Injection molding machine screw design & It’s Operations In depth guide [2023]

In this article i will talk about the Injection molding machine screw designs & operations in Injection molding machine.

Injection molding machine Screw

“screw is basically a helically flighted hard steel shaft that rotates within a plasticizing barrel to mechanically process and advance the plastic being prepared.” It’s rotating drive system can be powered by a hydraulic or electric motor. The use of electric motors tends to increase the melt-processing efficiency and thus the production rate.

General Purpose screw for Injection molding

There are general-purpose and dedicated screw designs. The type of screw used is dependent on the plastic material to be processed. 

The screws are usually a simple appearing device, but it accomplishes many different operations at the same time. These include 

1. Conveying or feeding solids;
2. compressing, melting, and pressurizing melt; and  
3.   Mixing, melt refinement, and pressure and temperature stabilization. 

A simplified version of the screw plasticating process follows and is divided into the three sections or zones as shown in above figure. 

• Feed section
• Transition section
• Metering section    

Feed Section

Unmelted plastic in pellets or another fermenter at the beginning of the feed section. The plastic is carried forward in the same manner as grain in a farm auger the screw flight is angled in the direction of travel through the solid resin particles.

As the resin proceeds further down the feed section, a densifying (compaction) occurs as the pellets or particles are pressed more closely together. The movement of solid is always enhanced by anything that increases the friction between the plastic and the internal surface of the barrel or decreases the friction between the plastic and the surface of the screw. In other words, it feeds well if it adheres to the barrel and slips on the screw. 

Material effects The form of the material entering the feed hopper or feed section has an effect on processing success. Powders and fluffy regrinds, for instance, generally lead to more feed and processing difficulties than pellets, cubes, and heavier regrinds. 

Transition Section 

The transition, or compression, section of a screw is the portion where the depth charges from the deep feed section to the shallow metering section. Because of a number of things that happen here, the design of the transition is critical to the performance of the screw.

Some of the functions performed by the transition are (1) melting, (2) compaction and elimination of voids, and (3) pressure build-up. 

Melting Although melting occurs in the feed and metering sections, most of it takes place in the transition section. As the channel depth is decreased, the solid plug of plastic is compressed and rubbed against the heated barrel surface.  This provides efficient frictional heating and melting plus some additional but less efficient conductive heating. Compaction and elimination of voids in the transition section the polymer changes from compacted pellets with air spaces between to melted polymer without air bubbles.

Metering Section 

The metering section controls the output of a properly designed and operated molding screw. The term “metering” comes from the idea of a constant-depth section metering out a smooth and exact amount of plastic. The concept is much like a mechanical gear pump metering out oil or any other fluid in precise and constant amounts.

The metering section should accomplish at least the first of the following: 

1. metering a uniform output, pressure, and melt temperature.
2. some final melting.
3. melt refinement pressure holding in the barrel.

Shear rate 

Most of the energy that a screwing part of the plastic material is by means of shear. The plastic is sheared between two surfaces moving in relation to each other. These surfaces are the barrel inner wall and root of the screw.

The actual shear rate at any single point along a screw can be calculated using the following formula.

“S=DN/19.1*h”

• Where S = shear rate (reciprocal seconds)
• D = screw diameter (in.)
• N = screw speed (rpm)
• h = screw channel depth (in.)

Pressure holding 

The pressure at which as the crew can pump through a die or fill an accumulator depends largely on the configuration of the metering section. The longer and shallower the metering section, the greater its ability to maintain constant output or sustain pressure created at the end of the transition section. 

FAQ about injection molding machine screw

Conclusion

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