For professionals in the rubber and plastics industries, calenders are extremely familiar equipment. They are indispensable for processes ranging from rubber sheeting and fabric coating to the production of PVC films and artificial leather. Calenders are heavy-duty, high-precision machines that demand exceptional accuracy and efficiency. So, what is the connection between this equipment and rotary joints? It is a crucial one.
Why are rotary joints essential for calenders?
During operation, calender rolls require heating or cooling to maintain precise temperatures. Consider this: whether producing rubber sheets or plastic films, poor temperature control results in uneven thickness and imperfect surface finishes-essentially turning the output into scrap. The challenge lies in the fact that the rolls rotate at high speeds, while the pipelines supplying the heat transfer medium remain stationary. How can heat transfer oil, steam, or hot water be conveyed from a fixed pipe into a rotating roll? This is where the rotary joint comes into play.
The rotary joint acts as the "link" connecting the stationary pipeline to the rotating roll. Without it, the heat transfer medium cannot enter, temperature control becomes impossible, and the production line must shut down.
What are the specific requirements for rotary joints used in calenders?
The operating conditions for calenders can be extremely demanding. Take plastic calendering, for instance: rigid PVC processing requires temperatures approaching 230°C. When using heat transfer oil, temperatures can easily exceed 250°C or even 280°C, with pressures reaching 1.8 to 3 MPa. Rubber calendering is relatively less intense; roll surface temperatures typically range from 90°C to 120°C, and steam or pressurized hot water suffices for heating.
Consequently, different operating conditions call for different rotary joint "specifications."
Calenders using heat transfer oil operate under high-temperature conditions and require rotary joints capable of withstanding temperatures above 280°C. Sealing components must utilize antimony-impregnated graphite, and some designs incorporate a spherical-seat seal; this design offers sealing rings with strength several times that of standard graphite, ensuring greater resistance to breakage and a longer service life.
For calenders using steam or hot water, operating temperatures are lower, allowing for a relatively simpler structural design-though sealing integrity remains just as critical. For instance, rotary joints featuring single or double spherical sealing structures rely on springs to press the graphite sealing ring against the mating surface; this design allows for automatic compensation for wear, thereby ensuring a leak-free seal. It is important to note that because calendering machine rollers expand and elongate when heated, a rigid connection between the rotary joint and the piping is not feasible; instead, a short section of flexible metal hose must be used as an intermediary. Without this, the thermal expansion and contraction would place excessive stress on the joint, quickly rendering it unusable.
It is also worth mentioning a specialized application for calenders: the production of photovoltaic (PV) glass. The molten glass reaches temperatures as high as 1,100°C, and since the calendering rollers come into direct contact with the melt, their surface temperatures can hit approximately 700°C. In this scenario, the rotary joint is not used for heating but rather to circulate cooling water, thereby lowering the roller temperature and preventing thermal deformation.
Calling it the "throat" of the temperature control system is no exaggeration.
Whether circulating high-temperature thermal oil or cooling water, the rotary joint acts as an "energy transfer hub" for the calender, facilitating the input or extraction of heat. If the unit leaks or seizes, the consequences range from product scrapping to a complete production line shutdown. Thus, describing it as the "throat" of the calender's temperature control system is entirely accurate.