Injection molding of plastics

Injection molding of thermoplastics is the most widespread technology in the processing of plastics. Are produced, in a discontinuous manner, pieces of different shape and size and weight varying from a few grams to several kilos.

The main advantages of this technology are:
  • the ability to produce parts with complex geometries;
  • high productivity;
  • lower costs of production;
  • the high automation of processes;
  • the possibility of producing pieces of very small size;
  • molds and presses can be used with various plastic materials;
  • the ability to print in combination with metal inserts;
  • production of manufactured goods with different colors and materials.
The main disadvantages are:
  • the high cost of the molds and presses;
  • the lack of knowledge of the relationship between process and properties;
  • the inability to immediately determine the quality of the printed particular;
  • the high competitiveness.

The plastic material to be transformed is the fundamental component that, after a possible pretreatment drying or dehumidifying, is aspirated through a system of power within the cylinder plasticizing.

Inside the cylinder is located a screw which, by rotating  and translating by means of hydraulic pistons, creates a friction that, combined contribution to the heat generated by the electrical resistors located on the plasticizing cylinder, causing the melting of the material.

After the material has reached the necessary viscosity, is injected at a certain speed within the mold (injection phase or filling), passing through appropriate channels and filling the cavity that represents negative in the piece.
Filled the cavity starts the maintenance phase during which the polymer is held under pressure for the purpose of compensate with other material density increases associated with the decrease in temperature and solidification that occur during  the cooling of the workpiece.

The molten polymer enters the cavity through the injection point or gate; solidification of the polymer to the gate determines the end of maintenance phase.
Once the gate has solidified, it can not enter more polymer in the cavity irrespective of the pressure that exerts  the screw and starts the cooling phase during which the artifact continuous solidification.

The molding cycle

Injection molding is a discontinuous process, then it will be composed from phases that constitute a cycle. The phases of a molding cycle are:

  1. Closure mold : in the phase of closing the movable platen of the press is closed by tightening the two halves of the mold according to the following sequence.
  2. Approach injection unit : the combination of the injection takes place during the first cycle performed semi-automatically.
  3. Injection : the molten material located in the plasticizing chamber is pushed by the screw within the cavity of the mold with a certain speed of lamination.
  4. Maintaining : the molten polymer that is inside the mold is kept under pressure so as to prevent the backflow of material from the mold to the plasticizing chamber.
  5. Dosage : the screw is rotated to allow the material to pass from the hopper to the storage area (the area next to the nozzle of the plasticizing cylinder) and finding the nozzle in front of the solidified material, the material for thrust pushes back the screw.
  6. Cooling : is the phase in which the press does not move. The duration of the cooling time is a function of the type of polymer used and the geometry of the article and must allow the material to solidify within the mold.
  7. Open mold and extraction : opens the mobile part of the mold with the movable platen of the press; arrived at the stroke end comes into operation the extraction of the press that acting on the extraction system of the mold ejects the workpiece printed.
  8. Closure mold : finished extracting the moving platen is closed again and the cycle starts again.