Small scale vacuum resin infusion.
26'th July 2006.
But working on the model scale is a little
different and that is what this page is about.
I have occasionally done something crudely resembling vacuum infusion by simply adding resin into the bottom of a plastic bag, placing my “job” next to it, then a resin trap made of tissue paper and shoving a vacuum line into the open end and then sealing the bag around it.
This kind of works (sometimes) but there is the same time pressure that you have with other wet layups - you have resin with a limited working time and the clock is ticking. When things go wrong things can get really messy really quickly. The other problem was the infusion was slow and unless I helped it along I could only get around 50mm infused before the resin gelled.
A bit of research (including the links above)
pointed to the lack of a resin transport/flow layer as the reason
to this slowness.
I tested hessian, chux wipes, paper towel and cotton flannelette. I would also have liked to test – cotton towel, wool, shade cloth, geo-textile and of course real flow media.
The set-up (click to enlarge).
Infusion port (yellow disk on the left).
I turned a disk of plastic on my lathe (LDPE, re-melted milk bottle tops). The disk has a hole bored in it which will take a syringe nozzle with a tight fit. I also machined a hollow around the hole. It might be beneficial to have some radial groves on the underside but this one doesn't. The disk is placed inside the plastic bag, after the vacuum is drawn and everything is sitting in place – a syringe is jammed into the hole. Today the plastic didn't puncture when I did this, so I had to remove it and break the seal with a drill bit. This has to be an air tight seal. The reason I machined a depression is so resin can be poured there as a leak test and leak inhibitor. Having a pool of resin there could also allow the hole to be unplugged under certain circumstances without letting air in - I will explain why later.
I choose this LDPE plastic because resin doesn't bond to it. I'm sure there are many alternatives to what I've done. I would suggest a rubber suction cap or a large pencil eraser could also be easily modified for this purpose.
Underneath the port I placed a strip cut from a scouring pad to give a low resistance path for the incoming resin.
vacuum port (white disk on the right).
trap (at the back).
I poured 25ml of resin into the 25ml syringe. The
resin infused fairly rapidly. I immediately mixed another batch
and only 5ml of the first batch was left when I was ready. The
pad rapidly saturated and the resin appeared to be boiling. Resin
pooled inside the bag near the port. As soon as the resin front
hit the hessian it was clear this was going to win the race. The
resin travelled along the hessian about twice as fast as through
the glass. The hessian is supplying resin to the surrounding area
as well so it would be even faster if the entire surface was
covered with it. The chux and paper towel didn't seem to help at
all, the chux is only in ahead because it is getting resin from
the hessian as well.
Something to ponder.
infusion the pressure inside the bag near the infusion port is
close to ambient. Resin pools here and the laminate is not being
compacted as we would like. The pressure in this region only
drops after the resin inflow has been stopped and excess resin is
removed from the area as it flows toward the exit.
Today's test laminate is carbon fibre, four layers of 135 GSM cloth (25/75 weave). The flow material is shade cloth. The enkafusion flow material is under the two ports. Note the the infusion port is still sealed in the above photo. Inserting the syringe punctured the plastic and the port worked perfectly.
This is what happened.
I mixed up a bigger batch of resin than yesterday (60ml). It infused at alarming speed and by the time I hurriedly mixed a second batch the resin had already reached the vacuum side of the laminate. The resin appeared to be boiling (and it probably was).
The mid line connecting the ports was fairly bubble free but further out towards the edges seemed to be mostly vapour. I don't know what was happening in the carbon. I made a little video clip which shows the bubbling accompanied by the sweet sound of a dynavac (1.3 meg).
It only took a few more minutes for bubbles to appear in the vacuum line. Here is a longer clip (2.6 meg).
After a while there didn't seem to be much point in continuing to let resin into the system. I tried reducing the vacuum a little by uncapped the second vacuum line but it didn't make much difference. When the flow media is as open as this - my previous comment about dual vacuum lines doesn't apply but I wanted to try it anyway. The flow of resin into the infusion port had slowed quite a lot. I pooled extra resin in around the infusion point and unplugged the syringe. I delayed a little to obverse the flow and a little air did get sucked in. I plugged in the second vacuum line and the boiling increased and resin immediately began to bubble through the line. More vapour appeared in the shade cloth even along the mid line. The resin in the resin trap was also boiling (supporting the theory that these weren't just air bubbles)
impatient I shone a desk lamp onto the job to warm it and speed
up the curing. Eventually the bubbles in the lines slowly down as
the resin thickened and the left line more so than the right. The
reason was probably that there was an air leak on the right side
due to a puncture from the enkafusion.
Once cured, the carbon was very difficult to separate from the other layers. In particular the enkafusion becomes rather tough to move once it is full of cured resin. This could be a real problem on a complex shape.
The laminate had some blemishes on the underside (the cutting board side). This was probably due to my hurried mixing on the second batch of resin. The top side looks as good as any I've ever made.
More to ponder.
Today's results are looking promising.
The initial infusion was too fast for comfort. If I use these materials again I will add a valve or clamp to reduce the vacuum in the early stages. I think I will still go to full vacuum in the later stages.
I don't really think I will use this enkafusion/shade cloth combination for the model work expect maybe for flat laminates. At the moment I think hessian is the top candidate because it drapes better. I think it will also be easier to separate it from the laminate. Strips of shade cloth or even enkafusion will be used but not over the entire surface. A better flow material may still be found and maybe a suitable release film.
I used a ridiculous amount of resin today. I mixed 120 ml to make a 23 gram laminate. Around 20 ml was unused and almost 40ml ended up in the resin trap. These aren't totally wasted because I can use the hard pieces for machining. I think the amount of resin wasted can be significantly reduced.
Sixty ml of resin was carefully mixed. The vacuum line was clamped to reduce the initial panic phase but was not needed. The initial infusion was rapid then progressively slowing down. There was a bit of boiling but very little compared to yesterday. I estimate it took under five minutes to reach the stage in the first photo.
It took another seven minutes to get this far. Notice how well defined and straight the resin boundary is.
almost forty minutes to reach the shade cloth at the vacuum end.
Five ml of resin still remained unused. Note the resin at the
edge is a little more advanced than further in, this is because
there is no CF at the edge.
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