Introduction to Polymer Manufacturing Processes
The conversion of raw polymers into finished products involves a series of polymer manufacturing processes. The first step consists of mixing additives into the polymer to achieve the required modification to the properties of the raw polymer.
The second stage is to create the desired shape. Inherent in the forming stage is the requirement to set or maintain that shape.
Forming can be conveniently divided into two-dimensional forming, where products have a relatively simple geometry, and also three-dimensional forming with complex geometry.
In most manufacturing there will be a number of finishing steps. The advantage of polymer processing over manufacturing with more traditional materials is that there are opportunities for cost savings through minimising finishing processes.
|1. MIXING (compounding)||Powder, melt, dispersion, solution|
|2.1 2-D FORMING||Extrusion, calendering, coating|
|2.2 3-D FORMING||Thermoforming Moulding: compression, transfer, Injection, blow; rotational|
Machining, decoration, assembly
Manufacture of a particular product may require more than one forming process.
A crucial feature of most polymer processes is the preparation of the polymeric material in a appropriately softened state to suit the forming stage. Usually the softened state is achieved by heating the polymer.
Setting the shape is achieved by either cooling or carrying out a chemical process (crosslinking) to achieve the necessary dimensional stability.
Polymer manufacturing processes can be divided into continuous processes and batch processes. In continuous process where raw material is fed in continuously and the product flow appears continuously e.g. extrusion, there is more efficient use of energy and it is easier to maintain a consistent quality.
For batch or cyclic processes, such as moulding processes, there is a higher probability of batch-to-batch variation and lower efficiency due to unproductive parts of the cycle (down time).
Polymer manufacturing processes – The common features of polymer processing are:
- Mass transfer
- Energy transfer (mainly heat energy)
- Flow and deformation (rheology)
Polymeric materials are characterised by high specific heat and also low thermal conductivity. Therefore, this makes them unsuitable for heating by conduction in thick sections. Consequently, the best form of feed is as finely divided granules or powders.
In conduction heating, the poor thermal conductivity results in undesirable temperature gradients and shear (frictional) heating is faster and also provides more uniform temperatures.
The energy required to raise unit mass of polymer from room temperature to processing temperature is defined as the enthalpy. Semi-crystalline thermoplastics have enthalpies almost double that of their amorphous counterparts.
Because of their susceptibility to thermal decomposition it is advisable to heat polymers up quickly, form quickly and cool down as soon as possible and avoid long residence times at elevated temperatures.
In the melt state thermoplastics show varying resistance (viscosity) to applied flow stress.
Viscosity (resistance to flow) :
decreases with increasing temperature
increases with increasing pressure
decreases with increasing shear strain rate (shear thinning, pseudoplastic)
increases with increasing molecular size (MW)
decreases with increasing lubricant content
increases with increasing filler content
Polymer dispersions (latex, plastisols) can exhibit both shear thinning (pseudolplasticity) and shear thickening (dilatancy).
Because of their high viscosities, thermoplastic melts rarely show turbulent flow. In most situations it can be assumed that flow is laminar.
In an isothermal channel, such as an extrusion die where the wall is at the same temperature as the melt, the flow front will be parabolic. The highest velocity will be at the centre of the channel and reducing to zero at the wall.
In injection moulding, where a hot melt flows into cooler channels, a “frozen” skin layer is established at the wall with the melt flowing inside the skin in an unfolding melt front (“fountain flow”).
Extrusion is a process in which polymeric materials, in the form of powder, granules, trip or melt, are converted into products of controlled cross-section in a continuous fashion. This is achieved by softening (plasticising) the material using heat and/or pressure, forcing the softened material through and orifice (die) and maintaining the desired cross-section by cooling or by chemical reaction.
1. Extruded products
|Profile:||rod, edging strip, window frame, curtain rail|
|Hollow Section:||tubing, ducting, pipe|
|Flat Section:||sheet, cast film, tubular film, tape, oriented film|
|Coating:||wire covering, sheet coating|
|Mixing:||compounding, palletising, recycling|
2. Extruder Design
2.1 Ram Extruder
In its simplest form an extruder is analogous to icing a cake. The polymer is fed to the cylinder (barrel) in a plasticised state. The cylinder is heated to maintain the softened state.
A hydraulically actuated ram (plunger) forces the material through a die clamped to the end of the cylinder. A ram extruder is not truly continuous. However, it is appropriate for short product lengths, eg strip for tyre tread. Also the temperature distribution, and hence viscosity distribution, is poor.
2.2 Gear Pump Extruder
Gear Pump mechanisms, meshing cogs, work well on pre-plasticised , low viscosity materials, giving a positive throughput. However, they are poor for plasticising. Gear pumps are now used between a single screw extruder and the die to maintain constant throughput.
2.3 Single Screw Extruder
A rotating Archimedean screw in a cylinder ensures the continuous transfer of material from the feed (hopper) end to the die end. Heat is supplied from external heater bands or jackets.
With a simple screw design the conducted heat sets up unwelcome temperature gradients in the material. A long cylinder would be required for operating at a reasonable throughput.
The limiting factor would be the length of the screw which is restricted by the drive torque it can sustain. On smaller extruders (35 mm diameter screw) length/diameter (L/D) ratios go up to 35:1 but on larger extruders (200 mm diameter) L/D ratios approach 40:1.
2.3.1 Screw Design
To maximise the heat input and also to produce more uniform temperatures, frictional (shear) heating, generated between the rotating screw and the stationary cylinder, is maximised by putting the material under pressure. The compression is achieved by reducing the available volume (flight volume) along the screw by progressively increasing the root diameter of the screw.
For design and operational purposes, the screw can be divided into three zones:
|(a) FEED ZONE||From the hopper to the compression zone, with a constant channel depth (constant root or stem diameter);
the material is gradually warmed up by cylinder heaters and air in the powder or granular feed is removed back through the hopper
|(b) COMPRESSION ZONE
|Decreasing flight volume further compresses the material and plasticises it by shear heating|
|(c) METERING ZONE||Constant channel depth, smaller in comparison to the feed zone;
its purpose is to minimise any non-uniformity in composition, temperature and viscosity before the melt is delivered to the die;
the metering zone also acts like a non-return valve to isolate the die from the compression zone
Screws are normally constructed from hard tool steel with hard wearing flight tips. However, they are still subject to wear and require refurbishing at intervals. Some screws have internal water cooling to assist the feed mechanism in the feed zone and to also aid the shear heating process.
2.3.2 Extruder Cylinder
Because of the high pressures experienced during plasticisation the cylinder must be of strong construction. Hardened steel liners are used to minimise wear and the cost of replacing the complete cylinder.
For easily softened materials (e.g. elastomers) steam or hot oil jackets are sufficient but for thermoplastics cylinder heating is normally provided by banks of electrical resistance heaters.
Where shear heating is very efficient, cooling channels or aluminium fins with air blowers are used to take away excess heat.
At the feed end the hopper should be designed to give a positive flow of granules to the throat of the extruder. Magnets in the hopper prevent metal contamination from reaching and damaging the screw.
For materials that absorb moisture (e.g. polyamides) it is important to pre-dry the material. This can be done either off-line or with hopper dryers that circulate warm dry air.
Vacuum hoppers remove air from powder feed and prevent blocking (bridging). The level of material in the hopper can be automatically maintained using pneumatic filling devices from bags, bulk containers or silos.
Dosing devices for mixing virgin polymer with masterbatch or recycled regrind can also be incorporated into the hopper.
At the delivery end there is a flange to which is located the die, by screw fixtures, bolts or quick release mechanisms for die change and maintenance.
2.3.3 Screw Drive
Electric motors (either AC or DC) are used for screw rotation, normally with gear reduction and drive belts to give a range of screw speeds. Modern extruders have infinitely variable screw speeds. The high pressures created at the die end will tend to force the screw back on its axis and thrust plates or tapered roller bearings are incorporated to prevent backward movement.
3. Die Design
The function of an extruder die is to:
- produce the required extrudate cross-section;
- break up the rotational flow coming from the extruder
- maintain laminar flow to the die exit
- ensure consistent and minimal residence time of melt in the die
- make allowance for die swell
- maintain the melt temperature
3.1 Breaker Plate/Screen Pack
To break up the rotational flow coming from the extruder cylinder and also to establish laminar flow, a perforated plate (breaker plate) is inserted between the extruder cylinder and the die.
The breaker plate also helps to filter out unmolten granules and contamination as well as increasing the back pressure in the cylinder. Therefore, this results in more efficient plasticisation.
The filtering function and back pressure effect is enhanced by placing layers of fine wire mesh (screen pack) behind the breaker plate. Screen packs have to be changed (sometimes with automated systems, to maintain process conditions.
3.2 Die Head
Within the die bodies, laminar flow of the melt is maintained by avoiding dead spots and abrupt changes of sections. Most dies are designed with converging sections to increase the pressure before the die exit.
In the final section (die parallel or die land) cross-sections are maintained constant to minimise die swell. Dies normally have some form of external heating to maintain melt temperatures. Higher die temperatures at the die exit can produce a better finish on the extrudate and minimise other extrusion faults.
3.3 Die Swell
When a viscoelastic polymer melt is subjected to shear or extensional stresses in the die, there is an elastic recovery (memory) in the extrusion direction after the extrudate exits the die.
This elastic recovery manifests itself as a contraction on the length (not visible in a continuous process) and a corresponding expansion in the cross-section, producing an extrudate with cross-sectional dimensions greater than the dimensions of the die exit.
Die swell can vary from 10% to more than 100% increase in dimensions. It depends on the material, melt temperature, extrusion speed and also die geometry.
Die swell can be minimised by increasing the length of the die land, increasing the melt temperature and also by reducing the die throughput (decreasing screw speed)
3.4 Types of Dies
Dies vary in design, size and also in complexity to produce a wide range of extruded products.
3.4.1. Solid section Dies
To produce rod, edging strip, curtain rail and window frame profiles, dies have a simple design with a tapering entry to the die exit and a long land length (at least 10 times the cross-section dimension).
3.4.2. Hollow Section Dies
To produce tubing or pipe, dies are designed with an internal mandrel (torpedo) to create the internal dimensions. The complexity arises from the method of locating the mandrel within the die head to allow for concentricity adjustments and also to minimise flow disruption.
In some cases the mandrel is mounted on three small legs (spiders) in a straight through die. Sometimes it is mounted more rigidly in the die head, in a crosshead die where the melt has to sweep round the mandrel and turn through 90o to the die exit.
In both cases there will be weld line weaknesses in the extrudate where the melt flow is recombined. A further complication in hollow dies is that there has to be an internal pressure in the extrudate during cooling to prevent the hollow extrudate collapsing.
This is normally achieved by an air line through the mandrel via one of the spider legs. However, a more elegant solution is to apply an external vacuum to the extrudate as it is cooling.
3.4.3. Wire Coating Dies
With a cross-head die it is possible to feed a substrate, such as a wire conductor, through a hollow mandrel so that the substrate accepts a continuous coating of polymer melt in the die-head.
Dies can be designed to give a constant wall thickness of coating (tubing or sheathing dies) or to produce a product of constant outer diameter (in-filling dies).
In the wire and cable industry, in-filling dies are normally used for primary insulation and tubing dies for the outer sheath of cables.
3.4.4 Sheet and Film Dies
To produce flat section products of varying width, slit dies have been developed to produce film and sheet up to and over 2 m wide and thicknesses down to 100 µm. Fish tail designs, with gradually increasing width and reducing height, have problems in distributing melt to the edges.
Incorporating manifold channels into the die encourages more uniform flow (coathanger die). Restrictor bars are adjusted across the width for fine control of sheet thickness.
In biaxial stretch film manufacture extruded sheet, while still hot, is stretched in the length and in the cross-direction. This gives very wide (14 m) and very thin (less than 100 µm) film for packaging.
3.4.5 Tubular Film Dies
Annular cross-section dies produce thin walled tubing with uniform thickness. By stretching the hot tube circumferentially and stretching the tube in the length the diameter is increased and the thickness reduced. Slitting the tube in the length produces very wide film.
Alternatively cutting and heat sealing the tubular film is a simple method of manufacturing polyethylene bags and sacks.
3.4.6. Co-extrusion Dies
Thermoplastic sheet and packaging films can be produced in co-extrusion dies (slit dies or tubular dies) consisting of individual layers of different polymers bonded together.
The different melt streams, prepared in separate extruders, are brought together either at the die exit or at an earlier stage in the manifold. The key to co-extrusion lies in the laminar flow of thermoplastic melts, with no tendency for mixing between successive layers.
4. Downstream Equipment
The success of manufacture using the extrusion lies as much in the design and operation of the rest of the production as in the extruder and die.
Cooling of extruded products is achieved using air, water bath and contact with cold metal rollers. During the cooling process the dimensions of the extrudate have to be maintained or, in some cases, modified to meet the product specification.
Calibration takes the form metal plates, calibration dies, or rollers. Extrusion is a continuous process. However. products are normally cut to lengths (over 100 m for flexible products but less than 1 m for rigid extrudates).
Extrudates are automatically cut to length using guillotine action, cutter wheels or saws. Flexible film is wound on a roller, again with automated roll change.
Other downstream equipment includes edge trim, lamination, embossing, printing and also on-line quality checks (eg thickness profile, pinholes)
1. Injection Moulding Principles
For thermoplastics the dominant process for producing complex shapes is injection moulding in which the polymer melt is produced efficiently in one part of the machine.
In a separate function, the measured volume of melt is then forced into the cavity between the two mould plates. These are locked together to prevent separation under the high hydrostatic pressure of the melt in the cavity.
The final stage is to extract heat from the moulding using a heat exchange system incorporated into the mould plates, before the mould halves are opened and the finished moulding ejected. For thin walled parts cycle times can be less than 10 seconds but for thicker sections the cycle times may be well over 60 seconds.
Melt temperatures can be as low as 150oC but for some thermoplastics they have to be over 350oC. In all cases injection pressures can be up to 120 MPa, consequently creating large hydrostatic opening forces in the mould.
Mould locking (clamping) forces can be as low as 100 kN (10 tonnes force) for small area mouldings and more than 25,000 kN (2500 tonnes) for very large mouldings.
The injection moulding process can be conveniently divided into four phases.
|Phase 1||Plasticisation||Transformation of thermoplastic powder or granules into a homogeneous melt state|
|Phase 2||Injection||Transfer of melt from the plasticisation unit to all parts of the mould cavity|
|Phase 3||Setting||Cooling of the melt in the mould cavity to below its heat distortion temperature|
|Phase 4||Ejection||Opening of the mould and also removal of the finished moulding|
To keep the cycle time short, some of these phases can operate in parallel, eg plasticisation for the next cycle can occur at the same time as cooling and ejection.
To optimise the filling process injection times are short (less than 5 seconds and sometimes less than 1 second). This accounts for the high injection pressures (up to 120 MPa).
As soon as the melt starts to cool in the cavity it will contract and lose compression. Volumetric shrinkage during the cooling phase will result in a moulding that will be slightly smaller than the mould cavity and lacking the detail of the mould cavity.
To minimise the mould shrinkage effect, pressure is maintained for a few seconds after the injection stage to pump more melt in to compensate for the shrinkage.
A high hold pressure will also minimise mould shrinkage but it cannot be eliminated. Typical mould shrinkage values range from 0.5% or less for amorphous thermoplastics and filled thermoplastics to more than 1% for semi-crystalline thermoplastics.
To meet dimensional specifications mould cavity dimensions have to be adjusted to allow for the shrinkage. Because the final mould shrinkage value depends on process variables such as melt temperature, injection pressure, hold pressure and hold time, this presents a major problem to toolmakers to anticipate the shrinkage factor.
2. Machine Design
2.1. Plasticisation Unit
Originally plasticisation units were designed with plungers that pushed granules from a hopper, through a heated cylinder to melt the thermoplastic and forced the melt through a nozzle and feed channel into the mould cavity.
The inefficient plunger design was eventually replaced by the in-line screw design. Here plasticisation is achieved by the rotation of an archimedean screw in a heated cylinder, using shear heating more than conduction heating to give a more homogeneous melt.
The melt collecting at the head of the screw forces the screw back on its axis (while still rotating) until the required volume of melt has been produced.
Screw rotation is stopped and a hydraulic cylinder brings the screw forward in a plunger action to fill the mould cavity. By separating the plasticisation and injection stages efficiency and also quality are greatly increased.
2.2 Mould Unit
The function of the mould unit is to:
- locate the two mould halves without warping and misalignment;
- open and close the parts of the mould;
- lock the mould halves together during injection;
- assist in ejecting the mouldings when the mould opens;
- assist cooling of the moulding before ejection.
To minimise mould distortion, misalignment and warping the mould plates are mounted on large steel plates (platens). These are aligned with a series of tie bars (two for very small machines and 4 for larger machines).
Most designs consist of 3 platens, the fixed platen (next to the plasticisation unit), a tail stock and a moving platen in between, running on the tie bars to effect the opening and closing strokes. One mould plate is secured to the fixed platen and the other mould plate to the moving platen.
During the injection, pressure-hold and cooling phases the mould plates must be locked together by a force that exceeds the mould opening force created by the hydrostatic pressure of the melt in the cavity and the effective area of the mould cavity (projected area).
In some machines the locking force is generated by a hydraulic cylinder between the tail stock and the moving platen (direct hydraulic).
In other designs (toggle) a system of levers provide rigid beams to counteract the mould opening force in the mould-closed position but also provides rapid opening and closing actions with the aid of small hydraulic cylinders.
Although the majority of injection moulding machines are designed with horizontal plasticisation units and in-line, horizontally opening mould units there are other possible configurations.
- Horizontal plasticisation, vertical mould unit
- Vertical plasticisation, horizontal mould unit
- Vertical plasticisation, vertical mould unit
- Horizontal plasticisation, horizontal mould unit opening at 90o
Blow Moulding of thermoplastics has its origins in glass blowing technology. It accounts for over 5% of plastics consumption in producing a variety of hollow mouldings, bottles, large containers and also toys.
The basic principle of blow moulding is the inflation of a hot viscoelastic thermoplastic in a simple form (parison) to take up the internal shape of a two-part hollow mould.
The metal mould serves to cool the formed thermoplastic to allow the mould to be opened and the hollow moulding ejected.
Parison preparation is achieved by:
- extrusion of a hollow tube followed immediately by inflation;
- injection moulding of a simple shape followed immediately by inflation;
- reheating of a previously extruded tube or a moulded parison.
1. Extrusion Blow Moulding
Discontinuous extrusion of parisons, to allow for cutting to length, insertion of a blowing stick, inflation, cooling and ejection, would place unreasonable demands on an extruder that is designed for continuous operation. Continuouis extrusion can be accommodated by using two or more mould stations.
A single mould station can be adequate if the extruder is raised to allow vertical extrusion while the previous parison is being blown.
The main problem with large parisons is “parison sag” resulting in thin walls where the hot parison stretches under its own weight.
To minimise the effect of parison sag, the extruder steadily feeds melt into a chamber (accumulator) from which the parison is extruded quickly using a hydraulic ram.
During the inflation stage the parison stretches by different amounts at different positions in the mould giving rise to variable wall thickness. To produce hollow mouldings with more uniform wall thickness it is necessary to start with parisons with variable wall thickness.
This is achieved by moving a conical mandrel axially within the extrusion die during the extrusion of a parison length.
One of the main disadvantages of extrusion blow moulding is the amount of waste associated with the “flash” at the neck area and also at the pinch-off area. There is also considerable waste in the production of containers with integral handles.
2. Injection Blow Moulding
To reduce the amount of waste and also to control the wall thickness variations in the parison (to suit the degree of stretching in the final moulding), parisons can be produced by an injection moulding process.
The test-tube shaped parison, complete with detailed neck geometry, is moulded round a blowing stick on which it is transferred while still I the melt state to the two part blowing mould. However there is the additional cost of the parison mould.
3. Injection Stretch Blow Moulding
To produce thin walled bottles in PET and polypropylene with high strength and good transparency it is necessary to control crystallisation by biaxially stretching the parison during the final forming stage.
Stretching in the circumferential direction occurs naturally with inflation. Stretching in the length is achieved by mechanically stretching the parison using the blowing stick.
Because of the low pressures in the inflation stage (air pressures of the order of 0.5 MPa [ 6 bar]), moulds can be constructed from cast aluminium and other alloys as well as steel. The critical features of the “pinch-off” (for cutting and sealing one end of the parison) and the neck insert are usually constructed in tool steel.
The inner surface of the mould is usually shot blasted to prevent air entrapment between the parison and the mould. For high speed production, moulds are water cooled.
Most thermoplastics can be blow moulded. However, for best results there are special grades that have the desired level of hot strength; too low will result in parison sag; too high will require higher inflation pressures. The most popular materials for blow moulding are, polyethylene, polypropylene, polystyrene, PVC-u, polycarbonate and also polyethyleneterephthalate (PET).
In the bottle and container market polyethylene and polypropylene are used for their chemical resistance, low cost and flexibility in containers for detergents, bleach, polishes, inks and also cosmetics.
Polystyrene has been used in yoghurt pots, cosmetics packaging and also medical packaging.
Transparent rigid PVC bottles was originally used for water, fruit juice, wine, vegetable oils, shampoo and disinfectant but this market has been mostly taken over by PET.
With its excellent impact resistance, polycarbonate (PC) has been used for large water bottles.
Polyethyleneterephthalate (PET) provides high clarity, high strength, lightweight bottles for many of the above products. The excellent barrier properties make PET particularly suited to packaging of carbonated drinks (Coke, lemonade, beer).
Today many blow moulded products are produced from multi-layer extruded parisons with up to 7 layers of different materials, each supplying a different property to the final container (strength, barrier properties, printability, heat sealability, chemical resistance etc)
Some very large containers (up to 2 m3) have been produced by blow moulding. However, the latest development is “technical blow mouldings” to produce twin skin structures such as car bumpers and refrigerator doors.
So, we hope you enjoyed this guide to Polymer manufacturing processes.
Finally, you may wish to consult the British Plastics Federation page on Plastics Processes for more information.
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