Injection Molding Machine

Injection Molding: Principle and Machinery

Injection molding concept  

               Melt plastic, flow into mold and take part shape, cool, demold

Characteristics:

1. Injection molding makes parts in discrete (discontinuous) process
2. More injection molding machines used for plastic processing than any other equipment
3. Almost all thermoplastic and some thermosets materials can be injection molded
4. Process is automated with little post-molded finishing and highly repeatable.
5. Injection molding parts are finished with little post molding operations
6. Very complex parts can be made

7.  Machines are expensive

8.  Molds are expensive, usually made of P-20 steel

9.  Relatively high amount of scrap (process residue)  - to make a regrind

Injection Molding Machine

The injection molding machine comprises of three units: 

The injection/plastication unit: to heat, melt and inject the melt into the mold

The Mold and Clamping Unit:

   - Mold: where the product part is shaped

   - Clamp Unit/Press: to press and close the mold, resisting the force from the injected polymer melt (force ~ 150-1000 MPa)

3) The Control Unit:

   computerized control

   all the machine elements

Some Key Terms: for Injection Molding Machines

The control panel

The Injection Units

The injection or plastication unit heats, melts (plasticates) and injects the melt plastic into the mold (through non-returen value and nozzle).

The unit consists of hopper, plasticating screw, non-return value and nozzle

Two general types of injection units are used :

    - the reciprocal-screw type – where the screw rotate-backward when injecting the melt

    - the ram type – where the ram (plunger) is used to push off (inject) the melt

There are also two-stage injection units where the plastication part and injection part are separated, each part can also be either screw or ram type.

Types of The Injection Units

1      
                    Reciprocal Screw Injection 

2                 Ram Injection

Check Valve or Non-return Valve

Check Value: prevent the resin flow when the screw is advanced during resin injection.

The Mold Part

The mold is a steel unit that is mechanically fasten (e.g. bolted) in the clamping unit, but can be replaced for different molding shapes.

 The mold components consist of

1) Cavity/Part/Impression: where the molded products take shaped.  Can have single or multiple cavities in a mold.

2) Channels: where the polymer melt flows

            - Sprue : a channel from nozzle,

            - Runners : run from sprue to individual cavities

            - Gates : narrow gaps at the end of each runner, at the entrance to the cavity. 

3) Cooling channels/Chiller : where cooling water flows to reduce heat

4) Ejector pins : to remove the molded parts from the mold cavities.

   * The mold is very expensive( need a great care & clean storage).

Components of The Mold

A-Plate: stationary plate,  holds cavity insert, sprue bushing, and runner systems

B-Plate: moving plate, holds the core insert and space for ejector pins.

 Picture of The Mold Base

Stationary and Moving plates of the mold:

If the threaded part is being molded, the moving plate needs to have a threaded core pin and unscrewing retraction is performed to reject the molded part.