PHASES OF OPERATION

In injection moulding, we can describe the following three phases of operation:

· Plastication
· Injection
· Cooling

We will discuss how to determine the optimum moulding parameters for each phase of operation. As always, it is a good idea to refer to machine manufacturer information when setting up the machine parameters on the machine that you will be using.

3.3.1 Plastication

We begin with the first stage of melting the plastics material.

3.3.2 Barrel Temperature Settings

When selecting barrel temperature settings, refer to the lowest processing temperature that the material supplier has recommended. Moulding at a low temperature results in a shorter cycle time and reduces the chances for material degradation.

This lower processing temperature requires a higher injection pressure to fill the mould. However, the efficiency of the operation increases with a slightly faster cycle time, and the final product will have better moulded-in properties.

3.3.3 Heat Profile Settings

The heat profile settings along the barrel will determine how the plastics will melt. Plastics such as polyethylene, polypropylene, ABS, and PVC can be set up so that the lowest temperature is in the feed zone and the highest temperature is in the metering zone. This type of profile is referred to as a forward profile and is the most common profile used in injection moulding.

For nylon, acetal, PET, and PBT, the zone temperatures are fairly constant, creating what is called a straight profile. For materials that have a tendency to drool, such as nylon, a reverse profile may be used- with the lowest temperature in the metering zone and the highest in the feed zone.

3.3.4 Screw Speed

The actual melt temperature of the plastics will be higher than the barrel temperatures, due to the shearing action between the barrel and the screw. As a result, screw speed is a critical injection moulding parameter to control melt temperature because 70 to 90% of the heat required to melt the plastics
comes from this shearing action. Screw speed determines the rate at which the plastics pellets melt. It is necessary to maintain a slow, steady, consistent speed to evenly melt the material.




To determine the correct screw speed, refer to the processing information supplied by the material manufacturer. It is important to use the lowest possible setting that will allow for uniform melting. A high screw speed may overheat and degrade the plastics material. This will cause dimensional problems as well as a reduction in the physical properties of the material.

3.3.5 Screw Back Pressure

Screw back pressure causes the screw to make more revolutions, and the additional revolutions of the screw create more shear heat, which can cause material degradation.

You should use little or no back pressure with glass or mica filled materials because it breaks up the filler and as a result, reduces moulded part quality. This increased back pressure also accelerates the wear on the barrel and screw.

For material using a color concentrate, extra back pressure can be used to assist in dispersing and mixing the colorant more evenly. Extra back pressure can also be used for screws with a short L/D ratio of 15:1. However, screws with higher L/D ratios require very little back pressure as more mixing takes place.

3.3.6 Suck Back

After screw recovery, suck back prevents the nozzle from drooling. Too much suck back may cause splay or bubbles on clear parts or black parts.