Misunderstandings and Improvements of First Piece Inspection in Injection Molding Factory
Time：2022-05-15 12:43:15 / Popularity： / Source：
First Article Inspection
It is a must-use method in on-site quality control. Generally, it refers to inspection of first or first few products processed at the beginning of each shift or after process has changed. (such as personnel changes, material change and tool change, machine tool adjustment, tool replacement and grinding, etc.) Generally, 3-5 products that are continuously produced must be inspected, and subsequent products can be processed in batches after passing. This control method is mostly used in production of parts and components in auto industry, but does the first article inspection we do play a role in quality control?
01. First inspection failure caseIn a factory, left front window control switch was repeatedly insensitive to lift glass. After investigation, height dimension of sliding block produced by factory exceeded lower limit. Data of 8-model products in piece inspection are: 9.78, 9.80, 9.77, 9.80, 9.77, 9.79, 9.77, 9.79. Production starts after the first piece passes inspection. Why are there still defective products?
┃ Let's take a look at how to do first article inspection?
Purpose of first article inspection is to confirm that process of this process is stable and has process capability to meet quality requirements. In layman's terms, it is to confirm that qualified products can be produced stably.
02. What are criteria for passing first article inspection?Many friends regard first pass as pass standard. If you go back a few decades, a large number of processes are processed by workers operating general equipment, and results of work mainly depend on skills of employees. Then purpose of first article inspection is to confirm that this process has ability to produce qualified products, and it is correct to regard qualified as acceptance criterion. With passage of time, most of today's enterprises use automatic equipment or even intelligent equipment for production. Operator is a skilled worker, and work results have little dependence on operator. Then first article inspection has evolved into identification method for process capability (stability), and it is very inappropriate to use qualified as a standard.
Please take a look at a case: size is out of tolerance
Machining workshop of a factory found that size of parts was out of tolerance (requirement: 12 (+0/ - 0.15), actual measurement: 12.01 - 12.02), 106 pieces were submitted for inspection, 35 were defective, and failure rate was 42.68%;
Note disqualification distribution, which is 12.01 - 12.02, with 48 pieces. Please estimate, where is size distribution of qualified parts? By the way, probably in range of 11.98 - 12.00.
Problem came out. Qualified standard is 12 (+0/ - 0.15), and variation range of processed product is 0.05. Why does equipment with such high processing precision have nearly 50% defective products?
Friends who do not understand statistical process control only need to remember that passing standard of the first piece should be limited to range of 1/5 - 1/3 of central value of tolerance zone to ensure quality stability of mass production.
Based on principles of statistical process control, if process is stable, then 2/3 of points will fall within 1/3 of control limits.
Therefore, in order to confirm that process meets quality requirements, corresponding size of first piece should be controlled within 1/3 of central value of specification limit, so calculated process capability is 1.0.
Such reasoning is because testing a part is a small probability event, and result should conform to a high probability distribution. Acceptance criteria for first article should be controlled within 1/5 of central value of specification limits if a process drift of 1.5 standard deviations is considered.
If everyone agrees with this reasoning, you should adjust your first pass criteria. Control principle of other industries is same, one-sided tolerance considers one-sided probability distribution.
03 . When will first article inspection be done?Start of each work shift
Replacement or adjustment of equipment, process equipment (including tool change)
Change technical conditions, process methods and process parameters (such as changes in roughness requirements, changes from reaming of inner holes to boring holes, changes in amount of cutting or speed in NC program, etc.)
After adopting new material or material substitution (such as material change during processing, etc.)
Replace or re-assay bath solution (such as phosphating, nitriding, etc.)
04. What should be recorded in first article inspection?First, record quality characteristic value of first piece. Second and most important thing is to record operating conditions for processing first piece, including but not limited to batch of raw materials, actual value of process parameters (not set value), environmental parameters, model of mold tool, etc.
In the case of CNC equipment, processing time can also be recorded, as long as it can be traced back to conditions for processing first piece.
Above records correspond to quality influencing factors of first piece and first piece of processing. If a defect is found in mass production, operating conditions of first piece and first piece can be compared on spot to find cause of defect.
There may be an embarrassing situation, that is, there is a defective product, but operating conditions of defective product are not much different from first one.
Congratulations, you have discovered a new quality impact factor, find a way to identify and control it.
05. Contradictions between first article inspection and production scheduleFirst piece inspection generally requires full-time inspection personnel to judge, and some even need to be sent to laboratory. Mass production cannot be carried out until first piece has not passed judgement. Of course, I have no choice but to make first piece of your site undecided and it is also in mass production.
Imagine a scenario where 10 stations have processed first piece after starting production one day, but inspection force is not enough, and operators are waiting for result.
To resolve this contradiction, there are three ways:
The first is first inspection by squad leader.
If you use him, you must trust him, but first piece released by squad leader must not be reordered, and it can only be circulated after full-time inspection and confirmation.
Second is to develop special inspection tools in a targeted manner to improve inspection speed.
Note here that rule for first special inspection tool is also a tolerance of 1/3-1/5.
Third is to use alternative characteristic test.
For example: CNC equipment does not need full-scale inspection, but only needs to inspect dimensions related to tool and positioning. Geometric tolerance and other dimensions are guaranteed by machine and software itself.
Comparing with previous first article operating conditions, if operating conditions are consistent, the first article inspection can also be replaced.
Note: First article inspection of engineering changes cannot be an alternative method.
06. SummaryWith application of advanced technology and progress of management thinking, classical inspection methods should also be given new meanings. There are still many such nodes in on-site quality management. I hope all friends will also start thinking about what inspection criteria need to be improved in today's digital factory.
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