Injection moulding

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

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.

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

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.

Plunger Type Injection Moulding Machine
Vertical & Horizontal Plunger Type Injection Moulding Machine

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

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.

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.

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.

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.

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.

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.

The mould opens for part ejection
The mould closes and the next cycle begins

Mould system
A typical (three-plate) moulding system

A two-plate mould. A three-plate mould.
The moulded system includes a delivery system and moulded parts.

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.

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 20:1 L/D ratio. But it may
range from 18:1 to 24:1
• In the case of Thermoset it may range from 12:1 to 16:1.

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.

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 1.5:1 to 4.5:1 for most
thermoplastic materials.
• Most injection screws classified as general purpose have
a compression ratio of 2.5:1 to 3.0:1.
• Thermo set screws have a 1:1 ratio.

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

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

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

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

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.

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)

Molecules lie in a definite fashion or regular arrangement

Molecules fall in Crystalline & amorphous pattern

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

Broad Molecular weight Distribution shows broad
Melting Points

Narrow Molecular weight Distribution shows sharp Melting Points

Plastic Product Properties can change 10% or more by
changing Process Conditions

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

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.

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.
TOGGLE TYPE CLAMPING

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.

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.
HYDRAULIC CLAMPING

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.

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.

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 * 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 * 0.90 / 1.05 = 68.57 gms of PP.

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 * 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.

Projected area is calculated by multiplying
length times width.
Determining Projected Area

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%.
Determining Clamping Force
(Tonnes)

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.

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.

Plastic flow
(a)Simple shear flow.
(b)Simple extensional flow.
(c) Shear flow in cavity filling.
(d) Extensional flow in cavity filling.

When Plastics flow in the cavity, the pressure decreases along
the delivery system and the cavity

Injection pressure as a function of melt viscosity, flow length,
volumetric flow rate, and part thickness

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

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

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

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

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.

Cycle time in injection moulding

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

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

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

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 Customer’s 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.

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

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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

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.

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

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.

Splay Marks (Silver
Streaking, Splash Marks)
Marks or droplet type imperfections
formed on the surface of a finished
part.

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.

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

Blush - Problems
• mould temperature too cold
• Injection fill speed too fast
• Melt stock temperature too high or too low.
• 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.

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.

Burn Marks - Problems
• Excessive Injection speed
• Excessive injection
pressure.
• Inefficient mould
temperature.
• Excessive amount of
volatiles due to improper
Venting.
• 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.

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.

Poor Weld Lines - Problems
• Material too cold.
• Injection speed too slow
• Entrapment of air at weld
line.
• Contamination of poorly
dispersed pigments.
• Core shifting.
• mould temperature to low.
• Injection speed too slow.
• Melt stock temperature to
low.
• Injection pressure too low.
• 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.

Voids (Bubbles)
An unfilled space of such size that it
scatters radiant energy such as light.

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.

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.

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.

Flow Lines and Folds
Mark visible on the finished item that
indicate the direction of flow in the cavity.

Flow Lines and Folds - Problems
• Stock temperature too low.
• Runners too small
• Improper gate size and/or location.
• Inadequate cold slug well.

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

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.

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

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.

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

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.

Brittleness - Problems
• Inadequate mould temperature
• Excessive screw rpm
• Excessive back pressure
• 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

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

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.

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

Blister - Problems
• Screw rpm too high
• Back pressure too low
• Gate improperly located
• Regrind too coarse

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

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

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

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

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

Low Gloss - Problems
• Inadequate polish of mould surface.
• Material or mould too cold.
• Air entrapment.
• Melt index of material too low.
• Wrong injection pressure.

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.

Short Shot
Injection of insufficient material
to fill the mould.

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

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

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