Heat curing a glueup with a pipe heating cable?

[Bow Bender]

I have a question for you electronic minded geniuses out there about designing a heating element for bow forms.  I've never used a heat strip on a form but from what I have read about them they are delicate and easily damaged and a quick search for them didn't yield any results. Sorry Kirk, but even though it may work, the thought of connecting a battery charger to metal strips to heat them causes me to have visions of smoke and flames.   I'm looking at the possibility of using a 110V water pipe heating cable for the heating element.  A channel of sufficient size to accommodate the heat cable could be cut with a router into the laminating surface of the form then a strip of wood or Formica could be laminated to the form to cover the cable that would lay in the channel. This would put the heating element directly below the laminations.  The factory installed thermostats on a heat cable senses the ambient air temperature and will turn it on and off around 38* so the factory installed thermostat would have to be changed.  I'm thinking that the rotary switch from an electric hotplate would work for controlling the temperature.
  As a safety precaution a temperature sensor like the one that Bingham sells could also be incorporated  in the circuit to prevent overheating.
  A cable that is too long could probably be cut to the desired length.
  The channel could be large enough to allow the cable to slide out to be replaced if necessary.
  If this would work it sure would make the heat curing process a lot simpler.
  let me know what you all think about this.
    This is a link to a heat cable that could possibly work. 
https://www.amazon.com/dp/B07QNP2FSG/ref=sspa_dk_detail_3?spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUExSlNLUUlWNDRXUEJLJmVuY3J5cHRlZElkPUEwMzkyNDQ2MTZCSU1PTFNPWlBZTCZlbmNyeXB0ZWRBZElkPUEwNTM5NDA5MVZJWERRV1dEVklTMiZ3aWRnZXROYW1lPXNwX2RldGFpbDImYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl&th=1

This is a link to a hotplate that has the type of rotary switch that could possibly work. Thrift stores would probably be a good source for finding one.
https://www.amazon.com/Electric-Haofy-Portable-Countertop-Stainless/dp/B07Q8RFC4L/ref=sr_1_7_sspa?crid=16DH1GPFYW2AE&keywords=electric+hotplates+110v&qid=1651347730&sprefix=electric+hotplate%2Caps%2C2099&sr=8-7-spons&psc=1&smid=A14DZMNNLWG1RZ&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEySUJWQzJQVldESDhGJmVuY3J5cHRlZElkPUEwNjczNDQ5MUxKMklPT0lQVUdJNSZlbmNyeXB0ZWRBZElkPUEwMTA2MjY0MUIzU0w3VTFYQjhCNiZ3aWRnZXROYW1lPXNwX210ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=

[wcasey755]

As someone who watched Kirks system first hand a handful of times. No smoke or flame. Very awesome system. But maybe I'm biased to my sensi lol

[Longcruise]

Just some thoughts.   Not meant to be criticism.

If the channel allowed the removal of the cable, then maybe the surface of the form would not be firm enough to prevent deforming the lay up?

Maybe a dimmer switch would be simpler and more economical as a control?

How would the Bingham thermostat work?  Mine works by tracking ambient air in the oven.

From my days n the insurance business,  I recall those wires getting credit for a few fires 🔥

[BO-R]

bow bender,
I do not think those elements will stand up to the heat generated for curing bows..
generally the temps run 140 to 170 deg.

I am not trying to convince you of anything but I also use the battery charger/regulator setup
with great results. Setup properly works great.
VJ.

[Crooked Stic]

Yeah first thing is will they get to that temps for curing and be okay.
If so a snap disc will work as a temp control.

[Shredd]

  If I wanted the heat directly on the bow in the form I would rip all the wire out of a electric blanket and put it in the air hose...  I know it would definitely get up to 120* or possibly more...

[Kirkll]

Good luck on your adventure brother…. I went down that road many years ago with a number of different products.  The best were silicone heat strips used with a reistat for temp control.  But… they were not very durable. To successfully pull off a heat strip cure it needs to have heat on both sides of the limb as evenly as possible. 160 -170 degrees in 20 minutes will completely cure EA 40.

The 22 gauge stainless steel pressure strips work well because of durability and can be sanded clean if you get glue on them without deforming the surface. Using them as heat strips at the same time is a bonus. But…. It’s tough on battery chargers.

I would be open minded to a different system in the future for a heat source, but for now this works well for me…. It Has for years….   

Good luck….. Kirk

[Bow Bender]

 Thanks to everyone for the comments and suggestions.  My goal is to eliminate the heat box so that once the layup is done just plug it in and walk away from it. I haven't set up a prototype experiment yet but I plan to soon and will let you all know how it turns out.
 BO-R and Kirk, you both use the battery charger and pressure strip setup for heat curing and it definitely piques my curiosity. Would it be possible for you all to go into more detail about this setup, such as the components used, amperage, how connections are made, any do's/dont's, safety precautions and any other knowledge about this that you can pass on?  Pictures/video would be  very helpful.  If anyone else uses this system I and I'm sure others would like to hear about your setup.  Honestly, I had not heard about battery charger/pressure strip heat curing until Kirk mentioned it in a post a while back.  I am aware that frozen water pipes can be thawed by connecting a welder to each end to heat the metal so I assume that this is the same principal.  I'm also aware of using an electric blanket for a heat source.  Are there any other heat curing setups that are being used that haven't been talked about that would eliminate the heat box?
    Thanks, Royce.

[Kirkll]

Let me see what I can find on my PC…. I think I did a write up in MS word with photos for some book material I’ve been accumulating for quite a few years. I told myself I was going to have a bow building book published years ago, but I still haven’t put all the materials together. Some day I’ll get around to it…

Kirk

[BO-R]

I will put together some pics,
VJ

[Kirkll]

Here ya go....

Bow Press Heat Strips

This system for building laminated fiberglass bows is the best I’ve come up with in 15 years. But…. There are a few details in getting set up and operation that needs to be documented here.

First of all you will need 22 gage stainless steel strips that are used both top and bottom in tour form. I had mine cut to 1.5” and 1.75” widths and 72”lengths prior to shipping.  You want enough length on these strips to have about 3” sticking out of each end of your form.   I purchased enough strip material to have a pair for each bow form I use. I put the bottom strip in the form and attach it permanently and leave it there. Here is the link to my supplier:
   https://www.stainlesssupply.com/order-metal-online/docs/g1c1045s1ss0p0/304-stainless-steel-sheet-4-finish.htm

……..

Next you will need a fairly large battery charger that has manual settings. These are difficult to find now with all the electronic safety over ride models they have available now. The automatic ones with circuit boards will not work for what we are using this for. Look at the photo of the charger I use.
   https://photos.app.goo.gl/SxemPLaXwT66Ad6B6
This one below will NOT work   
 https://www.amazon.com/Schumacher-SC1353-Wheeled-Battery-Charger/dp/B07BVFGVQ1/ref=sr_1_21?crid=2O3KDCXISAPSA&keywords=battery+charger+automotive&qid=1642610980&sprefix=battery+charger%2Caps%2C399&sr=8-21

This manual wheel  type below  is what you want.
https://www.amazon.com/Schumacher-Manual-Wheel-Charger-Engine/dp/B08PQ28NPB/ref=sr_1_27?crid=3G7TN835WBGWC&keywords=large+battery+charger&qid=1642611374&sprefix=large+battery+chargers%2Caps%2C195&sr=8-27
OK….. once you have your brand new battery charger, you need to tear it apart and rewire the cooling fan. There is a small fan that runs on 110 volts that needs its own power source.  Why?   Because the battery charger is going to run off of the voltage regulator at a reduced voltage….. That reduces the voltage to the cooling fan too, and slows it way down if it doesn’t have its own power source, and….. it WILL over heat the fan and the charger, and eventually burn up…. Trust me on this one….. The only other option is to attach another fan to the battery charger body that blows cool air into it while its running. This is real important.

See the extra lamp cord I have plugged in to 110 and notice the battery charger plugs into the voltage regulator.
https://photos.app.goo.gl/T5qa6trSMqEaEx7J8
…………
Next you need a voltage regulator. This will adjust the temperature of your heat strips.      Get the “10 amp” version…. The 5 amp model will be popping fuses all the time.   This is what you want.
https://www.amazon.com/LVYUAN-Variable-Transformer-Regulator-110V-120V/dp/B082KYKP9P/ref=sr_1_6?crid=3PF7ACKQJN850&keywords=voltage%2Bregulator%2B120v&qid=1642611136&sprefix=voltage%2Bregulator%2Caps%2C141&sr=8-6&th=1
https://photos.app.goo.gl/sKNmmsB6kfPizvtP9



The last item needed is an accurate thermometer. There are many different types of probe thermometers out there, but I like this one the best because it has a high temp alert built into it…. I can set the alarm for 180 degrees and if the strips get too hot it alerts me and I can turn down the voltage a bit.
Here is what I use.    https://photos.app.goo.gl/ZyzWkz1hz3vHBVfs5

OK….. so now you have all your parts and pieces and equipment, and have a set of heat strips ready to go….You install one in the bottom of your form, wrap your bow laminations in a thin painters plastic , and put them directly on the heat strip. Then put the other heat strip directly on top of the limb and use zip ties to hold everything tight to the form. Take care to align the heat strips so they are the same length coming out of the form, and use a small clamp, or needle nose vise grips to clamp the two heat strips together on one end, and the battery charger clamps red on one strip, and black on the other. Be sure these two do not touch each other. I separate them with a scrap piece of wood lam.
Next is your air hose and button up your form. Then slip your temp probe in between the air hose and the top of the heat strip and give that hose about 20-25# of pressure. Check your lamination alignment and let it set 60 seconds before going up to 50-60#s of pressure…. No more than that is needed. I’ve laid up limbs completely using 25# before and they came out fine…. That was an ooops though… Recurve limbs need that extra pressure. 

Now turn on the voltage regulator and adjust the voltage to about 40-45 volts. That is a good place to start for 2 -36” heat strips…. One piece bows with long heat strips will need more voltage, and going from 1.5” width to 1.75” width needs adjustment too. You will have to do some dry trials to establish the right settings. But what you want is the temp to slowly rise to 160 degrees in about 30 minutes and stay between 160-180 for another 20-30 minutes. I set my timer on the battery charger for one hour and its perfect every time once I have my voltage setting established. Btw…. Hot or cold temp in the shop can effect the setting a bit too.

The only other thing of note is to wait until the temp cools to 90 degrees before removing them from the form. You could easily lay up 3 sets of limbs per day with this set up.

Here are some of my bow forms I have set up.
https://photos.app.goo.gl/1ihdPa5SExkyBCma7


Kirk

[Longcruise]

@kirkll that's great info right there.   I can see setting something like that up eventually for sure.

I'm fine with the oven system but it's a PITA to store it and move it around.  I keep it in the shed and pull it out into the yard to use it.  Like I said,  PITA! 😀

For our OP, don't be in too big a hurry to complicate your systems.  Kirk is managing a business and time is money 💰 

Here's a lay up that I'm curing as this is typed.

The form covered with a blanket.

A small space heater blowing warm air over the form under the blanket.

A Bluetooth meat thermometer to track the temperature.

Even this is unnecessary.  Smooth on will cure at room temperature over 24 hours but at my house "room temperature" is in the 67* range.

[Kirkll]

The biggest thing i did not like about using a hot box was cooking my forms to death. The short TD limb forms were not too bad, but the one piece forms can twist on you easily after cooking them a bunch. Besides that the dad burn things are heavy lugging up and down the stairs. I keep my hot box in the loft of my shop. ...

I'd rather let them set for 24 hours with 65-70 degree temp than cook em..... btw.... i never cook my riser footings . Those i clamp up and let set over night before milling.     Kirk

[Bow Bender]

Kirk, great information!  Your description of the components and the instructions for setting up and using is very clear and easy to follow. I now have a greater understanding of how it works and it doesn't sound near as scary as I first imagined it to be. I like the way heat is applied from below and above at the same time.
  My first question, of what probably will be several, is about the stainless steel pressure strips. Will 22 ga galvanized or any other sheet metal work, does stainless steel have properties that cause it to heat up different than others? The reason that I am asking this is that I haven't found a source here locally and the supplier you referenced has an order minimum of $250.00 . Are the strips flexible enough to coil them for shipping? If not, the shipping would be outrageous.

[Kirkll]

Yes….The 304 stainless steel has properties that work well as a heating element. Number two is that it’s easily sanded to clean glue off of it without altering the smooth surface. These heat strips go right up to the glass with only a thin painters plastic between them. If you use to heavy of plastic to wrap your lams, any wrinkles in the plastic will be transferred to the glass like an etching.

When I purchased all my stainless steel on line I bought about $350 worth and set up a half dozen forms. So the minimum wasn’t an issue, and shipping wasn’t bad.
You could find a local steel fabrication company and have them cut your strips, but be careful to explain that a “shear” cut isn’t going to work. These things need to have a nice straight edge, and be flat. A shear will often cup the edge a bit, and sometimes introduce a slight twist to the strips. Yes a 6’ 6” length can easily be coiled up, but I think they shipped mine in a long package. Can’t remember actually… it’s been years ago.

Setting up a system like this is not cheap by any means. But when you are building 50-60 bows a year it’s the only way to fly……     Kirk

[Bow Bender]

 I finally did an experiment with a 6' heat cable that had been laying around for at least 22 years and I haven't need it since I first got it and used it so I don't feel that I am wasting anything here. The first thing that I did was cut away the plastic housing that contained the temperature sensor so I could see how it was wired.  The actual heating element appeared to be a very fine stainless steel wire coiled around a multi strand non conductive core.  I removed the temp sensor and connected the original power cord to the heat cable with wire nuts and then electrical tape to hold everything secure. I laid the cable on a 6' pine 1X4 and then a 1/4" strip of plywood on each side of the heat cable to create a channel for it and then topped it off with a 1x2 strip of ash. There was enough room in the channel that the cable would slide in it and if it burned out it would be able to be changed.
 I clamped it all together and inserted a thermometer in the groove between the 2 heat elements of the heating cable.

I plugged it in and started monitoring the temperature. The ambient room temp was around 60*.

At 10 min 115*           30 min 145*       50 min 156*         1:30  161*
    15 min 128*           35 min 149*       55 Min  158*         1:45  162*
    20 min 134*           40 min 151*       1 hr      159*          2 hr   163
    25 min 140*           45 min 153         1:15     161*     

     
I seemed like it maxed out at about the 2 hour mark but it probably would have gradually crept up if I had left it plugged in longer.

 It appears that this has possibilities. The top of the 1x2 felt warm to the touch but not hot. Wood is a terrible heat conductor but the longer that it would have heated the more heat the wood would have absorbed.  In a bow form, if a metal strip such as 22 ga galvanized was put directly over the heat cable it would absorb heat and spread it fairly even to the full width of the laminations. I know that EA 40 will cure in 24 hours at an ambient temp of 70*, but if this will cure it at a much higher temp if left for a few hours then it seems to me that it would be a lot better cure than room temp curing. This wouldn't be a setup for mass production but for an occasional build. If it eliminates the heat box then it is a plus. 
  I don't know what the wattage of the heat cable is but there may be higher ones out there now.
 I have an extra takedown limb form that I may experiment with. If its a flop then I'm not out much compared to a one piece form. If I get that done then I will post those results also.
 What do you all think about it?

 

[Mad Max]

Did you check the temp. at the other end?

[Kirkll]

My thoughts are the same you had about my set up. Setting a pipe heating cable into an enclosed piece of wood (Dado)  that is designed for wrapping around a pipe and being exposed to the cold air around it sounds like a melt down waiting to happen..... I don't think i'd walk away from that set up....

#2 is, you are only heating one side of the limb.... on a TD bow it might work ok, The temp is certainly high enough.....But with a one piece bow your belly ramps and one side of your fades are not going to get any heat and will require an over night setting time before pulling it from the form. Then you have a bow that has only been heat treated on one side.

#3 has already been stated.  The pressure strip could deform in the dado area depending on what type of clamping system you use..... 

those are my observations....   

You would be better off using these for just a few bows a year.

https://www.ebay.com/itm/281709798883?chn=ps&_trkparms=ispr%3D1&amdata=enc%3A1OCWaqcY5Tou8I0Ci9GTp8A61&norover=1&mkevt=1&mkrid=711-213727-13078-0&mkcid=2&itemid=281709798883&targetid=4581046489808873&device=c&mktype=&googleloc=&poi=&campaignid=418640322&mkgroupid=1239149842233245&rlsatarget=pla-4581046489808873&abcId=9300602&merchantid=51291&msclkid=1dc65b6586241abd938739e9a3f3fc7e

Here is the manufacturer info :      https://www.keenovo.com/


Kirk

[Bow Bender]

 Max, I did check both ends and the cable seemed to be heating up evenly from one end to the other.
 Kirk, I agree with you completely on all of your points and this was just an experiment that probably wont go any farther.

[Shredd]

  Why don't you try routing a slot in both pieces of plywood  1" to 1 1/4" away from the edge on your form where the bow will sit and insert your heat tape in slots before you glue up the form... Or fish it through form afterwards...

I would double that tape over on itself 1 or two times and wrap it tight with a towel and leave it on for 24 hours... If it don't burn up or have any damage, you are good to go... Got to do it in a safe location like big concrete slab or gravel driveway or attic of your house...