INJECTION MOULDING

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INJECTION MOULDING
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Historical Background

• A single-action hydraulic injection machine was
  designed in the U.S.A. in 1870 by Hyatt
• Heating-cylinder design was first recognised in a
  patent issued to Adam Gastron in 1932.
• Large-scale development of injection moulding
  machinery design towards the machines we know
  today did not occur until the 1950's in Germany

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Injection Moulding Process Over View

• Solid Wide neck, Flat Product is made like
  bucket, cabinets, Automobile Industrial parts
  etc. by injecting molten thermoplastic material
  in to a closed mould which is relatively cool.

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Type of Injection Moulding Machine

• Hand Injection Moulding M/C
• Plunger type Injection Moulding M/C
• Reciprocating Screw Type Injection Moulding M/C

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Hand Injection Moulding Machine

vertical machine consists of Barrel, Plunger,
Band Heaters along with energy regulator, Rack
Pinion system for Injecting the material by the
plunger, a torpedo and nozzle.
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Plunger Type Injection Moulding Machine

Vertical Horizontal Plunger Type Injection
Moulding Machine
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The Reciprocating Screw

• The feeding zone
• The compressing (or transition) zone
• The metering zone

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Machine components
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The Injection Process

• Plasticises the material by reciprocating Screw.
• Injects the molten material to a closed mould
• via a channel system of gates and runners.
• Cools the Mould.
• Refills the material for the next cycle.
• Ejects the Product.
• Closes the Mould for further cycle.

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Injection Moulded Items
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Injection Moulded Items
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Injection Moulded Items
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Advantages of Injection Moulding Process

• Parts can be produced at high production rates.
• Large volume production is possible.
• Relatively low labour cost per unit is
  obtainable.
• Process is highly susceptible to automation.
• Parts require little or no finishing.
• Many different surfaces, colours, and finishes
  are available.
• Good decoration is possible.
• For many shapes this process is the most
  economical way to fabricate.
• Process permits the manufacture of very small
  parts which are almost impossible to fabricate in
  quantities by other methods.

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Advantages of Injection Moulding Process

• Minimal scrap loss result as runners, gates, and
  rejects can be reground and reused.
• Same items can be moulded in different materials,
  without changing the machine or mould in some
  cases.
• Close dimensional tolerances can be maintained.
• Parts can be moulded with metallic and
  non-metallic inserts.
• Parts can be moulded in a combination of plastic
  and such fillers as glass, asbestos, talc and
  carbon.
• The inherent properties of the material give many
  advantages such as high strength-weight rates,
  corrosion resistance, strength and clarity.

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Limitations of Injection Moulding

• Intense industry competition often results in low
  profit margins.
• Mould costs are high.
• Moulding machinery and auxiliary equipment costs
  are high.
• Lack of knowledge about the fundamentals of the
  process causes problems.
• Lack of knowledge about the long term properties
  of the materials may result in long-term
  failures.

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Machine operation sequence
The mould closes and the screw begins moving
forward for injection.
The cavity fills as the reciprocating screw moves
forward, as a plunger.
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Machine operation sequence
The cavity is packed as the screw continuously
moves forward.
The cavity cools as the gate freezes off and the
screw begins to retract to plasticize material
for the next shot.
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Machine operation sequence
The mould opens for part ejection
The mould closes and the next cycle begins
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Injection Mould
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Mould system
A typical (three-plate) moulding system
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A two-plate mould.
A three-plate mould.
The moulded system includes a delivery system and
moulded parts.
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Screw Used in Injection Moulding Machines
The screw has three zones with a ring-plunger
assembly. The Feed Zone, where the plastic first
enters the screw and is conveyed along a constant
root diameter the Transition Zone, where the
plastic is conveyed, compressed and melted along
a root diameter that increases with a constant
taper and the Metering Zone, where the melting
of the plastic is completed and the melt is
conveyed forward along a constant root diameter
reaching a temperature and viscosity to form
parts.
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L/D RATIO

• The L/D ratio is the ratio of the flighted length
  (Effective Length) of the screw to its outside
  diameter. 
• Most injection screws use a 201 L/D ratio. But
  it may range from 181 to 241
• In the case of Thermoset it may range from 121
  to 161.

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High L/D Ratio results the following .

• More shear heat can be uniformly generated in the
  plastic without degradation
• Greater the opportunity for mixing, resulting in
  a better homogeneity of the melt.
• Greater the residence time of the plastic in the
  barrel possibly permitting faster cycles of
  larger shots.

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COMPRESSION RATIO (CR)

• The ratio of the first flight depth of feed zone
  to the last flight depth of meter zone ,
• Or,
• First Channel Volume of feed zone to last channel
  volume of metering zone,
• Typically ranges from 1.51 to 4.51 for most
  thermoplastic materials. 
• Most injection screws classified as general
  purpose have a compression ratio of 2.51 to
  3.01. 
• Thermo set screws have a 11 ratio.

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Higher the CR results the following .

• Greater shear heat imparted to the resin
• Greater heat uniformity of the melt
• High Potential for creating stresses in some
  resins
• High energy consumption

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Back Pressure (Kg/Cm2 or bar)

• Back pressure is the amount of pressure
  exerted by the material ahead of the screw, as
  the screw is pushed back in preparation for the
  next shot.
• Effect of Back Pressure
• More Homogeneous Mix
• Proper Melting
• More compact
• Sometime leads degradation

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Injection Speed (cm/Sec)

• The injection speed is the forward speed of
  the screw during its injection operation per unit
  time.
• Effect of Injection Speed
• Easy Injection of Material
• Avoid Short-Shot
• Some times leads more orientation burn marks

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Screw Rotation Speed

• The screw rotation speed (RPM) is the rate at
  which the plasticizing screw rotates.
• The faster the screw rotation result the
  following ..
• Faster the material is compressed by the screw
  flights
• Increasing the amount of shear heating
• Low residence time, some less melting

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Cushion

• The cushion is the difference in the final
  forward position of the screw and its maximum
  allowable forward position.
• More Cushion results more residence time, some
  time degrades.
• If the screw were allowed to travel its full
  stroke and stop mechanically against the nozzle,
  the cushion would be zero.
• With zero Cushion no hold on works.
• Typically a cushion of 3 to 6 mm is used.

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Materials for Injection Moulding

• Acrylonitrile butadiene styrene (ABS)
• Acetal
• Acrylic
• Polycarbonate (PC)
• Polyester
• Polyethylene
• Fluoroplastic
• Polyimide
• Nylon
• Polyphenylene oxide
• Polypropylene (PP)
• Polystyrene (PS)
• Polysulphone
• Polyvinyl chloride (PVC)

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Molecules lie in a definite fashion or regular
arrangement
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Molecules fall in Crystalline amorphous pattern
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Amorphous Polymer has
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While flowing in the channel or cavity of the
Mould. As the melt touches the surface of the
mould its viscosity increases because of lowering
of melt temperature, So it slides on the Surface
and the Molecules gets oriented
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Non Newtonian Plastics
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Non Newtonian Plastics
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Newtonian Plastic
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Broad Molecular weight Distribution shows broad
Melting Points
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Narrow Molecular weight Distribution shows sharp
Melting Points
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Plastic Product Properties can change 10 or more
by changing Process Conditions
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During Refilling
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Additive Function Examples
Filler increase bulk density calcium carbonate, talc, limestone
Plasticizer improve processability, reduce product brittleness phthalate esters, phosphate esters
Antioxidant prevent polymer oxidation phenols, aromatic amines
Colorant provide desired part application color 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 fibers
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TOGGLE TYPE 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.

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TOGGLE TYPE CLAMPING

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TOGGLE TYPE CLAMPING

• ADVANTAGE
• Low cost and lower horsepower needed to run.
• Positive clamp of the mould
• DISADVANTAGE
• Do not read the clamp force.
• Clamping is more difficult.
• Higher maintenance as lubricant is provided.

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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.

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HYDRAULIC CLAMPING
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HYDRAULIC CLAMPING

• ADVANTAGE
• Clamp speed easily controlled and stopped at any
  point.
• Direct a read out of clamp force.
• Easy adjustment of clamped force and easy mould
  set up.
• Low maintenance as part is self lubricated.
• DISADVANTAGE
• It is higher cost and more expensive than toggle
  system.
• None positive clamp.

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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.

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TIE-BAR LESS CLAMPING
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TIE-BAR LESS CLAMPING

• Much larger mould mounting area.
• Larger stroke compared to the toggle type
  machines.
• Full machine capacity can be utilised.
• Smaller machines can mould larger components.
• Saves floor space.
• Saves electrical energy because of reduction in
  the size of machine.
• Has the capacity to reduce weight of the moulded
  component because tie-bar stretching is not
  there.
• Machine becomes very flexible for future
  modification.
• Easy access to mould cavity's because of the
  absence of the tie bars.
• Robotic arm movement becomes easy.
• Fewer moving parts so lesser wear and tear so
  longer life for machines.
• Lower lubrication required.
• Removal of mould plates much simple.
• Greater stability.

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Theoretical Calculation
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Example 1 POM has an S.G. of 1.42. It is to be
moulded in an Injection Moulding Machine with a
shot weight of 80 gms (in PS). This machine has
a shot weight of 80 x 1.42 / 1.05 108.19 gms
of POM. Example 2 PP has an S.G. of 0.90. It
is to be moulded in an Injection Moulding Machine
with a shot weight of 80 gms (in PS). This
machine has a shot weight of 80 x 0.90 / 1.05
68.57 gms of PP.
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Example 3 Figurines made of UPVC (S.G. 1.38)
with a combined weight of figurine plus runners
of 40 gms. are to be moulded. What size of
machine is sufficient? Shot weight in terms of
PS 40 x 1.05/1.38 30.43 gms. Using the 85
guide line, the machine shot weight needed
30.43/0.85 35.80 gms. Example 4 The same
figurine in example 3 is to be moulded in a big
machine. What is the biggest machine that could
be used? Using the 35 rule, the biggest
machine that could be used has a shot weight
30.43/0.35 86.94 gms.
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Determining Projected Area
Projected area is calculated by multiplying
length times width.
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Determining Clamping Force (Tonnes)
Projected Area Length x Width and
multiplying that area by a clamp factor of
between 2 and 8. Most commonly factor 5 is
used. Clamp Force Projected Area x 5 For
every inch of depth the clamp force must be
increased by 10.
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Example 5 What is the residence time of UPVC
(S.G. 1.38) in a machine with screw diameter of
55 mm, injection stroke of 250 mm, shot weight
(PS) of 567 g, and a cycle time of 10 s moulding
shots weighing 260 g? Volume of melt in the
barrel is estimated to be two times the injection
volume 2 3.1416 5.5 5.5 25 / 4 1188
cm3 Barrel residence time 1188 1.38 10 /
260 63 s Example 6 A GPPS cup of diameter 79
mm is to be moulded. The cup is 0.6 mm at its
thinnest section. Find a conservative clamping
force which would be sufficient. The projected
area of the cup (and runner) is 3.1416 7.92 / 4
49 cm2. This cup belongs to the thin wall
domain. The conservative clamping force is 0.62
49 30.4 tonnes.
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Example 7 The same GPPS cup has a flow path
length of 104 mm. Find a more accurate clamping
force needed. Flow path to thickness ratio (L/T
Ratio) 104 / 0.6 173. From Figure 2, at 0.6
mm wall thickness, the cavity pressure is 550
bar. 1 bar 1.02 kg/cm2. The clamping force
550 1.02 49 27,500 kg 27.5 tonnes. The
above calculation has not accounted for
viscosity. It turns out to be still correct as
the viscosity factor for GPPS is 1.0. Example
8 The same cup as in the above example is to be
made out of ABS. Find the clamping force needed.
Using the viscosity factor of 1.5, the clamping
force needed 1.5 27.5 tonnes 41.3 tonnes.
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Plastic flow

1. Simple shear flow.
2. Simple extensional flow.

(c) Shear flow in cavity filling. (d)
Extensional flow in cavity filling.
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When Plastics flow in the cavity, the pressure
decreases along the delivery system and the
cavity
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Injection pressure as a function of melt
viscosity, flow length, volumetric flow rate, and
part thickness
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Setting Machine Process Conditions

• 1 Set the melt temperature
• 2 Set the mold temperature
• 3 Set the switch-over position
• 4 Set the screw rotation speed
• 5 Set the back pressure
• 6 Set the injection pressure to the machine
  maximum
• 7 Set the holding pressure at 0 MPa
• 8 Set the injection velocity to the machine
  maximum
• 9 Set the holding time
• 10 Set ample remaining cooling time

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Setting Machine Process Conditions

• 11 Set the mold open time
• 12 Mold a short-shot series by increasing
  injection volume
• 13 Switch to automatic operation
• 14 Set the mold opening stroke
• 15 Set the ejector stroke, start position, and
  velocity
• 16 Set the injection volume to 99 mold filled
• 17 Increase the holding pressure in steps
• 18 Minimize the holding time
• 19 Minimize the remaining cooling time

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Basic Process Factors in Injection Moulding

• Material Parameters
• Amorphous, Semicrystalline, Blends and Filled
  Systems
• Pressure-Volume-Temperature (PVT) Behaviour
• Viscosity
• Geometry Parameters
• Wall Thickness of Part
• Number of Gates
• Gate Location
• Gate Thickness and Area
• Type of Gates Manually or Automatically Trimmed
• Constraints from Ribs, Bosses and Inserts
• Manufacturing Parameters
• Fill Time
• Packing Pressure Level
• Mold Temperature
• Melt Temperature

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Residual stress
The development of residual flow stresses due to
frozen-in molecular orientation during the
filling and packing stages. (1) High cooling,
shear, and orientation zone (2) Low cooling,
shear, and orientation zone
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Process Controls

• Injection Moulding cycle can be broken down into
  four phases
• Fill,
• Pack,
• Hold, and
• Cooling/plastication

• These phases can be controlled by following
  variables
• Injection Speed,
• Plastic Temperature,
• Plastic Pressure,
• Cooling Temperature and Time.

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Cycle time in injection moulding
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Post Moulding Operation

• Heat inserting
• Chrome Plating
• In Mould Insert Moulding
• Post Mould Inserting
• Drilling
• Polishing
• Assembly

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Secondary operations

• Bonding
• Welding
• Inserting
• Staking
• Swaging
• Assembling with fasteners

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Secondary operations

• Appliqué a surface covering applied by heat and
  pressure
• Printing a process of making a mark or
  impression onto a substrate for decorative or
  informational purposes.
• Painting
• Hard coating
• Metallizing/shielding
• Surface treatment
• Annealing
• Machining

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Benefits of Post Moulding Operations

• Reduced costs by carrying out post moulding
  operations in house, and utilising lean
  manufacturing tools, we can greatly reduce
  component costs and the complexity of work that
  our customers would ordinarily undertake.
• High level of quality performing post-moulding
  operations on products helps ensure that a high
  level of quality is maintained. By checking parts
  from the moment they leave a press, to final
  assembly, quality levels can be maintained and
  ensure that components are only assembled to the
  highest standards.
• Reduction of Customers stock holding Assembly
  of components will reduce the cost of customers
  stock holding due to delivery of an assembly
  rather than a range of components.
• Reduced production times post moulding
  operations mean there is very little time between
  the production of components and their assembly.
  This means that a great deal of time can be saved
  when components would normally be transported, or
  stored, in between moulding and assembly
  operations.

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Heat inserting is the addition of inserts into a
part increases the functionality of a part by
which components can be assembled.
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Benefits of Heat Inserting

• Increased functionality by adding inserts to
  mouldings the part can more easily be used for
  its designed purpose. For example by adding
  threaded inserts parts can be easily be screwed
  to their fixings or other parts, increasing their
  functionality.
• Low part degradation the process of heat
  inserting means that the heating/melting of the
  part is very localised to where the insert will
  be pressed in. this means that parts do not
  suffer warping, or any other distortion effects,
  due to being heated again.
• High level of quality due to the known
  challenges with heat inserting extra measures are
  taken to ensure the processes is repeated to as
  high a level as possible, meaning part quality is
  kept very high.

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Chrome Plating
Due to the chrome plating process requiring the
part to be electrically conductive, a series of
steps are required before the chrome can be
deposited onto the surface of the product.
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Benefits of Chrome Plating

• Metal finish - Metal finishes can be very popular
  and, by coating plastics, advantage can be taken
  of characteristics from both materials.
• Wear resistant as chrome is a metal rather than
  a plastic its wear resistance properties are much
  greater than those of the plastic it covers. This
  means for applications where a part might be
  handled repeatedly, such as a shower handset, a
  chrome finish is likely to wear better than its
  plastic counterpart.
• Electrically conductive parts by chrome or
  nickel plating a part it is possible to give a
  plastic component the ability to conduct
  electricity. This gives the advantage of being
  able to create electrical components that are
  light weight and less costly to produce than
  completely metal parts.
• Attractive mouldings by applying chrome finish
  to mouldings a

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In Mould Insert Moulding
In mould insert moulding is the process by which
a metal, or preformed plastic, insert is
incorporated in to the component during the
moulding stage.
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Benefits Of In Mould Insert Moulding

• Reduced post-moulding operations With in mould
  insert moulding the need for post moulding
  operations is greatly reduced. This helps with
  ease of assembly and reduces the labour necessary
  for products.
• Increased part consistency Insert Moulding has
  major benefits in the consistency of parts
  produced. As the inserts are placed in the same
  locations in tools for every cycle each of the
  mouldings produced will be exactly the same. This
  helps reduce costs, as rejected parts will be
  kept to a minimum.
• Ease of assembly Due to inserts being
  incorporated into parts during the moulding stage
  this eases the assembly of the part. Instead of
  having to place fittings to attach parts fittings
  can be incorporated during the moulding stage so
  that parts can be simply clipped together.
• Reduced production time when vertical moulding
  machines, that are equipped with a rotary table,
  are used for production there is the opportunity
  to have two halves of the lower part of the tool.
  This means that production is almost constant
  with mouldings being formed at the same time as
  fresh inserts are being loaded into the second
  half of the tool. This lowers overall production
  times and can also reduce the amount of labour
  needed.

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Post Mould Inserting
Post mould inserting is the process by which a
metal, or preformed plastic, insert is
incorporated into a moulding by means of a
secondary process once the component has already
been moulded.
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Benefits of Post Mould Inserting

• Ease of assembly by adding inserts to a
  moulding the ease by which it can be assembled is
  greatly increased. Inserts such as clips or screw
  bolts can be incorporated into mouldings which
  greatly assist assembly operations and subsequent
  product performance.
• Increased part functionality besides adding
  inserts to aid assembly inserts that improve a
  parts functionality can also be used. For
  example, terminal fittings for wires, or seals to
  make parts watertight.
• Increased component value any second operation
  carried out on a part will add value to it. By
  adding inserts to help assembly or increase
  functionality, product value will be raised. This
  helps to compensate for the extra time involved
  in second operations and ensure products remain
  cost effective.
• Good part consistency to carry out post mould
  inserting jigs are used to hold mouldings while
  they are inserted. This means that the
  repeatability of the operation is very good and
  all parts inserted will be of the same quality.

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Drilling

• The drilling of parts is used to remove any
  unnecessary polymer that may have been necessary
  in the moulding process. By removing this extra
  material in house it means a ready-to-assemble
  moulding can be provided to the customer, or the
  part can be assembled with other mouldings.

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Polishing

• For products that have a high quality gloss
  finish a post moulding polishing operation is
  often a useful extra process. Even though the
  finish produced by the moulding tool may be of a
  very high quality, a polishing operation to
  remove any dust from the product before final
  packaging gives a part the high gloss finish that
  will have been specified.. Polishing operations
  are carried out on a soft-polishing wheel with
  high quality wax to ensure that a part is
  polished to a perfect finish without leaving any
  marks.

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Assembly

• For products that require assembly we are able to
  carry out this operation in our assembly
  facility. We can demonstrate examples of
  assemblies where we mould all the separate
  components in house and assemble the parts either
  as a whole in the assembly facility or as a step
  by step process on the press as each part is
  produced. By carrying out assembly in house we
  can reduce costs for our customers while still
  producing products to a high standard.

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Faults Remedies
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Sink Marks

• Depression in a moulded part caused by shrinking
  or collapsing of the resin during cooling.

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Sink Marks - Problems

• Resin feed inadequate
• Improper mould design.
• Parts cool too rapidly
• Rib section in part too wide.
• Temperature of mould surface opposite rib too
  hot.
• Entrapped gas.
• Nozzle too restrictive,
• land length too long.

• Pressure too low.
• Mould temperature too low or high
• Stock temperature too high
• Gate too small
• Improper gate location
• Nozzle and metering zone temperatures too high.
• Excessive cooling time in mould
• Unbalanced flow pattern.
• Bad check valve.

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Jetting

• Turbulence in the resin melt flow caused by
  undersized gate, abrupt change in cavity volume,
  or too high injection pressure.

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Jetting - Problems

• Excessive injection speed.
• Melt temperature too high.
• Melt temperature too low.
• mould Temperature too low.
• Nozzle opening too small.
• Gate and length too long.
• Sprue, runner, and/or gate size too small.
• Nozzle heating band malfunction.
• Inefficient gate location.

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Splay Marks (Silver Streaking, Splash Marks)

• Marks or droplet type imperfections formed on the
  surface of a finished part.

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Splay Marks (Silver Streaking, Splash Marks) -
Problems

• Obstruction in nozzle.
• Screw rpm too high.
• Back pressure too low.
• Melt temperature too high.
• Nozzle too hot.
• Nozzle too small.
• Gates too small.
• Sprue too small.
• Insufficient venting.

• Burr in runner or gate.
• Cracked mould.
• Trapped volatiles.
• Excessive moisture.
• Resin contaminated.
• mould cavity contamination.
• Excessive shot size.

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Blush

• Discoloration generally appearing at gates,
  around inserts, or other obstructions along the
  flow path. Usually indicates weak points.

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Blush - Problems

• mould temperature too cold
• Injection fill speed too fast
• Melt stock temperature too high or too low.
• Improper gate location
• Sprue and nozzle diameter too small.
• Nozzle temperature too low.
• Insufficient cold slug well.
• Sharp Corners in gate area
• Resin excessively moist.
• Inadequate injection pressure.

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Burn Marks

• Black marks or scorch marks on surface moulded
  part usually on the side of the part opposite
  the gate or in a deep cavity.

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Burn Marks - Problems

• Excessive Injection speed
• Excessive injection pressure.
• Inefficient mould temperature.
• Excessive amount of volatiles due to improper
  Venting.
• Improper gate location
• Front zone temperature too high.
• Screw speed too high.
• Excessive back pressure.
• Compression ratio of screw too high.

• Faulty temperature controllers.
• Frictional burring--gates too small
• Dead material hung up on screw or nozzle.
• Melt stock temperature too high or too low.
• Nozzle diameter too small
• Over-heated heater band
• Incorrect screw rpm.

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Poor Weld Lines (Knit Lines)

• Inability of two melt fronts to knit together in
  a homogeneous fashion during the moulding
  process, resulting in weak areas in the part of
  varying severity.

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Poor Weld Lines - Problems

• Insufficient mould venting
• Cylinder temperature too low.
• Injection back pressure too low.
• Nozzle diameter too small.
• Excessive screw flights in metering zone.
• Improper gate locations and/or size.
• Distance from gate excessive.
• Ineffective flow pattern.
• mould release agent (brittle weld lines).
• Inadequate flow.

• Material too cold.
• Injection speed too slow
• Entrapment of air at weld line.
• Improper mould design.
• Contamination of poorly dispersed pigments.
• Core shifting.
• mould temperature to low.
• Injection speed too slow.
• Melt stock temperature to low.
• Injection pressure too low.

128
Voids (Bubbles)

• An unfilled space of such size that it scatters
  radiant energy such as light.

129
Voids - Problems

• Injection pressure too low
• Packing time too short
• Insufficient feed of material
• mould temperature too low.
• Injection speed too high
• Excessive cushion
• At the side of a rib rib too thick.
• Runners or gate too small or badly positioned.

130
Delamination (Skinning)

• Surface of the finished part separates or appears
  to be composed of layer of solidified resin.
  Strata or fish scale type appearance where the
  layers may be separated.

131
Delamination - Problems

• Contamination of resin by additives or other
  foreign materials.
• Resin temperature too low.
• Non-uniformity of resin temperature.
• Wrong mould temperature.
• Excessive material moisture.
• Inadequate injection speed.
• Sharp corners at gate.
• Incompatible polymers.

132
Flow Lines and Folds

• Mark visible on the finished item that indicate
  the direction of flow in the cavity.

133
Flow Lines and Folds - Problems

• Stock temperature too low.
• Runners too small
• Improper gate size and/or location.
• mould temperature too low.
• Inadequate cold slug well.

134
Excessive Warpage/ Shrinkage

• Excessive dimensional change in a part after
  processing, or the excessive decrease in
  dimension in a part through cooling.

135
Warpage / Shrinkage -Problems

• mould closed time too short.
• Inefficient injection forward time.
• Ram speed too high or too low.
• Injection and holding pressure too high or low.
• Melt temperature inadequate.
• Excessive nozzle and metering zone temperatures.
• mould temperature too high (for thick wall
  sections).

• Parts cool unevenly.
• Parts underpacked.
• Improper gate location.
• Gate too restrictive
• Unequal temperature between mould halves.
• Non-uniform part ejection.
• Parts mishandled after ejection.
• Unbalanced gates on multiple gated part.
• Too many stresses in part.

136
Black Specks

• Particles in the surface of an opaque part and
  visible throughout a transparent part.

137
Black Specks - Problems

• Contamination of material.
• Holdup of molten resin moulding machine or mould
  runner system.
• Press Contamination.
• Local over-heating in the injection cylinder.
• Defective closure of the nozzle.
• Oxidation by occluded air or inadequate air
  venting
• mould contains grease.
• Trapped air
• Inefficient injection speed.

138
Brittleness

• Tendency of a moulded part to break, crack,
  shatter, etc. under conditions which it would not
  normally do so.

139
Brittleness - Problems

• mould temperature too high
• Inadequate cooling in gate area
• Gate section of item too thin (gate brittleness)
• Resin too cold.
• Non-uniformity of resin temperature.
• Undried material.
• Contamination.
• Poor part design.
• Material degraded.
• Non-compatible mould release.
• Packing the mould.
• Melt temperature too cold.
• Excessive amounts of regrind.

140
Brittleness - Problems

• Inadequate mould temperature
• Excessive screw rpm
• Excessive back pressure
• Insufficient venting.
• Improper gate location.
• Excessive injection speed.
• Excessive residence timed
• Melt temperature too high.
• Nozzle too hot.
• Injection pressure too low (weld lines).
• Runners and gates in adequate (weld lines).
• Dwell time in the injection cylinder too long
  (material degraded).
• Material degraded during drying or pre-heating

141
Flash

• Excess plastic around the area of the mould
  parting line on a moulded part.

142
Flash - Problems

• mould parting surfaces do not seal properly.
• Injection pressure too high.
• Clamp pressure set too low or projected area or
  item too large for clamp pressure of the machine.
• Injection temperature too high.
• Feed needs adjustment.
• Hold time too long.
• Inadequate mould supports.
• Oversize vents.

143
Blister

• Defect on the surface of a moulded part caused by
  gases trapped within the part during curing.

144
Blister - Problems

• Screw rpm too high
• Back pressure too low
• mould temperature too low.
• Gate improperly located
• Insufficient venting.
• Regrind too coarse

145
Crazing

• Fine cracks in part surface. May extend in a
  network over the surface or through the part.

146
Crazing - Problems

• Insufficient drying of the material.
• Contamination.
• Injection temperature too high (crazing
  accompanied by dis-coloring or yellowing).
• mould surface contaminated
• Inadequate injection speed.
• Inefficient injection forward time.
• Excessive injection pressure.
• mould temperature too low.
• Gate too large.

147
Cracking

• Fracture of the plastic material in an area
  around a boss, projection, or moulded insert.

148
Cracking - Problems

• Parts cool too quickly
• moulded-in stress
• Wall thickness too heavy for compound.

149
Low Gloss

• Surface roughness resulting from high speed fill
  which causes surface wrinkling as the polymer
  melt flows along the wall of the mould.

150
Low Gloss - Problems

• Inadequate polish of mould surface.
• Material or mould too cold.
• Air entrapment.
• Melt index of material too low.
• Improper mould design.
• Wrong injection pressure.
• Excessive injection speed.

151
Low Gloss - Problems

• Inadequate flow.
• Contamination
• Resin excessively moist
• Sprue, runners, and/or gate size too small.
• Pigment agglomerates.
• Oil or grease on knockout pins.

152
Short Shot

• Injection of insufficient material to fill the
  mould.

153
Short Shot - Problems

• Insufficient feed, cushion.
• Inadequate injection pressure.
• Inadequate injection speed.
• Insufficient booster or injection high-pressure
  time.
• Inefficient screw delay.
• Inadequate injection back pressure.
• Melt temperature too low.
• Cylinder temperature inadequate.
• mould temperature too low.

154
Short Shot - Problems

• Gates, sprues, and/or runners too small.
• Excessive screw flights in metering zone.
• Insufficient venting.
• Improper gate location.
• Melt index of resin too low.
• Excessive clearance between non-return valve and
  barrel.
• Screw bridging.
• Injection press of insufficient capacity.

155
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