Paint Pail Mould Cavities and Cores

  •   The cavity and core give the Paint Pail Moulding its external and internal shapes respectively, the impression imparting the whole of the form to the molding. We then proceeded to indicate alternative ways by which the cavity and core could be incorporated into the mould and found that these alternatives fell under two main headings, namely the integer method and the insert method. Another method by which the cavity can be incorporated is by means of split inserts or splits.

      Integer cavity and core plates

      When the cavity or core is machined from a large palte or block of steel, or is cast in one piece, and used without bolstering as one of the mould plates, it is termed an integer cavity plate or integer core plate. This design is prefeered for single-impression moulds because of the strength, smaller size and lower cost characteristics. It is not used as much for multi-impression moulds as there are other factors such as alignment which must be taken into consideration.

      Of the many manufacturing processes available for preparing moulds only two are normally used in this case. These are (a) a direct machining operation on a rough steel forging or blank usingthe conventional machine tools, or(b) the ‘precision’ investment casting technique in which a master pattern is made of the cavity and core. The pattern is then used to prepare a casting of the cavity or core by a special process.

      Based on the grade you are using, check for the coefficient of linear thermal expansion (CLTE). It will depend on at which stage your weld lines are formed. If you have a high molecular weight material and the weld area is during the initial stages of fill, the weld strength may be good in both climate. If your weld area is towards the end of fill, a grade with higher elongation property might help. If you face the problem in an assembly, you have to check the CLTE of both the matching surfaces. Without having the complete details, it would be difficult to provide appropriate solution.

      When you are molding a part, the mold temperature and melt temperature may be same irrespective of climate. So the environment at molding is uniform (assuming that the material is pre-dried if it has moisture). If moisture is present, the weld strength would be low and at the same time you will notice surface defects. The molecular chain would break and the material strength would be low; as good as low molecular weight material. The stresses developed due to climate change can cause problems. Thus, you have to analyze the problem whether it is in individual part or in an assembly.

      Ambient temperatures and R.H. WILL influence the precise conditions that the material experiences as it makes its way through the tool. I’m confident that if the tool was fitted with thermocouples just near to the mold surface near to weld-line location and also with melt temperature sensor (Kistler and others offer these, even combined with pressure transducer function: another useful parameter to monitor for this purpose) also near the weld-lines, you would see differences, even with exactly the same barrel temperature and cooling medium settings. For clarity, it’s arguably better to refer to ambient air conditions (temperature and R.H.) than “climate change” as this latter may trigger very different thoughts amongst some readers!

      Good points about location of weld-line (early or late in flow path), but as this is fixed and material is fixed, the variables under scrutiny here are surely related to ambient environment only.

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