how much excess plastic needed around a mold.

[JackCaper]

Hi, I am working with .060 High density styrene which I try to form over a dome shaped form. The dome is 20" in diameter and 11" high.

I was planning to use a sheet 24" square. Is this enough plastic to do a proper form or should it be larger?

[dpryor]

Jack,

I dont think the 24" will do it. There are various "rules of thumb" that range from 50% of the height to 100% of the height. If this is a gradual dome, you might get away with 50% but you're still looking at 31" min. Then you need to clamp onto the material also. If this is a 180° dome, you may need even more.

[spektr]

The question to ask is what is the required wall thickness after forming? That will determine the sheet size. If you cant handle thinning and you start with 060 you get large sheet requirements. If you up gauge to thicker material and have enough machine to do it, you could use 24 inch sheet and still have acceptable clamp areas. The real way to do this would be as a vacuum snapback process with a plenum box drawing the initial shape upward away from the tool surface. then you dump the plenum vacuum and apply vacuum to the tool to pull the preform to the tool and make the part. The advantages of this are reduced thinning of the top of the part and more uniform distribution of part thickness.

Scott Correa
SPEKTR PRODUCTS
Mount Vernon, Wa.

[kayaker43]

The rule I use is that the mold should be as far from the edge of the forming surface as it is tall at the edges. A pyramid could fill the table and your dome comes right down to the table so it can also be pretty close to the edge of the forming surface.

Having said that, a 24 x 24 sheet only yields around a 21 x 21 usable area depending on your clamp frame and clearance. Not ideal but it may work if your frame travels beyond the platen edge to make sure the plastic gets a seal. You will see thickness variations where it pulls materials from the corner but it may not matter to you.

Good luck