This is part two of a previous blog showcasing the most common defects that occur in molded parts and some ways to prevent them. You can find Part 1 here.
Description: As the term implies, short shots can be described as a situation where a molding shot falls short. This means that the molten plastic for some reason does not fully occupy the mold cavity or cavities, resulting in a portion where there is no plastic. The finished product becomes deficient because it is incomplete.
Causes: Short shots can be caused by a number of things. Incorrect calibration of the shot or plasticizing capacities can result in the plastic material being inadequate to fill the cavities. If the plastic is too viscous, it may solidify before fully occupying all the cavities and result in a short shot. Inadequate degassing or gas venting techniques can also result in short shots because air is trapped and has no way to escape; plastic material cannot occupy the space that air or gas is already occupying.
- Select a less viscous plastic with higher flowability. This plastic will reach and fill the hardest to reach cavities.
- Increase mold or melt temperature so as to increase flowability.
- Account for gas generation by designing the mold so that gas is not trapped within the mold and is properly vented.
- Increase the material feed in the molding machine or switch to a machine that has a higher material feed in the event that the maximum material feed has been reached.
Description: Warping (or warpage) is the deformation that occurs when there is uneven shrinkage in the different parts of the molded component.
Causes: Warping is usually caused by non-uniform cooling of the mold material. Different cooling rates in different parts of the mold cause the plastic to cool differently and thus creates internal stresses, which, when released, leads to the deformation called warping.
- Ensure that the cooling time is sufficiently long and that it is slow enough to avoid the development of residual stresses being locked into the part.
- Design the mold with uniform wall thickness and so that the plastic flows in a single direction.
- Select plastic materials that are less likely to shrink and deform. Semi-crystalline materials are generally more prone to warping.
Description: Burn marks are discolorations, usually rust colored, that appear on the surface of the injection molded prototypes.
Causes: Burn marks are caused either by the degradation of the plastic material due to excessive heating or by injection speeds that are too fast. Burn marks can also be caused by the overheating of trapped air, which etches the surface of the molded part.
- Reduce injection speeds.
- Optimize gas venting and degassing.
- Reduce mold and melt temperatures.
Description: Jetting refers to a situation where molten plastic fails to stick to the mold surface due to the speed of injection. Being fluid, the molten plastic solidifies in a state that shows the wavy folds of the jet stream on the surface of the injection molded part.
Causes: Jetting occurs mostly when the melt temperature is too low and the viscosity of the molten plastic becomes too high, thereby increasing the resistance of its flow through the mold. When the plastic comes in contact with the mold walls, it is rapidly cooled and the viscosity is increased. The material that flows through behind that viscous plastic pushes the viscous plastic further, leaving scrape marks on the surface of the finished product.
- Increase mold and melt temperatures.
- Increase the size of the gate so that the injection speed becomes slower.
- Optimize gate design to ensure adequate contact between the molten plastic and the mold.
Description: Flash is a molding defect that occurs when some molten plastic escapes from the mold cavity. Typical routes for escape are through the parting line or ejector pin locations. This extrusion cools and remains attached to the finished product.
Causes: Flash can occur when the mold is not clamped together with enough force (a force strong enough to withstand the opposing forces generated by the molten plastic flowing through the mold), which allows the plastic to seep through. The usage of molds that have exceeded their lifespan will be worn out and contribute to the possibility of flash. Excessive injection pressure may force the plastic out through the route of least resistance.
- Increase the clamp pressure to ensure that the mold parts remain shut during shots.
- Ensure that the mold is properly maintained and cleaned or replaced when it has reached the end of its useful lifespan.
- Adopt optimal molding conditions like injection speed, injection pressure, mold temperature, and proper gas venting.