Part Dimpling Vacuum Infusion

Hi All,

I'm an engineer at a company where we produce offroad expedition vehicles.
We are vacuum infusing our fibreglass parts, but we are having issues with dimpling in the finish of the part which requires finishing after pulling the part.

Our layup is as follows:
- Spray Gelcoat.
- Back up gelcoat with a brush when dry. Dry overnight
- 225 CSM handlay, using Polyester resin at 1%, allow to fully cure.
- 450 CSM handlay, using Polyester resin at 1%, allow to fully cure.

Dry layup below.
- 600 Biaxial.
- lay in foam.
- 300 CSM Powder bound.
- 400 double bias.
- 600 Biaxial
- Peel ply.

Infusion resin is polyester mixed at 1-1.2% (We aim for 45min gel time, perform a test before every infusion).

I don't believe this to be print through issue (I can't see any glass print), we previously had issues with print through but removed a 300 CSM powder bound from the dry layup for a 450 CSM in the handlay. This did also help reduce the dimpling.

I've attached photos of the part, but it is hard to see.

Some possible reasons I have thought of below.
- The mould is flexing between the steel framing of mould. The areas where the steel framing isn't, the mould may be flexing slightly under the vacuum. The mould is made with tooling resin, lots of glass and foam to be as stiff as possible.

- The resin in the tie layer is wrong. Vinylester would be preferred in the tie layer, will the vinylester resin help prevent the dimpling with it more supple properties and less shrinkage to polyester?

Any insight is greatly appreciated,

Thanks,
Ryan

 

Edit: We back the gelcoat up dry not wet.

 

@ondarvr is a forum expert

there are others, I'm not one of them, however I can tell you I'm having a hard time interpreting the issue as the pictures are hard to get an idea of the scale; perhaps this is my lack of knowledge

when you say dimpling; a golf ball comes to mind, but the pictures say something else is all...

the pictures look like much larger areas of defect than a dimple

I'd test the mold moving theory with a 1/10,000 gauge. If this is happening in the same place, mount a gauge and meausure the flex of the mould. Not sure you even need a part; just the bag. You haven't indicated the time the problem has existed or changed, that would also help debug. If the mold is degrading from movement; it may get worse overtime, i.e.

 

Is the foam in the laminate grooved or perforated to allow resin to pass freely?My suspicion is that the flaw is in the region of a join in the foam and I wonder if it may be that air beneath the foam has no easy escape route.Would it be a huge problem to take the extra time to infuse in two stages;1st stage being up to and including the foam and then 2nd stage remainder of the laminate?

 

Would it be feasible to use partial layups on the problematic section of the mold to test any theories? All that comes to my inexperienced mind is try reducing the vacuum and possibly destroy the resulting component to check its integrity.
Maybe give the wooden boat forum a go at this is you can't solve the issue, there are a lot of hands on lamination guys there but not sure about any from a mass production environment.
Good Luck from a former inhabitant of the 'Gong.

 

What is this backing up business? Are you applying a second coat of gelcoat into the mold? If yes, that's probably your problem, get someone who can spray a single coat of the recommended thickness, or switch to brushing alone.

How are you guaranteeing adhesion between foam and the 600 biax below without a CSM layer?

 

Hi there,

Yes, I think dimpling maybe wrong term, a wavey finish is more accurate. It doesn't look like pre-release. I will attempt to measurse the Flex under vacuum. The mold is new, having only 3 parts pulled.

 

Foam is grooved and perforated. I dont think it is air being trapped, can't hear any air gap when tapping on the part. Infusing in two stages is too time costly in production environment.

 

I think you may be on to something regarding reduced vacuum. Its something I am looking further into.

 

Spray single coat, brush on extra to corner ETC that may need further sanding.
Haven't had any issues with adhesion thus far.

 

If the wavy areas aren't near the extra gelcoat there are other possibilities. I heard it's possible to boil off to much styrene under full vacuum and get uneven cure rates, but I can't confirm.

 

Under cured and changing shape after its been de-molded. Up the skin coat catalyst level to at least 1.5 % or more. After infusing leave it in the mold longer, and make sure it cures very shortly after the infusion is completed.

The longer it takes to cure, the more it will change shape after demolding. Sometimes you need to change to a different type of resin to achieve a good cure in a thinner laminate.

 

OP ~ pay close attention to this fellow; he has years of debug experience...

I think his words here could be confusing for amateurs, but a faster cure and longer demold time is the production challenge.

 

Thanks Ondarvr, I also agree about the under curing. I am meeting supplies soon to fine the correct catalyst.