Questions on Deep Draw Vac Forming 1

[KENAT]

We are working on a product that includes some vacuum formed parts. They are fairly deep draw, the one I'm concerned with right now is about 12" or slightly more draw. We need to maintain reasonable thickness on the parts.

We had a vendor lined up, that did some good prototypes from this aspect, they used .187 start material and on the formed parts the thickest material was around .12 while the thinnest was around .09. So fairly consistent thickness, thinnest material was about 75% of the thickest.

However these parts were a bit too flimsy so we decided to look for .25 start material. We also had to change vendor due to problems with the first company on just about every other aspect of the project.

The new company supplied some first units yesterday. First off they ignored the drawing and made some of the units from .187" material. Then they rushed a sample to us made from .25" material.

All the samples showed severe thinning - in the order of the thinnest parts being only 35% the thickness of the thickest parts.

They are now being, shall we say, awkward about agreeing to improve on this.

Does this sound like something they should be able to improve on and get to a minimum thickness comparable to parts from our first vendor?

On the proto parts starting at .187 material thickness we were getting around .09-.11 in the thinner regions. So with .25" start Material were hoping to get around .11"-.14" formed thickness at the thinnest parts. The sample actually had around .05"-.06" at the thinnest parts.

Thanks,

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[btrueblood]

Kenat,

" order of the thinnest parts being only 35% the thickness of the thickest parts."

Do you mean there was variation from part to part, or variation in all parts from thickest to thinnest sections?

"Does this sound like something they should be able to improve on and get to a minimum thickness comparable to parts from our first vendor? "

Yes, provided they know what they are doing. Sounds like vendor #2 may not have increased the oven dwell time when pre-heating the thicker sheet material, or may not be doing a good job of process control (cycle time) from preheating to mold loading to application of vacuum.

Is this the same part you had trouble with before, and were looking (or were we talking to you about) to add doublers to stiffen it up?

 

[KENAT]

Sorry, while there is variation between parts this isnt' the issue. I'm talking about the thickest and thinnest sections of the same part.

Within reason we don't care how thick the thickest part is, we do care how thin the thinnest part is though.

The original vendor made a big deal out of the fact they did a pre stretch before forming - I don't remember the new vendor mentioning this. I'd like to talk to the new vendors technical guy about this but they are playing silly games and he hasn't been in on the last few calls.

I have asked related questions before but not about this aspect, so I think not guilty on the doubler. We did consider doublers though but ruled it out because it would be awkward and because the the original .187" parts were almost good enough so we though going to .25" material would fix things.

I meant to post this in "Plastics Engineering" but must have hit the wrong thread-minder link. Oh well it kind of fits here.

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[btrueblood]

Yeah, the usual suspects lurk here too. Pat and Chris should be along soon...

Good luck with it, I've given you what I know. It sounds like finding a better vendor is where you are going to end up, sorry to say.

Some things that might help others out - what material, what original sheet size, what are the open dimensions of the mold cavity. What lead-in radius on cavity edge and draft angle. Obviously, the bigger the lead-in radius the better. Preheated metal mold, or is it a quick-and-dirty foam board with gelcoat "prototype" mold? I do recall somebody saying a pre-stretch and/or pressure (blow)/vacuum combination molding process does a better job...

Any chance of changing to a rotomolded part?

 

[MikeHalloran]

I've been down this road a few times, most notably with a part that resembled an angel food cake pan, but in one piece, with a very narrow/tall central core. It was a bad design, but every vendor we contacted assured us that it would be no problem, and sure enough, every one of them bastards could make ... one.

Production lots were either thin-walled or torn through, and accompanied by buckets of promises and reassurances, none of which made the parts look or work any better.

Vacuum forming vendors are, in general, as politely as I can say it, scum. There might be an exception, but (s)he hasn't showed up on my doorstep yet.

The fix for the cake pan was a change to a simple cup, with an additional part, a big standoff, in the middle. I hate adding parts.




Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

Sorry KENAT

Vacuum forming is not my area.

I would imagine that 2 stage could help.

Localised heating could help but may be difficult to control.

The shape of the first draw may be critical.

Regards
Pat
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[loughnane]

We had ThermoFab swing by here about a year ago and showed some parts.

They had some thick walled stuff with a pretty decent draw (I want to say about 10"), but I can't really speak to their process.

Chris Loughnane - Product Design

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[patprimmer]

I have some info somewhere about acrylic bath tubs. I expect they are deep draw, however I think the design compensates by high draft angle and real big rad.

Regards
Pat
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[HDS]

Kenat

The real issue the ratio of draw to sheet size not just the depth. That is why tubs are easier than cake pans.

Did you transfer the tools or make new ones? A new tool could be as much of a problem as a new vendor.

Prestretching is done to improve the uniformity of thickness. As we have discussed before uniformity of thickness is one of the parts of the process that needs lots of attention.

If you need a professional 3rd party there are experts like McConnell and Thorn with excellent reputations. The sheet supplier will also have an expert that can help out. Both of these options have saved me in the past.

Don't give up yet you will get there.

Mike Halloran

I have to respond to your disparaging comments. I understand your feelings after those experiences. I feel the same way about lawyers...

Just like injection molders or any other manufacturer there are good thermoformers out there. However because of the low capital requirements and traditional focus on large parts, it does tend to be more of a regional operation with smaller less experienced companies. Yet, there are many successful national and international thermoformers. I suggest looking for someone involved with the SPE thermoforming division. They tend to be more technical and competative. It also give you a chance to see some of the parts they are showing off.

http://www.thermoformingdivision.com/

As someone else mentioned Thermofab is a good example as are Thermopro, Wilbert and many others.

 

[KENAT]

Well thanks all.

Material is ABS, the specific grade is one of the things the vendor is meant to be helping us define.

The mold is a '2 up' (as the part is open on bottom & one side) so ends up being something like maybe 20" x 18" or so - I haven't seen it in person.

The surfaces of the part are curved so the draft angle varies accordingly. The minimum is fairly small, though greater than 1° but gradually increases - more so on the largest side.

The lead in rad looks pretty tight from the untrimmed parts we saw, maybe 14" or less - though with a generous trim allowance.

The first vendor made their parts off of temporary wooden molds. The new vendor went straight to aluminum.

We're limited on our vendor selection because the part gets lined with molded foam and we want a single vendor that does both so we don't have to manage a supply chain etc. We've only found 3 places that did both thermo forming & polyurethane foam - this afer hours looking on the internet, traipsing round industry trade shows etc.

The 2 we've actually try both do work for aerospace and the like so we were hoping this would suggest they were slightly better. I'm wondering if we would have been better off going with the little garage shop!

Cost is a major driver on this part which is why we went vacuum forming in the first place. We have low volumes - if we're really lucky we may hit 100 a year when the economy picks up - so tooling costs are a significant factor. We did get quotes from some other processes (not rotomolding which I'm not sure it would be suited to) but they were generally more expensive or didn't have the foam capability. We tried to design the part around vacuum forming capabilities, and have been told by both vendors we did pretty well in that respect - though maybe they were sucking up.

My colleague finally got hold of them yesterday and they are going to do a pre draw and send us a sample early next week - so we'll see.

I'm pretty nervous though 'cause our time lines have become real tight due to the issues with the first, and now this, vendor.

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[KENAT]

1/4", not 14!

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[btrueblood]

Kenat, likely none of the following helps due to short timeline left, but:

1/4" seems pretty tight. 14" I thought was a bit overkill, but hey . It might be a fairly straightforward fix to machine the mold face back, and bondo/glue-lam some more material on the face of the mold, and machine a gentler entry radius (say 1/2") to see if it helps, and it is doubtful it would hurt...

A rotomold would just be a big cavity mold with your cover mirrored across the parting line, then you trim the part in half to create your covers. You still need some draft, but get a consistent wall throughout the part. You'd probably have the same issues as always with finding somebody to put the foam in as well. Usually, this ends up being either an internal process or yes, you find a garage shop willing to take on the work.

Third option, not that it helps as you've got a short time left, would be to layup a thermoset compound (glass/epoxy, e.g.) into your existing mold. Messy, handwork job, but there are shops that do it.

I know of a couple vacuum forming shops in Washington state that I'd trust. They would reject part designs pretty regularly as being unfit for their processes, and suggest changes (draft angles, radii increases) to improve them, or material changes. I'll dig a little and see if I can recall the name.

Hmm...Thomas gives me this link, I think it's one of the right ones.

http://www.fabriform.com/

They do RIM molding also, FWIW.

There's another little shop in Spokane, can't recall their name...I could dig if you want it.

 

[KENAT]

Thanks Btrue - I got an update earlier and it seems they're going to try and do a pre stretch and have agreed to a .110 minimum formed thickness which if they achieve it is probably fine. If they'd let me talk to their technical guy I'd discuss some of the ideas you mention. He actually seemed really good - it's the plan manager I'm having issues with.

With the timeline we're stuck making this place work - we're already late.

I'll keep the rotomolding idea in mind for future designs, though again the foam lining aspect may limit our choices.

Thanks for the help.

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[HDS]

It maybe time for s site visit if schedules are tight...

 

[KENAT]

Our purchasing guy visited them on Wednesday just gone. I offered to visit them Monday on my way in but it's not clear they'll have anything new to look at by then.

They're about a 3 hr drive away, which isn't terrible but means a trip to them is pretty much a lost day. There's so much else to do on this task, and we already spent over a week travel time between previous visits to them and more so the original vendor, that a visit is a bit of a hassle.

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[KENAT]

This link from the vacuum forming section in wikipedia has some relevant information.

http://support.knowlton.ohio-state.edu/files/FormechVacuumGuide.pdf

I should be getting parts to look at tomorrow.

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[KENAT]

Well, got the sample late this afternoon.

The uniformity is much better than on the .25 sample we got last week.

However, somehow, supposedly starting with .25" thick material they've managed to make the formed thickness everywhere slightly less than our previous vendor achieved with .187" thick material.

This seems a bit fishy to me, and sure doesn't meet our goal of making it a little stiffer than our previous vendors efforts.

I'm starting to come to Mike's point of view!

(How on earth did I get a star on my own thread?)

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[patprimmer]

Dashing good looks and charm can do it. ;-)

Regards
Pat
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[btrueblood]

Pat, you should get out more.

Kenat, ask them to send the trimmed-off scrap piece from the molded part as well. The outer edge of the original sheet should be at or near the original sheet thickness.

Better yet, have somebody there to witness.

 

[KENAT]

Well we did have someone there last week when they made the first part from .25 material. I offered to go down on Monday when they would have been making the new 'even thickness' .25 material parts but neither the overall project manager or the purchasing guy wanted me to.

The trimmed off material is an idea, but if they are lying about using different thickness material, then I wouldn't put it past them to lie about the trimmings too and send us some from the 'uneven' .25 material - mangled just enough so we can't compare thickness at the cut lines.

There's a bunch of internal politics at our end, and I think some at theirs too which is making it even worse.

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