About Injection Molding Machine Screw and Barrel System

Composition of injection molding screw barrel

The important 5-piece set of injection molding screw barrel includes: barrel, screw, rocket head, rubber meson, check ring.

Requirements and functions of various accessories for injection molding barrels

1. Three small parts of screw head

The three small parts of the screw head play the functions of sealing, kneading, plasticizing and other functions in the whole screw assembly, which is an important part. The quality problems of many plastic products are closely related to the three small parts of the screw head. Often, failure to deal with the details of many three small parts will greatly increase the scrap rate of plastic products. It is even forced to shut down for maintenance, causing a great loss of benefits to production.

The overall surface of the three small parts of the screw head is required to be smooth. Except for the assembly part, any place that can be seen from the front can’t be right-angled, acute-angled or dead angled, and the transition should be smooth. The size of the smooth R depends on the situation, because the wrong angle is not only easy to crack, but also easy to accumulate material. And that will make the flow of raw material not smooth enough. After a long time of use, it is easy to have black spots and turn yellow. The three small parts are the most prone to yellowing. Many PC, PMMA and other yellowing phenomena are caused by the abnormality of the three small parts.

2. The two ends of the rubber ring

This design is also very important. Now many injection molding machine manufacturers design the joint surface of the rubber ring and the meson to be flat and have no slope. This has two disadvantages:

 Planar contact is not as large as inclined contact, so the service life is much shorter than that of inclined contact.

The junction between the meson without the bevel and the screw head becomes a dead angle, and the raw material will have black spots and yellowing if it stays for a long time. The best slope is 30 degrees. 15 degrees is not big enough and it is easy to accumulate material. The 45-degree contact surface is large, and it is not easy to accumulate material. But it is easy to burst through the rubber ring when injecting glue.

 The gap between the left and right sides of the rubber ring should be moderate, depending on the raw material to be produced, choose a larger or smaller gap. The key point of glue injection of many large machines for thin-walled products is unstable because the gap between the left and right movement of the rubber ring is too large. There are also many PMMA and PC raw materials that make rattling noises during production. This is because the gap between the left and right movement of the rubber ring is too small, resulting in unsmooth discharge.

3. Screw head

The surface of the threaded part does not have to be as hard as other parts. If it is too hard, the thread will easily fall off and the strength of the thread will decrease. It should be blocked nitrogen protection before heat treatment, or the thread should be processed after nitriding. The undercut of the threaded part is preferably an R of the arc, rather than being cut directly by a right-angle turning tool. Then the stress is better, and many screw head fractures are in this part.

Injection molding screw structure

The relationship between screw and plasticizing quality

Main Factors Affecting Plasticizing Quality

L/D slenderness ratio   Compression ratio   Back pressure   Screw speed   Electric heating temperature setting.

① L/D slenderness ratio

L/D slenderness ratio = screw working length / screw diameter

If the slenderness ratio is large, the feeding will be even, but it is easy to overdo it. Plastics with better thermal stability can use longer screws to improve kneading performance without worrying about burning and yellowing. For plastics with poor thermal stability, a shorter screw or the screw with no thread at its end can be used.

Requirements for L/D slenderness ratio of various plastics:

  • Thermosetting: 14~16 rigid PVC, high viscosity PU, etc.
  • Thermal sensitivity: 17~18
  • General plastic: 18~22PC,PPS,PC+ABS, etc.
  • High temperature stability plastic: 22~24

Requirements of color mixing ability on L/D slenderness ratio:

  • For 12~16, it is advisable to form the dyed rubber particles to avoid color difference.
  • 16-18 color masterbatches are kneaded, dyed, and molded in the material tube with uniform quality and less color difference.
  • 20-24 Coloring materials are mixed and dyed in the material tube, and the dispersion is uniform, which has a better protective effect on the physical properties of the finished product.
Injection molding screw

② Compression ratio

Compression ratio = feed tooth depth / metering tooth depth

Consider the compressibility of the material, the degree of feed, reflow, the compactness of the product, heat transfer and exhaust.

Appropriate compression ratio can increase the density of the plastic and make the molecules more tightly combined. It helps to reduce the intake of air, reduce the temperature rise caused by pressure, and affect the difference in output. Improper compression ratio will destroy the physical properties of plastics.

The higher the compression ratio, the higher the temperature rise in the plasticizing process of the plastic in the barrel. In addition, it produces better mixing uniformity for the plastic in the gelation process, and the relative output volume is greatly reduced. High compression ratio is not suitable for fusible plastics, plastics with low melting viscosity and poor thermal stability.

1- Nozzle 2- Screw head     3- Check ring     4- Barrel

5- Screw     6- Heating ring     7- Cooling water ring

③ Back pressure

Increasing backpressure has the following effects:

  • It can increase the work done by the screw on the molten resin.
  • Eliminates unmelted plastic particles.
  • Increase the density and uniformity of raw materials in the material tube.
  • Reduces problems such as injection shrinkage and warpage.

Back pressure is used to improve the uniform mixing of plastics, and its effect is most obvious.

  • If the back pressure is too large, the plastics with high heat sensitivity are easy to decompose and cause color difference. For low-viscosity plastics, anti-adhesive phenomenon may occur.
  • If the back pressure is too small, there may be air bubbles in the finished product. The rotation speed of the screw directly affects the shear of the plastic in the spiral groove.

The groove of the small screw is shallow, absorbing the heat quickly enough to make the plastic soften in the compression section. The frictional heat energy between the screw and the tube wall is low, suitable for medium and high speed rotation, and increases the plasticizing capacity.

Large screws are not suitable for fast rotation to avoid uneven plasticization and excessive frictional heat. For plastics with high heat sensitivity, if the speed of the injection screw is too fast, the plastic will be easily decomposed. Usually, the screw of each size has a certain speed range, generally 40-250 rpm.

Leave a Comment

Contact Us