Injection molding is one of the most common manufacturing processes used today. It allows the mass production of plastic parts with tight tolerances, complex geometries, and detailed features.
As consumers and manufacturers become more environmentally conscious, there has been increasing interest in using sustainable bioplastics like polylactic acid (PLA) for injection molding applications.
So in this comprehensive guide, as a professional plastic injection molding manufacturer, I’ll walk you through everything you need to know about PLA injection molding.
By the end, you’ll understand:
- What is PLA?
- PLA material properties
- PLA injection molding process
- Pros and cons of using PLA
- Common defects in PLA parts
- How to optimize your process
Let’s get into it!
What is PLA Material?
PLA stands for polylactic acid or polylactide. It is a biodegradable thermoplastic derived from renewable resources like corn starch, tapioca roots, sugarcane etc.
The lactic acid monomers produced from these crops are linked together in a condensation reaction to form long chain PLA polymers. Biotech companies like NatureWorks produce different grades of PLA for use across industries.
Unlike conventional plastics, PLA is compostable under industrial conditions. It breaks down into carbon dioxide and water within 90-180 days. This makes it popular choice for single-use food packaging and disposable consumer products.
Early generations of PLA had issues with brittleness, heat resistance and processability. But things have improved tremendously over the past decade. Modern PLA grades can now match PET, PS and PP in properties while being sustainable.
PLA Material Properties
Let’s look at some of the important material properties of PLA that matter for injection molding-
1. Low melting point – PLA melts between 130 °C and 180 °C, which is lower than traditional polymers like PS (240 °C) and ABS (220 °C). This allows easy flow in thin sections and the ability to combine PLA with heat sensitive additives.
2. Fast crystallization – PLA can develop crystallinity quite fast compared to PET (30×) and PP (3×). Faster crystallization means reduced cycle times and higher production rates.
3. Low gas permeability – PLA allows much less oxygen and CO2 transmission compared to other bioplastics. This results in excellent shelf life for PLA bottles and blister packaging.
4. Good optical clarity – Amorphous PLA can achieve clarity close to PS and PET. This is useful for see-through medical devices and electronics housings. Clarity reduces once PLA parts become semi-crystalline.
5. High strength and rigidity – PLA products feel stiffer and tougher than PP and PET containers. The flexural modulus can range from 2.7 GPa to 7 GPa depending on the grade. Heat-resistance goes up to ~100 ̊C.
6. Poor impact strength – Neat PLA has low-impact strength due to brittle failure. So impact modifiers are usually added for durable applications. The izod impact strength varies from 0.5 kJ/m2 (rigid sheet) up to 30 kJ/m2 (toughened film).
How Does PLA Injection Molding Work?
Now that you know about PLA’s properties, let’s understand how to injection mold parts from it. The basic process consists for four major steps:
1. Drying
PLA resin is highly hygroscopic – meaning it quickly absorbs ambient moisture. Too much moisture leads to hydrolysis and molecular weight loss during melt processing.
So PLA must be properly dried to less than 250 ppm moisture before use. Industrial dehumidifying dryers with dew points of -40°C are necessary for this.
Recommendations depend on the type of PLA but drying overnight at 80 ̊C is usually sufficient. The dried resin must be stored in sealed containers before loading in the molding machine.
2. Plasticizing
This is where the PLA pellets are gravity fed from a top mounted hopper into the injection molding machine’s heated barrel.
Inside the barrel, the reciprocating screw subjects the pellets to mechanical shear and conductive heat. This plasticizes the pellets into a homogenous polymer melt having uniform temperature and viscosity.
The plasticizing temperature for standard PLA lies between 180°C to 230°C. For toughened PLA compounds, it may go up to 260°C. Excessive temperatures promote material degradation which must be avoided.
3. Injection
After sufficient melting and plasticizing near the front of the screw, the desired shot size is retracted and held ready for injection.
The screw then moves forwards rapidly, injecting the polymer melt into the temperature-controlled mold tool under high pressure.
Injection molding of neat PLA requires pressures ranging from 55 MPa to 110 MPa. And injection speeds around 150 mm/s to 300 mm/s fill the mold cavities properly.
Higher molecular weights and additive-filled PLA might need more optimized processing conditions for defect-free parts.
4. Cooling and Ejection
After the mold gets filled completely, a packing pressure is maintained to compensate for material shrinkage as the part solidifies and cools down.
This is where PLA’s crystallization kinetics makes a big difference. Optimized cooling combined with the right mold temperature, allows the molded PLA part to be ejected faster while maintaining dimensional stability.
Amorphous parts require the mold surface to be below 60°C. For semi-crystalline high heat resistance parts, mold temperatures around 90-120°C work very well.
The cooling water circuits regulate these temperatures precisely throughout the tool. Faster heat transfer also minimizes the PLA injection molding cycle time.
Benefits of Using PLA for Injection Molding
Now that you understand how PLA injection molding works, let’s discuss a few of benefits that make it an attractive alternative-
1. Environmental Sustainability – Made from annually renewable resources like corn and sugarcane, PLA has 90% lower carbon emissions compared to traditional plastics. It also reduces reliance on fossil fuels for producing raw plastic materials.
2. Enhanced Corporate Image – Offering eco-friendly bioplastic products caters to the environmentally conscious buyers of today. This allows brands to boost their green credentials and corporate social responsibility.
3. Lightweight Construction – With specific gravities between 1.21 to 1.25, PLA is 10% lighter than PS and almost 20% lighter than ABS. This aids fuel savings in transportation applications.
4. Easier Processing – Low melt viscosity provides better flow for intricate mold designs. Reduced processing temperatures save energy costs as well. The fast crystallization also enables shorter cycle times.
5. Lower Safety Risks – Negligible emissions and non-toxicity allows PLA products to be safely used for medical, pharmaceutical and food packaging purposes.
Challenges With PLA Injection Molding
Despite the many advantages, working with PLA poses some unique challenges during injection molding-
Moisture Sensitivity – PLA’s tendency to rapidly absorb ambient humidity makes drying a prerequisite before processing. Any residual moisture leads to hydrolysis, reduced molecular weights and mechanical performance.
Narrow Processing Window – The small gap between PLA’s melting and degradation temperatures provides a narrow processing window. This demands precise temperature control to balance productivity and part quality.
Faster Crystallization – While fast crystallization improves cycle times, sudden cooling and temperature variations on the mold walls leads to uneven shrinkage, warpage and structural stresses. Intelligent temperature regulation is required.
Lower Heat Resistance – In its amorphous state, PLA cannot be used beyond temperatures of 60 ̊C. Special nucleating agents and crystalline morphology is necessary to achieve higher heat resistance.
Limited Impact Strength – Neat PLA has very poor impact strength due to brittle failure mechanisms. This necessities the use of special impact modifiers or plasticizers for durable applications involving high loads.
Hydrolysis Susceptibility – Moist operating environments slowly hydrolyze PLA over months or years, needing replacement or disposal before mechanical failure. Use of stabilizers can somewhat prevent this issue.
Weld Line Strength – The weld lines created between converging melt fronts lead to much lower mechanical strength compared to ABS and PC. Proper mold gating/venting and reinforcement additives are required to overcome this issue.
How Can You Optimize PLA Injection Molding?
With a good understanding of PLA’s injection molding behavior, let’s now see how to optimize the process-
1. Prioritize proper drying before molten processing to prevent hydrolysis-based degradation of the bioplastic. Moisture levels must be kept below 250 ppm for stable results.
2. Utilize hot runner systems with externally heated manifold zones for low-shear PLA melt conveyance. This also prevents drooling and stringing of the material.
3. Correctly balance the filling velocities and packing pressures based on part thickness and geometry. PLA’s narrow temperature window necessitates very good process control.
4. Regulate mold surface temperatures diligently depending on required crystallinity levels and cycle time targets. Uniform cooling is also equally vital for good dimensional stability.
5. Consider using PLA compounds instead of the base resin to enhance flow, strength, heat-resistance and aesthetics based on application requirements.
6. Add wear resistant and anti-corrosion coatings to improve tool surface durability given PLA’s acidic nature after decomposition. Hardened cavities/cores also help.
7. Perform preventive maintenance with scheduled cleaning to remove PLA residue from injection barrels, hot runners and tool surfaces based on batches produced.
Conclusion
With growing interest in sustainable technologies, PLA promises to be a gamechanger for injection molding across industries in the future.
I hope this guide gave you deep insights into PLA’s behavior along with actionable best practices to injection mold parts out of it.
Optimizing the processing parameters and overcoming PLA’s sensitivity to moisture and temperature changes needs some effort. But the environmental and commercial benefits make it totally worthwhile.
Let me know if you have any other questions in the comments!
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