Wednesday, 22 July 2009

Thoughts on rafts

Erik asked me for details of how I do the rafts so here are my thoughts.

I am not totally happy with the way they are currently and keep fiddling about with them. At the moment they hold well and give a good flat surface on the bottom of the object, but they can be difficult to remove. I use a blunt penknife to remove them.

I use the long blade to remove the raft from the bed and the object. The smaller blade is handy for clearing out strings from internal areas.

It needs to be not too sharp otherwise it tends to cut into the object, or the bed, rather than prizing them apart, or scraping off strings.

I have developed a thick callous on my thumb while making the Darwin parts, and I frequently stab myself, another reason for not having it too sharp!

I think most people use more sparse rafts than I do. They will be easier to remove, but I find that gives a ribbed base on the object.

My rafts are orthogonal to the axes and the infill is at 45°. I find that convenient because you can tell where the raft ends and the object begins.

The bed I use for ABS is, I think, Foamex PVC foam board. It is a solid dense foam 3mm thick, not the type that is soft foam laminated with paper. I glue it to a piece of wooden floor laminate with Evostick contact glue. Now that Evostick has gone solvent free / water based I find it takes much longer to dry than it states on the tin. Blowing it with a gentle breeze from a fan makes it dry much faster.

Even when it is glued down, I find the warping force is strong enough to lift the edges, so I have a frame around the edge that is screwed down. The foam board is reusable over and over again. I only have to replace it when I have had an accident that makes the raft impossible to remove (head too low, or temperature too high).

Other people have reported good results with Acrylic sheets and of course an ABS sheet will work. I have yet to try these. For HDPE I use a PE-LLD chopping board from Ikea that is 10mm thick.

I find that I need at least three raft layers. I.e. each of the three layers has a definite function.

The first layer of the raft has to stick well to the bed but still be peelable. It also has to be thick enough to cope with bumps and troughs that develop on the bed with use and slight errors in the z-calibration.

The filament diameter I use for the base layer is twice the nozzle diameter or 0.8mm, whichever is the biggest. The height of the head is 0.7 times that. The pitch of the zigzag is 3 times the diameter. The head is relatively low, so that it gives a wide filament pressed against the bed. It is widely spaced so that when the bed has a bump it can spread further without merging. It is extruded at the maximum rate that the extruder will do, which works out at only 4mm/s with 1mm filament through a 0.5mm nozzle. The temperature is 225°C for ABS and 215°C for HDPE. The other layers of the raft are extruded at 240°C to bond strongly to the layer below.

The middle layer's function is to give the raft some strength and bridge the ridges created by the bottom layer. The filament diameter is 1.5 times the nozzle aperture. The height of the nozzle above the layer below is again 0.7 times the nozzle. The pitch is 1.2 times the diameter, so that gives closely packed threads that tend to merge.

The top layer aims to present a flat platform to the object but still be discrete threads so they can be picked off one by one. The diameter is the same as the nozzle and the height is 0.8 times that. The pitch is 1.8 times the diameter.

The raft is cooled back to room temperature with a fan before the object is placed on it. The first layer outline of the object is done at half the normal speed (8mm/s for ABS, 4mm/s for HDPE) and at a temperature of 215°C for ABS and 230°C for HDPE. The first layer infill is done at full speed and at 195°C for ABS and 205°C for HDPE. The rest of the object is 240°C.

One annoying thing is that when I peel the raft most of the top layer is left stuck to the object not the middle layer. This is despite the fact that it was bonded to the middle layer at a high temperature, and to the object with a low temperature. I think the reason is that the contact area is 100% against the bottom of the object, but the top of the middle layer is quite wavy so has less contact area. I think adding another dense layer between the middle and the top will fix that, but waste more time and plastic. It is on my very long list of things to try.

As Erik suggested, small objects do not need to be bonded as strongly to the raft as large objects. Something else I mean to try is some logic like this: -
If the length or the width is > 30mm and the height > 5mm then use a strong raft else a weaker one.


  1. Hi Nophead,

    Have you considered trying Adrian's idea with the oven bag? If it prevents warping, you might be able to go with less raft.


  2. Thanks for the write up. You've been really thorough! I had the same issue with a two layer raft, that it sticks best to the object instead of the raft layer below it. A three layer raft is probably worth the investment for bigger objects, I have been too impatient while not knowing that it was almost essential to having a good raft.

    While you're still fiddling with the rafts, they really do work. In your pictures the objects are really flat. That gives me confidence in you rafting strategy. I assume that there still is significant internal stress in the objects? I've been thinking about keeping the object heated A couple of times, I actually used a hair dryer during the making of the raft and first layers, but I'm not sure if that helped. It worked for Adrian... I've also tried upping the room temperature uncomfortably high, but besides me sweating a lot, made the controlling laptop crash in mid-build. I'd have to recommission my SD card module...

    You had some script generating the rafts for you, right, or do you use skeinforge for it now? Otherwise it would be nice to diff the skeinforge settings and see where the exact differences are.

    When I get back from my holiday I'll try your raft strategy and see if I can start building bigger objects!

  3. Hi Matt,
    I can't fit a bag over my machine because the base of the extruder rises past the bottom of the z-axis, so the beg would get torn off. I will be trying a heated bed though.

    I think the only way to make objects past a certain size is to build in an oven, as commercial machines do.

  4. Erik,
    Yes the objects have more stress in them when they are held flat by the raft. They curl a bit after they are removed, even if they were held completely flat while building.

    I am currently converting a small oven into a SMT reflow oven, so when I have that working I will try putting the object in the oven while it is still on the bed and putting it through a heat cycle up to Tg. That should releive the stress.

    No I don't use Skeinforge for anything other than the filament path.

    Have a good holiday.

  5. Hi Chris

    You might want to repeat toFlecther;s experiment with a heated washer plate 1mm back from the tip to control warping.