About Injection Molding Machine Screw

This post introduces all the issues about the injection molding machine screw including its materials, segments, parameters, quality evaluation standard, how to choose and how to use.

1. Classification of injection molding machine screw materials

(1) HPT fully hardened powder alloy screw

Using tungsten-cobalt powder alloy tool steel, it is formed by overall hardening treatment through a special process. The overall hardness of the inside and outside of the screw is the same, reaching about HRC65°. It is suitable for processing all kinds of transparent products, precision products and LCP, PPS, PES, PPA and other special high-temperature and highly corrosive plastics with added fibers.

(2) Bi-metal screw

SACM645, 38CrMoAlA is used as the base material, and then the crest or screw groove is ion-welded with tungsten carbide nickel-based alloy. The surface hardness of the screw is about HRC60°. It is suitable for processing various transparent products and engineering materials such as PA6 and PA66 with added fibers.

(3) Electroplating screw

SACM645 and 38CrMoAlA are used as the base material, and the outer surface is electroplated with a layer of hard chromium, nickel, titanium and other elements after molding. The hardness is around HRC60°. Suitable for processing PC, PMMA, PET transparent materials and corrosive plastic U-PVC materials.

(4) Stainless steel screw

The material is 9Cr18MoV special stainless tool steel, which adopts overall quenching + overall hardening treatment, and the overall hardness is about HRC55°.

(5) SKD61 screw

Using Hitachi SKD61 material, processed by special process, the product has good toughness and strong torsion resistance.

(6) Ordinary nitriding screw

SACM645 and 38CrMoAlA are used as the base material and formed by special processing, the depth of the nitride layer is 0.5-0.8mm, and the overall hardness is HV980~1080°. Suitable for processing ordinary plastics, such as PP, ABS, PS, AS, PVC and other common plastics.

2. Injection molding machine screw segment description

Injection molding machine screw three-stage diagram ▼

Injection molding machine screw segment

The screw of an injection molding machine can generally be divided into:

① Feeding section

② Compression section

③ Homogenization section (also called a metering section).

(Note: The ratio of the three sections of different screws is different, the depth of the screw groove is different, and the transition form of the bottom diameter of the screw is different)

(1) Screw feeding section

The depth of this section of the screw groove is fixed, and it is responsible for preheating, conveying and pushing of plastic solids. It must be ensured that the plastic begins to melt at the end of the feed section.

(2) Screw compression section

This section is the thread depth of the tapered screw. Its functions are plastic raw material melting, mixing, shear compression and pressurized exhaust. The plastic will completely dissolve in this section, and the volume will shrink, so the design of the compression ratio is very important.

(3) Screw homogenization section

This section is the fixed groove depth of the screw groove. Its main functions are mixing, melt conveying, and metering. It must also provide sufficient pressure to maintain a uniform temperature of the melt and stabilize the flow of molten plastic.

3. Injection molding machine screw parameter

(1) D—screw diameter (usually represented by Φ)

The size of the screw diameter directly affects the plasticizing ability and the theoretical injection volume.

(2) L/D—screw length-to-diameter ratio

L is the effective length of the threaded portion of the screw. Under the premise of a certain screw diameter, the larger the L/D, the longer the thread length. It directly affects the thermal history of the material in the screw, and also affects the ability to absorb energy.

If the L/D is too small, it will directly affect the melting effect of the material and the quality of the melt. If the L/D is too large, the transmission torque and the energy consumption will increase.

(3) L 1—length of feeding section

The length of L1 should ensure that the material has enough conveying space. Because the overly short L1 will lead to premature melting of the material, making it difficult to ensure stable pressure conveying conditions. So it is also difficult to ensure the plasticizing quality and plasticizing capacity of the subsequent stages of the screw.

(4) h1—the depth of the screw groove in the feeding section

If h1 is deep, it can accommodate more materials, which improves the feeding amount and plasticizing capacity. But it will affect the plasticizing effect of materials and the shear strength of the screw root. Generally, h1≈(0.12~0.16)D.

(5) L3—the length of the melting section

The length of L3 is conducive to the fluctuation of the melt in the screw channel, which can stabilize the pressure, making the material discharged from the head of the screw with a uniform amount of material, generally L3=(4~5)D.

(6) h 3—Metering section screw groove depth

Small h3 and shallow screw groove improve the plasticizing effect of the plastic melt and are beneficial to the homogenization of the melt. But overly small h3 will lead to overly high shear rate and overly large shear heat, which will cause the degradation of molecular chains and affect melt quality. If h3 is too large, due to the enhanced reflow effect of the screw back pressure during pre-plasticizing, the plasticizing ability will be reduced.

(7) S—pitch

Its size affects the helix angle, thereby affecting the conveying efficiency of the screw groove, generally S≈D.

(8) ε—compression ratio

ε=h1/h3, that is, the ratio of the depth h1 of the screw groove in the feeding section to the depth h3 of the screw groove in the melting section. When ε is large, the shearing effect will be enhanced, but the plasticizing ability will be weakened.

4. Quality evaluation standard for injection molding machine screw

Injection PC screw ▼

Injection PC screw

(1) Plasticizing quality

A screw must first be able to produce products that meet the quality requirements like the followings:

It has various properties that meet the requirements including qualified physical, chemical, mechanical and electrical properties.

It has the qualified appearance, meeting user’s requirements for air bubbles, crystal points, and uniformity of dyeing dispersion, etc.

Qualified plasticizing quality of the screw.

  • Whether the temperature of the melt extruded by the screw is uniform, how much is the axial fluctuation, and the radial temperature difference.
  • Is there a minimum melt temperature at which molding can take place.
  • Whether the extruded melt has pressure fluctuations.
  • Whether the dispersion of dyeing and other additives is even.

(2) Output

It refers to the output or extrusion volume through a given machine head under the premise of ensuring the quality of plasticization. As mentioned earlier, output is generally expressed in kg/hour or kg/rev. A good screw should have high plasticizing capacity (production capacity).

(3) Unit consumption

The unit consumption refers to the energy consumed per kilogram of plastic (rubber) extruded, generally expressed in N. Among them, N is the power (kW), and Q is the output (kg/hour).

The larger the value, the more energy is required to plasticize the same weight of plastic. That is, the more heating power consumed, the more mechanical work done by the motor enters the material in the form of shearing and frictional heat. Vice versa.

As a good screw, the unit consumption should be as low as possible under the premise of ensuring the plasticizing quality.

(4) Adaptability

It refers to the adaptability of the screw to process different plastics, match different heads and different products. Generally speaking, the stronger the adaptability, the lower the plasticizing efficiency.

Therefore, a good screw with both adaptability and high plasticizing efficiency is the best.

(5) Ease of manufacture

A good screw must also be easy to manufacture and low in cost.

5. How to choose injection molding machine screw

(1) Screw diameter (D)

Related to the required injection volume:

Injection volume = 1/4 × π × D↑2 × S (injection stroke) × 0.85.

It is inversely proportional to the highest injection pressure and directly proportional to the plasticizing capacity.

(2) Conveying section

Responsible for the conveying, pushing and preheating of plastic, and it should be guaranteed to be preheated to the melting point. Crystalline plastics should be long (such as POM, PA). Amorphous materials should be second (such as PS, PU, ABS). Heat sensitivity should be the shortest (such as PVC).

(3) Compression section

It is responsible for the mixing, compression and pressurized exhaust of plastics.

The raw materials passing through this section are almost completely melted, but not necessarily uniformly mixed.

In this area, the plastic gradually melts, and the volume of the screw groove must decrease accordingly to correspond to the decrease in the geometric volume of the plastic. Otherwise the material pressure is not solid, the heat transfer is slow, and the exhaust is poor. The compression section generally accounts for more than 25% of the working length of the screw.

But this section accounts for about 15% of the nylon screw, about 40% of the plastic screw with high viscosity, fire resistance, low conductivity, and high additives, and 100% of the PVC screw. , so as not to generate intense shear heat.

(4) Metering section

Generally, it accounts for 20%/25% of the working length of the screw. It is to ensure that the plastic is completely melted, the temperature and the mixing is uniform. If the metering section is long, the mixing effect will be better. But if it is too long, the melt will stay for too long and cause thermal decomposition. If it is too short, the temperature will be uneven.

Heat-sensitive plastics such as PVC should not stay for too long to avoid thermal decomposition. So a shorter metering section or no metering section can be used.

(5) Screw groove depth

The deeper the feeding screw groove, the greater the delivery volume, but the screw strength of the injection molding machine needs to be considered. 

The shallower the depth of the metering groove, the higher the plasticizing heat and the higher the mixing performance index. However, if the depth of the metering screw groove is too shallow, the shear heat will increase. 

Besides, the self-generated heat will increase, and the temperature rise will be too high. This will cause discoloration or burning of the plastic, which is especially unfavorable for heat-sensitive plastics.

  • Gradient screw – the compression section is longer, accounting for 50% of the total length of the screw. The energy conversion is moderate during plasticization. It is mostly used for plastics with poor thermal stability such as PVC.
  • Mutant screw – the compression section is shorter, accounting for about 5% to 15% of the total length of the screw. The energy conversion is more severe during plasticization, and it is mostly used for crystalline plastics such as polyolefin and PA.
  • Universal screw – can be adapted to the processing of various plastics.

6. How to use the screw of injection molding machine

Injection molding machine barrel diagram ▼

Injection molding machine barrel

1-Nozzle             2-Screw head            3-Check ring             4-Cylinder            5-Screw                6-Heating ring                                  7-Cooling water ring

(1) Do not start the machine if the barrel has not reached the preset temperature. The temperature of the newly opened electric heater is generally required to reach the set value for 30 minutes before operating the screw.

(2) If the machine is shut down for more than half an hour each time, it is best to close the discharge port and clean the material in the barrel, and set up heat preservation.

(3) Avoid foreign matter falling into the barrel and damaging the screw and barrel. Prevent metal fragments and sundries from falling into the hopper. If processing recycled materials, a magnetic hopper is required to prevent iron filings from entering the barrel.

(4) When using the anti-saliva, make sure that the plastic in the barrel is completely melted, so as not to damage the transmission system parts when the screw moves back.

(5) Avoid screw idling, slipping and other phenomena.

(6) When using new plastic, the rest of the barrel should be cleaned. When using POM, PVC, PA+GF and other materials, minimize the degradation of raw materials. And then, rinse them with ABS and other outlet materials in time after shutdown.

(7) Avoid mixing POM and PVC into the barrel at the same time, which will react at the melting temperature and cause serious industrial accidents.

(8) When the temperature of the molten plastic is normal but black spots or discoloration of the molten plastic are continuously found, check whether the screw check ring (passing apron, meson) is damaged.

7. Common problems and solutions

(1) Slippage

In the stage of screw pre-molding, when the screw rotates in the barrel to convey the material along the direction of the screw and retreats to accumulate the material for the next injection, the screw will also slip. If the screw starts to slip during the pre-molding phase, the axial movement of the screw will stop while the screw continues to rotate.

The main reason for the slippage of the injection molding machine screw is that it is difficult to press the material at the feeding port and the length of the barrel cannot form sufficient adhesion, so the screw will slip.

There are two solutions to the slippage of the screw of the injection molding machine:

Add a small amount of material to clean the end of the barrel, and check the melting temperature at the same time. A short residence time will cause the melting temperature to be lower than the set value of the barrel temperature.

Observe the molded products. If there are black spots or light stripes or marble patterns, it means that the materials have not been well mixed in the barrel.

(2) No cutting

During injection molding production, the screw may rotate but does not discharge, and normal production cannot be completed. The common reasons and solutions for the screw of the injection molding machine not to discharge:

The discharge port of the barrel is blocked – check whether there is a melted plastic block sticking.

The temperature control is inaccurate and the temperature of the rear end of the barrel is too high – adjust the temperature setting, and check whether the cooling water circuit is blocked.

Lugging occurs on the screw – the plastic wraps the screw and rotates together.

Too much oil is added to the plastic, causing the screw to slip.

Too many recycled materials are added to the plastic.

The wear of the screw and the barrel and the wear of the apron may cause the screw of the injection molding machine to fail to feed, resulting in plastic leakage and not being conveyed to the front of the barrel.

If the plastic particles are too large, bridging will occur – crush the plastic again.

If the newly replaced screw does not discharge, it may be that the design of the barrel and the screw discharge port is not appropriate:

For the screw, the screw groove in the feeding section of the screw is too shallow. So the screw cannot drive the plastic forward when it rotates.

It may be the problem that the amount of material conveyed is very small.

For the barrel, the cause may be the design of the discharge port.

Injection end point parameter of injection molding machine▼

Injection end point parameter

During the use of injection molding technicians:

The parameters must be set strictly according to the material melt temperature.

After shutting down, the gun barrel must be cleaned according to the regulations.

It is necessary to check whether the injection end point is stable every day.

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