Machining Marks

Machining Marks

Machining Marks

For maximum fatigue life,all bored and drilled holes should have a smooth finish, without any machining witness marks showing. Special drills are available that impart a smoother finish than a twist drill where this is necessary.

Reaming would give a better result but this can be difficult to do, as well as being an expensive operation, and is therefore rarely used. Mechanical honing is even more expensive and almost never used.

A more practical alternative is fluid bed-honing. In this process abrasive slurry is forced, under pressure, inside melt flow channels. This has the effect not only of increasing the fatigue life but also of improving the polymer flow.

Although it may seem that these are extreme lengths to go to, these factors become very important on tools subject to millions of cycles in preventing premature failure if maximum economy of production and trouble free operation is important, for example in very competitive markets.

Water cooling holes are nearly always left with a drilled finish. While this promotes turbulent water flow, it also promotes corrosion. The corrosion starts at the tool marks and progressively eats into the steel, creating a sharp-notched effect. As the corrosion effect cannot be seen, the problem does not become apparent until water leaks start occurring.

The walls of cavities are always highly stressed on every injection cycle, by the pressure of the incoming melt. Any channels or screw holes in the cavity insert can worsen this problem, with each feature of this type creating stress raisers.

If channels are too close together, this has the cumulative effect of acting like a series of perforations and will severely decrease the fatigue life. The designer can compensate for this by ‘overdesigning’ in most cases. Where space is at a premium, however, this is not possible and calculations should be carried out to ensure adequate strength. Such calculations can be tedious and the application of a few rules will enable the designer to avoid potential pitfalls. The most important of these are:

  • Do not make wall thicknesses too small.
  • Apply adequate safety factors.
  • Avoid excessive steel hardness.
  • Avoid water channels and holes being too close together.
  • Avoid poor finishes on holes.
  • Avoid sharp corners and notches.

Contributed by Steven,a China plastic mould maker

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