Polymer processing methods


Introduction

There are many methods for converting compounded plastic to the finished product. Which processing method is best suited to a given material is dictated primarily by the class of polymer (thermoplastic, thermoset, or elastomer), as this determines its physical form and cure mechanism. However, within each class, there will be processing variants which depend on differences in the thermal and melt properties of the compounded polymer.

Thermoplastics are, with few exceptions, supplied as pellets, about 3mm across. Many require drying before processing to eliminate absorbed water, which might otherwise result in bubbles in the melt which would weaken the finished product. However, as was implied in the ‘time and time again’ in polymer basics, any surplus material or scrap can be remelted or returned for reprocessing.

Thermoset materials are generally supplied as liquids. Some rely on chemical initiation of polymerisation, and are supplied as two components which have to be mixed immediately prior to use. However, for the moulding processes below, a more common form is a partially polymerised powder, referred to as a ‘moulding compound’. which is melted during the processing, but subsequently cures. Unlike thermoplastics, when thermosets solidify, chemical reactions during processing convert the material to a cross-linked, non-remeltable product, as we saw in polymer types.

Elastomers can be either solid or liquid and are processed in a similar fashion to thermoplastics and thermoset plastics.

Whilst this section applies broadly to all classes of polymers, there are differences between what is nominally the same material from different suppliers. The supplier should always be asked to provide data and guidance in both the design and processing of polymers for specific applications.

The processes which are described below have been grouped into:


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Making mouldings

Compression moulding

Compression moulding is a process where a resin is placed in a heated mould which is then closed, and heat and pressure are applied, causing the material to flow and fill the mould. If the material is a thermoplastic, the mould is cooled and the part removed; if a thermoset or an elastomer, then it is left in the mould to cure and removed after the resin has cured sufficiently to have hardened. Compression moulding has a low part cost, provides fast production rates, and produces little waste. It is not suitable for intricate parts, close tolerances, undercuts, or delicate inserts.

Injection moulding

Injection and transfer moulding are processes which are conceptually similar to each other, and can be described by the schematic diagram and flow chart of Figure 1.

Figure 1: Schematic and flow charts of a generalised plastic moulding process

Figure 1: Schematic and flow charts of a generalised plastic moulding process

The process sequence involves driving molten polymer into the moulds, allowing it to cure or solidify and cool slightly, and then opening the mould to be able to remove the mouldings.

In injection moulding, a thermoplastic polymer is preheated to reduce its viscosity so that it will flow easily, and is then forced under pressure through a nozzle into a closed mould cavity. A schematic of injection moulding equipment is shown in Figure 2.

Figure 2: Screw ram cylinder section (injection moulding)

Source: Grandilli 1981

Transfer moulding

In transfer moulding, a pre-measured quantity of thermoset material such as phenolic or epoxy resin is placed in a heated cavity and forced into the mould proper once the desired viscosity is attained. The process sequence is shown in Figure 3. The material may be fed into the press in granular form or loaded as a single pre-moulded pellet, to improve process control.

Figure 3: Pot or sprue type transfer moulding of thermosets

Figure 3: Pot or sprue type transfer moulding of thermosets

In both cases, the mould is opened, and the part ejected once the resin has solidified sufficiently. For thermoplastic materials, the mould has to remain relatively cold; thermoset resins are held under pressure in a hot mould until cured, but the partially cured part can be ejected while hot.

Advantages over compression moulding are that there is better control of material flow, parts have good dimensional accuracy and reproducibility, with little finishing of the part required, and thin sections and delicate inserts can be moulded easily. The very high production rate and low part cost are, however, offset by high tool and die costs. There are also size limitations resulting from the fact that injection pressures are high (up to 2,000 bar) and the press has to hold together the two halves of the mould against substantial internal forces. As a result, moulding presses generally have capacities in the 20 to 1,000 tonne range.

Cooling is particularly important for improving efficiency in the moulding process, as this stage often accounts for 80% of the cycle length. Heat from the hot plastic passes through the body of the mould by conduction to cooling channels filled with circulating fluid, generally water, which absorbs and carries off the excess heat. Improper cooling can affect the integrity (shrinkage and warpage) and strength of the part. Cooling system design has to take into account the size, shape, and mass of the part being formed, and the thermodynamic behaviour of the plastic and the mould material, as well as the physical configuration and constraints of the existing equipment.

Moulding is affected by variations both between material batches, and over time during a run, when machine parameters (especially mould temperature) tend to drift. Production machines vary greatly in the degree and sophistication of their control over this process. The more recent high-speed automated systems use closed loop process control, where the critical parameters of temperature and pressure are adjusted by providing feedback from the moulding results to achieve tighter control and less process variation.


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Film processes

Laminating

Laminating is a process that involves pressing together two or more layers of resin-impregnated fabric, paper, or fibre under heat and pressure to cure and consolidate the stack. The resin is the binding material, and can be thermoplastic or thermoset, although thermosets are usual for electronic applications. The reinforcement can be cotton, paper, glass, synthetic organic fibres, graphite fabric, and other inorganic fibres.

Laminating can be considered a special case of compression moulding. The process starts by impregnating the reinforcement with liquid resin (in solvent, melted, or a 100% solids liquid resin) and passing the impregnated web through a drying oven to remove the solvent and partially polymerise the resin. A schematic of the impregnation process is shown in Figure 4.

Figure 4: Horizontal treater tower for impregnation

Figure 4: Horizontal treater tower for impregnation

The dried impregnated fabric, which can range from very flexible to very rigid, is cut, stacked and pressed. A typical cycle is 35 bar/175°C for 1 hour, but pressures can range as high as 200 bar, with temperatures to 300°C. The process is inexpensive, production is rapid, and the product can be made to close tolerances.

Autoclave moulding

In autoclave moulding, heat (up to 300°C) and pressure (up to 70 bar) are applied to a part made by other methods (lay-up, winding, wrapping) to compact and cure it. An autoclave applies direct heat and pressure, but a transfer medium can be used: variants include ‘hydroclaving’, where water is the pressure-transfer medium, and ‘thermoclaving’, employing powdered silicone rubber which acts as a fluid under heat and pressure.

Advantages of autoclaving are that high pressures give good laminate consolidation and improved removal of volatiles for high-strength parts. Disadvantages are that capital and operating costs are high and that the size of the part is limited to the cavity of the autoclave. The process can be applied to most thermosets and some thermoplastics, and is in common use in making multilayer PCBs.

Thermoforming

Thermoforming involves forming a hot thermoplastic sheet into the desired shape by applying heat and pressure or vacuum to force it against a mould face (Figure 5).

Figure 5: Thermoforming

Figure 5: Thermoforming

There are many variations of the process: plug-assisted, straight forming, mechanical drawing, drape forming, matched-mould forming, snap-back forming, etc. Thermoforming enjoys low tooling costs, and large parts with thin sections can be produced, but is limited to parts of simple configuration, and produces high scrap. Amorphous polystyrene and PVC are typical materials used in thermoforming, a process commonly used to produce items such as packaging trays.


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Dispensing


Time-pressure dispensing

The principle of time-pressure dispensing is shown in Figure 6. In this simple syringe for a hand application, as might be used for solder paste dispensing during rework, an air-driven plunger drives the resin out of the nozzle.

Figure 6: Principle of dispensing by the time-pressure method

Figure 6:  Principle of dispensing

Dispensing equipment must be able both to control flow rate and to start and stop the dispense cycle as required. This may be easy to state, but is not always easy to achieve. Many adhesives are compressible, thixotropic and even tend to solidify under load. The principal factors likely to prevent correct flow rate being achieved are the viscosity of the material, the effects of temperature change on the viscosity, and hysteresis in the equipment.

Improved dispensing methods

For these reasons, more sophisticated mechanical methods are increasingly favoured, especially for the fast dispensing of glues and underfills. Figure 7 shows one solution, which uses an auger screw to drive predetermined quantities of material to the needle.

Practical dispensers have a considerable degree of automation in order to achieve high throughput rates. The needle distance is the critical factor in determining the dot profile and to reduce stringing, and the correct distance varies with needle size. In particular when dispensing glue, it is necessary both to maintain a constant syringe temperature, and to sample check dot size and profile, especially after a break in production.

Figure 7: Auger screw dispensing after Camelot Systems

Figure 7: Auger screw dispensing

Single-component materials can often be applied direct from the package, whether syringe, cartridge or drum. However, the need to proportion and mix two-component polymers before applying them results in equipment being quite complex. Some materials are supplied in ‘side by side’ and ‘coaxial’ cartridge dispensing units, where the two components are contained in a single package but only meet when dispensed through a mixing nozzle; more generally the two components are transferred from separate packing or reservoirs through metering units which feed an in-line mixer. The mixing action can be carried out by having a rotor in the mixing chamber, or by pumping the materials through a series of baffles. Once mixed the polymer has a finite working life, and mixer, down-stream pipe-work and nozzle all need to be cleaned from mixed resin.

Syringe-based dispensers apply pressure to the rear of a plunger to displace adhesive from the syringe. Until it is dispensed, the resin is not under pressure, so there is no surge effect. The dispense pressure is supplied from a controller unit which is normally fitted with a venturi valve. This produces a vacuum behind the plunger when dispense pressure is switched off, pulling the material back to assist cut-off.

There is generally a minimum pressure required to ensure adhesive flow through any nozzle, which depends on both the viscosity of the polymer and the diameter and length of the nozzle used. A relatively high dispense pressure is normally used to ensure that the compound is adequately dispensed. Note that it is also easier to regulate from a higher supply pressure, thus reducing dispensing variation due to pressure variations.

Self Assessment Questions

Describe with the aid of a diagram the process involved in forming a powdered polymer into a component package for a mounted silicon chip, such as a SOT-23.

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Health and safety issues

A range of safety hazards is associated with polymer materials and the processes involved in their use. The hazards are common to all the materials and care should be taken at all times in dealing with them. When using a material with which you are unfamiliar, check the appropriate health and safety documentation before starting work.

As there are a number of features common to the different polymers, a few general guidelines are suggested:

Uncured resins

Once fully cured or polymerised, polymeric materials present no health hazard, but care has be taken with all uncured resin systems. For example, for volume uses such as floor material, paints, or sealants, epoxy products are usually supplied in ‘two-component’ form, with separate resin and hardener which are mixed just before application.

Note that, once an allergic reaction has been suffered, the operator will remain sensitive (potentially for life) and be unable to work with epoxy.
The ‘single-component’ epoxies used in electronic assembly operations, where the materials are pre-mixed, are less of a hazard. This is because the materials are designed for long pot life and extended cure, so are generally less reactive. However, care should be taken to avoid skin contact, and a sensible level of hygiene is needed in order not to spread the substances and contain any uncured waste.

Solvents

Many polymers contain solvent additives to vary the viscosity and prevent premature curing. Solvents evaporate during dispensing as well as during the curing process. Solvents are generally classed as irritants and care should be taken not to touch the materials with bare skin and to provide sufficient ventilation (or preferably extraction) to remove the fumes. The solvent remains an irritant in vapour form and can affect people who suffer from asthma.

Dispensing

Dispensing can use relatively high pressure air lines and care should be taken to ensure appropriate screens or protective clothing in case of malfunction. Care should be taken to protect eyes, by the use of safety glasses or goggles where appropriate. The high volume dispensing machine is also mechanically dangerous as it travels at high speeds. Care should be taken to ensure that the appropriate safety procedures are followed when working with this type of equipment.

Moulding

The moulding process is not normally carried out as part of printed circuit assembly, but is common in the assembly of components. Moulding machines are sophisticated machines and incorporate many safety features, but care should be taken as the polymer materials within the machines is heated to high temperatures under very high pressure and are therefore dangerous if operated incorrectly or a machine fails.

Curing

UV light sources used in curing polymer adhesives are applied by controlled exposure units which are made to the required safety standards. These units should not be operated outside their controlled environment as exposure to high intensity UV light can damage your eyes. Read the operating instructions!
In the case of materials which require heating as part of the curing process, care should be taken to protect the operator with the appropriate protective clothing (e.g. safety glasses and gloves) to prevent splash damage, or burning by contact with hot surfaces.
In general the workplace is a safe environment if the appropriate precautions are taken and instructions are followed. Machinery with moving parts is fitted with many safety features: they are for your safety, so do not disable them.

Self Assessment Questions

What health and safety issues relate to those materials and processes in electronic assembly that use polymers?

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