There are various causes of mould defects. These include standard process errors, batch-to-batch variations in materials, changes in machine calibration after repair or maintenance, and environmental factors. Yes, experience can also play a role, but in very different forms.
For example, a long-time operator operating a machine for the first time may not be familiar with the quirks and peculiarities of the machine, which can lead to inevitable mistakes.
Some casting errors can be challenging to fix. Other problems can be avoided by adjusting the moulding process without redesigning moulds or replacing other production equipment.
-
Flow lines
Flow lines appear as an undulating pattern, often in a slightly different color than the surrounding surface, and usually in narrow areas of the part. They may also appear as annular bands on the surface of the product near the mold entry point or "gate" through which the molten material passes.
Flow marks generally do not affect component integrity. However, when found in certain consumer products such as luxury sunglasses, it can be unsightly and unacceptable.
Causes and countermeasures for flow lines:
Flow lines are most often the result of changes in the cooling rate of the material as it flows through the molding in different directions.
Due to the difference in wall thickness, the material cools at different rates and may leave streamlines. Bad injection molding.
For example, molten plastic cools very quickly during the injection process, and flow marks appear if the injection speed is too slow. The plastic partially hardens and becomes rubbery while filling the mold, revealing a wavy pattern.
Flow line measures for injection molded products:
Increase injection speed, pressure, and material temperature to ensure the material fills the mold before cooling.
Round the corners of the mold where the wall thickness increases to keep the flow rate constant and avoid flow lines.
Offset the mold gate to create a gap between the gate and the mold coolant to prevent the material from cooling too much during flow.
Increase nozzle diameter to increase the flow rate and prevent premature cooling
-
Burn marks
Burning usually appears as a black or rust-colored discoloration on the edges or surfaces of molded plastic parts. Injection Molding Defects Burnt usually does not affect the integrity of the part unless the plastic is burnt to the point of degradation.
Causes and prevention of burns
A common cause of burnt injection molded parts is air entrapment or overheating of the resin itself in the mold cavity during the injection. Excessive injection speed or heating of the material often leads to overheating which causes burns.
Measures to prevent burns on molded products:
Reduce resin and mold temperatures to avoid overheating.
Reduce the injection speed to limit the risk of air pockets in the mold.
Enlarge gas vents and gates to release trapped air from the mold.
Reduce mold cycle time to prevent trapped air and resin from overheating.
-
Warping
Warpage is the deformation that can occur in injection molded parts when different parts of the part shrink unevenly. Just as wood warps when it dries unevenly, plastics and other materials warp when uneven shrinkage during the cooling process puts too much stress on different areas of the molded part.
This excess stress causes the finished part to bend and twist as it cools. This is evidenced by parts that are meant to lay flat but leave gaps when placed on a flat surface.
Part distortion causes and avoidance:
One of the main causes of warpage in injection molded plastics and similar materials is cooling too quickly. Injection Molding Defects In many cases, excessive temperatures or poor thermal conductivity of the molten material can exacerbate the problem. Also, mold construction can cause warpage if the mold wall thickness is not uniform. Shrinkage increases with increasing wall thickness.
Common methods to prevent warpage in molded parts:
Make sure the cooling process is slow and long enough so that the material is not stressed unevenly
Decrease material or mold temperature
Try switching to a material that shrinks less when cooled
Redesigned geometry with uniform wall thickness and part symmetry to increase part stability during cooling
-
Vacuum voids/air pockets
Vacuum voids or air pockets are trapped air bubbles that occur in the finished part. Quality control professionals typically consider voids to be "minor" defects. Bad injection molding. However, large numerous cavities can weaken the molded part because there is air below the surface of the part where the mold material should reside.
Measures to avoid vacuum inclusions in molded parts:
One of the main causes of voids is insufficient molding pressure to force trapped air out of the mold cavity. In other cases, the material closest to the mold wall cools too quickly and hardens, pulling the material outwards and creating an abscess.
The material itself is particularly prone to voiding when its density changes significantly from the molten state to the hardened state. Voids become more difficult to avoid when the wall thickness of the part exceeds 6 mm.
Common ways to avoid tooth decay include:
Increase injection pressure to push out trapped air pockets
Choose a lower viscosity grade material to reduce the risk of air bubbles
Place gates near the thickest part of the mold to prevent premature cooling where the material is most likely to avoid.
5. Sink marks
Sink marks are small depressions or indentations on the flat, uniform surface of a part. These can occur when the inside of the molded part contracts and draws in material from the outside.
Causes and prevention of sink marks:
Sink marks are similar to vacuum cavities, but the cause and effect are reversed. Injection Molding Failure The material cools too slowly near the outside of the part instead of cooling quickly. The resulting shrinkage pulls the outer material inward before it cools sufficiently, creating a dent. Like blow holes, sink marks are more likely to occur in thicker parts of the component.
Steps you can take to avoid this error:
Increase holding pressure and time until the material near the surface of the part has cooled.
Increase cooling time to limit shrinkage
Design the mold with thinner walls for the component to allow faster cooling near the surface.
If you have problems with injection molding defects or want to learn more about quality assurance for molded parts, contact Dowell Molds, the best mold manufacturer in Vapi.