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Vacuum forming


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kato



Joined: 11 Dec 2010
Posts: 92
Location: 4505


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Vacuum forming

Has anyone used vacuum formed polycarbonate for a robot's chassis? I'm thinking about it as an underbody / chassis for a full body spinner / overhead weapon ant.

What thickness would be good? I was looking at Marto's kitbot thread and I liked the way the servos were just dropped into pockets in the printed robot body, and was thinking about carving a little balsa replica of the robot innards and vacuum forming over it to make a tight and fairly strong base that was light and needed very few fittings.

Thoughts?

Post Sun Mar 11, 2012 4:17 pm 
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MikeNCR



Joined: 05 Jan 2012
Posts: 54
Location: Norcross, GA USA


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I'd have to see a bit more on the design to be sure, but a few general thoughts-

Vac Forming PC is a pain, to do it properly you have to dry the material, which is a good deal more involved than it sounds.


quote:
Thermoforming: Polycarbonate sheet can be thermoformed on standard equipment, with vacuum forming, free-blown forming and line bending the most extensively used processes. While most standard forming techniques can be used, critical process modifications specific to polycarbonate sheet are necessary.

Polycarbonate sheet must be pre-dried prior to thermoforming and heating cycles need to be accurately controlled for uniform product quality. The thermoforming machine should be capable of generating and maintaining sufficient vacuum pressure throughout the thermoforming cycle. A minimum vacuum of 20 inch Hg throughout the entire vacuum cycle is necessary to retain part integrity.

Most commonly used vacuum forming machines with infrared heating elements perform well for polycarbonate sheet forming. Rotary and shuttle designs with automatic or semi-automatic controls are the most suitable because of their timer control accuracy, uniform heating sources and sufficient vacuum power. Single-sided heating has proven effective for polycarbonate sheet gauges up to 0.177 inches. For thicker gauges however, it is recommended that dual-sided heating ovens be used for effective radiation penetration.


That said, doing a simple bowl shape would be fairly doable. If you were trying to get the same sort of pocketing that you see with the 3D printed bots or thin stiffening ribs you'll have to spend a good amount of time tweaking the mold to make sure the vacuum is pulling on the right areas without tearing the plastic. Having some sort of draft angle to the sides will make removing the finished parts from the mold much easier.

Post Sun Mar 11, 2012 4:37 pm 
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dyrodium
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Joined: 24 Aug 2004
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Indeed, polycarbonate absorbs moisture from the air and when you heat it up to form that moisture bubbles and ruins the sheet pretty badly, hence needing to 'dry' the sheet.

Vac Forming is a bit of an art form, you can indeed get some very intricate and strong parts but certainly unlikely on your first go! Making sure parts come 'off' the mould is most of the battle, as well as making sure the details are captured using tiny holes in the mould.

You could try 3D printing yourself using shapeways white strong and flexible, I've seen that used on a few robot frames and if you account for the method of construction (layer by layer) some great strong and lightweight frames can be made. It's nylon 12.

All that said, once you had the mould fine tuned you could pump out hundreds of shells. Laughing

Post Sun Mar 11, 2012 4:42 pm 
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kato



Joined: 11 Dec 2010
Posts: 92
Location: 4505


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I was looking at a Snow Peak or Lifeventure dinner plate as the weapon - Titanium, 190mm diameter, already formed into a beautifully balanced circle, 63 g. All that would be needed would be to add or carve some teeth, and maybe a bit of drillium to keep the weight below the limit.

Put on the polycarbonate chassis a couple of servos, a RX, a brushless and ESC, and away you go - rocketproof from above and (hopefully) below. Of course, there's bound to be more to it than that.

For the Vacuum Forming, I was going to use a very basic rig like this instructable

Post Sun Mar 11, 2012 5:20 pm 
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miles&Jules
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Joined: 19 May 2010
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We use the intake of an air compressor to suck gas out of chemicals when we mix resin/silicone etc...this could be used as a suction device rather than a vacuum cleaner.

Just some thoughts. Never done vacuforming but plan to whip something up one day.
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Post Sun Mar 11, 2012 8:53 pm 
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Valen
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Joined: 07 Jul 2004
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we use the compressor from an old air conditioner
Its great, pulls a pretty hard vacuum makes ~120 psi and its near silent.

Also if you want your bot cut from plastic we can probably CnC it out of a block if you want, not quite as "i made it my self" but hey the offer is there lol.

I don't know about the need to pull a vacuum on the poly, i haven't heard that before, i'd be tempted to just try ovening it and see how that works out.
Alternately look at fiberglass.
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Post Sun Mar 11, 2012 11:33 pm 
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Nick
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Joined: 16 Jun 2004
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The only Polycarb forming I saw was pretty crap - and that was by a very experienced US builder. I would also go with 3d printing, but with a twist; leave some voids to glue in carbon fibre stiffeners.

Post Mon Mar 12, 2012 12:21 pm 
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kato



Joined: 11 Dec 2010
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Location: 4505


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Masters at Morayfield has 0.8 mm polycarb in a one metre wide roll for $20 per metre. That is cheap enough to have quite a few tries before writing the whole concept off. The cost of failure is very cheap compared to the potential fun if I succeed.

M&J, I have a project at work where I have to do exactly that with a compressor. I've re-plumbed a twin head compressor as a two stage vacuum pump to test packaging for air transport. You should see what it does to a marshmallow.

Jake, thanks for the offer. I'll wait to use up a favour when I have an idea worth burning one up on. Very Happy

Post Fri Mar 16, 2012 10:51 pm 
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seanet1310



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If you succeed or fail can you post what you do and what worked or didn't? Will be an interesting little experiment to follow
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Post Sat Mar 17, 2012 12:10 am 
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