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Sometimes known as flow lines, flow marks are a phenomenon that can occur during the process of injection molding. A flow mark is usually manifested as a line or series of lines which form a pattern that is slightly off-color from the rest of the molded material. There are several reasons why this type of pattern can appear, including issues with the speed that the injection is taking place.
The creation of a flow mark is usually an indication that there is some problem with the process used to created the plastics. Most commonly, flow mark patterns occur when the rate of injection is slower than it should be. When this happens, the plastic has time to cool during the injection, resulting in the uneven and somewhat wavy lines that appear in the molded material. Typically, increasing the injection speed will reduce the incidence of the flow mark patterns and allow the plastic materials to have an unblemished 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 make sure the injections are occurring at a speed that is in keeping with company standards. Quality inspectors usually examine samples from every lot of plastic goods produced in order to determine if there is any presence of flow mark patterns, and call for adjustments before the next lot is poured using the same equipment. When there is some sort of malfunction and a piece or several pieces are produced with prominent flow lines, they are often sold as second-quality goods, even though the pieces can normally be used with no apparent decrease in efficiency.
Businesses generally attempt to keep the incidence of those marks as low as possible. This is true when the production process calls for the production of plastic goods that are intended to have smooth surfaces with no lines or patterns in the plastic itself. There are instances in which the creation of the flow mark is intentional, adjusting the speed of the injection so that the plastic does have the chance to cool and form the pattern. By managing the injection at various speed throughout the process, it is possible to create an interesting pattern on the finished piece.