I am often asked to explain hot foil printing (sometimes called 'gold foiling') as it's something that our company uses for our wedding accessories.
First of all, it requires a 'hot foil' machine that is capable of heating up a plate to temperatures of up to 200 degrees C and applying pressure. This machine can only print when a negative plate is positioned and heated to the required temperature in order to 'release' the foil. Most hot foiling is done around the 100 degree mark, but printing on paper, card, leather and plastics can vary enormously.
The machines come in a variety of sizes from hand operated to fully automatic. They are all capable of applying the necessary pressure to hot foil and the larger the machine the more pressure, which means that they can also emboss (push the design out of the card or paper) and die cut (cut and crease the paper or card in one operation to form boxes, stationery, etc). The larger machines can exert several tons of pressure and be bigger and heavier than the average car.
The plates are usually made of magnesium (light, cheap, one use only) or copper (1000's of impressions and reuseable but expensive) and are etched to form a negative of the required artwork. They are bolted or glued to the hot plate of the hot foiler and then the item to be printed is aligned to match.
The foil in hot foiling comes on a roll which when sandwiched between the item being printed and the plate, releases the foil from the roll onto the item being printed on all areas that the design on the plate touches.
Hot foil is the only true way of printing glossy gold or silver as ink printing dries out matt. All colours are available and pantone colours can all be matched. The only possible drawback is that hot foiling can only be printed one colour at a time. It is not capable of full colour and although holographic and multi coloured foil is available, each impression will be different as the foil is pulled through.
Embossing is done on the same machines with hot foil plates being the female and plastic male plates being attached to the the other side so that the item being printed is fed between them so that the male plate forces the design into the female plate.
Obviously this is only a basic guide and there are lots of other 'need to know' things such as pressure, dwell, etc, but hopefully you will have a better idea of what is involved having read this.
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