Common defects/problems in plastic injection molds and their potential solutions (Chapter 1/8):

 1. Blisters: 

In injection molding, blisters refer to small, bubble-like defects that can form on the surface of a molded part. These blisters are typically caused by trapped air or gas within the plastic material during the injection and cooling process. When the trapped air expands, it creates these unsightly blemishes on the part's surface. Blisters can be a result of improper venting, excessive melt temperature, or other process issues. Addressing the root cause, such as adjusting the mold design, temperature settings, or injection parameters, can help prevent the formation of blisters and improve the overall quality of the molded parts.

Solution:

Reduce moisture content in the raw material by proper drying and storage, as trapped moisture can turn to steam during the injection process and cause blistering.

 

Adjust the injection parameters, such as injection speed, melt temperature, and holding pressure, to ensure complete and uniform filling of the mold cavity and prevent air entrapment that can lead to blistering.

2. Sink marks: 

Sink marks in injection molding are small, visible depressions or indentations that appear on the surface of a molded part. These defects occur when the plastic material in the mold cavity shrinks during the cooling process, but the surface of the part is unable to fully follow the shrinkage. This can happen due to factors such as uneven wall thickness, improper venting, or excessive cooling in certain areas. Sink marks can negatively impact the appearance and functionality of the molded part, so addressing their root causes is important in maintaining high-quality injection molding production.

Solution:

Optimize the part design by adding ribs, gussets, or other features to provide structural support and reduce the tendency for sink marks in thick or uneven sections.

Adjust the injection parameters, such as holding pressure and time, to ensure complete packing of the mold cavity and adequate material flow to compensate for volumetric shrinkage in problematic areas.

3. Weld lines: 

Weld lines in injection molding are the visible lines or marks that appear on the surface of a molded part where two or more flow fronts from the injection nozzle meet and merge. These lines can be caused by factors like improper mold design, uneven melt flow, or incorrect injection parameters. Weld lines can affect the part's appearance, strength, and overall quality, so they are an important consideration in the injection molding process. Minimizing or eliminating weld lines is often a key goal in part design and process optimization.

Solution:

Optimize the mold design by adjusting gate locations, adding flow leaders or melt guides, and ensuring uniform melt flow throughout the cavity.

Adjust the injection parameters such as melt temperature, injection speed, and holding pressure to improve melt viscosity and promote better fusion at the weld line.