Quality Assurance a generic overview

Quality management tools, techniques and their application to adhesive bonding

Adhesive bonding is an important joining technique which is used in a wide variety of industries (packaging, automotive, aerospace, general structural, building and construction are key ones).  In many products the performance and reliability of the adhesive joint is critical to the product's performance and, sometimes, to its safe operation.

The problem of assuring the quality and reliability of an adhesive joint 's the absence of suitable non-destructive examination methods, in all but a few special cases.  Therefore, inspection of adhesive bonds is not a feasible way of assuring quality and in modem manufacturing philosophy, it is after the event and is an unnecessary activity in a well controlled process.  The only way to provide assurance is to systematically manage and control the whole operation from design of the joint through to final assembly.  In this way, the possibility of poor quality joints being produced is reduced to the minimum because proven procedures are being followed at all times.

Every adhesive bonding situation is unique but all adhesive bonding situations have a number of common features.  Consequently, this generic model is designed to cover all situations.  All the major stages from design through to final assembly and inspection are listed which define the requirements, methods of quality verification and the remedial action at each stage.  Each stage can be accessed individually, progressively in more detail leading ultimately to the identification of the specific measures needed for an individual application.  The model [QUASIAT] therefore acts as guidance to the user in defining the specific quality requirements for a joint and the specific quality management actions needed to ensure the quality requirements are met.

The quality management of the adhesive bonding process relies on two major concepts:

1. Control of joint design and specification of materials and processes.
2. Process monitoring and/or inspection.

The application of these concepts relies on the definition of the requirements (unique for each case), the appropriate information flow/decision making process and the use of quality management tools and techniques at the appropriate point in the process (see Table below).

The key determinant of the quality of an adhesive joint is the design of the joint. Design in this context includes material selection (both adhesive and adherends) and the method of joining - in other words the complete specification of the joint. Unless the joint design is completely specified then there is no basis on which to use process monitoring and there would have to be reliance on inspection of the completed joint, which is known to be unsatisfactory. Therefore the quality management of the design is of primary importance.

The process monitoring and control of the bonding process is clearly dependent to a very large extent on the type of application. In reality, it is the manufacturing activity which will require differences in the model. The key features of the manufacturing activity which determine the quality management model are:

1. Production rate.
2. Level of mechanization of the process steps.

These, in their turn, are affected by product type, adhesive and adherend type and component design.  The criticality of the joint 's not a factor.  Clearly, the more critical the joint the more rigorously the quality must be controlled and assured but that is determined by the detailed application of the model. (Joint criticality is also a factor in overall joint/component design).

From the concept point of view, the assembly process can be characterized in one of four ways and these lead to the four sub-models.

1. High production rate, high level of mechanization e.g. automotive components or packaging applications
2. High production rate, low level of mechanization. e.g. some consumer goods or construction sealing
3. Low production rate, high level of mechanization. e.g. critical space or optical assemblies
4. Low production rate, low level of mechanization. e.g. airframe assembly

High production rate is considered as >10 joints/hour.
Low production rate is considered as <10 joints/hour.

Sub-models 2 and 4 relate to situations where operator skill is a determining factor (i.e. a craft based approach) whereas Sub-models 1 and 3 relate to more systems-controlled situations. It is, of course, possible that different levels of mechanization may be applied at different stages in manufacture.

The quality-related features of the different levels of mechanization can be summarized as
follows:

High level of mechanization:            Pre-qualified procedures.
                                                        On-line measurement and process control.

Low level of mechanization:             Training.
                                                         Inspection.
                                                         Pre-qualified procedures.

The generic model are shown in the form of activity, quality requirement, method of validation/control and corrective action in the Table below.
 

 

Generic flowchart of the design and manufacturing process [QUASIAT]

Activity	Quality requirement	Method of  validation/control	Corrective action
1. Specification review	a. Specification of operating conditions b. Performance requirements c. Test requirements d. Safety requirements e. Environmental / statutory requirements	Review of: Customer spec. Test work Records Published data National and international standards	Specification change
2. Design	Specification requirements achieved in an efficient, economic and consistent manner	Quality Function Deployment. Failure Mode and Effect Analysis. Taguchi loss function analysis.	Design changes
3. Verification of design	Does design satisfy requirements? Is design economic and efficient? Are proposed production and test methods satisfactory?	Qualification tests. Alternative calculations. Comparison with conventional fixing methods. Design review.	Design change
4. Selection and sourcing of materials	a. Joint design requirements:  strength, weight, environmental, appearance, dimensional control. b. Specified component requirements:  strength, weight, environment, ease of forming, appearance, durability, other properties specific to product.	Test records. Supplier certifications. Published data. Experience of previous use.	Return to supplier. Re-select. Design change.
5.  Selection and sourcing of adhesives	a. Joint design requirements:  strength, compatibility, environmental, durability, gap filling properties.  b. Production requirements:  Ease of dispensing, shelf life, tolerance to environment, cure time/temperature	Test records. Supplier certifications. Published data. Experience of previous use.	Return to supplier. Re-select. Design change.
6. Storage of adhesives	Requirements specified by adhesive supplier: Shelf life, packaging, temperature, humidity.	Inspection of packages. Batch/Data No. Control of storage facility.	Reject. Re-test and re-life.
7. Pre-treatment of surfaces	Requirements specified by (a) and (b) material properties: Cleaning, surface removal, dressing or chemical treatment	Use tested procedure (mistake proofed). Trained staff.	Re-treat.
8. Assembly	a. Component fit-up: Correct components Location b. Application: Type, mix, quantity, temperature	Inspection.  Use of jigs.   Metering by calibrated dispenser. Use of tested procedure (mistake proofed). SPC. Trained staff.	Re-jig or select correct components. Reject or re-apply.
9. Cure	Requirements specified by adhesive supplier: Time, temperature, pressure, heating / cooling rate.	Use tested procedure (mistake proofed).  Time / temperature records. SPC. Trained staff.	Reject or re-cure.
10. Final inspection	Joint meets design requirements: Strength, environment, appearance, reliability, durability.	Test programme. Review of process documentation and records.	Reject. Concession. Design change.
11. Pre-use storage	Joint meets design requirements: Strength, environment, appearance, reliability, durability.	Correct storage review of test reports, supplier information.	Reject. Design change.
12. Service	Joint meets design requirements: Strength, environment, appearance, reliability, durability.	Service monitoring. Joint failure rate.	Repair. Design change.

This model should be studied in conjunction with the Table below. How they will be put together is dependent on how the user will wish to 'see' the model.
 
 

Tools, techniques and their application

Process Step	Tools and techniques
Specification review
Design	Taguchi FMEA QFD
Design verification	Product testing
Selection and sourcing of materials	Product testing
Pre-treating	SPC Validated procedure
Assembly	SPC Validated procedure Inspection
Cure	SPC Validated procedure
Final inspection	Product testing
Pre-use storage	Product testing
Service	FMEA

Brief remarks on the quality control methods mentioned in the Table (QFD, Taguchi, FMEA) are presented in the following paragraphs.

QFD

The foundation of QFD (Quality Function Deployment) is the belief that products should be designed to reflect customers' desires and tastes.  Therefore this method is most suitable for applications that are based primarily on the detailed requirements and specifications of the customers and where interfunctional planning and communications among marketing people, design engineers and manufacturing staff are required.

Taguchi

The off-line Taguchi method defines quality control in relation to design for quality.  This is a continuous quality improvement programme that includes continuous reduction in the variability of product performance characteristics about their target values.  In this method statistically planned experiments can be used to identify the setting of product (Process) parameters that reduce performance variation.  Therefore, this method is less suitable for applications on which performance characteristics are not based on numerical measurements, but on an order categorical scale such as Poor, Fair, Good or Excellent.

FMEA

Failure Mode and Effect Analysis is an important method to identify all possible failures and their effect on the system.  The objective is to classify failures according to their effect.  FMEA provides an excellent basis for classification of characteristics.  When engaged in FNMA, it is wise to bear in mind that the severity of the failure is not the only important factor.  One must also consider the probability of failure.  One purpose of FMEA is to direct the available resources toward the most promising opportunity for improvement.  As far as adhesive bonding is concerned, this method offers a quality control technique that is most suitable for low rate production (i.e. highly cost sensitive production line) and where multiple processes are interlinked.

DFMA

Design For Manufacturing and Assembly reflects manufacturing problems in an assembly line and how these problems should be handled.  This technique is concerned with reducing the cost of a product through simplification of its design.  In a multi-part assembly line the best way to achieve this cost reduction is:

1 - to reduce the number of individual parts that must be assembled,
2- to ensure that the simplest process route has been selected.

References

Quality assurance in adhesive technology. Eureka Project EU716. Abington Publishing. Cambridge, England. ISBN 1 85573 259 9

 

Generic flowchart of the design and manufacturing process [QUASIAT]

Activity	Quality requirement	Method of  validation/control	Corrective action
1. Specification review	a. Specification of operating conditions b. Performance requirements c. Test requirements d. Safety requirements e. Environmental / statutory requirements	Review of: Customer spec. Test work Records Published data National and international standards	Specification change
2. Design	Specification requirements achieved in an efficient, economic and consistent manner	Quality Function Deployment. Failure Mode and Effect Analysis. Taguchi loss function analysis.	Design changes
3. Verification of design	Does design satisfy requirements? Is design economic and efficient? Are proposed production and test methods satisfactory?	Qualification tests. Alternative calculations. Comparison with conventional fixing methods. Design review.	Design change
4. Selection and sourcing of materials	a. Joint design requirements:  strength, weight, environmental, appearance, dimensional control. b. Specified component requirements:  strength, weight, environment, ease of forming, appearance, durability, other properties specific to product.	Test records. Supplier certifications. Published data. Experience of previous use.	Return to supplier. Re-select. Design change.
5.  Selection and sourcing of adhesives	a. Joint design requirements:  strength, compatibility, environmental, durability, gap filling properties.  b. Production requirements:  Ease of dispensing, shelf life, tolerance to environment, cure time/temperature	Test records. Supplier certifications. Published data. Experience of previous use.	Return to supplier. Re-select. Design change.
6. Storage of adhesives	Requirements specified by adhesive supplier: Shelf life, packaging, temperature, humidity.	Inspection of packages. Batch/Data No. Control of storage facility.	Reject. Re-test and re-life.
7. Pre-treatment of surfaces	Requirements specified by (a) and (b) material properties: Cleaning, surface removal, dressing or chemical treatment	Use tested procedure (mistake proofed). Trained staff.	Re-treat.
8. Assembly	a. Component fit-up: Correct components Location b. Application: Type, mix, quantity, temperature	Inspection.  Use of jigs.   Metering by calibrated dispenser. Use of tested procedure (mistake proofed). SPC. Trained staff.	Re-jig or select correct components. Reject or re-apply.
9. Cure	Requirements specified by adhesive supplier: Time, temperature, pressure, heating / cooling rate.	Use tested procedure (mistake proofed).  Time / temperature records. SPC. Trained staff.	Reject or re-cure.
10. Final inspection	Joint meets design requirements: Strength, environment, appearance, reliability, durability.	Test programme. Review of process documentation and records.	Reject. Concession. Design change.
11. Pre-use storage	Joint meets design requirements: Strength, environment, appearance, reliability, durability.	Correct storage review of test reports, supplier information.	Reject. Design change.
12. Service	Joint meets design requirements: Strength, environment, appearance, reliability, durability.	Service monitoring. Joint failure rate.	Repair. Design change.

This model should be studied in conjunction with the Table below. How they will be put together is dependent on how the user will wish to 'see' the model.
 
 

Tools, techniques and their application

Process Step	Tools and techniques
Specification review
Design	Taguchi FMEA QFD
Design verification	Product testing
Selection and sourcing of materials	Product testing
Pre-treating	SPC Validated procedure
Assembly	SPC Validated procedure Inspection
Cure	SPC Validated procedure
Final inspection	Product testing
Pre-use storage	Product testing
Service	FMEA

Brief remarks on the quality control methods mentioned in the Table (QFD, Taguchi, FMEA) are presented in the following paragraphs.

QFD

The foundation of QFD (Quality Function Deployment) is the belief that products should be designed to reflect customers' desires and tastes.  Therefore this method is most suitable for applications that are based primarily on the detailed requirements and specifications of the customers and where interfunctional planning and communications among marketing people, design engineers and manufacturing staff are required.

Taguchi

The off-line Taguchi method defines quality control in relation to design for quality.  This is a continuous quality improvement programme that includes continuous reduction in the variability of product performance characteristics about their target values.  In this method statistically planned experiments can be used to identify the setting of product (Process) parameters that reduce performance variation.  Therefore, this method is less suitable for applications on which performance characteristics are not based on numerical measurements, but on an order categorical scale such as Poor, Fair, Good or Excellent.

FMEA

Failure Mode and Effect Analysis is an important method to identify all possible failures and their effect on the system.  The objective is to classify failures according to their effect.  FMEA provides an excellent basis for classification of characteristics.  When engaged in FNMA, it is wise to bear in mind that the severity of the failure is not the only important factor.  One must also consider the probability of failure.  One purpose of FMEA is to direct the available resources toward the most promising opportunity for improvement.  As far as adhesive bonding is concerned, this method offers a quality control technique that is most suitable for low rate production (i.e. highly cost sensitive production line) and where multiple processes are interlinked.

DFMA

Design For Manufacturing and Assembly reflects manufacturing problems in an assembly line and how these problems should be handled.  This technique is concerned with reducing the cost of a product through simplification of its design.  In a multi-part assembly line the best way to achieve this cost reduction is:

1 - to reduce the number of individual parts that must be assembled,
2- to ensure that the simplest process route has been selected.

References

Quality assurance in adhesive technology. Eureka Project EU716. Abington Publishing. Cambridge, England. ISBN 1 85573 259 9