Category Archives: carbon fibre

3D Printing improves degassing

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So, Degassing is the process of getting all the air out of the mixed resin. This leads to stronger parts with a better cosmetic finish – you aren’t fighting tiny bubbles that appear in the resin, or that start to appear as the resin warms through the exotherm and the gas expands.

It’s simple to do – you put the resin in a chamber – evacuate it and watch the resin bubble up as the air comes out. As the bubbles near the surface, you let a little air in to equalise the pressure, the bubbles subside and you continue until under full vacuum, no bubbles appear.

However, it’s NOT SIMPLE TO DO: I lied. There are subtleties to this, and if you just open the tap carelessly,it’s easy to let the air in too fast and get resin everywhere. Also, you end up shooting air into the resin you were trying to clear of the damn stuff. This is also how you trash a degassing chamber, and they aren’t cheap.

The following youtube video I made illustrates this with water. Read on to see how I fixed this with a simple 3D Printed part.

So, I designed and printed a part that moved the air and vacuum over to where it was needed:

So, using some gum-tape to fix the air-guide in place, I now have a degassing chamber that lets air in without worrying about shooting it back into the resin.

Making a vacuum manifold

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I suppose your first question is “what are you even on?”,  and your second question, is “why bother?” If you need a vacuum manifold. Well, to answer them in order:

  1. I am on a chair, in my office.
  2. If you want to double bag, or hold down vacuum whist you degas, isolate a catch-pot or pat your head whilst rubbing your tummy, you need a vacuum manifold.

I mentioned making this in the infusion stations post and showed how I’d connected it to my vacuum reservoir, also known as “my old compressor tank”.

Basically, I’d hoped this doodad that I’d wombled off eBay would work (it’s a CO2 gas splitter with valves for the unclicking) and hoped it would fly straight out of the packaging, but it turns out it has non-return valves in, which means it won’t work for vacuum. 

I tested it against my vacuum gauge and it’s great – seals well and holds vacuum both ways.

Here it is, in the vice after I’ve taken it apart to see what gives with the valves, and can I somehow get them out.
They were only turned in with PTFE tape, so were an easy extraction. It’s worth noting that the steel is either polished stainless (probably) or chromed. Either way, the finish was great.

If you look at t’photo ont’ right, you can see the on-return valve, and as I hoped (but didn’t have a clue about before I dismantled it), the housing for the valve (ball on a spring) is a press fit. All I then did was get the right sized drill in there, and started drilling it out. Sure enough, it went pop, and the whole thing turned out.

Here it is, open and ready to go. You can easily see through to the white foam I used as a backing material for the shot.

 And the the detail-obsessed amongst you, here is the spring, cap, o-ring and ball to seal it. Very neat and simple.

Scrapping Tub 1.0

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So, here is the tub out of the mould. I had a lot of issues infusing this, and I’ve learned quite a lot whilst doing it. It’s gutting to scrap it, but it’s the right thing to do. Next time it’ll be right.
There are some high level mistakes that were mine, and they led to some lower level quality issues. It’s the first time I’ve done an infusion of this size and at the same time with a positive mould, rather than the usual negative mould. Doing it on a positive mould means I have to be very careful about the layer thickness otherwise I have a too large or too small gap between the part and the chassis rails I want to bond it to.

Some of the mistakes were down to layup technique, and some were down to getting the infusion wrong.
 I infused over a hexagonal core, and the hexagons were too large to not have a hole put through the middle.
Here’s another picture of the quality issues up close – now for the lessons learned:

Lessons Learned

  • I over engineered the part – too much carbon meant it was too heavy. When I cut it open I had great consolidation – all the issues are cosmetic.
  • In the spirit of making it ultra strong, I had core everywhere. Next time it’s going to be core in the floor, up the back and the top of the tunnel only. The core interfered with the close fit needed to bond it to the chassis. Again, this is due to a male mould. Normally I’d overlay layers in certain areas to achieve this. In this instance, I have to stick to the required layers to achieve a 2.5mm thickness up the sides and at the bonding areas.
  • More holes in the core, and a thinner core
  • The catch-pot imploded. I made one out of plastic. Air got in. A lot of air. I made one out of plastic and tested it for a week – it was strong. With the heat of the part (I had the tub heated to 35C) the pot weakened and imploded. Back to my all steel pot.
  • The tubing sucks up a load of resin. I had 6kg mixed and it flew into the part. This caught me off guard and I didn’t have enough mixed. A small amount of air got in. If I’d had really strong vacuum (i.e. No pot death) I might have recovered.
  • Have one KG of resin and hardener ready to mix. Even two.
  • I took 4 attempts to get the bag on – next time I’m going to make a huge envelope bag instead – it should increase my chances of a perfect vacuum.
  • No gel-coat. It’s a long layup and I was up against the clock. This drove some of the other mistakes. I’ll not be up against a schedule this time.

If I get another output that isn’t cosmetically great, I’ll spray it and be done. If it’s of a cosmetic quality, I’ll have it sprayed in two-part lacquer. This will give a great finish.

3D printed parts for resin infusion

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 This is a vacuum exhaust fitting. I’ve designed it to be deliberately flat so the release film will stretch around it. I’ve previously used these silicone connectors from easy composites. They’re quite good and just over 3 pounds each. The connector I’ve printed has a hole for a 6mm hose and a stop at the bottom so the hose just doesn’t sit on the mesh. They also have a larger surface area sat on the mesh to help pull vacuum out. Mine aren’t reusable, and only cost 19p each to print, and take about 15 minutes. 
I did a large infusion for the tub recently, and had no end of problems, to the point of scrapping the tub. I’ll write that up later, but one of the issues was that one of my pipes jumped off the clamp and sucked air in. Rather than use a simple OTS clamp, I’ve printed something specific for a 10mm hose and a 10L bucket. The hose is a snug fit and the height from the bottom of the bucket can be nicely set and held just with friction. No more springing hoses ever again. 

Thinking out loud for the resin load

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Area Cloth Layers Weight
4.5 600 4 10800
4.5 300 2 2700
Sum 13.5
Resin 5.4

 

So, I can expect to need 5.4kg of resin to infuse this part, assuming a load of 40% by the time you include pipework, mesh, etc. The actual part itself should only take a load of 30%, but I can measure the weight of the part afterwards to see what the actual load is.

Release Agent – pics of the shiny mould

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So, the Marbocoat mould-sealer claims to leave the mould more shiny than if it was just ordinarily treated. Well, if you look at the pictures above, you can see that it actually does. Five coats of sealer and five coats of release agent and you can see the result. 

I masked off the mould for about one inch (I know, thinking in old money – don’t care) around the perimeter so there wouldn’t be any issues with the tack-tape struggling for adhesion. As a comparison, masking tape applied to the treated surface just curls off. 
The mould is treated, the templates are made. The modifications to an old compressor to make a vacuum chamber are nearly done (subject of another post soon), and the spray-booth is complete. The next post illustrates just how Dexter I’ve gone.

A study in cores for my tub

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Problem

So, I need to decide which core to use in the sides of the tub, where I don’t really need huge stiffness like I do with the base, but I still need to do a trade-off between thickness, ease of working, stiffness and weight.

Weight wasn’t going to be my only trade-off here. I have some others to think about

  • Ease of preparation started to become important when I started calculating the time required to prepare a standard foam such as the Airex. It needs to be scored every 20mm as a grid, and then at each intersection, needs to have a hole pit in (only 2mm or so) to allow the resin to flow through to the other side of the core. I did a square metre of it, and it took over two hours. Very tedious to do accurately.
  • Ease of layup is also a significant factor. Some cores are very bendy, some are very rigid, and some will thermoform. I don’t want to choose a core of marginal better physical properties if it takes me days of frustration to get it into the stack. This isn’t just a simple flat sheet, but a large female mould with complex curves in multiple planes.
  • Physical strength

Method

img_0110-1

 

 

img_0111

  • do a flex-test to see how rigid it is (clamp one end to a flat surface – hang a standard weight off the other end). You can see how the steel rule is horizontal, and the part has deflected.. 24g is the weight.

Results

  • Weight – Soric (24g), 3D Core (23g), airex (29.5g). So, the airex is 2/5 thicker than the other two, and is 26% heavier.
  • Deflection (therefore infer stiffness) – Soric 30.9mm, 3D Core 31.6mm and airex – 20.5mm

Ease of layup

  1. Soric first – it’s more or less like a thick cloth, and will bend into most angles without being damaged
  2. 3D Core second. It’s a bit like hexagons of foam which are stitched together. It will easily follow simple curves, and can be thermoformed into the tighter ones. It’s not great in tight curves and will separate at the meeting points between the hexagons
  3. Airex last. It has some flex but does thermoform very well. However, getting the hot-air gun in there and pushing and shoving to thermoform it is going to be an arse.

Conclusions

  • I was surprised a bit between the 3D Core and the Soric. Deflections were very similar for very similar weight, which is what one would hope to see. This means the cores were performing as a function of the distance they separate the cloth. I thought the Soric would hold a lot more resin than the 3D core mind, and the results were so close.
  • The airex outperformed for stiffness, which is a symptom of the thickness it separated the cores. Interestingly, i could hear the fibers snap when i weighted this core, and it deformed at the end. Not sure why – further thought is needed.
  • I will use Soric for the side pieces. The ease of putting it into the stack and having it follow tight curves makes it far better for me than the foam. The time and effort to score, hole, form and shape these pieces isn’t worth the extra 600g of weight the chassis will carry as a result.

Hard Point design for the tub

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Here’s the design for my hard-point for harness mounting bolts. It’s 3mm thick steel, and the channels are 2x1mm cuts. What you’re looking at is the underside. The channels and holes are to allow resin to flow through and past the hard-point. The big hole in the middle is where the 7/16 insert will go and get welded. Each channel has been tapered down at the entry point so the resin can easily flow into the part.

I’m hunting around now for prices to get them CNC’d. I need 6 per side (six point harnesses) and I’m getting a couple of spare for welding practice.

When they actually go into the part, they will need a layer of glass-cloth either side to insulate them from the carbon to avoid any galvanic issues.

All hail Dolphin Glaze for filling holes

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So, in feeling around the part, I’ve found a few small undercuts where I’d repaired the gel-coat with more gel-coat. I went around it with my fingers to look for issues, and found a few where the repair I’d done accidentally still went in a little, like a dimple. I’ve started using Dolphin Glaze as filler – it’s really thin and spreads well. It also comes in a squeezy tube so application is easy. I also bought a pack of onion paper for mixing on – wish I’d done it much sooner.   

It’s styrene based though, but because I’m spraying in a barrier gel-coat first, I’m not concerned. 

So, next is to make a 2×2 frame and apply plastic sheet to keep the dust down, and then I’m making cloth templates.

First template for the core

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This is for the side. It’s a complex shape so multiple pieces of card cut-and-shut. Once I’ve transferred this to a 5mm core I will cut the core and heat shape it to the side. 

I’m using solid core rather than flexible in pursuit of minimum weight.