Polymer calendering

Introduction

A calender is a device used to process polymers into a more useful item. It’s a device that has been in use for over a hundred years and when first developed it was mainly used for processing rubber; however, nowadays it is used mainly for producing ^W sheets, coatings and films ^[1]. The calender never did become very popular when it was mainly used for rubber. There was a lack in the ability to be accurate in desired sheet thickness and and it was difficult to customize certain features like adjusting the desired gap between rollers. The process did not start to become popular until the 1930's^[2]. Calenders now achieve tolerances around ${\displaystyle \pm }$0.005mm if there is good control over the calender characteristics (ie. roller temperature, roller speed, etc.)^[2].

How it works

The calender concept is fairly easy to understand. The basic idea of the machine is that squishes a heat softened polymer between two or more rollers to form a continuous sheet. Before the material can be calendered it must go through blending and fluxing. In blending the polymer is created to the desired material properties. In fluxing the polymer is worked and heated until it is at an appropriate consistency to go through the calender^[3]. The thickness of the resulting sheet is dependent on the gap between the last pair of rollers. These rollers also dictate what the surface will look like, whether it’s glossy, textured , etc.^[1]. What has been observed with claenders is that the plastics tend to follow faster moving roller of the two that it's in contact with and it sticks more to the hotter rolls. That is why calenders typically end with a smaller roller at a higher speed to peel the sheet off. It is also why the middle roller is normally kept cooler so that the sheet won't stick to the other rollers nor will it split by sticking to both rollers which has been seen to happen^[4]. This splitting phenomenon makes a high friction ratio, between 5/1 and up to 20/1, a necessity^[4].

Uses^[3]

• floor tile
• continuous flooring
• rainwear
• shower curtains
• table covers
• pressure-sensitive tape
• automotive and furniture upholstery
• wall coverings
• luminous ceilings
• signs and displays
• etc.

Advantages

The only process that competes with the calender in sheet forming is ^W and the biggest advantage of calendering is that it will produce a sheet better-quality sheet film or coating to anything that extruding could produce. The calender also due is very good at handling polymers that are heat sensitive as it causes very little ^W, which is why it is commonly used for polymers like PVC^[2]. Another advantage to calendering is that it is good at mixing polymers that contain a high amount of solid additives. This is true because compared to extrusion the calender produces a large rate of melt for the amount of mechanical energy that is put in^[5]. This also means that because it mixes so efficiently that more filler can be added than would be with an extruder. Another advantage of the calender is that it is easy to change specifications like sheet thickness.

Disadvantages

Although the calendering process produces a better product than the extruding process there are a couple of disadvantages. One disadvantage is that the process is more expensive to perform which would be a major deterrent for many companies. The calendering process also is not as good at too high of gages or too low of gages. If the thickness is below 0.006 inches then there is a tendency for pinholes and voids to appear in the sheets^[4]. If the thickness is greater than about 0.06 inches though there is a risk of air entrapment in the sheet^[6]. Any desired thickness within that range though would turn out much better using a calender process.

Material Specifications

The best polymers for calendering are thermoplastics. This is because they soften at a temperature much lower than their melting temperature. This allows for a wide range of roller temperatures. They also adhere well to the rollers, allowing them to continue through the chain well, while not becoming stuck to the rollers. The last characteristic involves the polymer having a fairly low viscosity, but still being strong enough to hold itself together fairly well. Materias are also calendered if the are heat sensitive and could easily be subject to ^W and the melt must have a shear and thermal history that is constant across the width of the sheet^[7].

Types

There are 2 main types of calender: the L type and Z type

L Type

Z Type

The z type calender places each pair of rollers at right angles to the next pair in the chain. This means that the forces that are on each roller individually will not effect any other roller^[7].

Physics of Calendering

Fluid Mechanics

Using a newtonian analysis the process can be modelled fairly well. Certain assumptions had to be made to develop these equations though^[7]:

1. The flow is symmetrical between the two rollers
2. The flow is at steady state and is laminar
3. Incompressible fluid
4. There is no slip between the fluid and the rollers
5. The radius of the roller is much larger than the gap between the rollers that it can be assumed the flow is occuring between parallel plates.

The velocity of the fluid/melt against the rollers^[7]:

Where R is the radius of the rollers and ${\displaystyle \omega }$ is the angular velocity of the rollers in rad s^-1.

The velocity can also be found anywhere inbetween the rollers using the next equation^[7]:

Where is half distance between the two rollers x distance away, dP/dx is the pressure gradient, and y is the distance from halfway between the rollers that the velocity is being calculated for, and ${\displaystyle \eta }$ is the viscosity.

The volumetric flow can be modelled with^[7]:

Where w is the width of the sheet being produced

The maximumum pressure can be found with^[7]:

Where h₀ is half the distance between the rollers at when they are closest together and ${\displaystyle \lambda }$ is the pressure in between the rollers (so in the fluid) at a distance from where the rollers are closest together to where the melt stops touching the roller it leaves, but in the other direction.

This next equation is for the force, caused by the fluid, that acts to seperate the two rolls^[7]:

Where p is the pressure in between the rollers

Temperature effects

When looking at the thermal distribution in the fluid between the two rollers it is apparent that the temperature of it is highest at the rollers. This happens for two reasons i) the shear is highest at the sides in laminar flow and therefore friction and heat is also highest there and ii) the heat is added to the system through the rollers, and the fluid doesn't conduct it very well^[5]. The effects of this tend to grow in magnitude even more the more viscous the fluid is. If one were to raise the rolling temperature there would be changes in the above fluid mechanocs. It would decrease the viscosity and also decrease the pressure in the fluid. It would also lower the chances of a fracture in the fluid and mae the surface finish better, but this all comes at the price of making the increasing the chances of ^W[7].

Output Efficiency

The calender is able to produce the polymer sheeting at a fast rate. It will produce it at a rate between 0.1 - 2 m s^{^-1[2]}. Increasing the speed though has negative effects or the process would produce sheeting even quicker. By increasing the speed the heat has even less time to spread throughout the fluid from the rollers causing an even greater temperature variation. It also increases the pressure between the rollers which would mean the seperational forces between the rollers would also increase. It also causes an increase in shear forces in the fluid at the rollers, which increases the chances of surface defects like fractures^[7]. There are acceptable limits obviously it just depends on material, temperature, thickness of desired sheet, etc. to determine what those limits are.

References

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