Saturday, May 28, 2005


Extruding polymorph

Vik has succeeded in rolling accurate polymorph rods and putting them through a glue gun:

I decided to try some more extrusion experiments with it.

(Lest we all lose our sense of wonder, not - of course - at the RepRap project, but at the prodigious communication technology of the age, I'd just like to take a sentence to say how extraordinary it is to conduct experiments collaboratively with a person whom one has never met, never even talked to, and who is positioned antipodally as far away as it is possible to get and still be on the same planet; the results flash to and fro through light-fibres and a 12-hour time difference and appear instantly on a blog that is stored in a Google/ Linux box in San Francisco or Mountain View, CA; this again is about as far as it is possible to get from both of us... as Vik himself said, 'Fun this, innit?')

First I made a nozzle from an ordinary M4 screw:

I cut off all but a few mm of the thread, drilled out that thread from the back with a 1.5 mm drill almost but not quite to the top face of the head, then finished the hole with an 0.4mm drill (which is quite tricky to hold in a chuck, I can tell you...). Then I turned the head down to a shallow cone in a lathe.

Next I made an aluminium block with a 4mm hole almost all the way through, and tapped the last few mm at M4 for the nozzle.

I cut a slot in the block to fit the heater out of a glue gun. I greased a 4mm plastic rod with vaseline, put a tight-fit O-ring on it, and pushed it down the 4mm hole. In an oven set at 100 degrees C I cast a block of isocyanate/polyol thermoset round this to give a lead-in for the polymorph rod where it would be kept at a low enough temperature to maintain it as a solid. Here came a piece of serendipity: isocyanate+polyol normally needs to be cast in a vacuum to outgas it in order to get a decent result. But here the bubbles it generates as it sets became a positive advantage - they made the block weaker, but they lowered its thermal conductivity, keeping the right-end cool, even when the aluminium block was at full temperature. The isocyanate held the O-ring well, giving a good seal.

Not shown on the diagram above are 4 M3 screws. I drilled and tapped the aluminium block M3 where it would join to the thermoset and put four screws in with their heads protruding about 4 mm. These were to give a strong key for the thermoset to hold it onto the block.

Before I did all this I boiled up a scrap lump of isocyanate in cooking oil - it was stable and happy up to at least 180 degrees C. Here is the finished extrusion head together with a rolled length of polymorph. The hole bottom right is for a thermometer.

(Incidentally, glue gun heaters are really simple - they consist of a ceramic resistor sandwiched between two aluminium plates with a small spring plate to make the whole thing push-fit. The result is wrapped in a high-temperature polymer sheet for electrical insulation. The mains is connected across the aluminium plates. The resistor is about 800 ohms at 20 degrees, but this rises with temperature, giving an amount of negative feedback.)

As Vik found, the 4mm polymorph rods are very easy to make. You just put a lump of the stuff in boiling water to soften it, take it out, and roll it between two flat rigid plates using a 4mm diameter rod to get the thickness. You might imagine that you would get the best results with two such rods, one either side of the rolling polymorph, but it actually works better with just one. This gives a slightly undesized rod, as the plates are at a slight angle to each other. But the physics of the situation makes the polymorph roll immediately adjacent to the rod, so the reduction in diameter is very small. And, if you stop rolling before the polymorph fully sets it springs back a small amount, compensating for its reduced diameter.

I used polymorph granules from Maplin, but Vik also found a company (see below) making a similar material already in rod form: WFR / Aquaplast.

Finally I put the whole thing together, plugged in the heater, and pushed the polymorph rod through by hand. I would guess that I was using a force of about 70N.

The material extrudes well at around 120 to 130 degrees C. Note the change in colour as it sets. The final diameter was 0.5mm, indicating a small amount of die-swell, as would be expected.

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