Friday, 1 January 2010

New Year New Plastic

Just over a year ago a friend asked me to make replacement for a broken clip that was part of a light fitting. It was not too difficult to model and I made a copy in ABS with 0.3mm filament, 0.24mm layers.



It did the job mechanically, but with one obvious aesthetic problem: -



The original clips were made from transparent polycarbonate and all I had at the time was green ABS. I didn't use PLA because I worried the lamp could easily get hot enough for the clip to go soft and drop the shade. The only transparent thermoplastic that I could get hold of in filament form was PMMA (AKA acrylic / Perspex, etc), which is available in 1m rods. It is too stiff and brittle to use in my previous extruder, so I promised to have a go when I moved to a pinch wheel design.

The first attempt was a complete failure. It melts at 130 - 140 or 165°C depending where you read. It has a relatively high glass transition, 100-114°C, again depending where you read. I found I could extrude it with a fair amount of force at 180°C. It is very viscous with plenty of die swell. I couldn't get it to stick to anything, including itself, at that temperature though. It isn't sticky like PLA, so it wouldn't stick to masking tape. The obvious second choice was a sheet of acrylic as all thermoplastics will stick to themselves.

The general rule of thumb to make plastic weld to itself is that the average of the temperature of the hot part and the cold part has to be higher than the melting point. So to get it to stick to the base, which is at room temperature it would have to be extruded at twice the melting point minus the ambient temperature. The only plastic that seems to break this rule is PLA which melts at 160°C but will bond to itself at 180°C. I think it is something to do with it having a low glass transition and / or that it is sticky like a glue when it is molten.

I upped the temperature to 240°C but it started to hiss and smoke and still did not bond to the base. Lots of places quote the boiling point of PMMA to be 200°C! I dropped the temperature back to 220°C and it is much happier, but still does not stick .

So the only way to make it bond with itself is to raise the ambient temperature. Cue the heated bed. I set the temperature of my aluminium plate to 100°C, the hottest it can safely be below the glass transition. I taped a small scrap of 3mm acrylic sheet to the middle of the bed with Kapton tape. From my experiments before I estimate the surface temperature would be about 85°C. That gives an interface temperature of about 150°C and that seems to be enough to get it to bond to itself.



Here is a short video of 0.3mm PMMA filament being extruded at 16mm/s: -



Here is the finished object: -



It was not too difficult to release from the bed with a penknife once the bed has cooled that is, I keep forgetting that it is hot! The bed takes ages to cool unless I blow it with a fan.

I am very pleased with the final result. I only had 1 meter of 3mm filament to get this right and I managed to find a suitable bed material, temprature settings and make three clips. The build quality is excellent even if I say it myself.



So another useful material in the RepRap arsenal. Apart from HDPE I think it has the highest working temperature. It is very stiff and brittle though. I had a couple of jams due to it snapping where it enters the extruder barrel. The alignment is not quite right because being so hard it does not press into the worm pulley as far as other plastics. The extruder could do with an adjustment there perhaps, or a bigger entrance to the pipe.

It is a bit more transparent than PLA. It smells a bit more when it is extruded, but it is not an unpleasant smell, I would describe it as sweet and aromatic. The major downside is that it is only available in rod form, so the biggest object you can make in one go is 7 cm3 and at £1.49 per meter on eBay, it is comparatively very expensive.

11 comments:

  1. Wow nophead, your ingenuity and tenacity are a real boon to the home fab community. I think I would have just painted the green ABS white though...

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  2. Wow, never thought we can drive PMMA through an reprap extruder. You're lucky that you have a moving bed, fixed extruder. Otherwise I guess it would crack just due to the extruder movement.

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  3. Worth every bit of warped foam board an amazing achievement.. opening new frontiers in RepRapping.

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  4. This is such great news! Did you know that FabLabs have lots of leftover acrylic pieces from laser cutting?! Now we 'just' need a way to make filament from it :) I would love to start recycling soon!

    I discovered that PMMA would *probably* extrude when I crashed my print head into the acrylic once. Some 10mm of PMMA was extruded afterwards.

    Good to know how to get it to stick somewhat and actually build something!

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  5. That is brilliant, your scientific approach always amazes me - I am such an amateur in comparison!

    I have done some experiments with milk containers, I washed and thoroughly dried a 4 pint container, cut it into smaller parts and then put it into a food blender. This makes a fairly granular powder so what we need is a machine to extrude this powder into a continuous rod. Thats beyond my capabilities at the moment!

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    Replies
    1. hi there
      just wondering any forward on turning powder to continues rod??

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  6. Thees two last posts really prove that you are the master. I am really happy that I follow this blog.
    Although I don’t think that I will ever build something with a hot bed I am working my courage up to a mendel.. Just have to move from the flat first so that I can get a garage to work in...
    All the best for the new year from your fan in Sweden!

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  7. i have one question, so is there any chance to turn PMMA (Acrylic) to filament form. i am researching on suitable material for 3d printing for a green house. PMMA is better UV resistant than HDPE i think.

    anyone trying to 3d print a green house here???

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  8. It could be made into a filament but would have a very big minimum bend radius at 3mm. 1.75mm would be more practical I think.

    Polycarbonate filament is available and that is used for greenhouse windows, so I think that would be better.

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  9. is there any chance for PMMA to come as filament?? not PLA not ABA but PIMMA?
    is 3mm rod is the best is exist?

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