same as a lot of people lately... Have been reading this awesome website for some time now, finally posting my first message. Before the actual question, a big thank you to the creators and the community to make such a wealth of information available. It is simply unbelievable!!!
I am currently building a heated pneumatic press and I got some heat pads for cheap, with one slight problem... they are quite small (30" long).
I have 4: 2 for the bottom, 2 for the top. My idea was to use an aluminum sheet between the pads and the ski material to evenly distribute the heat over the entire ski length. I am not very well versed in heat transfer theory... what thickness should I use? What grade of aluminum? Is aluminum the best metal for this application?
Any help will be hugely appreciated!
Cheers
billl
Using an aluminum sheet to transfer heat
Moderators: Head Monkey, kelvin, bigKam, skidesmond, chrismp
hi Bill: welcome!!
look here for the thermal conductivity of popular metals:
http://www.engineersedge.com/properties_of_metals.htm
AL is 136 Btu / (hr - ft -°F), compared to mild steel: 26-38 Btu / (hr - ft -°F). copper has a higher value, but i think it will cost you more.
we've used AL with good success.
in terms of thickness, the AL sheet should be thin enough to bend and follow the contours of your mold. i know from first hand experince that 1/8" thick is a bit much. try in the neighborhood of 1/16" thick instead. also, make sure your mold surface is smooth. if you have ribs with gaps, cover it with masonite.
i hope this helps.
look here for the thermal conductivity of popular metals:
http://www.engineersedge.com/properties_of_metals.htm
AL is 136 Btu / (hr - ft -°F), compared to mild steel: 26-38 Btu / (hr - ft -°F). copper has a higher value, but i think it will cost you more.
we've used AL with good success.
in terms of thickness, the AL sheet should be thin enough to bend and follow the contours of your mold. i know from first hand experince that 1/8" thick is a bit much. try in the neighborhood of 1/16" thick instead. also, make sure your mold surface is smooth. if you have ribs with gaps, cover it with masonite.
i hope this helps.
-
Cadman
As far as Aluminum goes, the lower grade 5052-H32(used for aluminum cabinets) is softer and bends easier. but will take a set easier than the higher grades like 6061-T6. If you have access to a sheet metal roller, you can give your sheets a pre bend which makes life alot easier if you are going to stay at a reasonably consistant running length. Most airbag arrangements will take a fair amount of running length change (5-10cm) before you need to change sheets out. Anyway you can get by with a little thicker material (.08-,125 inches) if you are able to roll in a radius at each end. With thicker material you retain heat better.The heat blankets are normally around .055 thick so you can put an .06 inch spacer in between the two aluminum sheets that you sandwich the heat blanket in. Be carefull about sharp edges since you can short out a blanket very easily. Make sure that your blanket does not stick out into open space as this will cause the blanket to overheat.
Hi billl and welcome to the forum,
I, too, use two 30 inch long heaters (3 inches wide) for my bottom heater. I also have another two units for my top heater, but I haven't hooked them up yet. I recessed the heaters into the bottom mold, then covered that with a 7 inch wide sheet of regular ol' aluminum sheet that is .040 thick. 1/16 inch sheet was too stiff to bend into the mold very well, so I went with the .040. The aluminum conducts the heat very well... so well in fact, that I wouldn't recommend going much wider than your sandwich lay-up because you'd end up radiating a lot of your heat out into the the room air. I left about an inch space between where the heaters meet at the center of the mold. In that open area of the mold, I routed out a 1/2 inch deep void, into which sets a little block of aluminum that is attached to the aluminum sheet. Into that little block is threaded my thermo-couple (temp sensor). I also put a little thermal grease between the block and the sheet. The whole system works like a charm. I heat the ski for an hour after it reaches 170 F. A top heater could reduce the time down to about 20 minutes. Aluminum is about the best heat conductor you can get at a reasonable price. Gold, silver, and copper are all somewhat better thermal conductors. The price of copper around here has become very pricey lately. Have fun building the press.
G-man
I, too, use two 30 inch long heaters (3 inches wide) for my bottom heater. I also have another two units for my top heater, but I haven't hooked them up yet. I recessed the heaters into the bottom mold, then covered that with a 7 inch wide sheet of regular ol' aluminum sheet that is .040 thick. 1/16 inch sheet was too stiff to bend into the mold very well, so I went with the .040. The aluminum conducts the heat very well... so well in fact, that I wouldn't recommend going much wider than your sandwich lay-up because you'd end up radiating a lot of your heat out into the the room air. I left about an inch space between where the heaters meet at the center of the mold. In that open area of the mold, I routed out a 1/2 inch deep void, into which sets a little block of aluminum that is attached to the aluminum sheet. Into that little block is threaded my thermo-couple (temp sensor). I also put a little thermal grease between the block and the sheet. The whole system works like a charm. I heat the ski for an hour after it reaches 170 F. A top heater could reduce the time down to about 20 minutes. Aluminum is about the best heat conductor you can get at a reasonable price. Gold, silver, and copper are all somewhat better thermal conductors. The price of copper around here has become very pricey lately. Have fun building the press.
G-man
Cadman wrote:
G-man
I have a very hi-tech roller. I use an old 1 gallon paint can. Works very nice for really smooth, perfect radius bends in .040 material. I certainly agree that a little pre-bend makes things much easier. I use the same rolling technique and aluminum material for my cassettes. I think my 'regular ol' aluminum is actually 6061. I have to over bend it quite a bit to end up with the finish radius I'm looking for. As always, it's great to hear what others are doing, to learn about different rational, and learn about other tid-bits of information.If you have access to a sheet metal roller
G-man
i was just thinking about this: consider grounding the aluminum in case the heater wire(s) make contact with the aluminum. if not properly grounded, an exposed power wire in contact with the aluminum can put it at a very high (and lethal) potential (voltage). unfortunately, if you come in contact with the aluminum you may become the most convenient path to ground for large amounts of electric current -- on the order of amps!. it'll be one layup you won't forget; a shock can make you sweat and see stars, but worse, it can kill.
but if a short does happen and the aluminum (in this case chassis) is properly grounded, it will certainly channel all the current to ground, therefore trip a fuse. the experts call this a good safety ground.
since my press (made of metal) now has an electric heater, i have decided to properly ground the press frame and the aluminum sheet that covers the top of the heater blanket on my mold.
please be careful.
but if a short does happen and the aluminum (in this case chassis) is properly grounded, it will certainly channel all the current to ground, therefore trip a fuse. the experts call this a good safety ground.
since my press (made of metal) now has an electric heater, i have decided to properly ground the press frame and the aluminum sheet that covers the top of the heater blanket on my mold.
please be careful.
PID controller
Thanks for the advice about grounding the frame. I'll make sure that we are as electrically safe as possible.
Another question:
Have any of you guys dealt with some of the cheapo PID controllers that are all over eBay (e.g. coldfusionX, etc)? Do you know if they are any good, or should I stay well clear of them and drop the extra coin to get an omron or watlow?
Thanks
billl
Another question:
Have any of you guys dealt with some of the cheapo PID controllers that are all over eBay (e.g. coldfusionX, etc)? Do you know if they are any good, or should I stay well clear of them and drop the extra coin to get an omron or watlow?
Thanks
billl
I got 2 PID's from coldfusionX. They are working very well so far (1 year). I also ordered some extra thermocouples from coldfusion ($5 each) so that I can quickly switch the PID units to other devices like a dye sublimation heat transfer press, small test sample press, top heater unit for the main ski press, etc. Be sure to request the download link for the operating instructions. Instructions do not come with the unit, which can initially leave one feeling a bit lost.
Many of the PID units sold on e-bay are pretty much identical, with different product brand names on them... kinda looks like they may have all come out of the same factory in China. Programing each of the different units is very much similar.
G-man
Many of the PID units sold on e-bay are pretty much identical, with different product brand names on them... kinda looks like they may have all come out of the same factory in China. Programing each of the different units is very much similar.
G-man