Electroforming,Cavity Corrosion and Erosion,Gassing and Burning


This is a very similar process to (hut not the same as) electroplating. Whereas with plating a deposit of about 25 [im is the norm, electroforming can he millimetres thick.

Nickel or cobalt—nickel is deposited onto a former, which is made from an inert material, often acrylic. Other former materials may be used but if they are not electrically conductive they have to be made conductive by coating with chemically reduced silver.

The process consists of depositing a layer of cobalt—nickel up to 5 mm deep onto the former. Next a further layer of copper is deposited to increase the cavity wall thickness. At the end of the procedure the former is withdrawn and the composite cavity is inserted into a steel chase for support.

The advantage of this process is that a component accuracy of 1 micron can be achieved and there is no shrinkage involved, unlike in casting. The great disadvantage of this process is the time scale involved, which can he up to 10—12 weeks.

Cavity Corrosion and Erosion

When abrasive or corrosive materials such as glass-filled Nylon or PVC are being used, there is a danger that erosion or corrosion of the cavity will occur. These problems can severely damage the tool, which may then need expensive repairs.

The answer for both problems is to hard chromium-plate or nickel-plate the cavities. This gives a good level of protection to the cavity against both problems. When the plating begins to wear away it may he stripped off and the surfaces re-plated.

It should be remembered that,when using corrosive or abrasive materials, not only the cavity should be protected but also the runner and sprue bush as well.

Gassing and Burning

20160317102847When the melt enters the cavity, it has to displace the air in front of it. Often this presents no problem, as the air will escape through the split line or ejector pins or down the sides of core pins.

There are some situations, however,where the air cannot escape easily. This often occurs with blind cavities or when high injection speeds are used. In these cases the incoming melt will compress the air in front of it, causing the material to burn. The problem is worse when large volumes of air have to be displaced when using high injection speeds.

To overcome this, vents have to be included to provide an easier path for the air to escape, as shown above. These vent channels have to be very shallow, usually 0.015 to 0.025 mm to avoid the possibility of flashing. The land length of the channels has to be kept short, to allow the air to expand and cool as quickly as possible.

For more minor gassing and burning problems, venting can be provided by grinding very small flats on the sides of ejector pins. Alternatively, special venting pins can be located at trouble spots.

Maximum Metal Conditions

It is good engineering practice to make sure that mould tool cavities can be adjusted if necessary after the first sampling trials. Critical snap-fit features or features that have to mate with other parts are examples of where such adjustment may be necessary. Failure to ensure this can result in very expensive changes or replacements in the tool.

It makes sense to dimension the tool cavities and cores so that small amounts can be machined away from them if the moulding dimensions are incorrect. In fact, it is better to systematically make sure that critical sizes are slightly out of tolerance from the start, so that they be can adjusted after moulding trials, thus eliminating the possibility of remakes being necessary. This method is called using maximum metal conditions (MMC).



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