"Pressure-multiplier" mold configurations

[patrickjchase]

I've tried to search for this in the forum but this is a bit of an oddball question (it's difficult to figure out what terms others might have used to describe the concept) so I may have missed it. Sorry if this is a repeat.

I'm investigating the possibility of making my own skis, with the following assumptions and constraints:

1. CNC for cores and molds, accurate to 0.1 mm or less.

2. Firehose press with a maximum safe pressure of 50 psi

3. I want to be able to use effective pressures (at the ski topsheet) of up to 100 psi

Assumption (2) and requirement (3) are obviously in tension, so the purpose of this post is to get feedback on how to resolve that.

The mold stack is in equilibrium w.r.t. force, not pressure. This means that it's possible to increase pressure in some places by reducing it in others. In particular, if the firehose is significantly wider than the ski then the pressure can be increased by making sure that as much of the force as possible is applied to the ski topsheet and as little as possible to surrounding areas. I can think of two obvious approaches:

1. Use a "true" 3-sided mold base (one that constrains the sidewalls as well as the base), and design the mold top (which I will henceforth describe as a "pusher") so that it's as wide as the hose on top but precisely matched to the topsheet width on the bottom. Such a pusher will convert a low pressure on the hose side to a higher pressure at the topsheet. The issue here is symmetry/balance - If the hose is off-center to one side or the other then the pusher may tend to "lean over" and deliver non-uniform pressure.

2. Do the same as (1), but insert a compressible tamper (for example, a slab of closed-cell foam or an array of compression springs) between the pusher and the ski topsheet. Design the mold base and pusher so that the pusher's flange (the wide part that extends beyond the topsheet on either side) "bottoms out" onto the mold base, such that the tamper is uniformly compressed by a controlled amount. This guarantees that all points on the topsheet see basically the same pressure.

Has anybody experimented with either of these approaches? Am I missing another approach that I should be considering?

Thanks in advance...

[falls]

Does the cat-track not achieve some of this?
High contact width on the top side then smaller contact width on the bottom side where it presses on the topsheet. Pressure is higher at the laminate than in the airbag.

[twizzstyle]

(100psi might squeeze out too much epoxy and you'll have a low resin-to-fiber ratio)...

[patrickjchase]

Does the cat-track not achieve some of this?
High contact width on the top side then smaller contact width on the bottom side where it presses on the topsheet. Pressure is higher at the laminate than in the airbag.

[Facepalm] Yes, it does. That's what I get for posting at midnight.

The cat track is equivalent to the first configuration I described, but with the added constraint that the force is constant per unit length, which means you get variable pressure as f(topsheet width). Does anybody here use a cat track per ski, or is it always one wide track that covers both?

[vinman]

Usually 1 wide track

[Richuk]

It's worth taking a look at these guys: http://www.langzauner.at/de/Produkte/Co ... Skipressen. They're high end.

I can't quite imagine how option 2 would work. An alternative worth considering is the approach taken by Brazen - male female top and bottom mould.

The main issue to resolve isn't the pressure differential along the body of the ski, or between skis. but low presure at the tips. Using pre-preg (ultra high viscosity resins) or pre-cured fibre, will allow you row back on the technical requirements of the press design without affecting the quality of the laminate.

[gozaimaas]

I am always amazed when people talk about pressure in the hose without referencing the size of the gap between the molds.
50psi can mean a lot or a little force is applied depending on the gap. Pressure on its own means nothing.

[patrickjchase]

I am always amazed when people talk about pressure in the hose without referencing the size of the gap between the molds.
50psi can mean a lot or a little force is applied depending on the gap. Pressure on its own means nothing.

100% Agreed! That's actually what I was trying to address in the initial post, though admittedly in an uninformed way (and without having adequately considering the function of the cat track). Please allow me to try again:

In the cat track setup, the each hose contacts some width of the track above each ski. As I understand it, that width is typically 7-8" all along the ski. This means that at 50 psi hose pressure, the track is exposed to 350-400 lbf per longitudinal (along the ski) inch. Assuming a ski with 100 mm waist, this means that the ski is pressured to 90-100 psi in the waist, a bit less in the shovel and tail, and potentially several hundred psi in the tip. I can't help but wonder how many people are having tip delam problems for precisely this reason?

What I was proposing in the original post was 2 potential ways around this issue:

1. Replace the cat track with a profiled (i.e. width varies along the longitudinal axis) "pusher" built into the top half of the mold, and ensure that the hose only contacts the pusher surface (this can be done in a few ways). By doing this one could guarantee that the force per linear inch varies in proportion to the ski width. For example, if the part of the mold that's in contact with a 50 psi hose is twice as wide as the ski at all points, then the ski will see ~100 psi (minus some mechanical losses). Obviously there would have to be some mechanical tricks to avoid "toppling" or "tipping" the pusher, which is why I was asking if anybody else had tried it.

2. Put some sort of "springy" material with a known areal spring rate (lbs per unit deflection per unit area) between the pusher (or cat track) and the topsheet, and mechanically limit the deflection of that material via stops/flanges. By doing that one could VERY precisely limit the applied pressure. This is a very common design idiom in all sorts of applications by the way.

I actually just came up with a 3rd solution, that's much simpler but may be adequate. I'll post a picture shortly.

[gozaimaas]

Reading your post you sound like an over thinker
You are definately over thinking this. If you want maximum pressure applied to your laminate simply keep the gap small and perfectly uniform.

[falls]

If you are doing a 3/4 edge wrap you can leave the tip/tail base material uncut during pressing so you have more surface area in the tip/tail. Then shape the tip/tail after pressing. Doughboy has done this on his snowboards and there is a manufacturer video where they have templates they use to shape the tips after pressing.

With your "pusher" idea you will have to make sure it can conform to the lower mold and the airbags don't spill over the side. Possibly you would be best to use your airbags to push a perfectly shaped top mold down onto the layup. Maybe with a silicone rubber layer in between to iron out any slight defects.

[Richuk]

Just so we are clear, we're talking about pressure maps, not gaps. Pressure maps i.e. cat-track or not, is something we have discussed quite a lot in the forum.

The goal here is uniform pressure map. Uniform pressure is important, if the press is being used to define the resin ratio (assuming wet layup) and the quality of the laminate (void content). The drawback of this approach is the resin ratio for the upper and lower laminate are broadly the same. You don't get to optimise according to whether the laminate will be in tension or compression. The strength of the adhesive bond is a slightly different issue and it not wholly dependent on pressure.

Putting a bar across the end of the cassette the thickness of the tips resolves the pinch issue Falls. Low pressure, or lack of tip compliance is the issue to resolve in the tip section ... well, in my press.

[falls]

I agree. Poor tip conformation to mold where it transitions from camber to tip radius is a headache. Pneumatic rams. Elliptical rocker. Top mold more closely matching the contour of the particular bottom mold and core thickness.

Using a very wide airbag without a cat track should give you even pressure over your whole laminate.

[tufty]

Using a very wide airbag without a cat track should give you even pressure over your whole laminate.

Isn't that called a "vacuum bag setup"?

I'll get my coat.