Tips for applying PVA release agent on moulds

[axolotyl]

Its only half metalwork but I'm pretty sure some of you guys will know a
bit about this!


In point form:


1) Casting epoxy into a CNC machined mould.

2) Having problems with getting a consistent surface finish.

3) We are spraying the PVA release agent using a small touch up spray gun.

4) Sometimes we get small runs.

5) Sometimes part finish is matte due to the PVA drying too quickly.




Being a novice I think these problems are pretty much down to
application technique.

Any tips on what we can try to make the process of applying the PVA more
forgiving?



Alternatively would making the next mould from Teflon be worth trying?

I don't know if the machining finish would be smooth enough but my guess
is it could be used without a release agent.



Any ideas or experience?

[BobH]

A long, long time ago, I worked at a place that made parts for medical
equipment from glass filled epoxy. They used silicone rubber for the
molds with mold release compound from a spray can. The finish out of the
mold was pretty good, but the parts needed to be polished up with a
buffing wheel to look consistent. The molds were a pain to make because
you had to de-gas the silicone after mixing in a vacuum chamber, but
they lasted pretty well.

BobH

[Carl Ijames]

On 06/17/2016 11:18 PM, axolotyl wrote:
> Its only half metalwork but I'm pretty sure some of you guys will know a
> bit about this!
>
>
> In point form:
>
>
> 1) Casting epoxy into a CNC machined mould.
>
> 2) Having problems with getting a consistent surface finish.
>
> 3) We are spraying the PVA release agent using a small touch up spray gun.
>
> 4) Sometimes we get small runs.
>
> 5) Sometimes part finish is matte due to the PVA drying too quickly.
>
>
>
>
> Being a novice I think these problems are pretty much down to
> application technique.
>
> Any tips on what we can try to make the process of applying the PVA more
> forgiving?
>
>
>
> Alternatively would making the next mould from Teflon be worth trying?
>
> I don't know if the machining finish would be smooth enough but my guess
> is it could be used without a release agent.
>
>
>
> Any ideas or experience?

I've not used PVA mold release but I have used aerosol spray-on silicone
mold release from www.smoothon.com in aluminum molds to cast their
polyurethane, usually shore 70A rubber (part PMC780). They recommend
spraying the mold, then brushing with a small paint brush to get a uniform
film, then letting it sit for 30 minutes or more to flow out and dry. I was
casting small stuff, 1-3" max dimension, so used those disposable "battery"
brushes, 1/4" or 3/8" wide. If I did all of that the surface finish was
usually very good, as good as the mold. On prototype parts that I didn't
care so much about I would spray, wait a minute, then start mixing the
polyurethane. Those usually had a light fish scale look, with the
occasional run still visible :-). Anyway, maybe the same technique would
work with your PVA mold release?

-----
Regards,
Carl Ijames

[axolotyl]

Thanks Bob & Carl.

I'd not thought of pressure pack mould release. I'll check that out as
its a easy thing to try.

[Bob La Londe]

"axolotyl" <axol...@axolotyl.com> wrote in message
news:8didnXdDVf8FevnK...@westnet.com.au...

> Its only half metalwork but I'm pretty sure some of you guys will know a
> bit about this!
>

> Alternatively would making the next mould from Teflon be worth trying?

That is a thought, but I think I would go with HDPE as long as you are not
pushing the thickness limits of the epoxy and generating more heat than the
HDPE can tolerate. Its got high lubricity, machines well at the right
speeds and feeds (really fast), and it doesn't stick to epoxy all that well.

Sorry, I really don't know much about using PVA mold release, although I
have a bottle around somewhere for working boat repairs with poly resins.