1. Improper equipment selection.
When using the equipment, the maximum injection quantity of theinjection molding machine must be greater than the total weight of the plastic parts and the nozzle, and the total weight of the injection molding machine must not exceed 85% of the plasticization quantity
2. The supply is insufficient.
At present, the commonly used way to control the feeding is the fixed volume feeding method. Whether the roller feeding amount is uniform with the fruit grain of the raw material, and whether there is "bridging" phenomenon at the bottom of the feeding port. If the temperature at the feeding port is too high, it will also cause poor blanking. Therefore, it is necessary to dredge and cool the feeding port.
3. The fluidity of feed is poor.
When the fluidity of the raw material is poor, the structural parameters of the mold are the main factors affecting the under injection. Therefore, it is necessary to improve the stagnant flow defects of the mold gating system, such as reasonably setting the runner position, expanding the gate, runner and gate size, and using larger nozzles.
At the same time, an appropriate amount of additives can be added to the raw material formula to improve the fluidity of the resin. In addition, it is also necessary to check whether the amount of recycled raw meal in raw materials is excessive and reduce the amount of recycled raw meal appropriately.
4. Excessive lubricant.
If the amount of lubricant in the raw material formula is too much, and the gap between the ejector screw check ring and the barrel is large, the molten material will return seriously in the barrel, which will lead to insufficient feeding and underinjection. Therefore, it is necessary to reduce the amount of lubricant and adjust the gap between barrel and injection screw and check ring to repair the equipment.
5. Cold material impurities block the channel.
When the impurities in the molten material block the nozzle or the cold material block
The nozzle should be folded down to clean or enlarge the cold material cavity and the cross section of the runner.
6. The design of gating system is unreasonable.
When the first mock exam is multi chamber, the appearance defects of plastic parts often result from unreasonable design of gate and runner balance. When designing the gating system, we should pay attention to the balance of the gate. The weight of the plastic parts in each mold cavity should be proportional to the size of the gate, so that each mold cavity can be filled at the same time. The gate should be located at the thick wall, and the design scheme of balanced layout of the shunt channel can also be adopted. If the gate or runner is small, thin and long, the pressure loss of molten material along the flow process is too large, and the flow is blocked, it is easy to produce poor filling. Therefore, the runner section and gate area should be enlarged, and the multi-point feeding method can be adopted if necessary.
7. Poor exhaust of mould.
When a large amount of residual gas in the mold due to poor exhaust is squeezed by the flowing material, which produces a high pressure greater than the injection pressure, it will prevent the molten material from filling the cavity, resulting in underinjection.
In this regard, we should check whether there is a cold hole or whether its position is correct. For the mold with a deep cavity, we should add an exhaust groove or exhaust hole in the under injection part; on the mold closing surface, we can open an exhaust groove with a depth of 0.02 ~ 0.04mm and a width of 5 ~ 10mm, and the exhaust hole should be set at the final filling position of the cavity. When using raw materials with excessive moisture and volatile content, a large amount of gas will be produced, resulting in poor mold exhaust. At this time, the raw materials should be dried and the volatile matter should be removed.
In addition, in the process operation of the mold system, the poor exhaust can be improved by increasing the mold temperature, reducing the injection speed, reducing the flow assistance of the gating system, reducing the clamping force, increasing the mold clearance and other auxiliary measures.
8. Mold temperature is too low.
When the melt enters the low temperature cavity, it will be too fast to fill all corners of the cavity. Therefore, the mold must be preheated to the temperature required by the process before starting the machine. When starting the machine, the cooling water in the mold should be properly controlled. If the mold temperature does not rise, check the mold cooling system design is reasonable
9. The melt temperature is too low
Generally, in the range suitable for molding, the material temperature and filling length are close to a positive proportion, and the fluidity of low-temperature melt decreases, which shortens the filling length. When the material temperature is lower than the temperature required by the process, check whether the barrel feeder is in good condition and try to increase the barrel temperature.
When the machine is just started, the temperature of the cylinder is always lower than that indicated by the instrument of the cylinder heater. It should be noted that after the cylinder is heated to the temperature of the instrument, it will take a while to start the machine. If low temperature injection is necessary to prevent melt decomposition, the injection cycle time can be appropriately prolonged to overcome under injection. For screw injection molding machine, the temperature of the front section of the barrel can be increased appropriately.
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10 nozzle temperature too low
In the injection process, the nozzle is in contact with the mold, because the mold temperature is generally lower than the nozzle temperature, and the temperature difference is large, the frequent contact between the two will cause the nozzle temperature to drop, resulting in the melt freezing at the nozzle.
If there is no cold material cavity in the die structure, the cold material solidifies immediately after entering the cavity, so that the hot melt material behind the plug can not fill the cavity. Therefore, the nozzle should be separated from the mold to reduce the influence of mold temperature on the nozzle temperature and keep the temperature at the nozzle within the range of process requirements.
If the temperature of the nozzle is very low and cannot rise, check whether the nozzle heater is damaged, and try to increase the temperature of the nozzle, otherwise, too much pressure loss of the flowing material will also cause under injection.
11 insufficient injection pressure or holding pressure
The injection pressure and the filling length are close to a positive proportion. If the injection pressure is too small, the filling length is short and the cavity filling is not enough. Therefore, the injection pressure can be increased by slowing down the injection speed and prolonging the injection time.
If the injection pressure can not be further increased, it can be remedied by increasing the material temperature, reducing the melt viscosity and improving the melt flow performance. It should be noted that if the material temperature is too high, the melt will decompose and affect the performance of plastic parts.
In addition, if the holding time is too short, it will lead to insufficient filling. Therefore, the pressure holding time should be controlled in a suitable range, but it should be noted that too long pressure holding time will also cause other faults, and it should be adjusted according to the specific situation of plastic parts.
12.The injection speed is too slow
The injection speed is directly related to the filling speed. If the injection speed is too slow, the melt filling is slow, and the melt with low flow rate is easy to cool, which further reduces the flow performance and leads to underinjection.
Therefore, the injection speed should be increased appropriately. But it should be noted that if the injection speed is too fast, it is easy to cause other molding faults.
13. Unreasonable structure design of plastic parts
When the thickness and length of the plastic part are not proportional, the shape is very complex and the forming area is large, the melt flow is easily blocked at the entrance of the thin-wall part of the plastic part, which makes it difficult to fill the cavity. Therefore, when designing the shape and structure of plastic parts, it should be noted that the thickness of plastic parts is related to the limit flow length of molten metal during mold filling.
In the injection molding, the thickness of plastic parts is 1 ~ 3mm, and that of large plastic parts is 3 ~ 6mm. Generally, the recommended minimum thickness is polyethylene 0.5mm, cellulose acetate and cellulose acetate butyrate 0.7mm, ethyl cellulose 0.9mm, polymethylmethacrylate 0.7mm, polyamide 0.7mm, polystyrene 0.75mm and PVC 2.3mm. Generally, the thickness of plastic parts more than 8 mm or less than 0.5 mm is unfavorable for injection molding, and such thickness should be avoided in design.
In addition, in the process of forming complex structural plastic parts, necessary measures should be taken, such as reasonably determining the gate position, properly adjusting the flow channel layout, improving the injection speed or adopting rapid injection. Improve the mold temperature or choose the resin with better flow performance.
