Incomplete Plastic Molding of Products Made with Injection Moulding

This is a frequently encountered problem, but it is also relatively easy to solve. When the problem cannot be solved by technological means, it can be considered to be improved from the design and manufacturing of the mold, which is generally solvable.

Equipment aspects of products made with injection moulding

The temperature shown by the thermometer is not true, it is high but actually low, causing the material temperature to be too low

This is because the temperature control device such as the thermocouple and its circuit or the temperature difference millivoltmeter malfunctioned, or because the electric heating circle far away from the temperature measurement point is aging or burned, and the heating is ineffective and has not been discovered or repaired and replaced in time.

The inner hole diameter of the nozzle is too large or too small

If it is too small, due to the small flow diameter, the specific volume of the material strip increases, which is easy to cool down, block the material inlet channel or consume the injection pressure; if it is too large, the flow cross-sectional area is large, and the unit area pressure of the plastic entering the mold is low, forming a situation of small injection force.

At the same time, non-Newtonian plastics such as ABS cannot make the viscosity decrease due to the lack of large shear heat, which makes it difficult to fill the mold. The poor matching between the nozzle and the main flow channel entrance often causes the phenomenon of overflow outside the mold and the mold cavity is not fully filled. The nozzle itself has a large flow resistance or foreign objects, plastic carbon deposits and other blockages;

The spherical surface of the nozzle or the entrance of the main flow channel is damaged or deformed, affecting the good fit with the other side; pay attention to mechanical faults or deviations, causing the nozzle and the main flow channel axis to tilt or the axial pressure surface to detach. The nozzle sphere diameter is larger than the main flow channel entrance sphere diameter, and there is a gap at the edge, which gradually increases under the pressure of overflow and extrusion, which will cause the products made with injection moulding to be incompletely filled.

The plastic melt block blocks the feeding channel

Because the plastic is partially melted and agglomerated in the hopper dryer, or the temperature in the feeding section of the barrel is too high, or the plastic grade selection is improper, or there is too much lubricant in the plastic, it will cause the plastic to shrink when entering the feeding port. The position or the deep groove at the screw end melts prematurely, and the granules and the melted material are bonded to form a "bridge", which blocks the channel or wraps around the screw. When it rotates with the screw, it cannot move forward, causing feeding interruption or irregular fluctuations. This situation can only be fundamentally solved by chiseling the passage and removing the agglomerates.

The injection molding cycle is too short

Due to the short cycle, the material temperature cannot keep up and it will also cause material shortage. This is particularly evident when the voltage fluctuates greatly. The cycle should be adjusted according to the power supply voltage. When adjusting, injection and holding time are generally not considered, and the time from the end of holding pressure to the screw return is mainly considered. It does not affect the filling and molding conditions, but also can extend or shorten the preheating time of the material in the barrel.

Mold aspects of products made with injection moulding

There are defects in the mold pouring system

The runner is too small, too thin or too long, which increases the fluid resistance. The main runner should increase the diameter, and the runner and branch runner should be round. The runner or gate is too large, and the injection force is insufficient; the runner and gate are blocked by impurities, foreign objects or carbonized substances; the runner and gate are rough and have scars or sharp corners, and the surface roughness is poor, affecting the smooth flow of the material; the runner does not have a cold material well or the cold material well is too small, and the opening direction is incorrect;

For multi-cavity molds, the balance of the size distribution of runners and gates should be carefully arranged, otherwise the situation will arise where only the cavity near the main runner or the cavity with rough and short gates can be filled while the other cavities cannot be filled. The diameter of the runner should be appropriately increased, so that the pressure of the molten material to the end of the runner is reduced. The gates of the cavities far away from the main runner should also be enlarged to make the injection pressure and material flow velocity of each cavity basically the same.

The mold design is unreasonable

The mold is excessively complex, with many turns, improper selection of the feeding port, too narrow channel, insufficient number or inappropriate form of the gate; the local section ofproducts made with injection mouldingis very thin, and the overall thickness of the product or local thickness should be increased, or auxiliary runners or gates should be set near the under-filled area. The lack of exhaust measures in the mold cavity causing the phenomenon of incomplete formation of the product is quite common. This defect mostly occurs at the turning points, deep recesses, thin-walled parts surrounded by thick walls, and the bottom of the thin-bottomed shell molded by side gates.

The design to eliminate this defect includes opening effective exhaust passages, selecting reasonable gate positions to facilitate air discharge in advance, and specially making the local part of the mold cavity where trapped air is prone into a liner, so that the air overflows from the gap of the liner; for multi-cavity molds, uneven gate distribution is prone to occur, and the number of injection cavities should be reduced if necessary to ensure that other mold cavities produce qualified parts.