Taming Ceramics for Platen Heating

Original Article Appearance: Business News Publishing Co. Process Heating November/December 1994

Originally developed for forming of metals, heated ceramic platens and tools are finding a place for themselves in thermoforming plastics and composites. As the technology for such applications has expanded, ceramics have become more inviting to users.

One of the driving factors has been economics. Often, ceramic tooling can be designed and fabricated much quicker than metal tooling and at a fraction of the cost. Not only are they generally less expensive but in many circumstances, ceramic platens and tools perform more efficiently than their metal counterparts.

The high operational efficiency of heated ceramic tools can be traced, in part, to the fact that ceramics have a much lower thermal mass than metal. Properly designed, they require less energy to bring the forming surface up to temperature and to maintain that temperature. Similarly, ceramic platens and tools cool down faster, making finished-part handling easier and less hazardous. Personnel safety is also a consideration because the non-heated side od a ceramic platen will be cool to the touch, even when heated side is at full temperature.

Designers and manufacturers of heated platens not only realize monetary savings and operational advantages by using ceramics, they have a quicker tooling time because most ceramic tools can be fabricated rapidly. Additionally, in applications demanding very large tools, ceramic tools usually can be fabricated economically. In contrast metal tooling not only would be far more expensive but impractical as well.

As thermoforming requirements of plastics and composites change, ceramics can be considered an appropriate tool material for many demanding applications.

Ceramics as heaters
Although ceramic materials perform well as electrical and thermal insulators, they have a number of properties that make them excellent hot-surface materials. Such properties as ultra low thermal expansion and high compressive strength are important from a structural standpoint. Also high IR transmissivity and low heat capacity improve heating efficiency and uniformity.

When designing platens and tools from metal, designers can place the heating elements relatively far away from the actual heat transfer surface because metal conducts heat rapidly to the surface. Ceramics, however, have a very low thermal conductivity, so heating elements not only must be placed close to the surface but close together. The further away from the surface the elements are placed, the more the insulating properties of the ceramic come into play. Too great a distance not only results in poor heat transfer to the working surface but can cause heater failure. Unless the heater is near the surface, the ceramic material will cause the heat to remain locally, forcing a rise in heater temperature until it reaches a critical point and the heater fails.

Another reason for placing the heaters close to the working surface is to insure a uniform surface thermal profile. Heaters spaced too far apart cause "tiger stripping" in which the temperature profile varies dramatically across the surface and matches the heater pattern. The closer the heater elements are to the surface, the lower the temperature differential, delta T, is between the surface of the tool and the heater. This allows the heater to run cooler, maximizing heater life and allowing for fast heat up and response time.

Radiation Effects
Fused silica ceramics exhibit a phenomena that adds to their suitability as a heater substrate. Comprised of fused quartz particles, the material allows the transmission of heat not only by conduction but also by radiation.

The material's large grain size allows heat to be radiated from the heater to the surface in a manner similar to heat from the sun radiating through a window pane. Though not as efficient as clear glass, it's transmissive property is beneficial to heat transfer and lowers the delta T value. However, the farther the heater is away from the surface, the more the transmissive property is diminished. Denser, or higher alumina-content ceramics, do not display the same high level of transmissivity.

Depending on the specifics of each application, design considerations will change accordingly. Still, there are a number of general guidelines that help achieve successful results.

Heater Considerations
Because ceramic heating elements must be close to the surface and spaced closely together, IHE (INTERNATIONAL HEAT EXCHANGE INC.) recommends using a 0.22" dia, metal-sheathed tubular heater. To accommodate the heater, 0.25" dia holes should be cast into the ceramic plate. The recommended distance from the top of the hole to the surface of the platen or tool is 0.625", with a center-to-center hole spacing of 0.5".

The advantages of the metal-sheathed insertion heater includes ease of insertion and removal as well as ease of wiring. Also, the metal sheathed ensures that there is uniform heat distribution across the heater length.

In applications for curing composites and other materials up to 1400°F (760°C), Incoloy sheathed heaters are acceptable. For higher temperatures, Inconel 600 works well as the sheath material.

The wattage of individual heaters depends on the actual operating temperatures of the process. The higher the operating temperature, the lower the wattage, or watt density, on the heater surface. Up to 900°F (482°C), a watt density of 20W/in² is acceptable. At higher temperatures, a watt density of 11 W/in² is recommended.

The heater voltage rating is another important consideration. Because heaters are relatively small in diameter, the distance between the internal resistance wire and the outer metal sheath is small. This close proximity results in a low dielectric-insulation resistance that decreases even further with increases in temperature.

Therefore, it is a good design practice to use a low voltage power supply whenever possible. At temperatures up to 900°F (482°C), a heater voltage of 240 V is acceptable, but at higher temperatures, it should be reduced to 120 V. At higher temperatures, a 208 V, three-phase power source is commonly used with heaters rated at 120 V and wired in a wye configuration. It is important to allow for multiples of three heaters in each heater circuit to properly balance the load.

To facilitate wiring at one end of the tool or platen, the heaters should be grounded, with the sheath acting as one of the conductors. But because the heaters will be electrically live, care must be taken that they do not come in contact with any metal components of the assembly. In operation they are electrically isolated by the ceramic and pose no problems.

If this design is not suitable, you can electrically isolate the heaters with terminals on each end. Such heater designs typically are used in longer tools and platens , and in specialized applications.

Thermocouple Placement
When holes for the heater elements are being cast into the tool or platen, more holes should be reserved for locating a thermocouple for process temperature control. When the control thermocouple is appropriately positioned in its hole, it will give a very accurate representation of the actual surface temperature because it is approximately the same distance from the neighboring heater elements as it is from the surface. It is highly recommended that this be the only location used to control the process temperature. The designer may wish to locate additional thermocouples in the actual part or in auxiliary tooling, but the added sensors should be used only for recording and/or monitoring temperature and not for control. Ceramic surfaces heat extremely fast, therefore if control thermocouples are placed in the workpiece, heater and tooling surface temperatures overshoot the required process temperature, resulting in defective parts and/or heater failure.

It is recommended that a dual thermocouple be used instead of just a single one. One output should be for primary process control and the second for over-temperature and/or recording functions. For applications up to 1200°F (649°C), a type J thermocouple with either an Incoloy or stainless sheath can be used. Above 1200°F (649°C), a type K or N thermocouple with an Inconel sheath is recommended. Suitable high temperature thermocouple wire should be used for the terminations and the thermocouples should be ungrounded.

Temperature Control
It is highly recommended that an SCR or thyristor power controller be used with these heaters. The precision control provided by these devices insures maximum heater life. For large tools and platens, it is necessary to divide the heated surface into separate zones of control to achieve the required temperature uniformity. By grouping appropriate heaters electrically, you create zones that can be controlled separately. It is necessary to provide a separate control thermocouple for each group or heater circuit. Ideally each thermocouple should be located in the center of the heater group that forms a zone. A separate temperature controller and power controller circuit will be required for each zone.

For complex tooling, particularly where the part thickness varies over the platen area, take care in establishing zones. Properly controlling zone temperatures can be an effective way to ensure temperature uniformity across radical contours and varying masses.

Care and maintenance
On initial heat-up of the tooling or platens, a slow, controlled heat-up is recommended to allow any moisture that has been absorbed by the ceramic and/or heaters to bake out. Initially, operating heaters at half their rated voltage for two to three hours is a good way to ensure that the heaters will be dry and have maximum dielectric strength.

It is also recommended that the heaters be placed in a walk-in or portable oven and baked at 300°F to 400°F (149°C to 204°C) overnight, particularly if they have been stored for extended periods of time or if they have been subjected to high humidity environments. The internal magnesium oxide insulation is hygroscopic and will absorb moisture from the environment.

Excessive temperature should be avoided as both the ceramic and the heaters are subject to failure. It is recommended that a maximum of 1875°F (1024°C) surface temperature on the tool or platen be rigidly observed. This will help ensure maximum life of both the ceramic and the heaters.

Steve Grant: President of IHE (International Heat Exchange)

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