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            IC PROFESSIONAL TRAINING SERIES 

Last updated at AUGUST 2009
Copyright reserved by INDUSTRIAL CENTRE, THE HONG KONG POLYTECHNIC UNIVERSITY
                                                          Plastics Technology Practice




 Plastics Technology
 Practi c e
Objectives:

       To learn the practical applications of plastic technologies.
       To know the latest development of the plastic technologies.


1.     Introduction

In general term, plastic refers to the suitability for manufacturing and moulding
into different shapes. Technically, plastics are polymers of high molecular weight
by linking together many small monomers and may contain other organic, semi-
organic or inorganic chemical substances to improve performance and/or reduce
costs.

Plastics are widely used in packaging, building & construction, transportation,
communication, health, entertainment and many other industries for applications
such as glazing panel, plumbing fixtures, helicopter blades, airplane fuselages, car
bumpers, artificial hearts, food and drink containers, CDs, DVDs, electrical and
electronic products. Plastics are useful but littering is not. It is estimated that
everyday more than 60 million plastic water bottles are thrown away and most end
up in landfills or incinerators in US. Plastics are non-biodegradable substance that
degrade physically very slowly and prompt to pollute earth, air and water.

On the other side of the coin, plastic packaging offers a superior ability to protect
products against contamination; plastic pipes safely transport water or waste for
their superior corrosion resistance and high strength to weight ratio; plastic
vehicle parts consume less fuel for it weight to fuel impact; in electrical and
electronics enable plastics to make our living easier, safer, less expensive and more
fun for their ease of fabrication into complex shapes, insulation and colourful or
transparent aesthetic aspect.

Furthermore most plastics are petroleum base product and the energy required to
produce plastics is just half of the energy required in producing paper and 1/5 in
producing steel. Plastics can be firstly reused, replaced, and reduced and
ultimately recycled at the end of their useful life. Plastic parts are littered because
they are unfashionable rather than because they are worn out. Our living style is
harming the earth not the plastics.




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The European Union has enforced a series of directives to ensure the sustainable
development of mankind in using our nature resources.

      •   From January 2006 manufactures & retailers will be responsible for
          recycling waste electrical and electronic equipment under new EU
          legislation called the WEEE Directive (2002/96 EC).
      •   From June 2007, chemical substances are controlled through Registration,
          Evaluation, Authorisation and Restriction for their safe use under the
          REACH directives (1907/2006 EC).
      •   The EuP Directive 2005/32/EC on the eco-design of Energy-using Products
          encourages manufacturers to design products with the environmental
          impacts in mind throughout the product entire life cycle.

The use of plastics in our society should be undergone a holistic investigating or
valuation in assessing the social benefits against with the social & environmental
impacts starting from their raw materials extraction to final disposal: “Cradle to
grave”.

1.1 Plastic Industry in Hong Kong

Most of Hong Kong plastic manufacturing establishments have been blown up
and moved to China after the economic reform in 1978. The Hong Kong
entrepreneurs have been taking the leading role in transforming the Pearl River
Delta into the heartland of China manufacturing industries and radiating their
influences to the other provinces.

In the past thirty years, China has grown into a giant in the plastics industry,
ranking first in the world in the production volume of plastics processing
machines, second in the production of plastic products, and third in the
consumption of plastic resins and the largest importer of nature rubber. In 2007
annual plastics consumption in China is over 40 million tons and rubber
consumption will reach 3.8 million tons in 2008. China is developing into one of
the largest markets for plastic and rubber products in the world.


2.        Plastic Material

Plastics can be classified into three main types, namely the thermoplastics,
thermosets and the elastomers, accordingly to their physical or chemical
hardening processes.

2.1       Thermoplastics

Thermoplastic materials soften while heating and solidify while cooling.
Thermoplastic can be mainly classified into crystalline and amorphous (non-
crystalline type), they are different in molecular chain structure, such as linear,
branched, comb, star, cyclic, dendrimer or randomly branched as shown in the
following two diagrams. These chains associate themselves together through


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weak Van der Waals forces or stronger hydrogen bonding or stacking of aromatic
rings, while a highly crystalline structure is well order, an amorphous structure is
random. Most of the plastics are in form of semi-crystalline by a combination of
these two structures with certain degree of intra-molecular forces into a semi-
ordered structure.




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2.2    Thermosets
Thermoset materials are heat-sensitive
synthetic materials which, when subject to
heat and usually pressure, will undergo
chemical change with their molecules cross-
linked together to become permanently
insoluble and infusible. Thermosets cannot
be remelted and reformed after cured and
the process is irreversible. This reaction is
somewhat like cooking an egg: once cooked, it is set permanently.

2.3       Elastomers

Elastomers are natural or synthetic materials with rubbery properties that can be
stretched to at least 200 percent of their original length repeatedly (at room
temperature) and which will return with force to their approximate original length
when the applying stress is released. Natural rubber is an agricultural products
harvested mainly from Thailand, Indonesia and Malaysia in meeting the rapid
demand of automobile tyre industry, latex gloves, high pressure hydraulic hoses,
escalator handrails, rubber seals, rubber pad, elastic rubber thread and ribbed
rubber sheets. Thermoplastic elastomers (TPSs) is a kind of injection mouldable
plastics that are low modulus, flexible with both thermoplastic and elastomeric
properties in replacing traditional rubbers. The TPE is a class of copolymers based
on urethanes, polyesters, styranics and olefins. TPSs are found in products for the
consumers, medical, sports and leisure, automotive, lawn and personal care
market segments for their ease of processing and soft to touch texture.


2.4       Additives and Fillers

Additives and fillers are added to improve the performance or to reduce cost of
polymer during processing, or their servicing capabilities. The followings are some
common additives and fillers.

      •   Anti-microbial imparts protection against mould, mildew, fungi and
          bacterial growth to materials. Without anti-microbials, polymers can
          experience surface growths, causing allergic reactions, unpleasant odours,
          staining, embrittlement, and premature product failure.
      •   Antioxidants are used in a variety of resins to prevent oxidative
          degradation. Such degradation occurs by the initiation of free radicals,
          which possess unpaired electrons and are highly reactive. These radicals
          are created by heat, radiation, mechanical shear or metallic impurities.
          Free radicals may also form during polymerization, processing of
          fabrication. The function of antioxidants is to prevent the propagation
          steps of oxidation.
      •   Antistatic agents are additives used in plastics to prevent the buildup of
          excess electric charge. This electricity is formed during processing,


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          transportation, handling and final use. Secondary benefits of antistats can
          include improved processability and mould release. In fact, antistatic
          agents are used as lubricants, slip agents and mould release agents in
          some processes. Plastics are inherently insulated and do not allow built-up
          static electricity to dissipate easily. In most plastics, excess charges can
          linger or discharge, causing such problems as dust attraction, fire and
          explosion hazards, poor mould release, and damage to electrical
          components.
      •   Flame retardants additives for plastics are essential safety materials. The
          transportation, building, appliance and electronics industries use flame
          retardants in plastics to prevent human injury or death, and to protect
          property from fire damage. Fundamentally, flame retardants reduce the
          ease of ignitability and rate of burn of plastics.
      •   UV stabilizers are used in a variety of resins to prevent degradation
          caused by UV radiation from sunlight.
      •   Glass or Carbon fibres up to 40% (by weight) chopped Long and short
          glass fibers (GF) reinforced thermoplastic are added to a polymer matrix
          with distinguished good mechanical properties and high thermal
          resistance. Both Glass or Carbon continuous fibres are wound, weaved or
          braided into clothes and mats for transportation usage with superior fuel
          saving and reduction in production cost.
      •   Calcium Carbonate the least expensive and the largest mineral filler used
          (upto 70%) in thermoplastic to reduce shrinkage and offers good surface
          finish
      •   Barium Sulfate: the densest mineral uses in a few end products such as
          sound barrier or dampening applications.
      •   Talc enhance the stiffness and raise the heat deflection temperature
          significantly and better dimensional stability.
      •   Kaolin: clay or natural alumni-silicate provides good impact
          modification for automotive applications improves dimensional stability
          like talc. Clay provides better sound dampening but not as well as barium
          sulfate.


2.5       Selection of Plastic Material

In order to choose suitable plastic material for our application, we need to
understand the properties of different plastic material. The followings are some
common properties we need to consider before choosing the plastic material.

2.5.1     Physical properties considerations

Physical properties can be observed or measured without changing the
composition of matter. Physical properties are used to observe and describe
matter. The followings are some common physical properties.

      •   Density is equal to mass per volume. Density = mass (g) / volume (cm3), by
          knowing the volume of the material, the weight of the material can be


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        calculated. Smaller density means lighter in weight if the volume of the
        materials is the same. ISO 1183
   •    Water Resistant describes how well an plastic part resists to water
        passage. Especially in some outdoor electrical device waterproof should be
        considered.
   •    Dimensional Stability is the property of a polymeric part to retain its form
        when subjected to varying degrees of thermal, moisture, pressure, or other
        stress.
   •    Softening and Melting Temperature is the temperature when the
        material will soften and melt. Different environment and purpose may
        require different temperature range. ISO 75
   •    Flammability of plastics is tested accordingly Underwriters Laboratories
        UL 94 to measure the resistance of plastics to a fame source. UL approval
        is given for a particular product at a measured thickness with ratings
        ranging from least flame retardant to most flame retardant as HB, V2, V1,
        V0, 5VB and 5VA.
   •    Electrical properties are the resistance, insulation, dielectric strength,
        dissipation factor of the plastics. ISO 1325, ISO3915, ISO1325, ISO1325.
   •    Optical properties are the gloss, transparency, haze, colour and refractive
        index of plastics. Plastics can be product with a wide of range of colours to
        meet the lifestyle demand of people. ISO 489

2.5.2   Mechanical properties considerations

Mechanical properties describe how a material responds to the application of a
force or load. The followings are some common mechanical properties.

   •    Tensile Strength is the ability of a material to withstand forces pulling it
        apart. ISO527-1




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•   Impact Strength is the ability of a material to resist shock loading. ISO
    179, 180.




•   Flexural Strength is the measure of how much stress (load) can be applied
    to a material before it breaks. ISO 181, 871, 1210.
•   Ductility describes the extent to which a material can be deformed
    without fracture.
•   Hardness is the resistance to compression, indentation and scratch.
    Durometer hardness tester is used to measure the material resistance
    against the indentor spring load balance. The hardness is ranging from 1
    to 100 with Shore A. B, C, D, DO, E, M, O, OO, OOO,OOO-Sand R standards.
    The general Shore A standard is for normal elastomer and Shore D is for
    hard plastics (ASTM D2240 A and D testing standards). ISO 868.




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2.5.3   Practical application considerations

In practical application different usage have different requirement, the followings
are some considerations.

   •    Weather Resistance is the ability of a material to withstand the effects of
        wind, rain, or sun(UV) and to retain its appearance and integrity. ISO-4892 ,
        ASTM D-2565.
   •    Wear Resistance is the ability to resist removal of material from a surface
        as a result of mechanical action. ISO 2556, ISO 62,585, 960.
   •    Microwave Resistance is the ability of a material to resist microwave.
   •    Fire Resistance is the ability of a material to resist fire.
   •    Cost Factor is the amount of the money can be spent on the project or
        production.
   •    Manufacturability is the factors need to be considered in manufacturing
        like the method of manufacturing, shrinkage, tolerance.
   •    Environmental Factors is the impact on the environment when the
        material is disposed, is the material biodegradable? Can it be recycled?
   •    FDA Compliance is a certification of plastic materials that are used in
        contact with food certified by Food and Drug Administration (FDA) of USA.

2.5.4   Major Consumption of Plastics

Majority of plastics consumption (over 90%) are commodity thermoplastics such
as High Density Polyethylene (HDPE), Low Density Polyethylene (LDPE),
Polypropylene (PP), Polystyrene (PS), Polyvinylchloride (PVC), Polyethylene
Terephthalate (PET). Because of their popularity, they are respectively identified as
the recycling codes as shown on forms of plastic packaging in the following
diagram.




The second category of plastics (around 8% of total consumption) is known as
engineering plastics for their improved mechanical properties and load bearing
characteristics. Examples of engineering thermoplastics are Polyamides (Nylon),
Polycarbonates (PC), Polyoxymethylene (POM), Styrene acrylonitrile (SAN),
Acrylonitrile-butadiene-styrene (ABS) Polymethyl methacrylate (PMMA), cellulose
acetate (CA), Polyphenylene ether (PPE), Thermoplastic elastomers (TPS),
Polyurethanes (PUR) and the others.

Lastly, there is only less than 1% of plastics consumption that is classified as high
technology plastics. These plastics are with superior high temperature with much
improved mechanical properties such as liquid crystalline polymers (LCPs),
polyetheretherketone (PEEK), Polysulfones (PSU), Polyphenylene sulfide (PPS,
Polyarylates (PAR), Polyimides (PEI) and the others.



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2.5.5 Common Plastics

Thermoplastics

Polyethylene Terephthalate (PET): is used in beverage, food and other liquid
containers; thermoforming applications. For post consumer, PET is one the easier
collected and sorted in Mixed Plastic Wastes (MPW) for recycling purpose for its
dominated application and ease of identification as the bottles for drinks and
alcoholic beverage.

High Density Polyethylene (HDPE): is one of the most stable and inert polymers,
exhibiting very high resistance to chemical attack including alkalis, aqueous
solutions, non-oxidising acids and to a lesser extent, concentrated oxidising acids.
HDPE is used in hollow toys, playground equipment, tanks, milk bottles and water
pipe and very thin carry bags.

Polyvinyl Chloride (PVC): is commonly used as for the insulation on electric wires
and over 50% of PVC manufactured is used in construction.PVC is used as
magnetic stripe cards, window profiles, pipe, plumbing and conduit fixtures.

Low Density Polyethylene (LDPE): is used for plastic wrap, plastic bags, dispensing
bottles, wash bottles and food storage containers for its flexibility and soft
features.

Polypropylene (PP): has a melting point of ~160°C and is rated as 120°C operating
temperature and is suitable for food containers that need to be dishwasher safe.
Polypropylene is also very easy to add dyes to, and is used as hinges, food
packaging, textiles, laboratory equipment, automotive components, and polymer
banknotes.

Polystyrene (PS): Pure solid polystyrene is a colourless, hard plastic and brittle, can
be transparent for plastic assembly kits, plastic cutlery, rigid, economical plastics.
Expanded polystyrene for packaging is used as foam for protection.

Acrylonitrile-Buadiene-Styrene (ABS): is considered superior for its hardness, gloss,
toughness, and electrical insulation properties. The nitrile groups making ABS
stronger than pure polystyrene. The styrene gives the plastic a shiny, impervious
surface. The butadiene, a rubbery substance, provides resilience even at low
temperatures. ABS can be used between −25 °C and 60 °C.

Styrene-AcryloNitrile (SAN): exhibits outstanding transparency, good chemical
resistance, rigidity, dimensional stability and thermal shock resistance and
excellent resistance to outdoor exposure, aging and yellowing and is used as
appliance bodies, mixer bowls, water reservoirs.

Polycarbonate (PC): is used to create protective features, e.g. in banks bullet-proof
windows, lighting, lenses, sunglass/eyeglass lenses, compact discs, DVDs, and
automotive headlamp lenses for its impact resistance and good strength at



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elevated temperatures, to overcome the shortcomings of poor chemical and
physical weathering in an Ultraviolet light environment .

Polyamide (PA): is commonly known as nylon for its trade name, it is High
mechanical strength, rigidity and thermal stability, good impact resistance even at
low temperatures, advantageous sliding friction properties.

Polyacetal Copolymers (POM): is with a high degree of rigidity and mechanical
strength, outstanding resilience, optimal dimensional stability and excellent
resistance against a variety of chemicals and is used to make gears, bushings,
fasteners and other mechanical parts.

PolyMethyl Mehacrylate (PMMA): is commonly known as acrylic, Perspex or
Plexiglas for it clarify and transparent properties and is usually used as an
substitute or glass and cheaper but with inferior mechanical properties than
Polycarbonates.

Thermosets

Phenol-formaldehyde (PF): is the most widely used of all the thermosets for its
excellent dimensional stability under thermal cycling and high stress conditions,
low water absorption, and high surface hardness, compressive strength and highly
resistant to petrochemicals and hydrocarbons. An Modified injection moulding
method is by preheating, metering and plunging the PF resins into a mould that is
embedded with heaters to cure the resin after the injection.

Melamine – formaldehyde (MF): is usually formed by compression moulding
method for its lack of pourability, it is with wide range of colour and scratch
resistance and is often used in tableware, bowls and plates. However, it is not
microwave safe.

Epoxies: is cured by addition of a hardener to achieve total cross linking. It is used
as electrical connectors, encapsulating components of Integrated Circuits,
electronic components and coatings for its electrical, mechanical, chemical
properties at elevated temperatures and its very high moisture resistance.

Sheet Moulding Compounds (SMC) or Bulk Moulding compound (BMC)
Composite: is typically 20-30% lighter than equivalent steel parts resulting in fuel
saving and improved performance in transportation industries (automobiles and
airplanes). Composites are with different resin systems (Polyesters, polyimides,
expoxies and polyureas) and reinforcement (chopped or woven or filament
winding of organic, boron, glass or carbon fibres), combination designed to meet
different applications.    They can be moulded by convectional compression,
transfer and injection moulding and other techniques such as lay-up for
exceptional strength requirements.




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Elastomers

Silicones: is used as seals, gaskets, o-rings, terminal covers, lubricants in food
industries for its thermal stability, good flammability rating 94V0 in 1/16 in section,
non-stick, low chemical reactivity, low toxicity and good electrical insulation.

Natural rubber: is harvested as liquid suspension by scarping the barks of a rubber
tree and can be cured by vulcanization using sulphur, peroxide or bisphenol. It is
used as tyres, hoses, belts, matting flooring and dampeners in industrial uses and
gloves for its good elasticity.

Synthetic rubber: is made from the polymerization of monomers for a wide range
of physical, mechanical and chemical properties while maintaining the elasticity
properties.

Thermoplastics elastomers (TPE): is a physical mix of polymers as polyolefin
blends,    thermoplastic     polyurethanes,   thermoplastic     copolyester    and
theromoplastic polyamides with the flexibility of rubber, silent aesthetic and
pleasant to touch. The typical crosslinking processing vulcanization in the
thermosetting elastomers is through covalent bonding. While in TPE, the
crosslinking is a weaker dipole or hydrogen bond suitable for recycling and reuse.

General properties of Plastics
                                  Tm             Tg          Tensile
                  Density                                                         Elastic
      Name                       Melting        glass       strength
                  g/cm3                                                          limit %
                                   ºC            ºC           MPa
 PET              1.37-1.455       260           75           55-75               50-150

 LDPE           0.910 -0.940      98-115          -           8.0 -31

 PVC              1.30-1.58      100260         57-82         50-80               20-40

 HDPE           0.952-0.965      130-137          -         18.5-24.8                55

 PP             0.855-0.946        160            -           31-41                  15

 PS               1.04-1.05        240           95           45-60                 3-4

 ABS              1.04-1.05          -        105-115          29.6                  20

 SAN              1.06-1.1           -        102-104         32-40                   4

 PC               1.2-1.22         267           150          55-75               80-150

 PA Nylon 6         1.15           254            -           59-90                  50

 POM               1.4-1.5       165-178          -           18-97                  40

 PMMA               1.19         130-140          -           48-76                   5

 PF               1.30-1.51          -            -           50-55              0.45-2.3

 MF               1.41-1.49        345            -             45                    -


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3.      Plastics Processing

Many different methods are employed to convert plastics from their raw state into
finished products or to fabricate stock plastics materials into finished products. In
the industry the mass production processes are moulding and thermoforming.
Moulding includes injection moulding, extrusion, blow moulding, compression and
transfer moulding.


3.1     Injection Moulding

Injection moulding is the most important process used to manufacture plastic
products. Today, more than one third of all thermoplastic materials are injection
moulded.

3.1.1   Process Description

Injection moulding is the best process to use for high-speed, low-cost moulding
of intricate plastics parts required in high volume. In this process, thermoplastic is
fed from the hopper through an opening at the rear of the heated injection barrel
(charging). The resin is forced forward to the front of the heated barrel by the
rotation of a reciprocating screw, where the material is heated in various stages
until it reaches a molten state. The injection screw forces the measured amount of
molten resin into the shaped cavity of a closed mould through the
nozzle/sprue/runner/gate by a ram action. The molten resin cools and solidifies in
the mould cavity. After cooling, the mould is opened and the moulding is ejected.

Almost all thermoplastics can be injection moulded and even some thermosets are
being injection moulded with modified equipment. PE, ABS, nylon PA, acrylic and
polystyrene are amongst the leading thermoplastics used in injection moulding.
Typical injection moulded products include appliance housings, camera cases,
lenses, gears, fan blades, spoons, wastebaskets etc…




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3.1.2   Injection moulding machine

An injection moulding machine consists of three units; they are the plasticating &
injection unit, the clamping unit and the mould cavity.



                                            Plasticating and Injection Unit

               Clamping Unit




   •    Plasticating & Injection Unit : The major tasks of the plasticating &
        injection unit are to melt the polymer, to accumulate the melt in the screw
        chamber, to inject the melt into the cavity and to maintain the holding
        pressure during cooling.




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The recent development of all electrical or hybrid injection moulding machines
offer greater high speed performance, low power consumption and increased
precision, microprocessor control and robust & versatile in machine
configuration using the more accurate electric servomotors .




•   Clamping Unit

The major tasks of the clamping unit are opening and closing the mould, close
the mould tightly during injection. There are three clamping types: mechanical,
hydraulic and their combination.




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   •     Specification of Moulding Machines

   Shot size and clamping force are usually used to describe a machine. We need
   to consider both shot size and clamp tonnage when choosing a machine.

   i)       Shot size is the maximum amount of material the machine will inject
            per cycle (single shot) and the unit is ounces (oz) or grams (g). The
            standard for shot size measurement is general purpose polystyrene
            moulding in single shot.
   ii)      Clamping Force is the maximum force a machine can apply to a
            mould. The unit of clamping force is tons.


3.1.3    Part Design for injection moulding

Part design is a very important in injection moulding, good part design can reduce
the manufacturing cost and reduce the defects during manufacturing.

   •     Uniform Wall Thickness

   Uniform wall thickness should be the primary consideration in part design
   because different wall thickness causes different shrinkage which increases the
   difficulties dimension control and cause serious warpage in the injection
   moulded products.




   •     Draft Angles

Draft Angles are added in the internal
and external walls for the mould part to
be ejected from the mould. Draft angle
requirement are smaller in external walls
than internal walls.

   •     Radii/Fillet

   Internal sharp corners and notches are the leading cause of failure in injection
   moulded thermoplastic parts. To avoid the problem occurred, radii / fillet is
   commonly employed to all “sharp” feature.




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    Fillet radius is determined by the wall thickness and the carrying load. Fillet
    radius should be between 25 to 60% the nominal wall thickness. The larger
    fillet radius is suggested for load carrying features.

•   Ribs

Ribs are used to strength the structure and reduce the weight of the product.

                        t



                h




    Rib thickness should be 50 to 60% of the nominal wall thickness t, the rib
    height h should not excess three times of the nominal wall thickness.
    Spacing between two parallel ribs should be more than two times of the
    nominal wall thickness. Draft angle for ribs is 1 to 1.5°.

•   Bosses

Bosses are thermoplastic cylinders attached to a side wall or end corners. They
can be used for assembly with self-tapping screws. A boss should not be
attached directly to a side wall because it will cause sinks or voids.




The outer diameter of the boss should be two times the inner diameter of the
boss. The height of the boss should be less than three times of the outer
diameter of the boss. The distance between two bosses should be more than
two times of the nominal wall thickness t, the wall thickness at the base of the


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boss should less than 60% of the nominal wall thickness t. the minimum draft
angle on the outer diameter of the boss is 1/2° and inner diameter is 1/4°.

•       Snap-fit Design

Snap fits are commonly used as an assembly method for injection moulded
parts. Snap fits are very useful because they eliminate screws, clips, adhesives,
or other joining methods. The snaps are moulded into the product, so
additional parts are not needed to join them together. There are three main
types of snap fits: Annular, Cantilever, and Torsional.

    •    Annular snap fits are                     •   Cantilever snap fits are the
         generally stronger, but need                  most widely used type of
         greater assembly force than                   snap     fit.  There  is   a
         their              cantilevered               considerable     amount   of
         counterparts.      They     are               calculation and engineering
         basically interference rings.                 that goes into designing a
                                                       good snap fit.




    •    The torsional snap-fit relies for its spring effect on twisting rather than
         flexing like the other types. It is a good way of fastening a hinged lid on a
         box or container.




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3.1.4   Mould Design

The mould is an important element in the moulding machine, apart from
determine the shape of the product, the mould vents the entrapped gas, cools
the product and ejects the product. The quality of the product and the
manufacturing cost are largely determined by the mould.

The mould is comprised of mould base, core and cavity that determine the
feature of the product, sprue, runner and gate that deliver the melt to the cavity,
cooling system and ejection system.

   •    Mould Base: As a matter of fact, nearly all moulds consist of the same
        basic components. There are some standard mould bases in the market
        that provide cheaper and more reliable than custom design mould base.
        The followings are some standard mould bases.




   In very broad sense, moulds can be classified as cold runner and hot runner
   moulds. Two-plate mould and three-plate mould are most common in cold
   runner moulds.




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                                            ii)   Three-Plate Mould
i)   Two-Plate Mould
                                                  Three-plate mould has two
     This is the most basic and most
                                                  parting      line,   one      more
     common type of mould. Two-
                                                  intermediate and movable plate
     plate mould has a single parting
                                                  is introduced which increase the
     line, there are two plates in the
                                                  flexibility on gating locations.
     cavity plate, with the central
     sprue bushing assembled into
     the stationary half of the mould,
     the moving half of the mould
     contains the cores and ejector
     mechanism, and in most
     designs the runner system.




There are other types of cold runner moulds like external under-cut mould,
internal under-cut mould, side core mould, unscrewing mould, stack mould.

     iii)   Hot Runner Moulds

A hot runner mould refers to a mould in which the runner stays molten and is
not ejected during the moulding cycle. In hot runner mould, the runner is
eliminated so that the shot size, plastification time, runner cooling time and the
clamping force can be reduced. The hot runner system is comprised of two
primary components that are the manifold and the drop.




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•   Core and Cavity

A mould must consist of core and cavity. The core (male) is fixed on the
moving half of the mould and the cavity (female) is fixed on the stationary
half of the mould. Gate is always on the cavity.




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   •      Feed System

The feed system is the flow-way of plasticized material to the cavity; it consists of
a sprue, runner and gate.

   i)        Sprue: is the entrance of the feed system. It is a divergent taper and
             highly polished.
   ii)       Runners is the channels through which the plasticized material enters
             the gate areas of the mould cavities are called runner. Normally,
             runner is round or trapezoidal in cross section.When designing a
             runner layout, the runner length should be minimized and balanced.
             The large parts and small parts should not be combined.
   iii)      Gate provides the connection between the runner and the mould
             cavity. It must permit enough material to flow into the mould to fill
             out the cavity. The followings are common types of gates.




   •      Cooling system

   Cooling means to maintain the
   temperature of the mould/die
   evenly in the moulding process
   by cooling channel, poor cooling
   design will affect the functioning
   of the mould and the quality of
   the moulded part.


   •      Ejection system

   Ejection is necessary for part to be removed from the mould. The hot
   materials injected into the cavity will shrink and stick tightly onto the mould
   core. The ejector plate will be driven by the injection machine to carry the
   whole Ejection system travels sufficiently to clear the moulding from the
   mould.



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3.1.5 Moulding Defects

Injection moulding is a complex technology so defect may happen if it is not
careful or experience enough. The followings are some common defects.

   •   Burn Marks are caused by poor venting. It can be solved by adding
       venting at parting line.




   •   Sink Marks are caused by insufficient
       injection pressure and holding time. It can
       be solved by increasing holding pressure.




   •   Warpage is the shape deformation due to uneven shrinkage. It can be
       solved by increasing cycle time and using shrink jig.
   •   Weld line is due to insufficient air venting and injection speed too low.
       It can be solved by increasing injection speed and providing air venting.



   •   Air Trap is caused by poor venting, not
       enough injection pressure. It can be
       solved by adding air venting.




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3.2     Blow Moulding

3.2.1   Extrusion Blow Moulding

The extrusion-blow moulding process is extensively used for making bottles and
other hollow plastics parts having relatively thin walls. To blow mould a part, the
extruder first extrudes a hollow tube (parison) in a downward direction, where it
is captured at the proper time between two halves of a shaped mould. After
trimming the top and bottom of the parison, air is blown into the soft parison,
expanding it until it uniformly contacts the inside contours of the cold mould
and solidifies. Then the mould automatically opens, the part is ejected and a
new cycle begins.

PE, PVC, PP and PS are commonly used plastics for blow moulded articles. Typical
products include bottles, watering cans, display fruit and other hollow parts.




3.2.2   Injection Blow Moulding

In the injection blow moulding process, the polymer is injection moulded onto a
core pin; then the core pin is rotated to a blow moulding station to be inflated
and cooled. The process is divided into three steps: injection, blowing and
ejection.

The injection blow moulding machine is based on an extruder barrel and screw
assembly which melts the polymer. The molten polymer is fed into a
manifold where it is injected through nozzles into a hollow, heated
preform mould. The preform mould forms the external shape and is clamped
around a mandrel (the core rod) which forms the internal shape of the preform.
The preform consists of a fully formed bottle/jar neck with a thick tube of
polymer attached, which will form the body.

The preform mould opens and the core rod is rotated and clamped into the
hollow, chilled blow mould. The core rod opens and allows compressed air into
the preform, which inflates it to the finished article shape.




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3.3    Extrusion

Extrusion is common process in plastic manufacturing, nearly 40% plastic
product are made from extrusion. It is a process used for making indetermined
length of thermoplastics with constant cross-section. Pellets (or powder) are
drawn continuously from a hopper into a heated barrel by the action of a
rotating screw, where they are heated and softened as they progress through the
heated barrel. At the front end of the extruder, the melted plastics are forced
through a shaped die that determines the final cross-section of the extrudate,
after which it is uniformly cooled and carried away on a continuous basis. Length
can be cut as desired.

ABS, PE, PS and PVC are extensively used in extrusion. Typical product includes
piping, drinking straw, window track, wire and cable coating, film and sheet.




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3.4     Compression Moulding

Most thermosets must be moulded under heat and pressure to achieve a
satisfactory end product. The most widely used methods for moulding
thermosets are compression and transfer moulding.




In compression moulding a pre-weighed and preheated amount of thermoset
powder is loaded into a heated mould, the mould is closed and pressure is
applied to the powder. The powder melts under heat and pressure and flows
into all parts of the mould cavity, after which an internal chemical reaction
crosslinks the plastics chain, hardening the plastics into its final irreversible state.
The cured thermoset part is removed from the mould while still hot and allowed
to cool outside the mould.


3.5     Transfer Moulding

Transfer moulding is similar to compression moulding except that the heated
plastics powder is placed in a separate chamber in the form of a cylinder &
plunger, then transferred under heat and pressure into a closed mould where the
shape of the part is determined and the final crosslinking reaction takes place.

Phenolic PF, melamine formaldehyde MF, urea formaldehyde UF and epoxy EP
are common materials used in these processes. Typical products include pot
handle, electrical connector, button, dinnerware, knob and tool handles.




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3.6    Thermoforming / Vacuum Forming

This is a process for forming moderately complex shapes of uniform wall
thickness, particularly when walls are very thin and cannot be injection moulded,
or when parts are very large and too expensive to be injection moulded. The
thermoforming processes uses sheet plastic that is softened by heat until pliable,
then forced by vacuum, negative air pressure, or mechanical drawing against a
cold mould surface where the sheet cools and retains the shape of the mould.

Almost all thermoplastics sheet can be used in this process. The commonly used
plastics are HIPS, ABS, PVC, acrylic, cellulosic. Typical application includes blister
pack, suitcase and disposable plate.




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4.        Other Plastic Fabrication processes

In addition of the above mentioned mass production manufacturing processes,
single piece or small quantity of plastic models can be produced by others
processes, the following session will introduce three common techniques for
making plastic parts.


4.1       Plastic board Fabrication

In general, the working of plastic materials by hand tool or by machine usually
uses the same methods as those commonly employed for work on wood and
metals such as filing, drilling turning, milling , Hot Wire Bending, Engraving, Sand
blasting, Fastening, Laser Cutting, Mechanical fastening, Bonding or sawing.

4.1.1 Cutting Plastics - Plastics can be cut by methods commonly employed for
wood, metals and paper. Among the various cutting methods sawing is the most
effective one.

      •   Hand Saws - Many hand saws
          can be used to cut plastics. Hack
          saws work well for cutting rod,
          tube and sheet. Jig saws are
          useful for cutting intricate
          shapes and holes in plastic
          sheets.



      •   Circular Saws - Circular saws are suitable for straight cuts. A speed of
          about 1,500 m/min is a reasonable average for cutting plastics. Carbide-
          tipped saw blades will hold up longer with less maintenance, but hollow-
          ground cross-cutting blades with zero rake and 2-3 mm pitch will do
          many jobs well. All blades must be kept clean and sharp.




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•   Band Saws - Band saws are generally used for cutting curbes, irregular
    cuts, and thick materials. The advantage of using a band saw instead of a
    circular saw is that the cut is cooler. It is, however, more difficult to obtain
    as straight and as smooth a cut as with a circular saw.




•   Jig Saws - Power jig saws are more efficient than hand jig saws. They are
    suitable for cutting intricate curves and holes.




•   Sanding - Belt and disc sanding machines are effective finishing
    equipment for plastics. For parts which are too large to be worked by
    these machines, portable sanders or hand sanding may be employed.


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   •   Buffing - Buffing is a polishing operation using a cloth or felt that
       contains fine abrasive. The coarseness of the abrasive used depends upon
       the original roughness of the part and the degree of luster desired.
       Buffing will not ture a surface, but tends to round sharp edges and
       produce a lustrous appearance.




4.1.2 Cementing- There are two basic methods of cementing plastics, i.e.
Cohesive and Adhesive-bonding.

   •   Cohesive Bonding - Cohesive bonding is also known as solvent
       cementing, in which the surfaces of the joint are dissolved by a suitable
       solvent and then pressed together. However, this method is only suitable
       for thermoplastics and the material of the joint must be the same, e.g.
       acrylic with acrylic or styrene with styrene.

   •   Adhesive Bonding - This method is suitable for joining any materials,
       similar or dissimilar. It is necessary to find an adhesive which will stick to
       the materials involved. Although adhesive-bonding can be used with any
       plastics, it is generally not used where solvent bonding is satisfactory. This
       means that the most common application of adhesive bonding is with
       thermosets or where dissimilar materials are to be joined.

4.1.3 Mechanical Fastening
The use of screws, bolts and nuts for fastening two pieces of plastics is also a
common practice in joining plastics. The decision to use mechanical fasteners is
based on the strength of the plastic




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4.2     Welding

Welding by heat is only suitable for thermoplastics. The process consists of
heating joint areas to fusion and then presses the joint surfaces together. After
joining, the areas are cooled to their rigid forms.

4.2.1   High Frequency Welding

High-frequency Welding can only be carried out by a high-frequency (27.12
MHz) welding machine with a suitable welding electrode and a suitable plastics
material. The equipment consists of a high-frequency generator, a press which is
either pneumatic or mechanical operated, a machine table which forms the
negative electrode and a welding tool which forms the positive electrode. The
positive electrode defines the shape of the weld. When a material with a large
dielectric dissipation factor is subject to a high frequency direct current, the
molecules are forced to rearrange their polarities on either side of the positive
and negative electrodes in the electric field, and thus cause a fusion by molecular
friction heat.




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4.2.2   Hot Air Welding

Hot-air Welding involves the heating of the plastics joining area to fusion state
by a jet of hot air from a hot air gun and then filling up the joint by a filler rod
which is similar in properties with the plastics sheet being welded.




4.2.3   Ultrasonic Welding

Ultrasonic plastic welding is the joining or reforming of thermoplastics through
the use of heat generated from high-frequency mechanical motion. It is
accomplished by converting high-frequency electrical energy into high-
frequency mechanical motion. That mechanical motion, along with applied force,
creates frictional heat at the plastic components' mating surfaces (joint area) so
the plastic material will melt and form a molecular bond between the parts. The
following drawings illustrate the basic principle of ultrasonic welding.




4.3     Resin Casting

For Resin casting please refer to the rapid tooling section in the reading material
of Rapid Prototyping & Manufacturing



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References
•   Manas Chanda, Salil K. Roy, (2009) “Plastics Fabrication and Recycling”, CRC
    Press.
•   Erik Lokensgard, (2004) “Industrial Plastics Theory and Applications”, Thomson.
•   http://www.matweb.com
•   http://www.dukcorp.com/us/PPL_WhatIsUPA.htm
•   Osswald, T., Hernandez-Ortiz, J. P., (2006) Polymer Processing-modeling and
    simulation, Hanser
•   Hans-Georg, E. (2003) An Introduction to Plastics, Wiley-Vch
•   Friedrich Johnnaber, (2008) Injection Molding Machine- A Use’s Guide, Hanser
•   Osswald, T.,Turng, L.S., Gramann, P. (2008), Injection Molding Handbook,
    Hanser




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