Applying electrical heat to materials under vacuum can be an important manufacturing process to ensure product quality when drying, high-purity processing or when other vacuum-type heating is required.The use of vacuum promotes off-gassing and reduces the boiling point of trapped liquids. Unfortunately, vacuum alone may cause liquid to evacuate from the material or the part to freeze. What remains are ice particles that are not easily evacuated from the system.
Using heat and vacuum in tandem provides improved results compared to either process used separately. Applying heat with vacuum prevents ice formation and decreases the heat needed to sufficiently remove moisture and volatile compounds from parts. Three common applications utilizing the heat/vacuum combination include: vacuum drying, vacuum bakeout and vacuum heating for metal processing. To regulate heat applied in any vacuum application, a PID (proportional-integral-derivative) temperature controller is required to more accurately control the heating process.
Vacuum drying is a low-pressure operation that can be used to remove moisture from a substance. It is utilized when high temperatures may result in hazardous situations or potential damage to the product. The boiling point of liquids is reduced when subjected to vacuum pressure. A combination of vacuum and heat is used to reach the vapor pressure of water. The vacuum system evacuates the gases from the chamber, preventing them from condensing and rewetting the product while cooling. Products that can be processed with vacuum drying include:
Vacuum drying in a vessel requires a heating system with sufficient wattage and insulation to increase temperature from ambient to the set point for off-gassing. Not only does vacuum drying remove trace amounts of moisture, it also allows batch processing to be done while taking less time compared to conventional drying methods.
High or ultra-high vacuum applications, as well as other applications using high-purity parts, necessitate a vacuum bakeout process. Conventional cleaning methods should be used to remove manufacturing contaminants.
Vacuum bakeout elevates the surface cleaning efficiency by eliminating microscopic particles. Failure to remove the impurities will contaminate the product or spoil the vacuum. While water vapor absorbed from the atmosphere is the most common contaminant, oil particles from pumps and dust accumulated during assembly must also be removed. Without careful handling, fingerprints left on surfaces can also be problematic.
As heat and vacuum are applied to the part, particles are extracted from the surface and moved within the vacuum chamber. This movement creates pressure that works against the vacuum. Successive flushing of the chamber evacuates loose particles with vacuum pressures of <10-10 mbar.
Applications requiring high-purity vacuum bakeout include:
The process engineer has three things to consider when designing a vacuum bakeout system:
Ideal bakeouts may require temperatures of 300-400°C (572-752°F). Fiberglass-insulating tapes and high-temperature insulators can be used. During vacuum bakeout, temperature distribution is more critical vs. a drying application. Cold spots on the surface of objects being baked may not expel impurities, resulting in a poor vacuum.
Vacuum heating can be performed by placing products or a vessel within a vacuum oven fabricated from stainless steel, with openings for inserting items to be processed and for the vacuum system. Ovens can be small rectangular chambers or large cylindrical shapes that fit over coils of steel. Openings should be designed to prevent outside air infiltration when the oven is closed. Nondestructive testing and inspections must be used to ensure there are no leaks in the system.
Heating tapes may be an option and should be wrapped carefully around the chamber to contact as much surface area as possible. Silicone surface heaters can be used for application temperatures of 204-232°C (400-450°F). These can include insulating foam up to 1" (2.54 mm) thick with pressure-sensitive adhesive for installation. For higher temperature applications up to 593°C (1,100°F), cloth heating blankets are available. After securing heaters around the chamber, insulation should be applied, sufficient to maintain a touch-self temperature, typically 48-60°C (120-140°F). A durable cover may need to be placed over the insulation to preserve the integrity of the material.
Vacuum heating can be used in many metal processing applications. Processing under vacuum reduces the possibility of impurities during the melting and casting of alloys. Purity is also very important in the manufacturing of powdered metals used for 3D printing. Carburization—a process adding carbon atoms to the top layers of steel for surface hardening—is typically performed at 900-950°C (1,652-1,742°F). It can also be performed at temperatures below 600°C (1,112°F) when under vacuum. Other heat treatments, such as tempering and annealing, allow for heating to lower temperatures and cooling of metals without the risk of impurities or oxygen.
Cloth heating jackets are designed with durable polymer-coated fiberglass cloth and fiber insulation. Resistance wires are sewn to the interior liner, covering more surface area than tapes and providing superior temperature uniformity. The wires are covered with an insulating fiber and retained by a top cloth cover. Samox cloth can be incorporated into the liner, meeting Class 100 environments with exposure temperatures up to 593°C (1,100°F). Fiber insulation between the wires and the outer cover provide a safe and cool-touch surface meeting the SEMI S2 standard. BriskHeat is one of several companies that provides insulators and heating jackets to fit a variety of valves and pumps sold by popular high-vacuum component manufacturers.
Heat and vacuum can be used together to process materials that could not otherwise be accomplished. Vacuum drying can be completed at lower temperatures and in less time, which reduces cost. High and ultra-high vacuum removes even the most microscopic impurities from surfaces. As new metal alloys are developed, and metallurgical processes are required to enhance properties, vacuum heating provides repeatable product quality. No matter what the application, insulation and temperature controllers should be appropriate for the heaters selected. There are a variety of off-the-shelf solutions available; however, custom-designed heaters and insulators can provide superior performance. A qualified heating company can be consulted for application support.
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