Bio-based plastics promise a lower carbon footprint and, in some cases, compostability. They also behave differently in the press than the conventional resins they aim to replace. PLA and PHA are not drop-in substitutes for PP or ABS; they have narrow processing windows, they are sensitive to moisture and heat history, and they can degrade the tool or the part if run carelessly.

Figure 1. Bio-based plastic families with PLA and PHA processing windows.

This guide explains how the main bio-based families behave in injection molding, what changes in the tool and the process, and where they make sense.

The main families

• PLA (polylactic acid) — made from fermented starch or sugar. Stiff, transparent, good for rigid parts; low heat resistance (typically ~55–60 °C) unless crystallized or blended.
• PHA (polyhydroxyalkanoates) — produced by microbial fermentation. Tougher and more flexible than PLA, and — crucially — home-compostable and marine-degradable in some grades.
• Bio-based PE / PET — chemically identical to petro-based versions but made from renewable feedstock; process like their conventional counterparts.
• Starch blends and PBS — lower-cost compostable options, often blended with PLA.

The name "bioplastic" hides two separate ideas: bio-based (made from plants) and biodegradable (breaks down). Some bio-based plastics are not biodegradable; some biodegradable plastics are petro-based. Clarify which property the application actually needs.

PLA processing

PLA molds much like a stiff, low-shrink PS or PET:

• Drying is mandatory — PLA hydrolyzes readily; dry to below ~250 ppm, typically around 80 °C for 4 hours.
• Melt temperature — roughly 190–220 °C. Above about 230 °C, PLA degrades and loses molecular weight.
• Residence time — keep it short; long dwell at temperature causes degradation and yellowing.
• Mold temperature — around 20–50 °C for amorphous parts; higher (80–110 °C) only for crystallized parts, which also needs a nucleating agent and longer cycle.
• Shrinkage — low for amorphous PLA (~0.3–0.5%), higher for crystalline grades.
• Corrosion — PLA is slightly acidic and can be aggressive to un-plated steel over long runs; use corrosion-resistant tooling for high volume.

PHA processing

PHA is the trickier of the two:

• Very shear- and temperature-sensitive — narrow window, often around 160–180 °C. Overheating degrades it and releases odor.
• Slow crystallization — cycle times are longer; cooling and part robustness must be managed.
• Drying — required, but PHA can also be over-dried; follow the supplier's spec.
• Toughness — better than PLA, which makes PHA attractive for parts that must flex without cracking.
• Odor — some grades have a distinct smell; venting and post-conditioning may be needed.

Tooling changes

• Corrosion-resistant steel or coatings for acidic bio-resins, especially in hot runners.
• Generous flow channels — narrow gates and thin sections raise shear and degrade sensitive melts; open up gates and runners.
• Gentle hot runner design — avoid dead spots and long residence times; thermal uniformity matters more than for commodity resins.
• Cooling designed for slow-crystallizing grades — more cooling capacity and sometimes a heated mold for crystalline parts.
• Venting — good venting avoids trapped gas and burn marks, which are more visible on natural, light-colored bio-resins.

Process strategy

• Purge thoroughly when switching to and from bio-resins; contamination causes gels and weak parts.
• Keep the melt cooler than you would for PP, and shorten residence time.
• Dry rigorously and re-check moisture; splay is common with PLA and PHA.
• Watch heat history — regrind that has been reheated several times degrades quickly; limit regrind ratio.
• Set the cycle for the material, not the machine; slow crystallization cannot be rushed without warping.
• Validate mechanical properties on real parts after conditioning; bio-resins can be more sensitive to aging.

Where bio-based plastics fit

Good applications:

• Single-use and short-life packaging — cutlery, trays, containers, where compostability is the point.
• Consumer goods where the brand wants a renewable-feedstock story.
• Non-critical parts such as handles, cases and displays where heat resistance is modest.
• Parts destined for industrial composting streams with the right infrastructure.

Poor applications:

• Hot-fill or high-temperature service — PLA softens well below boiling water.
• Long-life structural parts where moisture and aging dominate.
• Precision parts needing tight, stable dimensions.
• Applications with no composting stream — a compostable part in a landfill is not automatically better.

Sustainability, honestly

Bio-based does not automatically mean low impact. Consider:

• Feedstock and land use — food-versus-fuel competition, fertilizer and water use.
• End-of-life — compostability only helps if the infrastructure exists.
• Performance trade-offs — a shorter-lived part may increase total material use.
• Carbon footprint over the full life cycle, not just the feedstock.

Figure 2. Quick reference: the key checks and reference values from this guide.

Checklist

• Application temperature checked against the resin's heat resistance
• Bio-based vs biodegradable requirement clarified
• Resin dried to spec and verified
• Melt temperature and residence time controlled and logged
• Corrosion-resistant tooling for acidic grades
• Gates and runners sized to lower shear
• Mold temperature set for the required crystallinity
• Purge procedure defined for material changeovers
• Regrind ratio limited to protect molecular weight
• End-of-life path confirmed (composting stream available)
• Life-cycle benefits assessed, not assumed

Bio-based plastics are a genuine option, but they are their own materials with their own rules. Respect the drying, the thermal window and the tooling, and PLA and PHA mold into good parts — treat them as drop-in replacements and the press will punish that assumption quickly.