BrE or Injection molding AmE, is a manufacturing
process for producing parts by injecting molten material into a mould.
Injection moulding can be performed with a host of materials mainly
including metals, (for which the process is called die-casting),
glasses, elastomers, confections, and most commonly thermoplastic and
thermosetting polymers. Material for the part is fed into a heated
barrel, mixed (Using a helical shaped screw), and injected (Forced)
into a mould cavity, where it cools and hardens to the configuration
of the cavity.:240 After a product is designed, usually by an
industrial designer or an engineer, moulds are made by a mould-maker
(or toolmaker) from metal, usually either steel or aluminium, and
precision-machined to form the features of the desired part. Injection
moulding is widely used for manufacturing a variety of parts, from the
smallest components to entire body panels of cars. Advances in 3D
printing technology, using photopolymers which do not melt during the
injection moulding of some lower temperature thermoplastics, can be
used for some simple injection moulds.
Parts to be injection moulded must be very carefully designed to
facilitate the moulding process; the material used for the part, the
desired shape and features of the part, the material of the mould, and
the properties of the moulding machine must all be taken into account.
The versatility of injection moulding is facilitated by this breadth
of design considerations and possibilities.
2 Process characteristics
4 Examples of polymers best suited for the process
5.1.1 Mould design
5.1.2 Mould storage
5.2 Tool materials
6 Injection process
Injection moulding cycle
6.2 Scientific versus traditional moulding
6.3 Different types of injection moulding processes
7 Process troubleshooting
7.1 Moulding defects
8 Power requirements
9 Robotic moulding
11 See also
13 Further reading
14 External links
Injection moulding is used to create many things such as wire spools,
packaging, bottle caps, automotive parts and components, gameboys,
pocket combs, some musical instruments (and parts of them), one-piece
chairs and small tables, storage containers, mechanical parts
(including gears), and most other plastic products available today.
Injection moulding is the most common modern method of manufacturing
plastic parts; it is ideal for producing high volumes of the same
Thermoplastic resin pellets for injection moulding
Injection moulding uses a ram or screw-type plunger to force molten
plastic material into a mould cavity; this solidifies into a shape
that has conformed to the contour of the mould. It is most commonly
used to process both thermoplastic and thermosetting polymers, with
the volume used of the former being considerably higher.:1–3
Thermoplastics are prevalent due to characteristics which make them
highly suitable for injection moulding, such as the ease with which
they may be recycled, their versatility allowing them to be used in a
wide variety of applications,:8–9 and their ability to soften and
flow upon heating.
Thermoplastics also have an element of safety over
thermosets; if a thermosetting polymer is not ejected from the
injection barrel in a timely manner, chemical crosslinking may occur
causing the screw and check valves to seize and potentially damaging
the injection moulding machine.:3
Injection moulding consists of the high pressure injection of the raw
material into a mould which shapes the polymer into the desired
shape.:14 Moulds can be of a single cavity or multiple cavities. In
multiple cavity moulds, each cavity can be identical and form the same
parts or can be unique and form multiple different geometries during a
single cycle. Moulds are generally made from tool steels, but
stainless steels and aluminium moulds are suitable for certain
Aluminium moulds are typically ill-suited for high
volume production or parts with narrow dimensional tolerances, as they
have inferior mechanical properties and are more prone to wear,
damage, and deformation during the injection and clamping cycles;
however, aluminium moulds are cost-effective in low-volume
applications, as mould fabrication costs and time are considerably
reduced. Many steel moulds are designed to process well over a
million parts during their lifetime and can cost hundreds of thousands
of dollars to fabricate.
When thermoplastics are moulded, typically pelletised raw material is
fed through a hopper into a heated barrel with a reciprocating screw.
Upon entrance to the barrel, the temperature increases and the Van der
Waals forces that resist relative flow of individual chains are
weakened as a result of increased space between molecules at higher
thermal energy states. This process reduces its viscosity, which
enables the polymer to flow with the driving force of the injection
unit. The screw delivers the raw material forward, mixes and
homogenises the thermal and viscous distributions of the polymer, and
reduces the required heating time by mechanically shearing the
material and adding a significant amount of frictional heating to the
polymer. The material feeds forward through a check valve and collects
at the front of the screw into a volume known as a shot. A shot is the
volume of material that is used to fill the mould cavity, compensate
for shrinkage, and provide a cushion (approximately 10% of the total
shot volume, which remains in the barrel and prevents the screw from
bottoming out) to transfer pressure from the screw to the mould
cavity. When enough material has gathered, the material is forced at
high pressure and velocity into the part forming cavity. The exact
amount of shrinkage is a function of the resin being used, and can be
relatively predictable. To prevent spikes in pressure, the process
normally uses a transfer position corresponding to a 95–98% full
cavity where the screw shifts from a constant velocity to a constant
pressure control. Often injection times are well under 1 second. Once
the screw reaches the transfer position the packing pressure is
applied, which completes mould filling and compensates for thermal
shrinkage, which is quite high for thermoplastics relative to many
other materials. The packing pressure is applied until the gate
(cavity entrance) solidifies. Due to its small size, the gate is
normally the first place to solidify through its entire
thickness.:16 Once the gate solidifies, no more material can enter
the cavity; accordingly, the screw reciprocates and acquires material
for the next cycle while the material within the mould cools so that
it can be ejected and be dimensionally stable. This cooling duration
is dramatically reduced by the use of cooling lines circulating water
or oil from an external temperature controller. Once the required
temperature has been achieved, the mould opens and an array of pins,
sleeves, strippers, etc. are driven forward to demould the article.
Then, the mould closes and the process is repeated.
For a two shot mould, two separate materials are incorporated into one
part. This type of injection moulding is used to add a soft touch to
knobs, to give a product multiple colours, to produce a part with
multiple performance characteristics.
For thermosets, typically two different chemical components are
injected into the barrel. These components immediately begin
irreversible chemical reactions which eventually crosslinks the
material into a single connected network of molecules. As the chemical
reaction occurs, the two fluid components permanently transform into a
viscoelastic solid.:3 Solidification in the injection barrel and
screw can be problematic and have financial repercussions; therefore,
minimising the thermoset curing within the barrel is vital. This
typically means that the residence time and temperature of the
chemical precursors are minimised in the injection unit. The residence
time can be reduced by minimising the barrel's volume capacity and by
maximising the cycle times. These factors have led to the use of a
thermally isolated, cold injection unit that injects the reacting
chemicals into a thermally isolated hot mould, which increases the
rate of chemical reactions and results in shorter time required to
achieve a solidified thermoset component. After the part has
solidified, valves close to isolate the injection system and chemical
precursors, and the mould opens to eject the moulded parts. Then, the
mould closes and the process repeats.
Pre-moulded or machined components can be inserted into the cavity
while the mould is open, allowing the material injected in the next
cycle to form and solidify around them. This process is known as
Insert moulding and allows single parts to contain multiple materials.
This process is often used to create plastic parts with protruding
metal screws, allowing them to be fastened and unfastened repeatedly.
This technique can also be used for
In-mould labelling and film lids
may also be attached to moulded plastic containers.
A parting line, sprue, gate marks, and ejector pin marks are usually
present on the final part.:98 None of these features are typically
desired, but are unavoidable due to the nature of the process. Gate
marks occur at the gate which joins the melt-delivery channels (sprue
and runner) to the part forming cavity.
Parting line and ejector pin
marks result from minute misalignments, wear, gaseous vents,
clearances for adjacent parts in relative motion, and/or dimensional
differences of the mating surfaces contacting the injected polymer.
Dimensional differences can be attributed to non-uniform,
pressure-induced deformation during injection, machining tolerances,
and non-uniform thermal expansion and contraction of mould components,
which experience rapid cycling during the injection, packing, cooling,
and ejection phases of the process. Mould components are often
designed with materials of various coefficients of thermal expansion.
These factors cannot be simultaneously accounted for without
astronomical increases in the cost of design, fabrication, processing,
and quality monitoring. The skillful mould and part designer will
position these aesthetic detriments in hidden areas if feasible.
American inventor John Wesley Hyatt, together with his brother Isaiah,
patented the first injection moulding machine in 1872. This machine
was relatively simple compared to machines in use today: it worked
like a large hypodermic needle, using a plunger to inject plastic
through a heated cylinder into a mould. The industry progressed slowly
over the years, producing products such as collar stays, buttons, and
The German chemists
Arthur Eichengrün and Theodore Becker invented
the first soluble forms of cellulose acetate in 1903, which was much
less flammable than cellulose nitrate. It was eventually made
available in a powder form from which it was readily injection
Arthur Eichengrün developed the first injection moulding
press in 1919. In 1939,
Arthur Eichengrün patented the injection
moulding of plasticised cellulose acetate.
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 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
coloured or recycled plastic could be added to virgin material and
mixed thoroughly before being injected. Today, screw injection
machines account for the vast majority of all injection machines. In
the 1970s, Hendry went on to develop the first gas-assisted injection
moulding process, which permitted the production of complex, hollow
articles that cooled quickly. This greatly improved design flexibility
as well as the strength and finish of manufactured parts while
reducing production time, cost, weight and waste.
The plastic injection moulding industry has evolved over the years
from producing combs and buttons to producing a vast array of products
for many industries including automotive, medical, aerospace, consumer
products, toys, plumbing, packaging, and construction.:1–2
Examples of polymers best suited for the process
Most polymers, sometimes referred to as resins, may be used, including
all thermoplastics, some thermosets, and some elastomers. Since
1995, the total number of available materials for injection moulding
has increased at a rate of 750 per year; there were approximately
18,000 materials available when that trend began. Available
materials include alloys or blends of previously developed materials,
so product designers can choose the material with the best set of
properties from a vast selection. Major criteria for selection of a
material are the strength and function required for the final part, as
well as the cost, but also each material has different parameters for
moulding that must be taken into account.:6 Common polymers like
epoxy and phenolic are examples of thermosetting plastics while nylon,
polyethylene, and polystyrene are thermoplastic.:242 Until
comparatively recently, plastic springs were not possible, but
advances in polymer properties make them now quite practical.
Applications include buckles for anchoring and disconnecting the
Injection moulding machine
Paper clip mould opened in moulding machine; the nozzle is visible at
Injection moulding machines consist of a material hopper, an injection
ram or screw-type plunger, and a heating unit.:240 Also known as
platens, they hold the moulds in which the components are shaped.
Presses are rated by tonnage, which expresses the amount of clamping
force that the machine can exert. This force keeps the mould closed
during the injection process. Tonnage can vary from less than 5
tons to over 9,000 tons, with the higher figures used in comparatively
few manufacturing operations. The total clamp force needed is
determined by the projected area of the part being moulded. This
projected area is multiplied by a clamp force of from 1.8 to 7.2 tons
for each square centimetre of the projected areas. As a rule of thumb,
4 or 5 tons/in2 can be used for most products. If the plastic material
is very stiff, it will require more injection pressure to fill the
mould, and thus more clamp tonnage to hold the mould
closed.:43–44 The required force can also be determined by the
material used and the size of the part. Larger parts require higher
Mould or die are the common terms used to describe the tool used to
produce plastic parts in moulding.
Since moulds have been expensive to manufacture, they were usually
only used in mass production where thousands of parts were being
produced. Typical moulds are constructed from hardened steel,
pre-hardened steel, aluminium, and/or beryllium-copper alloy.:176
The choice of material to build a mould from is primarily one of
economics; in general, steel moulds cost more to construct, but their
longer lifespan will offset the higher initial cost over a higher
number of parts made before wearing out. Pre-hardened steel moulds are
less wear-resistant and are used for lower volume requirements or
larger components; their typical steel hardness is 38–45 on the
Rockwell-C scale. Hardened steel moulds are heat treated after
machining; these are by far superior in terms of wear resistance and
lifespan. Typical hardness ranges between 50 and 60 Rockwell-C (HRC).
Aluminium moulds can cost substantially less, and when designed and
machined with modern computerised equipment can be economical for
moulding tens or even hundreds of thousands of parts. Beryllium copper
is used in areas of the mould that require fast heat removal or areas
that see the most shear heat generated.:176 The moulds can be
manufactured either by
CNC machining or by using electrical discharge
Injection moulding die with side pulls
"A" side of die for 25% glass-filled acetal with 2 side pulls.
Close up of removable insert in "A" side.
"B" side of die with side pull actuators.
Insert removed from die.
Standard two plates tooling – core and cavity are inserts in a mould
base – "family mould" of five different parts
The mould consists of two primary components, the injection mould (A
plate) and the ejector mould (B plate). These components are also
referred to as moulder and mouldmaker.
Plastic resin enters the mould
through a sprue or gate in the injection mould; the sprue bushing is
to seal tightly against the nozzle of the injection barrel of the
moulding machine and to allow molten plastic to flow from the barrel
into the mould, also known as the cavity.:141 The sprue bushing
directs the molten plastic to the cavity images through channels that
are machined into the faces of the A and B plates. These channels
allow plastic to run along them, so they are referred to as
runners.:142 The molten plastic flows through the runner and
enters one or more specialised gates and into the cavity:15
geometry to form the desired part.
Sprue, runner and gates in actual injection moulding product
The amount of resin required to fill the sprue, runner and cavities of
a mould comprises a "shot". Trapped air in the mould can escape
through air vents that are ground into the parting line of the mould,
or around ejector pins and slides that are slightly smaller than the
holes retaining them. If the trapped air is not allowed to escape, it
is compressed by the pressure of the incoming material and squeezed
into the corners of the cavity, where it prevents filling and can also
cause other defects. The air can even become so compressed that it
ignites and burns the surrounding plastic material.:147
To allow for removal of the moulded part from the mould, the mould
features must not overhang one another in the direction that the mould
opens, unless parts of the mould are designed to move from between
such overhangs when the mould opens (using components called Lifters).
Sides of the part that appear parallel with the direction of draw (the
axis of the cored position (hole) or insert is parallel to the up and
down movement of the mould as it opens and closes):406 are
typically angled slightly, called draft, to ease release of the part
from the mould. Insufficient draft can cause deformation or damage.
The draft required for mould release is primarily dependent on the
depth of the cavity; the deeper the cavity, the more draft necessary.
Shrinkage must also be taken into account when determining the draft
required.:332 If the skin is too thin, then the moulded part will
tend to shrink onto the cores that form while cooling and cling to
those cores, or the part may warp, twist, blister or crack when the
cavity is pulled away.:47
A mould is usually designed so that the moulded part reliably remains
on the ejector (B) side of the mould when it opens, and draws the
runner and the sprue out of the (A) side along with the parts. The
part then falls freely when ejected from the (B) side. Tunnel gates,
also known as submarine or mould gates, are located below the parting
line or mould surface. An opening is machined into the surface of the
mould on the parting line. The moulded part is cut (by the mould) from
the runner system on ejection from the mould.:288 Ejector pins,
also known as knockout pins, are circular pins placed in either half
of the mould (usually the ejector half), which push the finished
moulded product, or runner system out of a mould.:143The ejection
of the article using pins, sleeves, strippers, etc., may cause
undesirable impressions or distortion, so care must be taken when
designing the mould.
The standard method of cooling is passing a coolant (usually water)
through a series of holes drilled through the mould plates and
connected by hoses to form a continuous pathway. The coolant absorbs
heat from the mould (which has absorbed heat from the hot plastic) and
keeps the mould at a proper temperature to solidify the plastic at the
most efficient rate.:86
To ease maintenance and venting, cavities and cores are divided into
pieces, called inserts, and sub-assemblies, also called inserts,
blocks, or chase blocks. By substituting interchangeable inserts, one
mould may make several variations of the same part.
More complex parts are formed using more complex moulds. These may
have sections called slides, that move into a cavity perpendicular to
the draw direction, to form overhanging part features. When the mould
is opened, the slides are pulled away from the plastic part by using
stationary “angle pins” on the stationary mould half. These pins
enter a slot in the slides and cause the slides to move backward when
the moving half of the mould opens. The part is then ejected and the
mould closes. The closing action of the mould causes the slides to
move forward along the angle pins.:268
Some moulds allow previously moulded parts to be reinserted to allow a
new plastic layer to form around the first part. This is often
referred to as overmoulding. This system can allow for production of
one-piece tires and wheels.
Two-shot injection moulded keycaps from a computer keyboard
Two-shot or multi-shot moulds are designed to "overmould" within a
single moulding cycle and must be processed on specialised injection
moulding machines with two or more injection units. This process is
actually an injection moulding process performed twice and therefore
has a much smaller margin of error. In the first step, the base colour
material is moulded into a basic shape, which contains spaces for the
second shot. Then the second material, a different colour, is
injection-moulded into those spaces. Pushbuttons and keys, for
instance, made by this process have markings that cannot wear off, and
remain legible with heavy use.:174
A mould can produce several copies of the same parts in a single
"shot". The number of "impressions" in the mould of that part is often
incorrectly referred to as cavitation. A tool with one impression will
often be called a single impression (cavity) mould.:398 A mould
with 2 or more cavities of the same parts will likely be referred to
as multiple impression (cavity) mould.:262 Some extremely high
production volume moulds (like those for bottle caps) can have over
In some cases, multiple cavity tooling will mould a series of
different parts in the same tool. Some toolmakers call these moulds
family moulds as all the parts are related. Some examples include
plastic model kits.:114
Manufacturers go to great lengths to protect custom moulds due to
their high average costs. The perfect temperature and humidity level
is maintained to ensure the longest possible lifespan for each custom
mould. Custom moulds, such as those used for rubber injection
moulding, are stored in temperature and humidity controlled
environments to prevent warping.
Tool steel is often used. Mild steel, aluminium, nickel or epoxy are
suitable only for prototype or very short production runs. Modern
hard aluminium (7075 and 2024 alloys) with proper mould design, can
easily make moulds capable of 100,000 or more part life with proper
Beryllium-copper insert (yellow) on injection moulding mould for ABS
Moulds are built through two main methods: standard machining and EDM.
Standard machining, in its conventional form, has historically been
the method of building injection moulds. With technological
CNC machining became the predominant means of making
more complex moulds with more accurate mould details in less time than
The electrical discharge machining (EDM) or spark erosion process has
become widely used in mould making. As well as allowing the formation
of shapes that are difficult to machine, the process allows
pre-hardened moulds to be shaped so that no heat treatment is
required. Changes to a hardened mould by conventional drilling and
milling normally require annealing to soften the mould, followed by
heat treatment to harden it again. EDM is a simple process in which a
shaped electrode, usually made of copper or graphite, is very slowly
lowered onto the mould surface (over a period of many hours), which is
immersed in paraffin oil (kerosene). A voltage applied between tool
and mould causes spark erosion of the mould surface in the inverse
shape of the electrode.
The number of cavities incorporated into a mould will directly
correlate in moulding costs. Fewer cavities require far less tooling
work, so limiting the number of cavities in-turn will result in lower
initial manufacturing costs to build an injection mould.
As the number of cavities play a vital role in moulding costs, so does
the complexity of the part's design. Complexity can be incorporated
into many factors such as surface finishing, tolerance requirements,
internal or external threads, fine detailing or the number of
undercuts that may be incorporated.
Further details, such as undercuts or any feature causing additional
tooling, will increase the mould cost. Surface finish of the core and
cavity of moulds will further influence the cost.
Rubber injection moulding process produces a high yield of durable
products, making it the most efficient and cost-effective method of
moulding. Consistent vulcanisation processes involving precise
temperature control significantly reduces all waste material.
Small injection moulder showing hopper, nozzle and die area
With injection moulding, granular plastic is fed by a forced ram from
a hopper into a heated barrel. As the granules are slowly moved
forward by a screw-type plunger, the plastic is forced into a heated
chamber, where it is melted. As the plunger advances, the melted
plastic is forced through a nozzle that rests against the mould,
allowing it to enter the mould cavity through a gate and runner
system. The mould remains cold so the plastic solidifies almost as
soon as the mould is filled.
Injection moulding cycle
The sequence of events during the injection mould of a plastic part is
called the injection moulding cycle. The cycle begins when the mould
closes, followed by the injection of the polymer into the mould
cavity. Once the cavity is filled, a holding pressure is maintained to
compensate for material shrinkage. In the next step, the screw turns,
feeding the next shot to the front screw. This causes the screw to
retract as the next shot is prepared. Once the part is sufficiently
cool, the mould opens and the part is ejected.:13
Scientific versus traditional moulding
Traditionally, the injection portion of the moulding process was done
at one constant pressure to fill and pack the cavity. This method,
however, allowed for a large variation in dimensions from
cycle-to-cycle. More commonly used now is scientific or decoupled
moulding, a method pioneered by RJG Inc. In this the
injection of the plastic is "decoupled" into stages to allow better
control of part dimensions and more cycle-to-cycle (commonly called
shot-to-shot in the industry) consistency. First the cavity is filled
to approximately 98% full using velocity (speed) control. Although the
pressure should be sufficient to allow for the desired speed, pressure
limitations during this stage are undesirable. Once the cavity is 98%
full, the machine switches from velocity control to pressure control,
where the cavity is "packed out" at a constant pressure, where
sufficient velocity to reach desired pressures is required. This
allows part dimensions to be controlled to within thousandths of an
inch or better.
Different types of injection moulding processes
Sandwich-moulded toothbrush handle
Although most injection moulding processes are covered by the
conventional process description above, there are several important
moulding variations including, but not limited to:
Metal injection moulding
Thin-wall injection moulding
Injection moulding of liquid silicone rubber:17–18
Reaction injection moulding
A more comprehensive list of injection moulding processes may be found
Like all industrial processes, injection moulding can produce flawed
parts. In the field of injection moulding, troubleshooting is often
performed by examining defective parts for specific defects and
addressing these defects with the design of the mould or the
characteristics of the process itself. Trials are often performed
before full production runs in an effort to predict defects and
determine the appropriate specifications to use in the injection
When filling a new or unfamiliar mould for the first time, where shot
size for that mould is unknown, a technician/tool setter may perform a
trial run before a full production run. They start with a small shot
weight and fills gradually until the mould is 95 to 99% full. Once
this is achieved, a small amount of holding pressure will be applied
and holding time increased until gate freeze off (solidification time)
has occurred. Gate freeze off time can be determined by increasing the
hold time, and then weighing the part. When the weight of the part
does not change, it is then known that the gate has frozen and no more
material is injected into the part. Gate solidification time is
important, as this determines cycle time and the quality and
consistency of the product, which itself is an important issue in the
economics of the production process. Holding pressure is increased
until the parts are free of sinks and part weight has been achieved.
Injection moulding is a complex technology with possible production
problems. They can be caused either by defects in the moulds, or more
often by the moulding process itself.:47–85
Raised or layered zone on surface of the part
Tool or material is too hot, often caused by a lack of cooling around
the tool or a faulty heater
Air burn/gas burn/dieseling/gas marks/Blow marks
Black or brown burnt areas on the part located at furthest points from
gate or where air is trapped
Tool lacks venting, injection speed is too high
Color streaks (US)
Colour streaks (UK)
Localised change of colour
Masterbatch isn't mixing properly, or the material has run out and
it's starting to come through as natural only. Previous coloured
material "dragging" in nozzle or check valve.
Thin mica like layers formed in part wall
Contamination of the material e.g. PP mixed with ABS, very dangerous
if the part is being used for a safety critical application as the
material has very little strength when delaminated as the materials
Excess material in thin layer exceeding normal part geometry
Mould is over packed or parting line on the tool is damaged, too much
injection speed/material injected, clamping force too low. Can also be
caused by dirt and contaminants around tooling surfaces.
Foreign particle (burnt material or other) embedded in the part
Particles on the tool surface, contaminated material or foreign debris
in the barrel, or too much shear heat burning the material prior to
Directionally "off tone" wavy lines or patterns
Injection speeds too slow (the plastic has cooled down too much during
injection, injection speeds should be set as fast as is appropriate
for the process and material used)
Halo or Blush Marks
Circular pattern around gate, normally only an issue on hot runner
Injection speed is too fast, gate/sprue/runner size is too small, or
the melt/mold temp is too low.
Part deformed by turbulent flow of material.
Poor tool design, gate position or runner. Injection speed set too
high. Poor design of gates which cause too little die swell and result
Small lines on the backside of core pins or windows in parts that look
like just lines.
Caused by the melt-front flowing around an object standing proud in a
plastic part as well as at the end of fill where the melt-front comes
together again. Can be minimised or eliminated with a mould-flow study
when the mould is in design phase. Once the mould is made and the gate
is placed, one can minimise this flaw only by changing the melt and
the mould temperature.
Polymer breakdown from hydrolysis, oxidation etc.
Excess water in the granules, excessive temperatures in barrel,
excessive screw speeds causing high shear heat, material being allowed
to sit in the barrel for too long, too much regrind being used.
Localised depression (In thicker zones)
Holding time/pressure too low, cooling time too short, with sprueless
hot runners this can also be caused by the gate temperature being set
too high. Excessive material or walls too thick.
Short fill or short mould
Lack of material, injection speed or pressure too low, mould too cold,
lack of gas vents
Splash mark or silver streaks
Usually appears as silver streaks along the flow pattern, however
depending on the type and colour of material it may represent as small
bubbles caused by trapped moisture.
Moisture in the material, usually when hygroscopic resins are dried
improperly. Trapping of gas in "rib" areas due to excessive injection
velocity in these areas. Material too hot, or is being sheared too
Stringing or long-gate
String like remnant from previous shot transfer in new shot
Nozzle temperature too high. Gate hasn't frozen off, no decompression
of the screw, no sprue break, poor placement of the heater bands
inside the tool.
Empty space within part (air pocket is commonly used)
Lack of holding pressure (holding pressure is used to pack out the
part during the holding time). Filling too fast, not allowing the
edges of the part to set up. Also mould may be out of registration
(when the two halves don't centre properly and part walls are not the
same thickness). The provided information is the common understanding,
Correction: The Lack of pack (not holding) pressure (pack pressure is
used to pack out even though is the part during the holding time).
Filling too fast does not cause this condition, as a void is a sink
that did not have a place to happen. In other words, as the part
shrinks the resin separated from itself as there was not sufficient
resin in the cavity. The void could happen at any area or the part is
not limited by the thickness but by the resin flow and thermal
conductivity, but it is more likely to happen at thicker areas like
ribs or bosses. Additional root causes for voids are un-melt on the
Knit line / Meld line / Transfer line
Discoloured line where two flow fronts meet
Mould or material temperatures set too low (the material is cold when
they meet, so they don't bond). Time for transition between injection
and transfer (to packing and holding) is too early.
Cooling is too short, material is too hot, lack of cooling around the
tool, incorrect water temperatures (the parts bow inwards towards the
hot side of the tool) Uneven shrinking between areas of the part
Improper fusion of two fluid flows, a state before weld line.
Threadline gap in between part due to improper gate location in
complex design parts including excess of holes (multipoint gates to be
provided), process optimization, proper air venting
Methods such as industrial CT scanning can help with finding these
defects externally as well as internally.
Moulding tolerance is a specified allowance on the deviation in
parameters such as dimensions, weights, shapes, or angles, etc. To
maximise control in setting tolerances there is usually a minimum and
maximum limit on thickness, based on the process used.:439
Injection moulding typically is capable of tolerances equivalent to an
IT Grade of about 9–14. The possible tolerance of a thermoplastic or
a thermoset is ±0.200 to ±0.500 millimetres.
In specialised applications tolerances as low as ±5 µm on both
diameters and linear features are achieved in mass production. Surface
finishes of 0.0500 to 0.1000 µm or better can be obtained. Rough
or pebbled surfaces are also possible.
The power required for this process of injection moulding depends on
many things and varies between materials used.
Reference Guide states that the power requirements depend on "a
material's specific gravity, melting point, thermal conductivity, part
size, and molding rate." Below is a table from page 243 of the same
reference as previously mentioned that best illustrates the
characteristics relevant to the power required for the most commonly
Melting point (°F)
Melting point (°C)
1.12 to 1.24
1.34 to 1.95
1.01 to 1.15
381 to 509
194 to 265
0.91 to 0.965
230 to 243
110 to 117
1.04 to 1.07
Automation means that the smaller size of parts permits a mobile
inspection system to examine multiple parts more quickly. In addition
to mounting inspection systems on automatic devices, multiple-axis
robots can remove parts from the mould and position them for further
Specific instances include removing of parts from the mould
immediately after the parts are created, as well as applying machine
vision systems. A robot grips the part after the ejector pins have
been extended to free the part from the mould. It then moves them into
either a holding location or directly onto an inspection system. The
choice depends upon the type of product, as well as the general layout
of the manufacturing equipment. Vision systems mounted on robots have
greatly enhanced quality control for insert moulded parts. A mobile
robot can more precisely determine the placement accuracy of the metal
component, and inspect faster than a human can.
Lego injection mould, lower side
Lego injection mould, detail of lower side
Lego injection mould, upper side
Lego injection mould, detail of upper side
Fusible core injection moulding
Hobby injection moulding
Injection mould construction
Multi-material injection moulding
Reaction injection moulding
Design of plastic components
Direct injection expanded foam molding
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