# Can plaster be used to make molds for styrene plastic parts?



## Chuck_P.R. (Jun 8, 2003)

Can plaster be used to make molds for styrene plastic parts?

Or would the heat of the reaction be too intense on the plastic?

Can multiple pours be done in order to keep the weight on the plastic
from being too great?

I'm asking as I an considering molding a large surface area plastic
part(the top of a TOS E saucer that's about 8-9 inches in diameter) and then using the plaster mold to make a styrene vacuform copy.


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## terryr (Feb 11, 2001)

It should be okay as far as heat. But it doesn't make a very detailed surface. And it's crumbly, so it sticks to most things used for copies.

But if seal it with several coats of clear it should be okay for vacuforming. 

Many years ago I made a fiberglass copy of a saucer that way.


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## Chuck_P.R. (Jun 8, 2003)

Thanks for the advice.

Can't I use something like petroleum jelly for mold release?

I've done RTV rubber and bought expensive mold release spray
before and it didn't seem to work as well as warm petroleum jelly.

On RTV I've read that the wrong mold release can damage the mold
over time, don't know about using it with plaster though.

Plus, I've got a ton of plaster and don't think I'll need to make too
many pulls anyway.


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## SteveR (Aug 7, 2005)

Might talcum powder work as a mold release?


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## BWolfe (Sep 24, 2013)

Chuck_P.R. said:


> Can plaster be used to make molds for styrene plastic parts?
> 
> Or would the heat of the reaction be too intense on the plastic?
> 
> ...


Use Durham's water putty, it dries much harder than plaster and holds detail far better. You will need to apply some kind of lubricant to the surface of the part to keep the stuff from sticking to it.


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## Chuck_P.R. (Jun 8, 2003)

SteveR said:


> Might talcum powder work as a mold release?


Not sure. I've only had experience with RTV, which I don't think will be rigid
enough for vacuforming.

Though I'll be the first to admit that I may be wrong about that too.

RTV might withstand the 400 degree temperature necessary to melt styrene,
then again it might warp during the suction process.

Not only that, you'd need a ton of very expensive RTV to do a proper mold.


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## Chuck_P.R. (Jun 8, 2003)

BWolfe said:


> Use Durham's water putty, it dries much harder than plaster and holds detail far better. You will need to apply some kind of lubricant to the surface of the part to keep the stuff from sticking to it.


I'll check into that. Thanks!

Is it very expensive? I'd need a good deal of it.


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## kdaracal (Jan 24, 2009)

I want to pour Plaster of Paris into a mold of the Capitol Building pillars, then break them up for an effect of the Earth vs the Flying Saucers ship hitting the Capitol stairs of my Glencoe /Skyhook build.

_Hmmm..... _


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## BWolfe (Sep 24, 2013)

Chuck_P.R. said:


> I'll check into that. Thanks!
> 
> Is it very expensive? I'd need a good deal of it.


It is not much more expensive than plaster of paris. I looked on Home Depot's website and a 1.1 lb can is less than $2.00. I have actually used this stuff to make small parts, easier to work with in that application than resin.


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## phicks (Nov 5, 2002)

kdaracal said:


> I want to pour Plaster of Paris into a mold of the Capitol Building pillars, then break them up for an effect of the Earth vs the Flying Saucers ship hitting the Capitol stairs of my Glencoe /Skyhook build.
> 
> _Hmmm..... _


You can get miniature plastic pillars at cake decorating stores BTW. Should make it easy to make your molds.


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## Steve H (Feb 8, 2009)

BWolfe said:


> It is not much more expensive than plaster of paris. I looked on Home Depot's website and a 1.1 lb can is less than $2.00. I have actually used this stuff to make small parts, easier to work with in that application than resin.


I will also attest to the usefulness of that water putty.

Back in the day when I made prop guns for costumes I would mix this stuff up to a much more liquid than the standard 'putty' mix called for and pour it into the body and grip parts of the toy gun I had modified. It would dry rock-solid and give a nice heft to the prop. I don't recall much shrinkage when it dried. Nothing rattled, at least.


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## kdaracal (Jan 24, 2009)

phicks said:


> You can get miniature plastic pillars at cake decorating stores BTW. Should make it easy to make your molds.


I want to replicate the plastic pillars, (they're quite small at "N" scale). I figure if I use Plaster of Paris, I can get a good broken effect where the saucer hits the left Capitol exterior steps:











I picked up some 2-part silicon mold agent at Hobby Lobby to cast the kit pillars in. They are smaller around than a pencil. More like a smaller fast food straw, but thicker than a coffee stir.


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## CaptCBoard (Aug 3, 2002)

Plaster will not work for the job you want to do. Plaster actually shrinks a tiny amount and will hold on to you part, meaning the plaster will break before you get the part out. On the other hand, hydrocal will work, but it still won't do a perfect job. Since you are going to have to do a bit of hand-finishing anyway, you might get a decent part out of the hydrocal tool. My suggestion is to first do a thin coat of hydrocal, then do a 'backing coat' using hydrocal and some kind of fiber, for reinforcement. In a pinch, air-conditioning filter will work, but you really have to work the hydrocal into it. Fortunately, hydrocal has a long setting time.

You will have to let the hydrocal tool dry for about a week, to be sure all the water has evaporated. Do not try to bake the water out in an oven, the tool will warp. The water needs to be gone so that when the hot plastic hits the surface of the tool, steam is not generated. This will cause lifts in the part and you'll have a lot of filling and sanding to do to get the smooth surface back.

It will be less stressful if you drill the vents in the tool while it is still wet. You don't want to make them more than 1/16" in diameter or your part will have little bumps you'll need to sand out. You might get them anyway if you are using plastic sheet .040 or thinner. One thing to note here is you will not have any problem getting the plastic part out of the tool. With this 'negative' tool, you are pulling the plastic into a depression instead of a 'positive' tool, where the plastic is pulled over a tool (also called a buck, which is the term for a shape that will have plastic draped over it.) When styrene is heated, it shrinks as it cools. If the plastic was going over a buck, it would be really hard to get the plastic off without cutting the side to relieve the tension. Pulled into this negative tool, the styrene pulls itself away from the cavity, so it should just pop out. The other advantage to a negative tool is you can change the thickness of the styrene and not have the part get correspondingly bigger, because the additional thickness is on the inside of the part.

The better idea for this vacuform tool is to use a material called 1630, made by BJB Enterprises. Its not too expensive (like $80 for a 2-gal. kit) and you get enough to do a lot of jobs like this. This is a filled urethane resin and is hard as rock-- or harder. Very, very tough material. It is also very thick, so it really isn't good for most casting purposes. I use it to make protection masters because it doesn't shrink (enough to worry about...) If it sits unmixed for any time at all (like a couple of days), the filler settles to the bottom of the cans and it a real son-of-a-gun to mix after that. I take mine to a paint store and have them put it in a paint shaker. After a minute or two, everything is just fine. Don't get me wrong, you can use this stuff for casting, you just can't pour it in and walk away. You just need to be able to work it into the mold with a brush, to get rid of any voids. Fortunately, the stuff takes about 10 minutes to gel, so making castings is definitely possible.

Scott


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## Chuck_P.R. (Jun 8, 2003)

CaptCBoard said:


> Plaster will not work for the job you want to do. Plaster actually shrinks a tiny amount and will hold on to you part, meaning the plaster will break before you get the part out. On the other hand, hydrocal will work, but it still won't do a perfect job. Since you are going to have to do a bit of hand-finishing anyway, you might get a decent part out of the hydrocal tool. My suggestion is to first do a thin coat of hydrocal, then do a 'backing coat' using hydrocal and some kind of fiber, for reinforcement. In a pinch, air-conditioning filter will work, but you really have to work the hydrocal into it. Fortunately, hydrocal has a long setting time.
> 
> You will have to let the hydrocal tool dry for about a week, to be sure all the water has evaporated. Do not try to bake the water out in an oven, the tool will warp. The water needs to be gone so that when the hot plastic hits the surface of the tool, steam is not generated. This will cause lifts in the part and you'll have a lot of filling and sanding to do to get the smooth surface back.
> 
> ...


Fantastic advice, Scott!

I should have been more specific in saying the plaster I have is Hydrocal.
I've got a couple of half gallon cartons and a big ole' 20(or 25 maybe?) pound
bag as well.

I was thinking about trying to find a brush in resin material as well.

With the 1630 product you mention, will it be as sag free over time as
styrene?

Would making certain areas thicker in some place help or hurt?

I'm going to accuratize the saucer top from the Revell Germany 1/600th
TOS E and am considering making a mold of it afterwards.

I did the same thing years ago to make a solid cutaway 22" TOS E 
after creating the fused and finished part.

Didn't have much luck finding a resin that didn't sag, at least at anything
near the thinness of the styrene part, and thicker parts were just too
heavy.


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## Y3a (Jan 18, 2001)

kdaracal said:


> I want to replicate the plastic pillars, (they're quite small at "N" scale). I figure if I use Plaster of Paris, I can get a good broken effect where the saucer hits the left Capitol exterior steps:
> 
> 
> 
> ...


What size is your saucer? I have one that is about 6-7 inches across.


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## djnick66 (May 2, 2008)

Note to vacuuform something that large you will need holes drilled through the item itself. Plaster may crumble. Thats why larger vac kits have those little pips on the surface you have to cut off... they are artifacts of the vent holes needed on the master to help draw air out from around a big part.


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## kdaracal (Jan 24, 2009)

Y3a said:


> What size is your saucer? I have one that is about 6-7 inches across.


The smaller Skyhook version. About 3 inches, I guess.


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## CaptCBoard (Aug 3, 2002)

Chuck--

Resin sagging? At the thickness of a styrene part? Never ran across that problem and I've made some very thin parts.

Are you modifying a styrene part or replacing the part with what you want to vacuform? If you are modifying an existing part, making a rubber mold of the modified part is the way to go. If you are making a completely new part from scratch, using a vacuformed blank as the starting point, you still need to make a rubber mold of it once it is finished.

As for the 1630 material I mentioned, the way it would be used is to simply prep the part you want to duplicate by vacuforming, build a box around it so you have about 2 inches of clearance on all sides and then fill the box with the mixed 1630. No brushing needed. You will want to put a light coat of vaseline on the styrene part, so thin that it just makes the part shiny. After the 1630 has set (overnight is a good idea) you will have to drill a small hole into the part you just molded, so that you just pierce the part. You will then need to use a dusting nozzle (attached to a hose from a compressor) to force air into this hole. The styrene part should jump out of the hardened 1630. Follow this procedure if you decide to use the hydrocal.

I don't know the source of your vacuforming device, but this part should be made of .080 or thicker plastic. If you can't pull something that thick, then use thinner and make a couple of parts. Trim only one of the parts. Cut it into pie-wedges. Glue the pie wedges into the untrimmed part. You have cut the wedges because keeping them as a whole piece will not allow you to put that part snugly into the hero part. The last wedge will have to be trimmed to fit as closely as possible to the others. The end result of this is a vacuformed styrene part that is twice as thick. This will be essential if you plan to make copies of the finished replacement part you are making.

Scott


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