Definition of sink mark: partial collapse of surface of molded product (either in shape of dimples or in shape of ravines).

When rib is thick, place where rib and bottom plate meet is also thick, where plastic concentrates. When cooling, surrounding ribs and plates solidify first, center of intersection of this rib and plate remains liquid, plastic that solidifies later is ahead of plastic that solidifies first. It shrinks upward and has a sucking-in effect on surrounding plastic. If settling layer is weak in any one place (usually just at mold face opposite to rib), it may collapse into a sink mark at that place. Thickness of rib is preferably 50% of thickness of bottom plate, and can even be thinner.

 

If temperature of mold surface opposite to rib is higher than that of its vicinity (it is generally true, because nearby melted glue is concentrated, heat load is large, and mold temperature remains high), condensation layer is thin and rigidity is not enough. As central melt cures, residual stresses may pull thinner settling layer inward into a depression. Mold surface opposite to rib must be cooled to reduce temperature of mold, so that condensation layer is formed faster. When condensation layer is thicker, rigidity is greater and sink mark is not easy to occur.
When setting mold temperature, it can be set from recommended value of material manufacturer. Decrement (or increment) of each adjustment can be 6 ℃, and glue is injected 10 times. After molding situation is stable, it is decided whether to make further adjustments according to results.

If runner, runner or/and gate are too small, flow resistance will increase. If injection pressure is insufficient, cavity will not be filled, melt density will be low, and probability of sinking will be high.

Regardless of number or location of gates, flow length will be too long and flow resistance will be too large. If injection pressure is insufficient, cavity cannot be filled, melt density is low, and probability of sinking is high.
Sometimes due to defects in mold manufacturing, shrinkage of outer surface of injection molded part at position of cylinder is difficult to solve no matter how machine is adjusted. Even if it is solved without force, injection molded parts are already criticized by the whole body.
There are two main reasons for serious shrinkage of tube position:
Cylinder needle is too short or too thin, so that plastic part of injection molded part at the root of cylinder is too thick (for example, greater than 4 mm, while wall thickness of other parts of injection molded part is only 2 mm), problem of shrinkage is inevitable, the thicker it is, the more difficult it is to solve, even by adjusting machine. This is the case most often in production.
Cylinder needle is too long, so that wall thickness of injection molded part at the end of cylinder needle is too thin (for example, less than 0.3mm), resulting in very low heat intensity of plastic part here. When injection molded part is released from mold, cylinder needle is pulled out, and a vacuum is formed inside cylinder hole, and external atmospheric pressure will depress surface of injection molded part, which is like a shrinkage.
Therefore, before solving such a shrinkage problem, injection molded part should be cut and observed first, and it is found that injection molded part is too thick at this position. Generally, it should not be thicker than wall thickness of injection molded part, needle should be lengthened until shrinkage is OK; If it is found that recessed position is too thin (usually not less than 0.5 mm), barrel needle can be shortened with a grinder, and recessed problem can be solved.
In addition, spraying some release agent on cylinder can also be used as an auxiliary means to enhance effect of improving shrinkage. Because release agent can cool needle of cylinder, which enhances cooling effect at this position, thereby reducing degree of shrinkage. At the same time, it can also prevent formation of a vacuum when cylinder is ejected, because release agent will generate a little gas.

When temperature of barrel is too high, density of melted glue is low. When cooling, melted glue close to surface of cavity first solidifies into a frozen layer, volume of plastic shrinks, and density of melted glue in the center of cavity becomes smaller. When glue is gradually solidified, center of cavity will be hollow, and inner wall of cavity is full of tensile stress. If rigidity of coagulated layer is not enough, it will collapse inward and form a sink mark. Reduce material temperature, density of melted glue is high, and probability of sag is small.

If cooling time is not enough, plastic coagulation layer is not thick enough to resist pulling force generated when internal melt glue solidifies and shrinks, forming a sink mark. Material suppliers can provide suggested cooling times for different plastics and product thicknesses.

If holding pressure or holding time is not enough, plastic in cavity will not be compacted due to low pressure or insufficient supplementary materials, and density is low, so probability of sag is high. When buffer becomes 0, screw goes to the bottom and no longer moves forward. Pressure decreases when melted glue cools and shrinks, but screw cannot be pressurized, resulting in insufficient pressure holding and a high probability of sag. Buffer should be at least 3mm. Holding pressure should be sufficient. Hold pressure for at least 2 seconds.

 

Check valve prevents melted glue before screw in barrel from flowing back during injection stage. When screw pushes a certain amount of material forward, if check valve is worn, broken or improperly seated, melted glue may slip past gap between front end of screw, check valve and material tube to generate backflow, causing screw to push to the bottom, buffer disappears, and probability of sinking is high.
Remove check valve from front end of screw and check contact surfaces. If there is burnt plastic on the surface, use a wire brush to remove it; do not use a torch to burn plastic, because high heat will soften it valve metal, causing accelerated wear. If Nicks, Cracks or Pits are found on contact surfaces, defective parts should be replaced.
Device reason:
① Hopper cuts off material;
② Neck of hopper is partially or completely blocked;
③ Feeding amount is not enough;
④ Feeding control system is not operating normally;
⑤ Plasticizing capacity of injection press is too small;
⑥ Abnormal injection cycle caused by equipment.
Reasons for injection molding conditions:
① Injection pressure is too low;
② Injection pressure loss is too large during injection cycle;
③ Injection time is too short;
④ Injection full pressure time is too short;
⑤ Injection rate is too slow;
⑥ Material flow in mold cavity is interrupted;
⑦ Mold filling rate varies;
⑧ Abnormal injection cycle caused by operating conditions.
Reason for temperature:
① Increase barrel temperature;
② Increase nozzle temperature;
③ Check millivoltmeter, thermocouple, resistance heating coil (or far infrared heating device) and heating system;
④ Increase mold temperature;
⑤ Check mold temperature control device.
Mold reason:
① Flow channel is too small;
② Gate is too small;
③ Nozzle hole is too small;
④ Gate location is unreasonable;
⑤ Insufficient number of gates;
⑥ Cold material hole is too small;
⑦ Insufficient exhaust;
⑧ Abnormal injection cycle caused by mold;
Material reason: Fluidity of material is too poor.