Injection molding machine screw is generally divided into three parts: conveying section, compression section, metering section, processing different materials should be selected from the corresponding structure of the material rod, in order to obtain a good quality of workpiece processing.

First, the role of each section of the screw

1. The role of the screw conveying section

Injection molding machine screw conveying section is responsible for the transport of plastic, pushing and preheating, the material after conveying section should reach the melting point temperature, the general screw conveying section accounts for about 50% of the total length of the screw.

The deeper the conveying section of the screw groove, the greater the conveying capacity, but the actual design needs to consider the strength of the screw.

2. The role of the screw compression section

Compression section of the screw is responsible for plastic compression, pressure and exhaust, the material after this section has almost all dissolved, but not uniformly mixed, the general compression section accounts for 20-25% of the total length of the screw.

In this section of the region of the plastic gradually melt, screw volume must be gradually reduced, otherwise the material compression is not solid, slow heat transfer, poor exhaust, but also cause the formation of the workpiece porosity.

Screw compression ratio for the feed section of the last screw groove depth and metering section of the first screw groove depth ratio, compression ratio is certain, the longer the screw compression section, the smaller the relative shear heat, the same if the length of the compression section is certain, the smaller the compression ratio, the smaller the relative shear heat, so the compression section structure is the main part of the decision to shear heat size.

Three-stage combination screw

General low shear heat of the screw compression ratio of 1.8-2.2, h2 (metering section screw groove depth) select 0.09-0.1D (screw diameter)

3. screw metering section role

Screw metering section generally accounts for 20%-25% of the total length of the screw, mainly responsible for the mixing of plastic uniformity, to ensure that all plastic melt.

The metering section is long, the plastic mixing effect is good, too long is easy to make the melt stay too long and thermal decomposition, too short, the material temperature is not uniform.

In general, we select the injection molding machine processing products, the best product weight for the injection volume of 70%-80% of the best, such as injection volume is too large will cause the melt to stay in the measurement section for a long time, resulting in thermal decomposition of the material, the final product porosity, and even affect the mechanical properties of the product.

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Second, the impact of process parameters on plastic processing

1. Melting back pressure

Increasing the back pressure of melting plastic can increase the work done by the screw on the molten resin, eliminate the unmelted plastic particles, and increase the density and uniformity of the raw material in the material tube;

The increase of back pressure can increase the shear heat of plastic in the barrel, and then increase the temperature of the barrel, so the size of the back pressure plays a certain role in the shear heat

If the back pressure is too large, the plastic with high thermal sensitivity will be easily decomposed, and the plastic with low viscosity may produce flowing phenomenon, on the contrary, if the back pressure is too small, the plastic will not be compressed, and the workpiece will have pores.

2. Dissolving speed

Screw speed directly affects the plastic in the spiral groove shear, small screw groove shallow absorb heat source fast enough to soften the plastic in the compression section, the friction between the screw and the barrel is low, suitable for high-speed rotation to improve the shear heat and increase the plasticizing ability.

Large screws are not suitable for fast rotation, so as not to plasticize unevenly, resulting in transition friction heat, usually a certain range of speed for each size screw, the general speed of 100-150rpm is too low to melt the plastic, too high will be the material burn

3. Heating ring temperature setting

The temperature generated by the heating ring to melt the cold hard plastic trapped between the barrel and the material rod, in order to facilitate the rotation of the screw and provide part of the heat required for the melting of plastic.

In summary, there are several ways to reduce the shear heat of injection molding:

-Lower melt back pressure.

Negative effect: It may cause the melt density to decrease and the workpiece is prone to form pores.

-Reducing the melting speed.

Negative effect: Reduces productivity.

-Reducing the compression ratio or lengthening the length of the compression section.

Negative effect: Replace the screw and increase the cost.

-Increase h2 (depth of the screw groove of the metering section).

Negative effect: Not easy to process and requires post-processing surface treatment.

-Increase the temperature setting of compression section and metering section.

Negative effect: May cause material flow delay.

-Add a cooling fan at the metering section of the injection molding machine.

Negative effect: Affects the appearance of the equipment and increases the cost of electricity.