INJECTION MOULDING PROCESS
Injection moulding is a process of producing moulded objects by injecting molten plastic ingredients into a mould, allowing them to cool and solidify before being removed from the mould. Suitable for mass manufacture of items with complex geometries, this technology plays a significant role in the field of plastic processing and is becoming more popular.
WORKING PRINCIPLE OF INJECTION MOULDING
Injection moulding process is generally used to achieve high speed moulding of thermoplastic. It works on the principle that the molten thermo plastic is injected into a mould and a high pressure is applied the desired shape is obtained. Plunger system is used for achieving high pressure.
The material that is to be used for moulding is loaded into hopper from where it is transferred to the heating section. It is transferred using a feeding device where the temperature is about 150°C – 370°C.The material is melted and it is forced by an injection ram or by a plunger through a nozzle in a closed mould.
TYPES OF INJECTION MOULDING PROCESS
There are two types of injection moulding. They are,
- Ram or Plunger type Injection moulding process
- Screw type Injection moulding process
RAM OR PLUNGER TYPE INJECTION MOULDING
This type of injection moulding system has two units. They are the injection unit and clamping unit. In some cases these units may be split inorder to eject the finished component. First the selected thermosetting plastic is filled in a hopper. Then it goes to the heating section where the thermo setting plastic is melted and the pressure is increased. With the help of ram the heated material is injected under pressure. This makes the material to get filled in the mould cavity through the nozzle to get the required shape of the plastic. In this type of moulding system water is generally is used for cooling purpose.
SCREW TYPE INJECTION MOULDING
This type of moulding also has two units to split and eject the finished components. The two units are injection unit and clamping unit.
The injection unit consists of hopper, screw and the heating section. Mould cavity is present in the clamping unit. In this type of moulding machine the pellets are first fed into the hopper. The resins are pushed along with the reciprocating screw which is in the heated condition. The screw is moved forward to push the plastic material into the mould. The screw itself moves backward and allows the accumulation of enough material to fill the mould. The rotation of the screw provides the plasticizing action by frictional and shearing effects. The axial motion of the screw provides the filling action.
The jet moulding process is used for finding the problems occurring in the injection moulding process. The reaction moulding is the recent development in injection moulding. In reaction moulding, the low viscosity monomers are used in the mould. There is a chemical reaction taking place between the resins at low temperatures and a polymer is created.
In the jet moulding process the thermosetting plastic is preheated about 93°C in the cylinder surrounding nozzle. The reaction moulding is suitable for the production of polyurethane moulding.
It used in the manufacturing of parts of complex threads. Production of intricate shapes like thin walled parts is made easier using this process. Mainly used for the Production of electrical and communication components like telephone cables.
PROCESS STEPS OF SCREW TYPE INJECTION MOULDING
When it comes to evaluating component design, tool fabrication, and the effective manufacture of moulded plastic parts, the various steps of the injection moulding process must be taken into consideration. There are several elements and settings that we will not discuss in detail here, but the fundamental procedure remains the same. Let us begin with the fundamentals.
STEP 1: THE MOLD HAS BEEN CLOSED
When the mould is closed, the timer for the Injection moulding cycle starts. Each half of the mould is divided into two parts: a fixed component and a movable component. The mould design must make it possible for the pieces to be ejected with ease.
Install the mould in a certain equipment, such as an injection moulding machine, to ensure proper operation. The two halves of the mould are firmly pushed against one another throughout the moulding process.
In a plasticizing screw (also known as an endless screw), heated material is poured into a plasticizing screw. The pellets are softened by the movement of the screw in conjunction with the temperature, and the pellets are converted into molten plastic material. Prior to injection, the molten and flexible material is held in a reservoir at the front of the screw.
In other circumstances, such as when robots are used, the cycle goes “part to part,” which implies that the cycle starts and ends when the robot gets a new component or when the new part hits the conveyor belt, depending on which is correct.
STEP 2: INJECTION OF PLASTIC MOLTEN MATERIALS
Under the influence of heat, the heated softened plastic components are injected into the mould under high pressure. During the melting process, displaced air is expelled via vents in the injection pins and along the parting line when the melt enters the mould. During this step, it is critical to ensure that the mould is entirely filled with material before the substance begins to solidify.
In order to compensate for the withdrawal that happens as the material cools, we must continue to convey the material under pressure. The design of the runner, gate, and vent is critical in ensuring that the mould is adequately filled.
STEP 3: COOLING THE MOLDED PRODUCTS
After the material has been poured into the mould, the item is allowed to cool for the precise period of time required to solidify the plastic. The amount of time it takes to cool a component depends on the type of resin used and the thickness of the item. Each mould is equipped with internal cooling or heating lines, which circulate water through the mould in order to maintain a steady temperature throughout the mould.
STEP 4: PLASTICIZING THE RESIN
In this time, the barrel screw retracts and sucks more plastic resin into the barrel from the material hopper, all while the item is cooling. The heater bands ensure that the barrel temperature is maintained at the appropriate level for the type of resin being used.
Cooling of the whole ejected moulded item by use of cooling ducts inside the mould cavity. The item is released from the mould as a result of this process.
STEP 5: EJECTION OF MOLDED PARTS
Ejector pins are propelled forward when the mould opens up and the ejector rod is moved. The component slips out of the mould and is falled in a bin that is situated below the mould.
STEP 6: REMOVING THE RUNNER AND PACKING THE PRODUCT
Despite the fact that the injection moulding machine’s cycle comes to a stop at Step 5, the procedure continues. Periodically, the machine operators, or robots, remove the useable components from the remainder of the runner that was left over. Mold cavity filling is accomplished by means of the runner, which is the route taken by the plastic.
To decrease expenses and environmental effect, many runners are crushed and recycled after they have been used once or twice. It is then necessary to weigh and count the useable pieces in preparation for assembly or delivery.
Keep in mind that hot runner systems do not always produce trash or waste since the route is kept at a constant temperature throughout the moulding process.