August 2011 Archives

In the case of plastic injection mold, since it is necessary to maintain the temperature of the mold within a range of 20 to 200°C, and since there will be a difference between this temperature and room temperature, there will be thermal expansion of the parts of the mold (in some cases, there may be thermal contraction).

Although it is very common to carry out the adjustment of the assembly of a mold at room temperature in a constant temperature room, the temperature region differs during actual injection molding.

Each part of a mold has its own shape, and since a number of such parts are assembled in a complex manner, there will be sliding or friction movement.
Even the state of thermal expansion is extremely complex and it is very difficult to predict it by theory.

The practical method of using know-how accumulated by experience is very common regarding the problem of thermal expansion.
The method of supplying oil is modified, the contact surface area is reduced, and the amount of deformation is absorbed by providing an escape.
In addition, a technique is used of balancing the thermal bias by using additionally the temperature controlling function.
A cooling water flow path is provided near the heater or the manifold, or an air thermal insulation layer is provided, or a thermal insulation plate is provided.

Positioning rails are placed in the case of sliding parts.

Ball bearings are used in the case of parts lubricating which is difficult, or even a non-lubricating type alloy is used.

In order to balance the heat accumulation in the mold base, the excel thickness of unwanted parts are removed.

Regarding hot runners, it is particularly important to calculate and study thoroughly the thermal balance between heat supply and loss. It is necessary to give proper considerations about the balance between the quantity of heat on the fixed side and the quantity of heat on the movable side of the chip parts, manifold parts, etc.

It is also necessary to consider the amount of heat radiated from the platen of the molding machine.

After molding is carried out, it is necessary to carry out maintenance of the plastic injection molds such as disassembling and cleaning, etc. (maintenance and inspection work).
The "soot" constituents generated from the molding material, and the condensed gas deposits, etc. accumulate on the surface of the cavity or core and on the parting surface.
Although these constituents are still in the liquid state when the mold is still at a high temperature, but when the mold cools down they solidify, moisture content in air gets adhered because of these, eventually causing the generation of rust and corrosion of the mold.
When this state is reached, the surface quality of the molded item decreases, there will be fluctuations in the dimensional accuracy, insufficient air escape, which causes the generation of short shots.

The frequency of maintenance varies widely depending on the status of quality management of the molded item, and on the size of the mold, etc.
In the case of molds with short periods, it is necessary to carry out mold maintenance once in a few days, and about once in two months in the case of mold with long periods.

A common method of maintenance is to disassemble the mold, cleaning each of the part by ultrasonic cleaning, or by rinsing in an organic solvent, and to remove the rusted part by polishing and then electroplating.
In the case of parts having a lot of rust, the parts are replaced, or the inserting part is corrected.

It is necessary to modify tools, jigs, and equipment in order to carry out maintenance efficiently.
Cleaning brush, disassembling tools, wooden tools, cloth jigs, buffing tools and sand paper, polishing cream, lapping material, special chop sticks, bamboo spatula, bamboo comb, air tools, crane supplementary tools, Z light, magnifying glass, etc., need to be modified.

Even the work bench needs to be modified so that the work becomes easy. Modifications are to be done about the work bench height to improve ease of carrying out the work, walking space, crane position, air gun piping, etc.
It is also useful to use a table or white board for spreading the drawings, a digital camera, and a video camera.

In order to increase the efficiency of maintenance, it is necessary to give considerations from the point of designing the mold.
Holes are provided so as to facilitate disassembly of the mold, screws for hook bolts are provided, and supplementary screw holes are provided for assembling.
The cores should have the frame block construction so that they do not come apart, and it is also effective to use a construction of fixing using keys.
Very often the placement numbers of core pins are engraved, and tag cuts are made in order to fix the directionality.