Infill and warping

Now that I can create blocks with different infill densities I decided to experiment to see what effect it has on HDPE warping.

I have been using a standard test shape and a jig made of three nails to make comparative measurements.



I measure from the middle nail to the base with a pair of digital calipers and subtract the distance to a rule placed across the nails. The figure I get is an average of the amount each end warps upwards. Not very precise because the base is warped the other way as well.

The block is 40 x 10 x 20mm because you need about 40mm length before the warping becomes big enough to measure and 20mm height is about where things start to straighten out. Bigger shapes warp more but obviously take a lot longer to make. Each one of these takes about an hour including making the raft, extruding the block, separating it from the base and measuring it.

The block is held flat while it is stuck to the bed of the machine by the raft. It warps when I remove it. I have only recently noticed that it warps even more when left overnight, so some of my previous tests are not that accurate. For example I was quite pleased when I first produced this extruder sized block :-



But here it is again photographed some days later :-



Not easy to compare because of the angle but the uplift at each end probably increased from about 0.5mm to 1mm. It implies to me that HDPE creeps when under prolonged strain, not a very good engineering property. That is the main reason PTFE fails in the extruder.

I made the test blocks with different infill densities and left them overnight before measuring them :-



Here are the results: -

Density	Warp
20%	0.44 mm
25%	0.79 mm
33%	0.47 mm
50%	0.47 mm
100%	0.53 mm

The 33% value looks totally anomalous but that is because I tried a thicker base. Its base is 3mm of 100% fill including the raft, whereas all the other tests begin the sparse fill on the first layer above the raft.

I also tried 1mm filament 50% fill which gave 0.42mm warp showing that not stretching the filament does not give any improvement.

Conclusions: well sparser fill reduces the warping slightly. A thicker base, rather than resisting warping, actually contributes to it. I must point out that once you get less than 50% fill the object is considerably weaker than a solid block.

Finally here is a longer example, which illustrates how warping gets worse the larger the object is. This is 100 x 10 x 20mm with 20% fill. The first time I made it it lifted the raft away from the base. I got round that by increasing the raft temperature by 10°C to get a stronger weld. It was then quite hard work removing it and it caused some damage to the PP bed.



The 40mm section in the middle is only warped by 0.19mm but the ends are well over 1mm. That shows that you cannot compensate for the warping with a crowned bed because it is not a constant curvature. One could probably scan the shape of the base and lay down support material with the inverse curve. I expect it would then pull itself flat.

In my next experiment I will try filling the sparse blocks with polyurethane two part thermoset plastic.

Hot maths

As the power lost through the stainless steel barrel in my previous post seemed very low I decided to calculate it as a sanity check. I ignored the heat lost from the barrel by convection and radiation. They may be significant now but when I insulate it they shouldn't be.

The outside diameter of the tube is 6.35mm and the bore is 3.6mm, so that gives a cross sectional area of 2.15x10^-5m². Its length is 0.05m. The temperature difference over that length is 240°C-50°C = 190°C. The conductivity of stainless steel is 17 W/mK. So the heat flow is 17 x 190 x 2.15x10^-5 / 0.05 = 1.39W. That means it isn't very much compared to the total power required, so that matches my observation.

The amount of heat flowing into the heat sink is therefore 1.39W and it raises its temperature by 30°C, so the heat sink would have to be 22 °C/W. It was just a scrap one I had laying about so I don't have a spec but it is only 70 x 25 x 20mm so that seems in the right ball park.

Sanity checked!

A high temperature extruder?

The standard RepRap extruder can't quite handle the temperatures for HDPE for very long. I have found a high temperature replacement for J-B Weld. The main weak point remaining is the PTFE thermal barrier. PTFE is an excellent thermal insulator but it is not very strong mechanically. It also expands by about 0.5mm at 225°C. Worse than that it seems to slowly creep the more I use it, which makes a mockery of my z axis calibration. Since I got it working again I have re-calibrated it four times and each time it has grown: 0.3mm, 0.2mm, 0.15mm and 0.3mm. I.e. it is now 0.95mm longer than when I built it and a further 0.5mm when it is hot.

I have come to realise that stainless steel is quite a poor conductor of heat compared to other metals:-

Stainless Steel	Brass	Aluminium	Copper
17 W/mK	109	250	400

I bought some stainless steel pipes on eBay that have an outside diameter of 6.4mm and an inside diameter of about 3.5mm. I cut a 50mm length, tapped it and screwed in into a medium sized heatsink. I tapped the other end and screwed in my experimental high temperature heater. I applied heatsink compound to both threads.



I put a thermocouple in the heater and adjusted the power to get 240°C inside the brass part of the barrel. That only required 7.3W. I put another thermocouple at the top of the stainless steel barrel and that only reached 50°C.



Although this is just a lash up, it looks really promising. I can get the temperature even lower by using a CPU heatsink or a small fan. I will make a nozzle out of aluminium or copper with a built in heater and thermistor.

Not only will this stand temperatures up to the limit of the thermistor, which is 300C, but it is also much more rigid and does not change in length significantly with temperature. It should also reduce the amount of molten plastic because of the thermal gradient down the SS barrel. That should give less extruder overrun.