The Injection Unit's main objective is to heat the material to the specified temperature until it reaches a viscosity that will allow the material to flow into the mold while under force. You will notice on all injection machines there are two numbers that will give you an idea as to the size of the machine. The first number is the injection capacity. This will tell us what size part you might be able to make with regard to the volume of plastic that can be injected.
The second number is the clamping units size. This will tell you the amount of force available to keep your mold together. With these two numbers you should be able to determine the size of the machine. But first we will learn about the injection unit. The injection unit must follow a process in order to complete its task of injecting the melted plastic. That process will require the assistance of the following main parts.
End Cap on the Barrel
Non return valve
Motor to rotate the screw
Hydraulic cylinder to move the screw forward and back
Feed system to bring the plastic into the barrel
Control system for the temperature, time and speed
The injection of melt into the mold cavity is made up of three phases: the filling phase, the packing phase and the holding phase.
The injection phases can be vividly illustrated using the cavity pressure curve. Figure 9 shows the ideal curve which is achieved when the switch over to holding pressure is optimum. The switch over is sometimes called velocity to pressure transfer, where velocity refers to injection velocity and pressure to holding pressure.
Figure 9. Ideal cavity pressure curve
The filling phase starts at point 1. In the filling phase, the melt is injected into the cavity at a certain velocity. At point 2, the melt reaches the cavity pressure sensor. Due to the viscosity of the melt, pressure starts to rise. The cavity is volumetrically filled at point 3. Further screw advance compresses the melt up to point 4 when the machine switches from injection pressure to the much lower holding pressure. At the holding phase, the low holding pressure incrementally fills the cavity as the part cools to compensate for the shrinkage. At point 5, the sprue gate is frozen and the holding pressure can be removed (and the mold can be opened). 1-2-3 makes up the filling phase. 3-4 is the packing phase. points 4-5 is the holding phase. Further cooling occurs in 5-6.
Overpacking and underpacking
An overpacked cavity pressure curve is shown in Figure 10b. It is characterized by a pressure peak in the packing phase. The pressure peak is caused by the delay in switchover to holding pressure, so the high injection pressure is still applied after volumetric filling. The pressure peak is relieved at the switch over to the lower holding pressure. Here lies an often overlooked cause of flashing which is easily detected if one has cavity pressure sensing.
Figure 10. Underpacked and overpacked cavity curves
To remove the flashing, the straight forward thinking would be to increase the clamping force. Reaching the limit of the machine rated clamping force, one would even move the mold to a bigger machine. Even if the increased clamping force overcomes flashing, over packing adds weight and stress to the part and makes the part more difficult to remove. An alternative is to reduce the injection pressure. Too low an injection pressure causes defects such as sink marks. In actuality, the problem is easily and better solved by switching over earlier to get back to the ideal cavity pressure curve. In precision Injection Molding, over packing creates a reject.
An under packed curve is shown in Figure 10a. It is characterized by a pressure dip in the packing phase. The switchover occurs too early, before the cavity is volumetrically filled. Part of the filling takes place at the lower holding pressure. Subsequently, the screw advance increases the pressure. The part has reduced dimensions, is underweight, has sink marks and surface marks. Again a reject.
A device that switches over at volumetric filling would avoid the problems of over packing and under packing and produces the ideal cavity pressure curve. Switching is initiated at point 3 and completed in point 4 in Figure 9.
Methods of switchover
The available means to switch over in a modern
Injection Molding machine, in increasing order of accuracy, are
1. injection time
2. screw position
3. hydraulic pressure
4. nozzle pressure
5. cavity pressure
Injection time switchover
Temperature effects the viscosity of the melt, which presents resistance to the advance of the screw. Increased resistance slows down the screw and prevents the cavity from filling in the given injection time. On the other hand, reduced resistance would lead to over packing. Injection time switchover is the only means available in Injection Molding machines without screw position and pressure sensors.
Screw position switchover
Screw position switchover is not effected by temperature nor viscosity. This is the preferred method in machines with screw position potentiometer. Like injection time switchover, screw position switchover could be considered open-loop as screw position is not a direct measure of volumetric filling. A leaky nozzle misleads the machine computer into switching over before the cavity is filled. So could a worn screw valve and a worn injection cylinder. Furthermore, if the screw diameter is large relative to (the cube root of) the mold cavity volume, variation of 0.1 mm could give an over packed or under packed fill. Despite its deficiencies, this is the most widely used switchover method in a modern Injection Molding machine most probably because it is a standard (not optional) feature.
Hydraulic pressure switchover
The packing of the melt in the mold cavity has to be balanced by the hydraulic pressure driving the screw forward. A rise in the hydraulic pressure during injection could be used to signal the switchover. Due to a roughly 10:1 ratio between the twin injection cylinders and the screw cross sectional areas, the injection cylinder hydraulic pressure is less than the screw tip pressure by the same ratio. The pressure drop at the runners and sprue gate separates the cavity pressure from the screw tip pressure. The compressibility of the melt (between the cavity and the screw tip) delays the time the pressure is felt. As a result, hydraulic pressure is not an accurate detector of the volumetric filling point. However, hydraulic pressure switchover does have the advantage of the sensor working in a congenial environment (oil temperature below 50oC, oil pressure at system pressure, (usually 140 bars) and the sensing is independent of the mold (not attached to the mold). Hydraulic pressure sensor is usually an option in a modern Injection Molding machine.
Hydraulic pressure, nozzle pressure and cavity pressure sensing locations are shown in Figure 11.
Figure 11. Hydraulic pressure sensor in the injection cylinder
Nozzle pressure switchover
Nozzle pressure is also called injection pressure, which is the pressure of the melt in the nozzle or at screw tip. Nozzle pressure switchover is improved over hydraulic pressure as the compressibility of the melt cushion is avoided. The environment is harsher (melt temperature below 400oC, melt pressure at 1400 bars, the melt could be corrosive/abrasive), and the sensor face must be flush with the barrel interior wall. This switchover method is not often used except in research.
Figure 12. Nozzle pressure sensor
Cavity pressure switchover
The most accurate measure of volumetric filling is via cavity pressure. Two methods are in common use: direct and indirect. In direct cavity pressure measurement, a sensor in the mold senses the melt pressure in the cavity. Direct cavity pressure measurement is the more accurate of the two, but requires one to drill a hole at the mold for the sensor. Since it is inconvenient to remove the sensor, one needs to dedicate at least one sensor per mold. In a multi-cavity mold, cavity pressure measurement requires one sensor per cavity, increasing the sensor investment further.
Figure 13. Direct cavity pressure sensor
In indirect pressure measurement, a force sensor is placed behind an ejector pin the other end of which is in contact with the melt. Cavity pressure could be calculated from force/ejector pin cross sectional area. The temperature at the sensor is much less than that of the melt. With indirect cavity pressure switchover, the sensor is not dedicated to the mold (mold independent), which comes in handy when mold changing is often. It also reduces the sensor investment. Due to the friction at the ejector pin, indirect cavity pressure sensing is less accurate than its direct cousin.
Figure 14. Indirect cavity pressure sensor
Where the required quality on the surface of the molded parts does not allow marks either by the sensor or the ejector pin, a strain sensor that measures mold deformation could be used. After calibration in a test mold (which has a cavity pressure sensor), it may be used for cavity pressure measurement in the production mold (which does not have a cavity pressure sensor but has the calibrated strain sensor).
A device based on cavity pressure sensing can detect the volumetric filling point accurately. Switch over can be initiated by comparing the actual pressure with a set value equal to the cavity pressure at point 3 in Figure 9.