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Compression Molds

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Compression Molds --- Aheadmold

Thermosetting compression-molding compounds can be molded into articles of excellent rigidity and shape retention by supplying heat and pressure. Apart from the molding material, the mold itself is of great importance.
Compression molds nowadays are heated electrically exclusively. The mold is loaded with molding compound, by hand, with the aid of a filling device, or with pellets.
A construction drawing should be mandatory for every mold to be newly produced. Any new ideas concerning the mold, such as stability of the mold construction, optimum heating, aids to demolding and ejection, e.g., slides, split cavities, cores, etc., can be included in advance and given due consideration. The cost of such drawings will be more than justified as a rule by the ensuing efficient mold production and by fewer alterations and less finishing work on the completed mold. The more accurately details are incorporated in the design, the more finishing work is avoided, e.g., specification of the draft angle required and dimensional tolerances.
Because alterations to compression molds are always very expensive, it is of particular importance that the detail drawings be completely clear.
The mold must be of sufficiently solid and rigid construction to enable it to withstand the high pressures required with compression molding. The outer walls should be only slightly flexible. If the mold is too flexible, the result could be jamming of the two mold halves on opening, or troublesome ejection. High-walled parts may well exert the total compression a pressure on the side walls. The bottom of the mold must be well supported to absorb the pressure exerted on it and to avoid deflection.
As the material costs are comparatively low compared to wages, one can afford to have the mold solidly constructed without incurring any significant increase in cost. The higher steel content ensures a more uniform temperature distribution and temperature control, apart from the greater rigidity. A good polish of the molding areas is absolutely essential for trouble-free ejection and to give a satisfactory surface to the molded article. The mold surface should be glass-hard so that it can withstand the wearing effect the molding material exerts when flowing under pressure and so that it retains its polish. On the other hand, the tool steel needs to possess a tough core, as a slight distortion of the mold walls and the ribs is unavoidable. It is recommended to use a carburizing steel for the shape-giving mold parts. This has already proved itself in the construction of molds for the plastics-processing industry. The mold surface must be resistant to constant attack by chemicals, which is particularly prevalent with certain types of compression-molding compounds. A mold can be protected from chemical attack and frictional wear by chrome plating of the molding surfaces.
A further important requirement is that the mold consists of as few interlocking parts as possible. The fitting of several parts into each other is always fraught with danger because of the possible distortion caused by the high compression pressures employed. Should it not be possible to avoid working with inserts, it is then essential that the inserts always be fitted into the compression mold in line with the pressure and never across it.
A compression mold basically consists of an upper and a lower part. In normal cases the lower half is fitted to the table of the press and the upper half to the ram. Both mold halves are guided by hardened dowels. Asymmetrical parts cause large one-sided pressure loads to be exerted on the mold. They require compensation through special guides.
Ejection usually calls for special equipment. Parts such as flat dishes or plates are easily ejected by compressed air, which is already available on the machine for cleaning flash and material residue from the molds. In all other cases, ejection by ejector pins or ribs is feasible. For parts with a multitude of fibs and openings, ejector pins are essential because of the material shrinkage.

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