Introduction To Several Important Principles Of Plastic Extrusion---Part 3

1. The pressure at the end of the screw is very important

     This pressure reflects the resistance of all objects downstream of the screw: the filter screen and the plate that contaminates the crusher, the adapter conveying pipe, the fixed agitator (if any) and the mold itself. It depends not only on the geometry of these components, but also on the temperature in the system, which in turn affects the viscosity and throughput speed of the resin. It does not depend on the screw design unless it affects temperature, viscosity and throughput. For safety reasons, temperature measurement is important-if the temperature is too high, the die and mold may explode and harm nearby people or machinery.

    Pressure is good for mixing, especially in the last zone (metering zone) of a single screw system. However, high pressure also means that the motor must output more energy-therefore the melt temperature is higher-which can specify a pressure limit. In a twin screw, the two screws bite into each other, making it a more effective agitator, so no pressure is required.

    When manufacturing hollow parts, such as pipes made with a chuck mold that uses a bracket to locate the core, high pressure must be generated in the mold to help separate logistics reorganization. Otherwise, the product along the weld may be weak, and problems may occur in use.

2. Output = displacement of the last thread +/- pressure flow and leakage

    The displacement of the last thread is called positive flow, which only depends on the geometry of the screw, screw speed and melt density. It is regulated by the pressure flow, which actually includes the resistance effect of reducing the output (indicated by the highest pressure) and any feed overbiting effect that increases the output. The leakage on the thread may be in either direction.

    It is also useful to calculate the output per revolution per minute, because this represents any drop in the pumping capacity of the screw in a certain period of time. Another related calculation method is the output power per horsepower or per kilowatt. This represents efficiency and can estimate the production capacity of a given motor and drive.

3. Shear rate has an important effect on viscosity

    All ordinary plastics have shear drop characteristics, which means that the viscosity of the plastic decreases as it moves faster and faster. The effect of some plastics is particularly obvious. For example, when the thrust is doubled, the speed of some PVC will increase by 10 times or more. On the contrary, the shear force of LLDPE does not decrease too much. When the shear force doubles, the shear speed only increases by 3 to 4 times. The reduced shear reduction effect means high viscosity under extrusion conditions, which in turn means more motor power is required. This can explain why the operating temperature of LLDPE is higher than that of LDPE.

    The flow rate is expressed by the shear rate. In the spiral groove, the shear rate is about 100s-1; in most molds, the shear rate is between 100s-1 and 100s-1; the gap between the thread and the barrel wall and some small In the mold, the shear rate is greater than 100s-1. The number of melts is a commonly used method of viscosity measurement, but it is the opposite (for example, flow/thrust, not thrust/flow). Unfortunately, when the shear rate is less than or equal to 10s-1 and the melt flow rate is very fast, it may not be a true measurement.

4. The motor is opposite to the cylinder, and the cylinder is opposite to the motor

     Why is the control effect of the cylinder not always the same as expected, especially in the measurement area? If the cylinder is heated, the viscosity of the material layer on the cylinder wall will decrease, and the energy required for the motor to run in a smoother cylinder will also be reduced. The motor current (amperage) decreases. On the contrary, if the cylinder cools, the melt viscosity at the cylinder wall will increase, the motor must rotate harder, the current will increase, and some of the heat discharged through the cylinder will be sent back by the motor.

    Generally speaking, the barrel regulator does have an impact on the melt, which is what we expect, but the impact anywhere is not as great as the regional variable. It is best to measure the melt temperature to really understand what is going on.

    The last one does not apply to dies and dies without screw rotation. This is why external temperature changes are more effective there. However, these changes are not uniform from the inside to the outside, unless they are stirred in a fixed stirrer, which is an effective tool for melt temperature changes and stirring.

Nanjing Kelongweier plastic extruder manufacturer, located in Nanjing, China, covers an area of 7000 square meters, has 20 years of experience in technology, and has mature formulas. Welcome to consult.