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FrankZane
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Loss of bag tightness is caused by the pressure gradient between the vacuum port and the resin port. the resin port needs to be restricted to prevent it's pressure dropping to ambient. Link below shows this phenomenon. https://www.youtube.com/watch?v=4In9_Eyz6gs
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MarkMK
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I'd be a touch wary of taking-in everything you might see on You Tube and potentially introducing methods that are unnecessary and counter-productive I'm not doubting the findings, but I don't think that it offers anything in the way of delivering a better result and it is not necessary to restrict the inlet flow at all. It would be misleading to imply it was somehow necessary when infusing parts Looseness in the bag around the inlet when resin begins to flow is not an issue to worry about, as mentioned earlier and assuming that the bag is perfectly sealed, everything will tighten once the part is closed-off and pressure equalises. I've infused hundreds of parts to date, many of them quite small and relatively quick to wet-out and have always had a consistently perfect cosmetic finish. The use of resin breaks sized in proportion to the part size/infusion speed and allowing a small amount of resin flow after closing-off the vacuum port has been more than enough to ensure those consistent results. I've not yet encountered a single scenario where choking the resin flow would have been desirable. On smaller parts, in particular, choking the resin flow tends to lead to the gas in the resin becoming greatly expanded as the vacuum pressure at the front has a bigger impact. Whilst this might reduce when both ports are closed-off as pressure equalises over the whole part, it feels an undesirable method of trying to achieve a void free finish, especially where resin is likely to reach the exit fairly quickly. The appearance of the gas is likely to be far more prominent and not just at the front. At the extreme, it's only been the sudden appearance of lots of large bubbles across an part being infused that's alerted me to the fact that the bottom of the inlet tube has vacuum sealed itself to the base of the resin pot. As an aide and it's been debated a few times before, I don't bother with degassing any more either, prior to infusion, as I've found that it doesn't deliver a completely gas-free resin or impact on the part's cured cosmetic finish. Some swear by it, but I've yet to find any cosmetic difference in results, even on more complex-shaped parts. The OP's issue was voids and/or dry areas around the inlet, which seems to indicate a slight starvation of resin in this area, so it appears that his previous process might have been restricting the flow of resin towards the end of the infusion. I don't think that introducing a restriction of resin flow would help matters at all and would be more likely to introduce unwanted cosmetic issues
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mscomposites
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thanks for the advice again MarkMK We have been getting almost near perfect parts once we lowered our resin pot lower than the mold and restricting the resin flowing in just a little bit. Not to the point where the part looks like it’s starving but just enough to keep the bag tight through out the whole process. We also started degassing the resin upfront seems like that also eliminated a lot of the small surface pitting that we were getting. Those were the only two things we changed and we are getting better results now. Not sure if if it’s one or the other that gave us the results or both of them togeather. I usually like to only change one thing at a time but we just didn’t have anymore testing time, we had to start producing parts. We also noticed that the parts would come out pit free but as soon as we cleaned them up to spray our flat clear coat on them (per customers choice) the acetone we were cleaning the parts with was actually removing resin off the surface and breaking into the fabric and causing surface pores. Needless to say no more cleaning of the parts with knarly solvants.
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Hanaldo
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Not quite the right conclusion there regarding acetone and clear coating. If the parts are fully cured, they are completely inert - acetone will not remove any resin at all. It can hurt the gloss retention and dull the finish a bit, which is more evident on the 'high' spots: the tops of the fibres. Once clear coated though, this should not be evident at all.
If you are discovering porosity during clear coating, it was there before the acetone cleaning and indicates a still slightly lean component, or potentially another issue. Infused parts are actually quite notorious for this, they can look so perfect straight off the mould and even with close inspection you often miss the tiny porosity with the naked eye. Once you start spraying clear it becomes very evident.
I would suggest that your parts are still a touch lean, and if your customers are wanting them clear coated then increasing the resin content would spare you some work and improve the results.
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mscomposites
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thanks for your feedback Hanaldo, Yes thinking the part wasn’t fully cured then because the acetone cleaning before clear coated actually didn’t open up more pinholes and dryness look before applying clear coat. We are demolding the part the next morning and cleaning and then spraying clear coat on them. I know some resins are not fully cured for a couple days ambient and of coarse adding heat can speed that up. How do you suggest getting a more resin rich part? Looking for a thicker face layer or resin if that’s possible without putting down a face coat prior to laying down the fabric, and not having to do any coating after the infusion process.
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mscomposites
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I would really like to sit down and watch exactly how you do one part, step by step, so I can get down to the bottom of exactly what we’re doing wrong. haha
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Hanaldo
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My technique varies from part to part, based on what I am trying to achieve and the actual geometry of the part.
For the most part I agree with Mark and I don't like to restrict the resin inlet, it does have a tendency to cause issues. That said, it depends on the part and the layup - some parts do really benefit from slightly restricting the feed line. So it is virtually impossible for me to say "do X or dont do Y" and it will solve all your problems.
I suggest trying a part like you have been doing, as it sounds like you are nearly there, but try clamping off the vacuum line once everything is completely wet out and leave the resin feed open for a minute or two. Most of this excess resin will remain in the flow media, so the final part will only be a few grams heavier, but it can make all the difference for that surface porosity.
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mscomposites
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Ok will try that on the next one. We did lock off vacuum line and left inlet open for like a minute or two but maybe will go a little longer. We also switch over to using MTI hose for our vacuum/ exit line. And advice for that product if you have used it before?
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Hanaldo
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Yeh MTI hose is great for getting the hang of things, you can sort a lot of other parts of the process out while that takes care of more technical decisions like brake zone size and placement.
Once you've got the process sorted, I find MTI hose an unnecessary expense. You can get the same results without it, it just makes it a little bit more forgiving.
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mscomposites
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Yah I like the fact of keeping vacuum on without drawing out any resin. Will see what we get on these next couple of parts going to try locking off vacuum and allowing more resin to flow in before locking off the resin inlet.
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