best method for making a mold for big part, low quantities?

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#1

What is the best method to make a mold for a large part but it will only be used on a small quantity?

I want to sculpt my own car fenders. ill add/subtract material to my current fenders to modify them then use it as a plug. how ever to go through roughly 2 gallons of resin and 7-8 layers of 1.5 chop strand, to make a sturdy mold, Is there a method i can use to save cost since i only need to make one?

possibly use less layers and add foam ribs to make it more sturdy? Glassing in wood braces/ mesh screen and using 3-4 layers of fg? any suggestions?

#2

What surface finish do you wish? And what surface finish is the plug?

If both should be good enough for sanding and painting, I recommend to paint the plug, and make a polyester mould (after the 2K PU paint cured for 1 week, test test test).

Build up the polyester slowly, up to 4 layers, then add some bracing by using large blobs of MS polymer putty to glue a wooden brace on, which can also act to make the thing sit level on your workbench. (nothing fancy). This should keep it in shape for 1 part. (and very probably a lot more.)

Another technique, which is more difficult here, is to CNC the mould directly in a hard quality (5 lbs/ft3) EPS foam, then vacuum a flexible vacuum film in.

#3

can use tooling dough. Gel coat, lay a couple layers of glass, then tooling dough, then a couple more layers of glass.

#4

Polydur or Laminierkeramik. I still havnet decided which one to take but both resist 120-150ºC and are cheap and dont distort. If you ask please update us on prices of Polydur.

#5

Plaser. Ultracal 30 or 60 would work nicely. U.s.g gupsun has an inexpesive system with zero shrink. Best thing about plaster is cheap, zero(ish) shrink, can be applied against painted parts using vasoline as a release agent.

#6

I’m looking for a surface finish that thats equivelent to 220 grit? or better… basically the part is expected to be primered and painted but doesnt have to come out of the mold ready to paint.

Just to make myself clear, my main goal is cheapest method since i will only use the mold once or twice.

Kinda sounds like the normal process no? I’m looking to save some cost since the mold isnt going to be a production mold…

THIS is very interesting… Can you elaborate more on this method?

I assume:
wax plug
gel coat
2-3 layers of FG
then use the plaster?

#7

Google “chavant rayite mdm” step by step photos. Can be done with ultracal 30 or 60carbed also. Just lather on petrolium jelly on, brush or spray with cup gun a surfacecoat. cure a little, then plater with burlap or skrim or chopped glass mixed in.dimensionaly perfect but heavy. Pop of fender then seal with zinnser bullseye schellac(plaster absorbs the jelly). Schellac seals from the moisture from tool then applie filler if needed.wax then pva.make part. Ezpz! Usedto make molds off of oem motorcycle bodypanels using this way. Paint safe!

#8

Oh ya, after the shellac you can finish any way you wish. Urathane primers or clear, featherfill or duratec polyester surfacing techniques. Gelcoat also. Plaster is a useful technique to have in your arsenal but you probably should just go the fibergass route, cleaner and close on price.

#9

Can’t you just take your master fender and lay your fender up on top of it? Think of it as a male mold rather than a female one. You’ll have to do a little work on the outside surface but it would be faster than building a mold for one stinking part.

#10

Wyo- Yes but then i will have all that build up material underneath making it much heavier. and quiet frankly it would then be a hybrid of fiberglass, steel, and wood…

I think i may be able to get away with using 4 layers of 1.5 matt with some bracing. I’ll give that a try…

#11

You would apply release agent to your existing fender+modifications and layup on top of it and use it like a male mold rather than pull a mold from it.

#12

ohh, gotcha, but that would through the “fitment” off by a few… mm? would that be negligible? or so little that it wouldnt matter?

#13

If the part is going to be painted you would fill the weave in with a mix of epoxy and micro-spheres and some sanding. You could also add a layer of light 2 ounce glass as the last layer. This will give you some sanding room and something that would span the weave pattern of the previous heavier layer.

It’s not ideal, but it would be faster and cheaper than building a mold.

#14

thanks for the advice Wyo!

#15

Another technique, which is more difficult here, is to CNC the mould directly in a hard quality (5 lbs/ft3) EPS foam, then vacuum a flexible vacuum film in.

Herman, could you give some more hints on this flexible film? Is there anything apart of mylar or PTFE which would addapt to compounded curvatures easyly?

#16

Pva film would work best.

#17

Is there any specific supplier/brand to do some more research on it? Assuming a properly finished mould, what’s the finishing like on the part with this sort of films?

Many thanks!

Jordi