Calendering is a special process for producing high-performance, high-quality plastic films and sheets, mainly used for PVC and certain other modified thermoplastics.
How does it work?
The concept of a calender is fairly easy to understand. The basic idea of the machine is to collect heat-softened polymer between two or more rollers (this area is called the nip) to form a continuous sheet. To begin the process, the polymer needs to be mixed and fluxed before passing through the calender. Blending is a process that creates the desired polymer and flux and makes this blended polymer easier to handle on the calender. The polymer is then ready to pass through the calender and its thickness depends mainly on the gap between the last two rolls. ^The latter set of rollers also determines the surface finish; for example, they can affect the gloss and texture of the surface. One characteristic of calendered polymer is that the sheet passing through the rollers tends to follow the two faster-moving rollers in contact with it, and it also adheres more to the hotter roller. This is why calenders usually end up with smaller rollers at higher speeds to peel off the sheet. This is also why the middle roller is usually kept cooler so that the paper does not stick to the other rollers and split by sticking to both rollers. This splitting phenomenon forces calender operators to require a high friction ratio between the two rolls, ranging from 5/1 to 20/1.
advantage
High-quality plastic sheets are produced by calenders; in fact, the process that can compete with calenders in calender molding is extrusion molding. The calender is also very good at handling heat-sensitive polymers because it causes very little thermal degradation.
Another advantage of calendering is that it mixes well polymers containing large amounts of solid additives that do not mix or melt well. This is correct because the melting rate produced by a calender is fast compared to extrusion because the amount of mechanical energy is increased. Because the company is able to add more filler products to their plastics and save on raw materials. Calenders are very versatile machines, which means it is easy to change settings such as the size of the roll gap.
shortcoming
Although the calendering process produces a better product than the extrusion process, there are some disadvantages. One drawback is that the process is more expensive to perform, which is a major deterrent for many companies. The calendering process is also not very good at a pressure gauge that is too high or a pressure gauge that is too low. If the thickness is less than 0.006 inches, pinholes and voids will appear in the sheet. If the thickness is greater than about 0.06 inches, there is a risk of air entrapment in the sheet. Any desired thickness within this range will be better, albeit using a calendering process.
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