Many industrial processes - laminating, vulcanizing, extruding, curing, bonding and isothermal or superplastic forming - require the use of heated metallic platens and dies. For these applications, there are basic design criteria essential for efficient and effective results in processes ranging from 100 to 2,200°F (36 to 1,204°C). The main areas of concern in designing a heated tool or platen are platen/tool material; BTU requirements; temperature uniformity requirements; type and location of heaters; and method of temperature control.


Selecting Platen/Tool Materials
In designing metal components for thermal processing, it is important to consider structural, thermal and material compatibility. Over-designing, which is common, is not to be faulted, except in cases in which it leads to excessive cost with no return on investment.


Structurally, the compressive, tensile and shear strengths of the material should be evaluated for the application. Also, the material must offer acceptable thermal conductivity while withstanding the effects of the process temperature. It must be compatible with the process: It must resist oxidation at the desired processing temperature and avoid unwanted chemical reactions. If the process is under vacuum, the vacuum's effect must be determined on all materials being heated. For example, in high chrome bake out (chromizing) to occur, which can have serious consequences.


When choosing tooling material, consider the differentials in the coefficient of expansion of the material and platen. If dissimilar materials are used, the design should ensure that expansion differentials do not cause problems.


Determining BTU Requirements
When designing the heating system for platens and dies, calculate the total BTU or kilowatt requirement. This figure is determined by the weight of all heated components (including the material being processed), the specific heat of each component, maximum process temperature and heat up time required, and heat losses through convection, radiation and conduction.


The following equation can be used to calculate basic kilowatt requirements:
kW= Weight of Material (lb) x Specific Heat x ∆T

÷

3,412 BTU/kW/hr x No. of Hours to Heat

For example, to heat a 1,000 lb steel die with a specific heat of 0.12 BTU/lb/°F from ambient 60°F (15.6°C) to 900°F (482°C) in 1 hr would require 100,800 BTU, or 29.55 kW. For a 2 hr heat up time, 14.77 kW would be needed.


Once basic heat requirements are determined, heat loss factors must be added. Heat losses depend on many factors, including insulation thickness and K factor, exposed surface areas, heat removed by the material being processed, ambient conditions and emissivity of exposed surface areas. Total heat losses can be calculated and added to the basic heat requirement, but it is always recommended to include a safety factor (5 to 10% is common) to ensure against any unforeseen conditions. It is much easier to limit kilowatt output via temperature control than to add kilowatts if the designed rating is not sufficient.


Assuming reasonable insulation factors and operating conditions, heat loss can be estimated without having to run through numerous calculations. From 100°F to 1,200°F (38°C to 649°C), it is usually safe to add a 20% factor onto the basic kilowatt requirement to accommodate heat losses. However, this is a general guideline only. When excessive heat losses are anticipated, it is wise to run through full heat loss calculations, keeping in mind the basic modes of heat loss: convection, conduction and radiation.


Evaluating Temperature Uniformity Needs
Processes vary widely in their temperature uniformity requirements. Some materials can be processed under a wide temperature range while others require tighter tolerances. Temperature uniformity is influenced by two factors: the heat loss factor, in which heat is disproportionately lost from the die or platen, and nonuniform heating, which is due to improper placement and temperature control of the heaters or heat source.


As a rule, uniform masses can be uniformly heated by using equally spaced heaters of the same wattage controlled together as one zone. Half-spacing of the heaters on the edges helps offset losses that occur naturally due to the exposed surface area.


For large or nonuniform masses, temperature uniformity can be difficult to achieve. For large platens which tend to be of uniform thickness, a high degree of temperature uniformity can be achieved by sectioning the platen into separate zones of control. For example, a 60" x 60" platen can be divided into nine 20" x 20" zones (three zones front to back, three zones left to right). Each zone is controlled separately by a dedicated temperature controller receiving a signal from a thermocouple located near each zones center. In this case, nine thermocouples and nine temperature controllers are needed.


For dies that vary in thickness due to contours inherent in the design, the differences in thermal mass must be accommodated when specifying the type and location of heaters and the method of temperature control. Most female dies have greater mass toward the edges while male dies are designed with greatest mass at the center.


The key to successfully heating such dies is to analyze their cross-sections in terms of mass, isolate approximately equal mass areas and break these into separately controlled zones left to right across a die and several zones front to back as well, depending on die size and complexity.



Balancing Mass with BTUs
Zoning the heater according to thickness variations is critical because thickness variations dictate different BTU requirements. If heaters are spaced uniformly on the die with varying mass and heater output is controlled from a thermocouple located from the highest mass area, this area reaches the process temperature more slowly than smaller mass areas. When smaller mass areas reach process temperature, the large mass area would still be calling for more heat, driving lower mass areas to a temperature that exceeds setpoint. By zoning according to thermal mass, all zones achieve and maintain process setpoint individually without affecting other areas in the die.

For larger platens and dies, it also is important to consider insulation factors. Insulation assists in achieving better temperature uniformity, reducing process energy requirements and lowering operating costs. Insulation type and thickness depend on operating temperatures and other operation factors, including K values, vibration, space constraints, contamination, pressure and mounting methods.


Most platens and dies are contained within hydraulic presses, and insulating against conductive heat losses entails placing insulation between the platen or die and the press bolster. Depending on process temperature, insulation thickness can vary from 1" to 10" or more. Insulating around the edges of the platens or dies and the press's daylight opening reduces radiant and convective heat losses. Rigid heat shields consisting of reinforced insulating plates are used around the daylight openings for high temperature applications while fiber insulating curtains usually are sufficient for lower temperature processes. While the press is open, radiant heat losses impact the platen or die temperature uniformity. The higher the process temperature, the greater the potential radiant heat losses per square inch of exposed area. The areas of the platen or die most affected by radiant heat losses, in order of magnitude, are the edges, sides, and center.


While small platens and dies can recover relatively quickly from radiant heat losses, larger masses require more time. By zoning, the mass recovers to process temperature more quickly because the outer (edge) zones apply heat to compensate for radiant losses without driving the inner zones over the process temperature.


Overall, if uniform temperature requirements exist, the zoning approach should be considered for uniform masses over 24 in² and nonuniform masses on a case-by-case basis.



<>b>Choosing the Heater
When selecting the heater, consider heater type, watt density rating and applied voltage. In determining the voltage, it is imperative to understand the dynamics of the heater's insulation resistance and how it is affected by the process temperature.


For example, in metal-sheathed heaters, the internal resistance wire is electrically insulated from the sheath by a nonelectrically conductive insulating material with good thermal conductive properties. This allows the wire to efficiently transfer heat to the external sheath while preventing it from shorting to ground. Typically, magnesium oxide (MgO) insulation is used.

A heater can operate at a given voltage if the dielectric insulating material can provide sufficient protection, preventing the internal resistance wire from shorting to ground. Although fairly tight compacted in the manufacturing process, MgO insulation still will have small air spaces between the particles that aid in the insulation value. When the heater is energized, this internal atmosphere is expanded and reduced in volume, reducing its insulating value. The higher the temperature, the lower the volume and, therefore, the lower the insulating value. For this reason, nonsealed high voltage heaters exposed and energized under a vacuum will arc and short to ground. As a rule, the higher the process temperature, the lower the voltage should be. For applications below 1,200°F (649°C), 240 V to ground is acceptable.


When specifying heater wattage, remember that a heater will last only if it can dissipate heat as fast or faster than it generates it. An over-designed heater in terms of wattage will continue to heat to a critical level and will fail.

Thermal conductivity of the platen and die materials also dictates how quickly heat is dissipated from the heaters. Aluminum, for example, will conduct heat more readily than stainless steel. As temperature increases, the material's ability to transmit heat is reduced: therefore, designing heaters with the end process temperature in mind is essential.


Heater manufacturers have published data on the maximum allowable watt density, or watts per square inch of heater surface area. This data is based on empirical studies of different materials' transmissive qualities at various temperatures, and these guidelines should be followed. As with voltage, the rule is the higher the temperature, the lower the watt density rating of the heater.


For applications up to 400°F (204°C), 40 to 50 W/in² is acceptable in most metallic platens: from 400˚F to 800°F (204˚C to 427°C), 30 to 40 W/in² is acceptable: and from 800°F to 1,200°F (427°C to 649°C) 30 W/in² max. is recommended. These figures are guidelines: Specify each application according to its unique characteristics.


Several heater types are suited for use in metal platens and dies. Cartridge heaters work will for smaller platens and dies with process temperatures below 1,400 °F (760°C), and they can be shaded to distribute wattage over the heater's length to offset losses. Designed for a relatively tight fit in gun-drilled holes, it may be difficult to remove them if the heater fails.


Tubular heaters work in the same temperature range as cartridge heaters but offer a little better dielectric strength because the internal resistance wire typically is farther away from the heater sheath. Designed with one power terminal on each end, tubular heaters require wiring connections at both ends of the platen or die. They usually require a tight fit in drilled heater holes, but some applications require them to be centerless ground after manufacturing to ensure easy insertion. Due to the tight fit, tubular heaters can and do seize up, but this is more common in larger applications.


Split-sheath tubular heaters are an option when ease of insertion and removal is required but zoning is unnecessary. Constructed from a long tubular heater flattened on one side, compacted and bent back on itself, both terminals on this heater are at the same end. When inserted into an over sized hole and energized, the heater expands against the hole wall, forcing direct contact and providing good conductive transfer of heat. When power is disconnected, the heater cools and contracts, facilitating removal. With proper design, they can be used in applications up to 1,700°F (927°C).



Heaters for zoning
When the application calls for close temperature uniformity, loose-fitting heaters and high temperature capability, then consider zoned insertion heaters. Outwardly they resemble cartridge heaters, but internally they are very different. Four or six small diameter tubular heaters are inserted into an alloy tube, and this assembly is compacted by swedging into a finished heater. They can be designed with up to three independently controlled zones along their length by varying the internal wiring connections.


Instead of leadwires, zoned insertion heaters can have as many as six (one set for each heated zone), yielding good temperature uniformity. Unlike tubular and cartridge heaters that rely on conduction, zoned insertion heaters are designed to fit into over sized holes, transferring heat primarily through radiation to the hole wall. The heater hole typically is 0.125 to 0.1875" larger than the heater to ease insertion and removal.


Emissivity of the heater hole walls is important for efficient heat transfer. In essence, the heater is a radiating source, and heater hole walls should absorb this energy at a rate equal to the rate at which it is radiated. In highly reflective materials such as aluminum, loose-fitting heaters radiate to a bright body, which reflects this energy back to the heater, causing it to run increasingly hotter. Consequently, heater holes should be conditioned to a good black-body finish if loose-fitting heaters are used. With most metallic platens, this is achieved through the natural heating process. providing temperatures are sufficient to reach the metal's oxidation point: The presence of oxygen and temperature slowly oxidizes the heater holes. Injecting air into the heater holes while at temperature will accelerate the oxidation process. Black body coatings also can be used. For example, shiny aluminum holes can be treated with a black high temperature paint suitable for applications to 1,200°F (649°C).


In heated die applications, casting heater holes into the die or tool is a good approach. The casting process leaves the holes with a rough finish that increases the amount of surface area for the heater to radiate to, increasing heat transfer efficiency.


For the majority of platen and die applications, cartridge, tubular, split-sheathed tubular or zoned insertion heaters are the preferred choice. However, other types of heaters -- strip, ceramic band, ceramic fiber, silicone rubber and radiant heaters -- also can be used. These nonintrusive heating methods can be effective in situations where there are structural considerations or for heating small masses where it would be impractical to drill and insert heaters. Strip and silicone heaters mount easily but usually are limited to 600°F (316°C) max. Ceramic band and ceramic fiber heaters clamp around the part being heated and transfer heat through radiation and conduction. Ceramic-insulated band heaters can be used in applications up to 1,400°F (760°C) while ceramic fiber are suitable for applications up to 2,200°F (1,204°C). Radiant heaters are suitable when there is no room to mount heaters or where it is impractical to insert heaters. Depending on the type of radiant heater, process temperatures up to 2,000°F (1,093°C) can be achieved.



Locating, Installing Heating Elements

Heating element layout and the method of mounting or securing are critical. The main objective when determining heater placement is to ensure that the heat generated is uniformly dispersed through the mass of the platen or die.


For thin metallic platens operating at temperatures up to 400°F (204°C), metal or silicone strip heaters can be used. They easily mount to the base of the platen and, when controlled properly, provide uniform heating.

For higher temperature platens and dies, insertion heaters are the heater choice. They usually are placed along the centerline of the platen's thickness dimension with at least 0.75" space between the top and bottom of the heater hole and the platen surfaces. This space contributes to a uniform heat flux distribution to the platen surface and ensures the structural integrity of the platen. Usually, a single row of heaters -- spaced along the platen width on 2 to 2.5" centers -- is sufficient. Fro very thick platens or dies, several staggered rows of heaters help ensure that heat from one row does not overheat another row and that all heat generated is uniformly dispersed to the surface.



Managing Temperature Control
A closed loop control system will ensure good temperature uniformity and accuracy while maximizing heater life. Using current-proportioning temperature controllers and SCR power controllers is recommended.


Thermocouples must be carefully placed, especially in zoned platens and dies. To accurately control each zone, thermocouples should extend into the center of the zone. For thinner,uniform platens and dies, place thermocouples close to the work surface between the heaters and the surface. For thicker platens and dies, particularly where the mass is not uniform, place the thermocouples closer to the heaters to prevent temperature over shooting.


For thick platens and dies and applications in which dies are heated between platens, thermocouples can be inserted close to the work surface. In this case, the thermocouples should be used only to monitor surface temperature, not for control. Allow the process to soak up to the proper temperature to avoid temperature overshoot and excessive stress on the heaters.


Steve Grant: President of IHE (International Heat Exchange)

Using Heaters Effectively in Metallic Platens and Dies

Original Article Appearance: Business News Publishing Co. Process Heating March /1996

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