How to Avoid Welding Marks of Plastic Injection Molding Products? (Part Two)

When we can't improve or eliminate quality problems of plastic injection molding products due to welding marks by adjusting equipment's technical parameters and adding exhaust vents, we have to adjust the size of the cavity; of course, we need to accurately measure the corresponding thickness of the plastic part and adjust the cavity in the allowable range.

Wall thicknesses of plastic bumpers in different parts are not the same and they gradually vary; the reason is that when we take into account the specific shape of the plastic part and the mobility of the molten plastic, the gradually varied wall thickness is conducive to the forming. The wall thickness of the plastic bumper is usually from 2.60 to 3.50 mm.

Modifications of mold cavities' sizes can be classified into two kinds, that is, enlarging cavities' sizes and narrowing cavities' sizes. The first kind is easily to be realized by grinding the corresponding part of the mold's cavity based on the test data. The second kind is more complicated; first, we need to have the build-up welding on the surface of the mold's cavity and then grind the welding so as to achieve the purpose of narrowing the size of the cavity. The following specifically describes methods of narrowing sizes of molds' cavities.

We should firstly consider repair welding and grinding moving dies based on complexity, which is mush easier than those of fixed dies. Quality of fixed dies' cavity surfaces of injection molds will directly affect appearance of plastic parts; there will be a great deal of heat in the welding process; when there are no sufficient technical measures for heat, the heat will tend to change the constituent of the cavity's surface, resulting in the different hardness of the cavity's surface, and further affecting the appearance of the plastic part; therefore, we need to try to avoid modifications of fixed dies' cavity surfaces.

The general steps for modifying surfaces of moving dies' cavities are as the following.
(1) Approximately obtain thicknesses of cavities which need to be enlarged or narrowed by putting glues on surfaces of cavities in order to test molds.
(2) Carry out operation of repair welding and grinding.
(3) Test molds again and adjust sizes of cavities' surfaces according to molding effects. 

Among the steps above, the second step is difficult and the key point, and the detailed process of repairing the cavity's surface is as the following.
a. Select the base material to match the welding material, and determine the scope of the welding based on the grinding standard.
b. Alternately build-up weld moving dies; it should be noted that we shouldn't weld the whole area of the cavity's surface so as to avoid the internal stress causing the crack of the mold's cavity surface.
c. Grind the welding according to the reserved standard as well as pay attention to the protection of the surrounding part.
d. Fully weld the empty part to complete the modification of the cavity's surface after the measured height of the repair welding can meet the requirement.