Quality Control
Quality Control
The pressure from globalization has made manufacturing organizations moving towards three major competitive areas namely (i) quality, (ii) cost, and (iii) responsiveness. Quality is a universal value and has become a global issue. In order to survive and be able to provide customers with good products, organizations are required to exercise quality control practices. The quality of goods produced and services rendered has been monitored, either directly or indirectly, since time immemorial. Delivering products with high quality is a top priority for any organization.
If a product fulfils the customer’s expectations, the customers are pleased and consider that the product is of acceptable or even high quality. If their expectations are not fulfilled, the customers consider that the product is of low quality. This means that the quality of a product can be defined as ‘its ability to fulfil the customer’s needs and expectations’. Quality needs to be defined firstly in terms of parameters or characteristics, which vary from product to product.
A specification is the minimum requirement according to which the organization makes and delivers the product to the customer. In setting specification limits, the things which are to be considered are (i) the user’s and / or customer’s needs, (ii) requirements relating to product safety and health hazards as per the statutory and regulatory requirements, (iii) requirements provided for in national and / or international standards, (iv) the competitor’s product specifications, in order to gain marketing advantages In designing the product keeping in mind the capability of the processes, and the available equipment and technology. It is also necessary to maintain a balance between cost and value realization. The clearer the specification, the better the possibility of creating and delivering quality products.
Quality can be defined as fulfilling specification or customer requirement, without any defect. A product is said to be of high in quality if it is functioning as expected and reliable. However, there are a number of definitions of quality which are normally accepted. Quality can be variously thought of as (i) conformance to the specification, (ii) fitness for purpose, (iii) meeting customers’ requirements, (iv) doing things right first time, and (v) the features and characteristics of a product which bear upon its ability to satisfy a stated need.
From these definitions, there are nine dimensions (or characteristics) which describe the quality of an item. The nine dimensions are (i) performance which is the product’s primary operating characteristics, (ii) features of the product, (iii) reliability which is the probability of the product surviving over a specified period of time under specified conditions, (iv) conformance which is the degree to which the physical and performance characteristics meet established standards., (v) durability which is the quantity of use one can get from a product before it needs to be replaced, (vi) serviceability which is the ease with which the product can be serviced or repaired, (vii) aesthetics which means how the product looks, (viii) safety which means assurance that the customer is not going to be injured when using the product, and (ix) perceived quality which is a subjective assessment resulting from image, advertising, or brand names. Depending on the point of view of the organization, some of these dimensions can be of more interest to the organization than others. For example, aesthetics can be most important to the customer, conformance can be more important to the producing organization, and perceived quality can be most important to the sales distributor. Hence, how quality is defined depends on whether one is the designer, producer, distributor, or customer. For simplicity, ‘conformance to specification’ can be used as a working definition for quality.
Quality control is a system of routine technical activities implemented by the organizational personnel to measure and control the quality of the product as it is produced. It is a process which ensures customers receive products free from defects and meet their needs. It refers to the activities to ensure that produced items are fulfilling the highest possible quality. It is a process by which entities review the quality of all factors involved in production.
Quality control is concerned with sampling, specifications and testing as well as the organization, documentation, and release procedures which ensure that the necessary and relevant tests are carried out, and that materials are not released for use, nor products released for sale or supply, until their quality has been judged satisfactory. Quality control is not confined to laboratory operations, but is to be involved in all decisions which can concern the quality of the product. The independence of quality control from production is considered fundamental to the satisfactory operation of quality control.
ISO 9000 defines quality control as ‘a part of quality management focused on fulfilling the requirements of quality’. The ISO definition states that quality control is the operational techniques and activities which are used to fulfil requirements for quality. This definition can imply that any activity whether serving the improvement, control, management, or assurance of quality can be a quality control activity. What the definition fails to tell is that controls regulate performance. They prevent change and when applied to quality regulate quality performance and prevent undesirable changes in the quality standards.
Quality control is a process for maintaining standards and not for creating them. Standards are maintained through a process of selection, measurement and correction of work, so that only those products which emerge from the process meet the standards. In simple terms, quality control prevents undesirable changes being present in the quality of the product being supplied. Quality control can be applied to particular products, to processes which produce the products, or to the output of the whole organization by measuring the overall quality performance of the organization.
Quality control and quality assurance
There are two terms which are frequently used for ensuring the quality of a product or service. These terms are ‘quality control’ and ‘quality assurance’. These two terms are frequently being used interchangeably. However, these two terms differ in meaning. Quality control is product oriented and focuses on defect identification, while quality assurance is process oriented and focuses on the defect prevention.
Quality control is a system of routine technical activities, to measure and control the quality of the product as it is being produced. The quality control system is designed to (i) provide routine and consistent checks to ensure product integrity, correctness, and completeness, (ii) identify and address errors and omissions, and (iii) document and archive the product sample and record all quality control activities. The objectives of quality control in any production system are to (i) provide inputs for taking timely corrective actions, (ii) eliminate non-conformities and their consequences, (iii) eliminate re-work, wasted raw materials, and resources, and (iv) achieve the objectives at the lowest possible cost.
Quality control activities include general methods such as accuracy checks of testing instruments, calculations and the use of approved standardized procedures, measurements, estimating uncertainties, archiving information and reporting. Higher tier quality control activities include technical reviews of source categories, activity and data, and methods.
Quality assurance activities include a planned system of review procedures conducted by personnel not directly involved in the product production / development process. Reviews, preferably by independent third parties, are to be performed upon a finalized products following the implementation of quality control procedures. Reviews verify that the quality objectives are being met, ensure that the product represents the best possible implementation of the product specifications as per the standards, and support the effectiveness of the quality control.
Quality control is an evaluation to determine needed corrective action. It is an act of guiding a process where variables are kept under constant observation and control within a range of limits. It is based on measurement and control of the parameters which can affect the quality of product or services. On the other hand, quality assurance is an activity which gives confidence and make doubly sure that the things do not go wrong. Quality assurance takes place through a planned and systematic activity normally through procedures designed for meeting the quality requirements for a product.
The ISO definition states that quality control is the operational techniques and activities which are used to fulfill requirements for quality. This definition can imply that any activity whether serving the improvement, control, management, or assurance of quality can be a quality control activity.
Every organization is required to have a quality control department. This department is to be independent from other departments, and under the authority of a person with appropriate qualifications and experience. Adequate resources are to be available to ensure that all the quality control arrangements are effectively and reliably carried out.
Where the consequences are less severe or where other types of sensors are not practical or possible, human inspection and test can be used as a means of detecting failure. Where failure cannot be measured without observing trends over longer periods, one can use information controls. They do not stop immediate operations but can well be used to stop further operations when limits are exceeded. If one has no controls then quality products are produced by chance and not by design. The more controls one installs, the more certain the person is of producing products of consistent quality but there is balance to be achieved. However, one is to be beware of the law of diminishing returns.
It is frequently deemed that quality assurance serves prevention and quality control detection, but a control installed to detect failure before it occurs serves prevention such as reducing the tolerance band to well within the specification limits. Hence, quality control can prevent failure. Assurance is the result of an examination whereas control produces the result. Quality assurance does not change the product, while quality control does.
A defect is associated with a quality characteristic which does not meet certain standards. Also, the severity of one of more defects in a product can cause it to be unacceptable (or defective). The modern term for defect is non-conformity, and the term for defective is non-conforming product. The American National Standards Institute, the International Organization for Standardization, and the American Society for Quality provide a definition of a defect in ANSI/ISO/ASQ Standard A8402 (ASQ 1994).
Since the definition of quality involves meeting the requirements of the customer, these requirements need to be documented. A standard, or a specification, refers to a precise statement which formalizes the requirements of the customer. It can relate to a product, a process, or a service. For example, the specifications for an axle can be 2 ± 0.1 centimeters (cm) for the inside diameter, 4 ± 0.2 cm for the outside diameter, and 10 ± 0.5 cm for the length. This means that for an axle to be acceptable to the customer, each of these dimensions are to be within the values specified.
Definitions given by the National Bureau of Standards for different quality related documents are as (i) specification is a set of conditions and requirements, of specific and limited application, that provide a detailed description of the procedure, process, material, product, or service for use primarily in procurement and manufacturing (standards can be referenced or included in a specification), and (ii) standard is a prescribed set of conditions and requirements, of general or broad application, established by authority or agreement, to be satisfied by a material, product, process, procedure, convention, test method, and / or the physical, functional, performance, or conformance characteristic thereof, and a physical embodiment of a unit of measurement (for example, an object such as the standard kilogram or an apparatus such as the cesium beam clock). Acceptable bounds on individual quality characteristics (e.g., 2 ± 0.1 cm for the inside diameter) are normally known as specification limits, whereas the document which addresses the requirements of all the quality characteristics is labeled as the standard.
Quality control is a process for maintaining standards. Standards are maintained through a process of selection, measurement and correction of work, so that only those products or services emerge from the process which meets the standards. In simple terms, quality control prevents undesirable changes being present in the quality of the product or service being supplied. Quality control can be applied to particular products, to processes which produce the products, or to the output of the whole organization by measuring the overall quality performance of the organization.
Quality control personnel need to have access to production areas for sampling and investigation as appropriate. Finished product assessment is to embrace all relevant factors, including production conditions, results of in-process testing, a review of production (including packaging) documentation, compliance with the specification of the finished product, and examination of the final finished pack.
Quality control is frequently considered to be an activity which takes place after the event is over. This means that the quality is determined after the completion of the activity and the action to correct the deficiencies takes place post event for future activities. However, one can control results by measuring parameters by installing sensors before, during, or after the results of the activity is achieved. It all depends on where and when the measurement takes place in order to avoid the consequences of failure. Some failures cannot be allowed to occur and so are to be prevented from happening through rigorous planning and design. Other failures are not so critical but are to be corrected immediately.
Implementation of quality control needs resources, expertise, and time. The practical aspects of quality control are (i) quality control personnel are to work in complete harmony with all the connected departmental personnel especially with the personnel connected with operation of the process, (ii) technological discipline is very important for quality control since even small violation can have disastrous effect on the process, (iii) speedy communication is an important aspect for quality control and it is necessary that monitoring and testing data as well as inspection results are shared with the concerned personnel without any delays, (iv) quality control procedures are to follow the required standards and are to be comprehensive taking into considerations the process and product needs, facilities available, and the operating personnel’s requirements, (v) all the quality control personnel are to be well trained both in process and quality control activities, (vi) frequency of quality control checks are to meet the process as well as standard’s requirements, (vii) quality control data is to be properly and systematically recorded and controlled for analysis and for future use, and (viii) quality control functions are to be carried only by those personnel who have the required expertise.
Production system and quality control
The goal of any production system during its operation is to generate a product which is aimed at and which is useful. The product of the production system can be a physical object, service, or information. Every production cycle begins with inputs which are transformed by a process into a more desired state or into the product. Besides raw materials, every process needs resource (consumables, energy, utilities, and operating parts etc.). The production inputs to a process can be classified as (i) man (persons executing or controlling the process), (ii) machine (equipment, furnace or machinery used in the execution of the process), (iii) material (raw materials, resources or spare and operating parts required in the process), (iv) methods (procedures and sequence used to execute the process), and (v) information (work instructions, control parameters, data, and instrument’s readings that guide process execution).
In every process, large quantity of variations and discrepancies in the process parameters can cause non-conformities, with five undesirable consequences namely (i) scrapped or wasted raw materials and resources, (ii) degraded process throughput, (iii) contamination from undetected non-conformities causing degradation of the product to seconds or causing its total rejection, (iv) damage to the process equipment, and (v) unsafe working conditions for man and equipment. Different components involved in quality control are shown in Fig 1.
Fig 1 Components of quality control
Steps in quality control
The activity of quality control is carried out in the several steps consisting of (i) study of the process so as to have complete knowledge of raw materials including their complete parameters, the process including the equipments, instrumentation and automation, the resources and their specifications, and the products and by-products and their specifications, (ii) determination of the process parameters which are required to be monitored and controlled along with the decisions regarding the range of values within which these parameters are to be controlled, (iii) establishment of the sampling and testing methods for the raw materials, resources, and the products, (iv) establishment of the frequency of testing for the raw materials, resources, and the products along with the decision regarding the limits of acceptability and the units of measurement, (v) determination of the criticality of the monitoring and testing so that corrective actions can be taken in time to avoid process and product going out of control or becoming unsafe for operation, (vi) preparation of plans for control which specifies the means by which the process and product characteristics are to be achieved as well as variations detected and removed with the plans to be in the form of approved written documents (procedures), (vii) organization of resources to implement the plans for quality control. (viii) to ensure that gauges, instruments, and testing equipment are available in working order at appropriate points in the process to know the variance from specification, (ix) to collect and record and transmit the data to a place for analysis, (x) to verify the results and diagnose the cause of variance, and (xi) to take remedial action and to decide on the action needed for restoring the process to a healthy state. Fig 2 shows main steps in the process control.
Fig 2 Steps in process control
Evolution of quality control
Several of the quality control methods were initially developed to aid production because high volume production needs several repetitive steps involving a controlled sequence of operations. Since during operation of a process, the activities are frequently repeated, it is easier to recognize processing errors and identify appropriate control measures. Historically, the first quality control methods were based on inspections.
Good quality control laboratory practice
Control laboratory premises and equipment are required to meet the general and specific requirements for the quality control areas. The laboratory is to be arranged so as to minimize risk of the mix-up and contamination of the samples. Laboratory equipment is to be installed away from the high-risk areas to avoid accident and sample contamination.
The personnel, premises, and equipment in the laboratories are to be appropriate to the tasks imposed by the nature and the scale of the manufacturing operations. Laboratory documentation are to be control documents. An important part of this documentation which deals with quality control is to be readily available in the quality control laboratory. The document details include (i) specifications, (ii) procedures describing sampling, testing, records (including test worksheets and/or laboratory note-books), recording and verifying, (iii) a procedure for the investigation of ‘out of specification and anomalous’ results and ‘out of trend’ results, (iv) procedures for and records of the calibration / qualification of instruments and maintenance of equipment, (v) test reports and / or certificates of analysis, (vi) data from environmental (air, water and others utilities) monitoring, where required, and (vii) validation records of test methods, where applicable.
Any quality control documentation relating to a batch record is to be retained. Some kinds of data (e.g., tests results, yields, environmental control) are to be recorded in a manner permitting trend evaluation. Any out of trend or out of specification data is to be addressed and subject to investigation. In addition to the information which is part of the batch documentation, other raw data such as laboratory note-books and / or records are to be retained and readily available
The sample taking is to be done and recorded in accordance with approved written procedures which describe (i) the method of sampling, (ii) the equipment to be used, (iii) the quantity of the sample to be taken, (iv) instructions for any required sub-division of the sample, (v) the type and condition of the sample container to be used, (vi) the identification of batches sampled, (vii) any special precautions to be observed, especially with regard to the sampling, (vii) the storage conditions, and (viii) instructions for the cleaning and storage of sampling equipment.
Samples are to be representative of the batch of materials or products from which they are taken. Other samples can also be taken to monitor the most stressed part of a process (e.g., beginning or end of a process). The sampling plan used is to be appropriately justified. Sample containers are to bear a label indicating the contents, with the batch number, the date of sampling, and the processes from which samples have been drawn. They are to be managed in a manner to minimize the risk of mix-up and to protect the samples from adverse storage conditions.
Testing methods are to be validated. A laboratory which is using a testing method and which did not perform the original validation is required to verify the appropriateness of the testing method. All testing operations are to be carried out according to the approved methods. The results obtained are to be recorded, trended, and checked to make sure that they are consistent with each other. Any calculations are to be critically examined.
The tests performed are to be recorded and the records are to include at least the following data (i) name of the material or product, (ii) sample identification details, (iii) references to the relevant specifications and testing procedures, (iv) test results, including observations and calculations, and reference to any certificates of analysis, (v) date of testing, (vi) initials of the persons who performed the testing, (vii) initials of the persons who verified the testing and the calculations, where appropriate, and (viii) a clear statement of approval or rejection (or other status decision) and the dated signature of the designated responsible person.
All the in-process controls, including those made in the production area by production personnel, are to be performed according to approved methods and the results recorded. In case of wet analysis, special attention is to be given to the quality of laboratory reagents, solutions, glassware, and reference standards. Solutions are to be prepared and controlled in accordance with written procedures. Reference standards are to be certified, qualified, and verified as suitable for its intended use.
Laboratory reagents, solutions, and reference standards are to be marked with the preparation and opening date and the signature of the person who prepared them. Their in-use shelf life is to be established / documented and justified. The expiry date of unstable reagents is to be indicated on the label, together with specific storage conditions. In addition, for volumetric solutions, the last date of standardization is to be indicated.
Where necessary, the date of receipt of any substance used for testing operations (e.g., reagents, solutions and reference standards) is to be indicated on the container. Instructions for use and storage are to be followed. In certain cases, it can be necessary to carry out an identification test and / or other testing of reagent materials upon receipt or before use.
Product certification
If a product bears a recognized mark such as a national or regional, this gives an assurance to the buyer that the product meets the specifications to which the mark corresponds. In other words, the product can be considered a ‘quality / safe product’ by the buyer. ‘Product certification’ is defined as ‘a procedure by which a third party gives written assurance that a product, or process conforms to specified requirements. The product certification authorities normally permit the use of a mark on the product to demonstrate that the product meets a defined set of requirements, such as certain properties, safety, fitness for use and / or specific interchangeability characteristics which are normally specified in a standard. The mark is normally found on the product or its packaging. It also carries a reference to the number of the relevant product standard against which the product is certified. Ideally, a product certification mark is to demonstrate to the customer that a product meets the normally accepted standard for that product.
Product certification helps the customer to choose products which meet the requirements of the specified standard, are suitable for the purpose, and are safe from hazards to life and property. Product certification gives an organized purchaser greater confidence in the integrity of the product, saves unnecessary product inspection, and provides a convenient basis for concluding contracts. For the producer, product certification streamlines the production process and introduces a quality assurance system for ensuring conformity of the product to the standard. Product certification enhances the marketability of products and provides an opportunity for competing with similar products and building a better image of the product both in the domestic and the international market. Product certification helps reduce technical barriers to trade, as the World Trade Organization’s Agreement on Technical Barriers to Trade recognizes certification as an instrument to prevent technical barriers and as an important factor in the furtherance of international trade.
For certain products, it becomes a prerequisite for putting certain products on the market. A product bearing a mark carries a third-party guarantee that (i) the product has been produced according to an applicable standard, (ii) the production process has been supervised and controlled, (iii) the product has been tested in an independent laboratory, and (iv) If the customer finds that a marked product does not meet the declared standard, the customer can then approach the certification body for redress of the complaint.
Product certification carried out by third-party certification bodies (i.e., independently of the customer, seller, or buyer) is most acceptable to purchasers, importers, and regulatory authorities. Several national standards bodies in many countries provide third-party product certification services. In some countries, certification is carried out by trade or industry associations, governmental institutions, or private certifications bodies.
Benefits of quality control
Quality control during production can be a little tricky. Frequently, it is carried out at the end of the production process for segregating good products from the rejected ones. Customers expect and demand high-quality products. When customers receive quality products the organization is benefitted by (i) increase in customer loyalty, (ii) gain of repeat orders, (iii) gain of new customers from referrals, (iv) maintenance or improvement of the position of the organization in the market, (v) improvement in the safety, (vi) reduction in the liability risks, and (vii) contribution to the overall positive branding of the organizational product. Fig 3 shows benefits of quality control.
Fig 3 Benefits of quality control
Inspection
Inspection is an important tool to achieve quality concept. It is necessary to assure confidence to the producer and aims satisfaction to the customer. Inspection is an indispensable tool of the present-day production processes. It helps to control quality, reduces production costs, eliminate scrap losses, and assignable causes of defective work. The inspection and test units are responsible for appraising the quality of incoming raw materials and components as well as the quality of the produced product. It checks the components at different stages with reference to certain pre-determined factors and detecting and sorting out the faulty or defective items. It also specified the types of inspection devices to be used and the procedures to be followed to measure the quality characteristics.
The objectives of inspection include (i) to detect and remove the faulty raw materials before it undergoes production, (ii) to detect the faulty products in production whenever it is detected, (iii) to bring facts to the notice of managers before they become serous to enable them discover weaknesses and come over the problem, (iv) to prevent the sub-standard product reaching the customer and hence reducing the complaints, and (v) to promote reputation for quality and reliability of the product.
The purpose of inspection includes (i) to distinguish good lots from bad lots, (ii) to distinguish good pieces from bad pieces, (iii) to determine if the process is changing, (iv) to determine if the process is approaching the specification limits, (v) to rate quality of product, (vi) to rate accuracy of inspectors, (vii) to measure the precision of the measuring instrument, (viii) to secure products-design information, and (ix) to measure process capability.
Inspections are normally done on the products of a process and are made after the process has transformed inputs into a product. Inspections can be visual or are carried out with the help of gauges, instruments, and testing machines. During the inspection, the product can pass, segregated for reworking, downgraded, or rejected. Inspections are carried out based on applicable product standards. In order to ensure that the inspection results are consistent from one location to another, the gauges, instruments, and the testing machines used in inspection are to be traceable to national standards.
Inspection only measures the degree of conformance to a standard in the case of variables. In the case of attributes inspection only separates the non-conforming material from the conforming material. Inspection does not show why the non-conforming materials are being produced. Inspection is the most common method of attaining standardization, uniformity, and quality of workmanship. It is the cost art of controlling the production quality after comparison with the established standards and specifications. It is the function of quality control. If the said material does not fall within the zone of acceptability, it is to be rejected and corrective measure is to be applied to see that the items in future conform to specified standards.
By inspection, it is normally meant that, at certain stages in the course of production, a comparison is made between what has actually been produced and what should have been produced. The standard of reference can be a specification, drawing, or a visual quality standard. The check made is to be appropriate to the job and is to be made with suitable measuring instruments. Inspectors are not to waste time checking things which do not matter or fail to do a check which is important. Things which are unlikely to go wrong need little checking and those which are difficult to hold within limits need a considerable amount of attention.
It is a mis-conception that the inspector alone is responsible for quality. Quality results from a combination of quality of the original designs, the methods, equipment and materials used, and the skill and care of the operator. In spite of these, if the job is still wrong, no amount of inspection can put it right. As per the production flow, the inspection can be divided into (i) incoming inspection, (ii) in-process inspection, and (iii) final inspection. Types of Inspection can also be categorized as (i) floor inspection, (ii) centralized inspection, (iii) combined inspection, (iv) functional inspection, (v) first piece inspection, (vi) pilot piece inspection and (vii) final inspection
Incoming inspection concerns goods upon delivery from vendors and / or suppliers. It consists of inspection of raw materials, components, sub-assemblies, and so on. The aim of incoming inspection is to prevent goods which do not fulfil the quality requirements from entering the production process. The steps for the control of the quality of supplies during the incoming inspection are (i) preparation of a buying specification, setting out exactly what quality of material has to be obtained, and (ii) possible suppliers are checked for their ability and willingness to provide this quality which is called ‘vendor appraisal’ or ‘supplier evaluation’. If the results of the vendor appraisal are satisfactory, then the supplier is placed on an approved list and purchase orders are placed when goods are needed. When goods are received, they are subjected to some form of goods inward inspection. The results of the inspection are used to give each supplier a numerical rating, showing how satisfactory or otherwise his suppliers are. This is called ‘vendor rating’. The results at every stage are monitored and steps taken to improve or discontinue unsatisfactory suppliers.
In-process inspection aims to prevent products of unacceptable quality from being produced. It provides data for making decisions on the product (accept or rework or reject), as well as on the process (run or stop). In-process inspection can take the form of (i) first-piece inspection, (ii) patrol inspection, (iii) operator inspection. (iv) last-piece inspection, and (v) stage inspection.
Whenever a production run is started, it is prudent to check the first piece, the first assembly and so on before the main run commences. Many faults can be detected by checking the first piece off and this can prevent the whole batch from going wrong. For example, (i) first-piece inspection can check whether the machine, jigs, fixtures, moulds, temperature and so on are correctly set up, (ii) first-piece inspection can discover whether the operators have fully understood their instructions, and (iii) first-piece inspection can also identify any discrepancies between the drawing and the quality plan, which can be investigated to avoid any further damage.
While first-piece inspection ensures that the job starts correctly, the purpose of patrol inspection is to help the operator to make the whole run correctly. From time to time, the patrol inspector visits the machine or operator and if the quality of the sample checked during the visit is wrong on any point, then this is required to be corrected as quickly as possible. If the operators go wrong, they are to be told quickly. The operators are to be encouraged to regard the inspectors as a friend assisting them in the task of keeping defective work to a minimum.
Operator inspection means that instead of the inspector, the operator carries out the inspection at a pre-determined time during the production. Last-piece inspection is carried out on the last item produced in the lot. This allows action to be taken to rectify faults in the machine and / or tools before beginning of the next lot. Product quality is a guide for the next lot. If these faults are only detected when the next lot has started, there can be a risk of production delays.
Stage inspection involves inspection of products after every operation or group of operations. Stage inspection points are located on the shop floor itself, where components are tendered for inspection. Jobs found to be unacceptable are returned for rectification if they are rectifiable, otherwise they are scrapped.
Final inspection and / or testing is done after the production has been completed, with the object of making sure that the goods concerned are satisfactory for dispatch to the customer or maybe to another department for the next operation. Based on the product specifications, inspection instructions are prepared which lay down the details of the tests to be carried out, the measuring instruments or test equipment to be used, and the criteria for deciding acceptance of the product with respect to each characteristic. Inspection instructions are also to include details of the sampling plan such as size of sample and the criteria of acceptance to be followed.
Measuring instruments or test equipment used for inspection are to be calibrated periodically to verify their accuracy. It is necessary to exercise suitable control over the movement of the product through the inspection area in order to avoid a mix-up of accepted and rejected products. Ways to exercise such control include (i) provision of clear labels (preferably of different colours) for products awaiting inspection, accepted products, rejected products, products on hold awaiting the results of tests and / or inspection and so on, (ii) separation of accepted and rejected products, (iii) review of rejected products for rectification or repair or for sale as seconds, and (iv) the accepted product are only to be released to the next process or to the customer by a person who is authorized to do so.
There are two methods of inspection. They are (i) 100 % inspection, and (ii) sampling inspection. The 100 inspection involves careful inspection in detail of quality at each strategic point or stage of production where the test is involved is non-destructive and every piece is separately inspected. It requires a greater number of inspectors and hence it is a costly method. There is no sampling error. This is subjected to inspection error arising out of fatigue, negligence, difficulty of the supervision etc. Hence, complete accuracy of influence is seldom attained. It is suitable only when a small number of pieces are there or a very high degree of quality is needed.
In the sampling inspection method, randomly selected samples are inspected. Samples taken from different patches of products are representatives. If the sample proves defective, the entire concerned is to be rejected or recovered. Sampling inspection is cheaper and quicker. It needs lesser number of inspectors. It is subjected to sampling errors but the magnitude of sampling error can be estimated. In the case of destructive test, random or sampling inspection is desirable. This type of inspection governs wide currency because of the introduction of automatic machines or equipments which are less susceptible to chance variable and hence need less inspection. Sampling inspection is suitable for inspection of products which have less precision importance and are less costly.
The disadvantages of inspection are (i) inspection adds to the cost of the product but not for its value, (ii) it is partially subjective, frequently the inspector has to judge whether a products passes or not, (iii) fatigue and monotony can affect any inspection judgment, and (iv) inspection merely separates good and bad items and as such it is no way to prevent the production of bad items.
Quality control tools and techniques
Quality can be defined as fulfilling specification or customer requirement, without any defect. A product is said to be high in quality if it is functioning as expected and reliable. Quality control refers to activities to ensure that produced items are fulfilling the highest possible quality. Most of tools and techniques to control quality are statistical techniques.
Manufacturing organizations applies different quality control techniques to improve the quality of the process by reducing its variability. A range of techniques are available to control product or process quality. These include seven statistical process control (SPC) tools consisting of (i) cause-and-effect diagram (also called Ishikawa diagram or fishbone diagram), (ii) check sheet, (iii) control chart, (iv) histogram, (v) pareto chart, (vi) scatter diagram, and (vii) stratification. Besides SPC tools, other techniques which are available to control product or process quality are (i) acceptance sampling, (ii) quality function deployment (QFD), (iii) failure mode and effects analysis (FMEA), (iv) six sigma, and design of experiments (DoE).
Statistical methods were introduced in Japan as the third stage in quality management (after World War II), and later in Europe. Statistical methods are an important part of the analyzing methods of measured data. They are an essential tool in quality management. These methods are one of the many tools for improving the process of performance according to the DMAIC (define-measure-analyze-improve-control) which is the six-sigma improvement cycle. The appropriate application of simple statistical methods is necessary for management and for quality assurance and implementation of the quality management system.
Quality characteristics fall into two broad classes (i) variables, and (ii) attributes. Characteristics which are measurable and are expressed on a numerical scale are called variables. Examples of variables are the density of a liquid in grams per cubic centimeter and the processing speed of a computer. Prior to defining an attribute, it is necessary to define a non-conformity and a non-conforming unit. A non-conformity is a quality characteristic which does not meet its stipulated specifications. Let’s say that the specification on the width of a product width is 100 ± 3 millimeters (mm). If the product has a width of 95 mm, then the width is a non-conformity. A non-conforming unit has one or more non-conformities such that the unit is unable to meet the intended standards and is unable to function as required. An example of a non-conforming unit is a cast iron pipe whose internal diameter and weight both fail to satisfy specifications, thereby making the unit dysfunctional.
A quality characteristic is said to be an attribute if it is classified as either conforming or non-conforming to a stipulated specification. A quality characteristic which cannot be measured on a numerical scale is expressed as an attribute. For example, the colour of a product is either acceptable or is not. However, there are some variables which are treated as attributes because it is simpler to measure them this way or because it is difficult to obtain data on them. Examples in this category are several. For example, the diameter of a bearing is, in theory, a variable. However, if it is measured using a go / no-go gauge and classify it as either conforming or non-conforming (with respect to some established specifications), the characteristic is expressed as an attribute. The reasons for using a go / no-go gauge, as opposed to a micrometer, can be economic, that is, the time needed to obtain a measurement using a go / no-go guage can be much shorter and hence less expensive. Alternatively, an inspector may not have enough time to obtain measurements on a numerical scale using a micrometer, so such a classification of variables would not be feasible.
Statistical process control involves comparing the output of a process with a standard and taking remedial actions in case of a discrepancy between the two. It also involves determining whether a process can produce a product which meets desired specifications or requirements.
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