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Topic: HELP! Making fiberglass parts inside of plaster molds.  (Read 465 times)
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Beta Sigma Phi
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« on: January 05, 2009 04:12:58 PM »

My husband is making fiberglass motorcycle parts and he is using a plaster mold. The mold was made by pouring plaster over the outside of the original piece that was coated in corn oil and then popped out. Worked great, inside is perfectly smooth after spraying it with a simple clear coat.

Now... He tried to make his first fiberglass part by laying all of the fabric soaked in resin inside the plaster mold. As he took the part carefully out it stuck and ripped apart the inside of the mold. Can anyone give him any tips? Maybe a different type of mold release? He originally spent a lot of money on a little can of silicone mold release but the corn oil worked better. Any ideas or help at all?

.... I would love to tell him and his buddy that a bunch of women came up with the solution that they themselves have been trying to figure out for almost a month.

« Reply #1 on: February 03, 2009 05:18:35 PM »

In my sculpture class, we use Murphy's oil soap.  Brush a thin coat onto the plaster at full concentration.  I don't know how this would work with resin, but it might be worth a shot.

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« Reply #2 on: February 16, 2009 09:44:02 AM »

I think the problem is that the plaster is absorbing the oil, especially as the chemical reaction to cure the resin is exothermic (giving out heat) this would lower the viscosity of the oil making it thinner so it gets absorbed more easily. If I understand your original post properly, the first thing that was used as a mould, was the original part, this is plastic and non-porous. The plaster however is porous so that is where your problem is.

Plaster can be used for mould making but I would choose a mould release that has a much higher viscosity, ie a wax of some kind, a couple of layers of wax to polish the inside of the mould will help a lot. There are specialised waxes for this job, I dont know how well a regular floor wax would work, but its worth a try, give it a couple of coats and make sure that the mould is very smooth before you start, ie sand back the plaster before applying the wax, if there is any surface roughness.

Hope it helps

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