Defective parts

Risk-Based Inspection. Inspection programs developed using risk analysis methods are becoming increasingly popular (15,16) (see Hazard ANALYSIS AND RISK ASSESSMENT). In this approach, the frequency and type of in-service inspection (IS I) is determined by the probabiUstic risk assessment (PRA) of the inspection results. Here, the results might be a false acceptance of a part that will fail as well as the false rejection of a part that will not fail. Whether a plant or a consumer product, false acceptance of a defective part could lead to catastrophic failure and considerable cost. Also, the false rejection of parts may lead to unjustified, and sometimes exorbitant, costs of operation (2). Risk is defined as follows ... 

Prior to JIT, defective parts could be discarded and a replacement taken from inventory. Under JIT there is insufficient inventory to replace defective parts, often lea ding to a shutdown of the assembly process. The Japanese use this impact to heighten the awareness of nonconforming material... 

The process provides fast mol ding cycles, unHmited shelf life for the sheet, large part capabQity, and design flexibOity. The process also allows for scrap materials to be recycled. Trim waste from the mol ding operation and defective parts can be ground up and recycled into the basic sheet process in controUed amounts. Some of this waste has also been used as input for injection mol ding. 

Type of Data In general, statistics deals with two types of data counts and measurements. Counts represent the number of discrete outcomes, such as the number of defective parts in a shipment, the number of lost-time accidents, and so forth. Measurement data are treated as a continuum. For example, the tensile strength of a synthetic yarn theoretically could be measured to any degree of precision. A subtle aspect associated with count and measurement data is that some types of count data can be dealt with through the application of techniques which have been developed for measurement data alone. This abihty is due to the fact that some simphfied measurement statistics sei ve as an excellent approximation for the more tedious count statistics. 

Warranty and Guarantee - Ensures that the vendor stands behind the design specification and will replace any defective parts. 

Sullivan FM, Barlow SM. 1985. The relevance for man of animal data on reproductive toxicity of industrial chemicals. Prevention of Physical and Mental Congenital Defects, Part B Epidemiology, Early Detection and Therapy, and Environmental Factors. Alan R. Liss, Inc. 

This example clearly shows that developing and accepting a hypothesis based on accurate and complete information is necessary for setting an acceptable technical solution. If the plant manager could have persuaded the resin manufacturer to develop a new resin that was similar to the incumbent resin, then the defect would still be there, the cost of the troubleshooting process would have been extremely high, the supplier would have incurred unnecessary development costs, and a high level of defective parts would still have occurred because the root cause would not have been removed. 

SPC does require statistical quantities of product and automated data tracking. The best processes for SPC are those which are used to make large quantities of inexpensive parts. SPC is also difficult to apply to processes where the number of independent variables is large. Automated data acquisition is a must for SPC, but this is becoming inexpensive and common in the workplace. SPC is also a delayed control method. Many defective parts may be made before SPC corrects the process. The longer it takes to evaluate the results of the process, the more delay in the ability to react to process changes. Another important requirement of SPC is attention to detail on the part of the operator and/or process engineer. No battery of QC tests will detect every variation either in materials and process or in final quality. 

The process capability index, Cpk, is related to the so-called process sigma such that a 6a process corresponds to a Cpk of exactly 2.00, or 2.0 defective parts per billion (PPB), assuming N (0, a) quality variance distribution (can alternative calculation for process sigma estimates 3.4 defective parts per million for a 6a process). Examples of the correspondence between Cpk, process sigma, and defect rate for N(0, a) distributions are shown in Figure 2. The process capability (based on observed yield) of pharmaceutical manufacturers has been cited by some benchmark studies to be roughly 0.7 (2.1a) [28]. 

While industrial benchmarks clearly indicate that pharmaceutical manufacturers have many opportunities to improve quality control, direct comparison with other industries may be somewhat misleading. As opposed to such industries as semiconductor manufacturing, where defective parts are often readily apparent at some... 

If defect parts must be replaced it must be assured that the new part has the same specifications as the old one. 

A demand for a specially high level of significance of at least 0.9999 is present in rocket technology and spacecraft industry. In order to reach the mentioned level of significance, there has to be an almost disappearing level of (3 so that there is no chance of mounting a defective part into the mentioned crafts. 

Injection molding can be used with some thermosets in addition to thermoplastics, as long as the process can be controlled such that the crosslinking or curing takes place in the mold and not in the extruder barrel. It has been used effectively, for example, with thermosetting polyester resins. Scrap runners or defective parts must be discarded, however, as they cannot be remelted. 

Unfortunately, there are not many options available when you have a defective stopcock or joint. The best solution is to replace the defective part of the apparatus, or the entire apparatus if necessary. If the item is new, return it to your supplier and have it replaced. 

On the surface of manganese steel, the impact of the explosive layer onto the steel surface results in hardening the only objective of this process is that it enables repair work to be carried out on railway tracks in remote regions, and there is no need to convey the defective parts over long distances. In densely populated areas, forming explosions are difficult to perform. 

The safety valve is disassembled and examined, and all internal parts are blasted with glass shot to remove rust and surface accumulations. The valve body is then sandblasted and repainted, both internally and externally. During this stage, all defective parts are replaced or reconditioned to the manufacturer s specifications. Metal seats are machined to the desired flatness. All moving or guiding surfaces and all pressure-bearing surfaces are lightly lubricated. f8]... 

A machine can assemble 400 parts in half an hour. Of the 400 parts, 5% will be defective. If 2 such machines are working, how many non-defective parts will be assembled in 5 hours ... 

Risk has its place in a calculus of probabilities. It applies to a specific course of action. The risk of an action is the likelihood that it will produce an unwanted result. Risk lends itself to quantitative expression, as when we say that the chances offailing to strike oil in a field are better than fifty-fifty or that the chances of finding a defective part in a batch are two out of a hundred. In the framework of benefit-cost analysis, the risk of an innovation is how much we stand to lose if we fail, multiplied by the probability of failure. 

Trim waste (aftermixer, runner, gate, and flash) accounts for 10-15% of the Injection weight for PU-RRIM, and along with defective parts (2%), produce non-recoverable material which must be reflected In total material usage. 

Trim losses are negligible for SMC. Nonrecoverable raw material loss associated with prepreg fabrication and defective parts due mainly to poor material flow contributes about 4% and 3% respectively to the total material usage for this molding process. 

In effect this calculation normalizes all processes to a number of sigmas rather than absolute values. The relationship between DPMO and Z score is illustrated in Fig. 2. Z refers to the white area under the curve and the shaded area shows the area where there is a probability of failure. DPMO is the integration of the shaded areas, i.e., the proportion of the results beyond the calculated Z value. Obviously, as Z increases the defective part of the distribution shrinks. The exact probability associated with a specific Z score can be easily obtained from Z score tables or calculated with common software packages such as Excel or statistical packages such as Minitab . In the case of Excel, such calculations are not part of a standard package, but macros can easily be written to perform the needed calculations. 

The function V(( ) ) is however an ixnknown fxxnction, and we can only say that V(fg) = 0 at = 0. If we introduce the assvmption that the number or defect parts in a cell is distributed obeying a Poisson s distribution and the strength of a cell is decreased in proportion to the number of the defects contained in the cell, then the following equation can be obtained assuming a Poisson s distribution of the number of the defects in a cell (3). 

Employees must be trained in the proper use of respirators. Only persons who are physically capable of wearing respirators and of performing the clean up functions for which the respirator must be worn should be assigned tasks which carry the potential to require respirator usage. A medical evaluation should be performed to verify the employee s physical condition. Respirators should be fitted to each individual and potential effects of facial hair should be considered. Respirators should be cleaned and sanitized regularly after each use. Respirators should also be inspected and maintained according to manufacturer s instructions. Worn and defective parts should be replaced immediately. 

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