Undershot is also called short shot, insufficien […]
Undershot is also called short shot, insufficient filling, dissatisfaction with the part, commonly known as undershot, which refers to the partial incompleteness of the end of the material flow or the partial filling of a mold with multiple cavities, especially the thin-walled area or the end area of the flow path. The manifestation is that the melt is condensed before the cavity is filled, and the melt is not completely filled after entering the cavity, resulting in a product shortage.
The main reason for the short shot is that the flow resistance is too large, causing the melt to not continue to flow. Factors affecting the melt flow length include: part wall thickness, mold temperature, injection pressure, melt temperature and material composition. If these factors are not handled well, they will cause short bets.
1. Improper equipment selection
When selecting equipment, the maximum injection volume of the injection molding machine must be greater than the total weight of the plastic parts and nozzles, and the total injection weight cannot exceed 85% of the plasticization volume of the injection molding machine.
2. Insufficient supply
At present, the commonly used method to control the feeding is the fixed-volume feeding method, whether the amount of roll material 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. In this regard, the feeding port should be dredged and cooled.
3. Poor material fluidity
When the fluidity of raw materials is poor, the structural parameters of the mold are the main reason for the short injection. Therefore, the stagnant flow defects of the mold gating system should be improved, such as setting the runner position reasonably, expanding the size of the gate, runner and injection port, 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, check whether the amount of recycled materials in the raw materials is excessive, and appropriately reduce the amount of recycled materials.
4. Excess lubricant
If the amount of lubricant in the raw material formulation is too much, and the gap between the non-return ring of the injection screw and the barrel is large, serious reflux of the molten material in the barrel will cause insufficient supply and cause underfill. In this regard, the amount of lubricant should be reduced and the gap between the barrel and the injection screw and the non-return ring should be adjusted to repair the equipment.
5. Impurities in cold material block the forehearth
When impurities in the melt block the nozzle or the cold material blocks the gate and runner [/li][li], the nozzle should be folded down to clean or enlarge the mold cold slug and runner section.
6. The design of the pouring system is unreasonable
When a mold has multiple cavities, the appearance of the plastic part is often defective due to the unreasonable balance design of the gate and runner. When designing the gating system, pay attention to the balance of the gate. The weight of the plastic parts in each cavity should be proportional to the size of the gate, so that each cavity can be filled at the same time. The gate location should be selected at a thick wall, or a branch runner can be used. Design scheme of balanced layout.
If the gate or runner is small, thin, and long, the pressure of the melt will lose too much along the way during the flow process, the flow will be blocked, and poor filling will easily occur. In this regard, the runner cross-section and gate area should be expanded, and multi-point feeding can be used if necessary.
7. Poor mold exhaust
When a large amount of residual gas in the mold is squeezed by the flow material due to poor exhaust, and a high pressure greater than the injection pressure is generated, it will prevent the melt from filling the cavity and cause under-injection. In this regard, check whether there is a cold slug hole or whether its position is correct. For a mold with a deep cavity, an vent groove or vent hole should be added to the undershot position; on the clamping surface, a depth can be set The vent slot is 0.02~0.04mm and the width is 5~10mm. The vent 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 also be generated, resulting in poor mold exhaust. At this time, the raw materials should be dried and volatiles removed.
In addition, in the process operation of the mold system, poor exhaust can be improved by increasing mold temperature, reducing injection speed, reducing flow assistance of the gating system, reducing mold clamping force, and increasing mold gaps.
8. The mold temperature is too low
After the melt enters the low-temperature mold cavity, it will not be able to fill all corners of the cavity due to cooling too fast. Therefore, the mold must be preheated to the temperature required by the process before starting up. When starting up, the amount of cooling water in the mold should be appropriately controlled. If the mold temperature does not rise, check whether the design of the mold cooling system is reasonable.
9. The melt temperature is too low
Generally, in the range suitable for molding, the material temperature and the filling length are close to a proportional relationship, and the flow performance of the low-temperature melt decreases, which reduces the filling length. When the material temperature is lower than the temperature required by the process, check whether the barrel feeder is intact and try to increase the barrel temperature. When starting up, the temperature of the barrel is always lower than the temperature indicated by the indicator of the barrel heater. It should be noted that after the barrel is heated to the temperature of the indicator, it will take a period of time to start the machine.
If low-temperature injection is necessary to prevent melt decomposition, the injection cycle time can be appropriately extended to overcome short injection. For screw injection molding machines, the temperature of the front section of the barrel can be appropriately increased.
10. The nozzle temperature is too low
During the injection process, the nozzle is in contact with the mold. Since the mold temperature is generally lower than the nozzle temperature and the temperature difference is large, frequent contact between the two will cause the nozzle temperature to drop, causing the melt to freeze at the nozzle.
If there is no cold material cavity in the mold structure, the cold material will solidify as soon as it enters the cavity, so that the hot melt plugged behind cannot fill the cavity. Therefore, the nozzle should be separated from the mold when opening the mold to reduce the influence of mold temperature on the nozzle temperature and keep the temperature at the nozzle within the range of the process requirements.
If the nozzle temperature is very low and cannot rise, check whether the nozzle heater is damaged, and try to increase the nozzle temperature, otherwise, the pressure loss of the flow material will be too large and it will cause short injection.
11. Insufficient injection pressure or holding pressure
The injection pressure is close to the proportional relationship with the filling length. The injection pressure is too small, the filling length is short, and the cavity filling is not full. In this regard, the injection pressure can be increased by slowing down the injection advancement speed and appropriately extending the injection time. In the case that the injection pressure cannot be further increased, it can be remedied by increasing the material temperature, reducing the melt viscosity, and improving the melt flow performance. It is worth noting that if the material temperature is too high, the melt will be thermally decomposed, which will affect the performance of the plastic parts.
In addition, if the holding time is too short, it will cause insufficient filling. Therefore, the pressure holding time should be controlled within an appropriate range, but it should be noted that too long a pressure holding time will also cause other failures. The molding should be adjusted according to the specific conditions of the plastic part.
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 fills the mold slowly, and the low-speed flow of the melt is easy to cool, causing its flow performance to further decrease and cause under-injection.
In this regard, the injection speed should be appropriately increased. But it should be noted that if the injection speed is too fast, it is easy to cause other molding failures.
13. The structural design of plastic parts is unreasonable
When the thickness of the plastic part is not proportional to the length, the shape is very complicated and the molding area is large, the flow of melt is easily blocked at the entrance of the thin-walled part of the plastic part, making it difficult to fill the cavity. Therefore, when designing the shape and structure of the plastic part, it should be noted that the thickness of the plastic part is related to the limit flow length when the melt is filled.
In injection molding, the thickness of plastic parts is 1~3mm, and the thickness of large plastic parts is 3~6mm. Generally, the minimum thickness recommended is 0.5mm for polyethylene, 0.7mm for cellulose acetate and cellulose acetate butyrate plastics, Ethyl cellulose plastic 0.9mm, polymethyl methacrylate 0.7mm, polyamide 0.7mm, polystyrene 0.75mm, polyvinyl chloride 2.3mm. Generally, plastic parts with a thickness of more than 8mm or less than 0.5mm are not good for injection molding, and such thickness should be avoided in design.