"Dent" is caused by local internal contraction caused by gate sealing or lack of material injection. The depression or depression caused by the surface of the injection molded article is an old problem in the injection molding process. The dent is generally caused by a local increase in the shrinkage of the product due to an increase in the wall thickness of the plastic article. It may occur near the outer sharp corner or at a sudden change in wall thickness, such as a bulge, a rib or a support, sometimes In some unusual parts. The root cause of the dent is the thermal expansion and contraction of the material because the thermal expansion coefficient of the thermoplastic is quite high. The extent of expansion and contraction depends on many factors, with the properties of the plastic, the maximum and minimum temperature ranges, and the mold holding pressure being the most important factor. Also, the size and shape of the injection molded parts, as well as the cooling rate and uniformity, are also factors.
The amount of expansion and contraction during the molding of plastic materials is related to the coefficient of thermal expansion of the plastic being processed. The coefficient of thermal expansion of the molding process is called "molding shrinkage". As the molded part cools and shrinks, the molded part loses in close contact with the cavity cooling surface, at which time the cooling efficiency is lowered, and after the molded part continues to be cooled, the molded part is continuously shrunk, and the amount of shrinkage depends on a combination of various factors. The sharp corners on the molded part cool the fastest, harden earlier than other parts, and are farthest away from the thick part of the cavity at the center of the molded part, which is the last part of the molded part that releases heat. After the material is cured, the molded part will continue to shrink as the melt near the center of the part cools, and the plane between the sharp corners can only be cooled on one side, and the strength of the material at the sharp corners is high. The cooling shrinkage of the plastic material at the center of the part pulls the partially weakened surface of the partially cooled and sharpened corners inwardly. Thus, a dent is generated on the surface of the injection molded part. The presence of the dent indicates that the mold shrinkage here is higher than the shrinkage of its peripheral portion. If the shrinkage of the molded part at one place is higher than the other, the molded part causes warpage. In-mold residual stress reduces the impact strength and temperature resistance of the molded part. In some cases, adjusting the process conditions can avoid the creation of dents. For example, during the pressure holding of the molded part, a plastic material is additionally injected into the cavity to compensate for the mold shrinkage. In most cases, the gate is much thinner than the rest of the part. When the molded part is still hot and shrinks continuously, the small gate has solidified. After curing, the molded part in the cavity is not pressed. kick in.
The molded part of the semi-crystalline plastic material has a high shrinkage rate, which makes the dent problem more serious; the molding shrinkage of the amorphous material is lower, the dent is minimized; the reinforcing material is filled and maintained, and the shrinkage rate thereof Lower, the possibility of dents is smaller.
Thick injection molded parts have a long cooling time and will cause large shrinkage. Therefore, the large thickness is the root cause of the dents. Attention should be paid to the design. Thick-walled parts should be avoided as much as possible. If thick walls are not avoided, they should be designed as hollow. The thick parts are smoothed to the nominal wall thickness, and the large arc instead of the sharp corners can eliminate or minimize the dents generated near the sharp corners.
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