Analysis of the causes of under-injection of injection molded products

Under-injection, also known as short shot, under-filling, under-filled parts, commonly known as under-injection, refers to the phenomenon of partial incompleteness at the end of the material flow or a part of the filling of multiple cavities in a mold, especially the thin-walled area or the end of the flow path area. The performance of the melt in the cavity is not filled with condensation, the melt into the cavity is not filled, resulting in the product's lack of material.

The main reason for short injection is excessive flow resistance, resulting in the melt can not continue to flow. Factors affecting melt flow length include wall thickness of the part, mold temperature, injection pressure, melt temperature, and material composition. These factors can cause short injections if not handled properly.
 

1. improper equipment selection

In the use of selected equipment, the maximum injection volume of the injection molding machine must be greater than the total weight of the plastic parts and water spout, and the total weight of injection can not exceed 85% of the plasticized volume of the injection molding machine.
 

2. Insufficient material supply

At present, the commonly used method to control the feeding is a fixed volume feeding method, the volume of its roll material and the raw material of the fruit grain by whether uniform, the bottom of the filling mouth has no "bridge" phenomenon. If the temperature at the filling mouth is too high, it will also cause the material to fall poorly. In this regard, should be unclogged and cooling the feeding mouth.
 

3. The material liquidity is poor

When the liquidity of raw materials is poor, the structure parameters of the mold are the main reason to affect the under-injection. Therefore, should improve the mold casting system's stagnant flow defects, such as the reasonable setting of the sprue position, expand the gate, runner, and injection port size, as well as the use of larger nozzles. At the same time, we can increase the number of additives in the raw material formula to improve the flowability of resin. In addition, should also check whether the raw materials in the recycled material are excessive, appropriate to reduce its dosage.
 

4. Lubricant excess

If the amount of lubricant in the raw material formula is too much, and the injection screw stop ring and barrel wear gap is large, the melt in the barrel backflow will cause a serious lack of material supply, resulting in under-injection. In this regard, the amount of lubricant should be reduced and adjust the barrel and the injection screw and stop ring gap, repair equipment.
 

5. cold material impurities blocking the material channel

When the impurities in the melt block the nozzle or cold material blocking [/li][li] gate and runner, the nozzle should be folded down to clean or expand the mold cold material cavity and runner cross-section.

6. pouring system design is not reasonable

When a mold with multiple cavities, often due to the unreasonable design of gate and runner balance leads to defects in the appearance of plastic parts. When designing the pouring 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 location of the gate should be chosen at the thick wall, and the design scheme of the balanced layout of the manifold can also be used.

If the gate or runner is small, thin, and long, the pressure of the melt will be lost too much along the flow process and the flow will be blocked, which will easily produce poor filling. In this regard, the runner cross-section and gate area should be expanded, and the method of multi-point feeding can be used if necessary.
 

7. mold exhaust bad

When the mold is due to poor exhaust and a large amount of gas left by the flow of material extrusion, resulting in a high pressure greater than the injection pressure, it will prevent the molten material filled cavity caused by under-injection. In this regard, should check whether there is a cold material cavity or its location is correct, for the cavity deeper mold, should be in the part of the exhaust groove or exhaust hole; in the mold surface, can open a depth of 0.02 ~ 0.04mm, the width of 5 ~ 10mm exhaust slot, exhaust hole should be set in the final filling of the cavity at the mold.

The use of moisture and volatile content that exceeds the standard of raw materials will also produce a large amount of gas, resulting in poor mold exhaust. In this case, the raw material should be dried and the volatiles should be removed.

In addition, the mold system process operation can be improved by increasing the mold temperature, reducing the injection speed, reducing the pouring system flow assistance, and reducing the closing force, increasing the mold gap and other auxiliary measures to improve the exhaust poor.
 

8. Mold temperature is too low

After the molten material enters the low-temperature mold cavity, it will not be able to fill all corners of the cavity because it cools too fast. Therefore, before starting the machine, the mold must be preheated to the temperature required by the process, and when the machine is first started, the amount of cooling water passing through the mold should be properly restrained. If the temperature of the mold does not rise, the design of the mold cooling system should be checked to see if it is reasonable.
 

9. Melting material temperature is too low

Usually, in the range suitable for molding, the material temperature, and mold filling length is close to the proportional relationship, the low-temperature melt fluidity decreases, making the mold filling length shortened. When the material temperature is lower than the temperature required by the process, you should check whether the barrel feeder is intact and try to improve the barrel temperature. When the machine is just started, the barrel temperature is always lower than the temperature indicated by the barrel heater, so it should be noted that the barrel is heated to the instrument temperature and then it needs to be warmed for some time before starting the machine.

If you have to take a low-temperature injection to prevent the decomposition of the melt, you can extend the injection cycle time to overcome the under-injection. For the screw injection molding machine, the temperature of the front section of the barrel can be appropriately increased.
 

10. the nozzle temperature is 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 make the nozzle temperature drop, resulting in the molten material at the nozzle freezing.

If there is no cold material cavity in the mold structure, the cold material will solidify immediately after entering the cavity, so that the hot-melt material that helps to plug in the back cannot fill the cavity. Therefore, the nozzle should be separated from the mold during mold opening to reduce the influence of mold temperature on the nozzle temperature and keep the temperature at the nozzle within the range required by the process.

If the nozzle temperature is very low and does not rise, should check whether the nozzle heater is damaged, and try to improve the nozzle temperature, otherwise, the pressure loss of the flow material is too large will also cause under-injection.
 

11. Insufficient injection pressure or holding pressure

Injection pressure and mold filling length are close to the proportional relationship, the injection pressure is too small, the mold filling length is short, the cavity is not filled. In this regard, the injection pressure can be increased by slowing down the injection forward speed and extending the injection time appropriately. In the case of injection pressure can not be further improved, can be remedied by increasing the material temperature, reducing the viscosity of the melt, improve the flowability of the melt. It is worth noting that if the material temperature is too high will make the melt thermal decomposition, affecting the performance of the plastic parts.

In addition, if the holding time is too short, it will also lead to insufficient filling. Therefore, the holding time should be controlled within the appropriate range, but it should be noted that too long holding time can also cause other failures, molding should be adjusted according to the specific circumstances of the plastic parts.
 

12. Injection speed is too slow

The injection speed is directly related to the mold filling speed. If the injection speed is too slow, the melt is slow to fill the mold, and the low-speed flow of the melt is easy to cool so that its liquidity further decreases to produce under-injection.

In this regard, the injection speed should be increased appropriately. However, it should be noted that if the injection speed is too fast, it is easy to cause another molding failure.
 

13. the structure of the plastic part is not designed

When the thickness of the plastic part is disproportionate to the length, the shape is very complex and the molding area is large, the melt is easy to flow at the entrance of the thin-walled part of the plastic part is blocked, making it difficult to fill the cavity. Therefore, when designing the structure of the molded part, the thickness of the part should be related to the limited flow length of the melt when filling the mold.

In injection molding, the thickness of plastic parts is 1~3mm, 3~6mm for large parts, and the minimum recommended thickness is 0.5mm for polyethylene, 0.7mm for cellulose acetate and cellulose acetate butyrate, 0.9mm for ethyl cellulose, 0.7mm for polymethyl methacrylate, 0.7mm for polyamide, 0.75mm for polystyrene, and 2.3mm for PVC. Usually, the thickness of plastic parts over 8mm or less than 0.5mm is not good for injection molding, so such thickness should be avoided when designing.

In addition, when molding complex structure plastic parts, the necessary measures should be adopted in the process, such as reasonable determination of the location of the gate, appropriate adjustment of the flow channel layout, increase the injection speed, or the use of rapid injection. Increase the mold temperature or use the resin with better flowability, etc.