Molding Burst: Causes and Solutions
Molding burst is one of the most common failures observed in injection molding, and is typically caused by a number of different factors. This failure is characterized by the rupture of large sections of the molded part, often occurring around the thicker sections or corners. In some cases, small cracks can be observed, followed by catastrophic failure along the length of the part. The primary causes of molding burst range from improper material selection to design-based issues and improper injection molding machine settings.
Material Selection and Processing
Incorrect material selection is one of the most common causes of molding burst. The melting point, mold shrinkage, and flow characteristics of the material must be carefully considered when selecting a material for injection molding. Materials with a high melt flow can be particularly susceptible to molding burst since they exhibit more rapid shrinking after cooling. The molding cycle should also be optimized for the particular material selection, with flow rate and injection pressure balanced to provide adequate part filling.
Inadequate Tooling Design
Molding burst can also result from inadequate tooling design, characterized by insufficient part walls or inadequate flow channels. The wall thickness of the part should be appropriate for the loaded material, but overly thick walls can make it difficult for the material to flow and fill the cavity. The evolution of flow channels should also be considered to provide the necessary venting and fair part filling, particularly with highly filled or glass reinforced materials.
Incorrect Machine Settings
Incorrect machine settings can also lead to molding burst in injection molding. The injection speed and pressure should be balanced to provide adequate pressure to fill the cavity while minimizing potential shear damage in the material. In some cases, molding too quickly or with excessive pressure can cause rapid cooling of the material, resulting in rapid shrinkage or material degradation due to thermal stresses.
Inadequate Cooling
Adequate cooling is essential for preventing molding burst, since cooling determines the rate at which the material solidifies and shrinks back from the walls of the cavity. Insufficient cooling results in material that remains hot and plastic when the part is ejected from the mold, making it vulnerable to molding burst. The cooling channels should be optimized to provide even cooling throughout the part, avoiding opportunities for deviation from the intended shape and increasing the risk of appearance defects.
Conclusion
Molding burst can be a challenging issue to diagnose and resolve, but is often caused by material selection and processing issues, inadequate tooling design, incorrect machine settings, and inadequate cooling. By making the necessary adjustments to each of these elements, the potential for molding burst can be minimized and the quality of the parts improved.
