For an updated version of this blog entitled "What Causes Injection Molding Defects and How to Fix Them, read here."
Making injection molded prototypes is both an art and a science. The process has a few pitfalls, mostly caused from inexperience, and solutions are found in the expertise borne of experience and ingenuity.
Here are a few examples of defects that can occur in a molded part and ways to fix them:
Description: Flow lines are lines, streaks, or patterns that show up on the prototype, caused by the molten plastic passing through the mold or gate. They are commonly off-toned in color.
Causes: These defects are caused by the varying speeds of the molten plastic as it changes direction through the contours and bends in the mold. It also occurs when the plastic flows through sections with varying wall thickness. In addition, flow lines occur when the injection speed is too low and the molten plastic is solidifying and more viscous while in the mold.
- Injection speeds and pressure should be increased to the optimal level, which will ensure the proper filling of cavities while not allowing the molten plastic time to start cooling. The temperature of the molten plastic or the mold itself can be elevated to ensure the plastic does not cool down sufficiently to cause the defect.
- Corners and locations where the wall thickness changes should be rounded to avoid sudden changes in direction and flow rate.
- Locating the gate at a location with thin walls can also help against the defect.
Description: Sink marks are small craters or depressions that develop in thicker areas of the injection molded prototype when shrinkage occurs in the inner portions of the finished product. The effect is somewhat similar to sinkholes in topography but caused by shrinkage rather than erosion.
Causes: Sink marks are often caused when the cooling time or cooling mechanism is not adequate for the plastic to fully cool and cure while in the mold. It can also be caused by inadequate pressure in the cavity or by an excessive temperature at the gate.
- Mold temperatures should be lowered, holding pressure increased, and holding time prolonged to allow for more adequate cooling and curing.
- Reducing the thickness in the thickest wall sections will also ensure faster cooling.
Description: Vacuum voids are pockets of air trapped within or close to the surface of an injection molded prototype.
Causes: These are often caused by uneven solidification between the surface and the inner sections of the prototype. This can be aggravated when the holding pressure is insufficient to condense the molten plastic in the mold and force out air pockets. Voids also develop when a part that is cast from two halves of a mold is not aligned correctly.
- The gate location should be at the thickest part of the molding.
- Switching to a less viscous plastic will ensure that less gas is trapped.
- Holding pressure as well as holding time should be increased.
- Mold parts need to be perfectly aligned.
Description: Surface delamination is when thin surface layers appear on the part due to a contaminant material. These layers appear like coatings and can usually be peeled off.
Causes: Foreign materials that find their way into the molten plastic separate from the finished product because the contaminant and the plastic cannot bond. The fact that they can’t bond not only has an affect on the appearance of the prototype but also on its strength. The contaminant becomes like localized faults trapped within the plastic. An over-dependence on mold release agents can also cause delamination.
- Pre-dry the plastic properly before molding and increase the mold temperature.
- Smooth out the corners and sharp turns in the mold design to avoid sudden changes in melt flow.
- Focus more on the ejection mechanism in the mold design to reduce or eliminate the dependence on mold release agents.
Description: The weld line is actually more like a plane that appears in a part where molten plastics meet each other as they flow from two different parts of the mold.
Causes: It is caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic.
- Raise the temperature of the mold or molten plastic.
- Increase the injection speed.
- Adjust the design for the flow pattern to be a single source flow.
- Switch to a less viscous plastic or one with a lower melting temperature.
Stay tuned for Part 2