In addition to defects such as cracks caused by stress release, set deformation caused by uneven shortening, and fusion lines in some parts, medical precision injection molds also have various appearance defects due to improper mold design or process control.
Common surface defects include surface silver, bubbles, dark lines, dark spots, surface depressions, black corners, whitening near the gate, wrinkles and rubbing marks, poor gloss, surface flow lines or ripples, and uneven color, surface scratches and inclusions of unmelted plastic particles, etc. These can be prevented through process manipulation and mold design.
For example, silver wires and bubbles are caused by the volatilization of moisture and low-molecular-weight volatiles contained in the melt when heated, and are separated and elongated when the melt is moved during mold filling. The plastic should be fully dried before use, or prevent the melt temperature from being too high.
Dark lines and dark spots, uneven forehead color are all caused by overheating of plastic. If the material temperature is too high or the blocking time in the material is too long, the material will be oxidized and darkened or the color material will be decomposed.
Blackened or carbonized corners are caused by poor exhaust. If the exhaust is not good, when the melt quickly fills the cavity, the original air in the cavity will be compressed sharply, the temperature rise will exceed the carbonization temperature of the plastic, and the plastic in the place where the gas accumulates will be carbonized. this phenomenon.
The whitening, wrinkles and rubbing marks near the gate are all because the mold temperature is too low, the melt enters the cavity and cools too fast, and a thin condensation shell is formed as soon as it enters the cavity, and the shell is damaged during the continuous filling of the melt. When stretched, it will turn white or muddy due to stress. If the shell is stretched, moved or torn, it will show wrinkles or rubbing marks. Obviously, proper improvement of the mold temperature can prevent this problem and reduce the mold filling time. Shear rate that protects the condensed shell from excessive stretching.
The corrugation on the surface of the plastic is mainly caused by the eruption and filling that occurs when the small gate is selected, and appropriate measures should be taken to prevent it.