All about Plastic Extrusion and Helpful Tips for Buying ...

Plastic extrusion is a kind of continuous process for high volume manufacturing, where a certain thermoplastic material can be made in the form of pellets, powder, or granulates. Thereafter, they are homogeneously melted and moved into shaping die by applying pressure.

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For example, screws are made with the molten plastic flowing into the rotating molds. As your plastic melts, it will pass through the die, where it is going to acquire the shape of the die-hole shape, and will then leave the extruder. This extruded product is known as an extrudate.

A typical plastic extruder will consist of 4 zones:

1. Feed zone

The flight depth is constant in this zone. The flight depth is the distance between the main diameter of the screw at the top of the flight and the minor diameter of the screw at the bottom of the flight.

2. Transition zone/compression zone

In this zone, the flight depth begins to decrease. The thermoplastic substance is crushed and starts to plasticize as a result.

3. Mixing zone

The flight depth remains constant in this zone. A particular mixing device may be used to ensure that the material is completely melted and homogeneously blended.

4. Metering zone

The flight depth in this zone is lower than in the mixing zone, but it remains constant. In this zone, the pressure also forces the melt using the shaping die.

What can be created by using plastic extrusion?

Plastic extrusion is a technique that presses the molten plastic using a die to create tubes and plastic profiles. The procedure is used to make a wide range of goods and component parts for both industrial and home applications.

Thermoplastics will be utilized to force these plastics into shape because they are the best sort of material for melting and cooling to form a rigid shape. Two of the mainly included thermoplastics for plastic extrusion are polypropylene and PVC. Let us discuss here a few different applications for plastic extruder.

1. Tubing

Extrusion of plastic can be used to make a wide variety of pipes and tubes. Tubes can be extruded to transport vast amount of liquid or gases, such as sewerage or water pipes, on one end of the scale.

Tubes with a considerably smaller diameter, on the other hand, can be made for items like medical tubing or straws! Tubing is one of the most common things made by plastic extrusion, and it is also one of the simplest to make because it can be extruded to any length and cut to the desired size.

Extruded tubes can be made not just to a given diameter, but also can be thicker or thinner as needed.

Extruded tubes can be hard or flexible, depending on the type of plastic employed. This means that plastic tubes can be manufactured to remain rigid for use in plumbing and sewage while also becoming more flexible.

Garden hoses, medical tubing, and tubes that transport food, chemicals, or gases in a factory setting will all be made with more flexible plastic extrusions.

2. Profiles

To best suit the required design, cross-section shapes can be extruded in a range of various plastic types. Rain gutters, railing, trims, and seals for windows and doors are examples of profiles, which are forms rather than simple tubes.

Co-extruded elements, such as rubber seals on window trims, can frequently be added to profiles like these. Furthermore, adding another kind of plastic to the expulsion process.

3. Solid extrusions

Extrusions don't have to be hollow tubes or profiles and they can be solid shapes as well. Plastic planks and decking, as well as a variety of curved bar stock for industrial or automotive usage can be made in this way.

Solid form extrusion is beneficial for making outdoor furniture, such as benches and fences because the plastic is a weatherproof, low-maintenance alternative to wood that lasts far longer.

4. Insulation

Plastic extrusion can also be used to make the insulation that surrounds the electrical cable. This is accomplished by passing a wire through the die and then extruding plastic around it to form an insulating layer.

Other types of cable can also go through this extrusion process to make them waterproof and resistant to corrosion and abrasion, as well as easier to handle.

5. Sheets and film

Extruded sheeting and film can also be used as a protective screen or as part of window glass. Extruded plastic film can also be used to make plastic packaging like blister packs.

Few Different types of plastic extrusion line

There are several designs of extruders that are available in the market today. We will take up a few important ones here and briefly discuss them.

1. Single screw extruders

Single screw extruders are the most prevalent continuous extruders in the polymer extrusion sector because of their numerous advantages, including low cost, simple design, toughness, reliability, and a high performance/cost ratio.

Any standard single screw extruder will have 3 geometrically varying zones:

· Feed zone

· Metering zone

· Transition or compression zone

The consistent pitch of the screw, but variable channel depth creates the three zones. The compression phenomenon is caused by the screw channel's depth decreasing linearly from the feed zone to the metering zone.

The term 'single-stage' refers to screw designs having only one compression portion. For the same screw length and diameter, zone length, as well as maximum and minimum channel depths, may vary. As a result, several screw profiles are feasible.

2. Twin-screw extruders

Generally, twin screw extruder is categorised as continuous multiple screw extruders in general. The name comes from the fact that these extruders contain two Archimedean screws in their design.

There are several classes for twin-screw extruders since there are more design characteristics that may be adjusted in twin-screw extrusion, such as rotational direction, degree of intermeshing, and so on.

The twin screw extruder has 2 varieties and they are also further subdivided as follows:

a. Intermeshing extruders

· Counter-rotating

· Co-rotating

b. Non-intermeshing extruders such as

· Counter-rotating

· Co-rotating

· Co-axial

3. Tubing extrusion

Tubes and pipes are extruded using this method of extrusion. Air with a positive internal pressure can also be used in this process. After departing the die, the tubes or pipes are dragged into a cooling tank, where they are generally water-cooled.

4. Blow film extrusion

This type plastic extrusion line can be used to make plastic film tubes using a continuous sheeting process. The melt from the film tube is cooled before it leaves the die, resulting in a semi-solid tube that is blown to a suitable size and film thickness. This method is employed in the production of items like shopping bags.

5. Sheet film extrusion

In this plastic extrusion line plastic sheets or films that are too thick to be blown are extruded using this method. The sheets are drawn and cooled through a sequence of cooling rolls after exiting the die, which helps regulate sheet thickness.

6. Over jacket extrusion

Wire coating is done with this form of extrusion. The wire is drawn into the die's centre in this procedure. When a high level of adhesion between the wire and the coating is required, pressure tooling is employed.

The wire is covered with molten plastic while in the die and is pressured when it exits the die in this manner. Jacketing tooling is utilised if adhesion is not required. The melt covers the wire as it exits the die in this way.

How to select a plastic extruder?

The following are a few things that you must see while selecting any plastic extruder:

1. Track record of success

Reliability and experience are important things to consider when selecting a manufacturer. You must be confident that your chosen manufacturer will be there and able to assist you throughout the extruder's operational lifetime.

A long and successful track record is the best indicator of a manufacturer's dependability and industry experience.

2. Quality of components

Consider manufacturers who only employ high-quality components from globally recognised brand names when it comes to the numerous components used on an extruder machine. If a component has to be improved, repaired, or replaced in future, there will be no issues locating or integrating a spare part.

3. Quality of service and support

The term 'quality' refers not just to the extrusion machine itself, but also to the continual support provided at every step along the road. A good business will follow through on its promise of excellent customer service by offering assistance throughout the purchasing process.

Nanjing Cowin Extrusion Machinery Company Ltd was formed by the optimization and reorganisation of the twin-screw extrusion industry's leading team. It is a multinational, high-tech limited corporation specialising in the production of polymer material processing equipment.

In China, this company is considered to be one of the best extruder manufacturing companies. It was founded in Nanjing, Jiangsu Province, in . The company employs more than 50 people, including ten technical R&D engineers, twenty machining and assembly workers, 18 management and sales teams, and five after-sales support teams.

The choice of the right extruder size is based on many different factors and can have a decisive influence on the achievable product quality and also on the achievable productivity of the production. General statements are often not correct and must therefore be thoroughly questioned before they are implemented in day-to-day production.

When does the question of the right extrusion system actually arise?

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In many companies the situation is that the products to be extruded should not only be manufactured on a single line specially selected for this purpose, but that, depending on availability, one and the same product must be manufactured on different extrusion lines. The lines are then equipped with the appropriate extrusion dies as required and production is started with production parameters that match the line.

In such cases it happens that identical products are sometimes produced on a (e.g.) 45 mm extruder and on another day on a 75 mm extruder. It is imperative that very different processes are used, so that the product properties can vary greatly in some cases. If quality problems arise, the question is often raised as to the extent to which the extruder size can influence the product.

Another situation in which the question of the optimum extruder size regularly arises is when investing in new lines. Here a distinction must be made between whether the new line is to be purchased specifically for a continuous runner, i.e. whether it is to be optimally dimensioned for a special product, or whether the new line is to be suitable for a certain range of products, i.e. whether it is to be used as flexibly as possible.

In all cases, the choice of the right extruder size is an important task.

Extruder size, how is it described?

The size of an extruder can be described by the three main factors:

the screw diameter in mm
the screw length in multiples of the diameter
the drive power in kW

An extruder of type 45/35 (40kW) thus has a screw diameter of 45mm and a screw length of 1,620mm (45mm x 36). The drive power in this case is 40 kW.

Extruder size, relevant data:

The indicated screw diameter refers to the outer diameter of the screw (flight diameter). The usual dimensions for classic single-screw extruders for production are between 30mm and 150mm, but there are both smaller and larger systems.

It is not possible to make a general statement about the throughput that an extruder with a certain screw diameter can achieve, since the throughput that can be achieved can always depend on many other parameters such as the screw design, the material to be processed and other parameters such as the plant design or the drive power.

In practice, deep-cut screws with a large free volume (so-called throughput-intensive screws) can achieve higher throughput rates (with low pressure build-up requirements) than flat-cut screws with a smaller free volume (so-called pressure-intensive screws). With regard to the screw length, it can be roughly stated that longer systems improve the homogenizing and mixing performance of the extruder, but can lead to degradation due to the longer residence times of sensitive materials. In addition, the time and material required for 'rinsing' the system increases, for example when changing the colour.

The drive power of the extruder can also be a limiting factor, but in practice it is usually oversized rather than undersized.

The approaches presented in the following are therefore only intended to assist in the selection of an extruder size and do not represent general criteria. Nevertheless, we would like to try to define a small selection aid here.

Extruder selection:

For the selection of an extruder different aspects should be considered in the selection:

Desired throughput:

The throughput required in the application gives a rough idea of the approximate dimensions of the extruder. The following table shows a rough summary for throughput rates of different screw diameters with standard screw geometry (3 zones), in each case related to realistic maximum throughputs for processing PEHD and PPH.

Below is a diagram of the relationship between screw diameter and maximum achievable throughput, based on experience. This diagram is not general and not transferable, but serves only to illustrate the large dependencies.

(Special applications such as high-speed extrusion concepts with enormously high screw speeds (high-speed extrusion >1m/s peripheral speed) are not taken into account here).

On the basis of such information, which can usually be obtained from the various extruder manufacturers, a rough selection can be made as to the size of the screw diameter required to achieve the maximum throughput required. However, it is important that such data are always material-specific and cannot be easily transferred to other materials.

A further aspect that is of decisive importance when selecting an extruder is not only the throughput, but also the peripheral speeds of the screw occurring at the respective speed.

If, for example, an output of 400kg/h is planned, this can be achieved in the following configurations (exemplary data, not generally valid):

• 60 mm Extruder at 220 rpm  '>  peripheral speed: 0,691 m/s
• 75 mm Extruder at 160 rpm  '>  peripheral speed: 0,628 m/s
• 90 mm Extruder at 110 rpm  '>  peripheral speed: 0,518 m/s
• 105 mm Extruder at 80 rpm  '>  peripheral speed: 0,440 m/s

It is logical that a comparatively small extruder requires a higher screw speed to achieve throughput than a larger extruder. The screw speed, however, also directly increases the peripheral speed of the screw. If one now considers that high peripheral speeds can damage the material due to the high friction and shear in the barrel, it quickly becomes clear that gentle processing of the material is more feasible on slow-running lines. At the same time, however, the residence time of the melt in the system increases with larger extruders, so that sensitive materials can be thermally damaged.

The order of magnitude of the permitted circumferential speeds of the respective material can usually be found in the processing instructions of the raw material manufacturer.

Typical values for permitted circumferential speeds are in this order of magnitude:

With knowledge of the desired output of the extruder and considering the processing specifications of the material manufacturer, it is then possible to make a size selection for the extruder.

The following table and diagram show the resulting peripheral speed for different speeds and screw diameters:

Example (fictitious values, not universal):

A throughput of 300kg/h is to be achieved with a polycarbonate (PC) material. A critical peripheral speed of 0.3 m/s for this material can be taken from the manufacturer's processing instructions. A 60 mm and a 75 mm extruder are available.

The 60 mm extruder would probably be able to achieve this throughput at a screw speed of approx. 170 rpm. The circumferential speeds would be approx. 0.5 m/s.

The 75 mm extruder could achieve the throughput at speeds of approx. 100 rpm. Peripheral speeds of approx. 0.39 m/s would occur.

Both extruders would therefore not be suitable for processing the material with the desired throughput.

On the 60 mm extruder, the maximum peripheral speeds would be reached at approx. 85 rpm. At this speed a throughput of approx. 115 kg/h would be realistic.

On the 75 extruder, the maximum circumferential speeds would be reached at approx. 75 rpm, so a throughput of approx. 190 kg/h would be feasible.

Consideration of the drive power

A further aspect that still has to be considered when selecting or dimensioning the extruder is the design of the drive power. The first indication for the design of the drive power is the amount of energy required to heat the material to be processed to processing temperature.

If, for example, a PC is to be heated from ambient temperature to 270°C, a desired throughput of 200 kg/h would require approx. 25 kW of purely thermal power.

Due to the losses that occur in an extruder and the additional energy required to build up the pressure, today's extruders are usually equipped with at least 2.5 times this power.

Consideration of the residence time

Residence time is the time a plastic particle remains in the system from its entry into the extruder to its exit from the extruder. The longer the residence time, the more likely it is that the material in the system will be damaged and thermally degraded. In principle, it is advisable to extrude with the shortest possible residence times, unless the required homogenising capacity is not sufficient.

For a very rough calculation of the residence time, the quotient of the volume of the extruder filled with melt and the volume flow of the melt can be formed according to basic physical laws.

If, for example, it is assumed that there is a volume of 3,000 cm³ filled with melt in the extruder and is produced with a throughput of 100 kg/h, the average residence time (at an assumed density of 1,000 kg/m³) is approx. 108 seconds. (you can find an excel- tool for residence time calculation in our download section)

To determine the approximate residence time, however, it is necessary to describe the design of the screw and extruder as well as the degree of filling of the system as precisely as possible. The given calculation is therefore only suitable as a rough estimate.

In general, however, it can be said that the higher the temperatures of the melt, the shorter the residence times should be. Here, too, the processing instructions of the material manufacturer are an important point of reference.

Conclusion:

When selecting or designing an extruder, various aspects have to be taken into account, all of which are interdependent and generally have to be considered very material-specifically.

A universal extruder that can meet all requirements in the same quality is simply impossible. Each extruder dimensioning is a compromise of different sizes and must therefore be considered specifically for the application.

Nevertheless, it is important to have other aspects (circumferential speed, residence time) in mind in addition to the pure attainability of a desired throughput. In practice, neglecting these aspects repeatedly leads to considerable quality differences in the production of supposedly identical products when these are produced on different systems.

Register here (free of charge) as a premium member and gain access to our download area. There you will find an Excel tool for extruder design, residence time calculation and peripheral speed calculationas well as other tools, checklists, etc. In addition, as a Premium user you will always be informed about new contributions.

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