A process you might want to look for is called sublimation. The
metal pieces will have a special coating on them and you will have
to print your image on a special paper and apply to the metal with
a heat press. Good luck...
The special coating is polyester in liquid form, you usually dip or
spraycoat the item.
You'll need a special paper but also a special ink for your printer.
To top it off, I always recommend a second printer for the
sublimation ink because it will create less problems and maintain a
good economy. (Switching cartridges entails intensive cleaning of
As for a heat press - in the T-shirt business it's callerd
transferpress (at least in Europe) - it should not press hard like
you would with textiles but heat evenly which could become a problem
if you aim to do irregular forms. In a production situation there
are ways to solve this.
Please ask if you have any questions.