The barrel is a steel chamber that houses a reciprocating screw or plunger screw that feeds material. It is designed to handle the high pressures associated with injection molding. There are holes drilled into the barrel for placement of the thermocouples (T/C’s), and a larger opening towards the back end of the barrel known as the feed throat.
The two illustrations below (Figure 3.1 and Figure 3.2) show how different clamp styles, hydraulic and toggle, support the molds, and what effect each can have on the process. The one thing to remember, whether using hydraulic or toggle, is that the front half platen will always be the weakest point and create issues while molding. The reason the front half is the weakest is the large hole where the locating ring and the nozzle must enter the platen.
The ejection system on an injection molding press plays a significant role in the removal of parts from the mold. There are many different forms of part ejection. There is ejection from the press by which ejector bars run from the ejector plate through the platen and into the back of mold, tying into the ejector plate. Hydraulic cylinders are put on the mold to provide this ejector function by moving a stripper plate forward and back. There is also a mechanical function ejection system that operates with the movement of the mold.
The purpose of this test is to establish the optimum temperature for the rear barrel zone. This is an easy Excel chart that can show what is the optimized temperature for the fastest recovery. Use the recovery time to create the chart and label the X and Y axes correctly. In the example shown in Figure 5.1 below, notice that 480 °F and 6.6 seconds recovery time is the optimized temperature with the lowest recovery time.
The purpose of these tests is to optimize the process as much as possible, and to get an understanding of how each aspect will affect the part. Through this optimization process you can establish high and low limits along with the centerline process.
There must be two things available when talking about plastic flow: pressure and flow. Pressure enough that the process does not become pressure limited and sufficient flow available to achieve the velocity asked for.
The plastic pressure is what to measure on the process. It is calculated as the maximum hydraulic pressure multiplied by the intensifying ratio of the press (Ri). This will apply to the fill pressure, pack pressure, hold pressure, and back pressure. Even though the hydraulic pressure and the intensifying ratio will change from press to press, the plastic pressure (specific pressure) generated will remain the same because it is plastic pressure in front of the screw that we are trying to control.
The whole purpose of cooling is to run the fastest cycle possible with the most dimensionally stable part. Think of the cooling rate of the plastic and what effect it has on the physical properties of the plastic, fast cooling rate, slow cooling rate, and differential shrinkage. Let’s examine how and where the water lines are hooked up and their placement.
By performing this benchmarking procedure before the start of the next production run, the capability of the molding process to produce an acceptable product can be determined. This is part of the qualification procedure for injection molding tools.
Terms and definitions from RJG Master Molder series
Accumulator: An auxiliary oil source for fast delivery of plastic melt. Used on injection molding machines. Where oil is stored in a container under pressure and used on a molding machine to boost injection rate.
