Rigidizing thermoformed ABS?

[LenHodder]

I'm searching for a method of rigidizing a thermoformed ABS part. The thermoforming process thins the plastic quite a bit - to about 0.040" in places (started at 0.118"). The walls of this product, in selected locations, need to be supported. We're currently using a polyurethane foam, but the cost is prohibitive - both the material and labor. This is a very cost sensitive application, so a low-cost solution is required.

I'm wondering if there is a spray-on coating that would adhere well to ABS that would add some degree of rigidity. It can't be overly exothermic as it cures, as that would distort the plastic. Any ideas?

Thanks.
Len

 

[Demon3]

I am quite familiar with the process and the thinning you describe because that is exactly how fridge liners are made. You thermoform HIPS or ABS and the corners get thin. The rigidity comes from the polturethane you inject between the liner and the housing. Without that, the whole assembly is very flimsy.

The bad news is that fridges are made in the cheapest possible way and that way is the same one you just described. Therefore it is not going to be easy to reduce cost on it. Could you make some preformed corners of thing sheet and place those in the thermoform mold so that the part adheres to them as you draw down the sheet? I.e., small patches on the corners only for rigidity.

What mechanicals do you need? If you don't need the chemical resistance and impact resistance of the ABS then you could thermoform a thicker sheet (to keep good corner stiffness) of polystyrene. Polystyrene is cheaper than ABS so you could afford a thicker sheet for the same price and your ABS.

If you need the chemical resistance of ABS but not the impact resistance, then you could thermoform a sheet of SAN.

As a note, HIPS is polystyrene plus polybutadiene rubber for impact resistance. ABS is SAN plus polybutadiene rubber for impact resistance.


Chris DeArmitt

 

[patprimmer]

Make it from a thicker sheet or use acrylic or polycarbonate sheet

Regards
Pat
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[Pud]

I'm sure I've seen selective area heating to offset this effect. (I may of course have dreamed the entire episode!)

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[patprimmer]

Pud

That could work, but if you gain in one place you lose in another

Regards
Pat
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[Pud]

Hi Pat,

Indeedy, but the op said selected locations need to be thicker - implies some can be thinner...maybe.

If IR is used for heating, just try covering some areas with cooking aluminium foil for some of the heating cycle. Cheap, quick and dirty - I like it!

Cheers

Harry

Ps: Pat - you up late or risen early?

http://www.tynevalleyplastics.co.uk

 

[LenHodder]

Thanks for the suggestions, everyone.

Selective area heating will probably help the corners (good suggestion!), but I still need to rigidize the side walls. The polyurethane foam works well for that.

Thicker sheet is a problem because it adds cost to the overall product (the pan is quite large), while I only need that extra strength in certain areas. Alternative materials might improve it, but probably not enough to be the only solution.

As Chris noted, maybe foam is still the most cost-effective solution. I might look at the selecting heating for the corners, or adding something behind the corners to provide the strength, and then using a lower density foam to get some improvement in cost. Maybe some design changes to the plastic will improve the foaming process, too.

Again, I really appreciate the suggestions.

Cheers.
Len

 

[Pud]

Hi Len,

Could you not use some of your offcuts (commonly called scrap!) to make some inserts to place in the tool before thermoforming, using (maybe) double sided tape/adhesive so they adhere and stiffen the area? I realise the tooling will need recessing to accommodate them, but that's a one-off cost.

You will also be able to claim to be "green" by reusing your scrap!! LOL!

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[LenHodder]

Hi, Harry.

Well... its a good idea. Only problem is that it will affect the finish of the part. The mold is very smooth, and unless the inserts blend perfectly with the mold, it would leave marks on the finished part. In the right place (like a hidden surface), though, it would work well.

"Green" is important for this product, too.

Cheers.
Len

 

[patprimmer]

Depending on volume it may be cost effective to injection mould the part.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[HDS]

Can you use PS or PP beads and expand them for the foam instead of the PU?

Twinsheet is probably not an option because of cost.

How are coolers made? Are molded foam inserts put inside or is it foamed in place?

 

[btrueblood]

Coolers (the medium-cheap plastic shelled ones, not the styrofoam super-cheap ones) are foamed just before or after assembly of rotomolded and/or blowmolded shells. At least, that was how they showed it on the Discovery channel awhile back.

Could the OP not glue reinforcement material to the hidden side of the thermoformed shell? Assuming that a molded/formed reinforcement could be made cheaply. ABS solvent-welds fairly nicely.

Pud's idea of selective heating has merit, especially if you borrow a progressive forming (multiple draw) idea or two from sheet metal stamping processes. The idea being to rob some thickness from the center of the part by stretching it first, and then redraw in the opposite direction, slowly increasing the draw dimensions to the final form/shape ...

 

[griffengm]

Lenhodder,
If a sprayed-on material might be sufficient then would a strip of original material bonded to the out-of sight surface provide the additional reinforcement?

Griffy

 

[LenHodder]

Thanks for the additional info. We're going to try the selective heating method. That really makes a lot of sense for us. We've also found a material called Isotec, that's used on the back of ABS hot tubs. That might work, but I'm worried that the application machinery is really expensive.

We might be able to glue something to the back, but the irregular shape might make that difficult.

Cheers.
Len

 

[btrueblood]

Len,

I was thinking you might be able to make a seperate mold, or a mold off to the side of your existing ones. The new mold could form sheet into a smaller cavity, but having an i.d. profile similar to the o.d. profile (or whichever side is "hidden"). You would then cut the small mold up into corner pieces, and slap them onto your main part, with a little PVC or ABS pipe cement to bond them.

The idea of selective heating and/or progressive forming is (I think) to get the center of the sheet to stretch a bit first, "pulling" some material to the outside of the sheet before stretching the material down the vertical sides.. I have seen combined blow/vacuum molding done for this reason too - imagine a slight positive pressure behind the heating frame, bulging the sheet before it drops onto the vacuum form die to be shaped. The bulging causes a fair bit of stretch on the center of the part, where it would not normally see much stretch when making a big "pan" or "tray" shape.

 

[griffengm]

Time to consider rotational molding?

Griffy

 

[LenHodder]

Pre-stretching the part is an option. We use a vacuum box on the top (with the mold on the bottom), and draw the plastic up into the box, then release the vacuum on top, and draw the plastic down into the mold. This helps make the plastic thickness more uniform over the entire part, but also tends to create webs where the excess (stretched) plastic has nowhere to go.

We're going to try the selective heating first. I think that will help a lot. Unfortunately, adding extra parts to cut and glue will make the product too expensive. Add a minute or two of labor, and the margins are gone. (More than just the plastic is "thin"!)

I don't know much about rotational molding, but I believe that the cycle time to make a part is too long. We're looking to get to under 2 minutes per part.

Thanks again for the ideas.

 

[ornerynorsk]

There are 2 methods that I am familiar with. Glueing plywood or veneer material as backing, or expanded polystyrene pre-formed backups, as previously mentioned. One could also try canvas or ripstop applied with some form of contact adhesive, similar to frp layup, but cheap. You would still have some flexibility which would be a plus to prevent cracks caused by minor impacts or vibration. Good luck.

 

[ornerynorsk]

Just heard about a thermal spray process from a company called Xiom. Don't know if this would be of help or not. Interesting process, though.

 

[LenHodder]

Wow. You're right... Very interesting process. The heat would likely melt our plastic, but good to know, for future possibilities.

Cheers.
Len