Today we are back with two new common issues in plastic production. If you are new here, this post belongs to a series aiming to describe the 20 most common issues you might encounter in your Plastic Production. Our first post was about short shots.
What are silver marks?
Spatter lines formed on the surface of the plastic along the direction of melt flow are called silver marks.
Reasons and solutions
The emergence of silver marks is generally due to the rapid start of the injection which causes the air in the melt and mold cavity unable to be discharged quickly enough. This results in the appearance of silver filaments on the plastic surface. Silver marks not only affect the appearance of the plastic parts but also reduces the mechanical strength of them as well. The formation of silver marks is mainly due to the gases in the melt surface of the plastic part. By finding the source of these gases, we can find out the methods to solve the defects.
1. The plastic feed itself contains water or oil.
Exposure to air during the manufacture of plastics, as well as the inhalation of water, oil, or mixtures in the wrong proportion, causes these volatile substances to become gases at high temperatures in the solution.
2. The melt is decomposed by heat.
If the melt tube temperature, back pressure, and melt speed are set too high, or the forming cycle is too long, then the heat-sensitive plastics (such as PVC and PC, etc.) are easily decomposed into gas at high temperatures.
If the temperature of the melt barrel near the hopper is set so high that the surface of the plastic particles melt and stick together before compression, the air between the plastic particles cannot be completely excluded (bad degassing).
4. Poor melt plasticization
For this reason, the temperature of the barrel should be increased and the molding cycle should be lengthened. Internal heating injection port should be used as much as possible. The cooling well and the flow channel could also be increased.
The reasons and improved methods for the silver marks in the plastic parts are listed below:
Raw material contains water
Dry the raw material thoroughly (within the allowed moisture content)
Excessive material temperature (melting decomposition)
Lower melt temperature
Material containing other fillers (e.g. lubricant)
Reduce its usage or replace with other suitable additives
Pigment decomposition (Pigment has poor temperature resistance)
Choose a pigment with high-temperature resistance
Mixed raw materials or poor thermal stability
Replace raw materials or use plastics with good thermal stability
Excessive ash content
Reduce ash content
When the molten material flows from the thin wall to the thick wall, it expands. The volatiles vaporize and contact with the surface of the mold and become silver
Improved die structure design
Adjust the relationship between injection speed and position
The gate is too large/too small or in an improper position
Improve gate size or adjust gate location
Poor mold exhaust or low mold temperature
Improve mold exhaust or mold temperature
Injection speed too high (shear decomposition or air entrainment)
Make the injection speed lower
Air is trapped inside the barrel
Degass the mold
Low back pressure (poor degassing)
Increase back pressure appropriately
What is a Splay Mark？
Splay mark is a water wave pattern which can not be removed after the part is formed. It is mostly seen on the plastic parts with a smooth finish as shown in the figure below:
Reasons and solutions
Splay marks are caused by the first melt into the cavity cooling too fast while the the hot melted plastic is forced into the front to push the intitial melt forward, which causes the formation of water wave lines. They can be eliminated by increasing melt temperature and mold temperature, accelerating injection speed and increasing hold-in times. If the cold material remaining at the front end of the nozzle enters the mold cavity directly, it will also cause water ripple formations. Therefore, a cold material well should be opened at the end of the main channel to effectively prevent the occurrence of the splay mark.
The cause of splay mark and the improvement method are shown in the following table:
Poor melting and plasticization of raw materials
1. Raise the barrel temperature 2. Increase the back pressure 3. Increase screw rotation speed
The poor fluidity of melting material (low FMI)
Change material with good fluidity
Mold temperature or material temperature is too low
Raise the mold temperature or material temperature
The gate is too small or at an inappropriate position
Increase the gate or change the gate position
The flow channel is too long or too fine (melting prematurely cools down)
Shorten or thicken the runner
Slow injection speed at the splay mark
Increase the injection speed at the splay mark appropriately
The cold material well is too small or insufficient
Increase or increase the cooling well
Pressure too low or time too short
Increase pressure and pressure maintaining time
Free Resource: 20 Most Common Problems in Your Plastics Manufacturing
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