What is the cause of insufficient filling of plastic parts and how to eliminate it?

(1) Defect characteristics

Insufficient filling of plastic parts is a common defect. Its main performance is: the cavity is not completely filled, so that the plastic parts are not full, and the shape is incomplete.

(2) Causes of defects and their elimination methods

1) Injection mold

1. Mold exhaust structure is not good, when the plastic melt into the mold cavity, the gas in the mold cavity should be discharged in time, if not discharged in time, the melt is difficult to fill the mold cavity, resulting in insufficient filling. Therefore, it is necessary to modify or improve the exhaust structure.
2. The flow channel is blocked by impurities, so that the melt can not reach the various parts of the mold cavity unimpeded, resulting in insufficient filling, the need to dredge the flow channel in time.
3. If the gate is unbalanced, the quality of the plastic parts in each cavity is not proportional to the size of the gate, and the melt can not be filled with the cavity at the same time, which is easy to cause insufficient filling, so the balance of the gate should be ensured.
4. If the gate or runner cross-sectional area is small and long, the melt pressure loss increases, the flow resistance increases, and the flow is prone to poor flow and insufficient filling, so the pouring system needs to be corrected and improved.
5. If the location of the gate is located in the thinner part of the plastic part, it is easy to form poor flow and insufficient filling, so the location of the gate should be changed and the wall thickness should be located.
6. If there is no cold well in the mold, or the design of the cold well is unreasonable, the cold material will enter the cavity and condense first, resulting in poor melt flow and insufficient filling, so the reasonable design of the cold well is very important.

2) Injection molding process

1. The mold temperature is too low, when the high-temperature plastic melt into the low-temperature mold cavity, it will be cooled too fast to fill the cavity, so the mold temperature should be detected or adjusted to make it in the normal range.
2. The injection pressure is too low, the injection pressure is approximately proportional to the length of the filling mold, if the injection pressure is small, the length of the filling mold is short, and the cavity is difficult to fill. In this regard, the injection pressure should be appropriately increased.
3. The pressure holding time is too short, which makes the plastic melt in the cavity reflux and leads to insufficient filling. Therefore, the pressure holding time should be controlled reasonably. For thick wall products, the pressure holding time is longer, and for thin wall products, it is shorter, generally controlled at 30 to 120s.
4. The temperature of the plastic melt is too low, the fluidity decreases, the length of the filling mold becomes short, and it is difficult to fill the cavity. To that. The plastic melt temperature should be increased.
5. The injection speed is too slow, the plastic melt flows slowly, is easy to cool, and the melt fluidity further decreases, resulting in insufficient filling, for which the injection speed should be appropriately increased.

3) Injection molding equipment

1. The injection volume of the injection molding machine is insufficient, when the injection volume of the injection molding machine is less than the quality of the plastic parts, it is inevitable that the cavity is not full and the filling is insufficient, in this regard, the total injection amount should not exceed 85% of the plasticizing amount of the injection molding machine, and the quality of the plastic parts can not exceed the total injection amount.
2. If the nozzle is blocked by foreign matter, the flow of plastic melt is not smooth and the filling is insufficient, in this regard, the nozzle must be dredged.
3. If the nozzle hole is too small, the filling speed of the plastic melt will be reduced, and it is difficult to fill the cavity, in this case, the nozzle with a larger diameter should be replaced.
4. If the check valve is faulty, it will make the plastic melt flow back, resulting in insufficient melt supply, in this case, the check valve should be repaired.
5. If the injection stroke is not enough, the injection amount will not be enough and it is difficult to fill the cavity, for which the injection stroke should be appropriately increased.
6. If the supply of raw materials can not keep up with the requirements of injection materials, it will also cause insufficient supply, in this regard, it should be checked whether the particle size of the raw materials is uniform, and there is no "bridge" phenomenon of raw materials at the bottom of the feeding port.
7. If the gap between the ejector rod and the barrel is too large, the melt will return along the gap, resulting in insufficient supply. This gap is too large caused by excessive wear, to replace the relevant parts to repair it.

4) Plastic products

1. If the thickness and length of the plastic parts are not proportional, the shape is complex, and the molding area is large, the plastic melt is easy to be blocked at the entrance of the thin wall part of the plastic part, so that the cavity is difficult to be filled, and the thickness of the plastic part and the limit flow length of the melt when filling the mold should be corrected.
2. The thickness of the plastic parts has a greater impact on the filling situation, if the thickness of the plastic parts is more than 8mm or less than 0.5mm, it is unfavorable to the molding, and the minimum wall thickness corresponding to the different plastic varieties is also different, and the wall thickness should be selected according to different plastic varieties.

5) Plastic raw materials

1. If the water content of raw materials is too large, they turn into steam when heated, their appearance increases the burden of the exhaust system, if not drained in time, it will make it difficult for the melt to fill the cavity resulting in insufficient filling, in this case, the raw materials that are easy to absorb moisture should be pre-dried.
2. Some raw materials contain more volatile substances, become volatile gas free after heating, if the exhaust is not enough to cause poor exhaust, making it difficult to fill the cavity, in this regard, it is necessary to pre-treat such raw materials in advance or use less such raw materials.
3. If too much recycled material is added to the raw material, their fluidity after melting is poor, affecting the fluidity of the melt and insufficient filling, in this regard, the amount of recycled material added should be strictly controlled.
4. The fluidity of plastic melt is greatly affected by the formula, such as lubricants have a greater impact on the fluidity of the melt. Too little addition, poor fluidity, easy to form insufficient filling, too much addition, easy to flow, and easy to cause insufficient supply caused by backflow in the material brief, in this regard, the amount of additives that can affect the fluidity of the melt should be controlled.
5. If the impurities contained in the raw material are too much, mainly refractory impurities, they are easy to block the flow channel or nozzle so that the melt flow is not smooth, difficult to fill the cavity, in this regard, should try to remove refractory solid impurities in the raw material or choose high-quality raw materials.