1.
Introduction
1.1.Background
The injection moulding has seen steady growth since its beginnings in the late
1800’s. The
technique has evolved from the production of the simple things like combs and
buttons to major consumer, industrial, medical, and aerospace products.
The invention of an injection molding machine was achieved by John Wesley who
injected hot celluloid into a mold which resulted in billiard balls which were
used as a replacement for ivory which was based on the pressure die casting
technique for metals. The industry progressed slowly over the years, producing
products such as collar stays, buttons, and hair combs. The industry expanded
rapidly in the 1940s because World War II created a huge demand for inexpensive,
mass-produced products. In 1946, American inventor James Watson Hendry built the
first screw injection molding machine, which allowed much more precise control
over the speed of injection and the quality of articles produced. This machine
also allowed material to be mixed before injection, so that colored or recycled
plastic could be added to virgin material and mixed thoroughly before being
injected.
The main concept of plastic molding is placing a polymer in a molten state into
the mold cavity, so that the polymer can take the required shape with the help
of varying temperature and pressure. There are different ways of molding a
plastic some of them are blow molding, Injection molding, rotational molding
and compression molding. Each technique has their own advantages in the
manufacturing of specific item.

1.2 Injection Molding
Machine

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Injection molding
machines are normally general-purpose machines. An Injection molding machine is a machine for
manufacturing plastic products by the injection molding process.
Injection molding is a method of forming a plastic product from thermoplastics
by
feeding the material through the machine component called the hopper to a
heated chamber
in order to make it soft and force the material into the mold by the use of the
screw. In this
whole process pressure should be constant till the material is hardened and is
ready to be
removed from the mold. This is the most common and preferable way of producing
a plastic
products with any complexity and size.
Injection molding permits mass production net shape manufacturing of high
precision, three
dimensional of plastic parts.  It consists of two main parts,
an injection unit and a clamping unit.

1.2.1 Injection / Plasticizing unit: The
plasticizing unit melts, homogenizes, conveys, meters and injects the polymer
into the mold. Thus the injection unit has two main tasks: On one hand it
plasticizes the polymer and on the other hand it injects the melt into the
mold. The injection unit consists of hopper, barrel, screw/plunger
and the heating section. Barrel, screw and nozzle are
exchangeable. In doing so the injection unit is adaptable to the type of
polymer and the shot volume.

Types of
injection units-

Plunger type injection: The
design of this unit was based on a method used to mould rubber. Material is
metered by a dosage device and transported through the heated plasticising
cylinder until it is in front of the plunger at the correct temperature. The
material residence time in the cylinder is very long, making this method
unsuitable for heat sensitive material such as rigid PVC and thermosets. This
type of machine is not widely used as it was replaced by the reciprocating
screw piston injection unit.

Screw type injection:
Standard machines have typically reciprocating screws, which means the screw
acts also as an injection piston. Nowadays there are also approaches existing
which separate plasticizing and injection unit. Common screws for injection
molding consist of three zones: Feed section, Compression section, Metering
section the screw rotates in a heated barrel and a feed hopper delivers
granulate or powder into the barrel. Usually the injection unit is mounted
flexibly on the machine bed.

1.2.2 Clamping unit: The clamping unit
resembles to a horizontal press. The moving platen slides on four bars and the
halves of the mold are mounted on both platens.

Types of clamping
units-

 

Manual
Clamping: Manual
clamping in only seen in the case of manual or Hand Injection Moulding Machine
or some time in pneumatic Injection Moulding Machine. The proper clamping is
depend on the skill of the operator. The Clamping force is direct and not
measured. The position of the sprue with respect to nozzle axis is critical.
The ejection of the part from the mould is difficult.

Toggle
Clamping:

A toggle
is mechanically device to amplify force. In a moulding machine, which consists
of two bars joined, together end to end with a pivot .The end of one bar is
attached to a stationary platen, and the other end of a second bar is attached
to the movable platen. When the mould is open, the toggle is in the shape of a
V. When pressure is applied to the pivot, the two bars form a straight line.

Hydraulic
Clamping:

A clamping
unit actuated by hydraulic cylinder, which is directly connected to the moving,
closed the mould. In this case ram of hydraulic system is attached to moving
platen. There are two halves in hydraulic cylinder, which is actually inlet and
outlet of oil.

When oil
goes to the cylinder with pressure oil pushes the ram to forward direction by
which moving platen moves and mould closed and when oil comes from the cylinder
the ram come back and mould is open.

 

Hydro-Mechanical

This
Clamping System is combination of Toggle & Hydraulic Clamping System. To
move the toggle a hydraulic cylinder is operated.

TIE-BAR
LESS CLAMPING

Tie-Bar
less clamping system is basically Hydraulic clamping system without any tie
bar. The platen is moved on a rail system.

The main
advantage of this system there is no limitation of mould platen size. As there
is no tie bar so the mould dimension is not so important. Also mounting of the
mould is easy and it is very useful when products eject from the mould is
manual.

 

1.3 Injection Molding Process

The procedure of
injection molding is depicted in the figure

It is clear that all
procedure steps of injection molding take place one after another except for
the important cooling stage, which overlaps with the holding pressure, the
backward movement of the plasticizing unit, metering and plasticizing stage.
The machine control coordinates each process step and repeats it every
injection molding cycle. To achieve a high output and hence a high process
efficiency the cycle time should be kept as short as possible.

Metering the screw,
which rotates in the plasticizing cylinder, conveys the melt from the hopper to
the end of the screw. This way the material is going to be densified and
brought into a molten stage. While the screw conveys the melt, the screw moves
backward because the delivered material conglomerates at the screw tip. While
moving backwards a definite pressure is applied to the screw, which is called
back pressure. In doing so the screw has to convey against this pressure and
the homogenization and the dissipation heating improves. This back pressure is
adjustable.

At a certain screw
position the metering stops, because the mass of melt for filling the whole
cavities is reached. The position of the screw or the way the screw moves
backward respectively represents the metering volume and is called feeding
stroke or metering stroke. Both parameters have to be adjusted for each new
molding process.

 Injection During injection the screw moves
forward without rotating and injects the metered material through the nozzle
into the mold. In doing so the screw acts like an injection piston. A residual
melt cushion remains in front of the screw for applying the following holding
pressure. The maximum injection pressure is a prefixed parameter and an upper
limit for the applied injection pressure. Another prefixed parameter is the
injection speed. However it is variable during injection stage. It is also
possible to apply an increasing or decreasing velocity profile during injection
and holding pressure stage.

Melt flow in the
cavities during injection the melt flow can be divided into three stages,
depicted in Figure 8: A) Injection stage B) Compression stage C) Holding
pressure

During injection stage
the mold is going to be filled volumetrically. Afterwards the velocity slows
down and compression takes place. For densification more melt is conveyed into
the mold, while the pressure in the cavity rises rapidly and steeply. When
reaching a certain pressure value the holding pressure stage begins. When
switching too late from compression MSci PolySci P104 – Injection molding 11 to
holding pressure stage a high peak pressure arises in the melt that exhibits
still a very low viscosity. Thus overmolding becomes possible. While cooling
down the polymer shrinkage occurs. Hence more molten material has to be
conveyed into the cavity during holding pressure stage in order to keep the
volume of the molding constant. This stage is finished when the gate freezes.
The switching point from compression to holding pressure is of high importance.
When switching too early the density of the molding is too low and shrink marks
are visible on the surface.

Cooling stage Cooling
begins with injection and ends with demolding. Afterwards the molding has to
exhibit a certain temperature and enough stability. Cooling channels in the
mold nearby the cavities support cooling. A cooling medium mostly oil or water
flows through the channels. Its temperature is set at a definite value and
controlled by a cooling aggregate.

1.4 Material Selection

Material
selection depends to a large extent on the functional constraints of the part.
Both amorphous and crystalline thermoplastic resins are used in injection
moulding. Short glass fibres are commonly used as reinforcements.

Types of Plastic Materials:

Thermoplastics:-Thermoplasticsaretheplasticsthatdonotundergochemicalchangeintheircompositionwhenheatedandcanbemoldedagainandagain.Theyareeasilymoldedandextrudedintofilms,fibersandpackaging.Examplesincludepolyethylene(PE),polypropylene(PP)andpolyvinylchloride(PVC).

Thermosettingplastics:-Thermosettingplasticswhichareformedbyheatprocessbutarethenset(likeconcrete)andcannotchangeshapebyreheating.Theyarehardanddurable.Thermosetscanbeusedforautoparts,aircraftpartsandtires.Examplesincludepolyurethanes,polyesters,epoxyresinsandphenolicresins.

 

Commonly
used thermoplastics in injection moulding include:

?
Acrylonitrile butadiene styrene (ABS)

?
Acetal

?
Acrylic

?
Polycarbonate (PC)

?
Polyester

?
Polyethylene

?
Fluoroplastic

?
Polyimide

?
Nylon

?
Polyphenylene oxide

?
Polypropylene (PP)

?
Polystyrene (PS)

?
Polysulphone

?
Polyvinyl chloride (PVC)

 

The physical properties
of the materials (density, thermal conductivity, melting temperature, etc.)
must be considered in light of the required mechanical properties of the
finished part (strength, stiffness, hardness, etc.)

This schematic
illustrates the performance spectrum of a variety of plastic materials.

Various additives may be added to injection moulding
compounds to accomplish various purposes. This table summarizes some of them.
Additive

Function

Examples

Filler

increase bulk density

calcium carbonate, talc, limestone

Plasticizer

improve process ability, reduce product brittleness

phthalate esters, phosphate esters

Antioxidant

prevent polymer oxidation

phenols, aromatic amines

Colorant

provide desired part application colour

oil-soluble dyes, organic pigments

Flame retardant

reduce polymer flammability

antimony trioxide

Stabilizer

stabilize polymer against heat or UV light

carbon black, hydroxybenzophenone

Reinforcement

improve strength

E-glass, S-glass, carbon, Kevlar fibres

Polymer physical
properties dictate temperature processing window (Tg < Tproc < Tdeg) The operating temperature must lie in the range between the glass transition temperature and the degradation temperature of the polymer. 2.6.1 Resin data table Generic Name Melt Temperature (ºC) Mould Temperature (ºC) Min. Rec. Max. Min. Rec. Max. ABS 200 230 280 25 50 80 PA 12 230 255 300 30 80 110 PA 6 230 255 300 70 85 110 PA 66 260 280 320 70 80 110 PBT 220 250 280 15 60 80 PC 260 305 340 70 95 120 PC-ABS 230 265 300 50 75 100 PC-PBT 250 265 280 40 60 85 PE-HD 180 220 280 20 40 95 PE-LD 180 220 280 20 40 70 PEI 340 400 440 70 140 175 PET 265 270 290 80 100 120 PETG 220 255 290 10 15 30 PMMA 240 250 280 35 60 80 POM 180 225 235 50 70 105 PP 200 230 280 20 50 80 PPE-PPO 240 280 320 60 80 110 PS 180 230 280 20 50 70 PVC 160 190 220 20 40 70 SAN 200 230 270 40 60 80  

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