Common problems and solutions of blow molding molds

1、 Transition

It reflects the change in bottle wall thickness from thick to thin, such as the thickness change from the shoulder to the middle, or from the bottom to the middle. Solution to common problems: Symptoms of incomplete bottom molding, unsatisfactory contour of petal bottom, although slight incomplete bottom molding has little impact on filling, when it is severe, the stability of the bottle becomes a problem. It should be noted that it is not common for containers formed by blow molding molds to be perfect.

Diagnosis: Since not every product in the mold cavity is of poor quality, we assume that the cause is not simply attributed to insufficient blowing pressure, but rather to the possibility that the problematic mold cavity is due to insufficient blowing. If the temperature of the preform during blowing is lower and the blowing of the film bubbles is slower, the problem will be even more obvious. The light bottom also means that the material has reached its natural tensile limit before the entire container is formed.

Prescription:

1. Check if the high-pressure filter on the side of the mold is unobstructed to prevent it from affecting the supply of high-pressure gas;

2. Check the working condition of the circuit valve to ensure accurate positioning during blowing;

3. Check the airflow control components of the blowing air to ensure correct setting values and unobstructed internal flow;

4. Ensure that the blowing can be completed in a short time, otherwise, blowing the bottle will bring additional cooling to the PET material;

5. Ensure that all ventilation holes on the mold are unobstructed.

2、 Swelling of bottle mouth, blowing of bottle and gas leakage

Symptom: The screw area expands and spreads, transmitting to the anti air groove. It is more severe when the weather is hot, and due to the high temperature, it will occur during the bottle blowing process. At the beginning, there may be a sound of air leakage, which may resemble a short-term explosion (such as high-pressure gas supply until the end), and overheating greatly increases its danger.

Diagnosis: Due to the increase in temperature in the surrounding space, the cooling effect of the airflow passing through the oven is reduced, resulting in an increase in the temperature of the core rod, which is then transmitted to the threaded area.

Prescription:

1. Adjust the position of the cooling plate to provide stronger protection for the threaded area of the preform. The structure of the preform may make it difficult to adjust;

2. External cooling can be used to control the temperature of the threaded area and core rod when the preform passes through the oven;

3. A well-designed cooling plate may be a good solution for Z.

3、 Neck curvature

Symptoms: Usually manifested as difficulty in filling or sealing, displacement of the protruding ring under the thread of the bottle mouth, and non perpendicularity to the centerline of the bottle body.

Diagnosis: This problem is generally caused by mechanical operation, with excessive accumulation of material in the neck leading to deformation after mold closure.

Prescription:

1. If the material accumulation at the neck of the bottle is too thick, it will cause the bottleneck to become crooked when it is hit by the mold during the closing process;

2. The convex ring is clamped too tightly in the concave part of the top plate of the blowing mold, and the size of the preform needs to be fixed when it is still hot, with a thermal gap of 0.25mm;

3. The alignment between the conveying arm and the blowing center should also be considered as one of the causes of the problem.

4、 Bottom line turns blue

Symptom: Pearlescent stripes appear along the bottom cutting line. In terms of thickness, the area with pearl stripes on defective bottles is thinner than the same area on good bottles.

Diagnosis: Under normal circumstances, the material on the cross-section of this area should be thicker. This issue can manifest as mild or severe enough to generate waste.

5、 Bottom damage

Symptom: There is often a small, fine crack at the bottom where the material is too thick, which is difficult to detect without careful observation and may cause a burst during filling.

Diagnosis: This issue is not very common and is caused by a low temperature at the bottom of the preform. When the stretching rod pushes it towards the bottom of the mold, it forms micro cracks.

Prescription:

1. It is best to ensure that the end of the preform is neither too cold nor too hot between stretching and rupture (overheating can worsen the door gap);

2. The gap between the stretching rod and the bottom of the mold is also crucial, and only by using the necessary gap of Z can the positioning of the door be ensured.

6、 The screw part of the bottle mouth is damaged

Symptom: The exhaust groove in the threaded area has been subjected to undue stretching (from the sealing surface to the protruding ring). This phenomenon is more common in summer because the high temperature causes the thread area of the preform to overheat.

Diagnosis: The preform is fed into the mold cavity by a conveying arm, and when clamped, one side of the thread must be subjected to force. When the conveying arm is not accurately positioned and lifts the preform ring from above the mold, if the weather is cold, the blowing core will press against the closed surface of the preform, which will not cause damage. Unfortunately, in hot weather, the exhaust of the thread will be subjected to tensile force because the bottle blank is forcibly aligned by the blowing core that has been washed down.

Prescription:

1. Ensure precise and error free adjustment of the conveying arm; Also, the alignment of the blowing core field mold is also related;

2. Due to the fact that the problem is even worse when the temperature is too high, any decrease in temperature in the threaded area will have its consequences, as detailed in "Bottle Mouth Expansion, Blowing Out Gas". It should be remembered that the position of the preform is determined solely by the force of the spring when blowing the bottle core, and it is not sufficient to correct the alignment gap between the two. When the blowing starts, there is a considerable downward impact force, which can cause damage to the threads.