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JPR
07-18-2005, 04:33 PM
This is my first attempt at oven forming ABS plastic. I have a wooden form and a piece ABS plastic that I laid on top of the form. The spec sheet shows a temperature of 234 degrees. I have the oven set at 250F. Is there a problem with going higher? Only part of the shape is forming. Or do I need to have a two part mold with a top and a bottom?

[This message has been edited by JPR (edited 07-18-2005).]

DR
07-18-2005, 06:12 PM
ABS should form well.

What you're doing would be called slump forming I believe. This method won't give fine details.

No, you don't need a two part mold.

To get the part to pull down tightly over a form industrial applications use a vacuum. Sometimes in addition to the vacuum operating through holes in the form they use air pressure on the top side.

JPR
07-18-2005, 07:00 PM
It is gauge panel; however, each gauge sits on a hump so they are turned to face the operator.

Similar to this unit (http://photos.thedieselstop.com/showphoto.php?photo=25949&papass=&sort=1&thecat=998), except the one I am making will have four gauges and sits on the dash.

matador
07-18-2005, 07:37 PM
jpr,that is quite a deep moulding you're trying to "pull".be aware that you need at least 1" or more BELOW the lowest point of your mould.This is so that unformed corners can drape down,and you cut these of after forming.It may also help to have a "ring" around the part.e.g. you make a flat pieceof ply or whatever with a hole the shape of your mold,plus the thickness off your sheet,plus about1/16 extra.when your abs begins to slump,take out of the oven and quickly push the "ring" over the sheet/mould.you may have to reheat-push several times to get to the required depth.Don't rush it.good luck.

------------------
Hans

darryl
07-18-2005, 09:48 PM
ABS seem to be a little less tolerant of temperature than some of the other plastics. True, it forms at a lesser temperature than others, but it will 'lose it' easier as well. No big deal, just take the time to bring it up to temp while checking on it. Set the oven to around 230 and give the abs some time in there. If it doesn't reach forming temperature, crank it up another 10 and give it more time, check again. Once you reach the right point, note the setting on your oven's dial for future reference. This way you don't toast a piece of abs from the start, and you minimize any fumage that may develop.

I find it helps to lay a piece of aluminum sheet in the oven to lay the plastic on. That helps to even out temperature swings while the element cycles on and off, and eliminates hot spots in areas of the plastic.

Plastics differ, but you may find a borderline point where the plastic goes from not sticking to sticking to the aluminum. Sticking slightly is an indicator that the temperature is close to right for forming. If you can't easily raise a corner of the plastic, it's too hot.

If you want to lay it on the form in the oven, it might be wise to preheat the form first, then lay the abs on it, then put it back in for time enough to heat the plastic. If the form is cool, that will make it diffucult to pull the plastic into the details of the form. You will need pressure or vacuum, or a two part mold in lieu of, to get the plastic to form properly. It won't just lay down all over the mold by itself.
Another thing with abs- if it's got that textured finish, you can easily overheat it and that finish will start looking ****ty. This will happen before the plastic is so soft that it might almost lay down by itself. If there's that finish, you won't want a two part mold, so it's back to using vacuum forming.

[This message has been edited by darryl (edited 07-18-2005).]

JPR
07-22-2005, 01:14 AM
Thanks for the help, I got the front panel (http://img.photobucket.com/albums/v228/jpr2004/Misc/DSC00627s.jpg) formed. It will be cut and glued to the top/sides, which will be much easier to form.

[This message has been edited by JPR (edited 07-22-2005).]

ERBenoit
07-22-2005, 12:20 PM
Knowing nothing about plastics forming, (outside of machining) for a first attempt, I'd say that looks good. Nice Job!

Wirecutter
07-22-2005, 01:15 PM
It looks like you formed bubbles in the surface - is that what I'm seeing?

I made a gadget a while back that I wanted to be in a wood and clear lexan/plexiglass case. My first shot was to take a little sheet of it and warm it to low temp in the oven, then form it. It went very quickly from "clear and very hard" to "watery and full of bubbles". Damn - ruined.

Then I remembered... I used to do plastic injection molding. The bubbles formed as a result of the expansion of water trapped in the plastic. Before using the plastic pellets that they'd put in the machine's feed hopper, they'd often dry them in low temp ovens for about 24 hrs. (under 150F, I think it was) If you dried it just before using it, bubbles wouldn't form. I think it took about a day for most of the materials we used to soak up enough SW Virginia summer humidity at room temp before it had to be dried again.

Anyway, this trick worked for me. I put a sheet under a wooden box with a lightbulb inside overnight. Don't want it too hot. Then the next day, I made a crude "bending brake" out of smoothly sanded hardwood. Clamped the lexan down, and heated gently and evenly along the edge with a torch. (low flame, distance of 4" or so, waving back and forth along the edge.) After about 10 min, the edge got really soft. I put down the torch, made the bend, and held it while it cooled.
I'll see if I can post a picture of the end result. I definitely saw a difference when I pre-dried the sheet first. When I didn't dry, I would get fine bubbles along the bend line, especially if I heated too fast or too hot.

My 2 cents.

-M

Wirecutter
07-22-2005, 01:23 PM
<font face="Verdana, Arial" size="2">Originally posted by JPR:
Thanks for the help, I got the front panel (http://img.photobucket.com/albums/v228/jpr2004/Misc/DSC00627s.jpg) formed. It will be cut and glued to the top/sides, which will be much easier to form.</font>

I forgot to say earlier - that looks like nice form, and a cool project. Can you show/tell how you made the form, and what you made it from? This technique gives me all kinds of ideas...

-M

darryl
07-22-2005, 08:49 PM
Nice job, JPR. I see you used the textured finish stuff and it came out well. Good luck with the rest of the project.

As far as bubbles in heated plastic (not in this molding though) plexiglass hasn't done that for me, but lexan has. If you have some clear sheet plastic and don't know what material it is, heat some in the oven. Bubbling usually indicates polycarbonate (lexan) and sudden rapid curling and writhing is usually styrene or some blend of it. Plexiglass seems the most tolerant of slight overheating, and can be pre-shrunk before molding. This gives it a bit more 'give' when reheated for molding, and it will remain more dimensionally stable after molding.

ABS such as JPR is using is nice to work with but keep your fingers off it until it has cooled, unless you actually want to make a fingerprint mold.

[This message has been edited by darryl (edited 07-22-2005).]

JPR
07-23-2005, 06:06 PM
The bubbles are from the sealer that squeezed out. I ran a bead along the inside of the bolt holes prior to final assemble. A rubber gasket would have been much better.

The texture is close to the current texture in the truck. This the primary reason for using it rather than fiberglass.

The form is just a piece of 1x6 wood I had laying aroung. I used a 2-1/2 hole saw to cutout the four holes. Then I drilled three .063 holes in the edge of the board at each hole, two on one edge and the third on the other opposite edge. The plugs created from the hole saw where held in place while a 2-1/4" long brad was hammered into the three predrilled holes. I used plaster to seal the gaps.

Here are the parts (http://img.photobucket.com/albums/v228/jpr2004/Mold_disassembled.jpg) of the mold and excess plastic

A friend found this site (http://www.build-stuff.com/1002plans.htm) which has plans for building a dedicated vacuum oven. Seems very strange to lift the plastic up to the oven though.

[This message has been edited by JPR (edited 07-24-2005).]