www.TK560.com

[plasticfan]

Hi guys, love this place. Got some questions

Ive seen some curious designs but cant quite fathom a few things out of lack of experience so here are my questions.



-why the top heater so high up?. Unless a blower was attached or some reverse gravity device pushed the heat down this seems to be a moot application.



-why the angled cement box? is this supposed to condense the heat, trap it etc ---why not straight sides.



-why not make a literal oven if heat preservation is --as it would seem to be--so important. An oven with four sides, that has a hinged top that could swing over or slide up/down seems to be a great idea OR is this the use of the top heater source to clamshell the plastic? it seems as though the top heat source boxes are permanently affixed way up in the stratosphere so as to be moot in adding heating to the plastic so far below.


-and lastly, can somone provide a laymens terms chart for vac pumps and maximum table size/plastic thickness. In other words, if I wanted to build a big table, say 4 x 4 and wanted very good pull on thicker plastic, what pumps are best: brands and models, eg:

Model X will work on a 2x2 table
Model Y will work for a 2x3 table
Model Z will work for a 4x4 table
Model "best" will bring the heavens down


oh and last question: does anyone or CAN anyone do/sell/trade the electrical box work for me so all I have to do is attach the box, connect up a plug or something, wrap the heater coils and vac away. Im not an electrical guru



Great forum, again thanks
plasticfan

[jegner]

Many of the questions you are asking can be answered by picking up a copy of:

Thurston James' vacuum forming machine book The Prop Builder's Molding and Casting Handbook. http://www.amazon.com/exec/obidos/tg/detail/-/1558701281/qid=1086730486/sr=8-2/ref=pd_ka_2/103-4033176-3355803?v=glance&s=books&n=507846

Also, I detailed my constuction over on:

http://www.tk560.com/vactable4.html so if you have not read over that stuff, if will help answer a lot of your questions.

As for designs, I have made over/under designs, and well, if you want to be able to remove the parts and to pull tall parts, you need that vertical height.

As for the tapered walls on the oven, as for the flip flop designed machines, it is to have convection flowing hot air rise like in a chimney and that don't happen with straight walls. For over/under machines, this might be a moot point.

The oven needs to allow the plastic holding frames so that the drooping plastic does not come into contact with anything prior to the molds. Some folks just use the house oven/range but my wife does not like the smell fo melting plastic in the house. Also, most conventional ovens are not large enough for a 24x24 inch holding frame.

The table you ask for is non exsistent but the general rule is for a 24x24 frame, you need 23-30 gallon tank with a pump that can pull 6-10 CMF. The other frame sizes you will need to poll the other builders.

And wiring the oven for power is about as complex as wiring a light switch. Oh, wait, it is a light switch, on my machine!

Good questions.

Jim

BTW welcome to the board.

[plasticfan]

[jegner]

I'll defer comments on the over/under designs to those folks who have running machines in that format, but for flip-flop designs, the concept for the tapered oven box has to do with edge and corner heating. Assuming you made your oven 24 x 24 only, you get cold corners, and poor performance on your machine. By scaling up the oven to a 28 x 28 inch oven floor and a tapered walls to 24 x 24 at the top, as the heating up of the larger bottom, causes the hot air to circulate as it rises, [called a draft just as it happens in chimneys] more evenly heating all areas of of the oven, esp. the top and those darn hard to heat corners. The result is a more efficient heating design than one with straight walls only. Without the circulating heat effect, [called convection], just the heat directly from the heating coils [called radiant heat] are very uneven. The tapered walls makes a more efficient oven.

It's all about moving hot air and evenly heating the plastic. Thurston James talks about this in his book and does a better job explaining it that I can.

Jim

[crashmann]

[plasticfan]

baby, I can do the twist! ...and shout.


if its metal, I can do it.

[TD1035]

Can someone say SANDTROOPER BACKPACK!!!!

Todd
_________________
"If you're not living on the edge, you're taking up too much space!"

[drcrash]

I'm not sure I buy Thurston James' the story about the angled sides & convection.

I'd guess that the main reason it's important that the oven is wider at the bottom than the plastic is so that you can have heating elements past the edge of the plastic. You need that because the rest of the plastic is being heated from heaters on both sides (toward the center and away from it); for the edges not to be cooler, they need heating from both sides as well.

(If you just had a double strip or an especially hot strip right at the edge, rather than beyond it, it would significantly heat up the plastic a bit in from the edge, as well as the edge, and since that bit of plastic is being heated from the other side, it'd create a hot spot there.)

It's not clear to me that a straight-sided box wouldn't function pretty much the same as a tapered one, so long as there's a lip around the top that covers the same footprint (from the edge of the box to the hole the plastic sits over).

It's also not clear to me that the Thurston James design does a good job of engineering the convection that you want, to heat the edges of the plastic.

Here's a different idea:

(1) Drill some small holes in the bottom of the oven, near the center. This will let a little fresh air in, cooling the center a bit.

(2) Make a slight gap under the plastic-holding frames, so that the air can escape at the edges.

This should ensure that the air flow through the oven is from the middle out, so that the air gets warmer as it goes toward the edges.

(3) make a lid with a lip that fits around the frame, with a bit of room to spare all around, so that the hot air doesn't actually escape when it gets to the edges

(4) put some holes near the middle of the lid, so that the hot air can escape there, i.e., flow back over the plastic from the edges to the center.

This should heat the plastic from both sides, to a reasonable extent, when the lid is on, because actually hot air would flow into the gap between the plastic and the lid.

You'd want to tune the hole sizes to control the air flow. If you have too little, it won't do much good. But if you have too much, you'll just be letting your hot air escape before it does much good, and need significantly more wattage to heat the oven.

This is pretty much how a lot of forced-air convection ovens work; they just pump the air back through rather than letting it escape, so they can pump it faster without wasting a lot of heat.