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What should I do if the injection molded product is short shot?
2025-06-19
Incomplete molding of injection molded products is a common problem, but it is also relatively easy to solve. When the process can't solve it, improvements can be made in mold design and manufacturing, which can generally be solved.
1. Equipment:
(1) The plasticizing capacity of the Injection Molding machine is small. When the product quality exceeds the actual maximum injection quality of the injection molding machine, it is obvious that the feed volume is insufficient. If the product quality is close to the actual injection quality of the injection molding machine, there is a problem of insufficient plasticization. The material is not heated enough in the barrel, and as a result, the appropriate molten material can't be provided to the mold in time.
In this case, the only solution is to replace the injection molding machine with a larger capacity. Some plastics, such as nylon (especially nylon 66), have a narrow melting range and a large specific heat, so an injection molding machine with a large plasticizing capacity is required to ensure the supply of materials.
(2) The temperature displayed by the thermometer is not real, and it appears high but is actually 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 circuit or the temperature difference millivoltmeter, or due to the aging or burning of the electric heating coil far away from the temperature measurement point, the heating failure has not been discovered or has not been repaired or replaced in time.
(3) The inner diameter of the nozzle is too large or too small. If it is too small, the specific volume of the material strip increases due to the small flow diameter, which is easy to cool, block the feed channel or consume injection pressure; if it is too large, the flow cross-sectional area is large, the unit area pressure of the plastic into the mold is low, resulting in a small injection force.
At the same time, non-Newtonian plastics such as ABS can't reduce viscosity due to lack of large shear heat, which makes it difficult to fill the mold. The nozzle and the main channel inlet are not well matched, and the mold often overflows and the mold can't be fully filled.
The nozzle itself has great flow resistance or is blocked by foreign matter, carbonized plastic deposits, etc.; the spherical surface of the nozzle or the main channel entrance is damaged or deformed, affecting the good cooperation with each other; the mechanical failure or deviation of the injection seat causes the nozzle and the main channel axis to tilt or the axial clamping surface to separate; the nozzle ball diameter is larger than the main channel entrance ball diameter, and there is a gap at the edge. The axial push force of the nozzle is gradually increased under the pressure of overflow, which will cause the product to be not fully injected.
(4) Plastic lumps block the feeding channel. Due to the partial melting and agglomeration of plastic in the hopper dryer, or the excessive temperature of the barrel feed section, or the improper selection of plastic grade, or the excessive lubricant contained in the plastic, the plastic will melt prematurely at the reduced diameter position of the feed port or in the deep groove at the screw start end. The pellets and the molten material will stick to each other to form a "bridge", blocking the channel or wrapping the screw, and sliding in a circle with the rotation of the screw, unable to move forward, causing interruption of feeding or irregular fluctuations. This situation can only be fundamentally solved after the channel is drilled through and the lumps are removed.
(5) Nozzle cold material into the mold. Injection molding machines are usually equipped with straight-through nozzles to take into account pressure loss. However, if the temperature of the front end of the barrel and the nozzle is too high, or if there is too much material stored at the front end of the barrel under high pressure, "drooling" will occur, causing the plastic to accidentally enter the main channel entrance before the injection starts and harden under the cooling effect of the mold, thereby preventing the molten material from entering the cavity smoothly.
At this time, the temperature of the front end of the barrel and the nozzle should be lowered, the amount of material stored in the barrel should be reduced, and 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't keep up in time, which will also cause material shortage, especially when the voltage fluctuates greatly. The cycle should be adjusted accordingly according to the power supply voltage. When adjusting, the injection and holding time are generally not considered. The main consideration is to adjust the time from the end of holding to the return of the screw, which will not affect the filling and molding conditions, and can extend or shorten the preheating time of the pellets in the barrel.
2. Mold
(1) The mold gating system is defective. The runner is too small, too thin or too long, which increases the fluid resistance. The main runner should increase its diameter, and the runner and branch runner should be rounded.
The runner or gate is too large, and the injection force is insufficient; the runner or gate is blocked by impurities, foreign matter or carbonized matter; the runner and gate are rough and have scars, or have sharp angles, and the surface roughness is poor, which affects the smooth flow of materials; the runner does not have a cold well or the cold well is too small, and the opening direction is wrong; for multi-cavity molds, the runner and gate size distribution must be carefully arranged to be balanced, otherwise only the cavities near the main runner or with thick and short gates can be filled while other cavities can't be filled.
The runner diameter should be appropriately enlarged to reduce the pressure drop of the molten material flowing to the end of the runner. The gate of the cavity far away from the main runner should also be enlarged to make the injection pressure and material flow rate of each cavity basically consistent.
(2) Irrational mold design. The mold is too complex, with many turns, improper feed port selection, too narrow runner, insufficient number of gates or improper form; the local cross-section of the product is very thin, the thickness of the entire product or the local area should be increased, or auxiliary runners or gates should be set near the insufficient filling area;
It is not uncommon for parts to be unsatisfactory due to poor venting measures in the mold cavity. This defect mostly occurs at the corners of the product, deep depressions, thin-walled parts surrounded by thick-walled parts, and the bottom of the thin bottom shell formed with side gates. The design to eliminate this defect includes opening effective venting channels, selecting a reasonable gate position to make air easy to vent in advance, and deliberately making a certain part of the air-trapping area of the cavity into an insert when necessary, so that air can overflow from the insert gap;
For multi-cavity molds, unbalanced gate distribution is prone to occur. If necessary, the number of injection cavities should be reduced to ensure that parts in other cavities are qualified.
3. Process
(1) Improper feed adjustment, lack of material or excess material. Inaccurate feed metering or abnormal operation of the feed control system, abnormal injection cycle due to limitations of the injection molding machine, mold or operating conditions, low pre-molding back pressure or low density of particles in the barrel may all cause lack of material. For particles with large particles and many voids, and plastics with large crystalline specific volume changes such as polyethylene, polypropylene, nylon, etc., and plastics with high viscosity such as ABS, a higher feed amount should be adjusted, and a larger feed amount should be adjusted when the material temperature is high.
When there is too much material at the end of the barrel, the screw will consume extra injection pressure to compact and push the excess material in the barrel during injection, which greatly reduces the effective injection pressure of the plastic entering the mold cavity and makes it difficult to fill the product.
(2) The injection pressure is too low, the injection time is short, and the plunger or screw retracts too early. The molten plastic has a high viscosity and poor fluidity at a relatively low working temperature, so it should be injected at a higher pressure and speed. For example, when making ABS colored parts, the colorant's inability to withstand high temperatures limits the heating temperature of the barrel, which requires a higher injection pressure than usual and a longer injection time to compensate.
(3) Slow injection speed. The injection speed is of great significance for products with complex shapes, large thickness variations, long process, and plastics with high viscosity such as toughened ABS. When high pressure can't fully inject the product, high-speed injection should be considered to overcome the problem of incomplete injection.
(4) The material temperature is too low. The temperature at the front end of the barrel is low, and the viscosity of the molten material entering the cavity rises too early to the point where it is difficult to flow due to the cooling effect of the mold, which hinders the filling of the far end; the temperature at the rear end of the barrel is low, and the plastic with high viscosity has difficulty flowing, which hinders the forward movement of the screw. As a result, it seems that the pressure displayed on the pressure gauge is sufficient, but in fact the molten material enters the cavity at low pressure and low speed;
If the nozzle temperature is low, it may be that the nozzle has been in contact with the cold mold for a long time during fixed feeding and has lost heat, or the nozzle heating ring has insufficient heat supply or poor contact, resulting in low material temperature, which may block the mold's feeding channel; if the mold does not have a cold well, use a self-locking nozzle and adopt a post-feeding procedure, the nozzle can better maintain the required temperature; when the nozzle is too cold when the machine is just started, sometimes a flame gun can be used for external heating to accelerate the nozzle heating.
4. Raw materials
Plastics have poor fluidity. Plastics factories often use recycled scraps, which tend to show a tendency to increase viscosity. Experiments have shown that due to the increase in the unit volume density of molecular chain breaks generated by oxidative cleavage, the viscosity of the flow in the barrel and cavity is increased. Recycled scraps promote the generation of more gaseous substances, increase the injection pressure loss, and cause difficulty in filling the mold.
In order to improve the fluidity of plastics, it is necessary to consider adding external lubricants such as stearic acid or its salts, preferably silicone oil (viscosity 300~600cm2/s). The addition of lubricants not only improves the fluidity of plastics, but also improves stability and reduces the gas resistance of gaseous substances.