Plastics gys.. I need your input

[Thee Starry™]

We do 5 axis work on plastic parts. Currently we are using Thermo Cast to make our vacuum fixtures. The company, up till now, has poured the fixtures. We pour thermo cast into a formed part, build a base and go from there. The problem is that, as you can expect, the fixtures end up warped and twisted due to the heat generated from the cast material and the weight. We are thinking about machining our fixtures now. My question is, what do you guys do for your fixtures. What works well, what doesn't. Etc.. Etc...

Any advice or tips would be great.

[Mike Gates from Barefoot CNC]

Back when I was doing them, we sent a part out and had a fiberglass fixture made, then we attached a base that mounted it to the machine and plummed it for vacuum lines to hold the part. You could also look at laying up a mdf or foam core and then machining it and then drilling vacuum holes and plumming it. It's been a few years so some of the younger gen may have some better ideas.

[TR Mfg.]

I don't know if it would work for your parts but we make most of our fixtures from aluminum and use Mitee-Bite clamps or threaded holes in the part if it has any.

[Jim at Gentex]

We also use cast vacuum fixtures to trim polycarbonate lenses and other kinds of parts. Our practice is to cast the fixture slightly oversize, then machine to nominal. This takes care of any warping, twisting, shrinking, expanding, etc., caused by the thermal reaction of the casting material.

 

We also do 3D printed fixtures using the same approach. Depending upon finished size, we typically print the fixture with 0.05" to 0.06" additional material, then machine it to nominal.

 

HTH

 

edit:

I should also mention that we have a fully staffed model shop here, so we can either make the fixture castings from an existing part, or the model makers can make them from scratch.

[KalCam]

If its a new part I will machine fixture usually from Renshape or similar. So nice that everything is aligned perfect for adding o-rings, vac channels, revisions, ect.

 

I have done some large low volume part fixtures from MDF to save $$ but you need to seal it up to get a good hold.

[Thee Starry™]

I had the same thought. Pour the fixture, then machine to correct dims. We do parts that are generally between 1'x 1' x 1' cube to 5' x 5' x 3' size so the renshape can get pretty pricey for that scale. I did think about MDF due to the cost. Our largest part is 7' wide, 3' tall and 11' long and our smallest is probably 6" x 3" x 12". Generally average size part for us is about 2' x'2 x' 2' or so which means $$$$ in fixturing. Anyone know of a thermo cast, or poured casting material that generates less heat and is lighter weight? Some of our larger parts have trim fixtures that weigh 4-500lbs do to the cast material weight.

[KalCam]

How bout a laminating paste system. Ren 8169 or ? Check out FreemanSupply.com

 

Instead of pouring material, you apply a surface coat then pad in a layer of laminating paste. You will need a Hobart type mixer to mix this stuff.

[JohnW@Kauffman]

Michael,

 

We do trimming of plastic parts here, and we pour cast all our holding fixtures using the formed part to cast from. We use Repro 83 from Freeman Supply, brace the formed part, and block out large segments of the area to be cast, leaving anywhere from 1/2" to as much as 2 inches for the Repro. By keeping the casting material thinner, heat and shrinkage are minimal.

 

HTH

[Ployd]

Starry,

 

Check out our web page We use a steel frame mounted to foam with about 0.2" of fiberglass on top. Mdf is o.k. but it is heavy. Our molds and patterns are pretty lightweight. They bolt nicely to the table.

[Thee Starry™]

Hey SC, How does that whole deal work? You apply the surface coat, like RenGel 1124, let harden then fill with RenLam 569? How thick for both? Any tips?

[KalCam]

Michael,

 

Support your formed part, apply a release and yes paint "we use cheap brushes and cut down bristles to make brush stiffer" on a surface coat. The surface coat will be thin. Let coat almost harden but still tacky. Then pat on a layer 3/8" to 1/2" of laminating paste. Paste is like thick cookie dough. Add plywood ribbing as needed.

 

I only worked with this stuff on a small project to see how it worked, quickly found out that a big mixer is needed to mix it on a large scale.

 

The heat generated was minimal, dimensionaly accurate, light and very strong.

 

Try contacting your pattern materials supplier and ask then give you some sample material to try out.

[Thee Starry™]

Thank you...