Polymer Processing: Zenamarkos B. (PHD)

Polymer Processing
Zenamarkos B. (PhD)
Chemical Engineering Program

Faculty of Chemical and Food Engineering
Bahir Dar Institute of Technology (BiT)
Bahir Dar University
May, 2022
Contents
Chapter 4: Plastic Processing Technologies
 Introduction to plastic engineering
 Plastic processing
 Extrusion
 Injection Molding
2
Course Goal
Polymerization
EXTRUSION
4
Single Screw Extrusion
• Extrusion is one of the most widely used polymer processing operation.
• Aim: melting, homogenizing and pressurizing the polymer melt.
 Can be combined with various shaping steps.
5
Twin Screw Extrusion
 Contains two screws.
 Perform same elementary processing steps as single screw with improved
mixing due to screw-t-screw interaction.
6
Twin Screw Extrusion
 Use interchangeable screw elements, segmented barrel section.
 Modula design makes the machine flexible.
 Co-rotating and counter-rotating configurations are possible.
7
Polymer Processing
Polymer processing:
 engineering activity concerned with operations carried out on
polymeric materials or systems to increase their utility.
 deals with the conversion of raw polymeric materials into finished

products.
 involves not only shaping but also compounding and chemical

reactions leading to macromolecular modifications and morphology
stabilization.
=> ‘‘value-added’’ structures
8
Elementary Steps in Polymer Processing
Raw Material Finished Product
9
Melting
Melting:
the process of bringing polymers, commonly in particulate form,
from the feed temperature to the desired processing temperature
range, appreciably above the glass transition temperature (Tg) for
amorphous polymers and above the melting point (Tm) for
semicrystalline polymers.
 Melting in polymer processing is a very important elementary step, not

only because it is often the rate-controlling step, which consumes 70–
80% of the total processing energy input, but also because it determines to
a large extent the product quality related to homogeneity and stability
(e.g., injection-molding quality and film-thickness variation, respectively).
10
Melting
11
Melting
 Conduction melting
o suppose simple conduction.

o semi-infinite solid initially at temperature T0 is raised to T1 at time t=0.
o describe the temperature evolution.
12
Melting
 Compressive melting: polymer solids and melts are virtually
incompressible and very high pressures are
required to become significant.
 Deformation melting (viscous dissipation):

o proportional to viscosity
o very important melting mechanism in polymer processing operations!
 External source: e.g. use of ultrasonic energies, ... but limited in

polymer processing operations
13
Pressurization
14
Mixing
 mixing of highly viscous polymer melts
 compounding of solids, liquids, ... into a polymer for various reasons
 mixing => reduction on non-uniformities
 mixing => increase in interfacial area
 basic mechanism of mixing = stretching and folding
15
Compounding
 Achieve macromolecular modifications, creation of multiphase
structures.
 Polymer additives and modifiers used for different reasons:
 Improve chemical stability either during processing or during the
life-time of the plastic product (anti-oxidants, UV-stabilizers, …)
 Facilitate polymer processing operations (mould release agents,
slip agents, …)
 Functional towards material performances (mechanical
properties, thermal properties, flame retardants, …)
 Classification based on function, miscibility, and concentration.
 Additives can be rigid or deformable
16
Compounding
 Case study: additives for greenhouse covers.
 Problem: loss of heat during the night (emission of IR-radiation.
17
Elementary Steps in Polymer Processing
18
Shaping
Die Forming:
o Forcing a melt through a die
o Fiber spinning, film and sheet forming, profile forming, …
o Example of a steady, continuous process
19
Shaping
 Moulding:
o Forcing thermoplastic or thermoset into a mould.
o Injection moulding, compression moulding, resin transfer
moulding, …
20
Shaping
 Stretch Blow Moulding:
E.g., PET bottles
21
Shaping
 Calendaring and Coating:
o Oldest methods (rubber industry)
o Various coating methods such as knife coating
22
Shaping
 Mould Coating:
o Formation of a relatively thick coating on the inner or outer metal
surface of a mould
e.g., rotational moulding
23
INJECTION MOULDING
24
Injection Moulding
 Force melt into a cavity and reproduce its shape.
 ⁓30% plastics processed by injection moulding.
 Expensive and complex moulds, only efficient for large series.
 High pressures (⁓ 1000 bar) => surface limited to roughly 1m2.
 Thickness limited to roughly 5 mm (low thermal conductivity).
 Thickness typically larger than 0.5 mm otherwise difficult to fill).
25
Injection Moulding Cycle
 4 phases
26
Gas Assisted Injection Moulding
 high-pressure injection of a gas into a partially filled mould.
 Under the gas pressure, the polymer core melt is driven downstream of
the mould.
 Fountain flow forms a continuous melt skin and a gas core.
27
Co-injection Moulding (Sandwich Moulding)
 Use of fountain flow to introduce a core-skin sandwich.

 E.g., interesting when processing recycled materials: recycled material in
the core, and new material in skin.
 E.g., applications in which a different ‘feeling 'of the surface is required.
28
29

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Polymer Processing

Chemical Engineering Program

 deals with the conversion of raw polymeric materials into finished

 involves not only shaping but also compounding and chemical

=> ‘‘value-added’’ structures

 Melting in polymer processing is a very important elementary step, not

o suppose simple conduction.

 Deformation melting (viscous dissipation):

 External source: e.g. use of ultrasonic energies, ... but limited in

 Use of fountain flow to introduce a core-skin sandwich.

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