Discussion on Common Defects and Causes in PET Preform Production
Basic Background
PET preforms are "pre-forms" manufactured from PET (polyethylene terephthalate) material through injection molding, which are then heated and blow-molded into the final PET bottles. Their quality directly affects the performance and appearance of the finished bottles, therefore various defects frequently occur during the production process.
Raw Material Issues
- High Hygroscopicity Leading to Moisture Problems
PET is highly hygroscopic. If the raw material is not sufficiently dried (typically requiring less than 0.02% moisture), moisture vaporization during melting will cause defects such as bubbles, silver lines, and a hazy appearance.
- Raw Material Contamination or Impurities
Contamination of PET, or the mixing of other plastics or impurities, can lead to black spots, cloudiness, and opacity.
- Excessive Proportion of Recycled Materials or Inferior Materials
Recycled materials have complex thermal histories, which can reduce molecular weight, strength, and transparency, and make them more prone to color differences and cracks.
Injection Molding and Improper Process Parameter Settings
- Poor Temperature Control
Excessive temperature (exceeding the recommended range) can cause PET thermal degradation, yellowing, and burn marks.
Insufficient temperature results in insufficient flowability, leading to short shots (incomplete filling) and uneven wall thickness.
- Uneven Cooling
Uneven cooling can cause warping, uneven internal stress, and decreased transparency.
- Inappropriate Injection Pressure/Speed
Insufficient pressure can cause voids and short shots, while excessive pressure may cause flash or melt decomposition.
- Inappropriate Holding Pressure/Time Settings
Insufficient holding pressure can lead to shrinkage marks and uneven wall thickness.
Inadequate Production Environment and Equipment Maintenance
- Excessive Humidity in Raw Material Storage Environment
High humidity can cause dried PET to reabsorb moisture, increasing the risk of defects.
- Aging Equipment or Inadequate Maintenance
Wearing screws and inaccurate temperature control in old machines can affect product stability.
- Insufficient Operator Experience
Incorrect temperature/pressure settings and failure to clean molds in a timely manner are common human factors that frequently lead to scrap.
In addition, mold design and equipment issues, such as insufficient mold ventilation or poor cooling design, mold wear or misalignment, and improper hot runner/gate design, can all lead to inconsistent preform quality.
As a mold manufacturer, what can we do?
1. First, determine the specific cause of the preform defect.
2. Inspect the current usage of the injection mold and develop targeted solutions.
3. Determine whether the mold needs complete replacement or partial replacement.
4. Perform precise measurements and create drawings.
5. Produce and inspect the preform before shipping.
6. Track mold usage one-on-one and provide on-site debugging services.
About SZ-MOLDTECH
SZ-MOLDTECH (Jiaxing) Co., Ltd. is a professional manufacturer specializing in high-precision PET preform molds, integrating R&D, design, and precision manufacturing to provide customized solutions for the global plastic packaging industry.
- Core Business
Multi-cavity PET preform molds (up to 144 cavities)
Hot runner systems and core components
- Manufacturing & Quality
Advanced CNC and high-precision processing equipment
Key component tolerance ≤0.005mm
- Technical Strength
Experienced engineering and design team (22+ years expertise)
Strong customization capability for different production needs
- Advantages
High precision and long service life
Cost-effective alternative to imported molds
