Causes and solutions for incomplete molding of injection molded products This is a frequently encountered problem, but it is also easier to solve. When it can't be solved by the process, it can be improved from the design and manufacture of the mold, which can generally be solved.
First, the equipment:
(1) The plasticizing capacity of the injection molding machine is small. When the quality of the product exceeds the actual maximum injection quality of the injection molding machine, it is apparent that the supply amount is insufficient. If the quality of the product is close to the actual injection quality of the injection molding machine, there is a problem that the plasticization is insufficient, and the heating time in the barrel is insufficient, and as a result, the proper melting material cannot be provided to the mold in time. In this case, it is only necessary to replace the injection molding machine with a large capacity to solve the problem. Some plastics such as nylon (especially nylon 66) have a narrow melting range and a large specific heat, and an injection molding machine with a large plasticizing capacity is required to ensure the supply of the material.
(2) The temperature displayed by the thermometer is not true, and the height is low, causing the material temperature to be too low. This is due to the failure of the temperature control device such as the thermocouple and its line or the temperature difference millivoltmeter, or the aging or burning of the electric heating ring away from the temperature measurement point, and the heating failure has not been found or repaired and replaced in time.
(3) The diameter of the nozzle inner hole is too large or too small. Too small, because the flow diameter is small, the specific volume of the strip is increased, it is easy to be cold, the feed channel is blocked or the injection pressure is consumed; if it is too large, the flow cross-sectional area is large, and the pressure per unit area of ​​the plastic mold is low, forming a shot. The situation is small. At the same time, non-Newtonian plastics such as ABS are not able to obtain a large shear heat and cannot cause a decrease in viscosity to cause filling difficulties. The nozzle is not well matched with the main entrance, and the mold overflow is often caused, and the mold is not filled. The nozzle itself has a large flow resistance or is blocked by foreign matter, plastic carbonized deposits, etc.; the spherical surface of the nozzle or the main entrance is damaged and deformed, which affects the good cooperation with the other side; the mechanical failure or deviation of the injection seat causes the nozzle and the main shaft axis to tilt. Displacement or axial compression surface disengagement; nozzle ball diameter is larger than the entrance ball diameter of the main channel, due to the gap at the edge, gradually increasing the axial pushing force of the nozzle under the overburden of the overflow will cause the product to be not filled.
(4) The plastic frit blocks the feeding channel. Due to the local melting of agglomerates in the hopper dryer, or the temperature of the barrel feed section is too high, or the plastic grade is not properly selected, or the plastic contains too much lubricant, the plastic will enter the feed port to reduce the diameter or screw. The deep groove in the screw end is melted prematurely, and the pellet and the melt are bonded to each other to form a “cross bridgeâ€, which blocks the passage or encloses the screw, and slides along with the screw for circumferential sliding, which cannot be moved forward, causing interruption or irregular fluctuation of the supply. . This situation can only be solved after the channel is cut through and the block is removed.
(5) The nozzle cold material is fed into the mold. Injection molding machines usually only have straight-through nozzles for their pressure loss. However, if the front end of the barrel and the temperature of the nozzle are too high, or if the front end of the barrel is stored too much under high pressure, a "flowing" occurs, so that the plastic accidentally preemptively enters the main entrance when the injection is not started and the mold is opened. It hardens under the cooling of the stencil and prevents the melt from smoothly entering the cavity. At this time, the temperature of the front end of the barrel and the nozzle should be reduced and the amount of material stored in the barrel should be reduced. The back pressure should be reduced to avoid excessive density of the melt at the front end of the barrel.
(6) The injection molding cycle is too short. Due to the short cycle, the material temperature can not keep up with the lack of material, especially when the voltage fluctuation is large. The period should be adjusted according to the supply voltage. During the adjustment, the injection and holding time are generally not considered. The main consideration is to adjust the period from the completion of the pressure holding to the return of the screw, which does not affect the filling molding conditions, and can prolong or shorten the preheating time of the pellet in the barrel.
Second, the mold aspect
(1) The mold casting system is defective. The flow path is too small, too thin or too long, increasing fluid resistance. The main channel should be increased in diameter, and the flow path and the branching path should be round. The flow channel or the mouth is too large, the radiation is insufficient; the flow channel and the gate have impurities, foreign matter or charcoal blockage; the flow channel and the gate are rough and have flaws, or have sharp angles, and the surface roughness is poor, which affects the flow; There is no cold or cold well in the road, and the direction is not right. For the multi-cavity mold, the balance of the flow channel and gate size should be carefully arranged. Otherwise, there will be only the main channel or the gate is thick and short. The cavity can be filled while the other cavity cannot be filled. The diameter of the flow channel should be appropriately increased to reduce the pressure drop of the melt flowing to the end of the flow channel, and the gate of the cavity farther away from the main flow channel should be increased, so that the injection pressure and the flow velocity of each cavity are substantially the same.
(2) The mold design is unreasonable. The mold is too complicated, the turning is too much, the feeding port is improperly selected, the flow path is too narrow, the number of gates is insufficient or the form is improper; the partial section of the product is very thin, the thickness of the whole product or part should be increased, or the auxiliary is set near the insufficient filling place. Flow path or gate; the phenomenon that the exhausting measures in the cavity are not enough to cause dissatisfaction of the parts is not uncommon. Most of the defects occur in the corners of the product, deep depressions, thin-walled parts surrounded by thick-walled parts, and The bottom of the thin bottom case formed by the side gate. The design to eliminate this defect includes the establishment of an effective venting channel, the selection of a reasonable gate location to allow the air to be easily discharged in advance, and if necessary, a part of the trapped area of ​​the cavity is made into an insert to allow air to pass from the insert. Gap overflow; for multi-cavity molds, the imbalance of gate distribution is prone to occur. If necessary, the number of injection cavities should be reduced to ensure that other cavity parts are qualified.
Third, the process aspect
(1) Improper adjustment of feed, lack of material or more materials. Inaccurate metering or improper operation of the feed control system, injection molding machine or mold or operating conditions, resulting in abnormal injection cycle, small pre-plastic back pressure or small density of material in the barrel may cause material shortage, for large particles, Plastics with large voids and crystallographic specific changes such as polyethylene, polypropylene, nylon, etc., and plastics with higher viscosity, such as ABS, should be adjusted to a higher amount. When the temperature is high, the amount of material should be adjusted.
When there is too much material stored at the end of the barrel, the screw consumes an extra injection pressure to compress and push the excess material in the barrel during injection, which greatly reduces the effective injection pressure of the plastic entering the cavity. The product is difficult to fill.
(2) The injection pressure is too low, the injection time is short, and the plunger or screw is retracted too early. Molten plastics have higher viscosity at lower operating temperatures and poor fluidity and should be injected at higher pressures and speeds. For example, in the case of ABS color parts, the high temperature resistance of the colorant limits the heating temperature of the barrel, which is compensated by a higher injection pressure than usual and an extended injection time.
(3) The injection speed is slow. The injection speed has great significance for some products with complex shapes, large thickness and long process, long process, and plastics with high viscosity such as toughness ABS. When high pressure is not enough to fill the product, high-speed injection should be considered to overcome the problem of dissatisfaction.
(4) The material temperature is too low. The temperature at the front end of the barrel is low, and the melt entering the cavity causes the viscosity to rise prematurely to the point where it is difficult to flow due to the cooling effect of the mold, which hinders the filling of the distal end; the plastic in the rear part of the barrel has a low temperature and a high viscosity. The difficulty of flow hinders the advancement of the screw. As a result, it seems that the pressure displayed by the pressure gauge is sufficient, and actually the melt enters the cavity at low pressure and low speed; the low nozzle temperature may be the contact of the nozzle with the cold mold for a long time during fixed feeding. Loss of heat, or insufficient heating of the nozzle heating ring or poor contact, resulting in low material temperature, may block the feeding channel of the mold; if the mold does not have a cold well, use a self-locking nozzle, using a post-feeding program, the nozzle can maintain the necessary The temperature of the nozzle is too cold when the nozzle is turned on. Sometimes the flame gun can be used for external heating to accelerate the nozzle temperature.
Four raw materials
Plastics have poor fluidity. Recycled scrap is often used in plastics plants, and recycled scrap tends to reflect a tendency to increase viscosity. The experiment indicates that the molecular bulk density of the molecular scission due to oxidative cleavage increases, which increases the viscosity of the flow in the barrel and the cavity. The regenerated waste material promotes the generation of more gaseous substances and causes the injection pressure loss. Increased, making filling difficult. In order to improve the fluidity of the plastic, it is considered to add an external lubricant such as stearic acid or a salt thereof, preferably a silicone oil (viscosity of 300 to 600 cm 2 /s). The addition of the lubricant not only improves the fluidity of the plastic, but also improves the stability and reduces the gas barrier of the gaseous substance. Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn
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