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1. Global Procurement -
Supplier Quality
Plastic molding
Lise Robert SQS (Supplier Quality Specialist)
Rev. 01 – Nov 29th 2018
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2. Plastics processes
Injection molding
Extrusion
Blow molding
Vacuum Forming
Compression Molding
Calendering
Rotational Molding
Line Bending
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3. Injection Molding
 Injection molding machines have
many components and are
available in different
configurations, including a
horizontal configuration and a
vertical configuration. However,
regardless of their design, all
injection molding machines utilize
a power source, injection unit,
mold assembly, and clamping unit
to perform the four stages of the
process cycle
 Injection moulding is a highly
automated production process for
producing large quantities of
identical items. Granulated or
powdered thermoplastic material
is heated, melted and then forced
under pressure into a
mould. Once in the mould the
material cools, forming a
component that takes on the
shape of the mould cavity.
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4. Injection moulding
Process
 Plastic powder or granules are fed from a hopper into a hollow
steel barrel which usually contains a rotating screw.
 The barrel is surrounded by a jacket of heaters which melt the
plastic material as it is carried along the barrel by the screw
towards the mould. This part of the process is similar to the heating
and compacting stages in the extrusion process. The screw is
forced back as the melted plastic collects at the end of the barrel.
Once a sufficient charge of melted plastic has accumulated a
hydraulic ram forces the screw forward injecting the thermoplastic
through a sprue into the mould cavity. The diagram (left) shows a
typical injection moulding machine. This one is capable of exerting
forces of up to 250 tonnes. Pressure is kept on the mould until the
plastic has cooled sufficiently for the mould to be opened and the
component ejected.
Horizontal molding
machine
Vertical molding
machine
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5. Injection moulding
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6. 6
Injection Molding

7. 7
Possible defects of injection molding

8. 11 most common defects
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 Flow Lines
 Sink Marks
 Vacuum Voids
 Surface Delamination
 Weld Lines
 Short Shots
 Warping
 Burn Marks
 Jetting
 Flash
 Splay

9. Flow Line
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 Causes:
Flow line defects are caused by the varying speed at which
the molten plastic flows as it changes direction through the
contours and bends inside the mold tool. They also occur
when the plastic flows through sections with varying wall
thickness, or when the injection speed is too low causing
the plastic to solidify at different speeds.
 Remedies:
Increase injection speeds and pressure to the optimal level,
which will ensure the cavities are filled properly (while not
allowing the molten plastic time to start cooling in the wrong
spot). The temperature of the molten plastic or the mold
itself can also be elevated to ensure the plastic does not
cool down sufficiently to cause the defect.
 Round corners and locations where the wall thickness
changes to avoid sudden changes in direction and flow
rate.
 Locate the gate at a spot in the tool cavity with thin walls.
Description: Flow lines are streaks, patterns, or lines - commonly
off-toned in color - that show up on the prototype part as a
consequence of the physical path and cooling profile of the molten
plastic as it flows into the injection mold tooling cavity. Injection
molded plastic begins its journey through the part tooling via an
entry section called a “gate.” It then flows through the tool cavity
and cools (eventually hardening into a solid).

10. Sink Marks
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 Causes:
Sink marks are often caused when the cooling time or the
cooling mechanism is insufficient for the plastic to fully cool
and cure while in the mold. They can also be caused by
inadequate pressure in the cavity, or by an excessive
temperature at the gate. All else being equal, thick sections
of the injection molded part take longer to cool than thin
ones and so are more likely to be where sink marks are
located.
 Remedies:
Mold temperatures should be lowered, holding pressure
increased, and holding time prolonged to allow for more
adequate cooling and curing.
 Reducing the thickness of the thickest wall sections will
also ensure faster cooling and help reduce the likelihood of
sink marks.
Description: Sink marks are small craters or depressions that
develop in thicker areas of the injection molded prototype when
shrinkage occurs in the inner portions of the finished product. The
effect is somewhat similar to sinkholes in topography, but caused
by shrinkage rather than erosion.

11. Bubbles, Vacuum Voids
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 Causes:
Vacuum voids are often caused by uneven solidification between
the surface and the inner sections of the part. This can be
aggravated when the holding pressure is insufficient to condense
the molten plastic in the mold (and thereby force out air that
would otherwise get trapped). Voids can also develop from a part
that is cast from a mold with two halves that are not correctly
aligned.
 Remedies:
 Locate the gate at the thickest part of the molding.
 Reduce melt temperature
 Reduce mold temperature
 Reduce injection speed
 Increase pack and hold pressures
 Increase pack and hold times
 Dry material to suggested moisture levels
 Increase back pressure
 Reduce melt temperature
 Ensure that mold parts are perfectly aligned.
Voids Description: Vacuum voids are pockets of air trapped within
or close to the surface of an injection molded part.
Bubbles Description: When moisture and/or a gaseous byproduct
gets mixed with the melt and is injected in the mold cavity this
moisture or gas if embedded inside the melt can show up as bubbles

12. Surface Delamination
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 Causes:
Foreign materials that find their way into the molten plastic
separate from the finished product because the
contaminant and the plastic cannot bond. The fact that they
cannot bond not only has an affect on the appearance of
the prototype, but also on its strength. The contaminant
acts as a localized fault trapped within the plastic. An over-
dependence on mold release agents can also cause
delamination.
 Remedies:
 Pre-dry the plastic properly before molding.
 Increase the mold temperature.
 Smooth out the corners and sharp turns in the mold design
to avoid sudden changes in melt flow.
 Focus more on the ejection mechanism in the mold design
to reduce or eliminate the dependence on mold release
agents.
Description: Surface delamination is a condition where thin surface
layers appear on the part due to a contaminant material. These layers
appear like coatings and can usually be peeled off (i.e. “delaminate”).

13. Weld Lines
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 Causes:
Weld lines are caused by the inadequate
bonding of two or more flow fronts when there
is partial solidification of the molten plastic.
 Remedies:
 Raise the temperature of the mold or molten
plastic.
 Increase the injection speed.
 Adjust the design for the flow pattern to be a
single source flow.
 Switch to a less viscous plastic or one with a
lower melting temperature
Description: A weld or meld line is a weakness or visible flaw
created when two or more flow paths meet during the filling process.
Weld lines can be caused by material flowing around holes or
inserts in the part, multiple injection gates or variable wall thickness
where hesitation or "race tracking" can occur. If the different flow
fronts have cooled before meeting, they don't interfuse well and can
cause a weakness in the molded part. A line, notch and/or color
change can appear.

14. Short Shot
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 Causes:
Short shots can be caused by a number of things. Incorrect
calibration of the shot or plasticizing capacities can result in the
plastic material being inadequate to fill the cavities. If the plastic is
too viscous, it may solidify before fully occupying all the cavities
and result in a short shot. Inadequate degassing or gas venting
techniques can also result in short shots because air is trapped
and has no way to escape; plastic material cannot occupy the
space that air or gas is already occupying.
 Remedies:
 Select a less viscous plastic with higher flowability. This plastic
will fill the hardest-to-reach cavities.
 Increase mold or melt temperature so as to increase flowability.
 Account for gas generation by designing the mold so that gas is
not trapped within the mold and is properly vented.
 Increase the material feed in the molding machine or switch to a
machine that has a higher material feed in the event that the
maximum material feed has been reached.
Description: As the term implies, short shots can be described as a
situation where a molding shot falls short. This means that the molten
plastic for some reason does not fully occupy the mold cavity or
cavities, resulting in a portion where there is no plastic. The finished
product becomes deficient because it is incomplete.

15. Warping
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 Causes:
Warping is usually caused by non-uniform cooling
of the mold material. Different cooling rates in
different parts of the mold cause the plastic to
cool differently and thus create internal stresses.
These stresses, when released, lead to warping.
 Remedies:
 Ensure that the cooling time is sufficiently long
and that it is slow enough to avoid the
development of residual stresses being locked
into the part.
 Design the mold with uniform wall thickness and
so that the plastic flows in a single direction.
 Select plastic materials that are less likely to
shrink and deform. Semi-crystalline materials are
generally more prone to warping.
Description: Warping (or warpage) is the deformation that occurs
when there is uneven shrinkage in the different parts of the molded
component. The result is a twisted, uneven, or bent shape where one
was not intended.

16. Burn Marks
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 Causes:
Burn marks are caused either by the
degradation of the plastic material due
to excessive heating or by injection
speeds that are too fast. Burn marks
can also be caused by the
overheating of trapped air, which
etches the surface of the molded part.
 Remedies:
 Reduce injection speeds.
 Optimize gas venting and degassing.
 Reduce mold and melt temperatures.
Description: Burn marks are discolorations, usually rust colored, that
appear on the surface of the injection molded prototypes.

17. Jetting
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 Causes:
Jetting occurs mostly when the melt temperature is too low
and the viscosity of the molten plastic becomes too high,
thereby increasing the resistance of its flow through the
mold. When the plastic comes in contact with the mold
walls, it is rapidly cooled and the viscosity is increased. The
material that flows through behind that viscous plastic
pushes the viscous plastic further, leaving scrape marks on
the surface of the finished product
 Remedies:
 Increase mold and melt temperatures.
 Increase the size of the gate so that the injection speed
becomes slower.
 Optimize gate design to ensure adequate contact between
the molten plastic and the mold.
 Change gate location
Description: Jetting refers to a situation where molten plastic fails to
stick to the mold surface due to the speed of injection. Being fluid, the
molten plastic solidifies in a state that shows the wavy folds of the jet
stream on the surface of the injection molded part.

18. Flash
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 Causes:
Flash can occur when the mold is not clamped together
with enough force (a force strong enough to withstand the
opposing forces generated by the molten plastic flowing
through the mold), which allows the plastic to seep through.
The use of molds that have exceeded their lifespan will be
worn out and contribute to the possibility of flash.
Additionally, excessive injection pressure may force the
plastic out through the route of least resistance.
 Remedies:
 Increase the clamp pressure to ensure that the mold parts
remain shut during shots.
 Ensure that the mold is properly maintained and cleaned
(or replaced when it has reached the end of its useful
lifespan).
 Adopt optimal molding conditions like injection speed,
injection pressure, mold temperature, and proper gas
venting.
Description: Flash is a molding defect that occurs when some
molten plastic escapes from the mold cavity. Typical routes for
escape are through the parting line or ejector pin locations. This
extrusion cools and remains attached to the finished product.

19. Splay
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 Dry plastic to suggested
moisture levels
 Decrease injection speeds
 Decrease melt temperature
 Decrease screw rotation speeds
 Decrease back pressure
 Increase mold temperatures
 Increase venting
 Increase gate sizes
Description: A layer/ streak of a unwanted gaseous byproduct from
the melt or moisture in the material comes in between the melt flow
and the cavity walls preventing the texture from being picked up and
in addition eventually leaving a residue

20. Extrusion
 This process can be compared to
squeezing toothpaste from a tube. It is
a continuous process used to produce
both solid and hollow products that
have a constant cross-section: for
example, a window frame, hose pipe,
curtain track, garden trellis.
 This extrusion is part of a
window seal made from
 thermoplastic elastomer (TPE).
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21. Blow moulding
 Process
The cycle starts with the mould open (1). A hollow
length of plastic, called a parison, is extruded down
between two halves of the mould (2).
 The mould closes and compressed air is blown into
the inside of the parison which inflates it, pushing the
soft plastic hard against the cold surfaces of the mould
(3). The plastic is cooled by the mould, causing it to
harden quickly (4). The mould is then opened, the
moulding ejected and the waste (called flash) is
trimmed off with a knife (5).
Extrusion blow moulding is an automated process that is used extensively to
make bottles and other lightweight, hollow parts from thermoplastic materials.
(1) (2) (3) (4) (5)
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22. 22
Blow molding

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Blow molding

24. Vacuum forming
This process is used to manufacture a variety of
products in thermoplastic materials. These products
range in size from garden-pond liners to food trays
used in supermarkets. A typical industrial-size
vacuum-forming machine is capable of producing
vacuum formings up to 1.8m x 1.5m in size.
Once the plastic sheet has cooled down
to below its freeze point the air flow is reversed to lift
the forming off the mould. If this is not done quickly
the forming tends to grip onto the mould and
attempts to prise them apart often result in damage
to the forming.
A mould is attached to a platen (support plate).
The platen and mould are then lowered and rigid
thermoplastic sheet material is clamped onto an air-
tight gasket and usually heated from above.
Once the thermoplastic sheet is softened enough (i.e.
reaches a plastic state) then air is blown in to raise
the sheet in a slight bubble before the platen is
raised bringing the mould into contact with the
plastic.
Any trapped air remaining between the platen and the heated
plastic sheet is then evacuated by a vacuum
pump. Atmospheric pressure acting over the top surface
completes the forming process by pressing the plastic sheet
onto the mould.
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25. 25
Vacuum Forming

26. Compression moulding
This is, historically, the oldest commercial plastics moulding
process and is mainly used to make products from thermosetting
materials. A combination of heat and pressure is used to change
the material's form and chemical structure.
Materials
Typical thermosetting plastics
used in compression
moulding are urea
formaldehyde and phenol
formaldehyde. These
materials are different from
thermoplastics as they cannot
be reheated and reshaped
because a chemical change,
called polymerisation, has
taken place. Generally
thermosets tend to be harder,
stiffer and more resistant to
the effects of heat and
chemical attack than are
thermoplastic materials.
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27. Compression Molding
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 The raw materials for compression
molding are usually in the form of
granules, putty-like masses, or preforms.
 They are first placed in an open, heated
mold cavity. The mold is then closed and
pressure is applied to force the material
to fill up the cavity.
 A hydraulic ram is often utilized to
produce sufficient force during the
molding process. The heat and pressure
are maintained until the plastic material
is cured.

28. Calendering
 Plastic film, sheet and coated materials such as wallpaper and fabrics
are produced by the calendering process. It involves rolling out a mass of
premixed plastics material between large rollers to form a continuous and
accurately sized film.
 The main material used is PVC; others include ABS and cellulose
acetate. PVC ranges from flexible to rigid and the final product is
composed of a number of basic materials which must be combined in a
uniform mixture of measured ingredients. These ingredients include a
resin of a specified molecular weight, stabilisers, lubricants, reinforcing
materials, colorants and plasticisers.
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29. 29
Calendering

30. Line bending
Line or strip bending is used to form straight, small curvature bends in thermoplastic sheet material. The process is quite
straightforward. An electric element similar to that in an electric fire, is enclosed in a channel which has an opening at the top.
Thermoplastic sheet is placed across supports above the opening. By
adjusting the height of these supports the width of strip to be heated can be
altered. The supports are set to a low height for tight bends and higher, which
has the effect of widening the heated area, if a more gradual bend is required.
When using machines which have a single heating element it is necessary to
regularly turn over the thermoplastic sheet to ensure both sides of the plastic
are heated evenly. This will help to avoid blisters appearing on the surface of
the plastic. Heating times will vary according to thickness and colour of the
thermoplastic.
Bending jigs are used to ensure that each bend is identical on every product.This removes the need to measure and
mark out each bend and therefore speeds up the production process.
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31. Rotational moulding
Rotational moulding is a process used mainly to manufacture hollow-shaped products such as footballs, road cones and storage tanks
up to 3m³ capacity.
A measured weight of thermoplastic is placed inside a cold mould
like the one below (Station 1). The mould is then closed and moved
into an oven chamber (Station 2) where it is heated to a
temperature of 230-400°C whilst being rotated simultaneously
around both vertical and horizontal axes. Rotational speeds are
usually between 2 and 20 r.p.m.
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32. 32
Rotational molding