by Don Crockett
Glass lamination requires carefully controlled combinations of pressure and heat. For windshield and architectural glass applications, the lamination process involves bonding an interlayer of plastic between two lites of glass. The goal is to produce a clear lite of shatterproof safety glass. To do this, the lamination process must form a strong bond and remove air pockets from between the layers. With the special requirements of todays glass lamination industry, autoclaves are often the best type of processing equipment.
Over the years, autoclaves have been used to process a variety of materials. In addition to glass lamination, they are used for bonding composite materials, vulcanizing and curing rubber, laminating printed circuit boards, digesting pulp and paper, dewaxing investment castings, enriching uranium and sterilizing hospital waste and other materials.
Autoclaves are industrial, ASME-coded pressure vessels with quick-opening access doors. They are designed to apply pressure, heat and vacuum within a controlled environment. Unlike laminating presses, autoclaves are able to apply pressure and heat uniformly, regardless of the shape of the glass. This is important when laminating complex shapes, such as wrap-around windshields.
Two factors limit the amount of glass that can be laminated: the size of the batch and the length of the laminating cycle. One benefit that autoclaves have over presses is the ability to laminate large batches of glass quickly. The amount of glass that can be processed in a single lamination cycle is determined by the autoclaves work space, which can be sized to accommodate large batches. Autoclaves provide the rapid heat up and cool down needed for mass production.
Because of the high pressures and temperatures used for processing, autoclaves must be certified to meet American Society of Mechanical Engineers (ASME) Pressure Vessel Code standards. The thickness of the metal used to form the pressure vessel walls and the integrity of welds are important for safe operation.
Door design is a crucial factor because of the continual need to open and close the door, yet maintain an effective seal when the autoclave is pressurized. Smaller autoclaves can be equipped with a simple T-bolt door that is opened and closed manually. But for larger autoclaves, the sheer size and weight of the door make manual operation impractical. Generally, larger autoclaves are equipped with hydraulic systems to open or close and lock or unlock the door. Quick-opening doors typically feature a breech-lock design that operates with a single turn of a locking ring. An airtight seal around the door is initiated by an air-energized gasket. Once a seal has been achieved, it grows tighter as the autoclaves internal pressure increases.
Although standard units are available, an autoclave is generally custom-designed to meet the specifications of an individual process. Some industries require autoclaves with pressure and temperature capabilities of 700 pounds-per-square-inch gauge (psig) and 1,100 degrees F. For glass lamination, pressures of 200 to 250 psig and temperatures of 350 to 450 degrees F are typical.
An autoclave is pressurized by filling it with compressed air or an inert gas such as nitrogen or carbon dioxide. To shorten the lamination cycle, an air pressure accumulator can be used to pressurize an autoclave very quickly. The type of heating system that is specified often depends on what energy source is most readily available at the facility where the autoclave will be used. Options include electric-resistance heat, gas-fired heat, fuel-oil-fired heat and dry heat from steam or hot oil exchangers.
Uniform distribution of pressure and temperature is needed to form a strong bond between the laminated sheets of glass. Because its entire work space is pressurized, an autoclave automatically applies pressure uniformly across the surface of the glass or other material being processed. But for uniform temperature distribution, a fan and ductwork are required to circulate heated air throughout the pressure vessel. How the glass is loaded will determine the design of the autoclaves air circulation system. When glass is stacked vertically, a bottom-to-top airflow is most effective.
Heating and fan systems are usually located at the closed end of an autoclave. If the fan assembly is mounted inside the autoclave, it must be equipped with a cooling coil to protect it from the high temperatures applied during processing. Fan assemblies that are mounted externally require a pressure seal where the drive shaft penetrates the wall of the autoclave.
Autoclave controls range from simple microprocessor-based systems to the sophisticated systems that incorporate programmable logic controllers and personal computers. All are designed to provide precise control of the autoclave functions involved in a lamination cycle: heat-up rate, soak period, cool-down rate, pressurization and depressurization. More sophisticated control systems can be programmed to store a large number of ramp/soak profiles so that different lamination processes can be run without reprogramming the autoclave between each cycle.
Control systems also provide a range of data logging capabilities. By monitoring all thermocouple and pressure vessel operating conditions, computer-based systems can furnish continually updated historical trend displays. This operating data can be stored and used to generate reports to verify the quality of each lamination cycle.
Autoclaves can be supplied with material handling equipment to help load and unload the glass that is being laminated. To accommodate carts and trolleys, specially designed ramps and tracks can be built in. In some cases, conveyor systems have been installed to eliminate manual loading and unloading. To simplify maintenance between cycles, cleanout ports can be included for draining any excess plasticizer that builds up inside the autoclave.
Autoclaves should be inspected regularly, and any modifications need to be tested to make sure that they meet code standards. Because of the high levels of pressure and heat generated inside an autoclave, safe operating procedures should always be followed. Modern autoclaves are equipped with redundant safety systems to prevent them from being accidentally opened while they are still pressurized. Manual and electro-pneumatic lock pins must be in place before pressurization can begin. Once the autoclave has been pressurized, a zero-pressure switch prevents the door from being opened. A warning whistle sounds if there is an attempt to bypass these safety devices and open the door before the autoclave has been fully depressurized.
Don Crockett is sales manager for autoclaves and vacuum dryers, McGill AirPressure Corporation, Westerville, OH.
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