PET Preform System Product

Overview

PET preform injection molding is the first stage of a two-stage bottle-manufacturing process. Thermoplastic PET resin (polyethylene terephthalate) is injection-molded into small, rigid "preforms"—hollow shells 50–80 mm tall, 10–25 mm diameter, with a thick neck and threads. The preform is then reheated and stretch-blow molded into the final bottle shape.

This hybrid process (injection preform + stretch-blow final) combines the precision and complex geometry capability of injection molding with the strength and clarity of stretch-blow molding, resulting in lightweight, crystal-clear, strong bottles suitable for carbonated beverages, water, and premium packaging applications.

Modern preform injection lines are highly automated, producing 1200–4000 preforms per hour with 16–64 cavities per mold. PET's sensitivity to thermal history and moisture makes this one of the most tightly-controlled plastic manufacturing processes.

The Two-Stage Bottle Production Process

Stage 1: Preform Injection (This Machine)

PET resin is dried to <0.03% moisture, melted at 280–300 °C, and injected into a multi-cavity mold at 800–1200 bar. The preform is cooled in the mold (8–15 seconds), ejected, and cooled further to room temperature. The preform is a rigid, hollow shell with:

• Bottom: Solid, slightly domed base for structural support.
• Body: 1.0–1.2 mm wall thickness, thin for minimal weight.
• Neck: 2–4 mm thick, with molded threads or finish for bottle cap.
• Top: Open end, which will be the bottle mouth.

Stage 2: Stretch-Blow Molding (Downstream Equipment)

Preforms (stored at room temperature or chilled to slow any annealing) are reheated in an infrared oven to 100–120 °C (just below PET's glass transition), then transferred to a blow station. A stretch rod pulls downward 10–25 mm (axial stretching), while high-pressure air (30–40 bar) expands the preform radially against a cavity mold, forming the final bottle shape.

The biaxial stretching orients polymer chains, improving tensile strength 2–3×, improving oxygen barrier 5–10×, and improving clarity. The bottle emerges with 1.0–1.5 mm walls, light weight, and excellent mechanical properties.

PET Material Handling

PET is hydroscopic (absorbs moisture from air) and hydrolytically unstable (reacts with water at elevated temperature, breaking polymer chains). Preform injection requires strict moisture control:

Desiccant Drying

A [[pet-preform-injection-hopper-dryer|desiccant dryer]] is integrated or positioned immediately upstream of the injection machine. The dryer contains a drying tower (packed with silica gel or molecular sieve), a heater (140–180 °C), and an air blower (2–5 kW) that continuously circulates hot, dry air through the desiccant and over incoming resin pellets.

Target outlet moisture: <0.03% dew point (lower than -40 °C dew point air). At this moisture level, hydrolysis during injection is negligible (<0.1% chain-scission over an 8-hour production shift).

If moisture exceeds 0.05%, bottle preforms develop weak welds (where different melt streams join in the mold), voids, and brittleness, causing high scrap rates (typically >10%) and customer complaints about bottle failures.

Dryer maintenance: Desiccant must be regenerated (heated to 120–160 °C to drive off absorbed water) every 8–24 hours depending on ambient humidity. Over-used desiccant loses drying capacity and must be replaced (~$500–$1000 per tower per year in consumables).

Thermal History & Annealing

PET's physical properties depend on its thermal history (cooling rate after injection):

• Fast cooling (air-cooled or quenched): Amorphous (glassy), clear, flexible, lower density.
• Slow cooling (controlled cooling in warm mold): Partially crystalline, stiffer, slightly hazy, higher density.

For preforms, a balance is desired: crystalline enough for stiffness and thermal resistance, but amorphous enough for clarity and ability to reheat and stretch in the blow station.

Mold temperature is typically 70–90 °C, allowing a balanced cooling curve. Higher mold temperature (80–100 °C) increases crystallinity but risks crystallization haze (cloudiness from large crystal growth). Lower mold temperature (<70 °C) yields very clear, flexible preforms but poor thermal stability (preforms may deform or stick together if stacked while warm).

Mold Design & Cavity Arrangement

Multi-Cavity Mold

A typical preform mold has 16–64 cavities arranged in a balanced runner system. Runner channels deliver molten PET to each cavity simultaneously, ensuring even fill and consistent preform quality. Runner design is critical: unbalanced runner trees result in early-filling cavities (thin preforms) vs. late-filling cavities (thick preforms).

Core Pins & Threads

Each cavity includes two [[pet-preform-injection-core-pins|core pins]] (typically 3–8 mm diameter, hardened steel) that define the preform's inner bore and bottom. The top of each cavity is open, allowing the hot plastic to fill from above.

Threads or finish details may be molded into the neck; these are carried forward into the final bottle.

Ejection System

After cooling, the mold opens and [[pet-preform-injection-ejector-pins|ejector pins]] (spring-loaded or hydraulic) push the preform out of its cavity. Ejection must be gentle to avoid distortion or stress-cracking of the thin-walled preform.

Some molds employ a [[pet-preform-injection-mold-cooling-robot|post-mold cooling robot]] that sprays chilled water onto the mold cavity walls during the cooling phase, accelerating solidification and reducing cycle time from 15 seconds to 8–10 seconds. This robot also picks cooled preforms off the core pins and deposits them onto a conveyor, fully automating the unloading process.

Injection Machine Control

PET injection requires tight process control:

Barrel Temperature Profile

Typical setpoint across the three barrel zones:

• Feed section: 280–290 °C (minimize residence time, avoid premature melting)
• Transition: 290–300 °C (final melting, homogenization)
• Metering (nozzle): 295–305 °C (maintain flowability)

Temperature must be stable ±2–3 °C; variance >5 °C causes mold-filling defects. Many machines employ PID temperature controllers with tight tolerance bands.

Injection Pressure & Speed

Pressure is typically 800–1200 bar depending on preform geometry and mold design. Slow injection (0.5–2 seconds to fill all cavities) reduces shear heating and thermal degradation. Fast injection (>2 seconds) risks incomplete fill and "short shots" (partly-filled cavities).

Hold Pressure & Time

After initial fill, hold pressure (typically 300–600 bar) is applied for 2–5 seconds to pack the cavity, ensuring the preform is solid and dense. Hold pressure is gradually reduced (ramped) to avoid stress buildup.

Cooling Time

PET solidifies relatively quickly compared to other thermoplastics. Minimum cooling is ~8 seconds to allow a skin to form and the part to be ejected without distortion. Longer cooling (10–15 seconds) improves quality but reduces throughput.

Preform Design Specifications

Wall Thickness Distribution

Well-designed preforms have:

• Base: 2–3 mm, domed or with ribbing for stiffness and structural support.
• Body: 1.0–1.2 mm, thin to minimize weight.
• Shoulder: 1.5–2 mm, transitioning from body to neck.
• Neck: 2–4 mm, thick to support threads and prevent deformation.

Uneven preform wall thickness results in uneven stretch-blow (thin sections stretch excessively, thick sections under-stretch), compromising final bottle quality.

Gate Design & Vestige

The injection gate (where plastic enters the cavity) typically leaves a small stub or "vestige" on the bottom of the preform. Vestiges must be uniform in size (<1 mm diameter typical) so that during stretch-blow, they melt and blend imperceptibly. Oversized or off-center vestiges result in visible seams or weak points in the final bottle.

Robot Automation & Unloading

Modern preform lines often include a [[pet-preform-injection-mold-cooling-robot|6-axis collaborative robot]] with a spray-cooling nozzle and gripper:

1. Cooling spray: While mold is closed, robot sprays 5–15 °C water onto the mold cavity for 3–5 seconds, accelerating cooling.
2. Mold opening: Once cooled sufficiently, robot signals the machine to open the mold.
3. Part pickup: Robot gripper (vacuum cups or mechanical fingers) grasps cooled preforms and removes them from core pins.
4. Placement: Robot deposits preforms onto a conveyor, a storage rack, or directly into a downstream stretch-blow carousel.

Robot automation ensures consistent handling, reduces cycle time by 20–30% (via faster cooling), and improves safety by removing personnel from the mold area during hot plastic operations.

Quality Assurance

Preform Dimensional Control

Critical dimensions (OD, wall thickness, neck diameter, thread profile) are measured with:

• Automated vision system: Cameras and image processing check preform outline, detect short-shots and gate defects.
• Ultrasonic thickness gauge: Measures wall thickness at 4–8 points on each preform; data logged to SPC (statistical process control) charts.
• CMM (Coordinate Measuring Machine): Periodically samples and measures full 3D geometry to verify mold is in-spec.

Target tolerance: ±0.1 mm on neck diameter (critical for cap fit), ±0.2 mm on OD, ±5% on wall thickness.

Defect Classification

• Short shots: Incompletely-filled preforms, caused by low injection pressure, high melt viscosity (low temperature), or clogged runner. Rejected.
• Gate vestige defects: Oversized or off-center vestige, results in bottle seam or weak point. May be accepted if trim is performed in downstream blow molding.
• Sink marks: Localized surface depressions, indicate thick-wall region that cooled slowly. Usually acceptable for non-visible surfaces.
• Stress-cracking: Small cracks visible under bright light, indicate excessive hold pressure or rapid ejection. Rejected; causes premature bottle failure.
• Haze or discoloration: Indicates thermal degradation, moisture absorption, or contaminated resin. Rejected or downgraded.

Bottle Performance Testing (Downstream)

After stretch-blow molding, final bottles are tested:

• Burst pressure: Must exceed 3 bar (for carbonated beverages pressurized to 0.3–0.5 bar, safety factor ~6–10×).
• Drop/impact: Must survive 1.5 m drop onto concrete without cracking.
• Gas barrier: O₂ transmission rate <5 cm³/(m²·day) for carbonated beverage shelf life >6 months.
• Dimensional stability: Diameter variation <1%, consistent wall thickness profile.

Bottle failures typically trace back to preform quality: weak walls, off-center gate, or moisture-induced defects.

Operational Challenges

Moisture Contamination

Even brief exposure to ambient humidity (>60% RH) will increase PET moisture to unacceptable levels (<0.05%) within 1–2 hours. Prevention:

• Maintain hopper in a dry box (desiccant or nitrogen purge).
• Keep dryer operating at all times; do not skip drying.
• Transfer pellets in sealed bags immediately before use.

Resin that absorbs excessive moisture is considered scrap or downgraded for non-critical applications.

Preform Sticking in Cavity

If preforms cool too slowly or adhesion is high, they may stick to core pins and jam the ejection mechanism. Causes: high mold temperature, or sticky regrind (post-consumer PET resin). Solutions:

• Reduce mold temperature 5–10 °C.
• Apply mold release agent (typically a silicone-based spray) every 5–10 cycles.
• Switch to virgin resin if using high regrind content.

Thermal Imbalance

If mold cooling is uneven (one cavity cooler than others), that cavity's preforms solidify faster and may be under-filled (thin) or over-filled (thick), depending on when they solidify relative to hold pressure. Solution: check and clean all cooling water lines; verify all cavity thermocouple sensors are functioning.

Cycle Time Creep

Over a production run, if the machine operator increases temperature slightly to speed up fill, or decreases cooling time to increase throughput, preform quality degrades progressively:

• Thinner walls (less hold pressure, rushed cooling).
• Moisture absorption (higher temperature → faster moisture uptake).
• Thermal degradation (higher residence time in barrel).

Best practice: lock process parameters via the machine control system; require supervisor approval for any setpoint changes.

Economics & Throughput

A new 16-cavity preform machine costs $200k–$400k. A 32-cavity or 64-cavity machine (higher throughput) costs $400k–$1M+.

Preform mold tooling (multi-cavity): $30k–$100k depending on cavity count and complexity.

Production cost for a 28 g PET preform (typical 2-liter bottle preform):

• Material (virgin PET): ~$0.04–$0.06 (28 g × $1.50–2/kg)
• Energy (heating, cooling, air): ~$0.01
• Labor + machine depreciation: ~$0.03
• Total: ~$0.08–$0.10 per preform

Downstream stretch-blow molding cost: ~$0.02–$0.03 per bottle.

Total final bottle cost: ~$0.10–$0.13 (wholesale); retail price $0.50–$2.00 depending on brand and application.

High-volume beverage producers (Coca-Cola, PepsiCo) operate dedicated preform lines 24/7 with fully automated unloading, achieving cost <$0.08/preform through economies of scale and optimized processes.

Sustainability

Post-consumer PET (rPET) from recovered beverage bottles can be pelletized and mixed with virgin PET (typically 10–25%) for preform reinjection. However, rPET has:

• Lower molecular weight (weaker after each recycle).
• Potential contamination (different polymers, adhesive residue).
• Color variation (can be bleached or re-colored, but at extra cost).

Most beverage brands use <15% rPET in preforms to maintain strength and clarity. Some premium "green" initiatives target 50–100% rPET, but these require specialized mold design, robot cooling, and tight process control to compensate for rPET's reduced performance.

Chemical recycling (depolymerization of PET back to monomers, then re-polymerization) is emerging as a way to enable infinite recycling without property degradation, but it requires capital-intensive infrastructure not yet widespread in most regions.