Bake from a Cold oven??

[Banditperformance]

OK, are any of you operating a cold oven? I mean spraying and hanging parts in a cold oven, and then ramping the temp to cure?? I am having to do that with my big oven, I hang over 100 Pieces in for a baking. BUt, the ovens cold, because it takes 5+ Hrs to spray them. Now, I turn oven on, and it takes 45 min approx, maybe a bit more to reach 400. I am getting Sag sometimes, and more gloss on edges. I usually start the 400* cure time when the oven has got to 385 or so, for fear of overbake. What would you do? I know lower temps, longer bake, but what about a oven that basically takes the lower temps time to climb from low temp to 400*

[nuttyman]

hmmm. that shouldn't affect the finish that way. the sag usually comes from the spray being too thick, unless you're spraying candies over polished substrates in which case sagging happens until you get the touch down! what kind of powder are u using & where do you purchase it? are you absolutely sure the powder is good stuff?

[sdold]

Same here, I almost always put parts in a cold oven, and it takes a half hour or so to get up to temp. The only sagging I've ever had was from piling the powder on too much. Popeye mentioned good lighting in another thread, he's right on the money, bad lighting has caused more problems for me than anything else (too much or too little powder in areas).

[joker63]

i'm sure you know this, but i state it cause you didn't mention it. cure time starts when the part reaches cure temp, not the oven. and like the others said, sag is typically an indication of too thick. do you use a gauge?

[callmej75]

Do you have any buildup on the edges of parts? Like picture framing effect? Sometimes that is caused by too much KV's or too close to the part. Air pressure can affect it too. If you are spraying the parts close together, the powder tends to stray onto parts already coated.

[Banditperformance]

Ya, Picture framing is another problem I am having too. I am having problems with my Caswell adjustable gun, that I know. The Voltage goes Increadibly high sometimes. I somtimes get a 2+ inch spark off the tip, other times barely jumps 1/8". And, the gun is still charging when Not stepping on the pedal, although at a lower voltage and erratic. I also hate the way the powder builds up in the barrel, and clumps out when pointing the barrel down. Maybe it's time I move up to a more pro, fluidized bed/hopper gun

As far as powder, my client is supplying the powder, and some has been from EW, and some from Tiger. I use alot of Caswells, and sometimes have the same problem too, but like Nuttyman said, mostly with the translucients. This last batch I am refering to though, was from Cloverdale paint, and was a Wrinkle Black. I'd show some pics, but my client has requested me not take pictures, because he has people trying to copy his designs and artwork LOL

Oh, and using a gauge, that's after the part has baked, right, A scratch gauge? Destructive testing. If so, no chance of tha being able to use, cause of the varying nature of the parts I do.

[callmej75]

Hmm...might be the gun man. I know my EW gun works well...but at $700 it should! It still emits some electrostatic charge at the lowest setting too. Sometimes it sticks too...I just gotta hit the side of the gun and it stops. No buildup in the barrel tho. I get alot of the picture framing effect on totally flat, square surfaces tho. Sometimes I try to shoot everything hot and hit the trigger like I am touching up the part but coating it little by little.

[Popeye]

The oven times sound fine to me... and the picture framing, well sometimes I get the same thing IF I'm not paying attention and I'm using a fluidizing hopper with a powder pump. If there is a front and back to the part I'll spray the back first then spray the front trying to stay close to the center of the part and avoid the edges.
Most problems people have with these guns has nothing to do with the KV but more a result of powder delivery. IMO