12 common defects and solutions in bottle cap compression molding


Common defects and solutions in bottle cap compression molding

First. Spill

Overflow material, as the name implies, is that the pellets cut during the molding process cannot be completely put into the mold bowl, causing a part of the pellets to fall on the edge of the mold bowl, and forming a redundant part after the mold is closed. The biggest risk of spillage is that it may enter the beverage during filling, and larger spillage may also cause the filling line to jam.


Adjust the blowing of the material particles, check and clean the cutter, and adjust the star wheel turntable if necessary, so that it is accurately aligned with the mold, and try to make the material particles fall into the middle of the mold bowl.

Of course, the speed of the capping machine is very fast, and it is not easy for the naked eye to accurately determine the position and status of the pellets. There are many strobe lights on the market now, and the frequency of the flash can be adjusted manually. The price is usually around a few hundred dollars, such as capping The speed of the machine is 25000pcs / hour, which is equivalent to about 420pcs / minute. Therefore, we set the strobe speed to about 420pcs / min. At this frequency of the light, we can clearly see the movement track and state of the material particles. Quickly eliminating problems plays a big role.

Second. Water feature on bottle cap surface

The reason for the formation of water marks is that there is moisture on the surface of the mold, which is common when the machine is turned on. Due to the fast production speed of bottle caps, it needs to be formed at a lower mold temperature (10-20 degrees Celsius). Condensation water will form on the surface of the mold after shutdown. After the molten material falls into the mold bowl, the position that first touches the mold bowl will quickly cool in combination with moisture. After combining with water vapor, the cooling effect will be inconsistent with other positions, and the surface will form a gas pattern. .


1. Use compressed air to blow off the moisture on the surface of the mold before starting up, and then raise the temperature of the mold to make the moisture on the surface of the mold evaporate and then reduce to normal temperature to start production.

2. For those with strong capital, in order to ensure product quality more stably, it is recommended to consider installing a dehumidifier. Several domestic bottle cap factories will install dehumidifiers on the machines.

Third. The sealing plug has shrinkage pits

Some bottle cap factories are complaining and often receive complaints from customers about the poor sealing effect of the bottle caps. Water and air leaks lead to customer claims. When the cap was cut open, it turned out that the inner plug had shrinkage pits.

The shrinkage pits in the sealing plug are caused by poor cooling, which has a lot to do with the cooling design of the mold. Generally, there is no problem in the early stage of production, but as the production time is long, the heat will accumulate when the cooling water path is slightly poor. The more the pit problem appears.

Forth. The bottle cap is sunken

The normal surface of the bottle cap is usually relatively smooth. If the dent of the cap is found to be large during the production process, you must immediately find the number of the cap with the large dent and stop to check the corresponding mold. Generally speaking, the bottle cap is caused by depression due to poor cooling and large shrinkage.


1. Check whether the chiller is working normally and whether the actual temperature is consistent with the set temperature.

2. Shutdown and check the water path of the mold

3. If the mold that has been depressed for a long time is continuously produced, the mold will expand due to excessive heat accumulation, and the sliding parts will be stuck or burned to damage the mold. The bottle cap factories on the scale will require online technicians or quality inspectors to inspect the caps strictly according to the prescribed frequency.

Fifth. Bottle cap color mixing

Bottle cap color mixing is a headache for bottle cap factories. It usually occurs when the color is changed, and it takes a lot of materials to completely clean the previous color. Here, it is recommended that the bottle cap factory purchase some screw cleaning agents, such as brands such as Ai Sujie, mixed with plastic at a ratio of 10%, and increase the temperature of the screw to let it stay in the screw for about 15 minutes before extruding. This can significantly shorten the time for color transfer and reduce the cost of color conversion materials.

Sixth. Lack of thread or internal and external plugs

If the thread of the cap or the inner and outer plugs are suddenly missing during the normal production process, the reason is that the upper mold has leaked or seeped water, and the mold must be stopped immediately to check the seal ring.

This happens mostly because the sealing ring is not installed correctly or the sealing ring used is damaged when installing and maintaining the mold. If a mold leaks and is not found, it will cause a large number of bottle caps to be scrapped or need to be reworked. In order to reduce the chance of leaks, I will introduce a good suggestion:

The principle of using a mold water pressure test device is very simple. Manual pressure increases the pressure in the mold water channel. The mold after each maintenance is pressurized under the pressure test device for about half a minute to observe whether the mold leaks. Prove that the sealing effect is very good. Or simply connect a water channel specially used for testing molds in the cooling water channel of the factory, and use the corresponding quick connectors to test the molds.

Seventh. The overall size of the bottle cap is too large or too small

Molten pellets will be cooled and shaped after compression molding. During this process, the cap will shrink and the overall size will shrink. Based on this principle, summarize the following directions to solve the problem:

If the overall size of the bottle cap is too large, we can increase the cooling water temperature of the upper and lower molds, or appropriately increase the melting temperature to make the back shrinkage of the bottle cap larger, and the overall size of the bottle cap will be smaller.

If the overall size of the bottle cap is too small, we can lower the cooling water temperature of the upper and lower molds, or appropriately lower the melting temperature to make the back shrinkage of the bottle cap smaller, and the overall size of the bottle cap will be larger.

Eighth. Bottle cap drawing

Bottle cap drawing means that the edge of the cap has more filaments, which is easy to enter the beverage and has a greater risk. If this kind of problem occurs, you need to check whether there is a gap in the cutter, and check whether the height of the nozzle is suitable. As long as the material to be cut is a complete small column, this problem can be avoided.

Ninth. The side wall of the bottle cap is scratched

Side wall scratches are usually caused by poor demolding or poor alignment of the turntable and the mold. For example, when the demoulding is not good, the bottle cap will tilt away from the upper mold, and the time from falling to loading will be delayed, causing the turntable to scratch the wall of the cap. If this problem occurs, you need to check the upper mold and adjust the demolding air pressure to ensure that the cap is balanced and demolded.

Tenth. The surface of the bottle cap material grain printing

The traces of pellets are caused by uneven cooling, and the molten pellets first contact with the surface of the mold to cool sharply and form traces. Material traces can only be reduced in the compression molding process, but generally cannot be eliminated. To alleviate this phenomenon, the most effective way is to raise the temperature of the lower mold and slow down the degree of rapid cooling. The effect is very obvious. The bottle cap factory can try it.

On the other hand, in the years of experience, it is found that the traces of the bottle cap material produced by the large-sized nozzle are relatively slight. When the nozzle size is large, the cut pellets appear thick and short. When the thick and short pellets fall into the mold bowl, the contact surface with the mold is large. In this way, most of the marks on the surface of the cap produced are scattered to the side wall of the cap. The marks left on the top of the bottle cap become less noticeable.

Eleventh. The anti-theft ring and cover are too brittle and easy to crack

Once a bottle cap factory encountered customer complaints, a batch of bottle caps was easy to crack after packaging, especially the anti-theft ring part. The bottle cap factory is quite puzzled. It has been shipped normally for several years without any complaints. The raw materials and production process are unchanged. How can there be a cap or crack suddenly?

Later analysis found that the weld line of the anti-theft ring part of the bottle cap is very obvious. Obviously, the weld line is the weakest part of the bottle cap. The mold temperature is too low, which causes the plastic flow end to be effectively fused and molded. Usually the mold is produced after a few minutes. There is no such problem after the temperature rises. Later, after investigation, it was found that the batch of bottle caps was not discharged and inspected by employees when they were just turned on. Therefore, in order to completely avoid this problem, the cap factory needs to carefully check the quality of the cap a few minutes before starting production.

Twelfth. Lock wing lack of material

The wall thickness of the lock wing is usually thin, and it is difficult for molten plastic to pass smoothly. Generally speaking, you can consider raising the melting temperature and mold temperature, or increasing the molding pressure. In addition, the thickness of the lock wing edge should be considered in the design of the bottle cap, and the thickness should be increased as much as possible to ensure the folding effect, so as to ensure that there is a larger molding window during production.

12 common defects and solutions in bottle cap compression molding
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