Help! My former that took ages collapsed under test.

[flanajb]

Any seasoned vacuum pressers out there, I would really appreciate your help here. I made a former out of 18mm mdf and 18mm chipboard that I had lying around in the garage. The former looked great, but on the 1st test run before gluing up, it did not even get to full vacuum before the vacuum just destroyed it

I have attached a couple of pictures. Did I go wrong by using mdf / chipboard ?

Any pointers really welcomed

Thanks

 

[monkeybiter]

I'm not a vac. presser but hopefully some background knowledge might be constructive, sorry in advance if I'm teaching Granny to suck eggs;
A vacuum doesn't exert any force by definition, in this application it removes the equalising force from inside the bag so that the only forces left are from ordinary atmospheric air pressure ALL AROUND the outside of the bag, exerted in all directions. This means that on those large ends the air pressure is pushing inwards and at the weakest points the ends gave way. I would think that you should include some support struts horizontally from one flat end to the other to resist the external forces. HTH

 

[Anonymous]

plus had it been an MDF end it wouldn't have happened.

 

[jasonB]

The vacuum bag pulled the side pieces in you need to add packets between the ribs that come right to the edge., do these between all ribs not just the end ones. The way you have made it with the side stripps glued to the main ribs did not help either, you can see the glue joint has failed/flexed and the broken board is a continuation of the fail line.

This is actually going to be quite a difficult one to bend in the bag, what material have you decided to use for the laminations in the end?

You may also want to add a "skin" to the former befor laminating teh actual shape then there is less risk of the ribs ghosting through

Mark if you look its both the MDF and chipboard used on teh outer webs that has failed

J

 

[flanajb]

It did not fail on the glue line, the line you see is the 9mm slot that I cut into each panel. I did this so the individual formers could be removed. I did fail to support between the centers of the former horizontally. I think the collapsing happened here. I was amazed at the force and how it bent the 18mm inwards.

I was able to create the curve without issue using the following layers. It was pulling down nicely, before it all went pear shaped.

5mm flexi ply + 1.5mm skin ply + 5mm flexi + 1.5mm skin + 5mm flexi + 1.5mm skin

I did think of adding and outer skin, but came to the conclusion that I would struggle to get the bend in without any buckles by just screwing the skin to the former without any vacuum ?

The headache, is that it weighs a ton already and adding further bracing will make it hard to lift. If MDF is ok to use, I will scrap that one and make another using lightweight MDF and then do the bracing differently. The questions I am also struggling with are.

1. What the widest spacing you can have between the formers before the 5mm skin ply will start to sag between them when pulled down under vacuum presssure ?

2. Given MDF is pretty tough, how much bracing do I need to put in. Would a brace every 200mm not suffice or should I go every 100mm ?

3. Would making the former out of foam be a better option ?

Thanks

Justin

 

[Anonymous]

plus had it been an MDF end it wouldn't have happened.

Again very early in the morning and i didn't notice the broken MDF, sorry i wasn't of help.

 

[spg]

As you suck the air out then the bag will pull into the voids between the veritcal sheets. Those sheets with voids both sides will have equal pressure so will be stable as long as they don't crumble. The ones at the end only have the void on their inward facing side so will be pulled that way.

The solution will be to use noggins to stop the sheet from being able to pull inwards but depending upon strength of the vacuum then you might just end up with small areas failing between the noggins. So will also need some hardboard or similar to spread the load around.

HTH

 

[woodbloke]

I can't see the pics at the moment, but I'm guessing that the former in question is some sort of hollow form with bracing pieces. Fwiw, when I've seen formers made for pressing in the vac bag, they've always been completely solid. In other words, the first master is made from 18mm mdf and then the rest are bearing cut from it, usually using some cheap and nasty chipboard. When all the sections are complete they're bolted together with some long lengths of studding, after which the whole top surface is covered in some 6mm bendy ply - Rob

 

[flanajb]

I can't see the pics at the moment, but I'm guessing that the former in question is some sort of hollow form with bracing pieces. Fwiw, when I've seen formers made for pressing in the vac bag, they've always been completely solid. In other words, the first master is made from 18mm mdf and then the rest are bearing cut from it, usually using some cheap and nasty chipboard. When all the sections are complete they're bolted together with some long lengths of studding, after which the whole top surface is covered in some 6mm bendy ply - Rob

I agree a solid former would be the way to go, but I would end up with a solid 600mm * 500mm block of MDF and chipboard. That would equate to about 2.5 sheets of board. No way you would be able to lift that

 

[Anonymous]

I can't see the pics at the moment, but I'm guessing that the former in question is some sort of hollow form with bracing pieces. Fwiw, when I've seen formers made for pressing in the vac bag, they've always been completely solid. In other words, the first master is made from 18mm mdf and then the rest are bearing cut from it, usually using some cheap and nasty chipboard. When all the sections are complete they're bolted together with some long lengths of studding, after which the whole top surface is covered in some 6mm bendy ply - Rob

I agree a solid former would be the way to go, but I would end up with a solid 600mm * 500mm block of MDF and chipboard. That would equate to about 2.5 sheets of board. No way you would be able to lift that

you could make them as ribs, with a solid former in the middle and at either end. that cut the weight down drastically

 

[woodbloke]

I can't see the pics at the moment, but I'm guessing that the former in question is some sort of hollow form with bracing pieces. Fwiw, when I've seen formers made for pressing in the vac bag, they've always been completely solid. In other words, the first master is made from 18mm mdf and then the rest are bearing cut from it, usually using some cheap and nasty chipboard. When all the sections are complete they're bolted together with some long lengths of studding, after which the whole top surface is covered in some 6mm bendy ply - Rob

I agree a solid former would be the way to go, but I would end up with a solid 600mm * 500mm block of MDF and chipboard. That would equate to about 2.5 sheets of board. No way you would be able to lift that

Agreed, it would be a mite heavy :-" but it's the best way to guarantee that the former won't collapse. To be fair, when I've made these in the trade, there were always other makers around to help lug the things around, which did help somewhat. If you're on your own, it does present a bit more of a problem - Rob

 

[Boatfixer]

I would go with marks idea - solid ends with hollow spaces and ribs inside. I reckon you could get it lighter than it is now and re-use a lot of the former you have already made. You may need to put some breather holes in the end plates though to equalise pressures.

Graham

 

[flanajb]

I can't see the pics at the moment, but I'm guessing that the former in question is some sort of hollow form with bracing pieces. Fwiw, when I've seen formers made for pressing in the vac bag, they've always been completely solid. In other words, the first master is made from 18mm mdf and then the rest are bearing cut from it, usually using some cheap and nasty chipboard. When all the sections are complete they're bolted together with some long lengths of studding, after which the whole top surface is covered in some 6mm bendy ply - Rob

I agree a solid former would be the way to go, but I would end up with a solid 600mm * 500mm block of MDF and chipboard. That would equate to about 2.5 sheets of board. No way you would be able to lift that

you could make them as ribs, with a solid former in the middle and at either end. that cut the weight down drastically

I did think of that. I assume you mean ribs like that shown below

But you still need to add in some serious bracing between the solid formers, otherwise, they will just collapse ?

 

[Boatfixer]

I would be doing something like that but braced across the bottom as well. A solid end panel with a stack of hollow sections (say three or four) glued and screwed to it then another solid panel. Build up like that to get the length you need with a solid panel at the end and you have a substantial former that is essentially hollow so lighter. Just put some breather holes in the solid panels.

Graham

 

[flanajb]

I would be doing something like that but braced across the bottom as well. A solid end panel with a stack of hollow sections (say three or four) glued and screwed to it then another solid panel. Build up like that to get the length you need with a solid panel at the end and you have a substantial former that is essentially hollow so lighter. Just put some breather holes in the solid panels.

Graham

Your plan sounds like a good one, but just to throw a spanner in the works, I am now contemplating making the former out of extruded polystyrene, just because of the weight issue.

I have just calculated that a 500mm wide former requires 28 * 18mm ribs. With a ratio of 1:3 I have

8 full size ribs each having an area of 0.266m2 and a weight of 3.6Kg
21 cut down ribs. Worked on the basis that leaving making them 75mm from the former edge should be ok. 0.119m2 weight = 1.6Kg

This is based on a sheet of 18mm MDF weighing 40KG

Therefore 3.6 * 8 + 21 * 1.6 = 62.4Kg Still too heavy for one person to lift. I think foam is the only option

 

[Eric The Viking]

It looks like the end piece broke because of leverage - sideways pressure at the bottom of the bag forcing the bits outside your slots to bend inwards. Air pressure on its own probably isn't enough to do this, but it's amplified by the leverage.

Imagine causing those breaks by hand - where you would apply force to do it. You'd get a similar effect, but not quite identical, by pushing the bottom parts inwards (towards the middle ribs, I mean). If you had three hands (or more) and you pushed all along the nearest edge of the picture, it would probably go as it did.

I think you're on the right track - putting bracing blocks between the ribs and holes to equalise the pressure, just in case (they don't need to be big - air is pretty slippery!). My guess is that if you do that, with braces at the bottom of the outer part, particularly, it will take the pressure a lot better.

E.

PS: pressure/vacuums can have amazing effects. Years ago (1980s), the BBC in Bristol built a submersible robot for shooting video deep underwater. It wasn't cheap. The first version had a beautifully made frame of stainless steel tubing, enclosing the pressure vessel containing the camera, electronics and drive motors. When they tested it they had a nasty shock: It came back to the surface with the posh frame flattened as though it had been hit with a hammer. Water pressure at depth had done it. Obviously it had to be completely rebuilt, but the 'fix' was really simple - drilling small holes in the tubing wherever they had created a sealed piece in welding it up. This let the pressure inside and outside equalise as it descended. It was fine thereafter, although I think they never completely stopped the pressure vessel leaking!

 

[flanajb]

Thanks. I have decided to scrap what I have done and to make the mold using styrofoam. It will be much lighter and easier to handle than one made from MDF.

That is the plan, if I can find out where I can source the damn stuff and not have to buy a whole pallet load of it

 

[tomatwark]

Try this company

http://bagpress.com/Materials_and_Accessories.html

Tom

 

[promhandicam]

Therefore 3.6 * 8 + 21 * 1.6 = 62.4Kg Still too heavy for one person to lift. I think foam is the only option

best you get some of these if you can't lift 60kg - either to eat or alternatively use them as formers!

 

[Sgian Dubh]

Any seasoned vacuum pressers out there, I would really appreciate your help here. I made a former out of 18mm mdf and 18mm chipboard that I had lying around in the garage. The former looked great, but on the 1st test run before gluing up, it did not even get to full vacuum before the vacuum just destroyed it Thanks

Basically, your former was seriously underbuilt-- many years of experience at this end say that, not a mere pressing or three. It needed many more cross-bracing pieces than you put in. You need a cross brace about evey two inches of perimeter length, or less with chipboard or MDF.

The motto probably should be to build these kinds of formers for vacuum pressing to withstand something fairly significant... like a nuclear strike, or else expect to be disappointed. Slainte.