The principles of thermography are extremely simple. Thermographic powder is applied to the surface of a printed sheet and the surplus is removed by suction, vibrating or shaking, with the powder adhering only to the wet ink. The method of excess powder removal is dependant on the SUNRAISE Thermographer model that you purchased. The printed and powdered sheet is placed face upwards on to a conveyor belt which carries it under the heater. Thermographic powder has the characteristic, when heated of swelling slightly and fusing to form a raised glossy surface very similar to that produced by embossing. The powder is itself colorless so the embossed area takes the color of the ink. Powders are also available which contain gold, silver, or bronze ingredients. These obliterate the ink color and the appropriate metal color dominates.
The amount of heat applied is controlled by the speed of the conveyor belt and the heat settings. Unfortunately, it is not possible to give simple rules to specify the speed control setting which gives the correct amount of heat for a particular job, as this depends on several factors the most important the thickness of paper or card used. With thicker material more heat is required and a slower belt speed. Other factors are grade of powder. Coarser powder or boldness of type will require more heat. Atmospheric temperature, humidity of paper, reflectiveness of paper surface, metallic content of powder and the voltage of your electric supply all can be a factor in heat control. Taking all these factors in consideration the only way to determine the amount of heat required is by trial and error. With practice one can tell if a particular print is “undercooked” or “overcooked” and the belt speed can be adjusted accordingly. Thus, gross under heating is characterized by a matt crystalline finish in which the individual particles of powder are still visible. Slight under heating may be detected by a pock-marked or “orange peel” texture which indicates the fusing has commenced but has not been completed. Too much heat is characterized by resin running from the inked areas to give a “halo” effect around each letter filling in the loops and destroying the sharpness of the print. As the resin spreads the “raise” is lost and the shinny finish is diminished. With Thermographic powder there is a tendency for the basic color to be degraded towards brown, and the halo tends to have a brown color. Scorch marks on the paper are also a sign of overheating.
Scorch marks at the edges of the paper are more likely to be caused by paper curling up under the heater (see paragraphs (c) and (d) “Selection of Paper” for more information).
SELECTION OF PAPER
In principal, any paper of any thickness can be used for thermography. In practice there are some limitations which should be kept in mind.
(a) The best results are obtained with smooth surfaced paper. A heavily grained or textured paper may give a slightly ragged look to the thermography. This could even have some artistic merit, but it can be a difficult effect to reproduce consistently. A surface with loose fibers is also dangerous as grains of powder may be occluded in the surface.
(b) Gummed paper, envelopes, and bumper stickers have been successfully thermo graphed. How ever it is best to do a test run to insure the gum is not destroyed by heating. It is also necessary to be very certain that powder does not get into the envelopes.
(c) Paper should be kept in good condition. Too dry is better than too damp. Damp paper may curl up under the heater and burn or edges catch fire. Dampness in the paper or powder can also cause “craters” in the thermography where bubbles have been formed and burst. If paper is too damp it may be possible to salvage by drying it before printing. Do this by passing the blank sheets printing face down under the heater at increased speed or reduced heat. Caution: having paper too dry can cause it to hold a static charge and more likely scorch. The best prevention is to keep the paper sealed in the manufactures packet and store it in a temperate environment.
(d) Some makes of paper are predisposed to curl, even when at correct humidity. One solution is to turn the paper upside down and print on the back. If this is not acceptable (because the watermark may be upside down) then dry the back of the paper as described in (c) above.
(e) When thermo graphing very thin paper and if the maximum belt speed of the machine is insufficient to good thermography, simply overcome by reduction the heat.
(f) With thick cards it may be necessary to increase the heat. This is done by increasing the heat with your heat controller. Extra care should be taken to allow adequate cooling after increasing the heat, or the finished sheets will stick together in the receiving tray. To avoid this open the cooling restrictor and increase the cooling to the maximum.
For more information or assistance with your thermography project please contact us.