During plastic injection molding, there will be some problems on plastic parts, such as the color, gloss, size, filling shortage, uneven marks. Filling shortage problem is due to material is solid before filling the cavity. I will talk about some of my own opinions as below:
1. Injection molding machine
(1) Lack injection capacity
This is due to overestimation of the capacity of the injection machine, due to insufficient plasticizing capacity or insufficient injection volume can also occur. However, the insufficient plasticizing ability can be improved by extending the heating time, increasing the screw revolution and increasing the back pressure. If the injection volume is insufficient, the problem cannot be solved without replacing the machine with a large injection volume. For example, the weight of the product is 150g, and the machine with CJ80M3V will not work, and the maximum injection amount of CJ80M3V is 120-130g.The plasticizing amount or heating rate of the machine is uncertain, and the machine with large plasticizing amount and heating power should be selected. The actual injection amount is insufficient due to the material return (that is, reverse flow) caused by the wear of the screw and the barrel or the glue (the screw does not leave the material pad at the end of the stroke); The actual temperature of the barrel is too low due to the failure of heating system such as thermocouple or heating ring. The sealing element of the injection cylinder is worn, causing oil leakage or backflow and failing to reach the required injection pressure; The inner hole of the nozzle is too small, the part of the nozzle is sealed or the center of the nozzle is improperly adjusted, resulting in excessive resistance and pressure consumption or gap, resulting in overflow.
(2) Lack clamping force
Although it can be considered that clamping force is not related to filling shortage, it is sometimes the cause of filling shortage. Even if the injection volume of the same molding machine, sometimes there will be insufficient clamping force phenomenon. If the clamping force is insufficient, the moving die is slightly backward under the action of injection pressure, which will produce a flash burr and make plastic filling shortage, and also produce filling shortage phenomenon.
(3) Lack plastic supply
Although the injection machine capacity is sufficient, the injection of molten material from the nozzle does not reach the required amount, which can also produce filling shortage. The first reason is that the plastic sticky edge of the hopper can not fall into the barrel (because the plastic is partially melted and caked in the hopper dryer, so that the powder or irregular particles can not enter the hopper; Adsorbed on the wall of the cylinder because of electrostatic action, that is, bridge phenomenon); One is that when the screw injection molding machine is used, the plastic slips in the barrel and cannot move forward (caused by improper selection of plastic grade and excessive lubricant of granular material, such as changing to the correct ratio of raw materials can be solved).
2. Mold design is unreasonable
(1) Multi-cavity mold
The flow imbalance of each cavity caused by insufficient local filling If the injection molding capacity of the molding machine is sufficient, this defect is caused by the uneven flow imbalance of the aperture of the gate, that is, the unbalanced distribution of the mold cavity. The wall thickness of the parts is too thin, resulting in excessive pressure consumption and weak mold filling. The wall thickness of the whole part or part should be increased or an auxiliary runner or gate can be set near the filling shortage. Sometimes only the thick and short cavity near the main runner or gate can be completely filled, and the rest of the cavity is defective. This defect can be eliminated by achieving gate balance, that is, increasing the runner diameter, reducing the pressure drop flowing to the end of the runner, and increasing the gate farther away from the main runner cavity. The flow path of the mold is too small, resulting in pressure loss. Too large will appear the glue weakness; Too rough will cause dissatisfaction with the product. The size of the runner should be appropriately set, and the transition between the main channel and the diverter, the gate or the turn itself should be appropriately rounded.
(2) The melt flow is too large
The flow resistance is too large hinder the flow of molten material are nozzles, main runner, runner, gate and thin wall of the parts. The flow resistance of the nozzle can be reduced by increasing the diameter of the nozzle, increasing the temperature of the nozzle, and using the nozzle with less flow resistance. For the main runner, the diameter can be increased: for the runner, the semicircular runner with large flow resistance should be avoided, and the circular or trapezoidal runner should be used, if the diameter is increased, it is better, and the length must be the shortest.As for the filling shortage caused by too thin wall thickness, the entire wall thickness can be increased or the partial wall thickness can be increased, or auxiliary runner or gate can be installed near the filling shortage place to solve the problem. In particular, the molten material injected first by the nozzle is cooled by the main runner and runner, resulting in greater flow resistance, in which case a large cold hole should be opened. If the flow resistance increases due to the difficulty of feeding due to the low temperature of the local or overall mold, it is OK to increase the mold temperature. Changing the location of the cooling water inlet and outlet according to the specific situation of the mold, or changing the flow path of the cooling water will also achieve good results (because the mold temperature is not uniform).
(3) Poor venting in cavity
When filling the mold cavity, air is inserted, resulting in reverse pressure. When the molten material is injected into the mold cavity, the mold cavity is often closed at the beginning of the molten material, and there is residual air in the local unfilled place. Also because the filling is too fast, the air is sometimes too late to overflow from the parting surface and is compressed, resulting in local unfilled plastic and filling shortage of the formed parts. This defect is particularly prone to appear in corners of the mold cavity, deep depressions and thin-walled parts surrounded by thick-walled parts. That is, it mostly occurs in the thin-bottomed shell parts and the head of the long convex table when the side gate is formed.(3) Such enclosed air is adiabatically compressed to high temperatures, sometimes scorching parts (see burns and black streaks). The measure to eliminate this defect is to reduce the injection speed and give a certain exhaust time. If the air in the mold cavity is removed by a vacuum pump, it is generally very effective. The best way is to set the exhaust duct, select the gate position to make the air easy to discharge first, or consider the exhaust mode from the mold structure. If the mold cavity is partially made into an insert, the air is spilt from the insert gap, or the shallow slot is opened on the parting surface, and the gap of the ejector rod can be used to exhaust. It can make full use of the gap exhaust of the screw or reduce the clamping force by using the parting surface exhaust, and open the exhaust channel or air hole if necessary.
3. Improper process adjustment
(1) Improper injection molding process the temperature of the injection cylinder is too low, the injection speed is insufficient, the injection time difference is too short, and the back pressure is too small, resulting in a shortage of plastics.
(2) Excess plastic supply If too much plastic enters the barrel, the injection pressure is lost due to the compression of the particles, thus reducing the pressure necessary for injection molding to shoot the molten material from the nozzle, resulting in insufficient injection pressure. The solution is to adjust the quantity of feed, that is, the amount of melt glue. Make it just right for the amount of plastic needed for molding.
(3) The production cycle is unstable, the regular stop and start, and the production is not carried out in accordance with the normal production cycle, which leads to the storage time of some plastics in the cylinder is too long, the density and viscosity are reduced, and the plastic is short of material.
(4) The mold temperature is not appropriate, resulting in reduced injection speed and plastic shortage.
4. plastic selection
(1) Poor plastic fluidity
If the plastic fluidity is not good, it has been condensed before it flows to the end of the cavity or does not flow to the overflow tank, so it often causes filling shortage.
In order to eliminate this defect, the melt temperature can be increased, the mold temperature can be increased, the injection pressure can be increased, the injection speed can be accelerated, and the melt can flow to the end of the cavity before the melt condenses. In this case, the good flow of plastic is particularly important, so the replacement of good flow of plastic is also a solution. The length of the plastic flow is too large for the plastic to properly fill the injection parts. Change the position of the gate to reduce the length of the plastic flow.
(2) Try to make the return material and raw materials evenly mixed
If the mixing is not uniform, because the return particle is larger, and the density is larger than the raw material, it is easy to lead to a reduction in the amount of glue, and the amount of injection is naturally reduced.
5. Product design is unreasonable
In the design process of plastic products, we must follow the principle of uniform wall thickness. If the uniform wall thickness cannot be maintained, please change the mold design in time. Add diverting channels or reinforcing bars in difficult areas to avoid the defect of material shortage caused by too thin wall thickness.
In short, there are many reasons for the defect of filling shortage, and these aspects are mutually restricting and influencing each other. Reducing and correcting this defect requires an overall grasp of the relationship between these aspects to adjust, and it requires continuous practice and accumulation of experience to quickly find out the causes of filling shortage, so as to reduce the waste of resources and improve the qualified rate of products.
