the plastic cools off enough to become stiff. The hot steel base plate will keep the flange of the dome soft longer so the clamps should not be removed until the base plate is cool to prevent warping of the flange.
Here is the first dome I made. It is over inflated. The gauge arm left an impression in the top of the dome and it is not a good hemisphere. It still needs the holes drilled in the flange and the flange trimmed to size.
The dome is part of a larger project to make an improved enclosure for my All Sky Meteor Camera. Expect more articles on the rest of this project.