Manual of experiences ans tolerances in rope pump production


SHORT SELECTION

A.b. MOLDS FOR PISTON INJECTION:

For semi-industrial production, the injection process, in which the mold plays an important part, is used.
The function of the mold is to give an exact form to the piston; thus the final diameter of the pistons depends on the quality of the mold.

Each mold consists of two parts, as shown on page 9 of the photo-manual. The most logical design for these two parts is a bottom piece with the inside cone-shaped part of the piston and a pin on top with the diameter of the hole in the piston, and a top piece consisting in the outside shape of the piston. However, during the injection process, a seam is formed where the two pieces meet. A seam on the bottom part of the piston will cause friction in the pipe. The bottom part of the piston is slightly conical with a radius that is slightly inferior to the top part. (See Drawing A-03 in the technical drawing manual). A small seam on the higher part of the piston will not cause friction in the pipe and will disappear with use. This means that the separation between the two parts of the mold is not located at the bottom of the piston, but a little higher at the point where the piston becomes conical in shape.
Injection is done preferably through a central hole or canal and not from the side. The inside bottom part of the piston should have a rounded edge as shown in drawing A-01 of the technical drawing manual. This inside edge touches the rope when it passes though the guide and the wheel, and may cause the rope to wear.

TECNOLOGICAL ALTERNATIVES FOR THE MOLD:

There are molds which require injection through a side canal, producing a piston with a shorter life span and which do not permit intensive use.
Injection from the side will cause the injected material to enter the mold and separate into streams, which meet, again on the opposite side of the injection orifice. An almost invisible scar occurs at the spot where these streams meet, where the pistons tend to break with use. In fact, there are pumps installed with rope, but missing pistons where this type has been used. This occurs most frequently when slightly contaminated material has been used in reprocessing.
In high capacity injection machines, the molds can produce six or more pistons in one cycle, since the injection canal is a network. Injection in these cases is through the side and the material must be of good quality.
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