Hi musikmeister,
are you German? In English it would literally be "music master".
It seems to me that you have more machinery in your hobby shop than you are able to use properly.
Following 3V0's advice you should better use a good qualitiy laminator instead of an iron. With an iron you never know how hot a an area has become transferring toner from the paper print onto the copper clad. He further pointed out this is a process of learning - a pretty long and hard way sometimes.
Using photo positive method with UV-sensitive PCB material you should have a good laser printer, UV-exposure box and the necessary chemical products and a transparency to be used to expose and develop the "film".
For that method disable the printer setting "toner saving" and try to print an image of completely black traces and pads. (If necessary mount two films on top of each other. Traces and pads must be deep black!)
Do not "fill pads" using that option if your software offers it. Unfilled pads are easier to drill after etching, because the drill centers itself when hitting "bull's eye". (±0.25mm, otherwise kiss off the drill and say good bye to €7.50
)
Possible overexposure must be compensated for by reducing development time.
As soon as traces get visible check at short regular intervals for progress of development. When the PCB is developed to a point when no clouds of photoresist are visible anymore, remove it from the developer and rinse immediately and thoroughly. (Developer keeps on working until it is stopped by rinsing!)
When done drill the board as specified by drill diameters in your drill file.
Any more questions? PM me.
Regards
Boncuk