Phase 1: Injection Molding of the PET Preform
Before a bottle can be stretch-blown, raw polymer resin must be transformed into an accurately proportioned "preform." This phase takes place in high-pressure injection molding equipment utilizing specialized PET preform molds.
- Resin Plasticization & Injection: PET granules are dried to a moisture content below 0.005% to prevent hydrolytic degradation, melted, and injected under extreme pressure (often exceeding 800 bar) into the preform mold cavities.
- Concentricity and Wall Uniformity: The preform acts as the structural foundation of the final bottle. Any eccentricity or wall thickness variation produced during injection will multiply during the stretching phase, leading to weak spots. Yushun Machine equips its multi-cavity preform tooling with independent self-locking taper systems to ensure core-to-cavity concentricity within 0.05mm.
- Precision Neck Finish: Unlike the bottle body, the preform neck finish (thread ring and sealing surface) is molded to its final dimensions during this initial injection phase and undergoes no further deformation during blowing.
Phase 2: Reheating and Thermal Conditioning
In two-stage SBM systems (reheat stretch blow molding), cooled preforms are transported to the blowing machine, where they undergo precise thermal conditioning inside an infrared or microwave oven track.
Thermodynamic Precision: PET must be heated above its glass transition temperature (Tg ≈ 75°C to 85°C) into a thermoelastic state (typically 100°C to 120°C) without reaching its crystallization point, which would turn the polymer opaque and brittle.
The preforms rotate continuously while passing through infrared heating zones. Mandrel shields protect the neck finish from radiation to prevent thread distortion, while preferential heating algorithms adjust thermal absorption profiles along the preform body to accommodate complex bottle shapes (e.g., oval or square containers).
Phase 3: Biaxial Stretching and High-Pressure Blowing
Once heated to the optimal processing temperature, the softened preform is transferred via automated pick-and-place grippers into the bottle blowing machine molds. Here, the transformation into a finished container occurs through simultaneous mechanical and pneumatic forces.
1. Mechanical Longitudinal Stretching
As the mold halves close and lock under heavy clamping force, an internal stretch rod descends rapidly inside the preform. The rod pushes against the bottom base of the preform, stretching the hot polymer vertically toward the base cup of the mold cavity.
2. Primary Low-Pressure Pre-Blowing
Simultaneously, low-pressure compressed air (typically 6 to 15 bar) is injected through the blowing nozzle. This expands the preform radially while it is being stretched vertically, preventing the soft polymer from touching the cold mold walls too early and ensuring balanced, biaxial material alignment.
3. Secondary High-Pressure Final Blowing
Immediately after the stretch rod reaches its terminal position, high-pressure air (up to 40 bar) is unleashed. This forces the expanding plastic flush against the polished, CNC-machined inner surfaces of the blow molding machine molds, locking in every design detail—including ribs, logos, base contours, and fill lines.
Phase 4: In-Mold Cooling and Container Ejection
The speed and quality of the final stage determine total plant throughput. As the hot plastic meets the cold metallic surface of the mold cavity, rapid heat transfer freezes the polymer matrix in place, imparting crystal clarity and structural rigidity.
Yushun Machine's high-speed blowing cavities feature advanced conformal cooling channels routed close to critical geometry zones, such as the base cup and heavy shoulders. High-velocity chilled water circulating through these loops reduces in-mold residence time, allowing modern automated machines to complete the entire stretch-blow cycle in under 4 seconds while maintaining scrap rates below 0.2%.
Technical Summary: The 4-Step SBM Workflow
The table below summarizes the key operational variables and critical tooling responsibilities across every step of the stretch blow molding process:
| Process Stage | Primary Operational Variable | Pressure & Temp Ranges | Tooling Component (Yushun Machine Focus) |
|---|---|---|---|
| 1. Preform Injection | Core concentricity & resin melt distribution | Melt Temp: 270°C - 290°C Pressure: > 800 bar |
PET Preform Mold (Valve-gate hot runner, hardened S136 steel) |
| 2. Reheat Conditioning | Infrared profile & neck finish shielding | Polymer Temp: 100°C - 120°C | Neck protection masks & rotating mandrels |
| 3. Biaxial Stretch & Blow | Stretch speed, pre-blow timing & high-pressure lockup | Pre-blow: 6 - 15 bar Main blow: 30 - 40 bar |
Blow Molding Machine Mold (Conformal cooling, high-grade alloy base) |
| 4. In-Mold Cooling & Ejection | Water flow velocity & exhaust timing | Chilled Water: 6°C - 10°C Exhaust: 0 bar |
Precision base vent valves & conformal cooling networks |
The Yushun Machine Advantage: End-to-End Synergy
A flawless stretching and blowing sequence cannot be achieved if the injection tool and the blowing cavity are treated as isolated components. Minor variations in preform wall thickness or stretch ratios will cause structural failures during high-pressure blowing. Yushun Machine serves as your professional solution expert from China, engineering tightly synchronized combinations of PET preform molds and blow molding machine molds tailored to automated high-speed production lines.
By applying 3D mold-flow simulations, advanced CNC machining, and rigorous material testing, Yushun Machine ensures your packaging operation maximizes output, reduces energy consumption, and delivers superior container quality.
Partner with an industry leader to optimize your stretch blow molding process. Contact Yushun Machine today to consult with our engineering team and refine your tooling infrastructure.
