From PET Preform to PET Bottle: The Complete Stretch Blow Moulding Process Explained
Understanding the full journey from PET resin to finished bottle — through preform injection, reheat, and stretch blow moulding — is essential for anyone specifying preform moulds or optimising a packaging line. Here is the complete picture.
The Two-Stage Process: Overview
The production of most PET bottles follows a two-stage process: first, PET resin is injection moulded into a preform (Stage 1); second, the preform is reheated and blow moulded into its final bottle shape (Stage 2). The two stages can be performed on separate machines (2-stage or RSBM) or integrated into a single machine (1-stage).
PET Resin
IV specified
Dried to < 40ppm
Injection Mould
Preform produced
270–290°C
Preform Cool
Mould cooling
Post-mould tubes
Neck Crystallise
Finish crystalliser
140–160°C
Reheat Oven
IR heating
~100–115°C
Stretch Blow
Axial stretch
+ radial blow
PET Bottle
Biaxially oriented
Final shape
1-Stage vs. 2-Stage (RSBM) — Which to Choose?
- Preform moulding and blowing on separate machines
- Preforms can be stored and transported between stages
- Higher output; independent optimisation of each stage
- Preferred for high-volume commodity bottles (water, CSD)
- Hexamech preform moulds are RSBM-platform optimised
- Injection and blowing on the same machine
- Preform never cools fully — lower reheat energy
- Lower volume; higher per-unit cost
- Preferred for wide-mouth jars, specialty shapes
- Limited to lower cavity counts
How Preform Design Affects Bottle Quality
| Preform Design Variable | Effect on Final Bottle |
|---|---|
| Wall thickness distribution | Controls material distribution in bottle — thin preform base can cause base thinning in bottle |
| Preform length vs. bottle length | Stretch ratio — determines biaxial orientation intensity and mechanical properties |
| Gate geometry quality | Gate scar becomes the bottle base injection point — must be clean and centred |
| Preform IV (post-processing) | Higher IV in preform = better bottle mechanical properties and burst pressure |
| Crystallinity in preform body | Any crystallinity in preform body prevents even stretch — causes uneven wall in bottle |
Hexamech manufactures both injection preform moulds and stretch blow moulds for the food, beverage, and consumer goods industries. Specifying both from a single engineering partner ensures that preform design and blow mould cavity geometry are optimised together — eliminating the finger-pointing that occurs when preform and blow mould suppliers disagree about the root cause of bottle defects.
Complete PET Container Solutions — From Preform to Bottle
Hexamech supplies both preform moulds and blow moulds. One engineering partner, one responsibility, better outcomes.
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