Material choice must align with product performance requirements, aesthetic standards, and molding process constraints. Common thermoplastics include:
Polyethylene (PE): Ideal for low-cost, high-density applications (e.g., packaging).
Polypropylene (PP): Versatile for automotive and consumer goods due to its toughness and chemical resistance.
Polyvinyl Chloride (PVC): Suitable for rigid structures requiring flame retardancy.
Polystyrene (PS): Lightweight and transparent, often used in consumer electronics.
Acrylonitrile-Butadiene-Styrene (ABS): Combines strength, impact resistance, and processability for automotive and appliance parts.
Key material properties to consider:
Melt Flow Characteristics: Viscosity and shear thinning behavior affect mold filling and part quality
Shrinkage Rate: Varies by material (e.g., PP ≈1.0-2.5%, PA66 ≈0.5-1.5%) and must be compensated in mold design
Thermal Stability: Critical for high-temperature processes (e.g., PA requires a barrel temperature ≥275°C)
Many polymers (e.g., Polyamide (PA), Polycarbonate (PC)) must be dried prior to molding to avoid defects:
PA6/PA66: Dry at 80-120°C for 4-8 hours to reduce moisture content to <0.1%
PC: Dry at 120-130°C to prevent hydrolysis and maintain impact strength
PVC: Moisture content must be controlled to avoid voids and discoloration
Uniform particle size and shape ensure consistent melting and flow:
Granule Size: Typically 2-5 mm for standard machines; fine powders (<1 mm) may cause bridging in the hopper
Shape: Spherical granules improve flow characteristics; irregular shapes may lead to uneven packing and warpage
Finished parts must meet strict aesthetic criteria:
Defects to Avoid: Bubbles, weld lines, sink marks, and flash (excess material around edges)
Surface Finish: Achieved via mold polishing (B3/N4 standards) or texturing; critical for consumer products
Color Consistency: Use of masterbatch or pre-colored granules ensures batch-to-batch uniformity
Tolerances are typically controlled within:
±0.1-0.5 mm for general parts (e.g., automotive components).
±0.01 mm for high-precision applications (e.g., micro-molding)
Key Factors: Mold design (parallelism, cooling uniformity), material shrinkage, and injection speed
Testing must validate:
Tensile Strength: PA66 (80-120 MPa), ABS (30-60 MPa)
Impact Resistance: Izod test (e.g., PC ≥30 J/m, PP ≥1.5 J/m)
Flexural Modulus: Determines rigidity (e.g., PC ≈2400 MPa, PE ≈100 MPa)
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