Vacuum forming limits

[Paul Hunt]

Hello
I am new to the SawmillCreek Forum.
Actually I am new to Web Forums completely. I was in touch with Joe from Veneer Supplies, I am looking into building a Vacuum forming system primarily fro bending wood for prototyping.
I would love to here from anyone who has built a Vacuum system. I am looking at a ROBINAIR COOLTECH HIGH PERFORMANCE VACUUM PUMP 15601 only as it is conveniently available at a low cost.

My real question is about Vacuum forming limits I have a project I am about to embark on and want to know if Vacuum forming will work. The Item i wish to form is quite straight forward it has 3 sides at approximately 400mm x 400mm x 400mm and is 1200mm in length. the 2 edges are filleted with 70mm radii, I would like to achieve roughly a 20mm thickness. the aim is that the bent profile will retain the exact profile of the form without the need for additional structure. I was thinking about using 3mm birch ply with rightly 6 layers and an inner and outer veneer of my choice. is this possible, would it need to be veneered in stages? the next question is how much spring back will there be and can this be prevented. the finished form will need to be self supporting and capable of taking weight as it will be part of a side table.

147599

Any help would be greatly appreciated.

Many Thanks Paul

[Jamie Buxton]

The first question is whether your proposed laminate will bend around your proposed radius. This has nothing to do with how you're going to bend it, or how many laminates there are in your stack. It just is whether you can bend the laminate around the radius.
If your 3 mm birch ply is like what I see here, I think you'll find you can't bend it easily around the 70 mm radius you propose. Get a piece and try it. However, around here, I can buy plywood specifically intended for bending. The usual 3-ply plywood has a thick inner laminate and two thinner face laminates. Bending plywood is different. It has a thin inner laminate and thicker face laminates. As a result, it bends easily in one direction. A 3 mm plywood of this construction would easily bend around your 70 mm radius.

[Michael Flores]

Paul, You are starting off with the smartest idea possible and thats asking the questions before you drop money on a system that might not suit your needs. What you are trying to do can be done and be done very fast, however you might need to take a different approach. The tried and true method of doing what you want is done in a bent lamination form. That way you will be able to glue all the thin veneers at the same time and achieve your thickness of 20mm which is almost an inch thick.

I believe in giving credit where credit is due and i got this picture from this site.

http://images.google.com/imgres?imgurl=http://www.woodweb.com/knowledge_base_images/bah/lamination_clamping_2.jpg&imgrefurl=http://www.woodweb.com/knowledge_base/Clamping_Methods_for_Laminations.html&usg=__XRiHIyPgK22kMd-z91q-AAqyPb8=&h=540&w=720&sz=50&hl=en&start=62&sig2=iA-aE6khnLKDxkjE-5NFxg&um=1&itbs=1&tbnid=zuIDrSxQV-y38M:&tbnh=105&tbnw=140&prev=/images%3Fq%3Dbend%2Bform%2Blamination%26start%3D54 %26um%3D1%26hl%3Den%26safe%3Doff%26sa%3DN%26ndsp%3 D18%26tbs%3Disch:1&ei=X7y-S8a6Ao3qtAOIwJHLAw

[Paul Hunt]

Thanks Jamie and Michael for your response.
With regards to bending ply We have distributors and manufacturers within the UK. Does Bending PLY stiffen up when you laminate multiple layers together. I am trying to create a structure sturdy enough for someone to sit on although this is not the intended use.

The 3mm Birch PLY I intending to use has no problem bending into a 70mm Radius I have tried bending the radius the other day.

I am know in a dilemma with regards to Micheal's response I had original thought about using a negative and positive molds put was concerned about pressing the two together mainly due to the 90 degree right angles causing the molds to rub heavily on the ply and veneer , in addition releasing the mold as it has two parallel sides.Having to create a way of pressing the molds together. I become concerned this was not the best solution . I had come across Laminated Form work and was concerned i may end up with a slightly uneven surface where timber were clamped. As my unit is 1200mm in length i was further concerned the timber used would bend slightly not giving me a presently flat surface.

I would love to hear more regarding PROS + CONS The Laminated form work is the cheapest but does it give the results.
The Final result is important to this project as it has to mesh with a cast Concrete Form the full perimeter edges of the Bent ply structure is required to meet flush with its concrete Partner. I am hopping for as true a structure as possible.

[Jamie Buxton]

Okay, if you have bending ply, you're good to go.

Yes, laminating multiple layers of bending ply does produce a stiff structure.

On the springback issue, you reduce springback by having many laminates. That is, two layers springs back more than three, three more than four, etc. With seven layers you should have minimal springback. The assertion that seven layers gives minimal springback assumes that you have good thin glue lines throughout the assembly. If you have glueless gaps in the bend, you may get more springback. I'd definitely use a vacuum press for the job. The vacuum press assures that there are good thin glue lines.

If you put the bending form inside the vacuum bag, and it is hollow, remember that it must withstand enormous pressure.

To make your glue-up less stressful, use a slow-setting glue like a urea-formaldehyde or an epoxy with a slow catalyst.

[Earl Kelly]

Paul,

This is a big undertaking if you have no vacuum veneering experience. But I'll try to help.

First you need a pump that's at least 5CFM, 10 would be better. A small pump that only has 1-3 cfm capacity will take much longer to pump down. Then there's the question of an internal bag form or external, with only the glueup in the bag. Either will work. I prefer to have my form in the bag.

As to the lamination. I would use 2 layers of 3/8" bending luan and 5 layers of veneer. This should get you close to the 20mm thickness. The secret to getting the strength with the bending luan is run veneer between the bending layers at 90 degrees and on top and bottom under the face veneers. So when finished you will have a 7 ply construction. Each layer should be 90 degrees to the previous. This should be strong enough to sit on. This can be done in one shot, it would be difficult but possible, with planning, preparation and 2 people.

There will be some spring back. I would say from 0-1/16" at the bottom of the sides. There's a lot of factors, such as glue type time in press for curing and how easily materials bend around the form. If you are fitting to something you have to decide how much is acceptable.

As far as I'm concerned the only glue practical for this application is a urea resin glue. It dries rigid, unlike most PVA's.

If you can find a preformed plywood structure with needed dimensions, then you could just apply your face veneer. There's a few to choose from.

You may be able to get more info over at Vacuum Pressing Systems.

Here's a small piece I did based on a 3" radius.

Earl

[Mitchell Andrus]

Plan on building a traditionally made main structure, vac for the veneer. Cut solid radii parts, attach flat legs and top panel and you can make spring-back a non-issue - why do you need to bend the whole thing? A simple router sled and built-up shoulder parts... a few hours.

Fiberglass boat hulls are vac bagged, google images of boat mfgrs. for ideas on vacuuming veneer in 3D.
.

[Paul Hunt]

Many Thanks for the replies regarding Vacuum forming VS Bent Laminated Form work.

It sounds like Vacuum forming is the right choice.
I am intending to build a form out of 18mm MDF with a length of 1.5m, How frequent will i need to place ribs. instinctively I would put them every 300mm would this be enough to prevent the form from bending under vacuum pressure. I will try and draw the mold up tomorrow and post a drawing of my intent. the mold will be around 550mm high by 400mm wide by 1500mm in length how large should the base be would it be enough to extend the base out 300mm out around the perimeter of the mold?

I will have to get a vacuum system together as well. I have been advised by Earl that a 5-10CFM pump would be advised. I have been reading all the information posted at www.joewoodworker.com . Is this the correct way to go. I am still trying to get my head around all the info. So far i believe i require a Vacuum Pump rated around 5-10CFM AND then need to build up a EVS or V2 system. is this correct. Is this the best way to go?

Next question, how long do you leave the built up PLY in the Vacuum bag before removing? To reduce spring back I understand the amount of time you allow the adhesive to cure is a factor. I have been advised by Earl that urea resin glue would be the best way forward. how long do you have to work before the glue has gone off?

Many Thanks for all the input so far.
I will be going through a BIG LEARNING CURVE and would like to reduce the stress as much as possible.

Any one who who has experience building molds and pumps for projects of this type I would love as much info as i can get as mentioned i will try my best to get a drawing of the mold with DIM's up in the next 24hr. Would love the feedback.

Kind Regards Paul

[Jamie Buxton]

I think you'll find that ribs 300 mm apart will be insufficient. One atmosphere is 14.7 pounds per square inch. On a panel 30 cm (12") by 55 cm (22"), that amounts to nearly 4000 pounds.

[David DeCristoforo]

I would build the mould with ribs no farther apart than 6" I usually build up a 3/4" thick "shell" over the ribs using 1/8" poplar plywood. I glue and staple the first layer to the ribs and then laminate the subsequent layers using contact cement, stapling to the ends and ribs. Keep in mind that the mold needs a bottom that is "captured" between the ends and sides so as not to collapse. When the piece I need to form is not too large, I make my molds "solid" by building up layers of 3/4" plywood. Yes, you can use MDF if you feel the need to test your back. My "rule of thumb" for vacuuforming is that if you cannot bend the layers over the mold "by hand" they are probably too thick. Localized pressure is not that great in a vacuum press even though the total pressure over the whole surface might be huge. Per square inch, you can generate vastly more pressure with clamps.

[Paul Hunt]

Thank you for the replies I will be drawing up the mold tomorrow, like previously mentioned i will post the drawing as my method may be a bit unorthodox. I was going to build up to ridged MDF boxes taking into account the advice given to me by David and Jamie.

I have been planing to build a series of boxes staking them and then packing and filling the curved corners,the main box being roughly 500mm high by approximatly 400mm wide and 1500mm long then assembling a second MDF Box aprox 70mm high by roughly 260mm wide and 1.5mm Long centered on the top of the larger box. this would give a 70mm by 70mm setback to both sides along its lenght. I would then pack it out with stips of MDF to creat a rough 70mm steeped curve.this would then be filled with hard setting filler , to get the corect radious i was going to use a lasered taplate to creat the final radioused corners., then finish to acheave the required form.

I hope this does not shock anyone?

If anyone feels this is a disaster waiting to happen please say. I endeavor to get my drawing up tomorrow.

I have been looking at pumps. my next question is what should I be doing, being economical and buying a used non specific vacuum pump and going down the DIY route which i am still trying to figure out . or buy a complete Kit from a Veneer Press manufacturer? The UK purpose buit kits pumps look identical to a lot of heavy refrigeration Vacuum pumps.

http://www.airpress.uk.com/frame_set.htm?http://www.airpress.uk.com/UK/products_list_airpresses.htm


http://www.realwoodmachinery.co.uk/machinery_nabuurs.html



Kind Regrads Paul

[Jamie Buxton]

Making the whole structure in one big glue-up gives you two problems. First, the mold is extremely substantial, and so is the vacuum system. Second, the laminates are pretty big, and turn the glue-up into a wrestling match. I think I'd make the piece in several pieces. There would be three flat plates and two corners. The corners would be the 7 cm radius sections plus a few cm of straight beyond the end of the curve. The flat plates would be everything else. You'd need to make a bending form for the corner pieces, but it would be much much smaller than the mold you've proposed. The vacuum system would be much smaller too.

There would be some simple joints between the corners and the flat pieces. You could use a spline and glue, or biscuits and glue. You're going to have those cast concrete sections which will prevent anybody from stressing the splined joint, so it doesn't have to be really robust.

[Paul Hunt]

Thanks once again for your input, I have had several comments made with regards two building up my structure as components rather than one solid bent element.

I intend on reveling the edges and the fact that the structure is a bent laminated form it is important that it seen as bent ply.
In this case i am quite happy to go the difficult rout to achieve the the complicity that bent ply portrays. May not be saying that in a few weeks.

I have added a drawing of the my mould it will be glued and screwed together. I am hoping it will be more than strong enough. might even be over kill. Love to have feed back if at all possible.

I am under the impression i will need to place a base under the mould so that the bag does not crumple up around the mould or to help prevent.
how large should the base be?

Many Thanks once again Paul Hunt

[David DeCristoforo]

I would add a rib in the last cell on each end of the main (lower) section of the mold at right angles to the main ribs, centered, to stiffen the ends. I would also seriously consider making a custom bag specifically for use with this mold. It should be the same shape as the mold and only large enough to be easily slipped over the mold. I would make a table or platform several inches larger than the mold with the vacuum inlet coming up from the bottom. Then I would make a bag with a lip and a frame to clamp down over it to seal it. You would also need a gasket (http://www.m-powertools.com/products/vacuum/gasket.htm) between the bag and the platform to get good seal. This setup is often referred to as a "frame press". JW has a good article on the concept here: http://www.joewoodworker.com/veneering/vacuum-frame-press.htm
This would be more work at the outset but would be much better for making multiples. The main advantage is that you don't have to mess with trying to hump a heavy mold along with several layers of glue covered material into a bag. You can leave the mold on the table, get your laminations ready and drop the membrane over everything.

[Jamie Buxton]

There's no reason for a base like you've drawn. You do need a bottom to the mold to prevent the bag from crumpling into it. It'd also be good to have a hole into the hollow mold so that air can escape when you pump down.

You should test your mold and vacuum system with a dry run before you put glue on your laminates. It may prove educational.