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

When quality of plastic injection molding parts' surfaces can not meet design requirements, the technicians will solve quality problems by adjusting melt temperatures, injection speeds, pressure, flows, mold temperatures and parameters in other aspects.

Usually, values of melt temperatures, injection speeds, pressure, flows and mold temperatures can be slightly adjusted according to standards of molding conditions and be able to reach the ideal values through equipment.

Obviously, melt temperatures, flow rates, pressure, flows and mold temperatures these items can be adjusted by equipment, which is relatively easy to realize. The difficulty is that when the means above can not solve the problem, we have to modify the mold in order to achieve the desired effect, which is also the key point of this article.

Situations when we may need to modify molds are as the following. 
(1) There are bubbles at welding marks' parts and we need to add exhaust vents at corresponding parting surfaces.
(2) Locations of welding marks are in the middle of plastic injection molding parts and we need to adjust locations of pouring gates.

The reason of the bubble's formation at the welding mark' part is that when the two melts converge, the enclosed gas is not removed in time and remains in the interior of the plastic injection molding part, forming the pit on the surface of the welding mark.

The reason may be that the parting surface of the corresponding position is too tight, resulting in the gas not to be discharged; heights and sizes of cavities are excessively uneven(wall thicknesses of plastic injection molding parts have great differences). For the former situation, we often add or enlarge the exhaust vent to improve the problem we mention above. Taking the PP material which currently has a wide domestic application as an example, the perfect value of the exhaust vent's gap is from 0. 01 to 0. 02 mm based on the parameter of the flash' s gap being 0.03mm in order to avoid the flash forming the trimming.

Locations of exhaust vents will be set at parting surfaces of fixed dies and at parts which are filled last in cavities under most circumstances so as to clean cavities in mold processing and molding processes. Excessive unevenness of cavities' heights and sizes can only be adjusted by repair welding and grinding methods, which is the most difficult. The adjustment method of the cavity and the adjustment method of the welding mark that we are going to talk about are the same.