PET containers or bottles are made out of PET preforms. So, to make best bottles, it is very important that preforms are defect free. And to make preforms defect free, it is equally important to know the defects and its causes very well. So let us explain few important causes and solutions of defects that occur in production of preforms PET bottles.
To investigate these defect, let’s examine them carefully and understand their remedies.
1. Marble or Nacre tint
PET preforms majorly get rejected due to Marble Tint. Due to the Marble or Nacre defects, the bottle gives uneven appearance after some period of time.
Cause of defect - this usually occurs due to cold stretching or may be due to overstretching. Marble tint caused due to overstretching concentrates on one area of the bottle. On the other hand, due to cold stretching, the whole structure of the preform gets affected.
Remedy - this defect can be easily rectified by taking appropriate measures during preform manufacturing. A stringent quality check is required to perform. The cooling temperature needs to be controlled by adding some lamp lights. This increases the percentage of heating to a certain level.
2. Dim appearance of bottles
Oddly looking bottles in the whole lot can be easily identified. This happens due to the appearance of the bottle. People often get confused between marble tint and dim appearance. However, both are way different from each other. In the later one, the outer surface of the bottle becomes cloudy. This reduces the mechanical strength of the bottle by many folds. Also, make it appear dim.
Cause of defect - there are several reasons that lead to the dim appearance of the bottle. A preform appears to be cloudy white tinted if the temperature range and cooling range goes beyond a certain level. PET container is heated up to the range of temperature that goes beyond the chilling point of 125°C. Another reason for the defect is the improper blowing of the bottle. If the bottle kept for stabilization for a long time due to slow speed, the tinted effect appears.
Remedy - to rectify the defect, the heating temperature has to be decreased. This can be done by any of the following methods; reduce the number of lamps or intensify the furnace ventilation. This lowers the percentage of heating area and adjusts the rate of heating to reduce the temperature of performing below the critical temperature.
3. Folds around the stretching rod on the bottom of the PET bottles
PET bottles have defects in the form of folds at the bottom where un-stretched materials get collected. This creates folds on the lower bulge which create a cavity on the gate runner where the thickness of the material is more. The defect is initiated in the form of the cavity where the rod touches the bottom of the preform. The defects in the form of folds usually crack the bottles from the folded area.
Cause of defect - there are numerous causes due to which the defect in the form of folds occur at the lower part of the PET bottles, like:
• When a lot of material left on the bottom of the preform after the process of blow moulding, fold in the bottom is created.
• Due to insufficient blowing pressure,
• Delay in the process of pre-blowing,
• Inferior quality of preform or old perform
• Due to high amount of pressure,
• At the time of stretch blown
• The material is affected in the stretching rod in some places where the material is either thick or thin.
Remedy - this defect can be rectified by assuring the quality of the preforms using various test methods like:
• Use polarized lamp light to verify the quality of the preform with the help of testing instrument known as Polariscope.
• Decrease the temperature of heating from the areas that are adjacent to the gate runner where the fold is located,
• Enhance the pressure of blow moulding,
• Never forget to start the process of pre-blowing earlier,
• Enhance the rate of flow,
• Verify the heating temperature and ensure proper and equal material distribution.
These were some major causes and solutions of defects that occur in production of preforms PET Bottles. Prior to use in large-scale bulk moulding, if preforms are tested for internal strain, cost of rejection can be reduced to a large extent.
Polariscope is the perfect device to analyse the internal strain in the preformed caused due to external stress.