Plastic Selecting Process


While processing thermoplastic melts the following factors should be taken in to account in order both to process efficiently and obtain quality product.

  1. Water absorption of raw materials
  2. Physical form of raw material
  3. Thermal stability of polymer
  4. Flow properties
  5. Adhesion of melt to metal
  6. Thermal properties affecting, heating & cooling of melt
  7. Compressibility and shrinkage
  8. Frozen in orientation

Water Absorption:

  • Water/Moisture is the greatest enemy for processing of plastics
  • Hydgroscopic materials
  • Absorption phenomena Ex. Nylon, POM, PC
  • Adsorption phenomena Ex. HIPS, PS, ABS
  • All these materials should be pre dried
  • Non hygroscopic material Ex.PVC, Polyolifien etc
  • Need not be pre dried. Except when completely wet during monsoon.

Actions Necessary:

  • Granules as soon as the bag is opened
  • Pre drying ovens, Hoper drier, dehumidifying drier can be used
  • For PC-Dehumidifying drier preferable

Physical form of Raw material:

  • Powder form, Granular form, lumpy/slab form
  • Slab form calandering, compression molding
  • Granular form preferred uniform pallet size ensures even an faster feeding
  • Powder form difficult in feeding but shaving in cost because of the ability to avoid palleting stage, special feeder attachment essential to ensure proper feeding.

Thermal Stability of Polymers:

  • PVC thermally sensitive material: Little higher melt temp. May lead to deformation-HCL is released this can leads to corrosion and harmful to human being. PID temperature controlled can be used.
  • PMMA, POM up on depredation liberates MMA and formaldehyde respectively-MMA voltilize and cause bubbles-Formaldehyde gas causes eye irritation.
  • PVC & POM (Acetan) should never be process one after the other, this may lead to explosion.

Adhesion of Melt to Metal:

  • Waiting of the polymer melt against the metal wall of processing equipment can lead to strong adhesion of polymer to metal Ex. Difficulty in removing PVC-Mix form two roll mill.
  • PC has a strong adhesion to metal. It can take away the skin of the barrel if not properly purged.

Thermal Properties:

In the case of polymer melts the specific heat various with temperature. For crystalline polymers such as POM, Nylon etc. Latent heat of fusion and SP. Heat should be taken in to account. That is total heat content (enthalpy) = LH of fusion + specific heat.
Table: Thermal Properties

thermal properties of polymer
thermal properties of polymer

Cooling Shrinkage and compressibility:

When polymers are in molten stage the vibration of the molecules result in the polymer chain being pushed apart so that the volume occupied by a given polymer mass is higher than when the material is solid.

Table: Density of Polymer:

density of polymer
density of polymer

Frozen in Orientation:

When polymer melts are being shaped by either injection molding or extrusion the long polymer chains tend to be elongated or uncoiled in the direction of flow. After shaping, the melt is usually cooled rapidly and there is seldom time for the oriented molecules to return to a random coiled shape by the process known as relaxation. Some orientation is thus frozen the products. Such stress parts are very weak. Hence annealing is must.


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