PET containers or bottles are made from PET preforms. Therefore, it is very important that the preforms are free of defects in order to manufacture bottles of good quality. In order to keep the preforms defect-free, it is also important to have a good understanding of defects and their causes. So, let us explain a few important reasons and solutions for the defects that occur in the production of preform PET bottles. To investigate these flaws, let's examine them closely and learn about their remedies.
1. Marble or nacre shades
PET preforms were mainly rejected due to the marbled hue. The bottle will develop an uneven appearance over time due to marbling or nacre defects.
Cause of Defect - This usually occurs due to cold stretching or possibly due to overstretching. The marbled tone due to overstretching is concentrated in one area of the bottle. On the other hand , due to cold stretching, the entire structure of the preform is affected.
Remedy - This defect can be easily corrected by taking appropriate measures during the preform manufacturing process. Rigorous quality checks are required. The cooling temperature needs to be controlled by adding some lights . This increases the heating percentage up to a certain level.
2. The appearance of the bottle is dark
The odd-looking bottles throughout the batch are easy to identify. This happens due to the look of the bottle. People often get confused between a marble tone and a sombre look. but,
Both are different from each other. In the latter, the outer surface of the bottle becomes cloudy. This reduces the mechanical strength of the bottle many times. Also, make it look dark.
Causes of Defects - There are several reasons for the darkened appearance of the bottle. If the temperature range and cooling range exceed a certain level, the prefab appears cloudy white. PET containers are heated to a temperature range beyond the 125°C cooling point. Another cause of the defect is improper blowing of the bottle. If the bottle remains stable for a long time due to slow speed, a colored effect .
Remedy - In order to correct the defect, the heating temperature needs to be lowered. This can be done by any of the following methods; reducing the number of light fixtures or increasing the hearth ventilation. This reduces the percentage of heated area and adjusts the heating rate to reduce the performance temperature below the critical temperature.
3. Fold around the stretch rod at the bottom of the PET bottle
The disadvantage of PET bottles is the form of creases at the bottom, where unstretched material is collected. This creates a wrinkle on the lower bump, which creates a cavity in the gate runner with a greater thickness of material. Defects are initiated in the form of cavities where rods touch the bottom of the preform. Defects in the form of creases often cause the bottle to break from the folded area.
Causes of Defects - There are many reasons why defects in the form of wrinkles occur in the lower part of PET bottles such as:
Folding at the bottom when a large amount of material remains on the bottom of the preform after the blow molding process.
Due to insufficient blowing pressure,
delay during pre-blowing,
Poor preform quality or outdated properties
high pressure
while stretching and blowing
Where some material is thicker or thinner, the material in the stretch rod will be affected.
Remedy - This defect can be corrected by ensuring the quality of the preform using various testing methods such as:
The quality of preforms is verified using polarized light with the help of a testing instrument called a polarizer.
Reduce the heating temperature in the area adjacent to the gate runner where the fold is located,
Increase blow molding pressure,
Never forget to start the pre-blowing process early,
increase flow rate,
Verify heating temperature and ensure correct and uniform distribution of material.
These are some of the main reasons and solutions for the defects that occur in the production of preform PET bottles. If the preforms are internally strain tested before being used for large-scale block molding, the cost of scrap can be reduced to a large extent.
