Creating plastic injection molds requires a high level of technical expertise and attention to detail to prevent mistakes. These mistakes, known as molding defects, can end up costing companies a lot of money when mass producing parts for clients, such as in the markets that we serve. Luckily, our experts at Midstate know how to analyze and solve these defects when they appear in the plastic injection molding process. This article enlightens our readers on some of the molding defects that can occur during the injection process.
1. Flow Marks
Flow marks are lines or streaks that are visible on the surface of the molded part. They result from uneven cooling of the plastic. For this defect to occur, the molten material cools rapidly within the mold and becomes highly viscous. Once the melt starts to coagulate, it gets pushed by the melt injected afterward and forms the linear grooving or miniature bands on the surface of the molded part. It indicates the direction of material flow within the cavity.
Two Solutions for Flow Mark Defects
- Increase the injection pressure: By increasing the pressure, it will force the molten plastic against the mold cavity steel before the plastic solidifies—this will remove flow lines and duplicate the cavity finish.
- Increase the mold temperature: Generally, a hot mold will allow a material to stay molten longer because it allows the molecules to pack together correctly before they solidify. A cold mold produces the opposite, the molecules solidify before being packed together.
2. Burn Marks
Burn marks are discolorations, usually dark reddish brown streaks, on the surface of the final molded part. This defect has several possible causes, but it all results from trapped air being rushed into the cavities of the injection mold. During the injection phase (or filling stage), the air trapped inside the cavity mold is highly pressurized and becomes superheated that can scorch the plastic. The dark color is carbon residue from the burned front edge of the flowing plastic.
Two Solutions for Burn Mark Defects
- Reduce the injection speed: This will improve the ability of the tool’s vents to expel the trapped air.
- Size and location of vents: Consider if the vents should increase in size or the location is adequate for the tool and material.
3. Short Shots
A short shot is a molded shot that has fallen “short” of filling the mold cavity: This simply means the molten plastic has failed to fully occupy the mold cavity. In some cases, short shots are created intentionally to visualize the filling pattern in the injection stage. However, in this case we are discussing how we troubleshoot problematic short shots.
Two Solutions for Short Shots
- Mold or melt temperature: Increase either/or temperatures of these parts. This will help to increase the flowability that will fill the hardest-to-reach part of the cavity/cavities.
- Inadequate gas venting: Plastic material cannot occupy the space that air or gas is already occupying. Account for gas generation and design the mold so that gas is not trapped within the mold. It then can be properly vented.
At Midstate Mold, we have been producing plastic injection parts since 1965 for our global market. Our team of expert plastic injection molders are vigilant about quality and can manage molding defects should they arise. Contact us today to learn more about our quality control efforts and capabilities. Also, keep an eye out for part II of this blog discussing other molding defects.