Specifying Quality Requirements
SPECIFYING QUALITY REQUIREMENTS
This is a very important topic. Most disputes in the supply of diecastings, as in most other industrial products, stem from differences in understanding of what is required between the vendor and the specifier.
This does not mean that every aspect of a diecasting must be subjected to onerous written standards but it does mean that dimensions and attributes which are critical to achieving the castings function, including in this the specifiers aesthetic expectations of it, should be specified and agreed upon with the diecaster. Wherever possible the specification should be based upon measurable qualities. Where a subjective judgement cannot be avoided an effort should be made to achieve identical levels of discernment between the specifier and the diecaster. In such cases it is also important that the casting buyer, if this is a separate person, should also be made fully aware of this subjective quality requirement. This is to ensure that he or she can compare like with like when assessing competitive quotes.
Some casting attributes that may need to be specified are shown on the following page together with potential methods of checking them. Once a decision on which attributes need to be specified is made, further decisions on who will check them and how frequently, are required. For large volumes this may mean that statistical quality control may be specified for some attributes. Because of the cost implications it is quite rare for 100% checking of any aspect of a zinc alloy diecasting to be carried out.
Many zinc alloy diecasters are certified to ISO 9001:2000. Without wishing to denigrate this standard in any way it must be remembered that it is not a product standard. The certification of a company as complying with ISO 9001:2000 means that it has a competent quality system, and that you can feel confident that you will receive what you have specified from them. Nearly all especially capable zinc alloy diecasters are ISO 9001:2000 registered but many companies with more moderate casting expertise are also certified. Dealing with ISO 9001:2000 certified companies should at least mean that if the product quality specifications are too tight for them to meet they will inform you and allow you to take appropriate action.
Statistical Quality Control techniques are fairly widely practised in the zinc diecasting industry, true Statistical Process Control less widely. SQC deals with measurements of the product and SPC deals with process parameters. However this does not mean that SQC can only determine whether a batch of castings is acceptable. In practice, measurements of sample castings are made at regular intervals during production and the trends are plotted, enabling remedial action to be taken before out of specification castings are produced.
It should be born in mind that these techniques are only applicable to “in control” processes and that an initial process capability study will be required for each new die to establish that this is in fact the case.
For high volume components of a critical nature a combination of SQC and/or limited SPC is recommended as giving the closest possible approach to zero defects without the expense of 100% inspection. The cost of the statistical quality techniques is frequently justifiable by the reduction in scrap rates that result, both at the diecasting plant and during subsequent assembly operations.