Plastic extrusion 11 key principles introduced

1 mechanical principles
    Out the basic mechanism is simple - a screw in the plastic cylinder in turn and move forward. Screw is actually a slope or ramp, winding in the center layer. The aim is to increase the pressure to overcome the greater resistance. On an extruder, there are three kinds of obstacles to overcome: the solid particles (the feed) on the cylinder wall friction and screw rotation when the first few laps (feed zone) the friction between them; melting body in the tube wall adhesion; melt was moving forward when its internal logistics resistance.

    Newton had explained that if an object is not to a given direction, then the force on the object in this direction in the balance. Not to screw axial movement, although it may cross the circle fast turn around. Therefore, the axial force on the screw to be balanced, and if it is applied to the plastic melt a lot of forward thrust it also applied to an object the same as a backward thrust. Here, it is the role of the applied thrust back in the inlet bearing - thrust bearings.

    Most single screw is right-hand thread, as used in woodworking and machine screws and bolts. From the rear, they are reverse rotation, as they try to spin back out of the cylinder. In some twin-screw extruders, two screws in the reverse rotation of the cylinder and two cross-cutting, so one must be right-and the other must be left to the. Occlusion in the other twin-screw, two screws rotate in the same direction and therefore must have the same orientation. However, in either case have the ability to absorb backward thrust bearings, Newton's principles still apply.http://www.prmindustry.com/Abstract...Articles section 2 Hot principle

    Can be squeezed out of the plastic is thermoplastic - they melt when heated and cooled again solidified. The heat melted plastic come from? Feed preheat and cylinder / mold heater may work but also in the start is very important, but the electrical input power - motor to overcome the resistance of the melt viscosity when the screw rotation to generate frictional heat in the cylinder body - is The most important heat source for all plastic, small systems, low-speed screw and high melt temperature and extrusion coating applications, except plastic.

    For all other operations, recognizing the tube heater is not operating in the main heat source is very important, and thus out of role than we expected might be small (see Principle 11). After the cylinder temperature may still be important because it affects the teeth together or in the feed solids conveying speed. Die and mold temperature should normally be close to the desired melt temperature or the temperature, unless they are used for a specific purpose such as polishing, distribution, or fluid pressure control.

    3. Deceleration principle

    In the majority of the extruder, screw speed changes by adjusting the motor speed to achieve. Motor is usually the full speed of about 1750rpm, but it is a fast screw extruder. If such a fast rotation, will generate too much heat and friction of plastic retention time is too short to prepare uniform, good mixing of the melt. A typical deceleration ratio between 10:1 to 20:1. The first stage can either use the gear pulley, but the second stage gear and the screw located in the center of the last gear.

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

    Deceleration rate and the task of matching is sometimes wrong - there will be a lot of energy can not be used - and may change the maximum speed of the motor and a deceleration phase between the first to add a pulley.

    This is either the screw speed increased to more than the previous maximum speed limit or reduce the maximum speed allowed the system to a greater percentage of the run. This will increase access to energy, to reduce the motor amperage and avoid the problem. In both cases, according to the material and its cooling needs, the output may increase.

    4 play a coolant feed

    Extrusion is the electrical energy - and sometimes the heater - sent to the cold plastic, thereby convert it from a solid to melt. Enter the feed than the feed zone barrel and screw surface temperature low. However, the feeding area of ​​the cylinder surface is almost always above the plastic melting range. It is through contact with the feed grain cooling, but the heat transfer from the hot heat front and back to control heating and maintained. Even the current side to keep the heat by viscous friction heat input and does not require the cylinder, may need to open, heater. The most important exception is the groove into the barrel, almost exclusively for HDPE.

    Screw root surface cooling was also feed plastic feed particles (and particles of air) from the tube wall insulation. If you suddenly stop screw, the feed is stopped, and because the heat from the hotter the front backwards, the screw in the feed zone surface gets hotter. This may cause the adhesion of particles or in the root bridge.

    5 In the feed area, stick to slide the screw on the cylinder

    To make a smooth cylinder single-screw extruder feed zone to reach the maximum throughput of solid particles, the particles should be glued to the tube and slide the bolt. If the particles stick to the screw root, nothing can pull them down; the entrance channel volume and solid volume is reduced. In the roots of poor adhesion may be another reason for plastic hot in here and produce gels and similar refining pollution particles, or with changes in output speed intermittent adhesion and disruption.

    Most plastic slide naturally in the roots, because when they enter the cold, but not the root of the friction and heat to the cylinder wall as heat. Some materials are more likely than other materials adhesion: a highly plasticized PVC, amorphous PET, and some end-use in the adhesion properties are desired polyolefin copolymers.

    For the cylinder, the plastic is necessary here so that it adheres to the screw thread was scraped off and was moving forward. Between particles and the cylinder should have a high coefficient of friction, the friction coefficient, in turn, affected by the strong influence of temperature after cylinder. If the particles do not adhere to, they just rotate in place without moving forward - which is why a smooth feed bad reasons.

    Surface friction is not the only factor affecting the feed. Many particles never touch the cylinder or screw root, so the particles must have an internal mechanical friction and viscosity, and chain.

    Body with a grooved drum is a special case. Slot in the feed zone, feed zone and the rest of the cylinder is the depth of thermal insulation and is water-cooled. Threaded into the tank and the particles in a relatively short distance to form a high pressure. This increases the output of a low screw speed of the same bite fair amount of friction generated heat and thus reduce the front, lower melt temperature. This may mean that the cooling limit blown film production line in faster production. Tank particularly suitable for HDPE, which is in addition to sliding over fluorinated plastic than most common plastics.

    6. The most expensive materials

    In some cases, material costs can account for 80% of the cost of production - more than all other factors and - except over a small number of quality and packaging of products such as particularly important for medical catheters. This principle naturally leads to two conclusions: processors should be reused as much as possible instead of scrap and waste materials, and strict adherence to tolerance as much as possible so as not to deviate from the target thickness and product problems.

    7. Energy costs are relatively unimportant

    Despite the attractiveness of a plant and the real issues and rising energy costs at the same level, run an extruder is still needed energy in a fraction of the total cost of production.
Always the case because material costs are very high, the extruder is an effective system, if introduced too much energy so the plastic will become very hot very quickly so that can not be properly processed.

    8 screw at the end of the pressure is very important

    This pressure reflects the resistance of the screw downstream of all objects: filters and pollution tie broken boards, adapter pipeline, fixed agitator (if any) and the mold itself. It not only depends on the geometry of these components also depends on the temperature of the system, which in turn affect the resin viscosity and by the speed. It does not depend on the screw design, which affects temperature, viscosity and the amount of time except through. To security reasons, it is important to measure the temperature - if it is too high, die and mold may be near an explosion and hurt people or machines.

    Pressure for the mixing is beneficial, especially in the last single-screw system area (measurement area). However, the high pressure also means the motor to output more energy - and therefore a higher melt temperature - which can be specified pressure limit. In the twin-screw, two screws each bite is a more efficient mixer, so the pressure is not required for this purpose.

    In the manufacture of hollow parts, such as the use of stents on the core position of the spider mold tube, the mold must be produced in a high pressure to help separate logistic re-combination. Otherwise, along the weld line may be weak and in the use of the product may be a problem.

    9. Threaded output = final displacement of + / - pressure and leak Logistics

    Finally a thread called positive displacement flow, depends only on the screw geometry, screw speed and melt density. It is adjusted by the pressure of logistics, in fact, including the effect of reducing the output resistance (indicated by the highest pressure) and increase the output of the feed had bite any effect. Thread on the leak may be two directions in any one direction.

    Calculated for each rpm (rpm) of the output is useful because it means that the time of the screw pump capacity of any decline. A related calculation is used per horsepower or kilowatt output. This means that the efficiency and the ability to estimate a given motor and drive capacity.

    10 shear rate viscosity play a major role in

    All have a common plastic shear drop feature, which means faster and faster movement in the plastic when the viscosity becomes low. Some of the effects of plastic that was particularly evident. For example, doubling the number of PVCs in thrust when the flow rate will increase by 10 times or more. On the contrary, LLDPE shear drop was not too much reasoning, when doubling the flow rate increased by only 3 to 4 times. Reduce the effect of mean shear extrusion conditions to reduce the high viscosity, which in turn means more electrical power. This may explain why the operating temperature than LDPE LLDPE high. Shear rate of flow to said channel when the screw is about 100s-1, in most mold mouth is between 100 and 100s-1, in the thread and the cylinder wall clearance and small die gap greater than 100s- 1. Melt viscosity coefficient is a commonly used measurement method but it is reversed (such as the flow / thrust instead of thrust / traffic). Unfortunately, the measurement is in the shear rate in the 10s-1 or less and fast in the melt extruder flow rate may not be a true measurement.

    11 motor and cylinder opposition, confrontation with the motor cylinder

    Why is the control effect of the cylinder and expectations is not always the same, especially in the measurement of the area? If the heating cylinder, the cylinder wall at the smaller viscosity of the material layer, the smoother the motor in the cylinder body needs less energy to run. Motor current (amperage) decreased. Conversely, if the cylinder cooling, cylinder wall and the melt viscosity increases, the motor must be more hard to turn, increase the number of amps through the barrel to remove some heat when the motor has been sent back. Typically, the cylinder regulator does have an effect on the melt, which is what we expected, but the effect of any local area variables are not large. It is best to measure the melt temperature to really understand what happened.

    Principle 11 does not apply to die and die, because there is no screw rotation. This is why the external temperature variation, where more effective. However, these changes are so uneven from the inside out, unless a fixed mixer, stir it and stir to melt temperature is an effective tool.

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            Peter  Huang