A Guide to Injection Moulding

What is injection moulding?

Injection moulding is one of the most widely used manufacturing processes in the plastic industry due to its quick cycle of production, material and colour flexibility, low labour costs, design flexibility and low waste. Injection moulding is the preferred process for manufacturing thermoplastic and thermosetting plastics parts of diverse industrial sectors such as packaging, electrical and electronics and automotive, etc.

This process converts solid plastic pellets to viscous masses by thermal conduction and pressure which are then injected into a mould where the melt acquires its final shape by cooling down. The process cycle can vary, typically from a few seconds to minutes, depending on the size of the moulds and materials used.

How does an injection moulding machine work?

The main components of an injection moulding machine include: the injection unit, the mould and the clamp unit (Figure 1).

– Injection unit: in this unit the polymer is gradually melted by simultaneous
application of external heating from the barrel and pressure.
– Mould: the mould comprises a cavity and a cooling system. Here, the
molten polymer is injected under pressure into the cavity and solidifies by
controlled cooling, acquiring its final form.
– Clamp unit: this unit is essential to maintain the pressure inside the mould
and keep the mould closed during the cycle of injection.

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Figure 1: Schema of an injection moulding machine

Photo from the PhD thesis of Ryan Gosselin “Injection de mousses composites bois/plastiques d’origine post-consommination” http://theses.ulaval.ca/archimede/fichiers/22718/ch01.html

An injection moulding machine consists therefore of an injection unit which brings the feedstock into the screw and moves the material along the screw, forcing the molten polymer into the mould tool. After solidification, the clamp, which holds the two halves of the mould together, opens and ejects the final product.

Exploring the main components of an injection moulding machine

The injection unit includes the hopper, barrel, heater bands, rotating screw with checking ring (non-return valve), and a hydraulic system (motor).

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Figure 2: Schema of the injection unit

The polymer is fed into the injection moulding machine through the hopper which is used to store the feedstock. The melting of the polymer is carried out by heaters and internal heat from viscous energy dissipation via shearing by the rotational motion of the screw. The polymer is then conveyed forward along the screw, achieving the required uniformity at the end of the barrel. The check ring prevents the plastic from being flown back, generating the required pressure and compression of the molten polymer before injection.

The mould comprises two parts: one fixed (cavity side) and one moving (core side). The stationary side is commonly called the female part of the print and it is directly in contact with the sprue and the runner system, where the molten polymer is injected from the injection unit. On the opposite side, the moving part contains the ejector system. This is needed to eject the moulded part and it is represented by the male part of the print.

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Figure 3: Photo of a mould closed (a) and opened (b)

The cooling system removes heat by blowing air or circulating a cooling fluid (generally water) through the channels inside the mould. When the temperature of the molten material is reduced, the moulded part is then ejected from the mould.

The principal function of the clamp unit consists in keeping the mould closed with enough pressure during the injection phase and opens it when the plastic part is at the required cooling temperature (Figure 4).

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Figure 4: Two clamping system showing opened and closed mould a) electric mechanic b) hydraulic

The screw plays an important role in the plastic injection moulding process as it is used both to plasticise the polymer and to inject it into the mould. The major functions of the screw are achieved at different sections along its length, which are: feed zone, compression and metering zone (Figure 5).

The feed zone brings the feedstock into the injection machine and moves the material along the screw. The polymer is then conveyed forward and compressed before reaching the compression zone, where it is gradually melted by simultaneous application of external heating from the barrel heaters and also from the internal viscous shear (75% of the supplied heat is in the form of viscous shearing by the motion of the screw). Finally, in the metering zone, which is confined to the last few turns on the screw, the uniformity of the melt is increased before injection.

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Figure 5: Injection screw description

The purpose of the check ring is to prevent melt loss as the screw moves forward during the injection part of the cycle. The check ring acts as a non-return valve; the ring is forced back against the screw tip seat and seals against it. When the screw stops pushing, the check ring is then allowed to move forward and the screw brings new material forward for the next cycle (Figure 6).

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Figure 6: Check ring schema a) screw bringing new material for the next cycle b) check ring sealing