Whether it is the automotive industry or the medical industry, the manufacturers need several plastic parts to complete their productions. No doubt, injection molding is one of the excellent processes to meet your plastic part demands.
However, the technique is not as simple because of a series of complex steps that must be followed to attain the target product. This article covers the detailed working of the injection molding machines and equipment used in the process.
Definition of Injection Molding
The manufacturing technique that deals with producing molded parts by injecting heated molten plastic material into a mold under high pressure and controlled temperature is called injection molding. Once the molten plastic cools, it solidifies and acts as a finished product. Usually, this whole process takes place in specialized injection molding machines. The sizes of these machines vary according to the tons of clamping force required to hold the molds.
You can easily find injection molding applications in the automotive, electronics, packaging, medical, and construction industries.
What are the Basic Types of Injection Molding?
There are two significant types of injection molding:
- Hot runner injection mold
- Cold runner injection mold
Hot Runner Injection Mold
The hot runner injection mold is the type of injection molding that requires a temperature above the melting point of the plastic to heat the mold. The biggest plus point of this hot runner system is that the plastic present in the runners will never solidify. Additionally, there are fewer chances of material waste during the process, and it offers more complex-shaped products than the cold runner system.
However, the hot runner injection mold is expensive to set up and demands high maintenance costs.
Cold Runner Injection Mold
In a cold runner injection mold, cold temperature is used to cool the runner. As one production cycle ends, plastic and runner are ejected from the mold. Compared to the hot runner system, this type consumes much waste after each cycle. The cold runner injection mold is further divided into two types:
- Two-plate cold runner
- Three-plate cold runner
What Equipment and Tools are Involved in Injection Molding?
Injection molding machines utilize various components, including a power source, injection unit, clamping unit, and mold assembly, to perform different stages of the process. Let’s talk about some equipment and tools in detail.
1. Injection Unit
The injection unit is the equipment that not only plays a role in injecting the material into a mold but also heats the material into a molten state. The following are the components of the injection unit:
Hopper is a large compartment at the top of the injection unit that stores raw plastic pellets. The open bottom of the hopper leads the material into the next section.
Next comes the barrel, which offers both heating and injecting of the plastic into the mold. Usually, a ram injector or a reciprocating screw is responsible for heating and injecting mechanisms. The nozzle present at the end of the barrel introduces the molten plastic into the mold.
2. Clamping Unit
The clamping unit provides clamp force to the two mold halves for closing. While setting the injection molding machine, each half of the mold is attached to the platen (a large plate). The front half of the mold is called a mold cavity, while the rear one is known as the mold core. Typically, the mold cavity is fixed to a stationary platen aligned with the nozzle, and the mold core is attached to a movable platen along the tier bars.
While operating, the clamping motor activates the clamping bars; consequently, the movable platen moves towards the stationary platen. At this point, enough force is provided to these molds to hold them close. However, the rear half of the mold also carries the ejection system of the injection molding machine.
Now, let’s talk about the tools used in the injection molding process.
1. Mold Base
The mold base is the tool that holds the mold cavity and mold core. A support plate, sprue bushing, and a locating ring are on the front half of the mold base. At the same time, the other half contains the ejection system and a support plate.
2. Mold Channels
As the plastic is in a liquid state, it needs to flow in the machine. Thus, for its passage, various channels are manufactured while designing the mold. The first entrance of the molten plastic into the mold takes place through the sprue. Later, runners (additional channels) take the molten plastic from the sprue to different cavities. A gate is present at the end of each runner.
Furthermore, cooling channels are also present in these machines. Their purpose is to carry the water through mold walls in order to cool the molten plastic.
3. Mold Design
In addition to the structures mentioned above, there are many more designs of molds. For instance, it must possess complex features like undercuts or threads. The undercuts might be external as well as internal. Yet, for molding the threads into the component, you also need to have an unscrewing device.
How Does Injection Molding Process Work?
The injection molding process is not as simple as it seems to be. It involves many steps and parameters which need to be handled tightly to obtain positive results. The following are the major stages involved in working the injection molding process.
1. Choose the Right Thermoplastic and Mold
Thermoplastics and molds are the critical elements for creating the final products, so you need to be pretty conscious while selecting them. In order to make the right choice, it is essential to consider the mutual interaction of thermoplastic and mold. Sometimes the chosen plastic is not suitable for specific mold designs.
Typically, the mold tool consists of a cavity and a core. The cavity is fixed into which the plastic is injected, whereas the movable core is adjusted into the cavity to give the final shape to the product. The first step is to create a prototype mold tool so that you can test it with the selected thermoplastic in the machine. Doing so will inform you about the properties, temperature, and pressure resistances of the thermoplastic you’re using. Thus, you can go for the one which is most appropriate for your mold and project.
Most commonly, the following thermoplastics are used in the injection molding process:
- Nylons (PA)
- Acrylonitrile-Butadiene-Styrene (ABS)
- Polycarbonate (PC)
- Polypropylene (PP)
- Aliphatic Polyamides (PPA)
- Polyoxymethylene (POM)
- Polymethyl Methacrylate (PMMA)
- Polybutylene Terephthalate (PBT)
- Polytetrafluoroethylene (PTFE)
- Ethylene tetrafluoroethylene (ETFE)
- Perfluoroalkoxy (PFA)
- Polyvinyl chloride (PVC)
2. Melting the Thermoplastic
After deciding on the thermoplastic, it is converted into a molten state. The raw pellets of thermoplastic are inserted into the hopper of the machine. When the screw turns, the pellets slowly move into the barrel. Thus, the heat from the barrel melts the thermoplastic and turns it into a molten form.
Before moving towards the injection step, you must close the two halves of the mold attached to the injection molding machine using the clamping unit. The hydraulic pressure from the clamping unit provides enough force that keeps the halves of the mold closed while injecting the material into them. The larger the machine, the greater will be the clamping force.
4. Injecting the Thermoplastic into the Mold
Once you clamp the molds together, the injection step can be started. The molten plastic from the barrel reaches the end; as a result, the gate closes, and the screw moves back. As soon as the injection and clamp pressure reaches a certain point, the gate opens, which moves the screw forward, and the liquid plastic injects into the mold. The operators need to ensure that the temperature remains consistent throughout this stage.
After injecting all the molten plastic into the mold, the pressure is maintained for an estimated period. This can be termed as holding time that varies according to the thermoplastic type and characteristics of the component. Its primary purpose is to confirm the plastic packing of the tool and its best formation.
As the holding phase ends, the pressure is released, and the cooling time starts. The hot plastic is allowed to cool in the mold for a few seconds to some minutes to solidify it.
6. Ejection and Finishing
The next step after cooling is the ejection of the product from the tool. The plates in the machine eject parts into a container or, at the bottom of the machine, onto a conveyor belt. The final stage is to provide an excellent finish to the product by removing excess plastic. You can use various finishing techniques to enhance the appearance of the final part, including polishing and dying. Finally, the part is ready for customer use.
Moreover, the waste material during this process can be reinjected in the next cycle, resulting in material cost savings.
Finally, you are here with enough information about the injection molding process. You can produce pretty complex mold designs using this technique. However, you must go for an experienced and reliable service provider that can easily overcome any technical hurdles throughout the process.