Solution for bad exhaust of injection molded parts

Failure analysis and troubleshooting methods

1. Improper equipment selection

In the selection of 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 injection weight should not exceed 85% of the plasticization volume of the injection molding machine.

2. Insufficient material supply

At present, the commonly used method to control the feeding is the 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 "bridging" 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. Poor material flowability

When the material liquidity is poor, the structure parameter of the mold is the main reason to influence the under-injection. Therefore, the stagnant flow defects of the mold casting system should be improved, such as the reasonable setting of the sprue position, expanding 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. Excess lubricant

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 the gap between the barrel and the injection screw and the stop ring should be adjusted to repair the equipment.

5. Cold material impurities blocking the material channel

When the impurities in the melting block the nozzle or the cold material block the

When the nozzle and runner are blocked by cold material, the nozzle should be folded down to clean or expand the cold material cavity and runner section.

6. unreasonable design of pouring system

In the case of multiple cavities in one mold, the appearance defects of plastic parts are often caused by the unreasonable design of gate and runner balance. 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, and the location of the gate should be chosen at the thick wall, and the design scheme of balanced arrangement of the manifold can also be adopted. 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 adopted if necessary.

7. mold exhaust bad

When a large amount of gas left in the mold due to poor exhaust is squeezed by the flow material, resulting in a high pressure greater than the injection pressure, it will prevent the molten material from filling the cavity and cause under-injection.

In this regard, should check whether there is a cold material cavity or its location is correct, for the deeper cavity mold, should be set up 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, in the process operation of the mold system, we can improve the bad venting by raising the mold temperature, lowering the injection speed, reducing the flow assistance of the pouring system, and reducing the closing force, and increasing the mold gap.

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 just started, the amount of cooling water passing through the mold should be properly restrained. If the temperature of the mold does not rise, check whether the design of the mold cooling system is reasonable.

9. The temperature of molten material is too low

Usually, in the range suitable for molding, the material temperature, and mold filling length are close to the positive relationship, the low-temperature melt liquidity decreases, making the mold filling length shorter. When the material temperature is lower than the required temperature, check whether the barrel feeder is intact and try to increase 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 increased appropriately.

10. 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, the frequent contact between the two will cause the nozzle temperature to drop, resulting in the melt being frozen at the nozzle.

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, check whether the nozzle heater is damaged and try to raise the nozzle temperature, otherwise, too much pressure loss of the flow material 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, and 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 a suitable range, but it should be noted that too long holding time can also cause other failures, and should be adjusted according to the specific circumstances of the molded part.

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 fills the mold slowly, and the low-speed flow of the melt is easy to cool so that its flowability 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 other molding failures. 13.

13. The structure of the plastic part is not designed properly

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 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 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. Generally, 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.