Compression Molding Process,Injection Molding, Blow Molding & Transfer molding

Compression Molding process, Transfer molding, Injection & Blow Molding-PDF: The types of molding processes are based on the type of plastics we use. As we know that there are two types of plastics, one is Thermoplastics and the other is Thermosetting plastics and depending upon the nature of plastics, molding process is divided into 4 types.

1.Compression Molding Process:

  • Compression Molding is a molding method in which the molding material(especially Thermoplastics or Thermoset Plastics) are generally preheated and are placed in a mold cavity.
  • The heating or cooling given to the cavity is dependent upon the type of plastic you are using.Compression Molding Diagram:The below diagram represents Compression Molding Process for both Thermoplastics as well as Thermoset Plastics.
    Compression Molding Process-Thermoplastics & Thermosets,Applications
    Compression Molding Process-Thermoplastics & Thermosets, Applications

    Compression Molding for Thermoset Plastics:


    As It is used for both thermoplastics and thermosetting plastics, we can classify them under two cases and they are as follows.

    Case 1:(For Thermoset Plastics)

    • In this Process, the Thermoset liquid will be kept in the mold of a required shape of the component and is subjected to the heating process.
    • During heating, the liquid will undergo the following changes i.e. liquid will undergo in the form of gel and by the application of pressure, it turns to solid.
    • After that, the Compression load is applied at the gel condition of the liquid so that shaping of the component takes place.
    • Heating converts the liquid into solid whereas, compression load will give the shape to the component.

    Compression Molding for Thermoplastics:

    Case 2:(For ThermoPlastics)

    • The Thermoplastic molten liquid will be kept in the mold of the required shape of the component and subjected to compression followed by a cooling process of the mould.
    • Compression gives the shape of the component whereas cooling converts the molten liquid into solid.
    • Density and the Strength of the plastic can be controlled by varying the compression load “P”.
    • Small and simple shapes of the component can be produced with uniform density.
    • Larger and complex shapes of the components are difficult to produce with uniform density.
    • Difficult to insert metal pieces in the plastic part during the production of the component.
    • Compression Molding-Applications and Products:
      • Outer envelopes of a washing machine
      • Refrigerators
      • Automotive Exterior Panels
      • Gaskets
      • Helmets
      • Automobile auto parts etc.

      This is the detailed explanation of Compression Molding Process and is well explained by the help of Thermoplastics and Thermoset Plastics. If you have any doubt or if you want to add any material to the above concept feel free to ask from the comments section.


2. Injection Molding Process-Machine, Applications or Products and Defects:

Injection Molding Process-Machine, Applications or Products and Defects: It is a manufacturing process used for producing parts or components by injecting molten material into the mold cavity. Injection molding can be performed with only one of these materials like glasses and most common thermoplastic polymers.

Note: It is applicable for thermoplastic materials only.

Other Aspects of the Injection Molding Process:

Injection Molding Machine:

Injection Moulding Process
Injection Moulding Process-Machine, Applications or Products and Defects

The Injection Molding Machine consists of

    1. Hopper
    2. Hydraulic System
    3. Motor
    4. Screw
    5. Heater
    6. Mold

Injection Molding Process:

    • The spindle of the motor rotates in a clockwise direction which is in conjunction with the gear. Now the shaft which is attached to the gear also rotates which in turn rotates the entire screw due to the shaft of the motor.
    • The Plastic granules are to be poured into the hopper and they will be pass through the chamber due to the rotation of the screw.
    • The Heater heats the granules up to its critical temperature.
    • The thermoplastic molten liquid is pressurized(by the hydraulic system) at outside the Assembly and allowed to travel through a nozzle of small diameter(Dn)[injector] with high velocity and low pressure into the space between the molds.
    • The liquid will fill the mold with uniform compaction among the atoms and thereby density is uniform.
    • After filling the liquid in the mold, by the cooling process, it will be solidified.
    • Any shape and any size of the component can be produced with uniform density.
    • Density can be controlled by varying the pressure in the pressure line and thereby production rate is high and wastage is recyclable.
    • The component can be produced any number of times till it achieves the required shape.


    • The thermoplastic molten liquid has low viscosity and thereby it can flow easily and that indicates that the diameter of the nozzle is small.
    • In the case of thermoset liquids, the viscosity is high and thereby it is difficult to flow and that indicates that the diameter of the nozzle is large so as to avoid the obstruction.

Injection Molding Products:

The products which are produced are as follows.

    • Kitchen Products
    • Agriculture Products
    • Plastic Buckets
    • Industrial Pump Impeller
    • Water Cooler Parts
    • Medical Products

Injection Molding Defects:

The 10 Defects are as follows.

    1. Flash
    2. Short Shots
    3. Vacuum Voids
    4. Sink Marks
    5. Burn Marks
    6. Weld Lines
    7. Surface Delamination
    8. Warping
    9. Jetting
    10. Flow Lines

These are the Injection Molding Defects observed in products produced from Injection Molding Process.

3.Blow Molding Process-Pressure Distribution, Applications, Differences:

Blow Molding: It is a manufacturing process by which hollow plastic parts are formed. It is also used for forming plastic containers, plastic buckets, bottles or other hollow shapes.

It is applicable for thermoplastics only.

Blow Molding Diagram:

The figure shown below illustrates the complete Blow Molding Process in A, B and C segments. It generally consists of

Blow Molding
Blow Molding
  1. The feed from the Extrudes
  2. Melted Plastic
  3. Extrude Head
  4. Air tube
  5. Parison(Tubular shape of hot plastic)
  6. Mold
  7. Air Pressure
  8. End Product.

Blow Molding Process-Pressure Distribution, Applications, Differences, etc:

The Feed from the Extrude is given as an input into the Blow Molding Machine. The feed acts as the melted plastic and it is to be passed through the extrude head section and forming it into a parison.

The parison is a tube-like piece of plastic with a hole at one end through which compressed air can pass. The parison is to be passed through the mold. The Cope and Drag portions of mold are to be pressed against each other and at the same time compressed air is to be passed from the air tube.

By blowing high-pressure air on the soft sheet(parison), it will bend according to the internal shape of the mold and deposits on the internal surface of the mold and achieve the shape of the component. By the cooling process, the sheet will regain its hardness.

This is how you can get the final product in the segment C as shown in the fig.

Pressure Distribution(P):

  • Water Bottles ;P<1 bar
  • Plastic Buckets;P=1-2 bar
  • Large Liquid Containers; P=2-5 bar

The simple analysis of the component can be produced with a good dimensional accuracy of the product


  • Difficult to produce Complex geometry of the components with good dimensional accuracy.
  • To produce high thickness plastic parts, high blowing pressure is required.
  • If the shape of the mold is complex that implies result distribution is non-uniform that implies compaction of the film is non-uniform and that implies thickness is nonuniform and that implies dimensional accuracy is low.

Blow Molding Products/Applications of Blow Molding:

The applications are as follows.

  • Water Bottles
  • Plastic Buckets
  • Liquid Containers
  • Plastic Cups
  • Mugs etc.

Difference between Injection Molding and Blow Molding:

blow molding is used to produce hollow singular containers, such as bottles. On the other hand, injection molding is used to produce solid pieces, such as plastic products.

4.Transfer Molding Process:

    • One of the types of the molding process is Transfer Molding and is applicable only for thermoset plastics.
    • Thermoset liquid is pressurized at outside the mold assembly and transferred into the space between the molds with uniform pressure and velocity and that indicates compaction among the atoms will be uniform and that implies density is uniform and that implies the strength is uniform.
    • After filling the liquid in the mold by the heating process, it will be converted into solid.
    • The density and the strength of the plastic can be controlled by varying pressure “P”.
    • Any shape and any size of the components can be produced with uniform density.
    • Metal pieces can be inserted in the plastic part easily during production.

Ex: Electrical switches, plugs, sockets, etc.

This is the explanation of 4 Types of Molding Processes-Compression Molding process, Transfer, Injection & Blow Molding process which was explained in a detailed way.

Original Source

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