Extrusion Dies for Plastics and Rubber

Frontmatter

1. Introduction

Abstract

In the extrusion of thermoplastics into semi-finished products two units occupy a central position: These are the extrusion die – also known as the extruder head – which shapes the melt, and the former – usually mounted adjacent to the extrusion die –, whose function is to guide the molten semi-finished product whilst maintaining the desired dimensions, and providing the specified degree of cooling (Figs. 1.1 and 1.2).

Walter Michaeli

2. Properties of Polymeric Melts

Abstract

When we choose a theoretical description of the process correlations in the extrusion die and calibration unit for a reliable design of those systems, there are two things in particular to be considered:

• simplifications and boundary conditions based on the physical models always have to be analyzed critically with regard to the problem at hand.
• data pertaining to the processed material and which are being entered into the models become of key importance. These are data which characterize flow, deformation and relaxation behaviors and heat transfer; in other words, its rheological and thermodynamic data [1].

Walter Michaeli

3. Fundamental Equations for Simple Flows

Abstract

In this chapter, simple fundamental equations for the flow in channels with circular, rectangular and annular cross-sections will be derived. These allow in many cases at least an estimate of such factors as pressure consumption or throughput.

Walter Michaeli

4. Computation of Velocity and Temperature Distributions in Extrusion Dies

Abstract

The basis of the general mathematical treatment of flow processes are the balance equations for mass, momentum and energy. The flow can be fully described only when the velocity vector and the thermodynamic data as pressure, density and temperature are known at any time and at any point of the area of the flow.

Walter Michaeli

5. Monoextrusion Dies for Thermoplastics

Abstract

After having developed the fundamental theoretical considerations for the design of flow channels of extrusion dies in the preceding chapters, now in Chapter 5 the focus will be on types of design, applications and the layout of dies for the discharge of a single melt. These will be referred to as monoextrusion dies in contrast to coextrusion dies (see Chapter 6). In discussing this subject, the extrusion dies will be divided into groups according to the geometry of their exit cross section since as a rule dies with similar exit cross sections are practically equal in the way they are constructed, regardless of the extruded product.

Walter Michaeli

6. Coextrusion Dies for Thermoplastics

Abstract

Many products made from polymers cannot meet the requirements imposed on them when produced from a single material. In such cases the solution may be to combine different materials in a multilayer structure and, thus, create a new product that combines the positive properties of the individual components.

Walter Michaeli

7. Extrusion Dies for Elastomers

Abstract

Because of the diversity of products made from this class of polymers, many of the dies common in the processing of thermoplastics are found again in the extrusion of materials based on elastomers (rubber). The dies for thermoplastics were discussed in principle in the previous chapters. A major difference in the processing of elastomeric materials is the mass flow supplied by the extruder must be within the temperature range which allows a safe processing, i.e where no premature onset of vulcanization (so-called scorch) occurs in the extruder or in the die assembly [1].

Walter Michaeli

8. Heating of Extrusion Dies

Abstract

The distribution of temperature in an extrusion die critically affects the viscous and elastic properties of the straining melt, thereby, the flow pattern, the pressure loss inside the die as well as the elastic properties determining the swelling of the extrudate at the exit from the orifice.

Walter Michaeli

9. Mechanical Design of Extrusion Dies

Abstract

The mechanical design of extrusion dies, i.e. the calculation of the forces and deformations arising during the operation of the die, is important for two reasons: First, to assure that the die will not be damaged during the operations; second, to see that the distribution channel, particularly in distributor (manifold) dies, retains the geometry established by the rheological design also during its operation.

Walter Michaeli

10. Handling, Cleaning and Maintaining Extrusion Dies

Abstract

As a rule, extrusion dies are expensive, high-precision parts of the extrusion line that require, besides well thought-out, careful handling,. a frequently underestimated amount of care. To avoid malfunctions of the die that can result in periods of unplanned and costly stoppage [1, 2], it is necessary to operate the dies properly and to take preventive measures so that they are ready for use for as much of the time as possible.

Walter Michaeli

11. Calibration of Pipes and Profiles

Abstract

For the extrusion of profiles, pipes and similar semi-finished products, the calibration and cooling sections are connected to the die that has shaped the melt (Fig. 11.1). The goal is to solidify the melt by the contact with the calibration (sizing) die in the calibration section to a thickness sufficient for transferring the take-off forces and still maintaining the desired dimensions. The mean temperature T of the profile should drop below the solidification temperature (melting point) T_E as it passes through the cooling section, so that the remelting of the already solidified layers is avoided. The whole profile, as it reaches the saw or the flying knife, should have a temperature below the melting point [1]. The calibration and cooling equipment, therefore, has the task of fixing the dimensions of the extrudate and, consequently, it represents an integral part of an extrusion die.

Walter Michaeli

Backmatter