When it comes to plastic pipe extrusion, these 11 basic principles must be followed!

Ningbo Fangli Technology Co., Ltd. is a mechanical equipment manufacturer with nearly 30 years’ experiences of plastic pipe extrusion equipment, new environmental protection and new materials equipment. Since its establishment Fangli has been developed based on user’s demands. Through continuous improvement, independent R&D on the core technology and digestion & absorption of advanced technology and other means, we have developed PVC pipe extrusion line, PP-R pipe extrusion line, PE water supply / gas pipe extrusion line, which was recommended by the Chinese Ministry of Construction to replace imported products. We have gained the title of “First-class Brand in Zhejiang Province”.

01  Mechanical principles

The basic mechanism of extrusion is very simple - a screw rotates in the barrel and pushes the plastic forward. The screw is actually an inclined surface or slope, which would around the central layer. Its purpose is to increase pressure in order to overcome greater resistance. For an extruder, there are three types of resistance that need to be overcome: the friction of the solid particles (feed) against the barrel wall and their mutual friction during the first few revolutions of the screw (the feed zone); the adhesion of the melt to the barrel wall; and the logistic resistance within the melt as it is pushed forward.

Newton once explained that if an object does not move in a given direction, then the forces on that object are balanced in that direction. A screw does not move in an axial direction, although it may turn laterally and rapidly near the circumference. Therefore, the axial force on the screw is balanced, and if it exerts a large forward thrust on the plastic melt then it also exerts an equal backward thrust on the object. In this case, the thrust it exerts is on the bearing behind the inlet - the thrust bearing.

Most single screws are right-hand threads, like screws and bolts used in woodworking and machinery. If viewed from the back, they are counter-rotating because they are trying to screw out of the barrel as far back as they can. In some twin-screw extruders, the two screws rotate backwards and cross each other in both barrels, so one must be right-handed and the other left-handed. In other occluded twin screws, the two screws rotate in the same direction and therefore must have the same orientation. However, in either case there are thrust bearings that absorb the backward force and Newton's principle still applies.

02  Thermal principle

Extrudable plastics are thermoplastics - they melt when heated and solidify again when cooled. Where does the heat for melting plastics come from? Feed preheating and barrel/mould heaters may play a role and are important at start-up, but motor input energy - the frictional heat generated in the barrel as the motor turns the screw against the resistance of the viscous melt - is the most important heat source for all plastics,except for small systems, low-speed screws, high melt temperature plastics, and extrusion coating applications.

For all other operations, it is important to recognise that the barrel heater is not the main source of heat in the operation, and therefore plays a smaller role in extrusion than we might expect (see Principle 11). The rear barrel temperature may still be important because it affects engagement or the rate of solids transport in the feed. The die and mould temperatures should normally be the desired melt temperature or close to it, unless they are used for a specific purpose such as varnishing, fluid distribution or pressure control.

03  Deceleration principle

In most extruders, the screw speed is varied by adjusting the motor speed. The motor usually rotates at a full speed of about 1750 rpm, but this is too fast for an extruder screw. If it rotates at such a fast speed, too much frictional heat is generated and the retention time of the plastic is too short to prepare a homogeneous, well-mixed melt. Typical reduction ratios are between 10:1 and 20:1. The first stage can be either a gear or a pulley set, but the second stage is all gears and the screw is positioned in the centre of the last large gear.

In some slow running machines (such as twin screws for UPVC) there may be 3 deceleration stages and the maximum speed may be as low as 30rpm or less (a ratio of 60:1). At the other extreme, some of the very long twin screws used for mixing can run at 600rpm or faster and therefore require a very low deceleration rate as well as a lot of deep cooling.

Sometimes the deceleration rate is incorrectly matched to the task - there will be too much energy to use - and it is possible to add a pulley block between the motor and the first deceleration stage of changing the maximum speed. This either increases the screw speed beyond the previous limit or reduces the maximum speed allowing the system to run at a greater percentage of the maximum speed. This will increase the available energy, reduce amperage and avoid motor problems. In both cases, the output may increase depending on the material and its cooling needs.

04  Feed in coolant

xtrusion is the transfer of energy from a motor - sometimes a heater - to cold plastic, thereby converting it from a solid to a melt. The input feed is cooler than the barrel and screw surfaces in the feed zone. However, the barrel surface in the feed zone is almost always above the plastic melting range. It is cooled by contact with the feed particles, but the heat is maintained by heat transfer from the hot front end to the back end and by controlled heating. It may be necessary to switch on the rear heater even when the front end heat is held by viscous friction and no cartridge heat input is required. The most important exception is the slot feed cartridge, almost exclusively for HDPE.

The screw root surface is also cooled by the feed and adiabatic from the barrel wall by the plastic feed particles (and the air between the particles). If the screw suddenly stops, the feed also stops and the screw surface becomes hotter in the feed zone as heat moves backwards from the hotter front end. This can cause sticking or bridging of particles at the root.

05  The feed is glued onto the barrel or slid onto the screw

In order to maximise the solids particle transport in the smooth barrel feed zone of a single screw extruder, the particles should stick to the barrel and slide onto the screw. If the pellets stick to the root of the screw, there is nothing to pull them off; the channel volume and the inlet volume of solids are reduced. Another reason for poor adhesion at the root is that the plastic may thermo-condense here and produce gels and similar contaminating particles, or intermittently adhere and break off with changes in output speed.

Most plastics naturally slide on the root because they are cold when they enter and friction has not yet heated the root to the same level of heat as the barrel wall. Some materials are more likely to adhere than others: highly plasticised PVC, amorphous PET, and certain polyolefin co-polymers with adhesive properties that are desired for end use.

For the barrel, it is necessary for the plastic to adhere so that it can be scraped off and pushed forward by the screw thread. There should be a high coefficient of friction between the particles and the barrel, which in turn is strongly influenced by the temperature of the rear barrel. If the particles don't adhere, they just turn in place and don't move forward - that's why smooth feed is bad.

Surface friction is not the only factor affecting feeding. Many particles never come into contact with the cylinder or screw root, so there must be friction and mechanical viscosity linkage inside the particles.

Surface friction is not the only factor affecting feed. Many particles never touch the barrel or screw root, so there must be friction and mechanical and viscosity interlocking within the granulate.

The grooved cylinder is a special case. The groove is located in the feeding area, which is thermally insulated and deeply water-cooled from the rest of the cylinder. The thread pushes the particles into the groove and forms a high pressure within a relatively short distance. This increases the bite tolerance for lower screw speeds with the same output, resulting in a decrease in frictional heat generated at the front end and a lower melt temperature. This may mean that cooling limits faster production in blown film production lines. The groove is particularly suitable for HDPE, which is the smoothest ordinary plastic besides perfluorinated plastic.

06  The highest cost of materials

In some cases, material costs can account for 80% of production costs - more than the sum of all other factors - except for a few products with particularly important quality and packaging, such as medical catheters. This principle naturally leads to two conclusions: processors should reuse scraps and waste as much as possible to replace raw materials, and strictly adhere to tolerances to avoid deviation from the target thickness and product problems.

07  Energy costs are relatively unimportant

Although the attractiveness and real problems of a factory are on the same level as the rising energy costs, the energy required to operate an extruder is still a small part of the total production cost. The situation is always like this because the material cost is very high, and the extruder is an effective system. If too much energy is introduced, the plastic will quickly become very hot and cannot be processed properly.

08  The pressure at the end of the screw is very important

This pressure reflects the resistance of all objects downstream of the screw: filter screen and contamination crusher plate, adapter conveyor pipe, fixed agitator (if any), and the mold itself. It not only depends on the geometry of these components but also on the temperature in the system, which in turn affects resin viscosity and throughput speed. It does not rely on screw design, except when it affects temperature, viscosity, and throughput. For safety reasons, measuring temperature is important - if it is too high, the mold head and mold may explode and harm nearby personnel or machines.

Pressure is beneficial for stirring, especially in the final area (metering area) of a single screw system. However, high pressure also means that the motor needs to output more energy - thus the melt temperature is higher - which can specify the pressure limit. In a twin screw system, the interlocking of two screws is a more effective stirrer, so no pressure is required for this purpose.

When manufacturing hollow components, such as pipes made using spider molds with brackets for core positioning, high pressure must be generated inside the mold to help separate logistics recombine. Otherwise, the product along the welding line may be weak and may encounter problems during use.

09  Output

The displacement of the last thread is called normal flow, which only depends on the geometry of the screw, screw speed, and melt density. It is regulated by pressure logistics, which actually includes the resistance effect of reducing output (represented by the highest pressure) and any over bite effect in the feed of increasing output. The leakage on the thread may be in either direction.

It is also useful to calculate the output of each rpm (revolution), as this represents any decrease in the pumping capacity of the screw at a certain time. Another related calculation is the output per horsepower or kilowatt used. This represents efficiency and can estimate the production capacity of a given motor and driver.

10  Shear rate plays a major role in viscosity

All ordinary plastics have a shear force reduction characteristic, which means that the viscosity decreases as the plastic moves faster and faster. The effect of some plastics is particularly evident. For example, some PVCs increase their flow velocity by 10 times or more when the thrust is doubled. On the contrary, the shear force of LLDPE does not decrease too much, and when the inference is doubled, its flow velocity only increases by 3 to 4 times. The reduced shear force reduction effect means high viscosity under extrusion conditions, which in turn means more motor power is required.

This can explain why LLDPE operates at a higher temperature than LDPE. The flow rate is expressed as shear rate, which is approximately 100s-1 in the screw channel, between 100 and 100s-1 in most mold mouth shapes, and greater than 100s-1 in the gap between the thread and the cylinder wall and some small mold gaps.

The melt coefficient is a commonly used measurement method for viscosity, but it is inverted (such as flow rate/thrust rather than thrust/flow rate). Unfortunately, its measurement in an extruder with a shear rate of 10s-1 or less and a fast melt flow rate may not be a true measurement value.

11  The motor is opposite to the barrel, and the barrel is opposite to the motor

Why is the control effect of the barrel not always as expected, especially within the measurement area? If the barrel is heated, the viscosity of the material layer at the barrel wall decreases, and the motor requires less energy to operate in this smoother barrel. The motor current (amperes) decreases. On the contrary, if the barrel cools down, the viscosity of the melt at the barrel wall increases, and the motor must rotate more vigorously, increasing the ampere number. Some of the heat removed when passing through the barrel is then sent back by the motor. Usually, the barrel regulator does have an effect on the melt, which is what we expect, but the effect anywhere is not as significant as the regional variable. It is best to measure the melt temperature to truly understand what has happened.

The 11th principle does not apply to the mold head and mold, as there is no screw rotation there. That's why external temperature changes are more effective there. However, these changes are uneven from the inside out, unless stirred evenly in a fixed stirrer, which is an effective tool for changes in melt temperature and stirring.

If you need more information, Ningbo Fangli Technology Co., Ltd. welcomes you to contact for a detailed inquiry, we will provide you with professional technical guidance or equipment procurement suggestions.