The decision concerning how many impressions a mould tool should have is frequently given insufficient thought. The usual approach is as follows.
The production rate of any mould tool is given by the expression:
- 3600
- Production rate per hour = _____________ x Number of impressions
Cycle - When a new mould tool is being considered, however, the expression is rearranged to estimate the number of impressions necessary to achieve a desired production rate:
- Production rate per hour x Cycle
- Number of impressions = _____________________________________
3600If we take the example of a required production rate of 1800 per hour being necessary to meet production schedules, the number of impressions required to achieve this with an estimated cycle of 15 seconds is: - 1800 x 15
- _______________ = 7.5
- 3600Clearly this result would be rounded up to 8 impressions or sometimes a higher number to accommodate rejections and provide buffer stocks.
Having established this, the next stage is to establish which machine the mould tool will run on. Apart from the physical size of the tool, the other main limiting factors are the shot weight and the projected area of the mould. Once these factors have been resolved,
the mould tool may be designed to fit on the selected machine.
In many cases, this approach is satisfactory but a significant number of projects do not prove satisfactory in production when designed on this basis. This is often because the moulded parts do not meet the quality requirements. There are three main reasons:
- The mould tool design is unsatisfactory.
- The machine is not capable of producing the parts to the required quality.
- The machine is adequate but the number of impressions being used is too great for proper control.
If the mould tool design is incorrect, this can prove to be a very costly mistake. Major alterations to tools can be very expensive and time consuming. Worse than this, some tools that do not run properly due to errors in design cannot be rectified – new ones have to be made.
Putting the job on another more suitable machine, if one exists, may rectify the second reason. The third reason, however, is more difficult to resolve, and frequently even impossible.
Quality Versus Quantity
This is an age-old expression but it illustrates perfectly the situation described above. The term quality covers a wide range of conditions that a part may have to conform to in order to meet the customer’s requirements. Among these are:
- The appearance of the part
- The part geometry
- The drawing tolerances
Assuming the mould design is not at fault, a frequent cause of failure to meet quality requirements is due to using too great a number of impressions. In general, the greater the number of impressions a mould tool has, the less the control over the individual
impressions there is. Variations will exist between the impressions owing to differences in manufacture, pressure, temperature, gate sizes, and so on.
In order to minimise these differences, runner layouts and gate sizes may be balanced, but despite this there will still be finite variations in pressure, thermal gradients, impression sizes and other variations that make multi-impression tools sometimes difficult to run.
The three categories mentioned above are those which many moulders may have difficulty with through using too high a number of impressions.
More Cavities = Less Control
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