custom heating element with PID controller

Thread Starter

byakk0

Joined Nov 29, 2015
38
I'm building a vacuum forming table with an overhead heating element.
I'd like to make it 120V if possible as I don't have a 240 readily available.
I don't need the element to go over 500*F. I'm being generous here as I am mostly using just .093 ABS sheets, which soften enough at around 190*. I'm just wanting to be able use it on other plastics should the need arise.

The size of the heating box is 34"x18", and I figure it will snake across the 34" four times, so 30" to 32" x 4 gives a rough element length of 120".
I'll be using coiled Kanthal A1 wire (20-24 ga). Is there a formula to figure coil length?

So far I've come up with this list of items:
PID controller
K-type thermocouple.
SS relay
Thermocouple, rated to @ 500*
switch

Hope this makes sense.
Is it doable/recommended in 120v?
Would two 60" element legs be better than one full 120"
Other recommendations are welcome, if any.
Thanks.
 

mvas

Joined Jun 19, 2017
538
I'll be using coiled Kanthal A1 wire (20-24 ga). Is there a formula to figure coil length?
Thanks.
The length of wire = ( Circumference of each coil ) x ( # of coils per unit length ) x ( length of the coil )

Using Ohms per Foot and Voltage you can compute the Wattage.
 
Last edited:

Hymie

Joined Mar 30, 2018
739
Don’t forget to include a thermal cut-out/fuse wired directly in line with the heating element to prevent a fire in the event that a fault results in continuous power being applied to the heating element.
 

shortbus

Joined Sep 30, 2009
7,334
Don't most of them use infrared spot lights or heat lamps? Using resistance wire your heating places that don't need the heat. My youngest son worked for a while at a company that did both roto molding and vacuum molding. They used infrared heating elements, like a Calrod. Toaster ovens from second hand stores are a good source for them.

Heating

Heaters are generally infra-red elements mounted within an aluminum reflector plate. In order to obtain the best vacuum forming results, using any material, it is essential that the sheet is heated uniformly over its entire surface area and throughout its thickness. In order to achieve this, it is necessary to have a series of zones that are controlled by energy regulators. Ceramics do have some disadvantage in that their high thermal mass makes them slow to warm up (approx 15 minutes) and slow in their response time when adjustments are made.

More sophisticated quartz heaters are available which have less thermal mass enabling more rapid response time. Pyrometers enable accurate heat temperature control by sensing the melting temperature of the sheet and interacting with the operating process control. Precise temperature readout is also available with a computer controlled system working in unison with the pyrometers. Twin heaters are also recommended when forming thicker materials as they assist in providing more uniform heat penetration and faster cycle times.

Twin quartz heaters are advisable when forming high temperature materials with critical forming temperatures. By close control of areas of heat intensity, heat losses around the edges caused by convection air currents and absorption from clamp areas can be fully compensated for and consistent results achieved on a continuous basis. Cost savings can also be considerable if quartz heaters are specified, as there is an adjustable percentage power drop when the heaters are in the rear position during the forming process. From - https://formech.com/about/about-vacuum-forming/
 

Thread Starter

byakk0

Joined Nov 29, 2015
38
Thanks guys. I'll have to peruse all of these options. My current setup I'm using my propane bbq as a heating element. It works, but not very efficient.
 

Thread Starter

byakk0

Joined Nov 29, 2015
38
Yep. Love metalmeet. I need to get out there and do some more tin bashing.

As for my vacformer, I do want two zones, now that I think about it. Right side and left, so I can use smaller peices of plastic, so basically I need two elements. If I do the PID controller route, would I need two? Or can just one control each element.
 

shortbus

Joined Sep 30, 2009
7,334
Do you even need PID control for something like this? When you're heating by infrared, which is the normal way of this, and there is no direct contact with the thing being heated is PID effective?

The Kanthal or other type of resistance wire coils heat the air around themselves which means the plastic should be in a chamber that keeps airflow to a minimum, since it is the air that is getting hot from the resistance coils. Where with infrared the plastic it self is absorbing the heat, like a sidewalk on a sunny day. This is the difference between the toaster ovens that use heating coils and Calrods. Calrods give off infrared.

Now that the internet is out there, when doing something I haven't done before searching to see how others are or have done it and their mistakes is so much easier. So look at what others are doing and adjust accordingly to your project.
 

Thread Starter

byakk0

Joined Nov 29, 2015
38
I'm open to anything. I have just been researching and this is just one method I've seen.
I'll look into calrods, that's a new one for me.
 

shortbus

Joined Sep 30, 2009
7,334
They Calrods are or were pretty price when buying them out right, but they are used in many things that are either inexpensive new or next to nothing secondhand. Like toaster ovens or the so called quartz room heaters.

I looked on Youtube and most of the DIY ones are using "patio heaters" and they use the calrods. Like this, just the one I saw first, they could be cheaper on Ebay or Amazon. https://www.homedepot.com/p/Comfort-Zone-1-500-Watt-Infrared-Ceiling-Mount-Quartz-Electric-Portable-Heater-CZQTV5M/203558853?cm_mmc=Shopping|G|Base|D27E|27-23_PORTABLE_HEATING|NA|PLA|71700000034238966|58700003946879293|92700031953708174&gclid=EAIaIQobChMIu5rqtueq4QIVCrXACh3UWQcDEAQYASABEgKJxvD_BwE&gclsrc=aw.ds

And none of the guys were using any kind of thermostat, just going by the 'droop' of the plastic sheet. That is how they did it where my kid worked too.

Youtube videos -
There are tons more these were just ones I kind of liked and they were using thicker plastic like you. The last one was doing something that I kind of liked, he used foil to line the heat box. I think that using the foil backed insulation board instead of MDF would even work better.
 

Thread Starter

byakk0

Joined Nov 29, 2015
38
Yeah. I've seen most of those. Droop and softness is how I gauge it too.
I'm not trying to reinvent the wheel or anything, just was talking to someone recently and he mentioned making a heated box for melting ring making metals. Sounded like a decent idea at the time... Digital readout of the heat setting and all.
I've also considered taking elements from toaster ovens. Apparently that's the simpler idea.
Bottom line is, I need to heat a single 32x12 area, or two individually switchable 16x12 areas. I just want to make sure I get even heat on most of the surface. When I started this project my first incarnation was utilizing the front two burners on my stove. But, they are round and different sizes so each half of the plastic softened at different rates and yeilded a lousy part. I had to manually use a heat gun on the cooler areas, and what a pain that was.
 

shortbus

Joined Sep 30, 2009
7,334
Melting metal is a whole different thing, and so is metal heat treating, both of which use more like a kiln than a heater. Both also need to monitor the temperature for the metallurgy to be right.

I don't think you need to have your elements going completely to the edge of your box. Most of the heat wil come from convection directly under the elements but will spread by conduction to the edges pretty fast. And the plastic sheet usually isn't molded, shape changed over the whole surface any way.

The stove could have worked by turning the larger burner output down, so both were putting out the same amount of heat.

Have you considered using the tube type halogen bulbs for heat? The type used in work lights, they get really hot and are fairly cheap. Back in the early days of vacuum molding they used the old type heat lamps.

 

Thread Starter

byakk0

Joined Nov 29, 2015
38
I could give halogen a try. Equal heat distribution is the biggest factor, and time too.
When I did the stove burger method I found my white plastic would discolor, and I need to avoid that
 

Thread Starter

byakk0

Joined Nov 29, 2015
38
Haven't had a chance to work on it yet, but I came across this video. The machine is much larger than I need, but in general this is basically what I envision.
Looks like he's got calrods.

 

shortbus

Joined Sep 30, 2009
7,334
That's what it looks like. Also note the distance between them. You don't need full coverage of the sheet area.

but if doing it myself I'd probably go with the infrared patio heaters. I was surprised by how well infrared works. The fabrication side of my barn, workshop I used to heat with an overhead Reznor type heater, and it was always cold. I bought a gas infrared heater at an auction much smaller than the Reznor to add to the area. The infrared is all I use now it keeps the space hot so hot it gets shut off when working in the area.
 

Thread Starter

byakk0

Joined Nov 29, 2015
38
I had the time today to experiment with the single 500w halogen lamp I have. I'm impressed. Thanks for the suggestion. I should have explored this option years ago.
I built a small box and set my lamp inside, lined the box with foil and used a scrap piece of a failed part, as I'd rather not use a good piece of plastic for this test.
2 single lamps from HF are $27, compared to the cheapest 32" conrad elements I found on ebay yesterday for three, $65+ shipping. Wiring will be much simpler too. Looks like a new metal project is in my future for metalmeet :)


 

shortbus

Joined Sep 30, 2009
7,334
An idea that might make it even more efficient, instead of making the box sides perpendicular, make the angular. I mean start out at the size of the light and taper them out to the size of the frame you use for the plastic. This should make the most of the infrared from the lights.

I think I've said before, the old way of doing it was with the old fashion heat lamps. I can remember back in my high school shop class days people making things from plexiglass and using a heatlamp to allow it to bend.

The lights used to be available at Home Depot and Lowes as outdoor spot lights, with out the tubular frame and pretty cheap. I used them in the garage in the winter when the fluorescent's wouldn't turn on. If you make a box from metal be sure it is grounded with the light fixtures, and insulate it to keep the heat better contained.
 
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