Help with first acrylic bowl attempt

[Kimberly Connor]

Has anyone ever made an acrylic bowl? I have a customer that is wanting one that is triangle shaped and I'm having some trouble with it molding right after it is heated. I don't know if I need to adjust my shape before cutting or what I need to do different. This is my first attempt at something like this and I'm stumped.

Any ideas/tips would be greatly appreciated.

Thanks!!!

[Tom Bull]

Are you trying to cut out a shape and then bend it? Do you need to know the shape to cut that will yield the shape of the bowl?

[Kimberly Connor]

Yes, I am trying to cut the triangle shape out and then bend it over the bowl but it is scalloping very bad when I try to mold it. Not sure what I am doing wrong.

[Doug Griffith]

That would be drape forming. For acrylic, you will probably need a male tool with a female assist. The acrylic needs to be slowly heated all the way through to the point where it becomes completely plastic. Acrylic does not like to form over deep draws so the bowl you are using as a tool may not work.

Any pictures?

[Dave Johnson29]

Yes, I am trying to cut the triangle shape out and then bend it over the bowl but it is scalloping very bad when I try to mold it. Not sure what I am doing wrong.

You need to vacuum form it over a mold. A triangle is a tough one to do as it will pull seams at the corners. To prevent that use small blocks of wood back from the corners to spread the stretching.

You can set this up pretty simply but for a one-off it might not be worth it. Clamp the sheet between toy wooden or metal frames then heat in an oven until it sags in the center. At about 1/4" sag, drop it over the mold and hit the vacuum.

If it pulls to badly at the corners then let the next sheet sag a little further. When it has cooled cut it out. I have done a about a zillion of them in all shapes and it is a very simple process but you have to move quickly after heating the sheet. The difference between sag and plop is about 5 seconds. :)

Do some searching on vacuum forming.

[Doug Griffith]

I've done a zillion vacuum form jobs as well. I've also drape formed acrylic sheet quite a few times. For what Kimberly is looking for, drape forming should do the job. It is a lot more "doable" for the average joe and the forming window is far larger. It sounds like the draw is either too deep or the acrylic is not heated enough. I forgot the optimum number, but you need to heat it to between 275 degrees F to 375 degrees F all the way through. The problem is when you get it to the perfect temp, it picks up all the impefections in the tool. So basically, you need a polished tool.

[Rodne Gold]

You need to heat Cast to 180 degrees C and extruded to 170 degrees C to drape form.

[Michael Hunter]

Doug is right about the acrylic picking up imperfections from the former/tool, but it is possible to minimise the problem.

If the former is metal (or at least heat-conductive), then it needs to be warmed up first or the acrylic will cool too quickly and won't follow the shape. I found that the temperature of the former needed to be about 30C below that of the acrylic for the best results.
Heating the former to the same temperature as the acrylic guarantees that the imperfections will be transferred (which could be good of you had a patterned former).

[Kimberly Connor]

We were looking at vacuum forming for the final product if we get the job because it would be thousands of them in the end. A great production job if we can make it work. I was just trying to do it the other way for this prototype. From your comments I may need to just go ahead and do the vacuum forming. My husband has used this method before at other jobs but has been so busy he hasn't been able to help me with this one.

Another problem may be that I was using a wood bowl because it was the exact shape and size that I needed. That may be the problem as well. Also, I don't think I was getting it hot enough. I was putting it in a 250 degree oven for about 4-5 minutes and then molding. I don't think I am getting it hot enough all the way through. We use this time and temp for another product we do but it is much smaller and thinner.

I'll try adjusting the temp and time and I'll post some pictures when I finish to get your opinions.

Thank you for all the tips. If you have any more just let me know. I'll try anything on this could it has the potential to be a wonderful and money making job for us! We haven't had to many that had the potential to be this profitable and I don't want to loose it! :-)

Thanks again!!!

[Rodne Gold]

You need to heat to 350 - 250 is way too low.
The pex should come out the oven like a rubber sheet.

[Doug Griffith]

Another thing about vacuum forming is material yield. A triangle will be horrible. Vacuum forming machines need seal off around the perimeter as well as gripper. So picture putting the triangle shape within a rectangle, then add about 2 inches all the way around. After you trim the part, the rest is waste.

Also, trimming a triangle out of a compound curve would be best done with a 5 axis router. I'm sure a fixture could be built for a hand router but the results would be questionable.

If I were to get that job, I'd CNC a few cheap male and female tool pairs out of layered MDF. Small ventilation holes would be drilled across the surface and they'd be covered with fine felt. I'd then pre cut the triangles and heat them in an industrial oven laying on more felt. Once to temp, I'd load the acrylic onto one of the male tools and then place the female on top and allow gravity to compress them together. I might add a weight. An air hose would then be used to blow through the ventilation holes to speed up cooling. Multiple tools would speed up production since the parts need time to cool.

The trick is finding the optimum temperature that doesn't mark the surface. A heated tool isn't required for this type of drape forming because the part doesn't get as hot as vacuum forming. Besides, if the tool is heated, the part won't be allowed to return to a rigid state for removal.

Heated tools are required for vacuum forming to get more definition and prevent what's called chill. Chill is what happens when the tool cools the plastic too fast and it starts to solidify and shrink as it stretches.

Cheers,
Doug

[Kimberly Connor]

Thank you to everyone for your replies. I have almost all the material to make my first vacuum former. I'll post pics as soon as I have finished (of the vacuum former and the triangle bowl).

Hopefully that will solve our problem!

If you have any other hints before I'm finished just send them on over.

I love this place!!! You all give such great advice!!!

[Doug Griffith]

A few things to take into consideration if you're building your own former.

1) Heating the plastic is best done from both sides of the sheet.
2) For a male tool, moving the tool down into the drooping plastic will give a more even wall thickness than moving the tool up into the drooping plastic. Female tools are the other way around.
3) The oven(s) should extend beyond the grippers to allow full use of the sheet.
4) Mount a fan so it can cool the plastic at the end of the cycle.
5) Screens can be placed in the center of the oven to help give a more even heat coverage. It's a poor man's version of a zoned oven.
6) Small teeth can be incorporated into the grippers to help hold the sheet during forming and keep shrinkage from pulling the plastic out.
7) Timing is everything.

Also, since you are making bowls, the decorative side is the inside. Ideally, you would use a female tool. The surface of the tool should have a very fine bead blast which will allow air to evacuate.

Cheers,
Doug