Heat Blanket - Bottom Only

For discussions related to designing and making ski/snowboard-building equipment, such as presses, core profilers, edge benders, etc.

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dbtahoe
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Heat Blanket - Bottom Only

Post by dbtahoe »

I was chatting with Idris and mentioned I was planning on using only one heat source located on the bottom. I guess will increase camber.

Is anyone else using bottom heat only?

Short of reducing the camber of my press are there other pressing techniques that can be used to reduce camber?

Cure temp/Post Cure temp/time?

We will be using QCM EMV-0049 resin w/ECA-032 hardener.

Thanks for any insight!

TAHOE NEEDS SOME SNOW NOW!
Idris
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Post by Idris »

Make a pair, measure temp camber, camber bock etc. Adjust as required - press another pair.. Beware camber that comes from heat difference isn't always as long lived as that from pressing into a certain shape.... it tends to relax more readily from repeated flexing. Not exacty sure but I think this might be the case of the pair the Kam's built in the Make artical?

Cold press???? Heat both sides?
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bigKam
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Post by bigKam »

this is an interesting subject -- but one that i don't have an answer for yet. we use a heat blanket on the bottom mold and our molds have zero camber (e.g., the one i've been using with heat has zero camber). some of our skis have camber and the camber is bullet proof, that is, it doesn't go away. some skis, like the one Idris mentioned in the Make article lost their camber rather quickly. not sure why, but i strongly believe that the temp gradient between top and bottom of the ski can create camber in a ski. and what is puzzling is that the camber can be nearly permanent, or in some cases, the camber diminishes. i agree that there is temp-dependent camber and camber created by the mold shape itself. which one is 'more permanent' is unclear to me.

anyone out there care to comment?
Idris
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Post by Idris »

Not sure on why one ski keeps its camber and another dosen't. Adhesion between materials within the ski? I've seen skis that had bad resin mix in them delam internaly and just go floppy. Same happens with foam core skis degrading and going extra soft. Early Salamon 1080's were well known for doing this.

At PMGear we had heat blankets top and bottom. With one camber mold you can vary the camber by adjusting the heat on top vs bottom.

The more heat the more the composite layer on that side expands in the press. So it contracts on cooling.

More heat on top - less camber or reverse camber

More heat on bottom - more camber
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doughboyshredder
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Post by doughboyshredder »

Idris, what kind of a temperature variance or we talking about here? A few degrees, or significantly more?

I wonder how the temp difference effects the bond. The epoxy I plan on using has a specific temp setting for the best cure.

Most snowboards I have ridden seem to lose some camber after about 30-40 days on the hill. I have always assumed this is because the core itself is breaking down, and that is why it is much worse in foam cored boards. Carbon fiber strips seem to help with camber retention significantly.
Idris
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Post by Idris »

10 degrees F to begin with. 30 degress F if you are realy messing about. But always within the resins specifications for heat setting.
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iggyskier
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Post by iggyskier »

Idris - if we are only pressing with one blanket, then, and are looking to make a ski with a small amount of camber or reverse camber, do you think it would be better to place the blanket on top of the ski instead or continue press with the blanket on the bottom but make sure the mold has enough shape to get the camber we want?
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Dr. Delam
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Post by Dr. Delam »

I too had similar results when heating from the bottom only. My mold has zero camber from tail to toe piece area and rockered reverse camber to the tip. I used qcm 49 and 408, pressing at 180 degrees. The ski came out with about 4 millimeters of camber where it was supposed to be flat and the rockered area lost some rocker. Not a huge difference but when your results aren't what you designed, it is frustrating.

I haven't had a chance to ski them yet since there is no snow in Tahoe so I don't know what the camber will do.

I think the solution is to have 2 heat blankets and controllers to make necessary adjustments to produce results consistent with the mold design. I pretty much knew this going into it but I was hoping to not have to spring for another blanket. I also tried ramping the temp up very slowly but this didn't seem to make a difference. I believe my epoxy is designed for elevated temps so I don't want to press without heat either.
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bigKam
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Post by bigKam »

i've been thinking more about this, and below is an illustration of what i think is happening but there's still no proof, so check my reasoning carefully. if/when i get some time, i'll run a simple thermal FEA simulation...

Fig. 1: let's assume we have two different materials, A (top) and B (bottom), with zero thickness, and sandwiched together with wet (not cured) epoxy between them. (1) both materials have the same length, and it just happens that the interface layer between the two materials is of length L -- this is the epoxy layer. (2) add heat, but let's suppose the bottom layer material expands more than the top layer. also assume that the materials A and B expand to their steady state length before the epoxy cures. note that there's a mis-match between the amount of contact between the top and bottom layer as compared to the previous case. now wait until the epoxy cures. (3) then cool things down. as the materials cool, they will want to go back to their original lengths. but the cured epoxy causes the new interface between A and B to be of shorter length, i.e., less than L. think of stretching a balloon and then supergluing it to a piece of paper and relieving the tension.

Fig. 2: now let's apply this to our skibuilding exercise. assume we have a heat blanket on the base mold. it's reasonable to assume that the materials have a different coefficient of thermal expansion, such as base + edges and wood core will expand by different amounts for the same change in temperature. in fact, using a blanket on the bottom will certainly create a temperature gradient between the base and the top of the ski, so even if all the materials were homogenous, the amount of elongation over the vertical profile of the ski will be different. therefore, what happens is the interface layer between the different materials which gets bonded by the epoxy changes with temperature, and it's irreversible. if we also consider the geometry of the wood core and how it's vertically tapered, then we can probably explain the camber we've been experiencing using a zero camber, heated base mold.

like i said, this is just a hunch, so i welcome your thoughts.. (and as always, your corrections).

so how to you avoid this? one would have to heat the top and bottom mold sufficiently, such that in the steady state (and before the epoxy cures) the same boundary conditions exist between all the layers before and after heat.

as for skis losing camber prematurely, i think it's dominated by failure of the epoxy layer.

i have some comments about the importance of camber (my opinion, of course), but i'll save it for another time...

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Last edited by bigKam on Wed Nov 28, 2007 9:36 pm, edited 1 time in total.
hydrant71
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Post by hydrant71 »

i agree, exactly how i envisioned the process during many sleepless nights with building running through my noggin.

jason
dbtahoe
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Post by dbtahoe »

So if you are pressing bottom heat only what temps and duration are you using? Again, I'm using the QCM epoxies.

Why you ask?

After eating more than my share of pizza (right!) KT and I decided to do a quick test lay-up/full press trial before attempting our first ski. Not only was I light on the epoxy (just a little) but I pulled it out of the press before the top had cured. Not very inspiring.

14 oz epoxy per ski? Does that include hardener?
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bigKam
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Post by bigKam »

my process is about 120-130F for 30 minutes, then 190F for 45 to 60 minutes. some have used 212F. again, i believe the temp gradient between bottom and top surface greatly affects the camber. i've noticed more camber in skis when i press on colder days -- bottom mold heater only.
G-man
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Post by G-man »

Hi dbtahoe,

I use the same epoxy that you do, and I heat only from the bottom. It takes about 20 to 30 minutes for the PID read-out on my system to go from room temp (40 F on the read-out when I started pressing today) up to my 170 F curing temp. My temp sensor is mounted on an aluminum sheet that rests on top of the heat blankets, so I figure that, when pressing, it takes a good 30 minutes more before the top surface of the ski gets up to 170. Then, I maintain that temp for an additional 90 minutes to make sure that the entire ski gets up to heat for at least 20 minutes. I used to go for just a 60 minute heat time, but I got a bad batch of hardener from QCM a few months ago, and the resin at the waist of the ski didn't cure adequately and just went to powder when I took the ski out of the press... probably similar to your test lay-up. I think a longer heat period would have saved the ski because the thinner parts of the ski seemed to be okay. One thing to keep in the back of your mind is that QCM told me that they mix the hardener from large drums as they receive an order. So, it's quite possible that hardener composition may vary a bit from batch to batch. I suspected that something was different about my funky batch when I opened the can because the hardener was a lighter color and didn't have as strong of an odor as I was used to. So now, I do a little oven test every time that I get a new batch of resin or hardener. I put about 1/4 inch of mixed epoxy in a paper cup and bake in the oven at 170 for 20 minutes, then put the sample in the vice and bend it with a big ol' vice grip until the sample breaks. Generally, it's really tough to break. If it's not cured, it'll just shatter like sugar candy.

I think that I'll probably someday install a temp probe near the top mold so that I can monitor the actual temp at the top surface of the ski... someday.

I'd guess that about 2/3 of the resin that I use in a lay-up gets squeezed out during pressing. I'm using about 600 grams per ski. I've tried to use less, but the glass just doesn't look like it's well saturated as I'm doing the lay-up. It also seems that some types of glass wet out more readily than others. I don't like wasting all of that resin, so I'm trying to encourage myself to use a little less with each ski until I find that point where the finished ski actually suffers from being too dry. I'm using multiple layers of various uni and bi-directional types of fibers in my current lay-ups, and I think this approach inherently uses more resin that a woven triaxial glass would use.

Good luck on the first ski.

G-man
sleepycp
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Heat on top only

Post by sleepycp »

This is an interesting discussion, and I am curious how heating only on the Top only will affect camber. I was thinking of heating only on the top because it would make the problems of core alignment and all that easy to deal with, i.e just use a few nails like in an unheated press. My mold has 20 mm camber built in over the entire length, and shorter skis have somewhat less built in mold camber using my mold design. I expected that to relax to about half the molded in camber, and have gotten about that in the one pair I have pressed so far. That was without heat, using whatever kind of epoxy Snowboardmaterials.com sent me. I have now put together a heating system, which I could install top or bottom, and QCM epoxy. I was going to put the heater on the bottom, but was trying to figure out how to ensure precise alignment of all the parts, and decided it would be just as well to put it on the top, and do the layup on masonite, like I did before, and then I could easily nail in alignment blocks to hold everything in place. But now it appears that the temperature gradient produced by that system will have the effect of removing camber from the ski, which is not something that I think would be desirable. I am going for something with a lot of snap, which camber, or pretensioning if you will, contributes to.Sorry to ramble.
Idris
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Re: Heat on top only

Post by Idris »

sleepycp wrote:This is an interesting discussion, and I am curious how heating only on the Top only will affect camber. I was thinking of heating only on the top because it would make the problems of core alignment and all that easy to deal with, i.e just use a few nails like in an unheated press. My mold has 20 mm camber built in over the entire length, and shorter skis have somewhat less built in mold camber using my mold design. I expected that to relax to about half the molded in camber, and have gotten about that in the one pair I have pressed so far. That was without heat, using whatever kind of epoxy Snowboardmaterials.com sent me. I have now put together a heating system, which I could install top or bottom, and QCM epoxy. I was going to put the heater on the bottom, but was trying to figure out how to ensure precise alignment of all the parts, and decided it would be just as well to put it on the top, and do the layup on masonite, like I did before, and then I could easily nail in alignment blocks to hold everything in place. But now it appears that the temperature gradient produced by that system will have the effect of removing camber from the ski, which is not something that I think would be desirable. I am going for something with a lot of snap, which camber, or pretensioning if you will, contributes to.Sorry to ramble.
Yes it will have the effect of decambering the ski. This is the effect I have seen from either

1. The top heating pad suffers thermal runaway and gets realy hot.

2. The bottom heating pad fails and you have no heat.

keeping an eye on your temp readings whilst pressing is a good idea if you don't want nasty suprises. I've had both of the above happen and only find out when I take a bananna not a ski out of the press.
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