（20） Silver veins (including surface bubbles and internal pores) Injection Molding Process Defect Handling Manual (65 types)

The main reason for the defects is the interference of gases (mainly water vapor, decomposition gas, solvent gas, and air). If the screw rotates too fast and the back pressure is low, the amount of air trapped in the resin being plasticized will increase. As a result, striped bubbles appear on the surface of the molded product, and silver lines are easily formed. Resins are chemical substances that gradually decompose as temperature increases.
The higher the temperature of the resin, the more it decomposes, and the easier it is for the craze to appear. If the pre drying of the material is insufficient, the moisture and the original gas components in the resin will be carried intact into the molded product, which can easily form silver lines. When the gas is not completely discharged, bubbles will remain on the surface of the molded product, which is prone to crazing. If components different from the original resin are mixed due to insufficient cleaning or other reasons, and the temperature of the resin is low, gas may sometimes be generated and crazing may be induced.
1. Machine:
(1) The material barrel and screw are worn or there is a material flow angle in the rubber head and rubber ring, which is decomposed due to long-term heating.
(2) The heating system is out of control, causing excessive temperature and decomposition. Check whether there are problems with heating components such as thermocouples and heating coils. Improper screw design can lead to individual breakdowns or air entrainment.
2. Mold:
(1) Poor exhaust.
(2) The friction resistance of the runner, gate, and cavity in the mold is large, causing local overheating and decomposition.
(3) Uneven distribution of gates and cavities, as well as unreasonable cooling systems, can lead to uneven heating and local overheating or blocking of air channels. (4) The cooling passage leaks into the mold cavity.
3. Plastic:
(1) Due to high humidity of plastic, excessive proportion of recycled materials added or harmful debris (debris is easily decomposed), it is necessary to fully dry the plastic and eliminate debris.
(2) When absorbing moisture from the atmosphere or from colorants, the colorants should also be dried, preferably by installing a dryer on the machine.
(3) "The amount of lubricants, stabilizers, etc. added to plastics is excessive or unevenly mixed, or the plastics themselves contain volatile solvents.". Mixed plastics can also decompose when it is difficult to take into account the degree of heating.
(4) Plastic contaminated with other plastics.
4. Processing:
(1) If the set temperature, pressure, speed, back pressure, or rotational speed of the glue melting motor is too high, resulting in decomposition, or if the pressure, speed, and injection time, pressure retention, or back pressure are too low, and the gas cannot be melted due to insufficient density due to failure to obtain high pressure, resulting in crazing, appropriate temperature, pressure, speed, and time should be set, and multi-stage injection speeds should be used.
(2) Low back pressure and fast rotation speed easily cause air to enter the material barrel, and as the molten material enters the mold, if the cycle is too long, the molten material will decompose due to being heated too long in the material barrel.
(3) Insufficient material quantity, excessive feeding buffer, too low material temperature, or too low mold temperature all affect material flow and molding pressure, promoting the formation of bubbles.
Bubble
According to the causes of bubbles, the countermeasures for solving them include the following aspects:
1) When the wall thickness of a product is large, the cooling speed of its outer surface is faster than that of the center part. Therefore, as the cooling proceeds, the resin in the center part contracts while expanding towards the surface, resulting in insufficient filling in the center part. This condition is referred to as a vacuum bubble. The main solutions are:
a) Determine a reasonable gate and gate size based on the wall thickness. Generally, the gate height should be 50% to 60% of the wall thickness of the product.
b) Until the gate is sealed, a certain amount of supplementary injection material is left.
C) The injection time should be slightly longer than the gate sealing time.
d) Reduce injection speed and increase injection pressure.
e) Use materials with high melt viscosity grades.
2) The main solutions for bubbles caused by the generation of volatile gases are:
a) Fully pre dry.
b) Reduce the resin temperature to avoid generating decomposition gases.
3) Bubbles caused by poor fluidity can be solved by increasing the temperature of the resin and mold, and increasing the injection speed.
Gas bubbles
1. Apparent
There are many bubbles on the surface and inside of the product - mainly near the material head. The flow path is midway and away from the material head - not only where the product wall thickness occurs. Bubbles have different sizes and shapes.
Physical causes
Bubbles mainly occur in heat sensitive materials that must be processed at high temperatures. If the necessary molding temperature is too high, causing material decomposition through molecular splitting, there is a risk of thermal degradation of the melt, and bubbles are easily generated during the molding process.
If the cycle time is long, it may usually be the cause of too long residual time and insufficient travel utilization. It may also be due to overheating of the molten material in the barrel.
The causes and improvement measures related to processing parameters are shown in the following table:
1. Too high melting temperature reduces barrel temperature, screw back pressure, and screw rotation speed
2. The residual time of molten material in the cylinder is too long. Use a smaller cylinder diameter
The reasons and improvement measures related to the design are shown in the table below:
1. Unreasonable screw geometry using low compression screws