Silver streaks on injection molded parts include surface bubbles and internal pores. The main reason for this defect is the interference of gas (usually water vapor, decomposition gas, solvent gas, gas). The main reasons are analyzed as follows:
A. Machine level:
(1) The barrel and screw are damaged or the rubber head and rubber ring have logistics blind spots, which are decomposed by long-term heat.
(2) The heating system is out of control, causing the temperature to be too high and decompose. Check whether there are problems with heating elements such as thermal resistors and heating coils.
B. Mold:
(1) Poor exhaust.
(2) The friction resistance of the runner, gate, and cavity in the mold is large, resulting in partial overheating and decomposition.
(3) The gate and cavity are unevenly distributed, and the unscientific cooling system will lead to uneven heating, resulting in partial overheating or blocking of the gas channel.
(4) Water seeps into the cooling channel into the cavity.
C. Plastic:
(1) Plastic has high humidity, too much recycled material or contains harmful debris. The plastic should be fully dried and the debris should be removed.
(2) It absorbs nutrients from the air or absorbs nutrients from the colorant. The colorant should also be dried. It is best to install a dryer on the machine.
(3) The amount of lubricant and thickener added to the plastic is too much or the mixing is uneven, or the plastic itself contains volatile solvents. When the mixed plastic is heated to an insufficient level, it will also decompose.
(4) The plastic is contaminated and mixed with other plastics.
D. Processing level:
(1) When the set temperature, pressure, speed, back pressure, and melt motor speed are too high to cause decomposition, or when the pressure, speed, injection time, holding pressure, and back pressure are too low, because high pressure cannot be obtained, the density is insufficient, and the gas cannot be melted. The appropriate temperature, pressure, speed and time should be set, and a multi-stage injection rate should be adopted.
(2) Low back pressure and fast rotation speed can easily cause air to enter the barrel and enter the mold with the melt. When the cycle is too long, the melt is heated in the barrel for too long, causing decomposition.
(3) Insufficient material quantity, too large a feed buffer, low material temperature or low mold temperature will affect the flow and molding pressure of the material and promote the formation of bubbles.
The fundamental reason for weld seams in injection molded parts
When the molten plastic encounters the insert hole, the area with discontinuous flow rate and the area where the filling material flow is interrupted in the cavity, it cannot be completely melted and a linear weld seam will appear. In addition, pouring and filling the mold will also cause weld seams, and the strength of the weld seam is very poor. The main reasons are analyzed as follows:
a. Processing level:
(1) The injection pressure and speed are too low, and the barrel temperature and mold temperature are low, causing the melt entering the mold to cool too early and the weld seam to occur.
(2) When the injection pressure and speed are too high, there will be spraying and weld seams.
(3) Increase the rotation speed, increase the back pressure, reduce the viscosity of the plastic, and increase the density.
(4) The plastic should be dry and recycled materials should be used less. Excessive use of release agent or poor quality will cause welds.
(5) Reduce the clamping force and facilitate exhaust.
b. Mold:
(1) There are too many gates in the same cavity. The gates should be reduced or set symmetrically, or as close to the weld as possible.
(2) The exhaust at the weld is poor, and an exhaust system should be set up.
(3) The pouring is too large, the pouring system specifications are inappropriate, and the melt should be prevented from flowing around the insert hole as much as possible, or the insert should be used as little as possible.
(4) The thickness change is too large, or the thickness is too thin, and the product thickness should be uniform.
(5) If necessary, a welding well should be set at the weld joint to separate the weld from the product.
c. Plastic:
(1) Lubricants and thickeners should be introduced into plastics with poor fluidity or heat sensitivity as much as possible.
(2) Plastic contains various impurities and should be replaced with better quality plastic when necessary.
