Voids: Cause and Cure?

General Composites Discussions Composites Talk
#1

I have a part that I’m suddenly having trouble with. The process is vacuum infusion.

You can see the voids in the attached photo. 50% of my parts have voids, some worse than others. Funny thing is, the problems have come up only in the last few weeks. They seem to be getting worse.

The part is a skateboard with a foam core. The areas with the voids are the thickest areas - about 1.25 inches thick. There are about 10 layers of 5.8 oz cloth on either side of the core. The core is perforated.

On top of the part, I have peel ply, perf ply and flow media.

One of my suppliers told me the problems are air related. Either I’m not degassing enough or air is getting into the bag and at the end of the infusion when the resin supply is cut off, he thinks the air is migrating to the problem areas.

I don’t buy this for a number of reasons. 1) the voids are always in the same areas. I don’t think air would be that choosy. It would be more random. 2) I haven’t changed the bag material or the degassing process.

I’m thinking it’s the perf ply. Either I got a bad batch of perf ply or the laminator is overlapping or wrinkling the perf thereby blocking the holes.

I’m going to try another infusion with absolutely no perf. And another one with just strips of perf laid down perfectly flat. I need the perf to aid in demoulding.

Any thoughts?

#2

If you hate perf ply (who doesnt) look for Fibertex Compoflex material. (look it up on youtube).

As for the pinholes:
Is your mould airtight? Really?
Do you use absolutely all vacuum possible?
Do you have vacuum on the part for at least half an hour before infusion?

#3

I wouldn’t call these pinholes. More like voids. You can feel them with your fingers. They are also in the same general locations so it has to do with resin not getting fully saturated in those areas (bottom of the mould). If it was air getting into the mould, the voids would be all over. I’ve had that happen and know what it looks like.

The bag is super tight. I’m using a digital vacuum gauge and it shows excellent vacuum.

We de-gas the resin for 45 minutes and we de-gas the bag at the same time so it’s going for at least 45 minutes.

#4

What is your vacuum during degassing, infusion and after infusion?

A leaky mold still is a possibility. Draw a vacuum, and coat the backside.

#5

As with others, are you sure your mold is sealed? What kind of mold is it? I like what Herman said…pull a vacuum on it (just a layer of peel ply to the edge of the tacky tape is fine), and coat the underside if it’s wood, fiberglass, plastic, plaster)…if it’s metal, check for cracks.

Degassing depends on your resin. Looks like you already do for 45min. Do you let the bubbles rise up, and then collapse? If that doesn’t happen, then you aren’t really degassing. (i should video this one day)

How is your fabric stored, what is the weather like. Moisture might do this as well. If worried, hold your part under vacuum at RT for a day, or heated, for a few hours.

Obviously, check for leaks in your bag, and sealing techniques. ANY air coming in will cause voids like this.

Now, the voids being in the same AREA is a little strange. If the part is curved, and your foam is thick, and not conforming, maybe resin and voids pool up in the one spot.
Also, what is your distro-media layup? Do you have it attached to yoru resin line, and stopping at part edge? Do you have fast flow, or slow? Carbon likes slow. if you are racetracking with fast flow, you can trap air…the same place every time, if your parts/bagging is consistent.
if your perf is wrinkling…yes, you can cause issues.

#6

My experience with wrinkled perf film is that as long as the vacuum is super good, it is not that big an issue.

#7

Good news. The problem has been resolved. Turned out we were just letting the infusion go too fast. The resin was tracking across the top of the part and not fully saturating the mould surface. By cutting back the flow media and making sure the infusion takes 2 hours minimum, we are getting good parts.

Should have thought of this earlier. Seems obvious now.

Thanks for all the input.