When it comes to wide format thermo presses with flat heating plate, there are three most important parameters determining stable quality of thermal transfer:
EVEN HEATING OF THE WHOLE SURFACE OF PLATE
As it is known, sublimation printing is performed under a temperature about 200o C and it is of great significance the temperature to remain equal across the whole area of heating plate.
So, what is happening to print in case the condition fails? If some area of plate has lower temperature, this will cause the lighter color in such an area, which means a flawed item. Otherwise, in case of overheating of separate area of plate the image will be degraded, since sublimation inks lose their color under higher temperature.
In a word, even heating is a very important factor. Next, we will tell how we managed to achieve even heating of printing plate.
The heating elements in our thermo presses cover up to 60% of plate surface and are located in a way to take account of inertness of heat distribution and quickened cooling at the edges. Along the exterior perimeter of heating plate there are additional heating elements, which compensate deficiency of heating. In the center of plate there are, on the contrary, less heating elements, as far as heat is distributed towards the center.
It is known that the heat conductivity of aluminum is four times higher in comparison to steel. In such a way, if heating elements are located in proper positions, the thermal spots created on the surface of heating plate diffuse within its thickness and we get even temperature of working surface, which provides high-quality printing.
PRESERVATION OF PLATE GEOMETRY
Besides extra high heat conductivity aluminium features a high coefficient of thermal expansion as well. In the simplest words - under heating this metal differs with fantastic expansion capacity, especially in cases we deal with plates of wide format flat thermo presses. According to our measurements, aluminium plate sized 1 x 1,5 meters under heating from 20 to 200 degrees increases along its length by as many as 10 millimeters!
Such a property requires special approach to positioning of heating plate at the bed of thermo press. In case of its rigid mounting the plate and the bed will be deformed by expansion under heating.
So, we have developed our own system of heating plate separate mounts at the stage of our units’ designing: under heating it freely expands in all directions, not being restricted by the bed, which means there are no conditions for its deformation. To reach the goal we have applied handy and reliable system of flexible steel suspension units.
However, there is another one force having impact on plate causing its deformation. As a result of heavy pressure on printing table it tends to curve inwards the casing. If heating plate will be curved inwards, there will be a light spot of unprinted image in the center of print. In our thermo presses we use special restrictors preventing plate from incurvation even under the heaviest pressure.
Thus, due to our system of suspension units and restrictors the heating plate expands in a horizontal plane when heating and contracts when cooling. In a vertical plane the plate will never “slack” down, as far as it is permanently influenced by force pulling it up in several points. At the same time it will never be curved inwards due to special restrictors.
SUFFICIENT PRESSING FORCE
Even clamping of sublimation paper to fabric is the necessary condition for high-quality thermal transfer. To reach stable printing there is a necessity of printing liner being sufficiently tough. If we take into account the fact that in THOR 151 heating plate has a space of 15 000 sq cm, to provide pressure of 250 g on 1 sq cm we will need the load being equal to the weight of an ordinary road-roller having impact on the whole surface of heating plate.
The THOR thermo presses are equipped with powerful pneumatic cylinders providing pressure up to 250 grams on every square centimeter of heating plate surface, which means stable quality of printing.