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DEFECTS OF INJECTION MOLDING

Making injection molded prototypes is both an art and science. High levels of technical expertise are required to prevent small mistakes from costing companies big amounts of money when it comes to mass-production of parts. Most mistakes are caused by careless personnel who don’t have proper necessary experience or the right tools at their disposal. 

Flow Lines

Flow lines are streaks or patterns that show up as a consequence of the physical path the molten plastic makes as it flows into the injection mold tooling cavity. Injection molded plastic begins its journey through an entry section called a “gate.” It then flows through the tool cavity and cools. Flow line defects are caused by the speed difference at which the molten plastic flows as it changes direction and bends inside the mold tool. This also occurs when the plastic flows through sections with varying thickness. The temperature in the plastic injection molding machine of the molten plastic or the mold itself can also be elevated to ensure the plastic does not cool down sufficiently to cause the defect.

Sink Marks

Sink marks are small craters that develop in thicker areas of the injection molded dummy model when shrinkage occurs in the inner sides of the finished product. The effect is somewhat similar to sinkholes in land but caused by shrinkage rather than erosion. Sink marks are often caused when the cooling time or the cooling mechanism is insufficient for the plastic to get fully cooled while in the mold. They can also be caused by insufficient pressure in the cavity, or by an excessive high temperature at the gate. By reducing the thickness of the wall sections we can also ensure faster cooling and help reduce the risk ofsink marks.

Vacuum Voids

Vacuum voids are pockets of trapped air within surface of an injection molded prototype in a plastic injection molding machine. Vacuum voids are often caused by uneven solidification between the surface and the inner sections of the model. Voids can also develop from a part that is casted from a mold with two halves that are not correctly aligned.  This can be done by increasing the  holding pressure as well as holding time.

Surface De-lamination

Surface de-lamination is a condition where thin layers appear on the part due to an infected material. These layers appear like coatings and can usually be peeled off.  Foreign materials that get into the molten plastic separate from the finished product because the infected material and the plastic cannot bond together.  This can be prevented by Pre-drying the plastic properly before moulding.

Weld Lines

 Weld lines are actually more than a line that appears in a part where molten plastics meet each other when they flow from two different parts of the mold.  Weld lines are caused by the inadequate bonding of two or more flow fronts when molten plastic solidifies partially.  It can be controlled by raising the temperature of the mold or molten plastic, or by increasing the injection speed.

Short Shot

As the term says, short shots can be described as a situation where a molding shot falls short. This means that the molten plastic due to some reason does not fully occupy the mold cavity, resulting in a portion where there is no molten plastic in the plastic injection molding machine. The finished product becomes deficient because it is incomplete. Short shots can be caused by a number of things like if the plastic is too viscous; it may solidify before fully covering all the cavities and result in a short shot. The remedy to overcome this is by selecting a less viscous plastic which can flow faster or by increasing mold or melt temperature so as to increase flow ability.

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