Best way to hide seems on MDF layers

[marcinito]

I have a project - 5ft dome that is about 5in deep. I will have to glue MDF boards and 3d machine on my CNC router. I've done many molds with seems and used primer however when exposed to long run heat primer failed and small spots would come out.
My question is what would be best solution to hide MDF layer seems ?
Is it the way mold has to be sprayed or baked after words ? I'm pretty sure I wont find material thats 6' x 6' x 5" deep.

[bobsch]

Have you tried Bondo or another body filler?

[spektr]

You probably wont be able to hide the seams.. Everything you might use is harder than MDF. The real question is whether the Vac process is the one ot use. This almost sounds like you could make it by free blowing the unit. Thats the way you make skylights for houses.... sounds a lot like that in your description.

Scott

[marcinito]

The only way I can controll depth and shape on all formed faces would be mold. My problem is that I have to glue 3 layers of 2" thick MDF machine them into 3d dome with 1" border around. We have sprayed our seems with primers but I guess I'm looking for the best one out there.

[scobob]

Originally Posted by marcinito The only way I can controll depth and shape on all formed faces would be mold. My problem is that I have to glue 3 layers of 2" thick MDF machine them into 3d dome with 1" border around. We have sprayed our seems with primers but I guess I'm looking for the best one out there.

I've used Woodbond II glue in the past with pretty decent results.

[mcphill]

I don't know your final goal, costs, time, etc, but another option would be to use Styrospray: http://www.industrialpolymers.com/styrospray.html

You can machine the MDF to finish geometry, and then go just a bit deeper (~1/16 to 1/8"). Spray on a layer of Styrospray greater than the "deeper" you went, and let it cure. Then finish machine the Styrospray after it is cured. You will then have a single contiguous surface of material (the Styrospray) on the finished surface, with the MDF acting as only a support substrate. You could even use something much cheaper/lighter than MDF if you want to, and let the Styrospray layer be thicker if you need that support (you could use styrofoam instead of MDF, for instance).

[jdougn]

Originally Posted by mcphill I don't know your final goal, costs, time, etc, but another option would be to use Styrospray: http://www.industrialpolymers.com/styrospray.html ......

Interesting looking product. Have you used StyroSpray in a vac forming application? How does it hold up to relatively high temperatures (over 300 degrees for ABS) and vacuum forces?

EDIT: This product will not work for vacuum forming since it can not handle the high temps.

Here is a quote from the StyroSpray Technical Data Sheet:

Maximum working temperature of cured coating StyroSpray 1000 should not be used in applications where surface temperature will exceed 180F or fall below -20F. Above 200 degrees Fahrenheit EPS foams will begin to melt, shrink and revert back to solid plastic pellets.

[erikpurne]

Hi all.

I am having the same issue as the OP. We have several large tools machined from several layers (10+) of MDF, and we are having issues with the final parts showing the seams.

The tools were made by gluing the 0.75" layers together with Titebond II, clamping overnight, then machining the next day.

We have tried sanding, Bondo, cyanoacrylate, screws, wax paper...

Any suggestions?

Does anyone else use layered MDF for large tools? If so, what glue do you use? Do you have these issues? Tips on solving them?

Can anyone recommend some sort of coating for the tool to avoid this sort of thing, as well as to increase the tool life?

I'm new at this, so please, don't hold back any suggestions just because they seem obvious to you.

[scobob]

Originally Posted by erikpurne Hi all. I am having the same issue as the OP. We have several large tools machined from several layers (10+) of MDF, and we are having issues with the final parts showing the seams. The tools were made by gluing the 0.75" layers together with Titebond II, clamping overnight, then machining the next day. We have tried sanding, Bondo, cyanoacrylate, screws, wax paper... Any suggestions? Does anyone else use layered MDF for large tools? If so, what glue do you use? Do you have these issues? Tips on solving them? Can anyone recommend some sort of coating for the tool to avoid this sort of thing, as well as to increase the tool life? I'm new at this, so please, don't hold back any suggestions just because they seem obvious to you.

I've had reasonable success by coating the entire edge surface with Titebond glue. I've even thinned it a little with water and used an HVLP to spray it on. Experiment with different viscosities and see if you get the results you're looking for.

[scobob]

Originally Posted by erikpurne Hi all. I am having the same issue as the OP. We have several large tools machined from several layers (10+) of MDF, and we are having issues with the final parts showing the seams. The tools were made by gluing the 0.75" layers together with Titebond II, clamping overnight, then machining the next day. We have tried sanding, Bondo, cyanoacrylate, screws, wax paper... Any suggestions? Does anyone else use layered MDF for large tools? If so, what glue do you use? Do you have these issues? Tips on solving them? Can anyone recommend some sort of coating for the tool to avoid this sort of thing, as well as to increase the tool life? I'm new at this, so please, don't hold back any suggestions just because they seem obvious to you.

I've had reasonable success by coating the entire edge surface with Titebond glue. I've even thinned it a little with water and used an HVLP to spray it on. Experiment with different velocities and see if you get the results you're looking for.

[jdougn]

I tried a number of different approaches but nothing worked well for multiple pulls. Even though the MDF was glued real well it still started delaminating within itself and the edges would start showing again.

The commercial vac former that I visited used mahogany instead of MDF. Mahogany is dimensionally stable but even in #2 grade it's fairly expensive.

On my molds where overall dimension isn't an issue I made one nice clean pull then used that pull to cast BC8009 High Temp Urethane from BCC Products. BC8009 can be drilled, cut, and sanded so it was easy to resurface the mold to eliminate the marks from the MDF.