Vacuum Forming Positives

I saw a different thread on this, BUT it quickly went to a "Why is recasting wrong" thread, So i'll start a new one.

I have 2 questions.

1. The material used to creat the positives for vacuum forming armor can be many differnt things. MDF,Plaster,Metal. Does anyone how long a plaster positive will last(how many forms), I don't want to make a mold from plaster only to find that I have to repour the darn thing for every form.

2.Does anyone(Prefrably someone who has done this) know the BEST material to use for the positive. I would prefer Not to use wood or metal as this is a large time consuming prosses.
Thanx in advance


(Edited) Sorry I just found another NON-Hostile thraed about this also


Well-Known Hunter
It's always good to keep a master on hand, in case a mold breaks. You alreayd know that of course. :)

Instead of HydroCal, which I've heard is expensive, I used Fix-It-All, from Home Depot, for about $6 for a 20-lb bag. The stuff dries hard as a rock.

When my molds did get chipped, and I was too lazy to pour another, I used oil-based clay to 'doctor' it up, so that I could finish a run.


Active Hunter
I don't know where you heard Hydrocal is expensive, I buy it for $27.00 per 100 lb bag, and my Ultracal-30 (another gypsium cement plaster) is about $25.00 for 100 lbs. I've used both for Vacuum-form masters and they work great, if you add Burlap or sisal to the Ultracal-30 or Hydrocal
You both, seem very knowledgable on the subject. Do you coat the positive with anything before forming, Not like a release agent but more like a durability coat or is the material that you spoke off usual durable enough to hold up?

Doesn't the clay you used to patch the form smash on you when you form it or isn't there enough pressure?

Thanx. Man this forum is great


ANH trooper

New Hunter
I found plaster to be OK to form over if there wasn't too much detail where the plastic will lock onto and sometimes chip the mould on release.I've heard of coating the mould with talc to aid release but haven't tried it persoanlly.

I now have fibreglass moulds and they are perfect.I can knock them about without having to worry about breaking them.If they did manage to chip,a little bondo/filler will do the trick.

I don't think you will have to worry about moulds withstanding too much pressure on a home made former,only pro machines will break your moulds (I think).You can always reinforce your moulds if you are concerned about them breaking.

Hope this helps,



New Hunter
Hi I am a profesional vacuum former and we use a special 2 part resin material to produce production molds . It is specialy formulated to withstand the heat and stresses put on them . Its about $80 for 1.5 gallons of volume and is sold by the pound . It can be machined carved sanded or filled to shape and lasts about forever.


Well-Known Hunter
$80??? Where I used to work we used a 2 part Urethane Resin mix. You could get a gallon *thats a gallon of resin and a gallon of catalyst* for 35 bucks. Did the EXACT same thing as you say.