Shrinkage...

[Treadhead]

When doing resin casting, how do you avoid/minimize shrinkage?

[Umi_Ryuzuki]

I choose a resin that is rated for minimal shrinkage.

Tap Plastics Clear cast is horrible that way, and will not cure in silicone molds. When it does shrink and pulls away from the sides of a mold, the tacky surface leaves stringy strands for uncured resin across the gap.
These then fall back against the casting making an unruly mess.
I had some type one phasers that were ruined this way.

[Vince Hoffmann]

Also, avoid casting large solid parts. The more mass you cast, the more heat it creates (ruining your mold faster) and the more shrinkage you will have.

[Chacal]

To keep the resin from heating up too much, mix slightly less catalyst (hardener) to it. I don't have a chart or anything, but the heat "bottled up" inside a larger, thicker part will be more than enough to cure it, so why use more catalyst than needed? A little less hardener will let bigger parts cure as well as the reccomended amount. On the same token, a little bit more catalyst is reccomended for teeny tiny parts, because they lose heat too fast. Mind you, I'm talking 10-20% less for BIG parts (of about 2 pounds – I cast this – the 2nd place solid with some 20% less than the instructions for the resin – its intended use is in fiberglass, so it's thin layers) and some 30-40% more for tiny parts (a 1/72 pilot, for example).

[Treadhead]

To keep the resin from heating up too much, mix slightly less catalyst (hardener) to it. I don't have a chart or anything, but the heat "bottled up" inside a larger, thicker part will be more than enough to cure it, so why use more catalyst than needed? A little less hardener will let bigger parts cure as well as the reccomended amount. On the same token, a little bit more catalyst is reccomended for teeny tiny parts, because they lose heat too fast. Mind you, I'm talking 10-20% less for BIG parts (of about 2 pounds – I cast this – the 2nd place solid with some 20% less than the instructions for the resin – its intended use is in fiberglass, so it's thin layers) and some 30-40% more for tiny parts (a 1/72 pilot, for example).

That's quite a nice job you did I wish my stuff would come out as nice. Tell me, when using the 50/50 PartA and PartB resins, is Part B typically the catalyst? I'm thinking that it is, but not sure...

[Darkov]

"But the Pool was Cold......."



Sorry Couldn't resist

[Umi_Ryuzuki]

If you mix less than the recomended catalyst with polyurethane resins, any
resin that remains uncured is likely to weep out of the casting at its first exposure to heat. Sunlight, luxo lamp, hot water...

With Polyester resins, you can reduce the catylist, I woudn't recommend it with polyurethane mixes.

[Treadhead]

"But the Pool was Cold......."



Sorry Couldn't resist

Is that you, Costanza?

[modelnutz]

When dimentions are really critical....using slow cure urethane helps.
What really helps is using platinum cure silicones which tend to have minimal shrinkage.

As for polyester resins ( the standard fberglassing resin ) they shrink a TON!

Just a few cents worth for ya'

Modelnutz 8)