◆Jet patterns (serpentine patterns) and their solutions
Shrinkage marks are the obvious marks left at the boundary between features with significant differences in wall thickness in plastic parts, caused by uneven shrinkage of the two features, as shown in the figure below.
The causes and corresponding solutions for shrinkage marks on plastic parts are as follows:
① Improper control of the molding process. To address this, the injection pressure and injection speed should be appropriately increased to increase the compressibility of the melt, extend the injection and holding time, compensate for melt shrinkage, and increase the injection buffer. However, the holding pressure should not be too high, otherwise it will cause raised marks. If the depressions and shrinkage marks occur near the gate, they can be resolved by extending the holding time; when depressions occur in thick-walled areas, the cooling time of the plastic part in the mold should be appropriately extended; if depressions and shrinkage marks around inserts are caused by localized melt shrinkage, this is mainly due to the insert temperature being too low, and the insert temperature should be increased; if the surface depressions of the plastic part are caused by insufficient material supply, the material supply should be increased. In addition, the cooling of the plastic part in the mold must be sufficient.
② Mold defects. To address this, the gate and runner cross-sections should be appropriately enlarged based on specific circumstances. The gate location should be symmetrical as much as possible, and the injection port should be located in the thick-walled section of the plastic part. If sink marks and dents occur far from the gate, it is generally due to poor melt flow in a certain part of the mold structure, hindering pressure transmission. In this case, the structural dimensions of the mold gating system should be appropriately enlarged, ideally extending the runner to the area causing the sink marks. For thick-walled plastic parts, wing-type gates should be preferred.
③ Raw material does not meet molding requirements. For plastic parts with high surface finish requirements, low-shrinkage plastics should be used as much as possible. A suitable amount of lubricant can also be added to the raw material.
④ Inappropriate plastic part shape and structure design. When designing the shape and structure of plastic parts, the wall thickness should be as uniform as possible. If there are significant differences in wall thickness, this can be resolved by adjusting the structural parameters of the gating system or changing the wall thickness distribution, as shown in the figure below.
Silver lines (silver threads, decorative patterns) and solutions
As shown in the image below, the splash-like lines formed on the surface of the plastic part along the direction of melt flow are called silver streaks, also known as silver threads or material flowers.
Silver streaks are generally caused by the screw starting too quickly during injection, preventing air from escaping from the melt and mold cavity. This trapped air forms silvery, thread-like streaks on the surface of the plastic part. Silver streaks not only affect the appearance of the part but also significantly reduce its strength. The formation of silver streaks is mainly due to the presence of gas in the molten plastic. Identifying the root cause of this gas production will reveal solutions to the defect. The main causes and solutions are as follows.
(1) The plastic itself contains moisture or oil. Because plastics are exposed to air during manufacturing, they absorb moisture/oil, or incorrect proportions of components are added during mixing. These volatile substances turn into gas when the melt is heated.
(2) Thermal decomposition of the melt. If the melt barrel temperature, back pressure, and melt speed are set too high, or the molding cycle is too long, heat-sensitive plastics (such as PVC, Pyrite, and PC) are prone to gas production due to thermal decomposition.
(3) Air. Air is present between plastic particles. If the temperature near the hopper in the melt barrel is set too high, the surface of the plastic particles melts and sticks together before compression, preventing complete degassing (poor degassing).
(4) Poor melt plasticization. To address this, appropriately increase the barrel temperature and extend the molding cycle, and try to use internally heated injection ports, larger cold slug wells, and longer runners.
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| Cause Analysis | Solution |
|---|---|
| ① Raw material contains too much water | ① Raw material should be dried (keep within the allowed moisture content) |
| ② Material temperature is too high (materials are partially decomposed) | ② Lower the material temperature |
| ③ Raw material contains fat emulsifying additives (such as emulsifiers) | ③ Use less emulsifying additives or replace with other emulsifiers |
| ④ Color difference (color of colored casings is too light) | ④ Use darker-colored casings |
| ⑤ Injection speed too fast (air is trapped when cutting or vacuum mixing is insufficient) | ⑤ Reduce injection speed ⑥ Vent air a. Reduce vacuum degree b. Increase vacuum time and degree of vacuum |
| ⑥ South-North air in the material | ⑦ Cut and release the trapped air |
| ⑦ Mixed materials have poor thermal stability value | ⑧ Use thermally stable plastic casings |
| ⑧ Materials expand due to thick expansion, volatilization of spices, and poor surface energy absorption | ⑨ Adjust the formula a. Reduce the amount of starch (usually cold swelling) b. Adjust the expansion degree and position of the regulator |
| ⑨ After exiting the oven, the casing position is uneven when placed in a small position | ⑩ Adjust the size of the mouth or adjust the filling position |
| ⑪ Mold exhaust is insufficient or mold is too low | ⑪ Improve mold exhaust or increase mold height |
| ⑫ There are too many materials (materials stay in the hopper for too long) | ⑬ Reduce the amount of materials in the hopper |
| ⑬ Lower mouth processing temperature is too high | ⑭ Reduce tying temperature |
| ⑭ Low tying pressure (pressure insufficient) | ⑮ Appropriately increase pressure |
| ⑮ After filling, the casing is too loose | ⑯ Reduce the casing diameter, raise the lower mouth and remember to cool evenly |
| ⑯ The casing position is too large | ⑰ Reduce the casing diameter |
Water ripples and solutions
Water ripples refer to the traces of melt flow that cannot be removed after molding, appearing as wavy patterns centered on the gate. They are commonly seen on plastic parts injection molded using smooth molds, as shown in the image below.
Water ripples are formed when the initial melt flowing into the mold cavity cools too quickly, causing the subsequently injected hot melt to push the preceding melt along, creating a wavy pattern. The formation process is shown in the figure below.This can be mitigated by increasing the melt and mold temperatures, accelerating the injection speed, and increasing the holding pressure. Cold slug remaining at the nozzle tip can also cause water ripples if it directly enters the mold cavity; therefore, creating a cold slug well at the end of the main runner can effectively prevent water ripples.
Causes and solutions for water ripples
| Cause Analysis | Solution |
|---|---|
| ① Raw material contains too much water | ① Raw material should be dried (keep within the allowed moisture content) |
| ② Material temperature is too high (materials are partially decomposed) | ② Lower the material temperature |
| ③ Raw material contains fat emulsifying additives (such as emulsifiers) | ③ Use less emulsifying additives or replace with other emulsifiers |
| ④ Color difference (color of colored casings is too light) | ④ Use darker-colored casings |
| ⑤ Injection speed too fast (air is trapped when cutting or vacuum mixing is insufficient) | ⑤ Reduce injection speed ⑥ Vent air a. Reduce vacuum degree b. Increase vacuum time and degree of vacuum |
| ⑥ South-North air in the material | ⑦ Cut and release the trapped air |
| ⑦ Mixed materials have poor thermal stability value | ⑧ Use thermally stable plastic casings |
| ⑧ Materials expand due to thick expansion, volatilization of spices, and poor surface energy absorption | ⑨ Adjust the formula a. Reduce the amount of starch (usually cold swelling) b. Adjust the expansion degree and position of the regulator |
| ⑨ After exiting the oven, the casing position is uneven when placed in a small position | ⑩ Adjust the size of the mouth or adjust the filling position |
| ⑪ Mold exhaust is insufficient or mold is too low | ⑪ Improve mold exhaust or increase mold height |
| ⑫ There are too many materials (materials stay in the hopper for too long) | ⑬ Reduce the amount of materials in the hopper |
| ⑬ Lower mouth processing temperature is too high | ⑭ Reduce tying temperature |
| ⑭ Low tying pressure (pressure insufficient) | ⑮ Appropriately increase pressure |
| ⑮ After filling, the casing is too loose | ⑯ Reduce the casing diameter, raise the lower mouth and remember to cool evenly |
| ⑯ The casing position is too large | ⑰ Reduce the casing diameter |