What is injection cycle?

1. The cycle starts with- Closing the injection mould

In the simplest application, the injection molding machine closes the mold and applies pressure. The force of this clamping is set by the operator and is measured in kN and depends on the profile of the part, the size of the mold and the capabilities of the machine. During this process, the machine monitors how much pressure it applies as a safety check. Also, a closing "profile" can be used where the operator sets different speeds as the mold closes. This profile setting is used more as shapes become more complex.

In more advanced applications, injection molds will have other actions, hydraulics, side systems, and developing operations that work in tandem with the mold closing operation. They can work outside of time, position, or mechanically with form. In general, it is important to find a balance between fast closing and safety. It's not worth pushing the machine to its limits to produce a few more details in this rush, but it certainly has its place in the industry.

2. Injection

During injection molding, the screw is driven forward, pushing the melted plastic from the screw/cylinder into the mold cavities. The material and pressure must be of a precise amount to fill the mold normally. We try to balance speed with consistency of process and quality of detail. Injection that is too fast or too slow can cause a myriad of part defects. This concept seems simple enough, but it's actually a lot more complicated than that.

We can even create an injection profile and see a graphical representation of this on each cycle. If necessary, we can reduce or accelerate the speed of the screw in certain places. For example, you may want to slow down or speed up when the plastic reaches the inlet or certain areas of the part. Having such a high level of control can make all the difference in producing consistent, high quality parts.

A key aspect of injection molding is when to move to the next stage of pressing and holding under pressure. This is also known as transfer point or V-P switching. This means the point where you switch from pushing a volume of plastic into the mold at a certain rate to holding it with a certain amount of pressure, for a certain amount of time. As a general rule, the mold cavities are almost completely filled (approximately 95% or slightly more) during the injection molding phase. The remaining 5% is completed in the V-P phase or during the pressure hold. Of course, for details with a complex shape, there is an exception.

There are several ways the injection machine can know when to make this switch. This can be done by screw position (most commonly), pressure limit, time or cavity pressure. Although the position is common, the most modern and accurate way is to have cavity pressure sensors. Cavity pressure sensors control the injection molding machine on each cycle through feedback, relaying real-time information from inside the mold. The disadvantage of this system is that it requires an external control system and each form requires additional components, which increases the cost and.

3. Compaction and Retention under pressure

Now that the part is almost full, we need to compact it to the end and keep the pressure of the material under pressure until the hopper cools. Pressures that are too low can cause dent marks in the finished product, and pressures that are too high can cause burning or even damage to the mold. It is also important that the design and size of the sprue channel is properly matched to the part.

The sprue must be cool enough to insulate the part of the cylinder with the warm material. The presence of too large sprue channels can increase the cooling time. The sprues that are too small can freeze too quickly, interrupting material flow and causing short strokes or failure to fill the part.

The additional pressure is necessary to ensure that the molten plastic reaches all parts of the mold and assumes the correct (stretched) shape. In parallel, the cooling time of the already formed part also runs. Since the material enters the mold at a high temperature (160 to 320) degrees Celsius, it is necessary to ensure cooling in the tool itself. These are most often technological openings in the forming plates, through which coolant flows with a temperature guaranteeing proper and rapid cooling of the part. Cooling channels are also placed on other parts of the mold (inserts, jaws, punches, plates, etc.). On many injection molds, there is a cooling system even on the stationary/bottom and top plates.

All this is done in order to make the cooling time of the injected part as short as possible.

4. Cooling / plasticizing

After the densification and holding stage, the part is finally filled, but is still probably too hot to be removed from the mold. The cooling stage is simply time given to the mold to absorb heat from the plastic part. Once again, we try to balance cycle time against part quality and consistency. Large parts, thick-walled sections, or the use of materials with low thermal conductivity are more likely to deform or change after molding if adequate cooling time is not provided.

During the cool-down period, the injection molding machine begins reloading and compacting plastic material into the cylinder to prepare for the next cycle. The screw begins to rotate, which carries the raw material down the cylinder/auger. As the material moves forward, it pushes the screw back (see image). As the material is transported down the screw, both the heat from the cylinder heaters and the rotation of the screw are used to melt the plastic.

The main variables used for plasticization are screw speed and back pressure. RPM is simply the speed of rotation of the screw, and back pressure is the amount of resistance that the injection machine applies to the movement of the screw. Having high screw speeds and back pressure can mix the material better and raise the temperature of the plastic, but it can also degrade and overheat the plastic. Generally, there is enough cooling time in a cycle so that screw speed and back pressure can be set without worrying about adding cycle time.

5. Open the form

After the cooling phase is over and the injection molding machine is ready for the next cycle, the mold is opened. In its simplest form, the syringe mold opens, nothing exciting. However, more complex tools may require core pulling, air blowing and control, hydraulic core development, or opening profiles. These other aspects of the open mold are controlled by either form position or timing during the form opening phase. Like mold closing, the speed can be varied at different positions and the injection molding machine monitors the amount of force applied to open the mold.

6. Extraction of the work-piece

After the mold is opened, the details must be removed from it. This can be done with a single forward stroke of the ejection system or it may require multiple ejection strokes or even robotics. If a robot/manipulator is used, it is set according to the cycle of the injection machine, at which point the parts or the sprue should be picked up, during their removal from the mold. In some applications, the ejection step may occur while the mold is opening to reduce cycle time.

Once the parts are removed, this completes one cycle of the injection molding process. The mold and machine are then ready for the next shot. Cycle speed is determined by part design, operation and mold construction. This entire process can be done in seconds or over a minute.

Other process variables will include plastic material preparation, cylinder temperatures, tool cooling temperature, heat runner system control, integrated robotics, hydraulic drive systems, and interchangeable machine components.

The challenge is to adjust all the parameters of the injection molding process to consistently produce parts of the same quality. Most of the variables that the processor has control over affect the others as well. Small changes in one phase can have an impact on another phase of the process.

Fortunately for the plastic injection molding specialists, the new machines, the technology at their disposal and when used correctly, makes the process very precise and consistent.