Stretch Before Blowing (Stretch Blow Molding Process)
1. Process Principle
Stretch blow molding involves heating a pre-made preform (such as PET) to a softened state, then stretching it longitudinally using a mechanical rod while simultaneously injecting high-pressure gas for lateral expansion, causing the material to bidirectionally extend and form the final shape. Typical parameters are: stretching speed 0.5-1.2 m/s (according to the "Plastic Blow Molding Technology Handbook"), blowing pressure 25-40 bar.
2. Core Advantages
- High material utilization: Bidirectional stretching causes molecular orientation, increasing strength by 30%-50% (data source: American Plastics Industry Association), and reducing wall thickness by 10%-15%.
- Good transparency: Suitable for products with high appearance requirements, such as mineral water bottles and beverage bottles.
3. Limitations
- High equipment complexity, requiring precise temperature control (heating range 90-110℃) and simultaneous stretching and blowing actions.
- Only suitable for crystalline plastics (such as PET), poor results for PP/PE.
Blow-up followed by stretching (extrusion blow molding)*
1. Process Principle
Molten plastic is extruded into a tubular preform, then directly blown with air to expand it. After cooling and setting, a second stretching correction is performed. Commonly used in the production of HDPE milk bottles, cosmetic jars, etc., the blowing pressure is typically 5-15 bar.
2. Core Advantages
- High adaptability: Can process various materials such as PE, PP, and PVC; mold costs are 30%-40% lower.
- Flexible production: Suitable for small-batch, multi-variety orders; short adjustment cycle.
3. Limitations
- Poor uniformity of finished product wall thickness (tolerance ±0.2mm or more); transparency is lower than stretch blow molding.
- Weak molecular orientation of the material; compressive strength is 20%-35% lower than stretch blow molding.
Key Factors for Process Selection
1. Material Type: Stretch blow molding is mandatory for PET; extrusion blow molding is optional for PE/PP.
2. Production Capacity: A single stretch blow molding machine can produce 50,000-80,000 bottles (500ml standard bottles) per day, offering higher efficiency.
3. Cost Control: While the initial investment in stretch blow molding equipment is high (approximately 2-5 million RMB), long-term material savings are significant.
Technological Development Trends: New servo drive systems can reduce stretch blow molding energy consumption by 15%-20%, while multi-layer co-extrusion blow molding technology (such as EVOH oxygen barrier layers) is driving process integration. Future intelligent control systems may enable modular switching between the two processes.
