Polymer Technology

simple homopolymers, as made pure. Only a few of these are finally sold as “pure” polymers, such as polystyrene drinking cups and polyethylene films. Much more often, polymers are
sold with various additives. That the student may better recognize the polymers,
the most important additives are briefly discussed.
On heating, linear polymers flow and are termed thermoplastics.To prevent
flow, polymers are sometimes cross-linked (•):

THE MACROMOLECULAR HYPOTHESIS

The cross-linking of rubber with sulfur is called vulcanization. Cross-linking
bonds the chains together to form a network. The resulting product is called
a thermoset, because it does not flow on heating.
Plasticizers are small molecules added to soften a polymer by lowering
its glass transition temperature or reducing its crystallinity or melting temperature.
The most widely plasticized polymer is poly(vinyl chloride). The
distinctive odor of new “vinyl” shower curtains is caused by the plasticizer,
for example.
Fillers may be of two types, reinforcing and nonreinforcing. Common reinforcing
fillers are the silicas and carbon blacks.The latter are most widely used
in automotive tires to improve wear characteristics such as abrasion resistance.
Nonreinforcing fillers, such as calcium carbonate, may provide color or opacity
or may merely lower the price of the final product.

The main parts in Injection molding machine are:

1. INJECTION UNIT
2. CLAMPING UNIT
3. MOLD SYSTEM
4. CONTROL SYSTEM
5. HYDRAULIC SYSTEM

I. INJECTION UNIT

Parts which are belonging to injection unit are:

• Screw
• Check valve
• Cylinder (barrel)
• Heating Zones
• Nozzle

Tasks of the injection units are:

• Feeding of materials
• Transport of material
• Melting and plastification of granulates
• Homogenization of melted material
• Injection of melted material
• Compacting of injected material

Screw

• Task of screw:
• Feeding of granule
• Transport of material
• Plastification of granule
• Homogenization of melted material

Construction of a standard screw:

• Material of screw: nitride steel
• Roughness of screw surface Rt < 1 um
• L/D ratio -20
• 3 zones (feeding-, compression- and metering zone)

Check valve:
Function of check valve: to prevent the melting materials in front of barrel go back into feeding zone during injection

Heating zone

• Melting of plastic material
• Control of mass temperature
• Max. temperature < 500 C
• 4 heating zones ( 3 cylinder zones, 1 nozzle zone)

Nozzle
Type of nozzle:

• Open nozzle
• Shut-off nozzle (needle shut off nozzle, moved by spring, moved by hydraulic, slide shut off nozzle)

II   CLAMPING UNIT

Task of clamping unit:

• Fixing and guiding the mold
• Keeping the clamping force
• Ejecting the molded part

III MOLD SYSTEM

Mold system consist of:

• Runner system
• Cavity (forming the molded part)
• Cooling system
• Demolding of part for complex geometry
• Ejector system

Runner system
Types of runner system are:

• Cold runner: Bar sprue, pin gate, film gate
• Hot runner
• Insulating runner

Cavity
Task of cavity are:

• Forms geometry of plastic part
• Responsible for surface quality
• Equalizes the material shrinkage

Cooling systems
The significant parameter for cooling system are:

• Cooling channel (geometry and location)
• Cooling media
• Cooling time
• Cooling rate

Ejector systems
Ejector system consists of:

• Ejector pin
• Ejector plates

The [chemical] process by which [one or more] monomer(s) becomes a polymer

Addition Polymerisation

a polymer chain grows by addition of monomer to a reactive end-group

Condensation polymerisation

A polymer chain grows step-wise by reaction that  involve the elimination of a small molecule

The [Rise of the] Macro-Molecular Concept

• Hermann Staudinger

- 1917 proposed “macro-molecules”

gaint longchain molecules whose small-molecule constituents were linked together by chemical bnds no different to those in ordinary organic compounds

1953 Nobel Prize

• Wallace Hume Carothers

- DuPont 1920′s

-  synthesized high-molecular weight polymers

from bi-functional reactants, using normal organic condensation reactions

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Plastics – the beginnings

• 1839 Natural Rubber  – Charles Goodyear
• 1843 Vulcanite          – Thomas Hancock
• 1843 Gutta-Percha     - William Montgomerie
• 1856 Shellac              – Alfred Critchlow, Samuel Peck
• 1856 Bois Durci          - Francois Charles Lepage

History of Industrial Polymers

• 1939 PS             
  Eduard Simon[a German apothecary]
• 1926 PVC          
  Dr. Waldo Semon[BFGoodrich, hired to develop a cost-effective synthetic rubber]
• 1935 LDPE 
  Eric Fawcett and Reginald Gibson[ICI]
• 1941 PET          
  Rex Whinfield and James Dickson[Calico Printer's Association]
• 1951 HDPE 
  Robert Banks and and Paul Hogan[Phillips Petroleum]
• 1951 PP            
  Robert Banks and and Paul Hogan[Phillips Petroleum]

The Plastic Era – Thermosetting Plastics and Thermoplastics

• 1909  Phenol-Formaldehyde (Bekelite) – Leo Hendrik Baekeland
• 1926  Vinyl (PVC) – Walter Semon
• 1927  Cellulose Acetate
• 1935  Low-density polyethylene (LDPE) – Reginald Gibson and Eric Fawcett
• 1936  Acrylic or Polymethyl Methacrylate
• 1938  Polystyrene made practical
• 1938  Polytetrafluoroethylene (PTFE) Teflon – Roy Plunkett
• 1939  Nylon – Wallace Carothers
• 1941  Polyethylene Terephthalate (PET) – Whinfield and Dickson
• 1942  Low Density Polyethylene
• 1942  Unsaturated Polyester
• 1951  High-density Polyethylene (HDPE) – Paul Hogan and Robert Banks
• 1951  Polypropylene (PP) – Paul Hogan and Robert Banks
• 1958  Polycarbonate – Daniel Fox – GE  plastics
• 1964  Polyimide – DuPont – Noryl
• 1970  thermoplastic Polyester
• 1978  Linear Low Density Polyethylene
• 1985  Liquid Crystal Polymers

There are some models of fluid:

• Newton
• Bingham body
• Dilatant fluids
• Pseudoplastics fluids

The correlation between Shear viscosity-shear rate axis and the fluids will be shown in figure 3.

Figure 3:Shear Viscosity – Shear rate

The correlation between Shear stress-shear rate axis and the fluids will be shown in figure 4.

Figure 4: Shear Stress – Shear Rate

Descriptions for each fluid model:

»Newtonian fluid

• Have constant viscosity and not depend on shear rate
• For Fluid with low molecular weight (water, benzene)

» Bingham body

• Newton fluid after yield stress
• Highly filled polymer

»Dilatant fluids:

• Fluids, that the viscosity will be higher with increasing shear rate
• PVC plasticized

» Pseudoplastics:

• Fluids, that the viscosity will be lower with increasing shear rate
• Polymer molten

Those effects (Newton’s, Psedoplastics, dilatant) are depending on the shear rate but not depend on the time (time independent). Several fluids show the viscosity’s changing that depend on the time when it is loaded with constant shear stress.

» Thixotropy fluid: It’s the properties, that viscosity IS decreasing with constant shear stress (painting material)
» Rheopexy fluid: It’s the properties, that viscosity IS increasing with constant shear stress (Suspension PVC)

Several properties of-polymer are shown in figure5.

(a) Poly(methyl  methacrylate) at 240°C; (b) Polyethersulphone at 350°C; (c) LDPE (MFI 20 g/lO min) at 170 °C; (c) Polyamide 66 at. 285 °C.
Figure 5: Flow of curves of some typical Thermoplastics melts

The typical shear rate per second [1/s] of polymer melts In the processing as follows:

• Injection molding                10³ - 10⁵
• Compression molding           10⁰ - 10¹
• Calendering                       10¹-10³
• Extrusion                           10⁰ - 10³

By increasing temperature of polymer melts, the shear stress will be lower, so that the energy need to process will be lower for turning the screw. These properties will be shown in figure 6.

Figure 6: Shear Stress – Shear rate by different temperature