Wednesday, 24 January 2007

Plastics technology

INJECTION MOULDING
Russell Davison
Injection moulded parts can be produced with; ribs, varying thicknessand good surface finishes using all thermoplastic materials. Theorientation of molecules and reinforcement occurs during theprocess. High pressure, nonuniform polymer shrinkage andorientation can lead to warpage and shrinkage over ribs andbosses. Warpage is most apparent with crystalline materials andwith large, flat parts. Methods of controlling these effects aredescribed below.

Plastic granules are softened during injection moulding and forcedunder pressure into a cold mould through small orifices or gates. Pressure is maintained on the material after injection is complete soas to reduce shrinkage of the ribs and bosses as the materialcools. Pressure is higher at the gates because it will nottransfer effectively through the compressible and rapidly coolingmelt. The additional packing pressure leads to a higher densityof material near the gates and causes internal stresses. Thesestresses tend to be partially relieved when the part is removed fromthe tool, resulting in warpage.

The plastic melt must flow from the gates, through the narrow gapbetween cooled mould surfaces, to the edge of the tool. The gapbecomes narrower as the material flows because some of the meltsolidifies at the mould surface. The pressure, flow rate anddistance between the mould faces must be great enough, and the materialviscosity low enough, to fill the mould before the solidifying materialcloses off the flow path. For each material and part thickness, thereis a maximum practical flow length from a gate.

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

The maximum practical thickness of the part is about fourmillimetres. Above this thickness, cooling time becomesexcessive. The minimum normal thickness for injection moulding isabout one millimetre. Below this level, the party cools beforethe tool is filled and orientation is excessive.

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

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

Cycle times vary from less than a minute to five minutes. Injection moulding is the most useful thermoplastic processingmethod. However, there are size limitations and a tendencytowards warpage in flat parts. Shrinkage over ribs can bedesigned around.

INJECTION COMPRESSION MOULDING

Injection compression moulding is sometimes known as coining. Theplastic melt is injected into the tool, which is held to a slightlygreater opening than the ultimately desired part thickness. Asthe amount of injected material approaches the desired part weight, thetool is closed to compress the material and to fill out the tool.

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

Pressure requirements and orientation effects are less because materialflows into the tool with the tool surfaces further apart thannormal. The rate of injection can be higher because the flow pathis more open.

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

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

Injection compression moulding is most useful for large area parts upto 1.5 square metres and for reinforced components requiring minimumwarpage. Sinkage over ribs is bad, or worse than withconventional injection moulding, because packing additional melt intoribs and bosses is not practical. However, the ability to addreinforcement could overcome the need to use ribs.

Although it is not widely used, injection compression moulding doesoffer the opportunity to overcome some of the size, orientation andreinforcement limitations of normal injection moulding. Injectioncompression moulding avoids the pressure peak obtained during normalinjection. This allows larger parts to be made on the same tonnagemachines as smaller parts. Internal stresses are lower because of amore even pressure distribution.

Flash is minimised, but a vertical flash tool is necessary. Thiswould normally have only one large, centrally located gate. Orientationis nearly eliminated. The need to use a vertical flash tool for thisprocess limits its ability to be used for many parts, because of partshape.

HOLLOW INJECTION MOULDING

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

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

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

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

Hollow injection moulding may require heavier wall sections than normalinjection moulding and the process can be considered as being betweennormal injection moulding and foam injection moulding, with an improvedsurface. Various alternative processes, using the melt streaminjection of liquid or solid blowing agents, have been considered.

FOAM INJECTION MOULDING

Foam thermoplastic parts can be produced by adding a heat­activatedblowing agent to the plastic granules or by injecting gas into thepolymer melt in the injection moulding machine. Foaming does notoccur while the melt, containing the gas, is under high pressure in theinjection machine barrel. When the melt is injected into themould, the trapped gas can expand to produce a foam.

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

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

Typical foam parts have a surface made up of collapsed cells, giving aswirl pattern similar to wood. A major advantage of the processis that the foaming action completely fills large ribs and bosses,leaving a flat surface. This is an excellent system for articlesrequiring 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 materialan a core of a different material. The skin material is normallyunfilled and is chosen for its good surface characteristics, while thecore material is usually foamed to eliminate sinkage or reinforced toincrease stiffness.

The basic process relies on tow injection units connected, through aswitchable valve, to the gate system of a tool in a single clampunit. The skin material is injected and this is immediatelyfollowed by injection of the core material. The core material pushesthe skin material to the extremities of the mould, laying down asolidifying layer of skin on the cooled mouls surface as it passes.

With a reinforced core, the total thickness must be about onemillimetre thicker than for normal injection moulding. Sizelimitations are similar to those found in normal injection moulding,but multiple gating is difficult because the flow fronts always consistof skin material.

COMPRESSION MOULDING

Compression moulding is one of the few thermoplastic processing methodsthat 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 filledwith plastic and reinforcement, but little true control ofreinforcement orientation can be achieved, even though orientedcontinuous fibre is used in the starting material.

Stamping is the deformation of a heated sheet of plastic under thepressure of a cold mould with minimal flow of material or change inreinforcement orientation. Only single thickness parts arepossible.

THERMOFORMING

Thermoforming is the forming of heated plastic sheet by the applicationof air pressure (pressure forming) or a vacuum between the heated sheetand 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 lowpressure involved and the tendency of the sheet to tear. Orientationand crystallisation effects can be used to strengthen or modify thephysical properties of the plastic material, but such techniques are ofinterest only for low-cost food containers and similar items.

CONVENTIONAL BLOW MOULDING

Conventional blow moulding cannot handle reinforcement. The onlyeffect on the physical properties of the material caused by theprocessing is the tendency to orientate the molecules in the directionof the extrusion head. This can ultimately lead to failure of theproduct due too splitting in the direction of flow when subjected toimpact or stress corrosion.

INJECTION BLOW MOULDING

For injection blow moulding, an injection moulded preform is usedinstead of an extruded parison. The technique is particularlyuseful as a precursor for stretch blow moulding, in which blowing iscarried out at lower temperatures with mechanical means or preformshapes to ensure that biaxial orientation takes place. Stiffness andstrength are significantly increased by this process, as are otherproperties, such as resistance to the transfer of gases. Mostcarbonated beverage containers make use of this biaxial effect.

ROTATION MOULDING

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

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