The raw material for bottle preforms is mainly PET (polyethylene terephthalate), and its key performance parameters have a significant impact on molding:
1. Intrinsic viscosity (IV value)
Definition: An indicator for measuring the molecular weight of PET, the IV value of bottle grade PET is usually 0.78-0.85dL/g.
influence:
The IV value is too low → the melt flow is good, but the mechanical properties of the preform are poor (easy to break after blowing);
The IV value is too high → the melt viscosity is high, requiring higher injection molding temperature and pressure, otherwise it is prone to material shortage.
2. Moisture content
Requirement: PET has strong moisture absorption and needs to be dried to a moisture content of ≤ 0.005% (50ppm) before molding.
Impact: Excessive moisture content → hydrolysis reaction occurs during melting, resulting in a decrease in IV value (degradation), bubbles and silver streaks (white stripes on the surface) appearing on the preform, and excessive acetaldehyde content (affecting food safety).
3. Impurities and additives
If the raw materials contain metal impurities and dust, it can cause scratches on the mold cavity, or black spots or foreign objects on the preform;
Excessive additives such as antioxidants and release agents can affect the melt flowability of PET, leading to a decrease in the surface glossiness of the preform.
4、 Equipment status: Injection molding machine performance determines parameter execution accuracy
The stability and accuracy of equipment are the guarantee for the effective implementation of process parameters, and the core influencing factors include:
1. Injection molding machine screw and barrel
Screw compression ratio: The compression ratio of PET specific screws is usually 3:1-4:1, ensuring that the raw materials are fully melted and plasticized; If the compression ratio is not appropriate, it can lead to uneven melting (the appearance of unmelted particles in the preform).
Wear degree: Screw and barrel wear can cause fluctuations in melt adhesive pressure, affecting the stability of melt conveying, and requiring regular testing (replacement is required if the gap exceeds 0.3mm).
2. Locking force and clamping accuracy
Insufficient locking force → The mold is not tightly closed, resulting in burrs; Excessive clamping force → mold deformation, affecting the dimensional accuracy of the mold cavity.
Poor mold clamping accuracy (such as wear of guide columns) → misalignment between the mold cavity and core, severe uneven wall thickness of the preform.
3. Temperature and pressure control system
The precision of the temperature control system needs to reach ± 1 ℃, otherwise temperature fluctuations in the material barrel will cause unstable viscosity of the melt;
When the error of the pressure sensor exceeds ± 2MPa, the injection/holding pressure control is inaccurate, and the size of the preform fluctuates greatly.