Saturday, November 18, 2006

 

Support material extruder...

Some time ago Adrian noted that we would be using a syringe type extruder for support material for the near to midterm since there had been some problems with his peristalic pump approach. His system uses a putty sold in the UK and parts of the old British Empire called Pollyfilla. It's basically a mix of gypsum plaster (if I recall correctly) and cellulose fibre. I used tons of it to repair cracks in the plaster of my home in South Africa and can attest to it's excellence. It doesn't shrink at all.

Pollyfilla is difficult to come by in the US which is a shame. The generic term for Pollyfilla here is "spackling compound". You can get it in a variety of formulations. I'm looking at the water-based, vinyl/plaster compounds. It costs about $1/lb in gallon cans.

That gets us down to how we can bootstrap a spackling compound syringe extruder without having Adrian having to make one for us. Here's my suggestion. Head on down to your nearest feed store and head over to the veterinary supplies counter and get yourself a syringe and hypodermic needle. I'd suggest getting a sack full since they're not very expensive and gasoline for the ride is still pretty pricey.


I got some 12 cc syringes and some 1" #20 hypodermic needles. Actually, I made this purchase some years ago when I was trying to get my head around the ins and outs of blowing bubbles in molten HDPE. Carefully cutting the sharp end off of the needle and rounding it off with the grinder wheel on my little Dremel tool left me with a 0.635 mm orifice which is just about perfect for what I need. Mind, you can get just about any orifice size that you want by selecting your needle gauge. I used the #20's simply because I didn't want to make another rather long trip to a proper feed store from this rather overgentrified area that I live in.

A few words of caution. Whatever you do, don't ask your doctor for a syringe and needle and for sure don't try to buy them at the pharmacy. There are apparently federal laws about such places selling you such things and if you make them too nervous they could call the police down on you. In the merciful if sometimes schizoid manner in which American law works, however, feed stores are able to supply your needs in this area, or at least the ones in my neck of the woods can. Don't make trouble for yourself.

Also, the business ends of fresh hypodermic needles are sharper than broken glass. Be very careful when you try to saw the end off of one. I wasn't and have a plaster on the end of my middle finger for my troubles.

I acquired a quart (946 ml) of Ace Lightweight Spackling compound for $4.49 retail. It weighs in, container and all, at 280 grammes, which gives it a density of 0.3, which means that it is mostly air.


It doesn't expand much, if any, when it leaves the syringe.


Extruding it from the syringe it behaves very much like whipped cream. You have to take care not to apply too much pressure to the plunger if you want to get a steady thread. I expect that using a pseudostepper that should be no big problem. It doesn't seem to jam. You can apply a thread on top of a wet thread immediately and get no sagging. Further, because of their extremely low density they can be teased into modest three dimentional paths without a lot of effort.

There is a little bit of polymer in this mix, so when the thread dries and hardens after a few minutes it has a bit of tensile strength.

This material looks rather hopeful as a support material.

Comments:
http://wdass.net/reprap/Image6.jpg

Plastic syringe from a vet/pet store/pharmacy... this one was used for feeding liquid medicine to a pet (cat, rabbit or gunnie pig, I can't remember... as you can see, somebody got 7cc's of something on a regular basis!), but I've had smaller ones as well (10-15 cc's max, .1cc increments). I'm pretty sure I've seen the big ones (like this) at the local Osco/Walgreens (pharmacy store) on the shelf, they're harmless...

For the tip, as someone had mentioned in the past, the (removeable) end off a mechanical pencil, .5 in this instance... It 'fits' over the end of the plastic 'needle', just needs some glue to hold it in place... It also comes in .7! and you (probably) won't puncture your finger either! :)
 
A lot is going to depend on whether the spackling behaves like a non-Newtonian fluid like molten plastic does and expands after it leaves the nozzle. For the moment I have no idea whatsoever if spackling is non-Newtonian (I suspect it is) and how much expansion, if any, to expect.
 
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