Manufacturing Technology: Plastics Shaping Processes

Plastics Shaping Processes

• Plastics can be shaped into a wide variety of products, such as molded parts, extruded sections, films, and sheets, insulation coatings, and fibers for textiles.
• Plastic-shaping processes can be classified into 8 types based on the resulting geometry:
  • Continuous extruded products with constant cross section
  • Continuous sheets or films
  • Continuous fibers (filaments)
  • Solid molded parts
  • Hollow molded parts
  • Discrete parts made of formed sheets or films
  • Castings
  • Foamed products

The Polymer Melt and its Properties

• To shape a thermoplastic, it must be heated to a softened consistency, called the polymer melt.
• Important properties of the polymer melt include:
  • Viscosity, which affects the flow of the polymer in the cavity or extruder channels.
• The polymer melt can be mixed with fillers such as glass, asbestos, talc, and carbon to enhance its properties.

Limitations of Injection Molding

• Intense industry competition can result in low profit margins.
• Mold costs are high.
• Molding machinery and auxiliary equipment costs are high.
• Lack of knowledge about the fundamentals of the process can cause problems.
• Lack of knowledge about the long-term properties of the materials may result in long-term failures.

Types of Molding Processes

• Compression molding is used for thermosetting plastics and produces a wide variety of products, including electrical and domestic fittings.
• Transfer molding is used for thermosetting plastics and produces parts with small metal inserts and varying section thickness.
• Rotational molding is used to produce hollow objects, such as containers and toys, and is economical for large production quantities.

Product Design Considerations

• Designers must be aware of the limitations of plastic materials, including:
  • Strength and stiffness
  • Impact resistance
  • Service temperatures
  • Thermal expansions
  • Degradation

Injection Molding Process & Equipment

• Injection molding equipment evolved from metal die casting.
• The injection molding machine consists of two principal components: the plastic injection unit and the mold clamping unit.
• The plastic injection unit is similar to an extruder and consists of a barrel, screw, and non-return valve.
• The clamping unit operates the mold.

Film and Sheet Extrusion

• Sheets have a thickness ranging from 0.5 mm to 12.5 mm.
• Films have a thickness below 0.5 mm.
• The method of manufacturing is similar for both, using conventional extrusion.
• Cooling methods include water quenching and chill rolls.

Blown-Film Process

• Blown-film extrusion uses air pressure to form thin tubes.
• Used for high production of thin tubular films.

Blow Molding

• Blow molding is a molding process that uses air pressure to inflate soft plastic inside a mold cavity.
• Used for producing high quantities of thin-wall products, such as bottles.
• Consists of two steps: production of a starting tube (parison) and inflation of the tube to the desired final shape.

Thermo-Forming

• Thermoforming is a process that heats and deforms a flat thermoplastic sheet into the desired shape.
• Used for packaging and fabrication of large items, such as bath tubs and refrigerators.
• Consists of two main steps: heating the plastic sheet and deforming it into the desired shape.

Test your knowledge on the different types of plastic-shaping processes used to create a wide range of products, from molded parts to insulation coatings. Learn about the various methods used to shape plastics and their resulting geometries.