Replies

1. 
   ring | Dec 07, 2006 01:40pm | #1

   Dave,

   Whatever else you do, make sure you do a "dry run", that is, a clamp up without any glue. You will then see exactly how many clamps and where they are needed. Regarding the form, I've covered it with PVC packing tape to prevent sticking.

   David Ring

   http://www.touchwood.co.il/?id=1&lang=e

2. 
   art | Dec 07, 2006 06:15pm | #2

   I use a good coat of paste wax. Good luck and have fun. Art

3. 
   bps092 | Dec 07, 2006 06:27pm | #3

   For 2 laminations I wouldn't even bother, If you're careful you won't mess up the form that much. In a production run of a lot of radius moldings I've always just put a layer of wax paper down like you would use to bake with. Goes down fast and leaves the MDF form perfect.

4. 
   BigNat | Dec 07, 2006 06:54pm | #4

   Dave,
   The previous suggestions have all been good ones. A question to you;
   How many parts do you want to take from the mould? What kind of clamping system will yob be using? If the answer to the first question is one or two, packing tape and wax paper are an excellent choice. If the answer is lots (ie. a production run), my suggestion would be to cover the surface with a high-gloss post-forming laminate and put wax paper between it and the part. There's nothing worse than destroying a pefectly good mould simply because the part is firmly bonded to it.

   At 300mm wide (which is about 12" if I remember) you could certainly build the mould of solid MDF (For larger panels that can be impractical). To save money and materials, you can build the mould hollow (usually two layers on the outsides and one every
   100 - 150mm) and bend a piece of 6mm mdf around it to create a flat' curved surface. If you are using clamps for the assembly, the mould is fine without internal bracing, but if you intend to use a vacuum bag, you need to put lateral braces between the layers on a hollow mould (or risk catastrophic failure ie. a mould implosion. Sounds like a hand grenade going off)

   Lastly, with a springy wood like oak, it might be advisable to tighten the radius by a few mm from your intended final size to account for springback although, depending on the thickness of the part, the epoxy might be strong enough on it's own.

   Hope that helps.

   Nat

   Ps. I just realised you are only taking two parts off the mould, so disregard the bit about production runs.

   Cheers.

1. 
   Patto | Dec 08, 2006 05:09am | #8

   Thanks for all of the replies,

   The laminations are 4m thick and the end job will be 5 ply*65mm. The curve at each end finished radius will be 280mm inside the curve. Glue will be epoxy, and I had advice today about not clamping too hard or risking staving the glue line. Tennons wil be cut to capture the 3 middle plies.

   I had not really considered springback, but easy to do if I could guess the amount likely.

   The only other trap seems to be wheter to make the outside form in two parts and pull in from an angle on each end. I had not really considered making it a hollow form If I allowed 150mm wall thickness for the inside form the offcuts would be only 250*1200 which I would have little use for - rather not risk the strength of the form.

   Making this form seems a good project for the weekend if I can dodge the garden

   Dave

1. 
   DonStephan | Dec 08, 2006 04:45pm | #9

   Last week I tried to make 6" radius corners for table apron pulling the laminations into a concave mold (one piece mold) and the laminations kept breaking - needed matching convex mold and the laminations weren't uniform enough in thickness.Scrapped that and made convex mold which worked much better. Included a couple extra lamination strips and flexible iron sheet metal strap (1/32" thick?) on the outside to even out the clamping pressure. I drilled about 1" diameter holes in form to catch fixed end of Jorgenson bar clamps. Should have included outer caul 1" thick scrap over the straight ends to eliminate the minor in-and-out that resulted. With epoxy and two-part urea formaldehyde glues that dry rigid, you shouldn't have any spring back unless the laminations required great clamping pressure. Because epoxy has to be poured out into thin layer after mixing to avoid heat buildup I've found UF like Pro-Glue or Unibond to be easier to use. After mixing the UF I pour into squeeze bottle for dispensing and spread with disposable roller. After clampup, pour a bit of the excess adhesive into a baggie or similar and when it is hard the clamps can be removed.Biggest concern with UF glues is that they need I think at least 70 degrees F to cure.

5. 
   oldusty | Dec 07, 2006 06:59pm | #5

   Hi Dave  ,

               I've taped wax paper to the form with success . Also imo you should include some extra length in the glue up and then trim it to exact size on the ends , this will insure a good bond even on the very ends .

                          good  luck            dusty

6. 
   Jamie_Buxton | Dec 07, 2006 07:10pm | #6

   One approach I've just started using is to put the form outside the bag.   You use a few mechanical clamps to pull the laminates to the form in a few places.  The bag is used only to suck the laminates together.   The primary benefit of this approach is that it gives you more options in designing the form.  For instance, it can be hollow, and it doesn't need to have smooth cornners.  Another benefit is that no glue gets on the form.

    

7. 
   douglas2cats | Dec 08, 2006 04:37am | #7

   I dont know that I needed to do this but I've made all of mine with plastic laminate over the MDF and it worked real well. All my glue lams have been with plastic resin glue and any glue squeeze flaked right off the laminate just scraping lightly with a razor blade afterward. Not sure if epoxy would come off any harder or easier though. If you go that route, make sure to factor in the laminate thickness so that your form radius comes out where you want it.

   If you build it he will come.