Injection molding transition fillet
Injection molding transition radius is a key element in plastic part design, connecting different wall thicknesses or curved surfaces. Its proper design can significantly improve a product’s mechanical properties and molding stability. At corners, using right-angle transitions can easily generate eddy currents during melt flow, leading to increased localized pressure loss. Furthermore, stress concentration zones form inside the right-angle during cooling, making the part susceptible to fracture at the corner when subjected to stress. For example, a plastic toolbox originally designed with 90° corners was injection molded using PP. Drop tests revealed that 80% of the damage was concentrated at the right-angle corners, and the fracture surfaces exhibited distinct brittle fracture characteristics. Replacing the right-angle corners with a 2.5mm transition radius reduced melt flow resistance at the corners by 30%, shortened filling time by 5 seconds, reduced the drop test breakage rate to 12%, and transformed the fracture pattern into a ductile fracture, demonstrating that stress concentration was effectively alleviated.
The size of the transition fillet must be determined based on the material properties of the plastic part and the intended use scenario. For crystalline plastics like POM, due to their significant shrinkage, excessively small fillets can result in uneven shrinkage at the corners, creating sink marks. A gearbox housing made of POM was initially designed with a corner radius of R1mm. After production, significant concavity was observed on the inside of the fillet, with a surface roughness Ra reaching 1.6μm. Mold CAE analysis revealed that the melt at the R1mm fillet cooled 15% faster than the surrounding area, leading to sink marks. By increasing the fillet radius to R3mm and raising the mold temperature to 120°C, the cooling rate at the corners became consistent with the surrounding area, reducing the sink mark depth to 0.03mm and improving the surface roughness to Ra0.8μm, fully meeting the required assembly surface precision. Furthermore, the increased fillet radius increased the gearbox’s impact strength by 22% and extended its service life by 1.5 times.
In thin-walled plastic parts, the design of transition radius plays a crucial role in preventing warping and deformation. A mobile phone case, manufactured using PC+TPU dual-shot injection molding, is only 1.0mm thick. The original design for the frame corners was a 0.5mm radius. After demolding, the case began to warp inward, with a maximum deformation of 0.8mm, affecting its fit with the phone. Through simulation analysis, the technical team discovered that small radiused corners cause instability in the melt flow front during the filling phase, resulting in localized orientation differences. By adjusting the frame corner radius to 1.2mm and introducing a 0.5mm gradient transition at the junction of the radiused corner and the flat surface, the melt flow became smoother and orientation differences were reduced by 40%. Following these improvements, the case’s warpage was controlled within 0.2mm, the assembly yield increased from 78% to 96%, and the bond strength between the TPU soft adhesive and the PC hard adhesive increased by 18%, effectively preventing cracking during use.
The impact of transition radius on mold life and production efficiency is often overlooked. Improperly designed radius can lead to increased localized wear in the mold cavity and shorten repair cycles. A washing machine drum made of HIPS material had a radius of R0.8mm at the junction of the rib and the drum wall. After 50,000 cycles, the mold began to show noticeable hair picking at this location, resulting in scratches on the part surface and necessitating shutdown for mold repairs. By increasing the radius to R1.5mm and nitriding the mold cavity, the wear resistance at this location increased threefold, allowing the mold to continue production for 150,000 cycles without noticeable wear. Furthermore, the increased radius reduced melt filling resistance and injection pressure by 12%, saving energy and shortening the cycle time by 3 seconds per mold. Based on an average daily production of 20,000 molds, this increased annual production capacity by 2.19 million molds.
The design of transitional fillets for plastic parts with special structures requires innovative solutions. For example, in parts with undercuts, the coordination of the fillet and the core-pulling mechanism directly impacts the demolding process. A toy car wheel axle sleeve features an inner undercut. The original design had a 0.6mm radius at the base of the undercut, which resulted in significant resistance to the hydraulic core-pulling mechanism during extraction and prone to deformation of the plastic part. The technical team employed a variable-radius fillet design: the fillet on the side closest to the core-pulling direction remained at 0.6mm to ensure structural strength, while the radius on the other side was increased to 1.2mm to reduce core-pulling resistance. This improvement reduced core-pulling force by 25%, stabilized the hydraulic system’s operating pressure, and reduced part deformation from 9% to 1.5%. Furthermore, this design ensures more complete melt filling in the undercut area, reducing air entrapment. The pass rate for airtightness testing of plastic parts increased from 89% to 99%, meeting safety standards for toy products.
