Analysis of reasons for insufficient injection of injection molded products
Underinjection is also called short injection, insufficient filling and insufficient parts, commonly known as underinjection, which refers to the incomplete phenomenon at the end of the material flow or the insufficient filling of a part of the the first mock examination multi cavity, especially in the thin-walled area or the end area of the flow path. Its manifestation is that the melt condenses before filling the mold cavity, and the molten material is not completely filled after entering the mold receiving cavity, resulting in a shortage of material in the product.
The main reason for short injection is excessive flow resistance, which prevents the melt from continuing to flow. The factors affecting the length of melt flow include: part wall thickness, mold temperature, injection pressure, melt temperature, and material composition. If these factors are not handled properly, they can lead to short bets.
1. Improper equipment selection
2. When using the selected equipment, the maximum injection volume of the injection molding machine must be greater than the total weight of the plastic part and nozzle, and the total injection weight cannot exceed 85% of the plasticization volume of the injection molding machine.
3. Insufficient supply of materials
4. The currently commonly used method for controlling feeding is the constant volume feeding method, which checks whether the roller feeding amount is consistent with the grain size of the raw material, and whether there is any bridging phenomenon at the bottom of the feeding port. If the temperature at the feeding port is too high, it can also cause poor material discharge. In this regard, the feeding port should be unblocked 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 insufficient injection. Therefore, it is necessary to improve the stagnant flow defects in the mold pouring system, such as reasonably setting the position of the sprue, expanding the size of the sprue, 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 flowability of the resin. In addition, it is necessary to check whether the amount of raw materials in the raw materials exceeds the limit and reduce their usage appropriately.
4. Excessive lubricant
If there is too much lubricant in the raw material formula and there is a large wear gap between the injection screw check ring and the material barrel, severe reflux of molten material in the material barrel will cause insufficient supply, leading to underfilling. In this regard, it is necessary to reduce the amount of lubricant used and adjust the gap between the material barrel, the injection screw, and the check ring, and repair the equipment.
5. Cold material impurities blocking the material channel
When impurities in the molten material block the nozzle or cold material blocks the gate and runner, the nozzle should be folded down to clean or expand the cold material cavity and runner section of the mold.
6. Unreasonable design of pouring system
When the first mock examination has multiple cavities, the appearance defects of plastic parts are often caused by unreasonable gate and runner balance design. When designing the pouring system, attention should be paid to gate balance. 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 position should be selected at a thick wall, or a design scheme with balanced distribution channels can be adopted.
If the gate or runner is small, thin, or long, the pressure loss of the molten material along the flow process is too large, which hinders the flow and easily leads to poor filling. To address this, the cross-section of the runner and the area of the gate should be expanded, and if necessary, a multi-point feeding method can be used.
7. Poor mold exhaust
When a large amount of gas remaining in the mold due to poor exhaust is squeezed by the flowing material, producing a high pressure greater than the injection pressure, it will hinder the molten material from filling the mold cavity and cause underfilling. In this regard, it is necessary to check whether there are cold material cavities set up or whether their positions are correct. For molds with deeper cavities, exhaust grooves or holes should be added in the areas where there is insufficient injection; On the clamping surface, an exhaust groove with a depth of 0.02-0.04mm and a width of 5-10mm can be opened, and the exhaust hole should be set at the final filling point of the mold cavity.
When using raw materials with excessive moisture and volatile matter content, a large amount of gas will also be generated, leading to poor mold exhaust. At this time, the raw materials should be dried and volatile substances should be removed.
In addition, in terms of process operation in the mold system, auxiliary measures such as increasing mold temperature, reducing injection speed, reducing flow assistance in the pouring system, and reducing clamping force and increasing mold clearance can be taken to improve poor exhaust.
8. Mold temperature too low
After the molten material enters the low-temperature mold cavity, it will not be able to fill all corners of the cavity due to rapid cooling. Therefore, before starting up, the mold must be preheated to the temperature required by the process. When starting up, the amount of cooling water passing through the mold should be appropriately controlled. If the temperature of the mold cannot rise, the design of the mold cooling system should be checked for rationality
9. The temperature of the molten material is too low
Usually, within the range suitable for molding, the relationship between material temperature and filling length is close to a positive proportion, and the flow performance of low-temperature molten materials decreases, resulting in a reduction in filling length. When the material temperature is lower than the temperature required by the process, the material barrel feeder should be checked for integrity and efforts should be made to increase the material barrel temperature. When starting up, the temperature of the material barrel is always lower than the temperature indicated by the instrument of the material barrel heater. It should be noted that the material barrel needs to be heated to the instrument temperature for a period of time before starting up.
If low-temperature injection is necessary to prevent the decomposition of the molten material, the injection cycle time can be appropriately extended to overcome under injection. For screw injection molding machines, the temperature in 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. Due to the mold temperature being generally lower than the nozzle temperature and the large temperature difference, frequent contact between the two can cause the nozzle temperature to drop, resulting in the molten material freezing at the nozzle.
If there are no cold material cavities in the mold structure, the cold material will solidify immediately after entering the mold cavity, making it impossible for the hot melt material that is inserted behind to fill the mold cavity. Therefore, when opening the mold, the nozzle should be separated from the mold to reduce the influence of mold temperature on the nozzle temperature and maintain the temperature at the nozzle within the range required by the process.
If the nozzle temperature is very low and cannot rise, the nozzle heater should be checked for damage and efforts should be made to increase the nozzle temperature. Otherwise, excessive pressure loss of the flowing material can also cause insufficient injection.
11. Insufficient injection pressure or holding pressure
12. The injection pressure is closely proportional to the filling length. If the injection pressure is too small, the filling length will be short, and the cavity will not be filled properly. In this regard, injection pressure can be increased by slowing down the injection speed and appropriately extending the injection time. When the injection pressure cannot be further increased, it can be remedied by increasing the material temperature, reducing the viscosity of the melt, and improving the melt flow performance. It is worth noting that if the material temperature is too high, it will cause the molten material to decompose and affect the performance of the plastic parts.
In addition, if the holding time is too short, it can also lead to insufficient filling. Therefore, the holding time should be controlled within an appropriate range, but it should be noted that excessive holding time can also cause other faults. During molding, adjustments should be made according to the specific situation of the plastic part.
12. Injection speed too slow
The injection speed is directly related to the filling speed. If the injection speed is too slow, the molten material will fill the mold slowly, and the low-speed flowing molten material is easy to cool, further reducing its flow performance and resulting in underfilling.
In this regard, the injection speed should be appropriately increased. However, it should be noted that if the injection speed is too fast, it can easily cause other molding failures.
13. Unreasonable structural 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 flow of molten material is easily blocked at the entrance of the thin-walled part of the plastic part, making it difficult to fill the mold cavity. Therefore, when designing the shape and structure of plastic parts, attention should be paid to the relationship between the thickness of the plastic parts and the ultimate flow length of the molten material during mold filling.
In injection molding, the maximum thickness used for plastic parts is 1-3mm, while for large plastic parts it is 3-6mm. The recommended minimum thickness is generally; Polyethylene 0.5mm, cellulose acetate and cellulose acetate butyrate plastic 0.7mm, ethyl cellulose plastic 0.9mm, polymethyl methacrylate 0.7mm, polyamide 0.7mm, polystyrene 0.75mm, polyvinyl chloride 2.3mm. Usually, a thickness of plastic parts exceeding 8mm or less than 0.5mm is unfavorable for injection molding, and such thickness should be avoided in design.
In addition, when forming complex structural plastic parts, necessary measures should also be taken in the process, such as determining the position of the gate reasonably, adjusting the flow channel layout appropriately, improving injection speed, or using rapid injection. Increase the mold temperature or choose resins with better flowability.
