Vacuum Forming ABS the way to go? 1

[txaviator]

Good evening, and thanks in advance for reading my question. I am in the process of determining the best manufacturing method for a new product I am developing. If you picture a small roof to cover a golf cart, this is along the lines of the product. Nothing fancy as far as shape, etc. Essentially the product is a large rectangular piece of ABS (3/16") with side 'lips' that extend down from the top around all sides (approx 4" down).

Question: In an ideal world, I envision building around 500-700 of these parts. Therefore, based upon my initial research, injection molding is way out of line. I am seeking advice for the cheapest method of manufacturing these simple products, and my limited knowledge tells me that vacuum forming would probably be my best route?

My problem is that cost wise, I have NO idea what to expect. Can anyone provide even ballpark numbers? Not for the parts themselves, but rather, to set this up initially and make the first part. I have no idea what the molds cost, etc. ANY help and/or advice would be appreciated. The overall dimension will be approximately 60" long (picture a 60" x 40" rectangle built out of 3/16" ABS), and 40" wide, with a 4" lip around all edges.

Any help would be appreciated, and I thank you in advance for any help and direction you may be able to provide.

Sincerely,

Gary Robertson
Dallas, TX

 

[TheTick]

Those numbers are small, even for vacuum forming.

What about molded fiberglass? (think: bass boats, amusement park rides)

 

[txaviator]

Yes, I thought about fiberglass, and I have an extensive manufacturing history involving this product (a fiberglass shop has made hundreds of products for me in the past, although of a different nature). The problems with fiberglass are numerous, concerning this product- unfortunately.
1- EPA charges drive up the cost tremendously
2- Weight is higher than ABS which is higher shipping $ to end consumer, harder to handle being heavier, etc.
3- Labor is higher, to hand-trim every part to exact specs
4- The weight drives up my costs from manufacturer to my facility
5- Cure time...to make numerous products in a short period of time, multiple molds are required. While I realize a fiberglass mold may be considerably cheaper than a vacu-form mold, I would need 20-different fiberglass molds if I wanted my manufacturer to produce me 20-units per day. This also takes up a ton of room in a fiberglass shop. With vacu-form, I am betting they could build me 100 in a day, all with one mold.

Thanks for the tip though! MUCH appreciated!

 

[MikeHalloran]

If you start with 3/16" sheet, the lips will be thinner than that.

You can get ABS sheet with pre- textured surfaces (e.g. "hair cell") that hide scratches. The texture mostly survives forming if you don't stretch it too far.

You still need to trim the edges after forming, but ABS is easier to cut than FRP.

You need a substantial radius at the edges between the flat sheet and the lips, probably 2" would do.

You need a still bigger radius at the corners, I'm guessing 3".

You may want to add shallow steps or other features to the flat surface in order to prevent it from oil-canning noticeably. This could take the form of a large logo. Or you could put some compound curvature in the 'flat' part.

The tooling could be as simple as a shallow flanged plywood box, with the internal corner radius forms also serving as a manifold, a vacuum cleaner, and a bank of heat lamps. The box needs a smooth radius of ~1/2" between the edge and the flanges. You trim off the part of the sheet that doesn't get pulled down past that radius.

Take a close look at the geometry of a drop-in pickup bedliner to get an idea of what's possible.

Refrigerator liners are an extreme case that you can't duplicate with simple vacuum forming; the sheet is nonuniformly heated and/or stretched, and I'm sure they throw away a lot of bad ones before getting the process right.





Mike Halloran
Pembroke Pines, FL, USA

 

[TheTick]

Probably best to just submit a design for quote to a plastics house or two and see what you get back.

With small production numbers, you probably want to have setup costs quoted separately instead of amortized into piece price. You can decide how much to make at a time on your own.

I worked as a machinist for a plastics fabricator just prior to graduation from college. The vacuum forming operations always took a lot of setup time and wasted pieces while the mold reached operating temperature and thermal equilibrium.

 

[MikeHalloran]

Submitting the design for quotes is usually the best way to assess the design. At least with injection molders, it is.

I've had one bad experience with a vacuum molder, who encouraged my designer to ignore my advice and go ahead with a design that was impossible to vacuum form. The molder insisted he could do it, and expended our tooling money, and our time, proving that he couldn't. We had to go into a production with a suboptimal design with extra parts and extra fasteners.

Just keep the basics in mind; the part is drawn from a sheet of softened plastic, and you can't effect local increases in surface area without thinning it.



Mike Halloran
Pembroke Pines, FL, USA

 

[deper]

Mr. TX
For 500-700 parts you would probably want an aluminum tool. We have built V/F tools out of urethane tooling board (REN)for prototype and low-volume stuff but aluminum offers you a much longer tool life and you can add cooling to speed up your cycle time. I agree with TheTick, the best way to get a reasonable estimate is to submit your part design with an RFQ. Your part may also require a "snap-back" fixture to assist in the forming. If thinning is a concern, you could always form 1/4" material.

Doug Pryor
David Wolfe Design, Inc.
Akron, Ohio

 

[patprimmer]

If precision is not required, you can make a male mould from wood and drape the hot sheet over it and press into shape with blocks of wood or even by hand with leather gloves.

The hardest part will be the corners. If you are prepared to cut gussets from the corners, the job becomes real simple. lay the sheet over a flat surface with a radiused or bull nosed edge, heat the bend area with a strip heater and give gravity a little bit of hand assist.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[txaviator]

Thanks to everyone for your quality ideas and responses! I went and met with a plastics shop today, and ABS and Vacuum Forming is the way I am going. I already contracted to have my tooling / mold built, and they should be ready to pull the first parts within a week or so. The entire bid (per part) was MUCH cheaper than I could hav imagined...ie= more profits for me!

Thanks again to EVERYONE and their respectvie answers.

Sincerely,

Gary Robertson

 

[BradJH]

You might also want to look into injection molding with "nickel shell tooling". Google that for sources.

 

[HDS]

Most golf cart tops are thermoformed. You should pay the extra money for a weatherable material. You will find lots of examples on the Spartech webpage.