Process Challenges in Injection Molding of Biodegradable Plastics
With bio-based composition and natural degradability, biodegradable plastics have become mainstream alternatives to traditional petroleum-based plastics, widely used in packaging, disposable articles, tableware and daily necessities. Common types include PLA, PBAT, PBS and starch-based modified blends. Their molecular structure, thermal stability and rheological properties differ greatly from conventional PP, PE and ABS. During injection molding, these materials are prone to thermal degradation, poor flowability, difficult molding, unstable product performance and various appearance defects. Overall, they bring far more challenges to process control than ordinary plastics. Combined with practical production experience, the core process difficulties and corresponding problems are summarized as follows.
Poor Thermal Stability and High Risk of Thermal Degradation
Narrow heat resistance range and weak thermal stability are the most prominent problems in production. For instance, PLA will degrade rapidly when staying under high temperature for a long time. Broken molecular chains lead to decreased melt strength, which not only impairs the mechanical strength and toughness of finished products, but also causes yellowing, blackening, bubbles and peculiar smell on product surfaces. Slight overheating in barrel sections or excessive material residence time inside the barrel will accelerate material decomposition. Starch-based biodegradable materials tend to carbonize when heated, forming carbon deposits on inner walls of nozzles and runners and causing continuous product contamination. In addition, residual materials left in the barrel during standby or production switching will degrade quickly, resulting in batch defective products after restart. Strict temperature control and standardized machine standby management are required, and the temperature parameters for traditional plastics cannot be applied directly.
High Hygroscopicity and Multiple Defects Caused by Moisture
Most biodegradable resins and starch-modified blends absorb moisture easily. If damp raw materials are put into production directly, water will vaporize in the high-temperature barrel and form bubbles, silver streaks and voids inside and on the surface of products, which seriously damages appearance and structural integrity. Different from common plastics, biodegradable materials have a narrow drying window. Insufficient drying cannot remove moisture completely, while excessive drying temperature will cause material softening, agglomeration and partial degradation in advance. Opened raw materials reabsorb moisture quickly in normal workshop environment, leading to secondary dampness even after pre-drying. For blends with high starch content, moisture will also result in uneven melt and discontinuous material feeding, further increasing molding difficulty.
Special Melt Flow Characteristics and Difficulty in Filling and Runner Adaptation
Biodegradable plastics have complex rheological properties with obvious differences among various grades. PLA features high melt viscosity and poor flowability, so thin-walled products and parts with complex cavities often suffer from short shots and flow marks. PBAT has good toughness but soft melt, which easily causes flash under high injection speed. These materials are highly sensitive to shear rate; changes in screw speed and injection speed will alter melt viscosity obviously, and minor parameter adjustments may lead to unstable molding status. Conventional screws, runners and gates of injection machines are designed for traditional plastics, which further increases flow resistance in long runners and narrow gates. Without structural optimization and parameter adjustment, stable filling is hard to realize. Uneven filling and inconsistent quality among cavities are common problems for multi-cavity molds and slender parts.
Unstable Molding Shrinkage and Difficulty in Controlling Dimensional Accuracy
Biodegradable plastics generally have higher shrinkage rates than traditional plastics, and their shrinkage is highly affected by temperature, pressure and cooling speed, leading to large fluctuation range. Minor changes in holding pressure and holding time will cause sink marks and dimensional deviation. Uneven mold temperature and inconsistent cooling rate will result in product warpage and distortion. For blended biodegradable materials, different components have different shrinkage characteristics, forming complex internal stress. Products may deform gradually after demolding, even if the dimensions are qualified when ejected. It is difficult to maintain high dimensional precision for parts with strict assembly tolerances, and the rate of dimensional defects tends to rise during mass production.
Slow Cooling and Solidification, Reducing Production Efficiency
Biodegradable plastics have poor thermal conductivity, so their melt cools and solidifies much slower than PP and PE. If adopting conventional cooling time, products will remain soft after demolding, sticking to molds and suffering from ejection whitening, penetration and surface scratches. Extending cooling time will directly prolong molding cycle, reduce equipment productivity and increase production cost. Soft biodegradable materials gain hardness slowly after cooling, which requires precise control of demolding timing. Early demolding causes deformation, while delayed demolding further cuts productivity, creating a dilemma in production rhythm control.
Poor Demolding Performance and High Risk of Surface Damage
Melt of biodegradable plastics adheres strongly to mold cavities, and the soft material texture makes demolding more difficult. Simply increasing draft angle cannot solve the problem thoroughly. The use of release agent will contaminate product surfaces, affect degradability and cause fogging and spots. During ejection, ejector pins are likely to leave indentations and whitening marks on soft products, and large side surfaces may appear stretching and peeling. Uneven material toughness also increases cracking risks at thin walls, sharp corners and ribs during mold opening and ejection. Comprehensive optimization on mold structure, mold temperature and ejection speed is required to solve these problems.
Limited Recyclability and Difficulty in Controlling Material Waste
After one injection molding cycle under high temperature, molecular chains of biodegradable plastics are damaged, leading to declined mechanical properties and thermal stability. Reusing sprues and defective products will accelerate degradation during secondary heating, causing sharp drop of product strength, toughness and concentrated appearance defects. These materials allow only a very low proportion of regrind addition and are not suitable for large-scale recycling, unlike traditional plastics. Most sprues and waste generated in production have to be discarded, resulting in low material utilization rate, higher production cost and greater pressure on waste disposal. Cost reduction via material recycling is hardly achievable.
Poor Process Adaptability and Low Parameter Tolerance
In general, biodegradable plastics have extremely low process tolerance. Any slight deviation of temperature, speed, pressure and time will trigger a series of defects. Traditional injection molding has a wide parameter range, and minor adjustments will not affect product quality. However, biodegradable materials respond intensely to parameter changes, so repeated trial runs are necessary when switching machines or molds. In addition, fluctuations of workshop temperature, humidity and raw material batches will continuously affect molding conditions. Technicians need to conduct real-time monitoring and adjustment, which puts forward higher requirements for on-site process management and operator skills.
