Flow marks, or lines that appear faded from the rest of the printed material, can occur during injection molding due to slow injection rates. This is seen as a defect and manufacturers monitor equipment to reduce their incidence. However, flow marks can also be intentionally created for an interesting pattern.
Sometimes known as flow lines, flow marks are a phenomenon that can occur during the injection molding process. A flow mark usually manifests itself as a line or series of lines that form a pattern that is slightly faded from the rest of the printed material. There are several reasons why this type of pattern can appear, including problems with how fast the injection is happening.
Creating a flow mark is usually an indication that there is some problem with the process used to create the plastic. Most commonly, flow mark patterns occur when the injection rate is slower than it should be. When this happens, the plastic has time to cool during injection, resulting in the irregular, somewhat wavy lines that appear in the molded material. Typically, increasing the injection speed will reduce the incidence of flow notch patterns and allow plastic materials to have a spotless appearance.
For the most part, the presence of a flow mark is seen as a defect. This means that manufacturers will normally monitor the efficiency of the equipment used in the injection molding process to ensure that injections are occurring at a rate that complies with company standards. Quality inspectors typically examine samples from each batch of plastic items produced to determine if there is any flow mark patterning and require adjustments before the next batch is poured using the same equipment. When some sort of malfunction occurs and a part or parts are produced with prominent flow lines, they are often sold as second-rate goods, even though the parts can normally be used with no apparent decrease in efficiency.
Companies generally try to keep the incidence of such marks as low as possible. This is true when the manufacturing process requires plastic items to be produced to have smooth surfaces with no lines or patterns in the plastic itself. There are cases where creating the flow mark is intentional, by adjusting the injection speed so the plastic has a chance to cool and form the pattern. By running the injection at various speeds throughout the process, an interesting pattern can be created on the finished part.
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