www.TK560.com

[jegner]

Here is my over/under using the sunbeam grill:

http://www.tk560.com/vactable2.html



It works but as you can see from this pic



The cold spots show up really well.

Great for small parts, though.

[drcrash]

[drcrash]

Jim,

Did you bypass the thermostat on the Sunbeam, like Ralis Kahn did?

If not, that may be part of why it didn't heat well when you put aluminum between the elements and the plastic.

I've been fiddling with my Sunbeam and I think I've gotten it to heat more evenly. I put a diamond-shaped piece of perforated aluminum (from a disposable grill topper) over the middle of it, leaving the corners fully exposed. (I also left the slanted edge of the grill topper on, bending it a little more to use it as a 45 degree reflector around the edges.)

This seems to redirect some of the IR to the corners. (Basically, most of the IR bounces around between the backside reflector and this frontside perforated reflector, until it either comes out a hole, makes its way to an uncovered corner area, or is absorbed.)

The perforated aluminum is about 1/3 holes, and that seems to be roughly the right ratio---letting enough IR through in the middle diamond, and redirecting a fair bit toward the corner triangles, too.

[crashmann]

[drcrash]

[jegner]

A few things I noticed with the Sunbeam grill vac table. First, the heating element does not heat even at all. The center is much hotter than the edges. I used all sorts of tricks to get those corners hot, and nothing worked. About the best I could do was to bend some strips of sheet metal, around the element, and use that as a sort of extension to the element. The element is very round, and is not intended for this sort of application. I even went so far as to make a refractory mix of Perlite, and furnace cement, and coated the bottom of the black grill base, to no positive effect.

The grill actually sits on a tiny bit of a shelf made from aluminum strips, so NONE of the direct heat is lost due to the mounting frame. I wanted to be sure it would not burn my frame. LOL.

The thermostat kept shutting off right when I needed it to keep going. A rewire of that would be nice, but it defeats the ability to regulate the temp if I did. So that was never done.

The best you can hope for, IMHO with one of these grills is a small 8.5x11 size useable forming area. Good for scale models, and small parts like TOS communicator bodies, or tricorder parts, but not great for trooper armor.

Jim

[drcrash]

Jim,

I think pretty much any infrared heater should be able to heat pretty much any flat surface evenly, with the right reflector. The main trick is to reflect the IR to the plastic, rather than absorbing it. You shouldn't need a refractory coating, and it's probably counterproductive.

With a solid sheet of aluminum, the problem is that the IR must be absorbed by the aluminum and re-radiated or convected from the other side. Most of the IR will not be absorbed by the aluminum---it will be reflected and absorbed by the refractory coating on the other side of the heating elements. (And half of the re-radiated heat from the aluminum will go the wrong way, and mostly be absorbed by the refractory lining, too.) You'll heat the oven itself way more than you heat the aluminum or the plastic.

I think what you were doing would eventually work, with the right tweaks, but would be harder than building the right reflector to direct the IR to the plastic before it's absorbed.

As for the thermostat... I'd like to have a thermostat, too. I'm afraid the Sunbeam thermostat isn't very good, though. I used a grill thermometer and noticed the temperature inside the grill housing fluctuating by dozens of degrees in an on/off cycle. (Usually about 30 degrees, I think, but sometimes 40.) It may be better to control the heat manually, one way or another.

In your setup with the aluminum sheet, I'd think the thermostat would be a killer. Since the IR is mostly being absorbed by the rest of the oven, not the aluminum, the whole oven needs to get very hot before lots of IR starts re-radiating from the front side of the aluminum and heating the plastic. (And you'd need to insulate the rest of the oven really well, so that most of the heat comes off the aluminum rather than being re-radiated or convected from the rest of the oven.) The thermostat will keep that from ever happening.

Paul

[drcrash]

I did some tests, and the diamond-shaped reflector made out of a disposable grill topper works pretty well.

The heat is much more even---no big hot spot in the middle. I can tell this just from the red glow from the heating element, both visually and by putting my hand under it and feeling that it's pretty even.

It also shows up in the pulls. I did some pulls with a test plug, and now the plastic isn't annoyingly thin in the middle, and there's better detail further out. (The very cornerest corners are still a bit lame, but most of the 12' x 18" area seems quite usable.)

There are still a couple of hot spots about a third of the way from the end of the oven (not in the middle) which I want to tone down and try to get the heat into the corners. I probably should have made the diamond thing extend another inch or so toward the end with the two rounded bits, i.e., away from the plug/thermostat end.

Unfortunately, when I went back to the dollar store to get another pair of grill toppers to mess with, they didn't have them any more. I'm guessing they're a seasonal item. (There were plenty of turkey-roasting pans all the sudden.)

The heating element in my sunbeam seems to heat asymmetrically. One of the long sides is simply brighter/hotter than the other. I'll try to fix that with an asymmetrical reflector.

I'm thinking that to get a really good heat distribution, I'm going to need a skirt/shroud thing like Harley Guy's. That will let me put the plastic further from the heating element, where the IR illumination is more even, without much efficiency loss. It should also reduce convective cooling of the corners.

[drcrash]

By the way, once I got my Sunbeam inverted, I realized that the heating element wouldn't be recessed as far as I thought into the housing---so the edges of the housing block less of the IR than I thought, and it can illuminate/heat a bigger area.

I still think it could use better 45-degree reflectors around the edges than my grill topper hack is doing. (They don't go all the way to the back reflector, so a significant amount of IR goes right over them to the housing wall, rather than being reflected down to the plastic around the edges. That can't be good.)

[harley guy]

here is the info boxjoint