Thomas Performance Parts Vacuum Chamber
Chamber material and design fundamentals
Basic design outlines
Lets start with the Acrylic. This material is not suited for use in Vacuum Chamber Lid construction if you are using Alcohol to process, clean the lid or Chamber. The process is called crazing. Degassing exposes the lid to solvents and chemicals that will soften Acrylic over time. The problem is you don’t know when the Acrylic has been compromised enough to fail. This can cause catastrophic failure. Acrylic also doesn’t flex and this is not a good characteristic for Vacuum Chamber lids. Acrylic is a very brittle plastic not suited for threading or machining. The threads will crystallize in the plastic and fail. When machined the heat created by machining will cause the Acrylic to crack over time under stress. This will also happen when using a Laser to cut Acrylic.
Polycarbonate: This material is very well suited for use in Vacuum Chamber Lids. Polycarbonate is a soft plastic. It will scratch if cleaned with ruff materials. It should be cleaned with up to 50% Alcohol. It resists most chemical solvents. It flexes with under the stress load. This keeps it from shattering under load. It should not be threaded as the threads will tear and not hold position. It can be machined with good success. It can’t be Laser cut with a good finished edge.
Glass: Glass is not suitable for Vacuum Chamber Lids unless it is used in a Vacuum Oven where it is fixed into a door window. Because of the non-flexing properties of glass it will shatter easily. Tempered glass had a 5-10% defect rate as stated by the Industry Standards Instutute. This precludes it from lid use, as it is too dangerous.
Basic Lid Design: The design is of great importance no matter what material is used. You can have a bad design that turns a good material in to a useless product.
First lets look at why a lid design fails. Some use the wrong material for the job but lets use the same material too show design failure. Polycarbonate is the best choice. Our field studies have shown that the major causes of lid failure are holes in the lid. The lid flexes under Vacuum Load this creates tremendous pressures on the lid. The underside referred to as the inside of the Vacuum Chamber stretches. This is known as the tension side. The outside of the lid referred to as the compression side is pushed together. When you drill a hole in the lid you radically compromise the structure. It creates weak points that greatly increase a chance for failure. The lid stretches under load and the weak points fail. This causes spider cracking that over time the lid will leak. Placement of holes is also important as holes placed closer to the center of the where the great flexing pressure is will fail faster. This is why we changed our design on our Second Generation Chambers to have no holes in the lid.
As you can see by the diagram the flex under load tries to rip the lid in half. The arrows indicate force direction. Now add holes to the ripping force creating even weaker areas. The lid has no chance of surviving the force.