Product Structure Design Specifications
- Injection Molding Parameters and Defect Analysis
1. Introduction to the Injection Molding Process
Injection Molding Process:
- Feed: Plastic pellets or powders are added to the hopper.
- Heating and Melting: Pellets are heated and rotated in the barrel, melting into a liquid state.
- Injection: Molten plastic is injected into the closed mold cavity via a nozzle, using a screw or plunger.
- Holding Pressure: Pressure is maintained after filling the mold to ensure complete filling and reduce shrinkage and warping.
- Cooling: Plastic solidifies in the mold, usually with the aid of cooling channels.
- Mold Opening and Ejection: After solidification, the mold opens, and the product is ejected.
- Finishing: Remove the gate, burrs, etc., and perform post-processing like heat treatment or printing.
Characteristics:
- Efficient: Produces large quantities of complex, precise products quickly.
- Versatile: Applicable to both thermoplastics and some thermosetting plastics.
- Economical: Although mold costs are high, the cost per piece decreases with mass production.
- Precise: Capable of creating high-precision, complex structural products.
Design Considerations:
- Mold design and manufacturing are crucial as they affect product quality and production efficiency.
- Process parameters such as temperature, pressure, and time must be accurately controlled to meet material and product requirements.
2. Pressure Estimation in Molding
Estimated Injection Pressure Formula:
- Estimated Injection Pressure = Area of Projected Product × Injection Stress of Product × Factor of Safety
Units:
- Estimated Injection Pressure: kg
- Product Projecting Area: cm² (includes runners)
- Product Injection Pressure: kg/cm²
Injection Pressure Values:
- Poor Flow Materials (e.g., PC, PBT): 600 kg/cm²
- Average Flow Materials (e.g., ABS): 400 kg/cm²
- Good Flow Materials (e.g., PP, PS): 300 kg/cm²
Factor of Safety: Typically 1.2 or 1.3
Example Calculation:
- For an ABS product with dimensions 50mm x 80mm
3. Common Molding Defects and Their Causes
Flash (Burrs):
- Causes: Poor mold clamping, excessive injection pressure, improper mold design.
Shrinkage and Sinking:
- Causes: Uneven cooling, poor material melting, incorrect mold temperature, insufficient holding pressure, or uneven wall thickness.
Warping and Deformation:
- Causes: Uneven cooling, poor temperature control, uneven internal stresses, or inadequate ejection system design.
Bubbles and Voids:
- Causes: Poor material drying, moisture in the material, insufficient mold degassing, high injection speed.
Color Inconsistencies and Spots:
- Causes: Uneven dispersion of colorants, partial material degradation, poor mold temperature control, inconsistent screw rotation.
Short Shots:
- Causes: Low injection pressure or speed, insufficient material volume, high mold filling resistance, poor material flow.
Mold Sticking:
- Causes: Excessive mold temperature, insufficient cooling, improper use of release agents, poor part design (e.g., inadequate draft angles).
Cracks and Stress Whitening:
- Causes: High internal stresses, rapid cooling causing uneven crystallization, part design with significant thickness variations.
Flow Marks:
- Causes: High injection speed, material flow issues, poor runner design.
Dimension Instability:
- Causes: Machine instability, mold wear, fluctuations in process parameters, environmental temperature changes.
Solutions:
- To address these defects, consider optimizing process parameters, improving mold design, selecting appropriate materials, and enhancing production environment conditions.