The term “silver wire” is commonly used in injection molding to describe a defect where a thin strand or thread-like projection appears on the surface of the molded part. This defect is also known as stringing or cobwebbing. There are several causes for silver wire in injection molding:
1. High melt viscosity: When the melt viscosity of the polymer is high, it can result in excessive material flow during injection. As the material flows into the mold cavity, it can drag or stretch, leaving behind thin strands or threads on the surface. High melt viscosity can be influenced by factors such as high melt temperature, high molecular weight of the polymer, or improper processing conditions.
2. High injection speed: Too high can cause shear forces that lead to material stretching and stringing. The fast flow of molten material can create thin strands or threads as it moves through narrow gaps or features in the mold.
3. Insufficient cooling time: If the part is ejected from the mold before it cooled adequately, the material may still be semi-molten. This can cause material to stick to the mold surface or stretch during ejection, resulting in silver wire. Sufficient cooling time should be provided to allow the material to solidify entirely before demolding.
4. Improper gate design: The gate is the point where molten material enters the mold cavity. If the gate design is not optimized, it can create excessive shear forces during injection, leading to material stretching and stringing. Improving gate design, such as using an appropriately sized and positioned gate, can help minimize silver wire defects.
5. Material selection: The choice of material can also contribute to silver wire defects. Some materials have higher melt viscosities or greater propensity for stringing. Understanding the material’s rheological properties and selecting a suitable grade can help minimize this issue.
Optimizing processing parameters, such as melt temperature, injection speed, cooling time, and gate design, are essential to reduce or eliminate silver wire defects. Additionally, selecting the appropriate material and ensuring proper mold design can help minimize this defect. Conducting process validation and performing mold trials can help identify and address silver wire issues during injection molding.