(1) Defect characteristics Insufficient filling of plastic product in the plastic manufacturing process is a common defect. Its main manifestation is: the cavity is not completely filled, making the plastic parts not full, and the shape is incomplete.
(2) Causes of defects and elimination methods
1) Injection mold
- The mold produced by the plastic factory has poor venting structure. When the molten plastic is injected into the mold cavity, the gas in the mold cavity should be discharged in time. If it cannot be discharged in time, the molten plastic will be difficult to fill the mold cavity, resulting in insufficient filling. For this, the venting structure needs to be corrected or improved.
- The runner is blocked by impurities, which prevents the molten plastic from reaching all parts of the mold cavity smoothly and causes insufficient filling. The runner needs to be cleared in time.
- The injection mold produced by the injection molding manufacturer has an unbalanced gate. The quality of plastic parts in each cavity is not proportional to the size of the gate. The molten plastic cannot fill the cavity at the same time, which is easy to cause insufficient filling. Therefore, the balance of the gate should be guaranteed.
- If the cross-sectional area of the gate or runner is small and long, the pressure loss of the molten plastic increases, the flow resistance increases, and it is easy to have poor flow and insufficient filling. Therefore, the gating system needs to be corrected and improved.
- If the position of the gate is set at the thinner part of the plastic part, it is easy to form poor flow and insufficient filling. Therefore, the position of the gate should be changed and set at the thick part.
- If there is no cold feed well in the mold or the cold feed well is designed unreasonably, the cold material will enter the mold cavity and solidify first, which will cause the molten plastic to flow smoothly and insufficient filling. Therefore, the reasonable design of the cold feed well is very important.
1) Injection molding process
- When the plastic manufacturer custom plastic product, the mold temperature is too low. When the high-temperature plastic melt enters the low-temperature mold cavity, it will be unable to fill the mold cavity due to rapid cooling. Therefore, the mold temperature should be checked or adjusted to be within the normal value range.
- The injection pressure is too low. The injection pressure and the filling length are approximately in a positive proportional relationship. If the injection pressure is small, the filling length is also short, and the mold cavity is difficult to fill. For this, the injection pressure should be appropriately increased.
- The holding time is too short, which causes the molten plastic in the mold cavity to flow back and cause insufficient filling. For this, the holding time should be reasonably controlled. The holding time is longer for thick-walled products, and shorter for thin-walled products, which is generally controlled between 30 and 120s.
- The temperature of the molten plastic is too low, the fluidity is reduced, the filling length is shortened, and it is difficult to fill the mold cavity. For this, the temperature of the molten plastic should be increased.
- The injection speed is too slow, the molten plastic flows slowly, it is easy to cool, and the fluidity of the molten plastic is further reduced, resulting in insufficient filling. For this, the injection speed should be appropriately increased.
- injection molding equipment
- 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 part, it will inevitably not fill the mold cavity and cause insufficient filling. To this end, the total injection volume should not exceed 85% of the plasticizing volume of the injection molding machine, and the quality of the plastic part should not exceed this injection total volume.
- If the nozzle is blocked by foreign objects, it will cause the plastic melt to flow smoothly and cause insufficient filling. To this end, the nozzle must be cleared.
- If the nozzle hole is too small, the filling speed of the plastic melt will be reduced, and it will be difficult to fill the mold cavity. To this end, a nozzle with a larger diameter should be replaced.
- If the check valve fails, it will cause the plastic melt to reflux, causing insufficient supply of the melt. To this end, the check valve should be repaired.
- If the injection stroke is not enough, the injection volume will also be insufficient, and it will be difficult to fill the mold cavity. To this end, the injection stroke should be appropriately increased.
- If the supply of raw materials does not keep up with the injection requirements, it will also cause insufficient supply. To this end, check whether the raw material particles are uniform, and whether there is a "bridge" phenomenon at the bottom of the feed port.
- If the gap between the feed rod and the barrel is too large, the melt will flow back along the gap, causing insufficient supply. This large gap is caused by excessive wear. To this end, the relevant parts should be replaced and repaired.
4) Plastic Products
- If the thickness and length of the plastic parts are not proportional, the shape is complex, and the forming area is large, the plastic melt is easy to be blocked at the entrance of the thin wall of the plastic parts, making it difficult to fill the mold cavity. To this end, the thickness of the plastic parts and the maximum flow length of the melt when filling the mold should be corrected.
- The thickness of the plastic part has a great influence on the filling condition. If the thickness of the plastic part exceeds 8 mm or is less than 0.5 mm, it is not conducive to molding. The minimum wall thickness corresponding to different plastic varieties is also different. The wall thickness should be selected according to different plastic varieties.
5) Plastic raw materials
- If the moisture content of the raw material is too high, they will be vaporized into water vapor when heated. Their appearance increases the burden on the venting system. If they cannot be vented in time, it will make it difficult for the melt to fill the mold cavity and cause insufficient filling. To this end, the raw materials that are easy to absorb moisture should be pre-dried.
- Some raw materials contain a large amount of volatile substances. When heated, they become volatile gases and are released. If venting is not sufficient, it will lead to poor venting, making it difficult to fill the mold cavity. To this end, such raw materials should be pre-treated or used less.
- If too much recycled material is added to the raw material, they have very poor fluidity after melting, which affects the fluidity of the melt and causes insufficient filling. To this end, the addition of recycled material should be strictly controlled.
- The fluidity of the plastic melt is greatly affected by the formulation. For example, the lubricant has a great influence on the fluidity of the melt. If it is added too little, the fluidity is poor, and it is easy to form insufficient filling. If it is added too much, it is easy to flow and return to the barrel, causing insufficient supply. To this end, the amount of additives that can affect the fluidity of the melt should be controlled.
- If the raw material contains too many impurities, mainly infusible impurities, they are easy to block the flow path or the nozzle, making the melt flow smoothly and difficult to fill the mold cavity. To this end, the infusible solid impurities in the raw material should be removed as much as possible or high-quality raw materials should be purchased.