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Old 26-09-2015, 20:30   #1
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Having a little trouble removing part from male molds

I'm making fiberglass parts (have only made a few so far). These are misc. parts for the boat, not a production line. I'm having trouble removing the finished part from the male mold. The problem is not a mold release issue (plenty of waxing and PVA), but rather a problem of constriction.

I use a male mold when the finished part is concave. When the polyester resin/glass cures, it shrinks a bit, tightening around the male mold. When I try to pop that part off the mold, I have a hell of a time getting it off. I usually have to destroy the mold to get the part off. The last part I removed, I cut the inside corners of the male mold, being careful not to damage the part. That released the pressure just enough for the part to pop off. There was no noticeable change in dimension when I made the cuts to the corners.

I use a low catalyst to resin ratio to keep the reaction cool and slow.

I have other parts made off a female mold and those remove just fine.

Is there any way to make a part from a male mold without having to destroy the mold toe get the part off. I'm using polyester resin because epoxy is not compatible with gelcoat and epoxy degrades in UV.

Here is an example of one of the parts from a male mold. This part is a concave part for the manual salt & fresh water foot pumps in the galley.

https://picasaweb.google.com/1112749...Malemoldissues

Thanks for the help.
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Old 26-09-2015, 21:20   #2
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Re: Having a little trouble removing part from male molds

I've found that if I put a few holes in the bottom of the mail mold. I can put some air pressure in the holes and it will just pop the part off. Part of the reason the part will not come off easy is the perfect vacuum that is there when you try to take the part out. Mac
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Old 27-09-2015, 06:28   #3
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Re: Having a little trouble removing part from male molds

Ah ha!.

That make sense and that also explains why cutting the corners of the mold (which are on the inside) made the part come right off.

I will drill a couple of holes next time.

Thank you for that tip. Maybe now I'll be able to easily reuse a male mold.
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Old 28-09-2015, 06:17   #4
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Re: Having a little trouble removing part from male molds

'glass shop I knew years ago USTA glass-in a Schrader valve to the back of the mold. 'glass auto seat shells would pop right out
Very impressive project!
What gel are you using that's "incompatible" with epoxy?
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Old 28-09-2015, 06:38   #5
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Re: Having a little trouble removing part from male molds

Hard-rock-candy is right. Air pressure will relieve the vacuum post curing.
Drill a hole ( ie 3-5 mm) in the mould, glass a tube on it, strongly.

A shrader valve? Why not? Although I have not seen that done.

When preparing the mould, fill the drilled hole with a smear of wax, then PVA before the first (gel) coat. After curing put the hose or nozzle on the tube, and it will pop.
For a big mould you may need to drill and prepare more holes/tubes
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Old 28-09-2015, 07:06   #6
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Re: Having a little trouble removing part from male molds

Have you tried using plastic wedges around the outer rim?

I normally have success if I take a rubber mallet and smack the daylights out of the part around all sides starting at one corner while blowing air around the edge of the part.

If you do a layup with out gel coat, you can watch the part release as air makes its way between the part and the mold. Smacking the daylights out of the part is both therapeutic... and releases the bond against the mold surface.

Once you've got air between the two, a wedge and a rubber hammer normally gets it to move.

Next part you build, add a bit more draft angle to the sides!

Cheers,

Zach
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Old 28-09-2015, 08:52   #7
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Re: Having a little trouble removing part from male molds

Quote:
Originally Posted by HankOnthewater View Post
Hard-rock-candy is right. Air pressure will relieve the vacuum post curing.
Drill a hole ( ie 3-5 mm) in the mould, glass a tube on it, strongly.

A shrader valve? Why not? Although I have not seen that done.

When preparing the mould, fill the drilled hole with a smear of wax, then PVA before the first (gel) coat. After curing put the hose or nozzle on the tube, and it will pop.
For a big mould you may need to drill and prepare more holes/tubes


While I've glassed a fitting for air or water (which works much better) into many a mold, this will generally not be an option on a male mold (plug).
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Old 28-09-2015, 08:57   #8
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Re: Having a little trouble removing part from male molds

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Originally Posted by Zach View Post
Have you tried using plastic wedges around the outer rim?

I normally have success if I take a rubber mallet and smack the daylights out of the part around all sides starting at one corner while blowing air around the edge of the part.

If you do a layup with out gel coat, you can watch the part release as air makes its way between the part and the mold. Smacking the daylights out of the part is both therapeutic... and releases the bond against the mold surface.

Once you've got air between the two, a wedge and a rubber hammer normally gets it to move.

Next part you build, add a bit more draft angle to the sides!

Cheers,

Zach
A mold with improper angle is known as a "negative release". Sometimes a very small amount of negative release is intentionally designed in by necessity. You can't always just change the shape. Split molds are the answer with a female mold, obviously not an option with a plug.

For rough parts, ie parts which don't require a finish on the inside, use a foam which is compatible with your resin system of choice but can be dissolved with a solvent. Obviously this means a single use plug, but using this method you can build shapes on a plug with extreme negative release.
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Old 28-09-2015, 10:09   #9
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Re: Having a little trouble removing part from male molds

As Minaret said, the mold needs to have some relief to it. In other words, there needs to be an increasing gap between the mold and the part as it is being released. If you cannot add the relief to the mold because you require that specific shape, then think about making a mold that is more than one pc.. you can use regular modelling clay on the mold seam and wax/pva over. Good luck. James

Quote:
Originally Posted by scherzoja View Post
I'm making fiberglass parts (have only made a few so far). These are misc. parts for the boat, not a production line. I'm having trouble removing the finished part from the male mold. The problem is not a mold release issue (plenty of waxing and PVA), but rather a problem of constriction.

I use a male mold when the finished part is concave. When the polyester resin/glass cures, it shrinks a bit, tightening around the male mold. When I try to pop that part off the mold, I have a hell of a time getting it off. I usually have to destroy the mold to get the part off. The last part I removed, I cut the inside corners of the male mold, being careful not to damage the part. That released the pressure just enough for the part to pop off. There was no noticeable change in dimension when I made the cuts to the corners.

I use a low catalyst to resin ratio to keep the reaction cool and slow.

I have other parts made off a female mold and those remove just fine.

Is there any way to make a part from a male mold without having to destroy the mold toe get the part off. I'm using polyester resin because epoxy is not compatible with gelcoat and epoxy degrades in UV.

Here is an example of one of the parts from a male mold. This part is a concave part for the manual salt & fresh water foot pumps in the galley.

https://picasaweb.google.com/1112749...Malemoldissues

Thanks for the help.
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Old 28-09-2015, 10:18   #10
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Re: Having a little trouble removing part from male molds

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Originally Posted by Lokiyawl View Post
As Minaret said, the mold needs to have some relief to it. In other words, there needs to be an increasing gap between the mold and the part as it is being released. If you cannot add the relief to the mold because you require that specific shape, then think about making a mold that is more than one pc.. you can use regular modelling clay on the mold seam and wax/pva over. Good luck. James


Split molding is generally not possible with a plug, especially one with negative release, which is the only reason you'd want to do it.
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Old 28-09-2015, 10:31   #11
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Re: Having a little trouble removing part from male molds

Quote:
Originally Posted by minaret View Post
A mold with improper angle is known as a "negative release". Sometimes a very small amount of negative release is intentionally designed in by necessity. You can't always just change the shape. Split molds are the answer with a female mold, obviously not an option with a plug.

For rough parts, ie parts which don't require a finish on the inside, use a foam which is compatible with your resin system of choice but can be dissolved with a solvent. Obviously this means a single use plug, but using this method you can build shapes on a plug with extreme negative release.
===

Where a rough finish on the inside is not important, I've had good luck using a single ply from a polyethylene garbage bag as a release layer. When the glass work has cured I go around the outside edge with a large screwdriver, tap with a hammer, and it pops right off.
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Old 28-09-2015, 10:39   #12
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Re: Having a little trouble removing part from male molds

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Originally Posted by Zach View Post
...Next part you build, add a bit more draft angle to the sides!...
This.

There should be several degrees of draft on all surfaces which are ~parallel to the line of pull. And their surfaces must be smooth and fair.
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Old 28-09-2015, 11:10   #13
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Re: Having a little trouble removing part from male molds

It's possible but it does add more steps. Following the link provided in the original post, the mold shown appears to have plenty of relief in it so this does not appear to be the problem. James

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Originally Posted by minaret View Post
Split molding is generally not possible with a plug, especially one with negative release, which is the only reason you'd want to do it.
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Old 28-09-2015, 11:20   #14
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Re: Having a little trouble removing part from male molds

The mold shown in the link you provided appears to have plenty of relief so I have to wonder if despite the PVA and wax that you might still have some sticking issues. I have had some luck with building up the wax on a new mold, spraying on a thin coat of PVA/allowing it to dry fully and then applying a thin coat of wax over top of the PVA without wiping it off. It seems that the first several pull off of a new mold are always the hardest and once broken in the parts start coming off on their own as the shrinking occurs during curing. You can hear the little pops as the bond between the mold and the part separate as the polyester shrinks. The suggestions to vent, use air or water pressure can work but I am actually surprised that you are having any release problems with the the part shown in the link. Maybe try a different wax, PVA should not even be needed, especially after the mold is broken in. I have always used the Mequiars Mirror Glaze #8 wax myself. Good luck. James

Quote:
Originally Posted by scherzoja View Post
I'm making fiberglass parts (have only made a few so far). These are misc. parts for the boat, not a production line. I'm having trouble removing the finished part from the male mold. The problem is not a mold release issue (plenty of waxing and PVA), but rather a problem of constriction.

I use a male mold when the finished part is concave. When the polyester resin/glass cures, it shrinks a bit, tightening around the male mold. When I try to pop that part off the mold, I have a hell of a time getting it off. I usually have to destroy the mold to get the part off. The last part I removed, I cut the inside corners of the male mold, being careful not to damage the part. That released the pressure just enough for the part to pop off. There was no noticeable change in dimension when I made the cuts to the corners.

I use a low catalyst to resin ratio to keep the reaction cool and slow.

I have other parts made off a female mold and those remove just fine.

Is there any way to make a part from a male mold without having to destroy the mold toe get the part off. I'm using polyester resin because epoxy is not compatible with gelcoat and epoxy degrades in UV.

Here is an example of one of the parts from a male mold. This part is a concave part for the manual salt & fresh water foot pumps in the galley.

https://picasaweb.google.com/1112749...Malemoldissues

Thanks for the help.
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Old 28-09-2015, 14:52   #15
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Re: Having a little trouble removing part from male molds

normal polyester shrinks that's why you have problems
use polyester made for building molds it has a foaming agent in it so it swells when it hardens. Meaning the part gets bigger than the male mold or plug.
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