So...... After many months of acquiring equipment (we started our workshop from scratch), building jigs, a heated press, adjustable molds, making cores, acquiring materials, and spending countless hours reading every single entry into the skibuilders forums daily, we got together this past saturday and pressed our first "ski". Well, whatever you want to call it, I guess, as we won't be able to ski on it, but we sure had a ton of fun doing it and learned a whole bunch.
What went wrong:
- heat.
--> We are using 2 blankets on the bottom mold, and an aluminum sheet to transfer heat over the entire ski surface. Our mistake there was to put the thermocouple between the 2 blankets, i.e. in a location where temperature lagged a bit compared to directly over one of the blankets.
--> As a result we overheated over the blankets themselves, and hit the epoxy's foaming temp (65C). Note that we were controlling to 58C, but I guess the aluminum didn't conduct heat as fast as we thought it would. For our second attempt we will change the sensor's location, that way the controller will control the highest temp instead of the coldest temp.
--> second consequence of this was that the base expanded during pressing... This was unexpected! Is there a temperature over which we shouldn't press as far as the base material is concerned???
--> third consequence was that the top sheet blistered. I think it had a lot to do with the epoxy foaming, so this'll probably go away if we stay below 60C
- the base has taken the shape of the bottom mold's ribs... This surprised me quite a bit as we "only" pressed at 45psi, and have covered the ribs with masonite and a 1/16" aluminum sheet. I think we'll have to investigate that one a bit more. Any advice/experiences?
- the edges got pushed out at the tip and tail. We didn't use wrap around edges (stop right after the curvature starts...). We'll have to do a better job of gluing the edges on the base next time.
- the tip / tail spacers move during pressing. Any advice on making sure this doesn't happen?? How do you secure them to the core? (it's hard to get the tip spacer to be cut as the core's exact negative...)
Despite all this it came out looking like a ski, flexing like a ski and I think that we learnt a ton in the process. Any advice from you guys would be very appreciated, and we'll use our second ski as another learning experience.
Cheers
billl
First try
Moderators: Head Monkey, kelvin, bigKam, skidesmond, chrismp
Hey billl,
I just came in for a quick bite of lunch and saw your post. I'm needing to get back out to work, so I'll make this response short and do more later if needed/wanted.
First off, congrats on the first ski. It's a long road to that point, eh.
I also use two heat blankets on the bottom mold, one at each end of the press, under a layer of aluminum. At the center of the mold, where the blankets meet, there is about a 3cm gap between the blankets. In this space is where I have my temp sensor located, attached to the aluminum sheet via a little 1x1x2cm aluminum block that fits neatly into a little recess in the mold surface. My heaters are only 3 inches wide, but my aluminum sheet is 7 inches wide. My PID controller brings the temp up 170F over a time period of about 20 minutes. The aluminum is a quite good heat conductor, so I really don't think my sensor is lagging behind too far on the actual temp of the aluminum located adjacent to the heater surface... maybe just 2 or 3 degrees. I'm only relating all of this to you because our systems seem to be constructed similarly, and mine has never had the problems that you describe. So... how fast are you bringing the heat up? If you're coming up very fast, it could certainly be more possible to overshoot your target temp by a a larger margin. Are you using a PID to control temp rise, or some other device?
I generally get a bit of foaming of the excess run-off epoxy, but I've never had the epoxy foam within the ski layers. Are you pretty sure that the pressure of your bladder is actually being transferred evenly to you ski components? Was the blistering of the top sheet present over the entire ski?.. or just in certain areas?
There have been a number of reports from other builders of mold rib transfer to the ski. Even if you are using a layer of aluminum and masonite as smoothing layers, there is a lot of heat and pressure involved in pressing. If you integrate a full thickness 3/4 inch MDF bottom layer into you mold design, you won't get any of the transfer problems.
Stopping the edges where the tip and tail curves start will almost guarantee that the edges will pop loose. There is just too much force being applied to that area of the ski during pressing. I used to get weird kinks in the metal edge at that location (after pressing) until I stopped annealing the metal edge prior to bend/shaping. The difference in metal hardness where I stopped annealing caused the kink exactly where the annealing stopped. I no longer anneal the edge, and my tip and tail curves are now smooth as silk. So, unless you stop your partial metal edge farther back from the curves, I think you 'll always have a bit of a problem with them staying in place during pressing.
If you use router guide bushings and properly made templates, you can get tip spacers that are exact negatives of your core shape. You can even incorporate little locking tabs to keep things from shifting during pressing. Prior to CNC machines, router guide bushings were the only to get precision cuts for fine inlay work.
Hope this is somewhat helpful.
G-man
I just came in for a quick bite of lunch and saw your post. I'm needing to get back out to work, so I'll make this response short and do more later if needed/wanted.
First off, congrats on the first ski. It's a long road to that point, eh.
I also use two heat blankets on the bottom mold, one at each end of the press, under a layer of aluminum. At the center of the mold, where the blankets meet, there is about a 3cm gap between the blankets. In this space is where I have my temp sensor located, attached to the aluminum sheet via a little 1x1x2cm aluminum block that fits neatly into a little recess in the mold surface. My heaters are only 3 inches wide, but my aluminum sheet is 7 inches wide. My PID controller brings the temp up 170F over a time period of about 20 minutes. The aluminum is a quite good heat conductor, so I really don't think my sensor is lagging behind too far on the actual temp of the aluminum located adjacent to the heater surface... maybe just 2 or 3 degrees. I'm only relating all of this to you because our systems seem to be constructed similarly, and mine has never had the problems that you describe. So... how fast are you bringing the heat up? If you're coming up very fast, it could certainly be more possible to overshoot your target temp by a a larger margin. Are you using a PID to control temp rise, or some other device?
I generally get a bit of foaming of the excess run-off epoxy, but I've never had the epoxy foam within the ski layers. Are you pretty sure that the pressure of your bladder is actually being transferred evenly to you ski components? Was the blistering of the top sheet present over the entire ski?.. or just in certain areas?
There have been a number of reports from other builders of mold rib transfer to the ski. Even if you are using a layer of aluminum and masonite as smoothing layers, there is a lot of heat and pressure involved in pressing. If you integrate a full thickness 3/4 inch MDF bottom layer into you mold design, you won't get any of the transfer problems.
Stopping the edges where the tip and tail curves start will almost guarantee that the edges will pop loose. There is just too much force being applied to that area of the ski during pressing. I used to get weird kinks in the metal edge at that location (after pressing) until I stopped annealing the metal edge prior to bend/shaping. The difference in metal hardness where I stopped annealing caused the kink exactly where the annealing stopped. I no longer anneal the edge, and my tip and tail curves are now smooth as silk. So, unless you stop your partial metal edge farther back from the curves, I think you 'll always have a bit of a problem with them staying in place during pressing.
If you use router guide bushings and properly made templates, you can get tip spacers that are exact negatives of your core shape. You can even incorporate little locking tabs to keep things from shifting during pressing. Prior to CNC machines, router guide bushings were the only to get precision cuts for fine inlay work.
Hope this is somewhat helpful.
G-man
we are using a PID for temp control, but the gap between the blankets is wider than 3cm... Maybe that's our problem. We'll definitely put the sensor under a blanket on the next try, I think it'll help. I'll also read the PID manual to see if we can tune it for less aggressive / more undershoot behavior (currently used the auto-tune function).
The top sheet blistering happened over the blankets, so there definitely is a correlation to temperature.
I think that we will cut this ski and see what we can in its guts, so we'll see if it foamed inside or not.
We'll change our bottom mold design to get rid of the ribs.
How about base expansion? Any tips on max temp? It sounds that if you press at 170F (77C) we shouldn't have had any issues there...
As always, thank you G-Man so much for your input. Winter is coming
The top sheet blistering happened over the blankets, so there definitely is a correlation to temperature.
I think that we will cut this ski and see what we can in its guts, so we'll see if it foamed inside or not.
We'll change our bottom mold design to get rid of the ribs.
How about base expansion? Any tips on max temp? It sounds that if you press at 170F (77C) we shouldn't have had any issues there...
As always, thank you G-Man so much for your input. Winter is coming
billl,
Okay, more thoughts...
I also am using the auto-function on my PID. How many watts are you running. I think my heaters only put out about 2.5 watts per sq. inch ( I had the option of going with 2.5 or 5 watts/sq. inch when I ordered them) and they are only 3 inches wide, so I may be heating up much slower than you are just cause I'm not putting out as much heat. If you're running at a higher wattage, that could definitely cause more lag in your temperature readings, especially if your sensor is more than a few cm's from your blankets. It sounds like you have that part well under control now with the new location of your sensor.
I'm still pretty puzzled regarding some of the other problems you are having, seemingly at lower temps than I ( and many others) are pressing at. It seems that if over heated resin was the cause of your top sheet blistering, you would also be seeing some sort of similar problem with the bonding of the base, especially considering that your heaters are on your bottom mold. Theoretically, the top sheet should have been a bit cooler than the base, partly because the top pressing layers and the bladder function somewhat as a big ol' heat sink... but if your heaters are putting out a lot of wattage, you may have overwhelmed any heat sinking tendencies there. And if your temp sensor was lagging behind the true blanket temps by a whole bunch, then your new plan for getting a good handle on the true temp may solve most of those issues. I don't know what kind of pressing layers you are using above the ski, but one thing to consider (for all of us) is keeping the temp from getting too high on the fire hose. I think that mine is rated for 150F, and I'm sure it gets pretty close to that at times. I use 5 layers of pre-formed 1/8 inch masonite as my top pressing layer, and I think that helps a bit to protect the bladder, but the bladder still gets to hot to handle much of the time... a little spooky when its inflated to 50 psi.
Regarding expansion of the base material, what exactly are you seeing. I'm sure that my base material expands to some degree with heat, but I've never seen any untoward signs of it after pressing. Did your base material actually increase in overall dimension and stay that way after pressing? Maybe your press did just get really mo-fo hot and the result is all this unusual stuff you're seeing.
Maybe other builders can comment in this regard, but it seems to me that a slower rise in press temp might lead to fewer problems than a faster rise. My system isn't capable of a fast rise, so I haven't had any experience with rapid heating. Maybe I unknowingly lucked out by building a whimpy-ass system that takes 30 minutes to get up to speed. My cambers almost always come out just as they are molded. For what its worth, I leave my skis to cook in the press for one full hour after my temp sensor reads 170F. If I don't do that, the thicker portions of the ski don't always optimally cure... as in top sheet delamination in the binding area. If I cook em' long enough, making sure that everything gets up to heat, then I don't have delam issues.
I did get a bad batch of hardener a few months ago, with which no amount of heat would cure the resin... it would harden, but not cure. When I took the skis out of the press, the fiberglass matrix just turned to powder and the skis fell apart. That batch of hardener caused a bunch of problems and cost a lot of money in lost time and materials. The hardener came from a very reputable source, so it was a big surprise to get a bad batch. It made me realize that its not a bad idea to test each new batch of material on a sample before I use it on a ski. It'd be a hassle, for sure, but this was a leading resin supplier who I would've thought did adequate quality control on their products before they were sent to a customer. Not so, and it took them forever to make good on some new hardener. So, now, I just mix up a bit of resin in a plastic cup (about one cm thick) and stick it in the oven at 170F for 20 minutes. I let the sample cool and put it in a vise, grab it with some pliers, and try to break it. Most samples are pretty darned difficult to break. Anyway, you might try a test batch of your resin just so you can rule out a problem in that area.
I'll be looking forward to the report on the second ski.
G-man
Okay, more thoughts...
I also am using the auto-function on my PID. How many watts are you running. I think my heaters only put out about 2.5 watts per sq. inch ( I had the option of going with 2.5 or 5 watts/sq. inch when I ordered them) and they are only 3 inches wide, so I may be heating up much slower than you are just cause I'm not putting out as much heat. If you're running at a higher wattage, that could definitely cause more lag in your temperature readings, especially if your sensor is more than a few cm's from your blankets. It sounds like you have that part well under control now with the new location of your sensor.
I'm still pretty puzzled regarding some of the other problems you are having, seemingly at lower temps than I ( and many others) are pressing at. It seems that if over heated resin was the cause of your top sheet blistering, you would also be seeing some sort of similar problem with the bonding of the base, especially considering that your heaters are on your bottom mold. Theoretically, the top sheet should have been a bit cooler than the base, partly because the top pressing layers and the bladder function somewhat as a big ol' heat sink... but if your heaters are putting out a lot of wattage, you may have overwhelmed any heat sinking tendencies there. And if your temp sensor was lagging behind the true blanket temps by a whole bunch, then your new plan for getting a good handle on the true temp may solve most of those issues. I don't know what kind of pressing layers you are using above the ski, but one thing to consider (for all of us) is keeping the temp from getting too high on the fire hose. I think that mine is rated for 150F, and I'm sure it gets pretty close to that at times. I use 5 layers of pre-formed 1/8 inch masonite as my top pressing layer, and I think that helps a bit to protect the bladder, but the bladder still gets to hot to handle much of the time... a little spooky when its inflated to 50 psi.
Regarding expansion of the base material, what exactly are you seeing. I'm sure that my base material expands to some degree with heat, but I've never seen any untoward signs of it after pressing. Did your base material actually increase in overall dimension and stay that way after pressing? Maybe your press did just get really mo-fo hot and the result is all this unusual stuff you're seeing.
Maybe other builders can comment in this regard, but it seems to me that a slower rise in press temp might lead to fewer problems than a faster rise. My system isn't capable of a fast rise, so I haven't had any experience with rapid heating. Maybe I unknowingly lucked out by building a whimpy-ass system that takes 30 minutes to get up to speed. My cambers almost always come out just as they are molded. For what its worth, I leave my skis to cook in the press for one full hour after my temp sensor reads 170F. If I don't do that, the thicker portions of the ski don't always optimally cure... as in top sheet delamination in the binding area. If I cook em' long enough, making sure that everything gets up to heat, then I don't have delam issues.
I did get a bad batch of hardener a few months ago, with which no amount of heat would cure the resin... it would harden, but not cure. When I took the skis out of the press, the fiberglass matrix just turned to powder and the skis fell apart. That batch of hardener caused a bunch of problems and cost a lot of money in lost time and materials. The hardener came from a very reputable source, so it was a big surprise to get a bad batch. It made me realize that its not a bad idea to test each new batch of material on a sample before I use it on a ski. It'd be a hassle, for sure, but this was a leading resin supplier who I would've thought did adequate quality control on their products before they were sent to a customer. Not so, and it took them forever to make good on some new hardener. So, now, I just mix up a bit of resin in a plastic cup (about one cm thick) and stick it in the oven at 170F for 20 minutes. I let the sample cool and put it in a vise, grab it with some pliers, and try to break it. Most samples are pretty darned difficult to break. Anyway, you might try a test batch of your resin just so you can rule out a problem in that area.
I'll be looking forward to the report on the second ski.
G-man