Biodegradable plastics are mainly composed of PLA, PBAT, PHA and corn starch modified materials, featuring natural degradation and environmental friendliness, widely used in injection molded products such as disposable tableware, packaging products and daily necessities. Compared with ordinary PP and PE plastics, biodegradable materials have strong water absorption, poor melt fluidity, low heat resistance, unstable shrinkage rate and easy brittleness, which are prone to various appearance and performance defects during injection molding.
1. Product Brittleness, Poor Toughness and Easy FractureBiodegradable plastics are inherently brittle, especially PLA materials, which are easy to crack, break and edge chipping after molding. Properly increase the temperature of the middle and rear barrel sections to improve plasticization uniformity and reduce internal stress. Reduce injection speed and holding pressure to avoid stress concentration caused by high-pressure forced filling. Increase mold draft angle and polish cavity surface to prevent hidden cracks caused by demolding pulling damage. Fully dry raw materials in advance to avoid molecular chain degradation and toughness decline caused by moisture. Appropriately add toughening modifiers in formula to improve inherent brittleness of materials.
2. Short Shot and Incomplete FillingBiodegradable materials feature high melt viscosity and poor fluidity, easily leading to short shot and incomplete filling at the end of thin-wall and complex structural products. Appropriately raise barrel and mold temperature to reduce melt viscosity and improve flow performance. Increase injection pressure and speed to shorten filling time. Expand cross-sectional area of gates and runners, adopt side gate or fan gate to reduce feeding resistance. Optimize mold exhaust grooves to increase exhaust depth and width, avoiding trapped air hindering melt filling.
3. Product Warpage, Deformation and Uneven SurfaceBiodegradable plastics have uneven shrinkage rate and are sensitive to cooling speed, prone to arching in the middle, edge warping and overall distortion after molding. Balance layout of mold cooling water channels to ensure consistent cooling of front and rear molds as well as thick and thin product positions. Reduce holding time and pressure to avoid internal stress accumulation. Optimize product structural design to keep uniform wall thickness and avoid excessive thickness difference. Add shaping cooling station after molding and assist static shaping manually after ejection. Lower mold temperature to slow down cooling speed for more uniform shrinkage.
4. Surface Silver Streaks, Bubbles and WhiteningSuch defects are mainly caused by raw material moisture, poor plasticization and trapped air in the mold. Degradable materials are easy to decompose and produce silver streaks and internal bubbles when encountering high temperature and moisture. Strictly dry raw materials at 60 to 80 degrees Celsius for 3 to 4 hours. Lower front barrel temperature to prevent thermal decomposition and gas generation of materials. Slow down injection speed to allow orderly discharge of internal mold gas. Expand exhaust grooves at parting surfaces and weld line positions, and regularly clean sundries in mold exhaust. Reduce back pressure to avoid material degradation caused by excessive screw shear heating.
5. Sink Marks and Surface Depression
Sink marks and pits easily appear at rib and thick-wall positions of products. Appropriately extend holding time and moderately increase holding pressure to supplement volume shrinkage of melt. Optimize product structure to reduce rib thickness and avoid local material accumulation. Raise mold temperature to slow down surface cooling speed and facilitate feeding and shrinkage compensation. Adjust gate position to make thick-wall areas close to the feed inlet for smooth pressure transmission and shrinkage compensation.
6. Demolding Difficulty, Mold Sticking and Surface Pulling DamageBiodegradable materials feature high hardness and strong adhesion after cooling, easily sticking to cavities and causing demolding pulling damage and surface fuzzing. Increase draft angle at all positions and conduct fine polishing on cavities. Lower mold temperature to enable rapid shaping and shrinkage of products away from cavity surface. Reasonably increase the number of ejector pins and enlarge ejection area to balance ejection force. Properly use special release agent suitable for degradable plastics without excessive dosage to avoid affecting bonding and degradation performance.
7. Obvious Weld Lines and Low StrengthObvious weld lines easily appear at multi-gate and flow around positions, affecting appearance and structural strength. Raise barrel and mold temperature to improve melt fusion performance. Accelerate injection speed to reduce temperature drop of melt front. Adjust gate position to avoid appearance surfaces and key stress positions. Add exhaust at weld line positions for better melt fusion.
