Definition of Sink Mark: a local collapse (dimple or gully) on the surface of an injection molding product.
First: PRODUCTS
The rib’s wall thickness is too thick
When the ribs are thick, the joint between the ribs and the bottom plate is thick, where the plastic is concentrated. When cooling, the surrounding ribs and the plate solidify first. The central joint between the ribs and the plate remains liquid, and the post coagulated plastic contracts on the first solidified plastic, causing Sucking into the surrounding plastic. If any of the condensation layers is weak (usually on the die surface opposite the rib), it may collapse into a depression.
Suggestion: The wall thickness of the ribs should be 50% of the thickness of the bottom plate, or even thinner. The best wall thickness of ribs is 30% of the bottom plate.
Second: THE TOOLING
1. The die surface opposite the rib is too hot
If the mold surface temperature relative to the rib is higher than its nearby (generally true, because the near melt adhesive concentration, thermal load, mold temperature is high), the condensation layer is thin, rigid enough, the central melt adhesive curing, residual stress may be thinner condensation layer inward pull into depression. The mold surface relative to the rib must be strengthened to cool, reduce the mold temperature so that the condensation layer is formed quickly, when the condensation layer is thicker, the rigidity is larger, and the depression is not easy to produce.
Suggestion: Add cooling water road or Increase cooling water road
Suggestion: increase gate or runner or sprue or change gate location. relatively, increasing gate or runner is easy and costs very low. If using these ways can solve issues, looks nice.
3. Improper number or location of gates
No matter whether the number or location of the gate is inappropriate, the Flow Length will be too long and the Flow resistance will be too large. If the injection pressure is insufficient, the cavity can not be filled, the melting glue density is small, and the probability of sag is large.
Sometimes due to mold manufacturing defects, it is difficult to solve which injection parts are in the cylindrical (cylinder) opposite the surface of the concave.no matter use anyway. such as change injection molding technical data. Even without a strong solution, injection parts are already a whole flash.
Suggestion: change gate location has to according to the parts themselves discussing. as usual, the gate location fits part’s wall thickness very thick there.
The needle is too short or too thin, so the plastic part at the bottom of the injection part is too thick (more than 4 mm, for example; the wall thickness at other parts is only 2 mm). The problem of shrinkage is inevitable. The thicker the plastic part is, the more difficult it is to solve the problem, and you can't even adjust the machine. This is mostly the case in production.
The wall thickness at the end of the injection needle is too thin (less than 0.3mm, for example) because the syringe is too long, and the thermal strength of the plastic part is very low. Injection parts out of the mold, the division of the cylinder needle to pull out, the division of the cylinder hole inside the vacuum, the atmospheric pressure outside will be molded parts of the surface of the concave, the shape of the concave.
Suggestion: Therefore, before solving such a shrinkage problem, we should first cut the injection parts and observe and find that the position of the injection parts is too thick, generally not thick over the wall thickness of the injection parts, we will lengthen the cylinder needle, until the concave OK; If the concave position is found to be too thin (usually not less than 0.5 mm), the spinner needle can be ground short with a grinding machine, and the concave problem can be solved.
Third: INJECTION MOLDING TECHNOLOGY
1. The temperature of the material tube is too high
Feed tube at high temperature, melting glue density is small, cooling, close to the cavity surface melting glue solidified into first condensation Layer (Frozen Layer), plastic volume contraction, a cavity in the center of the melt glue density smaller, until in the middle of the melt glue also gradually solidified, the cavity is hollowing out, hollow wall riddled with tensile stress, if condensation Layer rigidity is not enough, will collapse inwards, Forming a depression. Reduce the material temperature, melt adhesive density, and small probability of sag.
Suggestion: this issue is simple, decreasing material temperature
2. The cooling time is insufficient
The cooling time is not enough, the plastic condensation layer is not thick enough to resist the tension generated when the internal melting glue solidifies and shrinks, forming a depression.
Suggestion: this issue also is simple, we can according to our experience increase the cooling time properly.
3. Holding pressure insufficient
Holding pressure or holding time is not enough, the plastic cavity because of the low pressure or insufficient supplementary material filling compaction, density is small, the probability of depression. When the buffer becomes 0, the screw reaches the bottom and no longer moves forward. When the melt glue cools and the pressure is reduced during contraction, the screw can’t boost the pressure, resulting in insufficient pressure holding and a large probability of depression. Holding pressure should be 30 sufficient. Hold pressure for at least 2 seconds.
Suggestion: According to experience increases the holding time and pressure
4.Non-return Valve failure
The check valve prevents the melt glue in front of the screw in the feed tube from back flow during the injection phase. When the screw pushes a certain amount of material forward, if the check valve is worn, broken, or improperly located, the melt glue may slip past the front end of the screw, the gap between the check valve and the material pipe will produce back flow, making the screw push to the bottom out, the buffer will disappear, and the probability of sag will be large.
Suggestion: Remove the check valve from the front end of the screw, check each touch surface, if have Burned plastics on the surface, use a Wire Brush to remove it; stop using a Torch to burn off the plastic, as high temperature heat will soften valve metal and accelerate wear. If found nicks, cracks or pits are on the surface, the parts with such defects should be replaced.
Conclusion
Injection molding condition reason
1)Injection pressure is too low
Suggestion: increasing injection pressure
2)The injection time is too short
Suggestion: increasing injection time
3)The injection total pressure time is too short
Suggestion: increasing holding time
4)Injection rate is too slow
Suggestion: increasing injection speed or rate
5)Injection pressure insufficient
Suggestion: increasing injection pressure
Material cause:
Poor material fluidity
Suggestion: increasing temperature, add material fluidity
Temperature reasons:
a.Improve the temperature of the barrel
b.Increase the nozzle temperature
c.Check millivoltmeter, thermocouple, resistance heating ring and heating system
Mold Reasons:
A.The flow channel is too small
B.The gate is too small
C.the nozzle hole is too small
D.The gate position is unreasonable
E.the number of gates is insufficient
F.The cold hole is too small
G.Lack of exhaust
Final opinion:
If happen sink mark or shrinkage when injection molding,
First: using injection technology. This is the best way, it is simple and cost low. Such as increasing injection pressure and time or holding pressure and time, or improve material temperature, etc.
Second: if not, have to change material, use better fluidity or smaller shrinking percentage material try again.
Three: if still not.change mold.increasing sprue or runner or gate or location or increasing cooling hole etc.
Four: If still haven’t worked it, have to change parts of original design, such as decreasing the rib’s wall thickness, increase the cylinder’s length.
Our team have more than 20 years experiences in injection molding field. If you have any questions or RFQ for plastic technology and custom plastic parts, welcome to contact us.
