YELLOWING ON MUGS AND METAL

[Jules]

Hi, I am wondering if someone can help me with a printing problem please.
When I print on a metal plaque or mug I am getting a yellowing in certain places on the items. Mugs I heat at 400 degrees F for 4 mins 30s and plaques at 400 degrees F for 50 seconds. It only seems to happen on bigger plaques and some mugs. Print heads are perfect and the image prints great on the paper.Any ideas anyone? I'm stumped! Thanks, Jules

[Paul]

hi. is that on white bits? or all over picture? i think you over cook your mugs. i dont know how much is 400F but i cook my mugs at 190C for 3 min and thats plenty enough. when i overcooked my mugs i found them yellow ish. born coating. hope this help.

[Jules]

Hi Paul, yes the yellowing is just on some white parts of the mugs. I will do as you say and reduce the heating time to see if this solves the problem. With the aluminium plaques...15cm x 15cm I use 190C for 50 seconds. Do you think this is too long?
many thanks for your fast reply, Jules

[JSR]

What mugs are they?

I've noticed yellowing on certain types of mug and I think it's all down to the coating. If the mug is a legitimate RN-coated mug then you can usually get away with murder, but if it's a mug with a soft-coating or a special coating (like colour change mugs) then they are far more picky.

For example, if I try to cook colour change mugs at the same time/temp as RN-coated mugs then they go yellow - and the yellow is far worse where the paper (TruPix) comes into contact with the mug. The only way to reduce the yellowing (I've not yet been able to get rid of it entirely) is to drop the temperature right down and increase the time to compensate.

Oddly, this wasn't a problem when colour-change mugs were produced by TAMS. But, then, there were a lot less mug problems in the days of TAMS.

190C is 374F, so your 400F is a little bit high. F = ( C x 1.8 ) +32

[Paul]

With the aluminium plaques...15cm x 15cm I use 190C for 50 seconds. Do you think this is too long?

I dont know. is that white aluminium? i only done silver and 190c for 60sec was long enough.

[bms]

190C for 50 seconds

Thats fine for white aluminium

[rudders18]

Hi
I used to get this when printing the aluminium travel mugs and I cured the problem by wrapping a piece of ordinary paper around the mug, over the the top fo the image paper, and then putting in the mug press. This sacrificial piece of paper stopped the slight "burning" problem you are getting.
Regards