INJECTION MOULDING

Injection moulded parts can be produced with; ribs, varying thickness and good surface finishes using all thermoplastic materials.  The orientation of molecules and reinforcement occurs during the process.  High pressure, nonuniform polymer shrinkage and orientation can lead to warpage and shrinkage over ribs and bosses.  Warpage is most apparent with crystalline materials and with large, flat parts.  Methods of controlling these effects are described below.

Plastic granules are softened during injection moulding and forced under pressure into a cold mould through small orifices or gates.  Pressure is maintained on the material after injection is complete so as to reduce shrinkage of the ribs and bosses as the material cools.  Pressure is higher at the gates because it will not transfer effectively through the compressible and rapidly cooling melt.  The additional packing pressure leads to a higher density of material near the gates and causes internal stresses.  These stresses tend to be partially relieved when the part is removed from the tool, resulting in warpage.

The plastic melt must flow from the gates, through the narrow gap between cooled mould surfaces, to the edge of the tool.  The gap becomes narrower as the material flows because some of the melt solidifies at the mould surface.  The pressure, flow rate and distance between the mould faces must be great enough, and the material viscosity low enough, to fill the mould before the solidifying material closes off the flow path. For each material and part thickness, there is a maximum practical flow length from a gate.

High pressures and narrow flow paths increase the orientation, which becomes greater as the gap freezes off.  Therefore, the orientation at the centre of the part wall is much higher than at the surface.  For the same reason, orientation is highest near the gates.  The gates should not be areas that are likely to suffer impact or other stresses, such as chemical attack.

The maximum practical thickness of the part is about four millimetres.  Above this thickness, cooling time becomes excessive.  The minimum normal thickness for injection moulding is about one millimetre.  Below this level, the party cools before the tool is filled and orientation is excessive.

The largest readily available injection moulding machines have a 3000 tonf clamping force, which restricts part size to about one cubic metre or less, for more difficult and filled materials. The flow length of the plastics from any one gate is limited to about 500mm with a 3mm wall thickness.    Therefore, multiple gates must be used for large parts. Gate design and position are very important for reducing part warpage and add to the complexity of the orientation effects.

The surface finish of injected moulded parts replicates the mould surface as it cools in contact with the surface, except over ribs and bosses.  Part design must be aimed at keeping ribs and bosses away form the back side of visible surfaces, reducing material in the rib root.  With filled or reinforced materials, the surface tends to be dull shows flow marks.

Cycle times vary from less than a minute to five minutes.  Injection moulding is the most useful thermoplastic processing method.  However, there are size limitations and a tendency towards warpage in flat parts.  Shrinkage over ribs can be designed around.

INJECTION COMPRESSION MOULDING

Injection compression moulding is sometimes known as coining.  The plastic melt is injected into the tool, which is held to a slightly greater opening than the ultimately desired part thickness.  As the amount of injected material approaches the desired part weight, the tool is closed to compress the material and to fill out the tool.

It is important for surface quality that the tool closure starts before injection stops and that the injection be completed before the tool is fully closed. This ensures that the material flow front does not stop.

Pressure requirements and orientation effects are less because material flows into the tool with the tool surfaces further apart than normal.  The rate of injection can be higher because the flow path is more open.

As the tool is closed down to the final part thickness, the melt is squeezed to the edges of the tool.  Orientation is less, because the final melt is not being forced through a narrow channel by high pressure from the gate.  Packing around the gate is eliminated as the injection is stopped before the tool is full.  Flash is reduced because their is no sudden pressure break, such as occurs in normal injection moulding when the tool fill is completed.

Long glass fibre, up to fifty per cent of fifty millimetre long, can be handled if properly formulated, because the lower injection pressures and larger gates allow the fibres to pass through more easily.  With lower built in stresses and less orientation, parts tend to exhibit much lower warpage when removed from the tool and less distortion and stress cracking in service.

Injection compression moulding is most useful for large area parts up to 1.5 square metres and for reinforced components requiring minimum warpage.  Sinkage over ribs is bad, or worse than with conventional injection moulding, because packing additional melt into ribs and bosses is not practical. However, the ability to add reinforcement could overcome the need to use ribs.

Although it is not widely used, injection compression moulding does offer the opportunity to overcome some of the size, orientation and reinforcement limitations of normal injection moulding. Injection compression moulding avoids the pressure peak obtained during normal injection. This allows larger parts to be made on the same tonnage machines as smaller parts. Internal stresses are lower because of a more even pressure distribution.

Flash is minimised, but a vertical flash tool is necessary.  This would normally have only one large, centrally located gate. Orientation is nearly eliminated. The need to use a vertical flash tool for this process limits its ability to be used for many parts, because of part shape.

HOLLOW INJECTION MOULDING

Hollow injection moulding is a relatively recent development.  High pressure gas is injected into the polymer melt flow at the nozzle of the machine or at the gates of a hot manifold system. The gas flows through the areas of lowest viscosity at the hotter centre of the melt.  Polymer injection is stopped before the part is full and this allows the gas to fill out the molten areas.  Final filling of the part is by gas pressure.

The molten areas must be designed to form a continuous path from the gate and along the ribs for the gas pressure to be effective to the extremities of the part.  Ribs must normally be widened at the root to allow for air passage.

Rib shrinkage is reduced, or eliminated, by this process.  Internal stresses and flashing are reduced, because the pressure peak is also eliminated, and this reduces warpage and finishing costs.  Pressure on the mould is also reduced.  Therefore, much lower machine clamping tonnage is necessary and the production of longer parts should be possible.

Surface finish is similar to that found in normal injection moulding, with the added advantage of reduced rib shrinkage. The process appears able to handle similar materials to normal injection moulding.  Limitations on reinforcement are similar to those of normal injection moulding.

Hollow injection moulding may require heavier wall sections than normal injection moulding and the process can be considered as being between normal injection moulding and foam injection moulding, with an improved surface.  Various alternative processes, using the melt stream injection of liquid or solid blowing agents, have been considered.

FOAM INJECTION MOULDING

Foam thermoplastic parts can be produced by adding a heat­activated blowing agent to the plastic granules or by injecting gas into the polymer melt in the injection moulding machine.  Foaming does not occur while the melt, containing the gas, is under high pressure in the injection machine barrel.  When the melt is injected into the mould, the trapped gas can expand to produce a foam.

To achieve foaming, the part thickness must be at least four millimetres and, for low densities, a minimum thickness of six millimetres is necessary to achieve a reasonable foam structure.

Cycle times are much longer than with other processes because of the greater part thickness.  This is sometimes balanced by feeding more than one tool from each injection unit.

Typical foam parts have a surface made up of collapsed cells, giving a swirl pattern similar to wood.  A major advantage of the process is that the foaming action completely fills large ribs and bosses, leaving a flat surface.  This is an excellent system for articles requiring a massive internal rib and boss system for stiffness, provided a high gloss finish is not required.

SANDWICH MOULDING

Sandwich moulding is used to produce parts with a skin of one material an a core of a different material. The skin material is normally unfilled and is chosen for its good surface characteristics, while the core material is usually foamed to eliminate sinkage or reinforced to increase stiffness.

The basic process relies on tow injection units connected, through a switchable valve, to the gate system of a tool in a single clamp unit.  The skin material is injected and this is immediately followed by injection of the core material. The core material pushes the skin material to the extremities of the mould, laying down a solidifying layer of skin on the cooled mouls surface as it passes.

With a reinforced core, the total thickness must be about one millimetre thicker than for normal injection moulding.  Size limitations are similar to those found in normal injection moulding, but multiple gating is difficult because the flow fronts always consist of skin material.

COMPRESSION MOULDING

Compression moulding is one of the few thermoplastic processing methods that allows the use of very long, or continuous, reinforcement.  Flow moulding and stamping are two forms of compression moulding.

Flow moulding involves heated plastic, moving in three dimensions, under the pressure exerted by the cold mould to fill the mould, carrying any reinforcements with it.  Ribs and bosses are filled with plastic and reinforcement, but little true control of reinforcement orientation can be achieved, even though oriented continuous fibre is used in the starting material.

Stamping is the deformation of a heated sheet of plastic under the pressure of a cold mould with minimal flow of material or change in reinforcement orientation.  Only single thickness parts are possible.

THERMOFORMING

Thermoforming is the forming of heated plastic sheet by the application of air pressure (pressure forming) or a vacuum between the heated sheet and the tool. The atmospheric pressure forces the sheet onto the tool, where it cools and retains the tool shape (vacuum forming).

It is impractical to form a reinforced sheet because of the low pressure involved and the tendency of the sheet to tear. Orientation and crystallisation effects can be used to strengthen or modify the physical properties of the plastic material, but such techniques are of interest only for low-cost food containers and similar items.

CONVENTIONAL BLOW MOULDING

Conventional blow moulding cannot handle reinforcement.  The only effect on the physical properties of the material caused by the processing is the tendency to orientate the molecules in the direction of the extrusion head. This can ultimately lead to failure of the product due too splitting in the direction of flow when subjected to impact or stress corrosion.

INJECTION BLOW MOULDING

For injection blow moulding, an injection moulded preform is used instead of an extruded parison.  The technique is particularly useful as a precursor for stretch blow moulding, in which blowing is carried out at lower temperatures with mechanical means or preform shapes to ensure that biaxial orientation takes place. Stiffness and strength are significantly increased by this process, as are other properties, such as resistance to the transfer of gases.  Most carbonated beverage containers make use of this biaxial effect.

ROTATION MOULDING

A plastic paste or powder is placed in a hollow metal mould, which is then heated and rotated so that the plastic melt coats the inside of the mould.  The mould is then cooled while still rotating. Parts produced in this way are difficult to reinforce because the fibres tend to separate from the plastics. Internal stresses are very low, but there is a risk of polymer degradation due to exposure to air. The process is used for decorative, nonstructural parts or as an alternative to blow moulding.