Case Study: Blow Mold

Processing PHA for Extrusion Blow Molding: Applications, Challenges, and Guidelines

As a compounded material, PHAs have a lot of Versatility. They can be engineered to be very flexible to very rigid, and can be both tough and elastic, making them suitable for a wide range of applications. 

Some of the material and application benefits:

  • Tensile Strength
  • Customizable
  • Lower carbon footprint
  • Lower energy consumption              
  • Suited for environmentally conscious brands


PHAs for Extrusion Blow Molding is new, leading-edge innovation. We will explore the opportunities presented by this innovative material, highlight the industries most likely to benefit, and provide recommendations on the most effective strategies for implementing related projects.

Defining the Blow Molding Process and Applications

Blow molding is used to manufacture hollow plastic containers through several processes.

Extrusion Blow Molding (EBM), a versatile process compatible with PHAs. EBM platforms include shuttle systems, reciprocating screw machines, accumulation head systems, and Graham Wheel machines. In EBM, molten plastic is extruded into a hollow tube (parison), which is then enclosed in a mold and expanded with compressed air to form the container. This process naturally produces tailings, requiring polymers that can be reground and reprocessed, an area where PHAs are suitable.

EBM is slower than Injection Stretch Blow Molding (ISBM) in terms of production cadence, but it requires lower capital investment and provides greater flexibility in wall thickness and material selection. For those seeking deeper process details, Ottmar Brandau’s: Extrusion Blow Molding, A Practical and Comprehensive Guidebook, remains the most authoritative resource on EBM.

http://www.blowmolding.org/Extrusion_Blow_Molding_Book.html

Applications of Extrusion Blow Molding

EBM is widely used across several industries:

  • Automotive: Windshield washer fluid containers, lubricants, additives containers.
  • Cosmetics: Shampoo, lotion, nail polish remover bottles.
  • Food & Beverage: Milk, dairy, and protein drink containers, often requiring multi-layer barrier technologies to prevent oxygen ingress and CO₂ loss.


The most promising near-term application for EBM containers, utilizing PHA, is in the cosmetics industry, a multibillion-dollar sector with recycling rates below 9%. In reality, almost none of this packaging is successfully recycled. Converting shampoo, conditioner, lotion, and mouthwash bottles into biodegradable PHA containers offers a clear sustainability advantage.

Cosmetics packaging is also less demanding in terms of barrier and performance requirements compared to food or automotive applications, making it an excellent entry market for PHA containers.

Conversely, automotive applications are not ideal due to the risks of leakage of hazardous fluids and the irony of packaging petrochemical products in biopolymers.

Food & Beverage Applications

While attractive for marketing, PHA is not a strong fit for mainstream beverage bottle packaging. PET remains superior due to its recyclability and performance. Multilayer dairy applications are unsuitable due to recyclability concerns.

A key limitation of PHA lies in its organic nature and production method. PHAs are made via bacterial fermentation of biomass sources (e.g., sugars, oils, agricultural waste, even sewage solids). After extraction and purification, residual odors often remain, sometimes strongly reminiscent of the feedstock (e.g., Post-Industrial cooking oils can impart a “French fry” smell). This poses sensory challenges for sensitive products like drinking water.

Supplements, vitamins and nutritional powders, typically housed in HDPE bottles are good candidates for conversion to PHAs.

Processing Challenges

Consumer Expectations

Replacement packaging must perform identically to the plastic it replaces. Consumers may pay more for biodegradable packaging, but they expect consistent functionality. Poor performance as seen in the “paper straw fiasco” damages consumer trust and brand credibility.

Material Sensitivity

PHAs are shear- and heat-sensitive. Excess shear generates heat (similar to rubbing hands together), leading to degradation. Residence time—the duration material spends above its softening point (~120 °C)—must be minimized. Production stops can quickly degrade PHAs, requiring operators to continuously purge or maintain low screw speeds.

Visual indicators of overheating:

  • Normal: Slightly opaque finish.
  • Warning: Pearlescent sheen (approaching thermal limit).
  • Critical: Loss of melt strength, liquefaction, off gassing.

Thermal Management Strategies

  • Pre-heating pellets at 70 °C for 3–4 hours reduces thermal shock.
  • Operate at the lowest possible extruder temperatures to avoid degradation.
  • Use purge compounds (e.g., LLDPE) for transition between materials, not excess heat.

Crystallization and Tg Properties

PHAs have a low Tg (–10 °C to 5 °C), enabling biodegradability by allowing bacterial hydrolysis in natural environments. This contrasts with PLA, which has a high Tg (~60 °C) and requires industrial composting (ASTM 6400) at 58 °C, conditions rarely found in nature.

Key Implications of Low Tg:

  • PHA products continue crystallizing for 48–72 hours post-molding, affecting performance tests.
  • Accelerated crystallization can be achieved by incubating parts at 70 °C for 4–12 hours.
  • Final crystallinity depends on compounding, not mold cooling rates.

PHA cannot be processed effectively in 1-step ISBM systems, as preforms crystallize during storage. Two-step ISBM may be theoretically possible but remains unproven.

Mold Cooling Recommendations:

  • Use hot water (55–70 °C) instead of cold water for mold surfaces.
  • Maintain contact surface temps at 60 °C+ to induce crystallization and rigidity.
  • For large-scale IBM, heated-air tunnels at 55–70 °C can stabilize parts post-ejection.

Operator Cheat Sheet (Quick Reference)

🔥 Heat & Shear

  • Start low, build up: Always run at the lowest temp that fully melts pellets.
  • Watch for pearlescence: If surface turns pearly, material is overheating.
  • Never stop “cold”: During downtime, keep screw turning slowly or purge.

🌡 Pre-Heating

  • Use dryer as pre-heater: 70 °C for 3–4 hrs before feeding.
  • Do NOT exceed 70 °C: Pellets can clump and block flow.

❄️ Mold Cooling

  • No cold water: Run hot water (55–70 °C) at the mold surface.
  • Target range: 60 °C is a good baseline; adjust up if parts stick or deform.
  • Large-scale tip: Use heated-air tunnels (55–70 °C) on takeout conveyors.

🧊 Crystallization

  • Allow for “post-crystallization”: Parts continue hardening up to 48–72 hrs.
  • Speed it up: Incubate molded parts at 70 °C for 4–12 hrs.
  • Always drop-test: Use proper water temp during QA — no skipping.

⚠️ Troubleshooting Quick Signs

  • Opaque = good
  • Pearly/shiny = overheating
  • Sticky/easy deformation = under-crystallized
  • Extensive shrinkage = mold was cut incorrectly

Conclusion

PHAs present both opportunity and challenge for extrusion blow molding. They offer ecological advantages, biodegradability, non-toxicity, and safety against plastic pollution, positioning them as a leading candidate for sustainable packaging. However, their processing sensitivity, crystallization behavior, and incompatibility with certain applications (notably beverages and automotive fluids) require careful consideration.

The best immediate applications lie in cosmetic and personal care packaging, where performance demands align well with PHA’s properties and the sustainability benefits can be fully leveraged.

Successful adoption will depend on:

  • Careful material compounding.
  • Operator training for thermal and shear management.
  • Realistic expectations about recycling, composting, and performance.

In short, PHAs are not a “silver bullet,” but they are a meaningful step toward reducing plastic’s environmental burden. They offer something petrochemical plastics never can: the ability to safely disappear when mismanaged. This quality alone makes them a vital material for the future of packaging.

To download the Extrusion Blow Mold Guide, click here.  

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