Adjustable mold - opinions?

[hallvardaase]

Hi

I am working on an idea for an adjustable mold based on experience from our first mold.

On our first mold we adjusted the amount of tip-rocker by simple placing bits of mdf under the mold to make it higher and increase the amonut of rocker.

I think that by making a pretty thin mold all flat exept from the tip and then making replaceable shims for camber, rocker and tail will work.




This design will of course place limitations on the hight of the tail, but we are not making twintips, so that will probably not be a problem.

Any opinions on wether or not this will work? Or even better; anyone who have tried this or similar designs?

- Hallvard

[sammer]

I've been doing something similar, but I vacuum press.
I'd be worried about the mdf mold breaking if I had a pneumatic press.
I can hear my mold cracking and popping if I use too big of spacer under it.
It's made me jump a couple times.

sam

[MontuckyMadman]

I dont think I fully understand how this would not break the mold under pressure or deform irregularly or behave unrepeatable.
perhaps another material like a plastic is in order other than mdf fpor teh main mold.
Perhaps PE or PP.

[hallvardaase]

Thanks for feedback:-)

I've been doing something similar, but I vacuum press.
I'd be worried about the mdf mold breaking if I had a pneumatic press.
I can hear my mold cracking and popping if I use too big of spacer under it.
It's made me jump a couple times.

sam

Yes, I have seen some of your posts and pictures of some of your setups. Actually our core profiler were inspired by your setup for core profiling.

But back to the mold idea.... How thick is your mold? And the spacers youre using, are they routed/milled to a curved shape? If you have a picture, I would love to have a look.

I dont think I fully understand how this would not break the mold under pressure or deform irregularly or behave unrepeatable.
perhaps another material like a plastic is in order other than mdf fpor teh main mold. Perhaps PE or PP.

I do understand your consern, and it is also my consern about the idea. As far as I can see the two main factors determening wether or not it willl work is the mold materials ability to deform elasticly and the thickness of the mold.

Everything can be bent to a certain radi without breaking, it comes down to material properties and cross-section properties (of course). An example is design of curving glulam beams. The lamellas of curving beams may (or may not) have to be thinner than the standard thickness to be bent to the desired radi without exceeding the ultimate stress capacity at the extreme fibres.

The radi of camber and rocker are quite gentle, so I think that if the mold is thin enough it will be able to deform without breaking. I have not started doing calculations on what radi will be possible for any given thickness, but maybe I will look into it.

Anyone tried using thin molds (for any reason, material savings or other)

- Hallvard

[sammer]

Here's a couple examples of simple vacuum mold designs.
Remember I'm only getting about 13.5psi
Most recent "boys" ski



Both used the same old too thin core.
One way gave me camber the other rocker
This pic the tip is propped up on a mdf block to give some tip rocker.



Even at 13 or so psi the gaps between the shims show up in the finished ski, if your looking for them.
I think if the "shims" under your MDF are continuous, no gaps between the pieces it may work.
The only way to find out is to build it and do a test press.
Sometimes trial and error is the best way to learn

sam

[Richuk]

If the MDF mould is thin, and the camber/de-camber is small you might get away with it. MDF might not be the prime choice of material, depending on the amount of heat and pressure being used, it may deform quicky in unsupported area and you may find it difficult to repeat your results over time (just like MM said). PP or PE, is likely to react in a similar fashion and is expensive.

Plywood is an option, or maybe you can skin your MDF in 3mm ply - perhaps doing the base of the mould is enough. Personally, I would do top and bottom.

Assuming you stay with MDF, have you considered whether this method, with gaps in it, would provide for even pressure across the layers of fibre. If the MDF deforms to provide the shape you require, will this mean you have slightly lower pressures across these areas? If you intend to do this without gaps ... then what's the point, you'll end up using more materials and not less.

[hallvardaase]

Thanks for pictures:-)

No, I dont think it will lead to lower pressure in certain areas. The bladder and cattrack will apply an even pressure. This will depend on how much the bladder is inflated. At least I think so.... Correct me if Im wrong:-)

Actually I think that if I do try this idea out I will make the main mold from wood and not mdf. Because of longitudial fibres, and ability to deal with tighter radius for the camber / rocker without breaking.

I used to be really skeptical about wooden-molds, but I think I will give
it a try. I think that the benefits of an adjustable modl will make up for the downsides.

Im sure that if the method works it will save both materials and work:-)[/img]

[Richuk]

You're creating positive bending stress, which will be resisted by the the aluminium mould, the core and the wooden mould below the core - what a mix!

Wood is more likely to have the elasticity you require - provided you don't go too thin, which would result in yielding and thinning of the aluminium bottom plate (depending on the thickness of the plate). Could this be spotted using a micrometer?

I need to withdraw the idea that you may suffer from low pressure spots ... provided you don't go too thin with the wood.

This is an interesting idea, and I'm looking forward to see the pictures of it in action!

[MontuckyMadman]

Im thinking you do what lish does if your trying to make this cheap and easy and are using these for prototyping.
.060" al/16ga slip rolled to reusable rise tip and tail. Vac bag the entire cassette and then put on shims with rocker/camber induced and cure that way.
Its how you build skis in a vacuum trailer.

[rockaukum]

This will and does work.
I use mdf on edge as a starting base. Next I place any shims needed. For camber, rocker, tip rise, whatever... Next is mdf (5/8') layed on the shims. This sets in between the tip and tail blocks. Next are Ski / Board layup(cassette) Cat track, hose, top mold. I'll be in the shop later today and will take a pic if you are interested.
ra

[chrismp]

yes, please take photos!

[rockaukum]

Okay, here are some pics of the set up. Let me know if you have any questions. I picked this up from another builder friend and he has had no problems either!
sorry for the clutter but the press is quite convenient for storing stuff on also.
You can see the shims for the camber here

here are shims for the early rise..

here is the press under pressure.

Here are the most recent pressing for my son and you can see the camber and tip rise (park rat skis)

[MontuckyMadman]

wow. Its hard for me to believe or understand that type of shimming doesn't translate dents in any significant way.
I wonder if heating added to this design would be really bad. Perhaps the fact that this is a room temp cure prevents it.
I thought about using profiled cores under the 5/8 mdf to create the camber in a flat mold setup.
Another important factor is that you are using 5/8 mdf rather than any thinner material.
Thanks for the pics and details.

[twizzstyle]

I used a bunch of Popsicle sticks to shim my mold for the last pair of skis, and it ruined my middle section of mdf. The underside is all dimpled from the sticks, and the top caved in, making my bases very convex. I use heat though, and had the heat blanket directly on top of the mdf.

[chrismp]

i'm so trying this!

what pressure are you using?