Plastics: Surface And Finish - Softcover

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ISBN 10: 0854045163 ISBN 13: 9780854045167

Verlag: Royal Society of Chemistry, 1995

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Plastics: Surface and Finish

By W. Gordon Simpson

The Royal Society of Chemistry

Copyright © 1995 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-0-85404-516-7

Contents

Preface, ix, 
Contributors, xv, 
Chapter 1 Introduction W. G. Simpson, 1, 
Chapter 2 Surface Recognition K. Nakajima and Y. Sato, 6, 
Chapter 3 Basic Finishing Techniques W. G. Simpson, 31, 
Chapter 4 Calendered Thermoplastics R. A. Fairbairn, 52, 
Chapter 5 The Sealing and Welding of Thermoplastic Materials C. Hughes, 71, 
Chapter 6 Adhesives for Plastics Fabrication P. J. C. Counsell, 90, 
Chapter 7 Decorative Laminates P. Allen and M. F. Kemp, 113, 
Chapter 8 Mouldings — Their Surface and Finish A. Whelan, 136, 
Chapter 9 Extruded Surfaces A. Whelan, 156, 
Chapter 10 Electroplating and Electroless Plating on Plastics A. C. Hart, 173, 
Chapter 11 Vacuum Metallizing R. R. Read, 190, 
Chapter 12 Painting Plastics T. A. Wilde, 210, 
Chapter 13 Surface Treatments for Plastic Films and Containers P. B. Sherman and M . P. Garrard, 221, 
Chapter 14 A General Review of Printing Processes for Plastics J. W. Davison, 237, 
Chapter 15 Vinyl Wallcoverings W. G. Niven, 281, 
Bibliography, 306, 
Subject Index, 311, 

CHAPTER 1

Introduction

W. G. SIMPSON

'Natural' and Other Finishes

The finish of an uncoloured plastic film or moulding is said in the industry to be 'natural'; such finishes can be serviceable and perfectly adequate in some uses (as in pulley wheels or other unseen parts in mechanical assemblies).

However, coloured or uncoloured materials may be finished in a great variety of ways. For certain purposes (even for 'natural'), quenching or annealing immediately after processing are necessary in order to obtain the molecular arrangements and therefore the mechanical or other properties required. In other words, the finishing is an important factor in deciding the suitability of the object for its purpose, and whether or not it will meet a specification.

The world would be more uniform, less colourful and stimulating, if it were not for the human habit of decorating and finishing objects of all kinds — both useful and ceremonial — often almost regardless of the types of substance from which they were made (like gilded or lacquered timber). Brooches and other items of jewellery, buttons, fasteners, seals, and more mundane things have been made for centuries by working bone or horn, and from malleable compositions like clays or filled resins, which were set hard just by drying under ambient conditions or were fired. A striking example of benefit from the habit was the skills developed in the nineteenth century in finishing a common material like cast iron (for a balustrade in the Royal Pavilion at Brighton, Sussex, for instance, this heavy dull metal was transformed by clever casting and painting into an unusually tough and long-lasting bamboo). Besides being converted by such means into a variety of 'timbers' iron was made to represent different types of stonework and decorative plaster (it was the 'versatile' building material of its day).

When 'natural', many plastics are transparent or translucent — which means, in effect, that they can be produced as transparent goods as well as in translucent or opaque colours. With materials that essentially are transparent, the range of colours possible is very wide. Other plastics are opaque, or dark in original colour — so that the opportunities for coloration by blending and mixing are more restricted.

With some plastics, the 'natural' appearance of the surface (especially in the case of opaque materials) is matt and not especially attractive; on the other hand, a polymer such as polystyrene in 'natural' form has a glossy surface which is described sometimes as 'glass-like'. Paradoxically enough, it may be the object of the technologist by finishing a material in a special way to make it more attractive and arresting in appearance than otherwise it would be — or, on the other hand, to make a glossy and rather brittle material less so. Success in finishing may mean the consistent production of a 'rough' finish as well as a smooth one.

It always is difficult to generalize, but apart from the desire to change and to improve the appearance of a substrate, perhaps to disguise it, there can be a variety of technical reasons for selecting methods of finishing (at times with objectives which may seem confusing or even contradictory) — such as to change the chemical, electrical, or physical properties, to match the appearance of some adjacent components, or merely to convey information by colour-coding or printing. Some possible approaches to and applications for finishing technology are summarized in the form of a list below. (It is not proposed in this volume to attempt to encompass the use of plastics and synthetic resins in finishing other substances — in forms such as paints, coatings, or coloured films applied (say) to protect timber boards, and sheets in ferrous or non-ferrous metal.)

(i) To provide a skin on the surface of a product and so to impart altered or improved properties (as examples, to make the surface conductive electrically, sensitive to light, more resistant to weathering — including degradation by ultra-violet light, and harder or stronger in a mechanical sense)

(ii) to provide a distinctive surface, either generally, or in a particular part (such as to abrade other substances, or to resist wear; to break or to tear easily in pre-arranged circumstances)

(iii) to assist in fabrication by marking out and cutting lines or shapes for sealing or welding later

(iv) to apply to the surface of the plastic a layer of another material — perhaps another plastic or something of different nature entirely (like organic fibres or silicates) — so that the resulting composite combines the properties of added materials with those of the original (typically, in packaging a film of another plastic might be applied in order to enhance resistance by the base material to permeation by gases, or to improve adhesion)

(v) to make for some specific purpose a new composite material (like a laminate in different colours-for engraving or otherwise machining for fancy effects, a flexible film to make signs that will reflect light, or a rigid but light 'sandwich' for structural purposes in aircraft and other applications where weight is an important consideration)

(vi) so that a surface will be more suitable for further processing (in preparation for electroplating, lamination, metallizing, printing, and so forth)

(vii) to mimic, deceive, or mislead, by giving the plastic the appearance of other materials (like brass, ivory, marble, leather, or wood)

(viii) to make an object similar in appearance (and therefore a match to or a contrast with) adjacent objects in other materials

(ix) to extend a material (and thus to reduce its cost) — perhaps physically by stretching it in some way, or (through the introduction of 'blowing agents' or gaseous expansion) by separating the particles or portions of the feedstock by means of closed or open voids

(x) to make decorative items such as prints for furnishings in domestic premises, hotels, and other public buildings, accessories for clothing, and so forth

(xi) for advertising and to convey information by printing logotypes, trade marks, instructions for use, details of composition, computer codes, etc.

(xii) as the reclamation and re-cycling of waste material become more important it will be an international requirement that all plastics are identified visibly by means of a standard code system — which, it is envisaged, will be printed or embossed on the surface.

Considerable ingenuity and ranges of equipment have been developed at various times for purposes such as these. For the continuous production of articles of a required quality perhaps two overall requirements are applicable in most if not all cases — the maintenance of consistency in the plastic to be finished, and standardization of the conditions of treatment. To comment briefly on both of these in turn:

Consistency in the Plastic

Techniques and plants for polymerization have become more precise and specific but there is a possibility still that similar grades of the same material made in different units may differ in practice (in features such as the distribution of molecular weights, and colour). It will be appreciated too that many polymers and copolymers are used in combination with other substances — stabilizers, fillers, and miscellaneous additives — all of which (and especially those occurring naturally, like China clay and some types of plasticizer) may themselves differ appreciably from batch to batch.

Differences between the ingredients in a formulation may be essentially of a chemical nature, but also may be physical (such as the ranges of particle sizes) — and quite often both chemical and physical variations are found in practice. In order to match a particular material consistently it is not sufficient merely to know even the precise formulation in terms of polymer, plasticizer, heat stabilizer, filler, 'anti-static' agent, colour, and so forth; each of the components in the formulation must be supplied consistently to a tight specification, and (furthermore) the blending and mixing must be carried out with similar equipment in the same manner each time.

When making special effects (as examples, 'marble' or 'wood-grain' sheets) the principal components may be quite different in appearance but still must be compatible and capable of being combined.

Lastly, besides inconsistency in the polymer and additives, there may be fortuitous differences between mixes of what supposedly is the same composition — such as the water content changing between summer and winter (or by night and day under some climatic conditions). Thus, provision must be made for drying and the removal of other volatiles, and (especially if the processing is by batches and components are held in store at intermediate stages) drying may be required at more than one point in the manufacture.

Standardization of Conditions

For the reasons indicated above, consistent and successful production of plastics articles at times in the past was regarded as more of an art than a science (with the production manager rather more of a master chef than a scientist or engineer).

The first hurdle on which success in production depended (assuming consistent raw materials, stored satisfactorily before use) was the blending and mixing of ingredients with some assurance that homogeneity was achieved and that obvious features (like the shade of colour) in the finished batch would be the same (or approximate closely to) its predecessors and those following. Since svery few polymers were used alone, effective blending and mixing (with the exclusion of all forms of contamination) soon were understood to be essential.

Typical contaminants include general dust and dirt, other foreign matter (pieces of sticking plaster, gloves, nuts and bolts, pins, insects, fragments of paper, feathers, and fibres) — almost anything. Ideally, all processing should be in automatic units with full air-conditioning but in practice this is not possible so great care must be taken not only by securing long hair and all clothing and equipment, but to locate suitable detectors before nips and other critical stages in the process. Such detectors always should be as close as possible to the inward ('feed') end, with automatic cut-outs.

Static electrical charges in the machinery and stock can be important in attracting contamination, and facilities for electrical discharge at appropriate points also will be needed.

In all types of thermal processing (calendering, extrusion, or whatever it might be) the control of temperatures is critical, but precise control of equipment all the time is difficult. There were engineering considerations — notably the heating and cooling of large volumes of metal and alloys both quickly and effectively, and economic ones; usually, greater precision in control called for more expensive facilities. A third factor of importance was the effectiveness of operators in keeping to the standards required — and, sometimes, their preparedness to do so at the expense of increased rates of output.

In some types of work there has been an increase, following the example of the pharmaceutical industry, in the taking of samples at intermediate stages in the production and testing them with a view to ensuring control of quality: many convenient instruments and techniques are available for purposes such as this.

Subject to satisfactory solution to the questions of engineering and related economies, the manufacturing operations in general today (not only thermal processing) may be controlled by means such as:

(i) mechanical devices (like spring-loaded cut-outs, trips, balances, levers, punched cards or paper rolls, pneumatic switches, hydraulics)

(ii) electrical and electronic devices (including sensors and computer programmes)

(iii) robots

(iv) human operators.

The first approach might be characterized as rather inflexible and suited to more simple routine work, although punched cards and rolls open the opportunity for variations in some fixed procedures: the second and third possibilities are more versatile but still somewhat less flexible than human operators. The approaches (i) to (iii), though capable no doubt of much further extension, are most appropriate for production in large numbers and for repetitive work: also, while they may well be far more precise in long runs than human operators, computers and robots can produce rejects in larger numbers more quickly. For all such methods of control the ability to unlock and to vary at times is necessary still — provided a variation is made deliberately and for a sound technical reason.

CHAPTER 2

Surface Recognition

K. NAKAJIMA and Y. SATO

Introduction

Plastics, like many industrial products, are manufactured with a view to being useful in society — and with the approach of a new century the words 'useful in society' have widening implications. In other chapters of this book emphasis is placed upon progress in science and technology leading to new processes and products, but as part of this advance the influence of technology on environments should be considered too. Still more, the product is required to satisfy not only physical and functional needs laid down but should respond to the sensitivity of those in contact with it. The implication is a technology in harmony with environments, and also with human feelings.

It is suggested therefore that the topic here — 'surface recognition' — can be seen as more than an approach to inspecting for surface defects but rather as a contribution towards goods offering genuine appeal for highly sensitive functions of human beings (like eyesight and touch). The range of materials and uses is wide and the present review can hardly cover all the details, but publications elsewhere (such as in the technical press) should help to meet further interests in this field.

In the first stages of assessing surfaces of plastics (and aside from the analysis of data) reliance is placed largely on human senses. Hence, in any transition to a mechanical system of recognition, the tendency is to regard the human inspector as the 'automatic' system and to use him or her as a standard for comparison. Similarly in this chapter we introduce first the method of recognition, with visual sensing and accepted software, then the technology intended to systematize and to integrate all aspects, including economy.

However, systems have been evolved with performance different from and in some ways superior to those associated with recognition by skilled inspectors. Specific schemes such as these have been brought into use, and in general the technology is being advanced rapidly. With a view to taking the pace of development into account it was considered appropriate here to pay particular attention to the principles that underly the design of satisfactory systems.

Types of Surface Defects

Defects are mentioned in other chapters in connection with plastics in a variety of forms, manufactured in various ways (extrusions, films, mouldings, sheet, and so forth). As demands for quality and uniformity become more stringent it is important to limit and to localize all defects, and as a first step it is necessary to identify and to classify them. Figure 1 is an attempt to do so for seven types of fault, as follows:

(i) 'flaw', occurring in many sizes, mainly in sheets, mouldings, and pipes

(ii) 'foreign matter', which can occur with any product, consequential upon contamination of the raw materials used

(iii) 'pinholes', which occur mainly in film and sheet, often from bubbles of trapped air

(iv) 'scratches' — marks made on the surfaces by tool edges or other hard objects

(v) 'segregation' — defects of transparency or colour most apparent in films

(vi) 'unevenness' — dents or protruberances at the surface

(vii) 'wrinkles' — faults in fiat-lying, again most apparent with films.

Types of Methods for Recognition

Many methods are available for classifying and appraising defects, and selection depends largely on the purpose and the nature of the system to be employed. Signals are obtained from the surface and from a defect by an appropriate method of 'sensing' — by the radiation, say, of electromagnetic or ultrasonic waves and then recording the transmission or reflection of such waves so that comparison is made with data for satisfactory surfaces and the size, shape, and position of any defect noted.

For the ultrasonic approach, receiving apparatus is arranged round the material under observation. The waves are generated and aimed at the surface, their reflection and transmission then being recorded. Defects are identified by differences between normal and abnormal wave shapes.

In methods with light, the surface to be examined may be irradiated uniformly, a photo-sensor being used to measure its distribution; alternatively, a laser may scan the surface in regular patterns, a sensor noting any differences in the amounts of light collected.

(Continues...)

Excerpted from Plastics: Surface and Finish by W. Gordon Simpson. Copyright © 1995 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Details zur Ausgabe

• Verlag: Royal Society of Chemistry
• Erscheinungsdatum: 1995
• ISBN 10: 0854045163
• ISBN 13: 9780854045167
• Einband: Tapa blanda
• Sprache: Englisch
• Auflage: 2
• Anzahl der Seiten: 344
• Herausgeber: Simpson W Gordon