Injection Molding



Introduction

Injection molding is the most typical method of processing plastics.
It is widely used for complicated plastic products.
In injection molding, the resin is heated, melted, and injected into the mold that has the cavity shaped as desired at high speed and pressure.
It then cools down and is ejected from the mold.
To manufacture products in optimal conditions requires the right combination of resins, dies, injection molding machines and peripherals that complement one another.
To provide guidance on this, we will introduce you to the injection molding methodology using the plasticity of plastics, injection molding machines, molding variables, molding processes, and standard injection conditions.



Selection of a Molding Machine

To be able to make quality products, you must choose an injector right for your product.
One of the things you have to take into consideration when selecting an injector is its capacity.
  + Injector capacity: The capacity of an injector is determined by the maximum clamping force and the maximum injection quantity.
  + Clamping force : The maximum pressure required to keep the die from opening when the molten resin is fed into the die.
  + Injection quantity : The maximum injection quantity per injection
 Mold plate area : The maximum area onto which the die can project.
You should be able to manufacture products of normal quality with 40-70% of the maximum clamping force and 40-70% of the maximum injection quantity (30-80% for large-sized products).
You can mold larger products, but it may hurt the product stability and the life of the molding machine.
Therefore, it is desirable to stick with the above percentages.
The required fastening force can be calculated by means of the required unit injection pressure for individual resins (kg /㎠) and the projection area of the product. Use the stability coefficient of 1.2.



Structure of a Molding Machine

The most common screw injection molding machine consists of the fastening device, the injector, the hydraulic device and the electrical device.

Clamp : It is the device that opens and closes the die of the molding machine by applying pressure during resin feeding to keep the die from opening.

There are the hydraulic clamp, the toggle or mechanical clamp, the combination clamp, and the electric clamp.

Injector : It plasticizes the raw material and injects it.

It is composed of the screw, the nozzle, the hopper, the heater, the die and the peripherals.

Hydraulic device : It provides locomotion for the injection of the injection molding machine and for the forward and reverse movement of the individual parts of the clamp.

It consists of the hydraulic pump and the plumbing hydraulic pump.

Electrical device : It refers to the electric control board.




Injection Molding Processes


1. Clamping the mold to the die.
    This process readies the injection machine for operation.

    injection Molding Processe 01.gif


2. Injection
    The revolution of the screw is stopped.
    It is a high-speed, high-pressure rectilineal movement.
    If the screw turns at this stage, the check ring, a.k.a. shut-off ring is worn or cracked.
    Most of the cavity (about 95%-97%) is filled at this stage.

     injection Molding Processe 02.gif


3. Holding pressure
    The resin shrinks as it cools down in the die. Pressure is applied to compensate for this residual stress.

     

4. Cooling, plasticizing, and measuring
    The screw turns, melts the resin and moves measured amounts of the molten resin to the end of the screw 

    with the nozzle in contact with the die, while the plasticized resin is cooling down in the die. 

    At this time the melt packs the screw, and reverses the screw by its repulsive power.

     injection Molding Processe 03.gif


5. Ejection
    The molded product is ejected as the die is opened.
    Ordinarily, when the measurement is done and the screw stops, the die opens while the nozzle recedes.
    However, in the event of serious drolling, the screw stops without the recession of the nozzle.

     injection Molding Processe 04.gif





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