When you are trying to decide whether a larger extruder can get the output you need, keep these calculations convenient. #Best Practices

Sooner or later, every processor will consider expanding the size of the extruder to get more output. Extruder designers know how increasing the inner diameter of the barrel can increase potential output. But for many others, it is not so easy to understand. The old "resistance flow" or output equation can be used to approximate the output at open discharge (no head). For a single screw, it is:

Resistance flow = [0.95 × π2 × D2 × H × (LW/L) × RPS × (sin Ѳ × cos Ѳ)] 1/2

D = Aperture, inches.

H = depth of metering channel, inches.

RPS = screw speed per second (revolutions per second)

Note that the aperture (D) is squared in the equation. So if we keep all other terms the same, use the same polymer, and only increase the pore size from 3 inches to 4 inches, what is the potential output difference? The effect of head pressure must be considered first, because it will be subtracted from the output.

The result of just squaring the hole size and keeping all other terms the same is (4)2/(3)2 = 1.77. This means a potential output of 4 inches. The extruder is at least 77% larger than the 3-inch extruder. Extruder.

However, you also need to increase the channel depth (H) proportionally for larger screw diameters to maintain similar shear rate, melting rate, melt temperature, and melt quality. Designers have a variety of methods to expand the channel depth to match the expansion of the aperture. Basically, it specifically matches the shear rate in the metering channel. Using channel depth estimation for amplification when processing the same polymer will further increase the expected output by 1.3:1, totaling (1.77 x 1.3) = 2.3. So start with 3 inches. To 4 inches. It is expected that the pore size will more than double the output.

This method has been used many times in the past to obtain a good initial estimate of the zoom screw output. I used it in designs ranging from 3 lb/hr to 120,000 lb/hr for screws with diameters ranging from half an inch to 18 inches, and it worked well. Again, this is an estimate rather than a complete answer, but it is very useful. Assuming the same L/D, many other factors need to be considered in the final design, such as the geometry of the feed and compression parts, head pressure and mixing.

The smaller the zoom in/out, the more accurate it is.

After the output is estimated, it can also be used to calculate the required drive power to achieve a similar motor load designed to scale. Power is a strictly defined quantity, which is almost always monitored on the extruder, and because the extruder is a relatively closed system, the power to heat and melt the polymer with an appropriate screw design is largely related to The output is proportional.

Once the screw starts to rotate, almost all the energy entering the polymer comes from the shear heat generated by the screw rotation, not from the heat conducted by the barrel, because the polymer has poor thermal conductivity. If this is not the case, you can simply turn on your barrel heater and start the screw as soon as the set point is reached, instead of waiting several hours until it "immerses".

The smaller the scale-up/down ratio, the higher the accuracy, because a very large step size may not explain some differences in heat transfer, and in a very small screw barrel, due to the small distance, heating can contribute more energy input于 Heat transfer.  

About the author: Jim Frankland is a mechanical engineer who has been involved in various types of extrusion processes for more than 50 years. He is now the President of Frankland Plastics Consulting, LLC. Contact jim.frankland@comcast.net or (724) 651-9196.

Most extrusion operations generate waste, and in the case of thermoformed sheets, it can exceed 70% of the total output.

If you know the drive power of the machine and the thermal characteristics of one of the materials that will be run, you can roughly estimate the potential output.

A key goal during extruder startup and shutdown is to prevent degradation of the polymer remaining in the extruder and downstream components (such as screen changers, adapters, and molds).

© 2021 Gardner Business Media, Inc. Privacy Policy [Login]