Page 332 - Plastics Engineering

Processing of Plastics 315

Calendering can achieve surprising accuracy on the thickness of a sheet.
Typically the tolerance is f0.005 mm but to achieve this it is essential to have
very close control over roll temperatures, speeds and proximity. In addition,
the dimensions of the rolls must be very precise. The production of the rolls
is akin to the manufacture of an injection moulding tool in the sense that very
high machining skills are required. The particular features of a calender roll
are a uniform specified surface finish, minimal eccentricity and a special barrel
profile (‘crown’) to compensate for roll deflection under the very high presurres
developed between the rolls.
Since calendering is a method of producing sheedfilm it must be consid-
ered to be in direct competition with extrusion based processes. In general,
film blowing and die extrusion methods are preferred for materials such as
polyethylene, polypropylene and polystyrene but calendering has the major
advantage of causing very little thermal degradation and so it is widely used
for heat sensitive materials such as PVC.

4.5.1 Analysis of Calendering
A detailed analysis of the flow of molten plastic between two rotating rolls
is very complex but fortunately sufficient accuracy for many purposes can
be achieved by using a simple Newtonian model. The assumptions made
are that
(a) the flow is steady and laminar
(b) the flow is isothermal
(c) the fluid is incompressible
(d) there is no slip between the fluid and the rolls.
If the clearance between the rolls is small in relation to their radius then at
any section x the problem may be analysed as the flow between parallel plates
at a distance h apart. The velocity profile at any section is thus made up of a
drag flow component and a pressure flow component.
For a fluid between two parallel plates, each moving at a velocity Vd, the
drag flow velocity is equal to Vd. In the case of a calender with rolls of radius,
R, rotating at a speed, N, the drag velocity will thus be given by 2nRN.
The velocity component due to pressure flow between two parallel plates
has already been determined in Section 4.2.3(b).

1 dP
v - - -(y2 - (h/212>
- 2~ dx
Therefore the total velocity at any section is given by