Heating Blankets
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Heating Blankets
Has anyone had success with lower currents/power/voltage heating blankets? 3000W is a lot of power. From what I've seen regarding silicone blankets, they can go up to about 500 degrees, but I don't know if anyone has done any calculations/tests to figure out the efficiency or actual heat that is transferred to the ski materials. I was also thinking of using one blanket on top as well as one on the bottom. I don't know the specific heat, etc. of all the materials, so if anyone knows this for UHMW, ABS, epoxy, and so on, that could be useful as well. Finally, while heat is pretty important for curing, from some of the pictures I've seen of production presses, I can't see any heat system.
Hi camhard,
I use two heat blankets on the bottom mold of my press. They measure 3 inches x 36 inches each. They have a power consumption rating of 2.5 amps per square inch. So, that's 270 watts per blanket, and 540 watts total power consumption. Those blankets are only about 1/16 inch thick and are recessed 1/16 inch into the bottom mold... that may be why you don't see them in pictures. My system, when pressurized, heats the bottom aluminum plate to 170F in about 10 minutes, with much of the heat also being transfered to the ski layup during that time. So, in my opinion, 600 to 900 watts is plenty. A 3000 watt unit would pull 27 amps of current (at 110 volts), so you'd have to make sure that your circuit protection could handle that much load... if you decided to go that large on the heat blanket. Epoxy that is formulated for heat cure only requires a temperature of 160F for 20 minutes. I want to make sure that the entire ski layup gets at least that much heat for that much time, so I heat to 170F for 90 minutes. I don't know of a benefit that would result from using a higher heat, but someone else may present one.
Good luck,
G-man
I use two heat blankets on the bottom mold of my press. They measure 3 inches x 36 inches each. They have a power consumption rating of 2.5 amps per square inch. So, that's 270 watts per blanket, and 540 watts total power consumption. Those blankets are only about 1/16 inch thick and are recessed 1/16 inch into the bottom mold... that may be why you don't see them in pictures. My system, when pressurized, heats the bottom aluminum plate to 170F in about 10 minutes, with much of the heat also being transfered to the ski layup during that time. So, in my opinion, 600 to 900 watts is plenty. A 3000 watt unit would pull 27 amps of current (at 110 volts), so you'd have to make sure that your circuit protection could handle that much load... if you decided to go that large on the heat blanket. Epoxy that is formulated for heat cure only requires a temperature of 160F for 20 minutes. I want to make sure that the entire ski layup gets at least that much heat for that much time, so I heat to 170F for 90 minutes. I don't know of a benefit that would result from using a higher heat, but someone else may present one.
Good luck,
G-man
Heating blankets
G-Man -
I was wondering about your specific set up. I also have two 3" x 36" heating blankets, 5 amps each. My controller has an internal 10 amp relay. I was wondering, when wiring this together, if I should put the blankets in series or parallel. At first I thought in series, which is easier to route the wires, but then I wondered about the resistance from the first blanket affecting the temperature output of the second blanket. I am no electrical wiz, so I could use a little advice.
I was wondering about your specific set up. I also have two 3" x 36" heating blankets, 5 amps each. My controller has an internal 10 amp relay. I was wondering, when wiring this together, if I should put the blankets in series or parallel. At first I thought in series, which is easier to route the wires, but then I wondered about the resistance from the first blanket affecting the temperature output of the second blanket. I am no electrical wiz, so I could use a little advice.
Hi sleepycp,
Good question. Nor am I an electrical wiz, but as a very bright friend of mine says, "With access to the internet, we don't have to know the answers to the questions... we just have to know how to ask the questions". So, in that regard, I give you:
http://en.wikipedia.org/wiki/Series_circuit
I have my heat blankets wired in parallel. That way, each blanket sees 110 volts AC (actually 121 volts at my meter). Theoretically, at 110 volts, the resistance in the wires of your blanket will allow about 5 amps of current to flow through each blanket (so, your 108 sq./in. blankets must have a 5 watt per sq,/in. rating for a total of 540 watts per blanket... volts x amps = watts). A parallel circuit will usually divide the available current across the various loads, but because your available current is only limited by the size/rating of your circuit breaker, your 5 amp blankets will get all the current they need. So, your total current is the sum of the currents through the various components... your two heat blankets.
As I understand series circuits, if you wired your heat blankets in series, the voltage across each blanket would only be 55 volts. Ohm's law says that this would cut your amperage by 50%, thus reducing your heat output (wattage consumption) by 50%.
One word of caution regarding your controller's internal 10 amp relay... If your actual house line voltage is greater than 110 volts (as mine is), let's say 120 volts, your heat blankets might actually pull more than 5 amps each, which might in turn damage your internal relay. Bigger, external relays can handle a bit of line voltage fluctuation, but your internal relay may be more sensitive. I'd suggest using an ohm meter to get an accurate reading regarding the resistance of your blankets, and a volt meter to check your actual line voltage. Then, use Ohms law to figure out just how much current you'll be trying to pull through your internal relay. I think you may be running kinda close to your relay's limit and I'm betting that you'd be pretty unhappy if you blew your controller, eh?
Good luck,
G-man
Good question. Nor am I an electrical wiz, but as a very bright friend of mine says, "With access to the internet, we don't have to know the answers to the questions... we just have to know how to ask the questions". So, in that regard, I give you:
http://en.wikipedia.org/wiki/Series_circuit
I have my heat blankets wired in parallel. That way, each blanket sees 110 volts AC (actually 121 volts at my meter). Theoretically, at 110 volts, the resistance in the wires of your blanket will allow about 5 amps of current to flow through each blanket (so, your 108 sq./in. blankets must have a 5 watt per sq,/in. rating for a total of 540 watts per blanket... volts x amps = watts). A parallel circuit will usually divide the available current across the various loads, but because your available current is only limited by the size/rating of your circuit breaker, your 5 amp blankets will get all the current they need. So, your total current is the sum of the currents through the various components... your two heat blankets.
As I understand series circuits, if you wired your heat blankets in series, the voltage across each blanket would only be 55 volts. Ohm's law says that this would cut your amperage by 50%, thus reducing your heat output (wattage consumption) by 50%.
One word of caution regarding your controller's internal 10 amp relay... If your actual house line voltage is greater than 110 volts (as mine is), let's say 120 volts, your heat blankets might actually pull more than 5 amps each, which might in turn damage your internal relay. Bigger, external relays can handle a bit of line voltage fluctuation, but your internal relay may be more sensitive. I'd suggest using an ohm meter to get an accurate reading regarding the resistance of your blankets, and a volt meter to check your actual line voltage. Then, use Ohms law to figure out just how much current you'll be trying to pull through your internal relay. I think you may be running kinda close to your relay's limit and I'm betting that you'd be pretty unhappy if you blew your controller, eh?
Good luck,
G-man
Internal relay, etc.
G-Man -
Thanks for the heads up. That's why I hire an electrician to work on my house.
Thanks for the heads up. That's why I hire an electrician to work on my house.
-
hugocacola
- Posts: 191
- Joined: Thu Mar 22, 2007 8:00 am
Hi hugocacola,
The heat blankets are about 2mm in thickness, so I just routed a 2mm deep recess into my bottom mold (and just wide enough for the heater to sit in), then placed two layers of approximately 1mm thick aluminum sheet (20 cm wide and 200 cm long) over the top of that. The heat blankets are designed to take quite a lot of pressure, and the aluminum sheets help the distribute the pressure over a wider area. So far, the system as been working just fine.
G-man
The heat blankets are about 2mm in thickness, so I just routed a 2mm deep recess into my bottom mold (and just wide enough for the heater to sit in), then placed two layers of approximately 1mm thick aluminum sheet (20 cm wide and 200 cm long) over the top of that. The heat blankets are designed to take quite a lot of pressure, and the aluminum sheets help the distribute the pressure over a wider area. So far, the system as been working just fine.
G-man
G-man;
I'm about to set up a heating system. I see you are using a 3’’x36’’ heater and a 20x200cm aluminum sheet on top of that. How does the heat transfer to the ski during curing? Have you noticed any temperature difference between the different parts of the ski? I’m specially thinking of the tips. How long skis do you press with this setup?
M
I'm about to set up a heating system. I see you are using a 3’’x36’’ heater and a 20x200cm aluminum sheet on top of that. How does the heat transfer to the ski during curing? Have you noticed any temperature difference between the different parts of the ski? I’m specially thinking of the tips. How long skis do you press with this setup?
M
The aluminum conducts the heat really well, but it does take a bit of time for the heat to conduct through the entire mass of the ski. I'm getting ready to install a temp sensor in my top pressing layer because I'm thinking that the top pressing layer and bladder my be functioning as more of a heat sink than I have previously thought, drawing needed heat away from the top portion of the ski assembly. I just want to be sure that the top composite layer is getting cooked adequately. I've been pressing for 90 minutes at 170 F (temp at aluminum sheets) and 50 psi. Also, I use two 3 x 36 inch heaters, end to end, with about 1 inch space in between to allow for the temp sensor mount.
G-man
G-man
I got a quote from OEM on a blanket 190cm x 10cm, 120V and about 600W. I decided that this would be a good size for one ski and was planning on getting two. However, as they are one of the more expensive components, I would be much happier if I could get away with one. Just wondering how the heat distributes through aluminum or steel sheets. If I placed the blanket between the two skis, would I end up with a discrepancy between the outside and inside edges of my skis?
Thanks a lot.
camhard
Thanks a lot.
camhard