Injection moulding (injection molding in
the USA) is a manufacturing process for producing parts by injecting material
into a mould. Injection moulding can be performed with a host of materials,
including metals, glasses, elastomers, confections, and most commonly
thermoplastic and thermosetting polymers. Material for the part is fed into a
heated barrel, mixed, and forced into a mould cavity, where it cools and
hardens to the configuration of the cavity. After a product is designed,
usually by an industrial designer or an engineer, moulds are made by a
mouldmaker (or toolmaker) from metal, usually either steel or aluminum, 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.
Process characteristics
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 former being considerably more prolific in terms of annual material volumes
processed. 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.
Injection moulding consists of high
pressure injection of the raw material into a mould which shapes the polymer
into the desired shape. 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 aluminum
moulds are suitable for certain applications. Aluminum moulds typically are
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, aluminum 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
pelletized raw material is fed through a hopper into a heated barrel with a
reciprocating screw. Upon entrance to the barrel the thermal energy 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 homogenizes 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. 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. 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 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.
Solidification in the injection barrel and screw can be problematic and have
financial repercussions; therefore, minimizing the thermoset curing within the
barrel is vital. This typically means that the residence time and temperature
of the chemical precursors are minimized in the injection unit. The residence
time can be reduced by minimizing the barrel's volume capacity and by
maximizing 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. 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.
