Gareth McGrath is Section Manager of Advanced Materials and Processes. He has been involved with research and development of adhesives, composites and plastics for more than 15 years.
All joining processes, welding, brazing, soldering, mechanical fastening and adhesive assembly are enabling technologies, which have a critical effect on product cost, and performance. Because they literally hold the product together, writes Gareth McGrath, they influence choice of materials, design, manufacturing sequence, productivity, production costs and service life.
In many industries, joining processes are well established and entrenched. Managers and operators are familiar with the processes involved. Joining has become part of the background scenery and is not considered when products are redesigned.
This could be a serious mistake.
Under modern competitive conditions there is continual pressure to improve product function and quality, while reducing the costs of production and ownership. Business and technologies do not stand still, and there is a danger of obsolescence if production and production developments are neglected.
This article recommends consideration of adhesives for industrial fabrication and assembly tasks. No matter which industry is involved, adhesive assembly may offer commercial and technical benefits.
Why use adhesives?
The simple answer is to make money by offering better products. The exact combination of reasons will vary from case to case, but advantages should be sought in the following areas:
Reduced production costs
These may result from:
- Increased production speed
In comparison with other fabrication methods, adhesive assembly is essentially fast. Even if curing is required, this can often be accommodated off-line, or combined with other processing stages such as paint curing.
In contrast with welding, adhesives allow a wide freedom of choice during material specification. It is possible to mix and match material combinations to suit product function and save production costs in ways which have been impossible in the past.
Traditional materials may be combined with new metal alloys, plastics, composites and ceramics to give distinctive product advantages.
Absorbing the full potential of this new freedom is perhaps one of the biggest challenges in finding significant market opportunities.
Adhesive assembly offers significant cost savings if material costs can be reduced and production operations simplified. There is ample evidence that this can be the case if design and manufacturing functions co-operate to design or redesign the production with manufacturing in mind.
New approaches can be taken to the manufacture of subcomponents, and castings may be combined with extrusions, sheet components, pultrusions, and parts produced in a variety of other ways.
- Better production sequences
Traditional assembly methods, such as welding, impose fairly rigid sequences during production, and frequently demand intermediate processing to remove contamination or rectify distortion. As adhesives may be applied more flexibly within the production sequence (after finish painting for instance) they allow new approaches to production planning, shop layout and workflow. Bottlenecks can be removed, unnecessary operations can be eliminated, and work in progress can be reduced.
Many adhesive operations involve manual application and the use of adhesive packages which have built in applicators. Even when mechanised or automated application is justified for high volume work, the equipment is usually lighter than would be used for welding.
This is a source of significant confusion. Costs per tube of some adhesives may be high, but this should not be confused with all-in production costs or cost per product. Cost comparisons should be based on the costs of the whole joining process, including plant, preparation and other pre-assembly costs, production and rectification expenses.
All-in cost assessment of this type provides a basis for accurate comparisons, and adhesive assembly may often give significant benefits.
Improved product features
In addition to price, order-winning product features centre on design and function, and it is in this area that adhesive assembly offers the most exciting opportunities.
In addition to the hidden advantages of design for manufacture, adhesive assembly offers more apparent benefits to the consumer in terms of product form and function. Traditional mechanical fastenings or welds which are all too obvious may be replaced by discrete or invisible seams, allowing more design freedom, and an integration of production and consumer interests.
Adhesively bonded structures may be designed to give increased rigidity, improved sound deadening, enhanced impact, fatigue and corrosion resistance in comparison with conventional fabrication methods.
As adhesives allow significant flexibility in assembly, they may be used to enable production of modular products which are based on common core components with a series of add-on parts. This may be extremely valuable where market conditions demand frequent updating.
Product quality
Quality has become a basic market qualifier in most industrial sectors, and adhesives offer significant advantages.
This is particularly important in the assembly of bearing bushes and other axially symmetrical components, where traditional interference fit methods may lead to distortion, especially if through-hole drilling is impossible. The use of slip-fit assembly with an adhesive to take up machining inaccuracies has been found to offer big quality improvements.
Similar benefits have been achieved in the fabrication of machine bases, where slideways may be located accurately in jigs while being bonded to rough-machined base tables.
Traditional welding processes often produce spatter and surface contamination which has to be removed before painting or plating. In addition, the heat of the welding process results in distortion which either has to be rectified or tolerated.
Adhesives in contrast can be applied cleanly to finish-painted parts without damaging the appearance of surrounding surfaces. This is of particular importance in consumer goods, but is becoming necessary in industrial products as an indicator of basic quality management.
Perhaps the biggest conceptual barrier to adhesives is in the field of reliability: there is a common belief that glues don't work. The fact is that adhesive assemblies are as good as the engineering and management that go into their use; and this is true of any other assembly method.
There is ample evidence, from fields as diverse as Victorian cutlery, the WWII Mosquito, motorway bridge reinforcement, electronics, footwear, airliner skins, gearbox assemblies, papermaking rolls, car bodies and jewellery, that correctly used, adhesives offer reliability and safety in long service beyond the levels obtainable with competing technologies.
The secrets of success
Although adhesives applications vary widely, it is possible to propose a sequence of activities which will improve the changes of successful implementation. Throughout the process, help is available to enable the correct choices to be made.
Recognise the task
Adhesive technology is just that: a technology. It has to be taken as seriously as welding or machining.
As indicated above, it impacts on all the technical and commercial aspects of production from design to final product disposal. It demands careful planning and availability of skills, which are well within the scope of an average manufacturer, but which have to be acquired and maintained.
Once the transition to adhesive technology has been completed, the processes involved are almost invariably simpler than traditional welding and fastening methods.
Start small
As adhesives may be viewed with some scepticism by management and workforce alike, it is important that the first application is a success, and recognised as such. A low risk entry approach is recommended to overcome barriers. This should involve a guaranteed win application which does not stretch existing production methods too far, but which will offer a worthwhile commercial advantage.
Typical applications in this class are the attachment of name plates to machine tools, or the securing of bushes and bearings.
The advantages of the low risk approach include:
- Teething troubles may be overcome at minimum cost.
- Skills and experience may be obtained without fuss.
- Major programmes are not put at risk.
- Modifications to company procedures may be foreseen and put in hand slowly.
- The company will gain confidence in the new area.
Absorb the technology
In due course, the technology becomes normal, and is accepted by all staff. As experience and confidence grow, it is possible to consider more ambitious applications which are closer to the company's main product strategy.
A key issue here is the presence of windows of opportunity in the design process which allow the introduction of changes in materials or production methods.
By the time around 7% of development costs have been spent, some 96% of final production costs have been committed, and it is too late to consider significant changes in production. This early stage forms the window of opportunity.
The size of the window may be very limited, but it is here that the most significant influence may be exerted on product function and costs.
Seek help
Assistance is available at all stages of the above process. Adhesive suppliers have extensive experience with industrial applications and help is also available from a number of other bodies.
Most adhesive suppliers provide a wealth of information, including on-site technical service and laboratory documentation. However, there is no guarantee that the user will follow this advice and when problems develop, the cause is often someone not following instructions.
The opportunities for error are endless, including: use of out of date materials, incorrect mixing ratios, rejected materials are despatched by mistake, use of alternative adhesive, inadequate surface preparation, the wrong adhesive is selected or curing is not carried out correctly!
The solution to these problems is adequate training for employees, process specifications for the operation and good quality management.
To assist in the implementation of good practice, TWI has five programs: adhesive selection, joint design, economic evaluation, quality assurance and training.
Adhesive selection
When using adhesives, it is critical to begin the choice of adhesives for manufacturing in the design phase of a product. Products designed for other joining methods cannot be readily adapted to adhesively bonded joints. In the design stage, the major decisions involving substrate materials, manufacturing process,
etc, are still flexible enough for change, if change is found to be necessary. While in the design stage of a product, the interaction of engineering, design, manufacturing, marketing, cost and other administrative functions should develop a set of criteria that reflects both what is needed and desired by the product.
The definition of the entire set of parameters that impacts performance is vital for a new product. However, where processes are well defined in a given industry, such as tyres, furniture or aircraft, it is not necessary to re-invent the wheel, unless there is a desire to change from commonly accepted practices. The criteria that are considered most important include, but are not limited to, the following: substrate, assembly, cost and service.
Substrate
It is important to know what materials are going to be used or are proposed for use. This includes detailing the thickness of material, the mechanical or chemical finish to each surface and any other data that can be obtained that will affect bondline characteristics. It is also important to list any alternative materials under consideration.
Assembly
If there is a preferred method of applying adhesive to a surface, or if the use of existing equipment is important, economic justification may not be necessary. While it may be difficult to assess assembly requirements in the absence of the right adhesive, a starting point is appropriate for estimating manpower, capital and marketing requirements. However a high degree of flexibility is essential because of the radically different nature of alternative adhesive types. Time, temperature and pressure are all necessary ingredients of the bonding process, but in the early stages of product design, these factors can only be estimated.
Service
Definition of the environment that the assembled product will experience is mandatory. Both mechanical stresses and environmental exposure can be, and should be, detailed completely. Will the product be subjected to peel, shear, impact, vibration or other mechanical forces? Will the product have to resist outdoor exposures, water, temperature, chemicals, gases or vapours, or some other environmental condition? Will these stresses be constant, cyclical or occasional? What is the expected useful life?
Developing answers to all of the criteria listed above can be frustrating and time consuming. The frustration often comes from a lack of knowledge about adhesive types and processes available to produce the designed product. However, the more complete the initial analysis is, the better the answer that will be obtained from the next phase of the project.
EASel, a Windows based selector system, has been designed specifically to assist in solving these complex interactions and is available from TWI.
Joint design
Finite element analysis is used to examine stresses in computer generated models of bonded joints. Applied at the design stage, it helps ensure that joints meet their service requirements.
ABAQUS: a general purpose FE analysis program. This is used to run the FE analysis.
FAM and ABAQUS are used at TWI to produce and analyse FE models of all kinds of component and structure, not just adhesive joints.
The dialogue box program works by asking the user simple questions about the joint's shape, size, materials and loading. The program then sends instructions to FAM and ABAQUS to tell them how to build and run the model.
Since the whole process of producing the model and running the analysis is largely automated. FE models can be produced very quickly. On TWI's fastest computer it only takes 10-15 minutes to produce the model, run a linear elastic analysis and obtain the results. This makes it practical to run several 'what if . . .' analyses to investigate, for example, effects of variations in joint geometry on stresses in the adhesive layer.
Cost
If the process and application are well defined and the choice is between relatively equal adhesives, the cost per gallon or kilo may provide a valid analysis. More valid for most new production applications, however, is an analysis on a cost per bonded unit basis. This includes material cost, freight cost, capital and labour cost, overhead, clean-up and disposal cost, scrap and rework rates and the potential impact of warranty claims. Relatively small differences in application rates can cause wide variation in unit cost. Performance trade-offs are a necessary part of this analysis. A product that costs 50% more on a per gallon basis may provide usage improvements of 3:1 or 4:1.
This analysis may be undertaken by a software program written by TWI: Techno-economic analysis.
Quality assurance
To complement these, a further program: QUASIAT, (Quality Assurance in Adhesive Technology) has been developed to provide the user with a methodology for quality management.
A procedure for quality assurance throughout design and manufacturing is a vital part of overall product quality management. Successful quality assurance should be based on two principles. First, quality assurance aspects must be introduced at the design stage, and control must be maintained throughout the development phases, to enable quality assurance of the finished product. Second, when considering the introduction of adhesive bonding as an enabling technology, the adhesive must not be considered in isolation. Instead, the adhesive system should be evaluated with other factors such as those illustrated in Fig.3. QUASIAT aims to help industry improve the quality of adhesive bonding.
QUASIAT was a three year EUREKA project with two key objectives. First, to gain improved reliability in adhesively bonded products; second to complement the more general quality management systems already in place in the manufacturing industry.
All industries participating in QUASIAT considered their products to be design-critical - whether the product is a thrust reverser for an Airbus engine made by Westland Aerospace, a sealed-beam headlamp sold on by Carello Lighting to Nissan, or flexible laminated packaging by CarnauldMetalbox for freezer-to-microwave meals.
Business performance
The quality and reliability of bonded products influences safety, client satisfaction and ultimately business performance. Compared with other materials joining methods, adhesive bonding can be more complex to control. For example, on a car assembly line, spot welding operations may be controlled fully at only one station. In contrast, up to five points along the line can contribute to success or failure of the bonding process.
Adhesive bonding fits into ISO 9000 as a special process, the results of which cannot be fully verified by subsequent inspections and testing. Therefore, continuous monitoring and compliance with documentation procedures are required.
In the project's formative stages, the emphasis was on production engineering. But as the work progressed it became increasingly apparent that it was necessary to include quality considerations during design (specifying materials, adhesive process), not simply when adhesive is applied in production.
Quality management model
A computer-based, generic quality management model has been developed to assess all the major stages from design through to final assembly and inspection. Although designed to accommodate a wide range of applications, it can be used to provide a detailed quality plan for an individual assembly process. The model has been evaluated in real manufacturing processes by industry.
Many quality tools and techniques have been developed to help the manufacturing industry, almost all defined by groups of initials such as FMEA, QFD or SPC. Worked examples of techniques considered to be of value in adhesive bonding have been collected from participating companies. By exemplifying these in relevant product assembly using adhesives, it is hoped to dispel myths and assist their uptake.
Ultimate objectives are to achieve an appreciation of key quality issues and to supply practical tools (a software package and text) for production.
Versatile software
The software not only caters for continuous, high speed production lines for packaging or electronics assembly. Equally important is its applicability to labour-intensive, on-site work such as pipe bonding and sealant application where there may be no factory process control but reliance on operator training.
Manufacturers and users of bonded products are set to benefit from higher levels of reliability as a result of technical achievements and new tools and know-how from QUASIAT.
Training
An interactive, multi-media distance learning program for adhesive engineers.
The program covers: Introduction and awareness of adhesive bonding; adhesive selection; surface preparation; health and safety. The program is designed for use in the office or work place, enabling on-site training to be undertaken and thus avoiding the need for costly down time as employees attend training workshops. Each unit contains case studies, worked examples and self assessment questions, enabling the user to monitor their performance.
Introduction and awareness module
This is the first of a number of modules designed to inform the professional engineer of the most important aspects of adhesive technology, both from a technical and business point of view. The first section, the overview, shows the importance of adhesive technology today. The second section explains how adhesives stick or bond and how they harden. The third section covers the physical form of adhesives, how they are supplied and a brief description of the most important types of adhesive. The fourth section covers the advantage of using adhesives when compared with other joining methods and some of the limitations of the use of adhesives. There are case histories, detailing where and why adhesives were chosen and illustrations of how some adhesives are applied in production and finally the summary of the position of adhesive technology in the world of joining.
Adhesive selection module
This module on chemically cured adhesives deals with the selection of adhesives in five sections.
- Chemically cured adhesives
- The prevention of curing reactions by chemical blocking
- Polycondensation adhesives
- Polymerisation and polyaddition.
- Future developments.
Surface preparation
This module is an introduction to surface preparation and is presented in four sections:
- Why pre-treatment is necessary
- Surface cleaning methods
- Surface modification
- Quality control and costing.
Health and safety
This module covers the health and safety characteristics of various types of adhesives and of equal importance, it gives a basic understanding of potential hazards associated with adhesives and adhesive technology and an appreciation of the practical procedures to make sure they are used safely. In addition, the module contains laws and regulations about the safe use of adhesives in different countries.
The module is divided into sections:
- An overview.
- Hazards and adhesives.
- Communication and interpretation.
- Handling adhesives safely
This multi-media package is complemented by text based modules: design, surface preparation, bonding practice, value management and dispensing.
Conclusions
The resistance of traditional engineering practice to the use of adhesives is being broken down. Adhesives have now been developed which clearly demonstrate the contribution they can make to the work of both the structural and mechanical engineer. Perhaps of even greater significance is the growing understanding of adhesives as materials in their own right and the need to consider joint design based on the properties of adhesives selected as an integral part of product development.
TWI has a comprehensive toolkit to assist you, the engineer, implement the change in manufacture to adhesive bonding:
EASel - adhesive selection
Gluemaker - joint design
Techno-economic analysis
QUASIAT - quality assurance
Multi-media training.
In addition, detailed documentation exists for design, surface preparation, bonding practice, value management and dispensing.
