Heating ABS to Forming Temperature W/O Burning

I purchased a used 3-phase 220v QVAC PC1824, which was designed as a skinner, but has the vacuum forming option installed. I first converted the 3-phase to single phase and run it off a 50 amp dedicated 220v circuit. I was told this machine would pull up to .125 inch material with the vacuum former option, but found out it needed more vacuum capacity. I changed out the 1/2 inch valve and lines to 3/4 inch and added a 60 gallon vacuum tank. The vacuum and capacity are now sufficient, but I have another issue: When I set the heater, which is on top of the plastic, to 475 F degrees or so, it will smoke the ABS just before the bottom the material gets to forming temperature of around 300 F degrees. When I set the heater down I can prevent this, but it takes longer time to get the material to 300 F degrees on the bottom before forming.

I was told that most good VF machines have heater element above and below the plastic. Should I use additional infrared heaters underneath the material to make up the temperature, or should I try to redesign another heater like the one above that also slides in and out below the the frame? Of course, I can also try to get by with .093 ABS material instead of the .125 ABS material, since presumably it would not insulate the heat and heat up to 300 F quickly and more evenly.

Your thoughts?

Thanks in advance.

Ray
Monterey, CA

It sounds like you have an issue with the timing and not an actual issue with burning the material (anymore). Without knowing about that machine, yes plastic needs to be heated relatively slowly. Without know exactly how fast it is going now, its hard to say if you have a real issue or not. Upper and lower heaters will always speed things up as will thinner gauge but you may still be getting the best result for your set up. Depends what you're doing and how much and the value to decide whether the upgrade makes sense.

GF

Thanks for the response.

Since my 1st post I made a couple of adjustments-- lowered the temperature to 400F and used a thinner material (.090 vs .125). The result is that the time increased to get to 300 F on the bottom of the ABS, but the top does not burn now.

From experimenting I find the ABS starts to smoke near 425-450F, and setting the heaters to 400 prevents the problem.

Now I'm working on mods to the mold and seeing how many parts will stretch on a sheet of ABS 16x24x.090. I've been told I will likely have to make a stretch assist plug.