This is a common problem, but it is also relatively easy to solve. When the process can not be solved, we can consider the improvement from the design and manufacture of the die, which can generally be solved.
I. Equipment:
(1) The plasticizing capacity of injection moulding machine is small. When the product quality exceeds the actual injection quality of the injection moulding machine, it is obvious that the material supply is insufficient. If the product quality is close to the actual injection quality of injection moulding machine, there will be a problem of insufficient plasticization, insufficient heating time in the barrel, the result can not provide the appropriate melt to the mould in time. This situation can only be solved by replacing the injection moulding machine with large capacity. Some plastics, such as nylon (especially nylon 66), have narrow melting range and larger specific heat. It requires a large plasticizing capacity injection moulding machine to ensure the supply of materials.
(2) The temperature shown by the thermometer is unreal and the material temperature is too low. This is due to the failure of temperature control devices such as thermocouples and their circuits or temperature difference millivoltmeters, or to the ageing or burning of electric heating coils far away from the temperature measuring points, which have failed to be found or replaced in time.
(3) The diameter of the inner hole of the nozzle is too large or too small. If the flow diameter is too small, the specific volume of the strip will increase, it will be easy to refrigerate, block the feeding passage or consume injection pressure; if the flow section is too large, the unit area pressure of the plastic feed die will be low, resulting in a situation of low injection force. At the same time, non-Newtonian plastics such as ABS can not make the viscosity decrease because of the lack of large shear heat, which makes filling difficult.
The mismatch between nozzle and main inlet often results in overflow and unsatisfactory filling in the die. The flow resistance of the nozzle itself is very large or blocked by foreign matter or plastic carbonized sediment; the spherical surface of the nozzle or main channel 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 axis of the main channel to produce tilt displacement or the axial compression surface to separate; the spherical diameter of the nozzle is larger than the spherical diameter of the main channel entrance, because of the gap at the edge, it is driven by the overflow. Increasing the axial push force of the nozzle will result in unsatisfactory injection of the product.
(4) Plastic frit blocked the feeding passage. Due to the partial melting and agglomeration of plastics in hopper dryer, the excessive temperature in the feeding section of barrel, the improper selection of plastics grade, or the excessive lubricant contained in plastics, the plastics will melt prematurely in the shrinkage position of feeding inlet or the deep groove of screw starting end. The bonding of granular and melt will form a "bridge" to block the passage or wrap the screw and rotate with the screw to make the circumference slippery. Motion, can not move forward, resulting in interruption or irregular fluctuation of feeding. This situation can only be fundamentally solved by drilling through the passage and removing the block.
(5) Injection of cold material into the nozzle. Injection moulding machines usually only have straight-through nozzles because of pressure loss. However, if the temperature of the front end of the barrel and the nozzle is too high or the material stored at the front end of the barrel is too much under high pressure, the "salivation" will occur, which will make the plastic accidentally rush into the main channel entrance and harden under the cooling effect of the template without starting injection while the mould is open, thus hindering the smooth entry of the melt into the cavity. At this time, the temperature of the front end of the barrel and the nozzle should be reduced, and the storage of the barrel should be reduced, so as to reduce the back pressure and avoid the excessive density of the melt at the front end of the barrel.
(6) Short injection cycle. Because of the short cycle, the material temperature can not keep up with the shortage, especially when the voltage fluctuation is large. The period should be adjusted according to the supply voltage. When adjusting, injection and holding time are generally not taken into account. The main consideration is to adjust the period from the end of holding pressure to the screw return, which does not affect the filling conditions, but also prolongs or shortens the preheating time of the particles in the barrel.
2. Mold aspect:
(1) The gating system of the mould is defective. The runner is too small, too thin or too long, which increases the fluid resistance. The diameter of the main channel should be increased, and the circularity of the flow channel and the diversion channel should be better. The runner or gate is too big, and the shooting force is insufficient; the runner and gate are blocked by impurities, foreign bodies or carbides; the runner and gate are rough and scarred, or have sharp angles, and the surface roughness is not good, which affects the material flow.
The runner does not have a cold material well or the cold material well is too small and the opening direction is not right. For the multi-cavity mould, the runner and the gate size distribution should be carefully balanced, otherwise only the cavity near the main runner or the gate thick and short can be filled, while other cavity can not be filled. The diameter of the runner should be appropriately increased to reduce the pressure drop of the melt flowing to the end of the runner, and the gate of the cavity far from the main runner should be increased so that the injection pressure and the material flow velocity of each cavity are basically the same.
(2) The die design is unreasonable. The die is too complex, with many turning points, inappropriate selection of inlet and outlet, too narrow runner, insufficient number or form of gate, and the partial section of the product is very thin, so the thickness of the whole product or part should be increased, or auxiliary runner or gate should be set near the filling deficiency.
It is common that the unsatisfactory parts are caused by inadequate exhaust measures in the die cavity. Most of these defects occur at the turning point, deep depression, thin-walled part surrounded by thick-walled part and the bottom of thin bottom shell formed by side gate.
The design of eliminating this defect includes opening an effective exhaust passage, choosing a reasonable gate position to make the air easy to discharge in advance, and specially making a part of the trapped area of the cavity into an insert when necessary, so that the air overflows from the gap of the insert; for the multi-cavity mould which is prone to the unbalanced distribution of the gate, the number of injection cavity should be reduced when necessary to ensure other types of mould. Cavity parts are qualified.
