Monday 22 September 2008

Going green

Getting a bit bored of natural ABS, which is cream coloured, I ordered some black and some green from New Image Plastics in the USA. Despite the high cost of shipping it still worked out cheaper than buying it in the UK. I chose green because it should be easier to photograph than black or white and it the RepRap logo colour.

I was surprised to find that it does not behave the same as the natural that I got from the RRRF. It does not seem to stick to itself, or to the base material, as well as the RRRF ABS did at the same temperature. I have raised all the temperatures by 15°C and, although it is better, I think it still needs bit more. It's appearance is more glossy, so that might account for it being less sticky, or it may just have a slightly higher melting point. I have no idea if the green dye has an effect or whether the base ABS is different. It will be interesting to try the black.

I made a new version of my screw top pot in order to perfect the thread before posting the STL files on the RepRap Objects wiki. It turned out to be nightmare to get it to print. Besides the problem of it not sticking to the bed, my extruder kept breaking GM3 gearmotors, but that's another story.

The ABS version came out about 1mm bigger in diameter than the HDPE version due to lower shrinkage.

I modified the thread to make it easier to start. The original thread started abruptly at the top of the pot and the bottom of the lid. I modified it in CoCreate by continuing the helix past the top of the pot and then milling it back to 2mm below the rim. I did the same to the lid's thread. That allows the lid to overlap 4mm before the threads engage, making it easier to line up. The thread now tapers out to a thin sliver to make it easy to engage. The result works a lot better than the previous attempt.

Monday 1 September 2008

Screw Top Pot

A day before going on holiday I decided I needed a container to store the lens adapter for my camera. The lenses have caps on each end, but they require and adaptor which is a bit delicate: -

I knocked up an HDPE pot with a screw top lid that just the right size to hold it: -

The outside diameter of the adapter is about 48.5mm so I made the pot I.D. 50mm to allow some clearance. It actually shrank to 48.5mm, so it is a snug fit. Lucky it didn't shrink any more!

I made the thread using the helix tool in CoCreate. You draw a 2D profile and then use the helix tool to spin that round an axis, specifying the pitch. The dimensions were just a stab in the dark: I made the crest of the thread 0.8mm as that is two layers (Nyquist sampling theorem) and made the sides 1mm long 45° slopes, so that made the crest height 0.7mm. The minimum pitch with this profile would be about 3mm so I made it 3.5mm to give some clearance. I also made the the lid 1mm bigger radius so there is 0.3mm clearance from peak to corresponding valley.

A couple of things I missed which would make it easier to engage: -
  1. The thread starts abruptly, but it should have a tapered lead in.
  2. Both the pot and the lid should have a few millimetres with no thread to aid lining them up before the thread engages.
Despite this it works surprisingly well for a first attempt with an arbitrary profile and dimensions.

I am slowly homing in on getting rafts peel-able for HDPE. I made this with the first layer outline hotter than its infill :-
HDPE.raft_temp = 215
HDPE.first_outline_temp = 230
HDPE.first_layer_temp = 205
HDPE.layer_temp = 240
Most of it peeled with a little encouragement from a chisel at one side.

Tuesday 26 August 2008

Bearing fruit

My wife has been asking me to make something to prop up the overladen branches of our dwarf apple tree for a few weeks now. I put it off while I was set up for ABS because I knew I did not have enough to finish my Darwin. Now that I have switched the machine to HDPE it is no problem, but it is now a few days late as one large branch has already snapped off!

We have lots of plastic covered metal poles so all I needed to do was make some Y-shaped end pieces. My first attempt went a bit chaotic while making the arms: -

I wasn't watching it but I figured it got too hot when doing the small pieces so I made the arms thicker.

Better but still very rough, it should look like this :-

I cleaned it up with a penknife and it was functional but it felt more whittled than extruded.

I made a couple more with even thicker arms but I was around to observe what was going wrong this time: -

When building the curved arms Enrique's software switches to 100% fill because it decides part of the layer is two layers from an outer horizontal surface, which a thin sliver down each side is. That would not be a problem in itself but because I have the infill overlap option set it ends up with slightly too much plastic on the 100% layers. As the height increases this excess builds up until the nozzle is actually submerged in the object while it is building it. Amazing that it manages to make anything resembling the correct shape!

What really needs to happen is that if the infill overlap parameter is set then the head needs to lay down the infill slightly faster so that the amount of plastic is still correct. I ran into the same problem with ABS when making an object with 100% fill.

I made a fourth version with the infill overlap set to zero and it was a lot better: -

Still very blobby but all the blobs are down to extruder overrun and easier to carve off. Overrun is worse with HDPE because it seems to be a more non Newtonian liquid than ABS. I.e. it compresses and expands more than ABS does, so when the extruder stops it oozes for longer.

I haven't tried anything to stop ooze yet. Simply stopping the extruder before the end of the line like the RepRap host does should improve it and is easy to do. Reversing the motor drive should also help. Simply stopping causes the extruder flow rate to fall exponentially but backing up a little should stop it completely in a finite time. The shaft encoder can then be used to go back at full speed to where it was before it backed up. There will still be some ooze without a valve but I think it could be a lot better.

Here is the final version cleaned up: -

And here is the tree with four crutches installed although only three are visible from this view though: -