Tuesday, August 28, 2012

Heater blanket project

I've started putting the controller together that was designed by Nik Stanton, a Freelancer.com engineer (see previous post). Soldering surface mount component is one tough gig but I am getting close.

The controller plugs into a standalone, single chip computer that I built from a kit called the 'Maximite'. Its a very cool little device that has a built in BASIC interpreter and 20 I/O pins that you can individual configure as digital or analog input or outputs. This is the 'intelligence' and the controller I'm soldering up will allow 3 mains loads to be switched on an off based on 2 pressure and 4 temperature sensors in the controller.

One load will be the newly acquired vac pump and the others will be for heating. Originally the idea was to use 500W workshop lamps as the post curing temperatures for the resin I use is only <50 degrees. But then I came across this thread on the ski-builders website for people building their own silicone heat blankets.


One guys, a kiteboard builder!!, also used his as a dye sublimation press which requires around c. 200 degrees C. So the blanket has the potential to be used for curing and doing graphics. A double whammy!!!

The blankets are pretty straight forward. Resistance wire sandwiched between fibreglass matting and wetted out with 2 part silicone rubber that you can get from mould making suppliers. A light dimmer can be used to control the maximum temperature. I'll be able to use the new controller to switch the blanket on and off to maintain the temperature. You can buy standalone PID controllers or you might even be able to get a timer to switch it on and off on a regular cycle once you know how fast the heat dissipates in the mould.

I think the blanket will end up costing around $100 dollars, most of which is the silicone. I believe it will be necessary to use Addition Curing Silicone Rubber as its rated to a higher temperature due to the platinum catalyst it uses. I believe this is what is used for low melt temperature metal casting.


1. Ni-chrome resistance wire ( about 60m for my 150x50 cm blanket). I've order 32SWG which has resistance of 13.68 ohms/m and will space the wires 1.5 cm apart. Ordered from the states via ebay for a total of $14!
2. Fibreglass matting, 2 pieces the size of the blanket plus extra length to accommodate the connectors
3. Drywall tape - this is the fibreglass tape used on Drywall ( Gyprock here in Aus). Used for holding the heating wire in place prior to pouring the rubber.
4. 1-1.5 kg of addition curing silicone rubber. On skibuilders, people are using rubber with a Shore A hardness of around 40. My local sources only stock 28A but given I am not using a press I am not too worried about the mechanical resilience of it. Goes for about $65 / kg but is much cheaper than buying silicone rubber sheets. I did come across some Teflon rubber BBQ mats on eBay that are 150x50cm and would b perfect but the challenge of bonding the fibreglass/wire element to it has got me stumped.
5.  A connector bar for the mains. For less than about 150 degree you could solder the resistance wire (or at least solder tube crimped over the end of it ) to a bus bar of some sort. People appear to have used Aluminium strip but am a bit iffy about that for 200 degrees.
6. Circuit break is probable a very good idea too in case it shorts ....

UPDATE: It seems that you get to 200 degree c you need power density of 0.5 W/ cm^2. Add that up over the size of the proposed blanket and it translates to about 3 W/in^2 or 3.7KW. It seems that the highest rating for a single outlet on the mains power is 2.4kw. However, this limit may be due to the rating of materials in the outlets themselves as its clearly possible to have more than one heavy use device running at the same time in the house. So this may mean that I need to have 2 circuits in the blanket each with a separate plugs so it can go into two outlets to get to dye sub temps. I need to check this one with an electrician. If not possible to do this economically then I will change my plan to make 2 x 1 kw blankets. Based on the ski builders thread this is enough to get to 90 degrees C no problem which will cover off all epoxy curing , maybe even re-gelling pre-preg resin and the dye sub press will have to use some other tech.

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