Solving the problem of injection molding sink marks


For injection molding specifications, sink marks can ruin a part’s aesthetic look and finish. Sink marks are a common issue in the injection molding process, but it doesn’t have to be this way. With the proper set of considerations, you can reduce or eliminate sink marks from your parts using intelligent choices for design details like gate location, wall thicknesses, and draft angles. To help ensure successful performance goals with minimal product variations due to issues stemming from poor surface quality or secondary operations such as polishing, in this blog post we will discuss strategies that can help manage or even eliminate injection molders’ most feared defect: sink marks. 

Continue reading to understand how proper management of injection mold temperature and pressure controls along with design changes can solve most problems associated with these cosmetic blemishes easily.


What are injection molding sink marks and what causes them?

Injection molding sink marks occur when the injected plastic cools before it has had a chance to fully fill the injection mold. This creates a visible imperfection on the final injection molded part, typically appearing as shallow dimples or depressions in the molded object. 

Sink marks can be caused by many factors such as insufficient injection pressure, incorrect injection speed and injection temperature, uneven material distribution within the injection cavity, excessive wall thickness, and inadequate mold venting. Minimizing sink marks is essential in injection molding since they decrease product quality and reliability; however, with proper injection parameters development and efficient production of high-quality parts can be achieved.


How do you prevent sink marks?

Sink marks can ruin an injection molded part, making it unusable and expensive to remake. The best way to prevent sink marks is to prioritize injection molding sync. This means adjusting injection speed and holding pressure profiles of the injection molding cycle to ensure that material is flowed into all cavities evenly, reducing any thinning or non-uniform filling of the parts. By ensuring injection molding sync, part uniformity will be improved and all materials should fill out, creating a usable finished product with little to no sink marks.


Surface Finish and Part Color:

The injection molding process helps manufacturers be able to create parts with a wide variety of surface finishes and colors. Depending on the material that is being injection molded, creating different textures and colors is both possible and cost-effective. The injection molding syncs easily with colorant materials to give a uniform look and finish. By using injection molding, manufacturers can minimize costs by reducing waste and cutting down on post molding operations. This efficient production process offers customers of injection-molded parts the ability to customize their products while maintaining reasonable production times.


Mold Design

Injection mold design is an integral part of the injection molding process. It involves synchronizing many components to ensure that injection molds are created with precision and repeatable results. Working closely with injection mold manufacturers, injection mold designers create drawings and 3D CAD models which help in creating injection molds for parts that need to be mass produced. Mold designers have the difficult task of significantly impacting injection mold assemblies by manipulating different features like parting line areas and injection gate locations, making sure that the injection molds produce a high quality product consistently over time.


Insufficient packaging, holding pressure, and/or time:

Ensuring injection molding processes are in sync is crucial to success, with insufficient packaging, holding pressure, and/or time have potentially disastrous consequences. 

Quality control must be monitored through proactive methods such as automated temperature measurements, enabling process technicians to adjust injection velocities before injection molding temperatures deviate from the desired level. Likewise, it’s important to ensure injection molds are routinely inspected for any signs of wear or damage that could lead to injection problems along the injection lines. 

Doing so ensures injection processes remain within tolerances and that insufficient packaging or flawed parts due to improper holding pressure or time will not be produced further down the line.


How do you get plastic parts without sink marks?

A superb part design is crucial, but the success of your plastics project also depends on choosing the correct injection molder and tooling maker. To reduce sink marks and guarantee high-quality components, Fictiv is your operating system for custom manufacturing, and our technical experts are prepared to help you through the whole injection molding process.