Mould release with compressed air

Hi all

I use compressed air to release parts normally. Anybody have an experience of parts being permanently distorted by high compressed air pressures used during mould release? I've been monitoring production parts & there is considerable random distortion of parts from often the same mould. It seems unlikely that the force of air alone would cause such a problem, considering some folk use hydraulics which can create massive pressure, but I would like to get anyone elses thoughts on this please.

Many thanks

EDIT *Thought I would add, parts are popped after 12-14 hours, so they have usually gone off fairly well*

 

I can't see how momentary air pressure would permanently distort. If it did anything, it might break something.

If they are all ready distorted when pulled from the mold, it sounds like too fast a cure, ie they are laid up too "hot" or post cure temps are actually too hot. If they distort after demolding, they are not cured enough. It would help to know how big and what sort of shape to the parts, how much glass, what kind of resin and how much catalyst, shop temps, cure temps. The parts might need more structural support in the form of more glass or added ribs, cores, etc.

 

Thanks SamSam

Appreciate the reply. They are already distorted when we pull them. They are large flat panel structures, polyurethane in a sandwich between a few mm of laminate. They have no ribs due to later products having to be laminated into the back of them, perhaps this is the problem. Catalyst is somewhere between 1-2% depending on ambient temperature. Chopped glass rather than matting. I did wonder about the air though, as the distortion seems random, is witnessed increasing towards the middle of the pieces *EDIT* The air valve is located here --> in the middle**. The only variables I can see are the ambient temperature & the difficulty in pulling the pieces which have a zero draft.

The loss is somewhere around 1-1.5cm bows/metre of piece. Always a bow towards the back of the laminate, away from the gelcoat. Could just be the 7% volumetric shrinkage? Shrinkage is the bane! Spray up is heavy towards glass content. As much glass as possible without affecting wetout.

*EDIT* glass is 2400 tex rovings. Polyester orth resin. Cure @ room temperature, which is seasonal for Europe.

 

Are the inside and outside layers of glass equal thickness? If your inside layer is thicker, there is more resin to shrink. (I used paint rollers to sop up any leftover resin after rolling out.) I'm guessing your chopper man needs a gauge.

Are the parts stored perfectly flat after demolding? It was long ago, but I recall keeping flat panels clamped to flat surfaces for a "postcure".

I can't imagine the air distorting the panels, but I never used high pressure to demold - always low pressure. (Unless it was a new guy in the shop applying the air - the loud pop would scare the bejabbers out of them.)

 

Inner & Outer

Yes, slightly I think the inner layer has more. But its fractional. Within a mm I would say. I'll check with a gauge like you suggest & see if a perfectly balanced top & bottom makes the difference. Its only on production pieces larger than 5ft or 6ft that its clearly evident. The larger the piece the more the warp. On the 2-3ft pieces nothing visible really.

*EDIT* Pieces are in bulk, we make lots of them, so clamping them is not practical, but they are stacked against each other. My problem is more the warp that is evident immediately after de-moulding.

And thanks for the reply

 

Maybe the release agent is too good and you're getting pre-release before the laminate has set good enough. Run something lightly over the back or lightly tap before putting the air to it to see if sounds hollow in places.

If an unbalanced laminate is the culprit, maybe intentionally unbalancing it the other way would solve it.

You might want to doublecheck on how much catalyst the chopgun is actually putting out. They are very touchy.

If you are putting laminate against the mold, then a core, then more laminate on the backside, all in one session, the first laminate will have a different curing environment than the second, ie more heat and less ventilation to waft away the styrene.

Ortho polyester is kind of like the hot dog of resins, there is no telling what's really in it. The cheaper it is, the more fillers, batch leftovers, dregs and old expired stuff you get. The same brand of stuff can vary between batches.

Have you been doing this long enough to tell if it is a seasonal problem or is it just sometimes any old time or does it always happen?

 

Good idea as regards over laying the back

I think the idea of more heavily laying the back of the laminate sounds a good idea. I'll try this. Its a nice even warp if anything, so perhaps giving an over shrink on the back may help pull things straight. The Polyurethane is 20mm, perhaps extra polyurethane would help create more leverage on the front as well. I remember something about moments in Physics!

I could well be pre-release. The semi-perms are great, but things just fall out of the moulds! I hadn't considered this angle. I would like to stay with semis as they seem fool proof. The guys can tape test them, whereas wax relies on some skill, I can't be in all places at once, to make sure they really wax properly. The worst they can do with a semi is haze the mould surface with a bad application. But yeh, I guess pre-release it could be. Some of the forms are thick, so hollow may be hard to test, but its a good idea.

Hotdog! I know what you mean about cheap production Orthos. Currently I'm running resins out of Croatia (no brand names). Strange behaviour for a resin, wetout problems etc. You can lay it out, and if you get into rolling within 30 secs. its okay, anymore than this and it doesn't wetout properly. But I compete against cheaper plastics popped out of a machine at one a minute, so I can't be picky!

Its been the same problem for a few years, back then I used steel forms, now polyester ones. Some heavy, some not, some braced some not. Too many variables to hit on exactly where the problem is coming from!

Thanks for the advice, I'll certainly try flipping round which side of the sandwich the heaviest laminate is sprayed.

 

Man, I didn't even think of pre-release. Could be a combination of factors. Lucky you, eh?

 

Lucky me

Lucky me

 

pre release

 

What I meant was likely drag something like a screwdriver or the back of your fingernails over the part, or lightly tap the part, not the form/mold, before you de-mold it.

If the problem is pre-release, perhaps vacuum bagging would help. We had a vaguely similar situation out in the woods once, and solved it by doing the lamination, covering it with plastic sheeting/visqueen and then piling sand on to hold the laminate down until it set. My friend called it dirt bagging.

 

samsam sorry for long reply

Hi Samsam

TY for reply. I came out to UK to assemble the parts I had to deliver, so I have been off forums for a while. I spoke just now to the release agent Chemtrend folk, they seemed to think if the problem is intermittent its not a release agent problem. But this does seem a likely area. The only way I'm going to rule out pre-release is to run one of the forms that has exhibited warpage on pulled pieces and wax over the semi-perm. Then run a series of pieces & monitor results. If the problem vanishes - then I know.

I like the idea of weighting the piece on the back. Since the bow is always in that direction it must help. I'll let you know how that goes.

I have no experience of vacuum bagging, why would this help to solve the warp?

I find it unlikely to be a shrinkage problem, I have just cut up & examined a competitor piece & the warpage is v. much lower. Perhaps a different ortho manufacturer would be the way to go as well.

 

Vacuum bagging would hold the piece against the mold until you let off the vacuum. Basically you would spread a piece of plastic film over the wet laminate, seal the edges and draw a vacuum. Atmospheric pressure would then take over, putting something like 14.7 lbs. of pressure per square inch over the piece. The vacuum would have to be held until the part cured enough to hold its shape. The advantage would be it is "weightless" pressure, whereas to actually put something in the mold to come up with the same pressure would take 2116 lbs per square foot and is almost certain to cause problems.