The quality of the preform directly determines the qualification rate of subsequent blow-molded finished bottles, and the following six core indicators need to be controlled:
Dimensional accuracy
includes the length, outer diameter, inner diameter, and wall thickness of the preform (especially the size of the threaded part of the bottle mouth), and must meet the mold design tolerance (usually the error is controlled within 0.1mm);
Dimensional deviations can lead to: asymmetrical bottle shape during blow molding, poor sealing of the bottle mouth and cap (leakage).
Uniformity of wall thickness The wall thickness of the
side wall and bottom of the preform should be uniform to avoid local thickness or thinness;
Too thick can cause uneven stretching during blow molding ("dents" in the finished bottle), and too thin can reduce the strength of the finished bottle (easy to break).
Appearance quality
No obvious defects: such as bubbles, black spots, impurities, scratches, deformation, gate residue that has not been cleaned, etc.;
Transparent materials (such as PET) need to ensure high transparency, without turbidity or yellowing (yellowing may be caused by raw material degradation or excessive heating).
Crystallinity (for semi-crystalline plastics such as PET)
The crystallinity of PET preforms needs to be controlled at a low level (usually 10%-15%), if the crystallization is too high, the preform will be milky white and brittle, and it cannot be effectively stretched (easy to crack) during subsequent blow molding;
Crystallinity is mainly controlled by the cooling speed: the faster the cooling, the lower the crystallinity and the better the transparency of the preform.
The melt index (MI) reflects
the fluidity of the plastic melt, and different materials and preforms for different uses need to match a specific melt index (for example, the MI of PET preforms is usually 25-35g/10min);
High MI: The melt fluidity is too good, which can easily lead to "flash" (melt overflowing mold gaps). Low MI: Poor fluidity, melt cannot fill the mold cavity, resulting in a shortage of preforms.
Hygiene and safety Preforms
used in food and pharmaceutical packaging must comply with national food safety standards (such as China GB 4806.7-2016 "National Food Safety Standard for Plastic Materials and Products for Food Contact");
The use of recycled materials (or the "proportion of recycled materials") is prohibited, and the raw materials must not contain harmful substances (such as BPA, heavy metals) to avoid migration into food.