Injection molding is a formative manufacturing technique. Subtractive and additive techniques have become more popular in recent years. CNC machining is subtractive in that you start out with raw materials and cut away at them until you have a finished product. 3D printing is additive in that you start out with small raw materials and put them together to form a finished product. With a formative manufacturing technique, you actually form parts out of material either by heat or pressure.
Injection molding is formative in that you form a part by melting plastic and pouring it into a pre-created mold. The plastic then cools and hardens inside the mold leaving the desired part. When the part is then taken from the mold you can reuse the mold as many times as needed. While this is a quick and relatively inexpensive form of manufacturing, there are some potential defects that you have to watch out for. Five of those common defects will be discussed below.
Warping
Warping is caused when different parts of a molding shrink or harden unevenly. Warping makes an object slightly off-kilter and it may never lie flat or straight. The most common cause of warping is that the plastic cools too quickly whether because of the shape of the mold or the quality of the plastic. Some key ways to prevent warping are:
Ensuring a gradual and long cooling process.
Lowering the temperature of the molding or plastic.
Switching to a material that shrinks less or cools at a higher temperature.
Redesigning the molding with uniform wall thickness.
Sink Marks
Sink marks are small recesses or depressions in an otherwise flat surface. It is often caused when the material is cooled too slowly or parts of the product are too thick. Some keys to preventing sink marks include:
Increase the time and pressure within the mold.
Increase cooling time so that the item does not shrink.
Design the mold with thinner walls.
Drag Marks
Drag marks occur when the plastic shrinks and applies pressure on the mold. The term "drag" is applied because the finished product looks like it has been dragged along the ground while it was being shaped. This generally happens when the mold is removed from the plastic prior to its hardening completely, and parts with vertical walls and no draft angle are most susceptible. Some keys to preventing drag marks include:
Allow the plastic to cool and harden sufficiently.
Use designs with geometric angles rather than soft curves.
Knit Lines
Knit lines occur when two flow lines meet. Knit lines are not just aesthetically problematic, but they can also signify weakness in the design. Some keys to fighting against knit lines include:
Increasing injection speed, pressure, and temperature.
Rounding mold corners where walls are the thickest.
Increasing nozzle diameter.
Short Shots
A short shot occurs when the injected material does not fully fill a cavity in the mold. Short shots produce an incomplete finished product and is classified as a major defect. The most common causes are narrow or blocked gates or that the mold is too cool before injection. The keys to fighting against short shots are:
Redesigning the mold with wider gates.
Increasing injection speed or pressure.
Increasing mold temperature.
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