Ski Builders Forum

holmtech wrote:OK, I gave it a shot, but I'm not sure that my first blanket is a go. Any help would be appreciated. Here's the details:

-9oz biax fiberglass
-50A silicone mold rubber
-5ohm nichrome wire

1st attempt:
-I first laid the fiberglass over a sheet of melamine. Measured everything out. Placed nails to string the wire as many have done. I figured out that I could not solder directly to the nichrome wire, but found that if I twist the nichrome wire with the braided copper, the copper would "drink" up enough solder to get a very solid solder to the nichrome.
-I taped the wire down with the sheetrock tape. When pulling the nails, the nichrome wire curled and pulled the tape right up. I spent some time trying to get it to work, but ended up ditching this and starting over. That batch of nichrome wire is now in a big ball.

2nd attempt:
-I laid the fiberglass over the melamine and mixed up some silicone. Spread 500 grams (plus hardener so 550g total) over the fiberglass and thought there was no way that was enough to soak the glass. Mixed another 500, spread it out and let cure.
-Some areas looked thin on that sheet, so I laid down the 2nd sheet of fiberglass and mixed about 1300g of silicone. Spread that out and thought it looked good.
-nailed up the thinner sheet and wired it up with nichrome wire. Placed tape over the wires noticing that it didn't stick to the silicone, but did stick to the wires so I thought it was still helping.
- I cut the heads off of each nail so that I could lay the 2nd sheet over the top to help hold the wires down.
- Mixed another 550g of silicone (200, then 200, then 150) and spread that over the wires and tape. Laid the top sheet over the whole sandwich punching through the nails. Pulled the nails and laid the 2nd piece of melamine over the whole thing. Requisite Volkswagen wheels over the top of the whole thing for weight.

Here's where the 2nd one went wrong. Obviously the whole thing is WAY too thick. It doesn't roll up at all, but more bends at weak points. It might have worked, but being that thick I'm not sure how durable it would be once it starts to kink and such. The main problem is that I plugged it into the light socket (it's wired for 220v, so 110 at the socket is about 700 watt vs 2800 watts at 220v) and I could see that the wire in my last circuit slipped and crossed over it's self. It runs the length of the blanket 6 times, so the crossing over shortens that circuit and increases the wattage of that circuit significantly. I'll test it on an actual controller, but I think this one is a waste.

My questions.
- Are folks having similar issues with the wire wanting to curl up? I tried to straiten it out as much as possible. The stuff I used came on a small spool and that might be my problem.
- Are people having better luck making two sheets, then wiring up wire in between, then a 3rd layer of silicone to glue it together? silicone it all at once? I though all at once would be the best, but how do you really get the wires to quit moving once you pull the nails?

Thanks for the help. Sorry for the long post.

Go back and have a look at my pics. My mats come up like a professional job.
You need to do it in stages over a few days, make the sheets, let them cure and trim them to rough shape.
Then fold back the ends to allow clearance for the nails, run your wires and glue the top sheet on. Then remove the nails and unfold the ends and glue the ends together.

Thanks gozaimaas. That makes sense and should work. I've got some thinner wire coming as well that should resolve my curling issue.

Btw, you don't always have to use resistance wire as the heating element. I've successfully built blankets with enamelled copper wire. This type of wire usually has very low resistance and therefore often only one length of wire is required (ie I built a blanket with 60 lengths of wire with 2m length each, so 120m wire in total; the wire I used was 0.35mm thick and has a resistance of only 0.18 ohms/m which results in a blanket with roughly 2200 watt). With this type of wire you just need to make sure that its sealed well from oxygen as they burn up real quick when they oxydize.

Hey is anyone on here using blankets of around 1000 watts?
I've been playing around with different circuit layouts. I won't be doing any sublimation with it and I don't want to build something that heats up in 5 minutes while drawing 8 amps per mat. So I was aiming for around 1200 watts but the best layout I get with my wire around this is 960 watts.
I saw a post by chrismp somewhere that he was only setting his blankets to 700 watt so I thought I might see what people are using.

Cheers.

I think the watts per inch is the important part to heat the laminate in a resonable amount of time. My blanket at 200cmx35 cm is 1400 watts. Takes about 25 min to get to temp. The mei one i have pulls the same amps but get to temp in about 2/3rds the time so obviously they made it better than mine. Mine cost 90bucks and the mei was 450 so go figure.

gav wa wrote:Hey is anyone on here using blankets of around 1000 watts?
I've been playing around with different circuit layouts. I won't be doing any sublimation with it and I don't want to build something that heats up in 5 minutes while drawing 8 amps per mat. So I was aiming for around 1200 watts but the best layout I get with my wire around this is 960 watts.
I saw a post by chrismp somewhere that he was only setting his blankets to 700 watt so I thought I might see what people are using.

Cheers.

what's your wires spacing?

Here is my:

http://boardbuilders-forum.1077691.n5.n ... d1064.html

It goes to 35-40C under 2-3mins and my fuses are only 3A.

MontuckyMadman wrote:I think the watts per inch is the important part to heat the laminate in a resonable amount of time. My blanket at 200cmx35 cm is 1400 watts. Takes about 25 min to get to temp. The mei one i have pulls the same amps but get to temp in about 2/3rds the time so obviously they made it better than mine. Mine cost 90bucks and the mei was 450 so go figure.

Yeah it looks like I am going to have to go with about 1400 watts too. PM'd chris and he had also had some good points. Oh well looks like I am just going to put an extra 15amp circuit in my shed, it needs it anyway.

dbabicwa - I want a bit more heat than that though mate. How long do you need to keep your boards in the press at 40 degrees?

if you go 220v all needed 6 to 7 A per blanket

has anyone had any issues with their homemade blankets being to lumpy and creating uneven bases when you bottom heat?? Or do I need a thicker cassete if I am having these problems??

After reading over this thread and some of the confusion with figuring out the electrical bits of the mat I’ve written out some of the basic equations needed to calculate everything when I built mine. Most of this has been covered in the thread, so this is mainly a summary. This won’t cover how to actually build the heat blanket, just figuring out the electrical aspect of it. I start with a desired wattage, but all of this can be reworked to start with a wire you already have. I’ll post an excel calculator for this later.

My press is kept in a garage that averages about -10°F during winter, and my last 1000W heat blanket would keep the edge of the ski about 15° cooler than the core. I decided to build a 3000W blanket to keep a more even heat. 3000 watts sounded like enough power to cook most anything. Getting 3000W of power at 120V requires 25A of current, which is well above a recommended load for a wall socket (not to mention all of my wall sockets are on 20A breakers). I opted instead to use 220V, which to achieve 3000W would only require approximately 13.64A of current and is easily accessible in my garage. For builders who will be using 120V, all of these equations still work.



The basic design of the mat is at the top are two bus bars or lead wires, to which resistive wire will be attached and then run down and back the length of the mat. Googling ‘resistive wire pelican’ is a good place to look at for different wire information. On my previous commercial mat, the resistive wire was spaced about every 1cm and used 26AWG wire. I used this for the upper limit of how far the wires should be spaced apart. The closest I am capable of laying a wire without shorting them together is about 1/8” or .3175cm. As to the width of the wire, I used this as my upper limit for wire diameter to make sure the mat stayed very flexible and I wouldn’t have to worry about wires breaking from metal fatigue. Larger wire can probably be used, this was just my arbitrary limit. I am not sure what a reasonable lower limit for the wire is. Most of these wires can be cycled thousands of times before failure so that’s not an issue, but the potential for the wire getting too hot as it cycles may be of concern.

By nature of how the wires will be laid in the mat (run parallel to the length of the mat) and how they connect to the bus bars, each resistive wire will be added in a parallel circuit. I wanted my mat to be about 8” wide (21cm) and for the wire to have a .5 cm buffer with edge. The number of loops I can place on this mat will be equal to the width minus the buffer divided by the wire spacing (multiplied by two because the wire loops back).



Figuring out the range for the number of loops you can theoretically build is a critical part in figuring out what resistance wire you should use. Since parallel resistance adds inversely it’s not as intuitive for calculating what resistance you’ll need. In this case the number of parallel resistive loops will have to be 10 or greater, but equal or less than 32. Later after we’ve determined the proper resistance wire, we will calculate the number of loops we’ll actually use.

To calculate the resistance of the wire needed, we start with the very first equation shown. To achieve 3000 watts of power, we needed 13.64 amps of current. The equation power equals potential times current can also be expressed as power equals current squared times resistance. Using this equivalent expression we know that the combined resistance of all our loops must be 16.13 ohms. By simplifying and rewriting the equation for equivalent parallel resistance, we get that multiplying the number of wire loops by the equivalent resistance gives us what the resistance of each wire should be. In this case if we have ten wire loops, each wire should have a total resistance of 161.3 ohms. For thirty two loops each wire should have a total resistance of 516.3 ohms.



The heat blanket will have a working surface 220cm long, plus a small additional length of wire is needed to connect to the bus bars and to account for the bend at the bottom where it loops back. In this case each loop has a length of 446cm. The wire I was looking at came rated in ohms per foot. After converting from meters to feet, we just divide this by the total resistance of a single wire by how long it is. We get another range, and now know the wire should be between 10.96 and 35.13 ohms per foot.



Cross referencing the product catalog on Pelican Wire I found a wire (Alloy 875 resistive wire) that had the properties I was looking for. At 34AWG (.16mm) it is thick enough to handle, and at 21.97 ohms per foot the resistance is in the middle of the acceptable values.

Now that we have a wire chosen, we can rewrite a few of the above equations to figure out how many loops are needed to get an equivalent resistance of 16.13 ohms, and what the spacing of these loops should be. Using these equations we get that 20 loops are needed to get the proper resistance and each wire should be spaced .5 cm apart. Since rarely will you get an answer that are a whole number, round to the nearest integer. Since we are not able to create a fraction of a resistive loop, we cannot achieve the exact wattage we were aiming for. Subbing in the values we have though, we see that with 20 loops we will produce 2995W of power, which is a 0.16% difference from the target goal. Pretty close.

Great write up! Thank you!

A bit off topic, but as the initiator of this thread and probably the first one to build their own heat blanket, I just wanted to let you guys know that I just ordered three blankets from http://alirubber.en.alibaba.com/ as at their prices (just paid USD 180 for a 2000x450mm heated area blanket) it is actually no longer worth the time to build blankets myself.

@chrismp: Which wattage did you order? still thinking if its usefull to use a 4kw blanket for sublimation AND pressing. With a good controller and enough heatsink it should be possible. One the other hand ... with these prices...
But maybe we should continue in a new thread

P.s. first post and halfway offtopic, sorry guys

chrismp wrote:Great write up! Thank you!

A bit off topic, but as the initiator of this thread and probably the first one to build their own heat blanket, I just wanted to let you guys know that I just ordered three blankets from http://alirubber.en.alibaba.com/ as at their prices (just paid USD 180 for a 2000x450mm heated area blanket) it is actually no longer worth the time to build blankets myself.

Hmm strange voice of reason from the deep....

I've order them all with 2500w. In my sublimation press I successfully use the same amount of power as it is a well insulated vacuum press.

@chrismp: did you get your ordered blankets already? happy with the quality?