Injection molding of plastics
Injection molding of thermoplastics is the most widespread technology in the processing of plastics. The main feature of this technology is the continuous production of pieces of different shape and size and weight varying from a few grams to several kilos.
- the ability to produce parts with complex geometries , helped and supported by the know how and the experience of a reliable supplier that has good knowledge about the relationship between process and properties of the raw material;
- high productivity;
- lower costs of production than can compensate in short time the cost of the tool to produce the parts;
- the high automation of processes supported by mould flow analysis to create the better tool to join the best product desidered;
- the possibility of producing pieces of very small size;
- molds and machines can be used to plastify various materials;
- the possibility to overmould parts and various inserts;
- production of manufactured goods with different colors and materials. high competitiveness.
The plastic raw material to be transformed is the fundamental component that, after a possible pre-treatment 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, causes the melting of the material.
After the material has reached the necessary viscosity, it is injected at a certain speed within the mold (injection phase or filling), passing through channels called “runner” and filling the cavity, that represents negative in the piece.
Filled the cavity the maintenance phase starts : in this phase the polymer is hold under pressure for the purpose of compensate with other material the part.
The molten polymer enters the cavity through the injection point or gate; the 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 even if there is some pressure . In this moment starts the cooling phase during which the solidification completes.
The molding cycle
Injection molding is a process composed of different phases that constitute a cycle. The phases of a molding cycle are:
- Closure of the mold : in the phase of closing ,the moving part of the machine closes against the fixed part and press the two sides of the mold one against the other.
- Approach of injection unit : the combination of the injection takes place during the first cycle performed semi-automatically.
- 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 and it fills the cavity .
- 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.
- Maintaining : the molten polymer that is inside the mold is kept under pressure to prevent the backflow of material from the mold to the plasticizing chamber.
- Cooling : is this phase the tool remains closed to cool the part inside and make it stable. 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.
- Open mold and exjection : the mobile part of the mold with the moving side of the machine opens ; the ejection system of the mold ejects the parts and they fall down.
- Closure of the mold : when the ejection phase has finished, the tool is closed again and a new cycle starts again.