Platen Issues

Thermal printheads are meant to be used in thermal printers where they produce dots of heat in order to form an image on a thermal sensitive media. The media is either a direct thermal paper or a thermal transfer ribbon and a receiver. In either case, the media must be pressed against the thermal printhead so that heat can flow into the media. Then the media must move relative to the printhead to position it for the next print line, so that a two dimensional image is eventually formed. Almost all thermal print mechanisms have a platen roller to press the media against the printhead. This pressure and the platen roller are discussed below assuming the use of direct thermal paper, with a section at the end to explain the differences with thermal transfer ribbons.

 Platen Pressure and Heat Flow

Pressure is required for efficient heat flow. On a microscopic level the surface of paper is composed of hills and valleys. If lightly pressed against the surface of a heater element, only the tops of the hills are in contact with the heater and the valleys are filled with air, which is a poor conductor of heat. If the platen pressure is increased, the hills are somewhat flattened, the width of the valleys is reduced and the percentage of heater surface area in actual contact is increased. Since there is a greater effective area for heat flow, more heat flows into the thermal paper and less heat is retained in the printhead.

No matter how smooth the paper, some contact pressure is required and heat flow will improve with increasing contact pressure, with diminishing returns. Printhead pulse life is improved from lower peak temperatures due to efficient conduction of heat away from the heater. Printhead abrasion life is shortened with increased contact pressure.

 Platen Pressure Specifications

Kyocera thermal printhead specifications give two values for the force to be used to press the platen roller against the printhead. In both cases they read "platen pressure" but the true meaning is "lineal force" or "force." In the "maximum ratings" section, the maximum lineal force is usually recommended to be 0.3 kgw/cm. The kgw is a unit of force equal to the weight of one kilogram at the Earth's surface. The cm means centimeters and refers to the width of the thermal paper or the width of the heater line, whichever is smaller. For example a KPA-104 printhead has a heater line 104mm or 10.4 cm long. The maximum recommended platen force would be 0.3 x 10.4 = 3.1 kilograms of force. If 50 mm wide thermal paper were used, this short paper would support most of the force, so the maximum total force is calculated as 0.3 x 5.0 = 1.5 kgw. To the extent that the platen roller distributes some of the force to the printhead on either side of the paper, a higher total force will be required. The abrasion life is shortened when using paper narrower than the full width and points of abrasive failure are where the edge of the paper rubs the printhead.

In the "operating conditions" section of a printhead specification, a total platen force is recommended, such as 2.6 +/- 0.5 kg/HEAD. This value varies from specification to specification and depends on the length of the heater line and on the type of glaze. The recommended value plus the variation (2.6 + 0.5 = 3.1) will be consistent with the maximum platen force for long abrasion life. The negative variation indicates that there is a minimum platen force necessary for good thermal transfer and long pulse life. For wide printheads, the springs that press the printhead against the platen roller should be spaced along the print width to evenly distribute the platen force.

 Platen Roller Construction

A platen roller consists of a metal shaft coated with a softer material. The metal shaft is usually steel and must be rigid enough that it does not bend when the platen force is applied along its length. The soft material makes the thermal paper conform to the top of the glaze mound on the printhead, so that the full surface of every heater on the heater line makes good contact with the thermal paper. Currently Kyocera recommends silicone rubber as the softer material, because it has better heat stability than chloroprene rubber that was previously recommended. A softer rubber is more forgiving of alignment differences between the platen roller, the heater line and the top of the glaze mound. A harder rubber keeps the platen pressure more focused on the top of the glaze for better heat transfer and higher speed.

The hardness of the platen rubber is measured by a durometer instrument in units of degrees shore A. The "operating conditions" section of a printhead specification will recommend a durometer range for the rubber, such as 40 - 50 degrees shore A. Kyocera recommends a harder rubber (45 - 50) for use with thin glaze. The harder rubber usually means that a printhead position adjustment mechanism is required. If a hard-surfaced direct thermal media is used, such as transparent film, the printer will act as if the rubber is hard, whether it is or not.

Kyocera has traditionally recommended a rubber thickness of at least 4mm. This recommendation dates back to the printhead models that allowed a 20mm platen diameter. Recommended rubber thickness is reduced to 2mm on modern printheads where the maximum platen roller diameter is 12mm, to leave enough steel cross section to make a rigid platen roller. Clearly, an intermediate rubber thickness could be used for an intermediate diameter. To somewhat compensate for reduced rubber thickness, these printhead specifications expand the durometer range to 35 - 50 to allow softer rubber.

Platen roller rubber can confound the relationship between platen force and true contact pressure on the heater surface. The rubber will pile up slightly on the upstream side of the heater line, reducing the actual contact pressure on the heater line. This effect will also be slightly different, depending on whether the paper is pulled past a non-driven platen roller or driven by a platen roller connected to the drive motor.

The dynamics of the platen roller are very complex, so platen roller selection is usually done by trial and error.

 Platen Roller Diameter

For real edge and corner edge printheads, there is no maximum platen roller diameter. For conventional flat style printheads, if the platen roller is too thick, it will rub the paper against the driver ICs. Reducing the cost of thin film printheads has meant reducing the size of the ceramic wafer, which reduced the distance between the line of heaters and the line of driver ICs, which in turn reduced the maximum platen roller diameter. In 1997, Kyocera broke free of this relationship by developing a way to interlace driver ICs and FPC solder pads on the same line. This new design permits larger diameter platen rollers to be used with the KPB, KPC and KFA series printheads.

platen diameter chart

 Platen Roller Length
On real edge and corner edge printheads and on KGT series printheads, electric current is brought to the common bus by FPC straps. But on conventional style flat printheads, all electric current goes around the ends of the heater line to reach the common bus side. The current travels through a relatively thick conductor pattern. If the platen roller is too long, it can rub away this thick conductor pattern. Consequently, there is a limit on the length of the platen roller described in the "operating conditions" section of the printhead specification. The danger is when the pattern is rubbed by paper pressed by the platen force. There is not so much of a problem if the paper overhangs an edge of the platen roller rubber and lightly brushes over the thick conductor pattern.

 Thermal Transfer Printing
Thermal transfer printing uses a thin ribbon as the thermal sensitive media, between the printhead and the receiver paper. Thermal transfer ribbons typically have a lubricating backcoat that allows the ribbon to slide easily over the printhead. This generally extends the abrasion life of printheads, compared to printing on direct thermal paper. Thermal transfer printing introduces a new difficulty called "ribbon wrinkle" that is sensitive to platen pressure, ribbon tension and print energy. Because of the backcoat, the printer can operate outside of the platen force range recommended in the printhead specification, and operation within this extended pressure range may be required to avoid ribbon wrinkle.
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