Introduction to Polymer Science and Technology

Polymer processing

3 Polymer processing

"Everything flows and nothing abides, everything gives way and nothing stays fixed."

Heraclitus of Ephesus(c. 535-c. 475) describes nature and life as continuously changing and nothing remaining still, and uses the changing /flowing river image in his arguments. The propensity to change for materials and products can be desirable or otherwise: during processing the change in material is actively accelerated in order to achieve meaningful productivity, however, once the product is formed the change is not desirable. Rheologyis the Greek word for "to stream" and is used to denote the study of the flow/deformation behaviour of material, both in liquid and solid states.

3.1 Concept of rheology

In polymer processing, viscosity is experienced under various states of deformation, for example in injection moulding the polymer melt is subjected to significant shear stresses and strains and therefore shear viscosity is of concern.

Shear stress (x)= (r|) x (rate of strain (dy/dt)) л Shear viscosity (r\) = (x) / (dy/dt).

Many low molecular weight simple liquids behave in accordance with Newtons Law of viscosity, where the viscosity is independent of the magnitude of shear stress, x, and strain, y. For high molecular weight liquids, e.g., polymer melts and solutions, x and dy/dtare not proportional over all ranges of x and у and the relationship becomes non-Newtonian. Some non-Newtonian flows are described in Table 3.1 and Figure 3.1.

Table 3.1Different flow behaviours

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Figure 3.1Shear stress vs. shear rate for Newtonian and non-Newtonian behaviours

Under non-Newtonian conditions that exist in polymer melts, the viscosity is no longer a material constant and therefore is termed as an "apparent viscosity". Variation of apparent viscosity with shear rate is shown in Figure 3.2. Most polymer melts behave in pseudoplastic manner. Dilatant behaviour is experienced in mixing some pigments/fillers into polymer resins/solutions, which can cause processing difficulties. Dilatant behaviour can be demonstrated by adding water to cornstarch and stirring it.

A simple power law relationship is popularly used to describe non-Newtonian behaviour seen in polymer melts.

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x = к (dy/dt)ⁿ

where, к and nare the power law indices, called the consistency index and the flow behaviour index, respectively.

For Newtonianliquids n = 1and к = r\,

n > 1 for dilatantand n < 1for pseudoplastic.

pseudoplastic

dy/dt

Figure 3.2Variation of apparent viscosity with shear rate

Polymer melts are processed under different processing conditions. The rate of shearing applied to the melt depends on the type of process used as outlined in Table 3.2. Polymer melts exhibit a wide range of viscosities (10²-10⁶ Pa.s) mainly depending on the polymer type, shear rate and the melt temperature. At low shears polymers tend to behave like a Newtonian liquid but at high shear rates their behaviour becomes pseudoplastic.

Table 3.2Shear rates involved in some polymer processes

3.2 Processing and forming thermoplastics

Prior to covering processes, it might be useful to include a few basic characteristics of some common thermoplastics listed below.

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Polyethylenes Polypropylene Polystyrene Polyamides/nylons - Polyvinyl chloride (PVC)

Saturated polyesters (e.g., polyethylene terephthalate (PET))

Acrylonitrile-butadiene-styrene (ABS) copolymer

Others include fluorinated plastics (e.g., polytetrafluoroethylene, PTFE), polycarbonate (PC), acrylics (e.g.,

polymethylmethacrylate (PMMA)), etc.

Polyethylenescome in a number of different well known grades, depending on their density as influenced by the degree of micro-structural crystallinity. Low density polyethylene(LDPE) is flexible and very strong, used for the less expensive end of the commodity market such as bowls, buckets and bottles. It burns only slowly and softens at approximately 50 °C and, therefore, does not resist boiling water. Normally it is optically translucent. High-density polyethylene(HDPE) is used where more rigidity is required. It softens at approximately 80 °C. Optically it is less clear than LDPE. There are several other grades of polyethylene.

Polypropylene(PP) is similar to PEs but more versatile and sturdy; some grades only soften at as high as 140 °C, therefore, suitable as steam sterilisable hospital ware; not affected by environmental stress cracking, and exhibits outstanding resistance to fatigue on flexing. It is clear that the polyolefins (a generic name for the aliphaticpolymers such as PEs and PPs) offer a range of plastics of increasing softening point, rigidity, gloss, and chemical resistance. Therefore why is not polypropylene used for more applications usually associated with LDPE? Mainly because of flexibility requirements and depending on market circumstances cost may become a factor.

Polyvinylchloride(PVC) is one of the few plastics to which plasticisers can be added, thus, exists as a rigid and as a flexible material. Unplasticised PVC (uPVC) is a hard, rather brittle (not as brittle as polystyrene) and resistant to many solvents (soluble in ketones, esters and chlorinated hydrocarbons). Furthermore, it is one of very few polymers with a reasonable inherent resistance to catching/spreading of flame, offers excellent electrical insulation and softens at about 80-100 °C.

Polystyrene(PS), readily identified by the metallic noise when dropped onto a hard surface, basic PS is colourless, transparent, hard and brittle, softens at 85-95 °C, resists aliphatic H/Cs, but is soluble in aromatics (e.g., benzene) and like ordinary PEs, it is not expensive. The lightweight PS (structural foam PS or expanded PS (EPS)) is an excellent heat insulator, but, since PS dissolves in aromatic solvents as display/insulation panels it should only be painted with emulsion paints.

Polyamides/nylonsare extensively used in textiles and engineering, e.g., as self-lubricating bearings (especially in food processing, where the presence of lubricating oils might lead to contamination). Some nylons offer a good barrier to gas permeation, therefore used as film for packaging cheese slices, etc. Although demonstrating good chemical resistance, it is susceptible to high water absorption.

Cutting edge examples of various engineering applications of nylons and their copolymers as well as other thermoplastics such as polyester, acetal (homo- and copolymer polyoxymefhylenes), polyimides and thermoplastic elastomer can be found in the DuPont knowledge centre (plastics.dupont.com) website: http://www2.dupont.com/Plastics/en US/index.html.

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The molecules of thermoplastics do not cross-link on heating and, thus, can be maintained in a softened state while being made to flow under pressure into a new shape. There are forming methods designed for thermoplastics and others for thermosets, although the barriers between these methods are becoming rather blurred. The processes/forming methods that are normally associated with thermoplastics include:

• Injection moulding

gas-assisted injection moulding

- blow moulding

• Extrusion

- blow moulding
calendering sheet/film
extrusion + thermoforming
fibre melt spinning

mesh (e.g., "Netlon") multi-layer extrusion tubular film/blown film

• Thermoforming/vacuum forming

• Rotational moulding

• Coating

• Dispensing foam

• Machining/joining of plastics

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Some of these processes will be expanded upon in the succeeding sections. More detailed information can be found in text books by Strong (1996), Morton-Jones (1989) and Groover (1996).

Processes of infection moulding and extrusion involve material handling,which basically entails the transportation of raw material often as granules/pellets in a satisfactory form.

Polymer granules undergo a series of handling steps from the raw-material producer to the processing machinery:

• conveying

• drying (typically via spin drying)

• conveying to the customer

• storage in the shipment packaging (bags or boxes)

• unloading (pneumatic conveying/silo storage)

• conveying to processing machinery

• further drying (with hygroscopic polymers)

• blending and feeding .

The transportation processes can cause deformation/degradation of pellets into undesirable products such as clumps of pellets, streamers/angel hair (thin ribbon of plastic caused by friction that melts and smears the pipe surface, which then peels off) and fines/dust, see Figures 3.3 and 3.4.

Figure 3.3Types of degradation in delivery/conveying

Figure 3.4granules and streamers

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The presence of these degraded pellets/granules can lead to numerous problems in the subsequent processes and defects in products: e.g., clogging of filters, inconsistent feeding into the machine resulting, for example, in variations of profile and film thicknesses, gels/specs in films, colour inconsistencies and presence of black specs, and safety issues such as a greater risk of dust explosion in the dust-collection system and respiration concerns for operators. Special-purpose separators (Angels hair separator) and elutriators (particle separator) are placed in delivery lines to remove these impurities.

Plastic particles small enough to pass through a 30 mesh screen (i.e., 30 openings or 30 wires per inch of the screen) are considered as fines and dust particles. The explosive concentration range of the plastic fines and dust is related to particle size.When dealing with plastic fines and dust, the important parameters are:

the Kst(the deflagration index) is the maximum rate of pressure rise, which is a measure of explosion severity.

There are different Kst values depending on the particle size.

the MIEis the minimum ignition energy required to ignite a dust cloud.

As particle size gets smaller, the Kst values increase and the MIE values decrease. Note that deflagration means the extremely rapid burning of a material. This is much faster than normal combustion, but slower than detonation.

Plastic exposed to the atmosphere can pick up moisture,and can cause air pockets that hinder injection/extrusion processes, lead to poor appearance (roughness and silver-strikes in surfaces and internal bubbles) and degradation of some mechanical properties. In their susceptibility to moisture absorption, polymers are identified as hygroscopic and non-hygroscopic.

Non-hygroscopic polymers(e.g., РЕ, РР and PVC) do not absorb moisture; however, they can pick up surface moisture which can lead to processing problems. It can be removed with a simple hot air dryer. Hygroscopic polymers(e.g., Nylon, PET, TPU and PC), have a strong affinity for moisture, and water molecules can become chemically bonded to the polymer chains. Usually a dehumidifying dryer is required to remove moisture successfully from hygroscopic polymers.

3.2.1 Injection moulding

The process entails injection of molten polymer into a closed mould, which is normally cooled to facilitate rapid solidification, to produce discrete products. Thermoplastics (TPs) that can be moulded easily are PS, PE and PP, and those which require greater care are rigid PVC, nylons and PMMA. The moulds can be single or family moulds.The machines are rated by their clamping forceand shot capacity.Machines exist with shot capacities ranging from a few grammes to tens of kg. Basic elements of an injection moulding machine are illustrated in Figure 3.5.

Introduction to Polymer Science and Technology

Polymer processing

Figure 3.5Basic elements of an injection moulding machine

The screw rotates and draws back during charging of plastics granules through the hopper, and once there is sufficient molten charge ahead of the screw, then the screw stops rotating and acts as a piston to advance the melt into the mould through the nozzle. A ring check valve is positioned in a suitable recess at the head of the screw behind the spider tip and retreats (is dragged back) onto the ring-valve seat and ensures that during the injection cycle the melt moves forward into the mould and does not leak/squeeze back onto the screw. The role of the ring is reversed during the charging when the screw rotates and retreats to leave space for the plastic melt, the ring is pushed forward by the transported melt against the shoulder of the spider openings, allowing the melt to flow over the spider openings to the space between the screw tip and the nozzle. Figure 3.6 shows the details of the spider and the sliding ring check valve.

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In a typical operation the melt flows through a conduit system, which normally incorporates a sprue, runnersand a gate(s),prior to entering the mould cavity and taking up the shape of the product. Figure 3.7 shows a sprue and runner system with pin gates for a multi-cavity family mould for the production of eight components with one shot. Pin and tunnel (submarine and banana) gating facilitate automatic trimming of the components from the moulding. Most of the other types of gates have to be trimmed off manually, and may leave behind a sizable mark, see Figure 3.8. Much information on various gate types is given in Strong (1996, p561) or http://www.dsm.com/en US/html/dep/gatetvpe.htm. Another feature of the mould is the cold slug well(s),see Figure 3.7, which is an extension of the sprue and runners (where runners change direction) and traps/captures the cold leading front of the plastic melt, allowing the hotter plastic to flow into the rest of the runner system, and it can also trap any other solidified plastic that enters the mould with the melt. For example, plastic that is left in the nozzle from the previous shot and may have solidified between shots.

Figure 3.7Photograph of a multi-gate sprue and runner system for a TPE product

The design of gating and the runner system in a multi-cavity mould for the production of a component should be balanced,i.e., the runners to all the cavities should be the same length and diameter, in order to ensure that all the cavities are filled evenly and the parts produced are uniform. Figure 3.9 shows examples of balanced and unbalanced runner layouts.

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Figure 3.9Examples of (a) balanced and (b) unbalanced runner systems

The size and positioning of gate(s) is also critical for ease of trimming and for avoidance of flaws such as jetting and weld lines or for minimising the impact of such flaws. Jettingis an initial squiggly narrow stream of melt which is followed by an expanding melt front, causing it to fold and gather up, see the short-shot moulding in Figure 3.10. The jetting is caused by the position of the gate such that the melt is injected straight into an open cavity and it does not make contact with the mould. By feeding the melt sideways and aiming at an obstacle as in tab gating, see Figure 3.11, the jet can be interrupted and normal mould filling should ensue.

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Figure 3.11Tab gate (source: http://www.dsm.com/en US/html/dep/aatetvpe.html

Figure 3.12 shows that jetting can also result depending on the position of the gate in the moulding: filling the cavity from the thinner end has resulted in jetting in a moulding of carbon fibre reinforced polyetheretherketone. The problem was alleviated by gating into the part from the end with the greater cross section, Akay and Asian 1995.

Figure 3.12Jetting when the mould is filled from the thin end (the moulding is an experimental PEEK/CF hip prosthesis)

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Weld lines(knit lines) can occur where a mould requires more than one gate to fill it or the mould includes an insert that splits the melt into streams, and subsequent merging of the melt flow fronts generates a line. This can be a source of mechanical weakness in the moulding produced, particularly with polymers containing fibres, as well as an appearance problem. Figure 3.13 shows the generation of weld lines by filling a cavity using two gates, positioned for melt flow fronts to advance adjacently or head on. If the generation of a weld line is inevitable then the tool design should ensure that its potential harm is minimised. Using a mould-flow software package, the placement of a weld line may be identified. Empirically the use of short shotscan also be informative. Short shots can also provide useful information on mould filling patterns and establishing the shot size for injection. A small shot size results in under packing and therefore sink marks and voids, and on the other hand, a large shot size can result in flashing, see Figure 3.14, and possible denting of the tool parting surfaces.

Figure 3.13Generation of weld lines: (a) short shots and (b) complete mouldings (from Akay (1993))

flashing

Figure 3.14Generation of flashing, and an example of submarine gates (facilitates automatic separation of parts and runner systems)

The sprue and runner system has to be separated from the part and this generates a large amount of scrap. Therefore, sprueless gating would be desirable and is achieved with hot runner gates,the nozzle of the machine is extended to a sprueless mould and the melt is injected through a pinpoint gate.

Introduction to Polymer Science and Technology

Polymer processing

Gas-assisted injection mouldinghas resulted in advances in the way in which injection moulded components are manufactured. Enhanced quality, reduced cycle times and component weight reductions, therefore cost reductions, can be achieved by the process.

Techniques have been developed whereby inert gas nitrogen is injected into the still molten plastic in the mould cavity. Acting from within the component shape, the gas inflates the component and counteracts the effects of the material shrinkage. The effect is to keep an internal pressure on the material until it solidifies and skin forms at the mould cavity surface. This is independent of any gate freezing. In addition, with the material being pressed against the mould surface by the gas until it solidifies, the moulding will have better surface definition and will be more likely to be dimensionally correct. In thicker components the resultant hollow core, can save up to 30% on the material used. Figure 3.15 shows the achievement of hollow cores with gas injection in some sections of a polypropylene kettle that would otherwise result in unnecessary thickness and extra weight.

Figure 3.15A section of a conventional kettle showing the hollow cores

Another major benefit is the reduction in machine cycle times that can be achieved. With no molten core to solidify, the material in the mould cavity solidifies quicker thus enabling the component to be ejected sooner. BPF lists benefits of internal gas injection moulding:

Inert gas nitrogen is injected into the molten plastic in the mould cavity. Acting from within the component shape, independent of any gate freezing, the gas:

inflates the component and counteracts the effects of the material shrinkage, and therefore avoids sink marks

keeps an internal pressure on the material until it solidifies and forms skin at the mould cavity surface

enables reductions in product weight, power consumption and cycle time

reduces in-mould pressures by up to 70%, and therefore reduces clamping forces, enabling larger mouldings

on smaller machines

reduces in-mould pressures, and therefore less wear on moulds

reduces moulded-in stress, and therefore improved dimensional stability with no distortion.

The British Plastics Federation (BPF) web site provides excellent information on gas injection moulding as well as all other plastics processes for thermoplastics and thermosets. The web site includes animations that make it much easier to understand the concepts.

Date: 2015-12-11; view: 3583