First, the equipment aspect
(1) The plasticizing capacity of the injection molding machine is small
If the quality of the product exceeds the actual maximum injection quality of the injection molding machine , the supply amount will not be sufficient; if the quality of the product is close to the actual injection quality of the injection molding machine, the plasticization is insufficient, and the heating time in the barrel is insufficient, and the mold cannot be properly provided in time. Melt. It is necessary to replace the injection molding machine with a large capacity.
(2) The temperature displayed by the thermometer is not true, and the height is low, causing the material temperature to be too low.
Heating failure occurs due to failure of temperature control devices (such as thermocouples and their lines or temperature differential millivoltmeters), or aging or burning of the heating coils away from the temperature measurement point. Need to repair and replace 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.
If ABS does not obtain a large shear heat, the viscosity cannot be lowered, resulting in difficulty in filling the mold. 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) Plastic frit blockage feeding channel
Due to the local melting and agglomeration of the plastic in the hopper dryer, the temperature of the barrel feeding section is too high, the plastic grade is improperly selected, and the excessive amount of lubricant contained in the plastic will cause the plastic to enter the feeding port to reduce the diameter or screw the screw end. Premature melting in the deep groove causes the pellets and the melt to adhere to each other to form a “crossing bridgeâ€, blocking the passage or enclosing the screw, and sliding 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 fundamentally after the channel is cut and the block is removed.
(5) nozzle cold material into the mold
Injection molding machines usually only contain straight-through nozzles due to pressure loss, but if the front end of the barrel and the nozzle temperature are too high, or too much material is stored at the front end of the barrel under high pressure, "flowing" occurs, so that the plastic does not start to be injected. When the mold is open, it accidentally preemptively enters the entrance of the main channel and hardens under the cooling effect of the template, thereby preventing 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 multiple 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 returned 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) Slow injection speed
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. Difficulty in flow hinders the advancement of the screw, resulting in a pressure that appears to be sufficient for the pressure gauge to actually enter the cavity at low pressure and low speed.
If the nozzle temperature is low, it may be that the nozzle is in contact with the cold mold for a long time, and the heat is insufficient, or the nozzle heating coil is insufficiently heated or the contact temperature is low, which may block the feeding passage of the mold; if the mold does not have cold material Wells, with self-locking nozzles, using the post-feeding procedure, the nozzles can maintain the necessary temperature; when the nozzles are too cold, the nozzles can be heated externally to accelerate the nozzle temperature.
Fourth, raw materials
Mainly because of the poor fluidity of plastics. Recycled scrap is often used in many plastics plants, and recycled scrap tends to reflect a tendency to increase viscosity. In order to improve the fluidity of plastics, it is considered to add an external lubricant such as stearic acid or its salts, preferably silicone oil (viscosity 300~600cm2/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.
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