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After 50 years of experience in plastics processing, designing, and manufacturing of tools and equipment, in the mid-1990s I got involved in the development of a business for the design and manufacturing of plastics extrusion dies. To develop the business, I needed to convince my customers, with a good reason, that my approach was different to that of anybody else. With this in mind, I searched a lot of literature. I found that there is a lot about the theory of the flow characteristics of molten plastics through the die channels, but there are not any solved examples of the die design. Some of the formulae and equations given in the literature are so complex that most of the practical engineers would find it difficult to solve these equations.

In the computation of the melt flow characteristics through the die channel, there are several influencing factors, such as the rheology data and the thermodynamic properties of the materials involved. Coupled with them are the die geometry and the processing conditions, such as the output requirements and the temperature variations in the melt as well as in the die. The derivatives of these are the pressure loss through the die channel, the velocity profile, the shear rate, the shear stress, etc. Commercial available computer software takes all the hard work of computation from the die design engineers. With the use of such software, die designers can design a die that is fit for its purpose, in terms of materials being used and outputs required for the given sizes of the products.

In this book, I have used the computer simulation software named "Virtual Extrusion Laboratory™, Polymer Processing Simulation Software" by Compuplast International, Inc. Most of the dies given in the book are based on the spiral type of distributor and the reason for using such distributor is also explained in detail. All the dies illustrated in the book have been simulated with the aforesaid software, and the results of the simulations are also given as a guide for the design of other similar dies.

The range of dies covered in the book is monolayer and multilayer tubes and pipes from Ø1 mm to Ø315 mm. Larger or smaller sizes can be designed on the same principles. In the last chapter, the choice of commonly used steels for the manufacture of dies is also discussed. Most of the solved examples of the dies given in this book are used in the manufacturing of specific products and are well proven. The above-mentioned software has been used as a tool for the simulation of the melt flow variables in the dies.

This book has been written as a practical guide for engineers and designers associated with the extrusion processes of polymers for the manufacture of plastic tubes and pipes. As the calculations of the melt flow through the dies are very complex and time consuming, the use of simulation software is highly recommended.

Finally, I thank Dr. Mark Smith and Dr. Julia Diaz-Luque of Hanser for their help and support in the publication of this book.

Sushil Kainth, MBA, B.Sc., CEng, MIET.

While studying part time at Gosta Green College of Advanced Technology (now Aston University), Birmingham, UK, Sushil Kainth was working full time in industry. By the time he qualified as a B.Sc. in Production Engineering in 1967, he had five years’ experience as a Production Engineer (Injection Molding) in the plastics division of Joseph Lucas Industries in Birmingham. In 1973, he set up his own plastics extrusion company in India for the manufacture of plastic pipes for water, for industrial and domestic use.

In India there were many problems for the industry, such as short supply of plastics polymers, and shortage of electricity for the industry, coupled with bribery and corruption at every walk of life; he decided to dispose of the operation and return to the UK in 1978. After working as a project engineer for a short time, he was appointed Technical Manager of a large plastics injection molding operation. Here he developed the process for manufacturing kitchen sinks from the material known as Asterite. This process was started based on 0.5% of the UK market, and during the 1980s it grew to 55% of the market.

In 1989, he set up his own company to manufacture high-quality cultured marble products and this business was eventually taken over by a large international company, which appointed him Technical Director of three companies. In 1995, he started the business of plastic extrusion die manufacturing in partnership with another well-established machine shop. Since then, he has designed and manufactured over 500 dies and tools for industries varying from medical tubing to water, gas, and petroleum pipes. Coextrusion dies have been his specialty for the last 20 years.

His approach to the design and manufacture has been to solve production problems at the design stage using computer aided techniques for the design and simulation of flow of plastics through the dies. As a result, the dies, which he has designed, manufactured, and supplied to many customers around the world, work the first time without having to waste a lot of valuable production resources in trialling and proving the dies.

The author is a member of the Institute of Engineering and Technology and takes active part in the activities of Aston University in Birmingham.

1.1	Die Definition and Purpose

In the extrusion process, the extrusion die, sometimes known as extruder head, is an essential piece of equipment for the production of the desired product. In the extrusion line, it is the die that takes the plastic melt from the extruder and shapes it into the finished or semi-finished product. The product can be tubes, pipes, rods, profiles, coated wires, cables, filaments, film, or sheet. Whatever the product may be, the purpose of the die is to shape the plastic melt into a desired form.

The purpose of the extruder to which the die is attached is to plasticize the granules (or powder, in some cases) of plastic into a homogeneous melt. The melt is pushed through the die to roughly the desired shape. The final finished product shape is formed by the downstream equipment, which consists of calibration, cooling, haul-off, and cutting equipment. In many instances, the same extruder and downstream equipment can be used for multiple products as long as the equipment has the capacity to produce the products. For instance, the extruder which makes pipes can be used for wire and cable coating, profiles, film, and sheet extrusion.

For every type of product mentioned above, a special die is required. A die which makes tubes of certain sizes will not be suitable for pipes of larger sizes. The terms "tube" and "pipe" are synonymous, with the difference being that any round and hollow section below 25 mm in diameter is known as tube and any above 25 mm in diameter is known as pipe. This definition of tube and pipe is not always followed, as hose pipes of 19 mm in diameter are not normally called hose tubes and conduit pipes of similar sizes are also not called conduit tubes. Nevertheless, the die which makes 20 mm tubes can make 30–35 mm pipes in some cases, but may not be suitable for 50 mm or greater diameters because of the differences in the geometry of the die, as is shown in Chapter 3.

Hence, for every type of product there has to be a special die. For this reason, there are tube dies, pipe dies, profile dies, film dies, sheet dies, etc. For each type of die design there are several different design aspects to be considered. Here the design aspects of tubes and pipes will be discussed in detail. Within the scope of designing these dies, monolayer and multilayer dies will also be explained.

Monolayer pipes and tubes are normally extruded through in-line (longitudinal) dies, and in special cases side-fed dies are used. Multilayer pipes and tubes are extruded through a combination of both in-line feed and side feed. Side-fed dies are called crosshead dies. Crosshead dies are also used when the products to be covered are fed through the middle of the die—for example, wire coating, cable covering, filament coating, pipe coating, etc.

The design aspects of some parts of different types of dies can be the same or similar and others can be quite different. A simple type of tube or pipe die is taken here to explain these differences, principles of design, and definition of the parts. This discussion can be extended to more complicated monolayer, multilayer, and crosshead dies.

A simple die with which most extrusion engineers are familiar is a mandrel support annular die, also known as a spider support die or simply a spider die, as shown in Figure 1.1.

Figure 1.1 Spider die for pipes

1.2	A Spider Supported Die Head

There are several names for this type of die, such as mandrel support die, ring support die, spider supported die, or spider die. The latter is the most commonly used term in the industry and will be employed here throughout this discussion.

This type of die is normally used for extruding PVC or similar types of thermally unstable materials. For other materials that are not so heat sensitive, such as polyolefins (LDPE, HDPE, and PP), polyamides, and many more, a spiral type of distribution is recommended because of its superiority in homogenizing the flow of plastic melt in the die head, as shown in Chapter 5.

In a spider die, a melt stream of plastic from the extruder enters the die head through the breaker plate or connecting ring (not shown in Figure 1.1) into a round channel. The spider cone or torpedo spreads the melt into an annular shape, as shown in Figure 1.1, before it is divided into several sections by the spider legs. Then, these melt sections are joined together by the converging angles of the connecting mandrel (04), the connecting ring (05), the die bush (07), and the mandrel (08). Finally, the melt is forced through a parallel annulus (more commonly known as a die land) between the mandrel and the die.

The term "die head" is used for the complete unit to distinguish it from the die or, as sometimes referred to, the bush. The die or bush is a part of the die head that forms the outside shape of the product to be extruded. To avoid any confusion, the term "die" will be used for the part 07 of the die head and "pin" for the mandrel (08).

The names of the die head parts used here are commonly known in the extrusion industry, and almost everybody involved in plastics processing is familiar with the function of these parts. Nevertheless, a brief description of these parts is given here, and more details are given in Chapter 3.

1.2.1

Flange Adapter

The flange adapter (01 in Figure 1.1) is sometimes known as flange connection and, as the name implies, on one side this part forms a connection to the extruder flange. On the other side it is attached to the spider, which holds the whole die head together. At the extruder end there is a recess to locate on the breaker plate or connecting ring.

The flange part is identical to the extruder flange and both the extruder flange and the flange adapter are clamped together with a clamp not shown in Figure 1.1. The melt from the extruder enters into the flange adapter from the breaker plate in the form of strands, which are joined together into a round slug. In the case of a spider die head, the slug of melt is spread around the spider cone or torpedo, as shown in Figure 1.1.

The flange adapter normally has a heater and a thermocouple probe fitted to control the temperature of the material in this region. In some cases, a pressure transducer to monitor the pressure in the die head is also incorporated in this part. The design of the flange adapter varies with the design of the other parts of the die and the geometry of the extruder flange to which it is fitted, as can be seen in Chapter 3 and Chapters 5 to 13.

Figure 1.2 Flange adapter

1.2.2

Spider Cone or Torpedo

The spider cone or torpedo (02 in Figure 1.1) is a conical part attached to the inner section of the spider. It is used for dividing the flow of melt from a round slug to an annulus form, which is pushed through the spider channels. The design of this part is discussed in Chapter 3.

Figure 1.3 Torpedo

1.2.3

Spider

The spider (03 in Figure 1.1) is the heart of this type of die head. It is a bridge between the flange adapter and the connecting ring on the outside, and the torpedo and the mandrel or pin on the inside. More importantly, it divides the melt stream into channels around the spider legs. The melt is again joined by the compression in the connecting ring and the mandrel or pin. The inside and outside annular part of the spider is kept together by the spider legs, the number of which varies between four and eight depending on the size of the die head. The gap between the outer ring and the inner section—in other words, the channel height—is designed to suit the output required, considering ease of manufacturing and a minimum residence time for the melt. A narrow channel increases the pressure in the die head and is difficult to machine and polish. On the other hand, a large section height increases the residence time and reduces the shear rate, resulting in degradation of the heat sensitive materials in this region. The shape of the spider legs is designed to divide the flow of melt stream and to make it join easily in the chamber between the connecting ring and the pin, as shown in Figure 1.4.

Figure 1.4 Section through spider leg

Figure 1.5 Spider

1.2.4

Connecting Mandrel

The connecting mandrel (04 in Figure 1.1) is connected to the inner section of the spider on one end and the pin is connected to it on the other end. The shape and dimensions of this part are dependent upon the shape and dimensions of the other adjoining parts, namely, the inside section of the spider and the diameter of the pin. The lengths of the connecting mandrel and of the corresponding connecting ring are designed to suit the characteristics of the material. These days, a spider die head is very rarely used for processing polyolefin materials like polyethylene and polypropylene. However, there are instances when, for very short runs and for economic reasons, a spider die head is used for processing these materials. In these instances, the lengths of the connecting mandrel and of the connecting ring are made considerably greater, to diffuse the melt disturbance caused by the spider legs and to minimize the flow lines.

Figure 1.6 Connecting mandrel

1.2.5

Connecting Ring

The connecting ring (05 in Figure 1.1) is an intermediary part between the spider and the die. In conjunction with the connecting mandrel (04), it is used for streamlining the melt coming out of the spider, and for guiding it to the entry of the die. The design of this part is quite simple, as the inside diameters should match the diameter of the spider on one hand and the inside diameter of the die on the other. The outside diameter corresponds with the outside diameter of the spider body and of the die-adjusting ring on the other side. In some instances, the connecting ring incorporates another part called the choke ring. The choke ring was considered to be an essential part of the design when the flow of material had to be adjusted by a trial and error method. The incorporation of the choke ring in the connecting ring is a good idea and gives the design flexibility for making certain changes; it can also be replaced with a striping ring, when different color stripes are required on the product.

Figure 1.7 Connecting ring

1.2.6

Die Clamp or Die-Adjusting Ring

This part (06 in Figure 1.1) clamps the die (07) and the connecting ring (05) together and is also used for adjusting the radial gap between the mandrel and the die, as illustrated in Figure 1.1. The number of bolts connecting the die-adjusting ring to the connecting ring varies according to the size of the die head. The number of adjusting bolts on the circumference also varies according to the size of the die head. Some designers prefer to incorporate the die-adjusting function of the die clamp into the main body of the die head and to replace this component with a simple clamp plate. The disadvantages of this system is that, if there is any wear or damage around the die-adjusting bolts, the repair or replacement of the die body is more expensive than the replacement of the die-adjusting ring.

Figure 1.8 Die clamp ring

1.2.7

Die or Bush

This part (07 in Figure 1.1) of the die forms the outer shape of the pipe or tube. In other types of products, such as profiles and cable covering, the same part produces the desired outer shape of the finished product. The two sections of the inside of the die are the die land and the converging section. The converging section guides the melt from the die connecting part or spider to the land section. The land section bears direct relationship to the final shape. This part of the die head is heated by external heaters to control the temperature of the melt in this region. The detailed design of the die or bush is given in Chapter 3.

Figure 1.9 Die bush

1.2.8

Pin or Mandrel

This part (08 in Figure 1.1) of the die head forms the inside diameter of pipes and tubes. In other types of products, such as cable covering and profiles, the same part determines the inside perimeters of those products. As the discussion here is about pipe and tubes, the shape concerned is round and the dimensions of this part are related to the inside diameter of the finished product. There are three main sections of the pin: the two sections corresponding to the die are the land and the converging section. The third section is the location of the spigot and the threaded part, which connects the pin to the connecting mandrel. The first two parts corresponding to the die are important in shaping the geometry of the final product. The design aspects of the pin are shown in Chapter 3.

These are the main parts used in the construction of a spider supported die head; any additions or alterations can be made to suit the process or product variations. The purpose of this section was to familiarize the reader with the parts and terminology employed in further discussion.

To start the design of the die is not quite as simple as outlined above. The designer needs to collect a lot of information about the product, the materials to be processed, the equipment installed, and other processing facilities and limitations. Therefore, the next chapter looks into the collection and analysis of the information of all aspects affecting the design of the die head.

Figure 1.10 Pin or mandrel

Figure 1.11 Complete spider die in 3D

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