PLA holds commercial importance as a biodegradable thermoplastic that can be processed using injection molding as effectively as the more conventional synthetic commodity plastics like HDPE and PP.
Nonetheless, to effectively process PLA seamlessly in the high shear, high stress injection molding operation without any defects like warpage and sink marks it is important to optimize the process specifically for PLA. Simply adopting the same process parameters and troubleshooting for other plastics even with similar mechanical properties and application often leads to inefficiencies and losses.
If you’re working with PLA, an optimized injection moulding of PLA requires understanding the peculiarities of PLA and how that pertains to its injection moulding. In this article we discuss the key parameters you must put into consideration, the common problems to watch out for and some advanced optimization tools you can employ.
PLA Properties Pertaining to Injection Molding
The properties of the final product are closely linked to the crystalline structure it assumes in the process of solidification towards the end of the injection molding cycle from filling of the mold, to packing and then the cooling of the part in the mold until it gets ejected. The stress distribution across the material, the residual stresses resulting from the pressure and temperature experienced by the material during processing influence the physical properties of the final product. How every material is affected by these parameters is closely related to the chemical structure of the polymer. Hence why the optimization of the injection molding process needs to be polymer specific.
Molecular orientation of a polymer varies with the temperature and pressure. PLA transition temperature Tg and the thermal conductivity affects the stress distribution in the material as it cools and solidifies in the mold. How fast it exchanges temperature with the mold and how the heat is distributed over time across the material determines how the final product forms.
Residual stress occurring as a result of changes in temperature and pressure during the injection molding process from the feed to the ejection of the molded part from the cavity is distributed on the surface of the polymer as well as along the thickness. How uniformly this stress is distributed across the material depends on the micro and macro structure of the polymer. This in turn determines the occurrence of defects like warpage or anisotropic mechanical properties.
For example a more crystalline polymer with a rigid structure might distribute this residual stress more evenly compared to a more amorphous material in a pliable state.
Therefore the optimization of PLA injection molding should give consideration to the chemical structure of the particular formulation of PLA. The crystallinity, molecular weight, polydispersity, additives in the particular formulation, the D and L -lactides ratio and the date of manufacturing are all important properties of the PLA batch to be processed.
Common Problems and Defects in Injection Molding of PLA
Warping, brittle parts, burn marks and discoloration as a result of thermal degradation, sink marks and short shots are some of the common defects often encountered in the injection molding of PLA. While these are also common in the injection molding of other polymers, PLA is particularly prone to these due to its inherent properties.
For example, HDPE has a degradation temperature of around 400 to 500oC, its processing temperature is around 180 to 280oC typically. This leaves plenty of room for temperature fluctuation along the barrel. For the PLA however, not so much. PLA begins to experience thermal degradation around 200oC. Meanwhile, the melt temperature of PLA injection molding can vary between 170oC to 230oC.
This leaves little room for temperature fluctuation. Therefore injection moulding of temperature needs temperature and other parameters influencing temperature to be more tightly controlled. Temperature control systems, sensors, indicators and feedback systems need to be calibrated with high accuracy and precision.
Key Process Parameters
Key parameters that are known to affect warpage, hardness, tensile and flexural properties of plastics during injection moulding are; injection temperature, packing pressure, packing time and specimen orientation.
Injection temperature control has the additional challenge of varying at different points along the barrel down to the mold cavity. For PLA it is particularly important to keep tight control on the temperature due to its biodegradability as well as its tendency to have non uniform stress distribution.
The packing pressure and packing time are related to the volume to fill the mold. This must be well managed to prevent leakage, flash, short shot or other related defects. The mold itself must also be protected from excessive pressure or abrasion.
While this might be overlooked by less trained engineers, in the product development and mold design the specimen orientation affects the stress distribution in the part and mold. This is an interplay between gravity, pressure and temperature that will determine how well the product gets formed.
Targeted applications, balance of parameters and Trade-offs
The extent to which each influences a defect varies, for example warpage is more linked to injection temperature and the orientation of mould while the hardness and ultimate tensile strength of the finished part is more linked to packing pressure and injection temperature. Furthermore, it is important to not be overly focused on optimizing the system to reduce one defect to the extent that it compromises the properties and performance or function of the finished part. For example an injection moulding system that has been optimized to eliminate warpage should not as a result compromise the tensile strength of the product.
Polymers like PLA can vary significantly. PLA can be semicrystalline or amorphous. The formulation might include organic or inorganic fibres which can have different aspect ratios. The formulation may be toward specific application or achieving specific aesthetics. Therefore the injection molding process must be specifically optimized towards a specific application and functionality. In some cases there would have to be trade-offs in some properties or aesthetics to meet the desired product application.
The targeted application of the product might also call for complex dimensional parameters which makes it prone to certain defects. Optimizing the injection molding process, particularly the filling of the mold cavity to the ejection stage, can significantly prevent losses in the actual injection molding process.
Optimization Tools and Strategies
It is therefore evident that there is a complex combination of factors that influence the efficiency of injection molding. To this end advanced tools are employed to aid the optimization of injection molding. These include mathematical tools like ANOVA and Taguchi’s design tools and simulation tools like Solidworks. Significant time and costs can be saved with these optimization tools.
With advanced computer aided engineering, computations that would typically require advanced coding skills and expertise can be implemented within mold design and development facilities. This allows mold designers and manufacturers to better optimize the injection molding process for specific materials like PLA. Not only that, the injection molding process can be optimized for specific blends of PLA. So choosing to incorporate additives and fillers should not compromise the performance and efficiency of the product or process.
Tools that allow the stress and temperature distribution across the injection moulding stages to be analyzed and visualized are very valuable to the optimization of injection molding of a material like PLA since the room for fluctuation is less compared to other polymers like HDPE.
For injection molded parts parameters like part thickness are more complex to measure as many injection molded parts have complex dimensions. These optimization tools make the computation and variation of these parameters less cumbersome.
Engineering softwares like Solidworks are used to simulate and analyse the filling of mold cavities. Occurrence of degradation, sink marks and other defects can be identified using parameters such as stress and temperature at different points in the cavity. For example if in a simulation the temperature at any point in the cavity exceeds the degradation temperature of PLA, then the parameters must be modified to prevent this.This way before even going into production processors can fully visualize the process in advance and tell what would result from set parameters.
A key strategy for avoiding defects in injection moulding is use of high quality, well pretreated polymers. Formulations can be prepared with components that have been proven to reduce certain defects. For example use of glass fibre as reinforcement in PLA can significantly reduce shrinkage and shrinkage related defects. Other parameters such as polydispersity, molecular weight and moisture content should be optimal for successful injection molding. PLA can be prone to moisture problems if left in long storage, therefore consider a drying stage prior to injection molding
Troubleshooting
Even after optimizing the process for PLA injection molding some other problems might occur that require immediate troubleshooting. Some common causes of defects in PLA injection molding may include:
- Low thermal stability
- Moisture in the resin
- Wrong melt temperature
- Trapped air
| Problems | A possible cause | Fixes |
|---|---|---|
| Burn marks | Low thermal stability | Add stabilizers to feed or in PLA formulation Avoid storing for too long before use Modify temperature settings |
| Brittle part | Moisture in Resin | Include a drying stage before feeding into injection molding machine |
| Warping | Wrong temperature and processing parameter settings | Check that the properties of the resin matches that of the optimization program and that the settings are correct |
| Short shot | Trapped air | Check vent is working properly and is in the right location. Also ensure there isn’t excessive volatiles released from melt |
One way to address defects in injection molding in the packing phase is to inject additional material into the cavity to mitigate against shrinkage. The packing time can also be increased to prevent shrinkage. However if not properly done, this can be counterproductive and cause increased residual pressure which then results in warpage. PLA is particularly more prone to warpage than it is to shrinkage.
Studies using ANOVA to optimize the injection molding of PLA has shown that coolant temperature, packing time and injection temperature have a significant impact of shrinking and warping of the moulded part. Therefore when troubleshooting for these defects, these are the parameters to inspect.
Additives can be introduced to address problems in the injection moulding of PLA.
The stress distribution can be controlled by for example modifying the plasticizer or the filler content. The optimization program identifies the optimal values for parameters such as tensile strength, melt viscosity and yield stress. The PLA formulation can then be modified to meet the specified parameters.
Conclusion
While PLA can achieve processability and performance comparable to synthetic commodity thermoplastics like HDPE and PP, it does present its own peculiar challenges in injection moulding. These are mainly owing to its lower thermal degradation temperature, melt properties, and moisture sensitivity.
Successful injection molding of PLA requires optimized processes that have strategically integrated these factors.
Implementing optimization strategies and employing optimization tools like simulation softwares and advanced computing can significantly save time and costs in PLA injection molding.