Injection moulding is one of the most common processes by which plastic parts are manufactured.
In the injection molding process, a vast variety of components of various sizes, shapes, and complexities can be created.
The three main requirements of this process are:
- An injection moulding machine,
- Raw plastic material, and
- A mould
The whole process, in short, is this: the plastic material is first melted in the machine with hot runners, and then poured into the mould. There, it cools and solidifies into the desired component.
Injection moulding is used to create a variety of thin-walled plastic parts for use in household appliances, electronic goods, toy manufacturing, etc.
Errors made during the process of injection molding or due to the materials used in the process can turn out to be very costly for your business. Defects in the final product could range from harmless and superficial quality issues to bigger problems that could affect the safety and efficacy of the product.
If proper care is not ensured during the molding process, there can be many defects that are formed on the finished plastic product. In this article, we are going to discuss all the possible injection moulding defects, what causes them, and how they can be avoided.
Understanding what the possible defects could be and what causes them is the first step in ensuring that the products developed in the process consistently reach acceptable standards.
This could reduce losses incurred due to products that cannot be sold, or products that are returned after sale due to the defects.
Here are some of the most common injection moulding defects, along with their causes and what can be done to avoid them.
Defects In Injection Moulding
Possible Defects Caused Due To Process Problems
Sometimes, the problem lies not in the raw materials or the tools used in the process, but in the injection molding process itself. When you are sure that the defects are being caused by the process, all you need to do is adjust certain important factors like the wall thicknesses, and they can be easily avoided.
Some of these factors could be the flow rate, the pressure, or the temperature settings of plastic injection molding.
Let’s take a look at some defects that are caused purely due to faults in the injection mould process.
1. Flow Lines Or Flow Marks
What it is:
Flow lines (also called flow marks) are waves, streaks, or lines that are discoloured when compared to the area surrounding them.
These are usually observed near the ‘gate’ points or gate locations of the mould, where molten plastic enters it through plastic injection. They do not affect the functionality of the finished product, but are unpleasant, especially when found on products that need to be aesthetically pleasing.
What causes it:
Flow lines are caused by variations in the cooling rate of the melt plastic as it flows through the heated barrel and mould and solidifies. This also happens if there are thin walls and thick walls instead of uniform wall thickness, as the material cools at different speeds.
A slow injection speed or low pressure injection molding can also cause flow lines, as the plastic cools very quickly and becomes thicker, forming molding defects like wavy patterns or streaks.
How to rectify it:
- The simplest answer to this problem would be to increase the injection speed of the injection molding cycle.
- The pressure and temperature of the material injected into the mold can also be adjusted so that the type of plastic is able to completely fill the mould before solidifying.
- If flow lines appear due to sharp contours or lack of uniform wall thickness in the molding machine, it would help to round the corners of the mold cavities so that the pressure injection flow is consistent.
- Another strategy would be to change the location of the mould gates of the injection units where the molten plastic begins to flow in during the manufacturing process.
- Increasing the size of the nozzle can help increase the flow rate and avoid premature solidification of the plastic material by increasing the cooling time.
- Applying a lubricant to the surface will also assist the flow of the molten thermoplastic material in the runner system.
2. Burn Marks
What it is:
Burn marks are dark coloured (either black or rust coloured) marks on the edges or on the surface of the moulded product that are not part of the product design. They are usually harmless unless the burn has degraded the plastic.
What causes it:
The most common cause behind burn marks is the overheating of either trapped air bubbles or resin in the mould cavity during cycle times. This could be due to high injection speeds or overheating of the material.
How to rectify it:
- Burn marks can be avoided by reducing the injection speed and holding pressure of the molding tool. This will prevent air bubbles getting trapped inside the mould of the injection machine.
- Gas vents can be enlarged in order to facilitate easy escape of trapped air during low pressure injection.
- Reducing the temperature of the mould and hydraulic cylinders can also help prevent burn marks.
3. Warping
What it is:
Warping occurs when different parts of a product cools and solidifies unevenly during plastics processing. The resultant compression molding product is disfigured with bends and twists.
What causes it:
The main reason warping occurs in plastic products is due to premature cooling rates of the raw material. This could be either due to overheating or no and no uniform walls of the mould.
How to rectify it:
- Make the cooling process gradual so that there is no stress on the molten plastic material as it cools.
- Adjust the mould temperature during mass production.
- Use materials that do not shrink excessively during cooling.
- Ensure that the mould has a uniform thickness of its walls so that there is uniform cooling throughout the metal casting and clamping units.
4. Vacuum Voids / Air Pockets
What it is:
These are trapped air bubbles in the finished product. Although this is not considered to be a major defect, it could lead to a weakening of the final product during product development.
What causes it:
One of the most common reasons for vacuum voids in plastic products is insufficient pressure in the mould during lowvolume production. The air bubbles that are trapped in the mould are thus not pushed out while the plastic cools and solidifies in the insert moulding process.
How to rectify it:
- The blow molding injection pressure needs to be increased in order to force trapped air bubbles out of the mould.
- Select a material with low viscosity so that air bubbles are not formed in it easily.
- Ensure that there are gates near the thickest parts of the mould so that the plastic does not cool too soon.
5. Sink Marks
What it is:
These are small depressions formed in the plastic product. This usually takes place when the inner part of the component solidifies fast and shrinks, and the outer material is forced inward due to shot weight.
What causes it:
Sink marks occur because the molten plastic material cools too slowly in the mould. Thus, the outward material is pulled inward before it has a chance to cool completely, causing a recession in the material.
This usually occurs in the thickest parts of the mould due to uneven cooling of the plastic material.
How to rectify it:
- A simple way to rectify this problem would be to increase the pressure so that there is more time for the material to cool and shrinkage is limited.
6. Weld Lines
What it is:
These are lines that appear on the surface of the plastic product.
What causes it:
It happens when molten plastic converges from different channels at a certain point in the mould and bonds weakly to each other. This results in a weakening of the plastic component at the weld line.
How to rectify it:
- The material temperature needs to be increased so that there is uniform solidification of the material throughout.
- The injection speed and pressure can be increased to ensure that there is no premature cooling inside the mould.
- A material with lower viscosity can be chosen as the raw material so that it flows easily and does not cool prematurely.
7. Jetting
What it is:
Jetting is a sort of deformation in the plastic product that can lead to weakness of the part.
What causes it:
Jetting, as the name suggests, is caused by a sudden jet of molten material entering the mould and cooling faster than the rest of the material.
This could be due to high injection pressure, resulting in the material squirting in through the gate into the mould instead of filling it in gradually.
This jet of molten plastic starts to cool before the rest of the cavity is filled, leading to wavy formations on the surface of the finished product.
How to rectify it:
- One way to rectify jetting in plastic components is to reduce the injection pressure. This will prevent squiring of the molten material into the mould.
- The temperature of the mould and material can be increased so that the jet stream does not solidify faster than the rest of the material.
- Optimal designing of the mould and reciprocating screw so that jetting does not occur is another solution.
Possible Defects Caused Due To Material Use Or Storage
Sometimes, defects in the injection moulding process can occur due to the material being used.
Here are some of the defects related to the plastic resins used or its storage prior to use.
8. Discolouration
What it is:
Discolouration is when the finished plastic product is a different colour than it is supposed to be.
The discolouration is usually present in one part of the product or as just a few streaks of a different colour on the surface of the component.
Discolouration is purely superficial and does not affect the strength or functioning of the finished product.
What causes it:
One probable cause of discolouration could be left over pellets of plastic material from a previous run in the machine. It could also be due to some residual material stuck in the nozzle or mould due to low pressure.
Other reasons could be that the colouring agent has not mixed well with the raw material.
How to rectify it:
- Make sure that all parts of the injection moulding machine like the hopper, nozzle, mould, etc. are thoroughly cleaned before you run a fresh batch.
- Purge the machine regularly to remove any excess colour or residual material.
- Make sure that your supplier uses colouring agents with sufficient thermal stability and that your raw material is properly mixed with the colouring agent.
9. Surface Delamination
What it is:
Delamination is when thin layers from the finished product start to peel away from its surface.
This is a relatively serious problem, as flaking of the surface material would eventually cause the product to weaken and lose its functionality.
What causes it:
Usually, surface delamination occurs because of the contamination of the raw materials with particles of a foreign substance like sheet metal.
The surface of the product starts flaking when the material cannot properly bind together.
This is especially the case when two or more different plastics are combined to create a product.
This is very dangerous if the finished product is a crucial component in terms of safety.
Another reason for surface delamination could be that the material has not dried properly prior to use and has moisture on its surface.
How to rectify it:
- Make sure that the material has been dried properly before loading it into the hopper of the machine.
- You can also increase the mould temperature to ensure that the material is dried.
- Be very careful in the storage and handling of the raw material to avoid possible contamination.
- Redesigning of the nozzle also would help to make sure contaminants do not enter the mould easily.
Possible Defects Caused Due To Faults In The Mould
There are some defects that occur in finished products due to poor design or improper maintenance of the mould.
10. Short Shot
What it is:
During a run, if the molten plastic material doesn’t fill in the mould cavity completely, the final product will have defects and be incomplete after it cools and solidifies. This is short shot. It is a crucial defect that greatly affects the functionality of the product and increases production cost.
What causes it:
Blocked passages or gates in the machine are one of the main causes of short shot. This could especially take place when the gates are narrow.
If the material is too viscous, or if the mould is not at a high enough temperature, the mould will not be completely filled up before the material solidifies.
Trapped air in the material can also affect the flow of the molten resin. Another reason could be insufficient pressure during injection of the material.
How to rectify it:
- The best way to rectify short shot is to redesign the mould with larger gates so that the resin can flow in easily.
- The injection speed and pressure can also be adjusted to improve the flow.
- Adding a thinner base material will aid in the movement of the hot fluid plastic throughout the mould.
- The mould temperature can be increased to prevent premature cooling of the material.
- Increasing the number of air vents can facilitate the escape of any trapped air inside the material.
11. Flash
What it is:
Sometimes, there is a small flap of thin extra plastic material at the edges of the finished product. This is called flash, or spew, or burrs.
The reason for flash occurrence is that the plastic material has flown outside of the mould and solidified there.
Flash is usually not a serious issue unless it affects the aesthetics of the finished product. It can be easily trimmed off from the component.
What causes it:
A low cost mould that is not properly designed, or that has been worn out due to over-usage could be the problem behind flash.
A very high mould temperature or pressure during injection can also cause it.
Another contributor to flash is the insufficient clamping force of the plates containing the resin.
How to rectify it:
- The mould may need to be redesigned if it is faulty or worn out. The temperature and pressure can be adjusted to prevent flash.
- Sometimes, the clamping force of the plates needs to be increased to prevent it.
12. Silver Streaking
What it is:
Silver streaking can be described as streaks of silver or white in the finished product. They usually occur in the direction of the material flow.
What causes it:
These silver streaks are usually caused by excess water that is absorbed in the material.
It could also be due to contamination of the material with a foreign substance or air bubbles that are trapped inside it. Excess additive in the plastic material could also cause this.
How to rectify it:
- Ensure that the mould is thoroughly cleaned before starting a new run.
- Pre-dry the raw material and investigate any possible contamination.
- Reducing the injection speed, increasing mould temperature, and applying a back pressure can also help avoid formation of silver streaks.
- You could also try adjusting the size of the gates or their positions.
13. Product Stuck in The Mould
What it is:
This occurs when the finished product does not separate from the mould parts.
Usually, after a product has been moulded, it sticks to the moving part of the mould and is removed using ejector pins when the mould is opened. When that does not happen, it could lead to broken ejector pins.
What causes it:
This could happen due to any of the following reasons:
– The residual pressure of the mould is too high,
– There is shrinkage in the product,
– The mould draft is not large enough, or
– The mould finish is at an irregular angle
These reasons could cause malfunctioning of the ejector pins, and the product gets stuck inside the mould.
How to rectify it:
- Decreasing the temperature and pressure settings could help get the finished product unstuck.
- Increasing the size of the mould draft or redesigning the mould to have a better finishing could prevent this from happening.
- Sometimes, mould parting agents are used to avoid this problem.
As you can see, there are many possible defects that can take place due to faults in the moulding machine, the various tools, and the raw material that is introduced into the injection moulding machine.
It is important to ensure that your mould is designed and finished properly to avoid the additional costs of redesigning a faulty mould.
Ensuring the quality of all finished products will guarantee their safety and efficiency.
