Top 5 Common Defects of Injection Molded Parts

Injection molding pressure

In injection molding, defects can arise from a variety of factors such as design, material, or processing. This blog post highlights the top 5 common defects in injection molded parts and provides insights on how to identify, troubleshoot and prevent them. Whether you are a product designer, engineer or manufacturer, understanding these defects is crucial for producing high-quality, defect-free parts.

Modo Rapid has relied on 18 years of mold manufacturing experience to summarize the top 5 most common defects and their causes. We share these insights with designers and product development personnel in hopes of increasing awareness of plastic injection-molded parts.

Introduction

injection molding and its importance

Injection molding is a manufacturing process that involves injecting molten material, typically plastic, into a mold cavity. The molten material is then allowed to cool and solidify, forming a finished part or product.

The injection molding process is widely used in manufacturing due to its efficiency, speed, and ability to produce large quantities of identical parts with high precision and accuracy. It is commonly used to produce a wide range of products, from small precision parts to large structural components, and can be used with a variety of materials, including plastics, metals, and ceramics.

Some of the key benefits of injection molding include the ability to produce complex shapes with high precision and accuracy, the ability to produce parts with a high level of consistency and repeatability, and the ability to produce large volumes of parts quickly and efficiently. Additionally, injection molding is a highly automated process, which helps to reduce labor costs and increase production efficiency.

Overall, injection molding is an important manufacturing process that is used across a wide range of industries to produce a diverse range of products. Its efficiency, precision, and versatility make it a popular choice for manufacturers looking to produce high-quality products quickly and cost-effectively.

Understanding common defects is crucial

Understanding common defects in manufacturing is crucial because it can help manufacturers identify and address potential problems before they occur, which can save time, money, and resources in the long run.

Common defects can occur in any manufacturing process, including injection molding, and can result in issues such as product failures, increased scrap rates, and reduced product quality. Some common defects in injection molding include sink marks, warpage, flash, short shots, and flow lines.

By understanding these common defects and their causes, manufacturers can take proactive measures to prevent them from occurring. This can include adjusting machine settings, changing the design of the mold or part, or modifying the manufacturing process to ensure that the defects are minimized or eliminated.

Additionally, understanding common defects can help manufacturers improve their overall manufacturing processes and quality control measures. By identifying and addressing defects early on, manufacturers can ensure that their products meet customer expectations and are of a consistent high quality.

In summary, understanding common defects in manufacturing is crucial because it helps manufacturers prevent problems, improve product quality, and optimize their manufacturing processes. By taking a proactive approach to defect prevention, manufacturers can save time and money, reduce waste, and improve overall customer satisfaction.

Injection molded part

Sink Marks

Definition of sink marks and how they occur

Sink marks are depressions or indentations on the surface of a plastic part that occur during the cooling stage of the injection molding process. They are caused by localized shrinkage in areas of the part that are thicker than others.

During the injection molding process, molten plastic is injected into a mold cavity under high pressure. The plastic then cools and solidifies, taking on the shape of the mold. However, as the plastic cools, it shrinks slightly, and if there are areas of the part that are thicker than others, they will cool more slowly and shrink more than the thinner areas. This can result in a depression or indentation on the surface of the part, which is known as a sink mark.

Sink marks can be unsightly and can affect the function of the part in some cases. They are more likely to occur in parts with thicker sections, and can also be exacerbated by certain factors, such as high mold temperatures, slow injection speeds, and high melt temperatures.

To minimize sink marks, injection molders can adjust their processes by optimizing the part design to reduce thick sections, using cooling or heating elements to control the mold temperature, adjusting injection speeds, or using different materials with lower shrinkage rates. By taking these measures, sink marks can be minimized or eliminated, resulting in a higher quality finished product.

Causes of sink marks, such as insufficient cooling or thick sections

Sink marks are depressions or indentations that can occur on the surface of a plastic part during the cooling process. There are several factors that can contribute to sink marks, including:

Insufficient cooling: When plastic parts are not properly cooled, the material in thick sections of the part may not fully solidify before the rest of the material does. This can create a depression or sink mark in the surface of the part.
Thick sections: The thicker the section of a plastic part, the more likely it is to experience sink marks. This is because thicker sections take longer to cool and solidify, increasing the likelihood of the material in those areas sinking or contracting.
Poor design: The design of a plastic part can also contribute to the occurrence of sink marks. For example, if a part has areas of varying thickness, it may be more prone to sink marks.
Inadequate packing pressure: The pressure at which the plastic is injected into the mold can affect the occurrence of sink marks. If the packing pressure is too low, the plastic may not fill the mold completely, leaving gaps that can result in sink marks.
Material selection: The type of plastic used can also contribute to sink marks. Some materials are more prone to sink marks than others, and the processing parameters may need to be adjusted accordingly to reduce the likelihood of sink marks.
Molding conditions: The temperature and humidity of the environment in which the plastic parts are molded can also affect the occurrence of sink marks. Higher temperatures and humidity can lead to more sink marks.

Overall, sink marks can be caused by a combination of factors related to the design of the part, the material selection, and the processing conditions used during manufacturing. By optimizing these factors, manufacturers can reduce the likelihood of sink marks and produce high-quality plastic parts.

Solutions to prevent sink marks, such as adjusting the gate location or reducing the part thickness

There are several solutions that can help prevent sink marks from occurring during the plastic molding process. These include:

Adjusting gate location: Changing the location of the gate, where the plastic enters the mold, can help distribute the material more evenly and reduce the likelihood of sink marks. By placing the gate in a thinner area of the part, the plastic can flow more easily and evenly throughout the mold, reducing the risk of sink marks.
Reducing part thickness: As mentioned earlier, thicker sections of a plastic part are more likely to experience sink marks. By reducing the thickness of the part, the plastic can cool and solidify more quickly, reducing the likelihood of sink marks.
Increasing cooling time: Insufficient cooling can also contribute to the occurrence of sink marks. Increasing the cooling time, by either increasing the cooling time in the mold or using a cooling system, can help the plastic solidify more evenly and reduce the likelihood of sink marks.
Adjusting packing pressure: As with gate location, adjusting the packing pressure can help ensure that the plastic is evenly distributed throughout the mold. By increasing the packing pressure, the plastic can be forced into the mold more evenly, reducing the likelihood of sink marks.
Using additives: There are various additives that can be added to plastic to reduce the likelihood of sink marks. These additives can help the plastic solidify more evenly and reduce shrinkage, which can contribute to sink marks.

Overall, preventing sink marks requires careful consideration of various factors, such as part design, material selection, and processing conditions. By optimizing these factors and using appropriate solutions, manufacturers can reduce the likelihood of sink marks and produce high-quality plastic parts.

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Warpage

How it affects part functionality

Warpage refers to the deformation or distortion of a material or object from its original shape due to the internal stresses, uneven cooling or heating, or other factors during the manufacturing or use of the object.

In the context of parts and components, warpage can have significant effects on their functionality. For example, if a plastic injection-molded part warps, it may not fit or function correctly with other parts in an assembly, leading to malfunction or failure. In the case of electronic components, warpage can affect the accuracy and reliability of the device by causing misalignments of the components, creating short circuits, or even breaking the connections entirely.

Warpage can also lead to decreased strength, durability, and fatigue resistance of the material, making it more prone to cracks, fractures, or other types of damage. In some cases, warpage may be acceptable or even desirable, such as in the case of curved or shaped components, but it must be carefully controlled to ensure the desired performance and functionality of the part.

Low Volume Manufacturing Optimizing Efficiency and Quality

Causes of warpage

There are several factors that can cause warpage in materials and components. Some of the most common causes include:

Uneven cooling: When a material cools down too quickly or unevenly, it can cause differential shrinkage, leading to warping or distortion. This is especially common in plastic injection-molded parts, where different sections of the part may cool at different rates, resulting in varying degrees of shrinkage.
Residual stress: Materials can experience internal stresses due to various manufacturing processes, such as molding, machining, or welding. If these stresses are not relieved or balanced, they can cause the material to warp or deform over time.
Material properties: Different materials have different thermal expansion coefficients, which means that they may expand or contract at different rates when exposed to temperature changes. This can cause warpage in materials that are made of multiple components or layers with different coefficients of thermal expansion.
Inadequate tooling or design: Poor tooling or design can cause localized stresses or uneven pressure distribution, leading to warpage or distortion. This is especially true in parts with complex geometries or thin sections that are difficult to mold or machine.
Environmental factors: Exposure to high temperatures, humidity, or other environmental factors can cause materials to expand or contract, leading to warpage over time.

It is important to identify and address the root causes of warpage to prevent defects and ensure the quality and functionality of the final product. This can involve adjusting the manufacturing process, selecting different materials, or modifying the design or tooling to reduce stresses and improve the balance of forces during production.

Solutions to prevent warpage

There are several solutions to prevent warpage and ensure the quality of the final product, including:

Optimize the mold design: Designing the mold with proper venting, cooling channels, and gate locations can help to ensure even cooling and reduce internal stresses. This can help to prevent warpage and improve the dimensional accuracy of the final part.
Adjust processing parameters: By adjusting the processing parameters, such as the temperature, pressure, and cycle time, it is possible to achieve more uniform cooling and reduce the risk of warpage. This can involve using a slower cooling rate, lower mold temperatures, or reducing the injection pressure.
Use appropriate materials: Different materials have different thermal expansion coefficients and mechanical properties, which can affect their susceptibility to warpage. Choosing a material with a lower coefficient of thermal expansion or better dimensional stability can help to reduce warpage.
Use post-processing techniques: Post-processing techniques, such as annealing or stress-relieving, can help to reduce internal stresses and improve the dimensional stability of the part. This can help to prevent warpage and improve the long-term performance of the part.
Improve part design: Modifying the part design to reduce thin sections, sharp corners, or other features that can cause localized stresses can help to prevent warpage. This can involve using more gradual curves, fillets, or thicker sections to improve the balance of forces during production.

By taking these measures to prevent warpage, manufacturers can produce higher quality parts with improved dimensional accuracy and better long-term performance. It is important to carefully analyze the root causes of warpage and select the appropriate solutions based on the specific material and process conditions.

Short Shots

Short shots consequences

Injection molding process

Short shots are a common defect that can occur in plastic injection molding when the material does not completely fill the mold cavity during the injection process. This results in a part that is incomplete, with missing features or sections that have not been filled with plastic.

The consequences of short shots can vary depending on the part and its intended use. In some cases, short shots may result in a part that is structurally unsound or otherwise unfit for its intended purpose. Short shots can also lead to functional defects, such as leaking, poor fit, or reduced strength.

In addition to functional defects, short shots can also result in cosmetic defects, such as surface blemishes or visible lines where the material did not completely fill the mold. These defects can reduce the overall aesthetic appeal of the part and affect its perceived quality.

Short shots can be caused by a number of factors, including improper material selection, inadequate pressure or temperature control, or poor part design. It is important to identify and address the root cause of short shots to prevent defects and ensure the quality of the final product. This may involve adjusting the processing parameters, modifying the mold design, or selecting a different material with better flow properties.

Causes of short shots

Short shots can be caused by a variety of factors, including:

Insufficient material: One of the most common causes of short shots is a lack of material in the injection molding process. This can occur when there is not enough material in the barrel to completely fill the mold cavity, or when the material is not flowing properly due to a blockage or other issue.
Air traps: Air can become trapped in the mold cavity during the injection molding process, preventing the material from completely filling the cavity. This can result in a short shot or other defects, such as bubbles or sink marks.
Improper mold design: Poor mold design can also contribute to short shots. This can include a lack of vents or gates to allow air to escape from the mold cavity, or an improper gate location that prevents the material from flowing properly.
Processing parameters: The temperature, pressure, and cycle time used in the injection molding process can all affect the likelihood of short shots. For example, a low injection pressure or insufficient packing pressure can prevent the material from completely filling the mold cavity.
Material selection: The properties of the material being used can also affect the likelihood of short shots. Materials with poor flow properties or high viscosity may have difficulty filling the mold cavity, leading to short shots.

It is important to identify the root cause of short shots in order to prevent them from occurring in the future. This may involve adjusting the processing parameters, modifying the mold design, or selecting a different material with better flow properties. By addressing the underlying cause of short shots, manufacturers can ensure that their injection molded parts meet the necessary quality standards and functional requirements.

Injection molding temperature

Solutions to prevent short shots

There are several solutions to prevent short shots in plastic injection molding, including:

Increasing the injection pressure: Increasing the injection pressure can help to ensure that there is enough material in the barrel to completely fill the mold cavity. This can be particularly effective when short shots are caused by a lack of material.
Modifying the mold design: Modifying the mold design, such as adding more gates or vents, can help to improve the flow of material and prevent air traps. This can be particularly effective when short shots are caused by poor mold design.
Adjusting processing parameters: Adjusting the temperature, pressure, and cycle time used in the injection molding process can help to optimize the flow of material and prevent short shots. For example, increasing the mold temperature can help to improve the flow of material, while increasing the packing pressure can help to ensure that the mold cavity is completely filled.
Choosing the right material: Choosing a material with better flow properties or lower viscosity can help to prevent short shots. Materials with a lower viscosity will flow more easily and fill the mold cavity more completely, reducing the likelihood of short shots.
Reducing wall thickness: Reducing the wall thickness of the part can help to improve the flow of material and prevent short shots. This can be particularly effective when short shots are caused by poor part design.

By implementing these solutions, manufacturers can improve the quality and consistency of their injection molded parts, reducing the risk of defects and ensuring that their parts meet the necessary functional requirements. It is important to carefully analyze the root cause of short shots and select the appropriate solutions based on the specific material and process conditions.

pressure or improving the venting system

Increasing the injection pressure or improving the venting system can also help to prevent short shots in plastic injection molding. Here’s a bit more detail on each:

Increasing injection pressure: Increasing the injection pressure can help to ensure that there is enough material in the barrel to completely fill the mold cavity. This is particularly effective when short shots are caused by a lack of material or poor flow properties of the material. Higher injection pressure can also help to improve the flow of material into more complex parts of the mold, reducing the risk of short shots and other defects.
Improving the venting system: A well-designed venting system can help to prevent air traps and ensure that the mold cavity is completely filled with material. This is particularly effective when short shots are caused by air being trapped in the mold cavity. The venting system should be carefully designed to allow air to escape from the mold cavity while preventing the escape of material. It is important to note that the venting system should not be relied upon as the primary means of preventing short shots – it should be used in conjunction with other solutions, such as increasing injection pressure or adjusting processing parameters.

Overall, the most effective approach to preventing short shots will depend on the specific material, mold design, and processing conditions. Manufacturers should carefully analyze the root cause of short shots and implement the appropriate solutions to ensure that their injection molded parts meet the necessary quality and functional requirements.

Flash

Flash impact

In plastic injection molding, flash refers to excess material that escapes from the mold cavity and creates a thin layer or thin protrusion on the surface of the part. Flash can occur when the mold is not properly closed or when there is excessive pressure within the mold. Flash can negatively impact the quality and aesthetics of the part in several ways:

Reduced part strength: Flash can weaken the structural integrity of the part by creating thin areas or sharp edges that are prone to breaking or cracking.
Poor aesthetics: Flash can also negatively impact the appearance of the part, as it creates a rough or uneven surface that may be unsightly or interfere with other parts of the assembly.
Increased cost: Removing flash can add additional costs to the manufacturing process, as it requires additional labor and time to remove the excess material.
Functional issues: Flash can interfere with the function of the part, particularly if it creates a blockage or prevents other components from properly fitting together.

To minimize the impact of flash on part quality and aesthetics, it is important to properly design and maintain the mold and to optimize the injection molding process to minimize the risk of flash. This may include adjusting the injection pressure, cycle time, and other processing parameters, as well as properly cleaning and maintaining the mold. Additionally, flash can be removed through a variety of post-processing techniques, such as trimming or sanding, to ensure that the final part meets the necessary quality and aesthetic standards.

Causes of flash

Improper mold clamping and excessive injection pressure are two common causes of flash in plastic injection molding. Here’s a bit more detail on each:

Improper mold clamping: If the mold is not properly clamped, excess material can escape from the mold cavity and create flash. This can occur if the mold is not fully closed or if there is uneven pressure across the mold surface. Proper mold clamping is essential for creating high-quality parts and minimizing the risk of flash.
Excessive injection pressure: If the injection pressure is too high, excess material can be forced out of the mold cavity and create flash. This can occur if the material viscosity is too low, the mold temperature is too high, or the injection speed is too fast. It is important to carefully monitor and adjust the injection pressure to ensure that the material is properly distributed within the mold cavity.

Other factors that can contribute to flash include poor mold design, insufficient cooling time, or inadequate venting. These factors can create areas of high pressure or cause the material to flow unevenly within the mold, leading to excess material escaping from the mold cavity.

To prevent flash, it is important to carefully analyze the injection molding process and identify any factors that may be contributing to the problem. This may involve adjusting the mold design or processing parameters, such as reducing injection pressure, increasing cooling time, or improving venting. By addressing the root cause of flash, manufacturers can improve the quality and consistency of their parts, while minimizing the need for post-processing and reducing overall production costs.

Injection molding defects

Solutions to prevent flash

Adjusting the mold clamping force and reducing the injection speed are two effective solutions to prevent flash in plastic injection molding. Here’s a bit more detail on each:

Adjusting the mold clamping force: The mold clamping force is the pressure applied to the mold halves to keep them securely closed during the injection molding process. If the clamping force is too low, the mold may not be properly sealed, leading to excess material escaping and causing flash. Increasing the mold clamping force can help to ensure that the mold is properly closed and reduce the risk of flash.
Reducing the injection speed: If the injection speed is too high, it can create excess pressure within the mold and cause material to escape and create flash. Reducing the injection speed can help to ensure that the material is properly distributed within the mold cavity and reduce the risk of excess material escaping and causing flash.

Other solutions to prevent flash include improving the mold design, increasing the cooling time, or improving the venting system. By optimizing these factors, manufacturers can improve the overall quality and consistency of their injection molded parts and minimize the need for post-processing to remove flash.

It is important to note that the optimal solution for preventing flash will depend on the specific material, mold design, and processing conditions. Manufacturers should carefully analyze the root cause of flash and implement the appropriate solutions to ensure that their injection molded parts meet the necessary quality and functional requirements.

Burn Marks

How burn marks affect part appearance and functionality

In plastic injection molding, burn marks refer to discoloration or blackening on the surface of the part caused by excessive heat or overheating during the molding process. Burn marks can negatively affect both the appearance and functionality of the part in several ways:

Poor aesthetics: Burn marks can make the part look unsightly, and can significantly reduce its appeal or marketability. Burn marks can be particularly problematic in applications where appearance is critical, such as consumer electronics, automotive, or medical devices.
Reduced part strength: Burn marks can weaken the structural integrity of the part by creating areas of weakness or micro-cracks on the surface. This can lead to reduced part performance, or even failure in critical applications.
Increased costs: Removing burn marks can add additional costs to the manufacturing process, as it requires additional labor and time to address the issue.
Functional issues: Burn marks can also interfere with the function of the part, particularly if they occur in critical areas such as sealing surfaces or mating surfaces. This can lead to issues with product performance or reliability.

Injection molding machine

To minimize the impact of burn marks on part quality and functionality, it is important to optimize the injection molding process to prevent overheating or excessive heat buildup. This may include adjusting the processing parameters such as melt temperature, mold temperature, injection speed, and pressure. Additionally, proper venting, and mold design can help to prevent trapped air or gases that can lead to overheating and burn marks.

Proper maintenance and cleaning of the mold and equipment can also help to prevent burn marks. If burn marks do occur, they can be addressed through various post-processing techniques such as sanding, polishing, or surface treatments to ensure that the final part meets the necessary quality and aesthetic standards.

Causes of burn marks

Burn marks in plastic injection molding can have several causes. Some of the most common causes include:

Overheating: If the mold temperature, melt temperature, or injection pressure is too high, it can cause the material to overheat and burn, leading to discoloration or blackening on the surface of the part.
Poor venting: Trapped air or gases within the mold can cause localized overheating, resulting in burn marks. Proper venting of the mold can help to prevent the buildup of trapped air or gases and minimize the risk of burn marks.
Resin degradation: Overheating or prolonged exposure to high temperatures can cause the resin to degrade, leading to the formation of carbon and other impurities that can cause burn marks.
Mold contamination: The presence of impurities or contaminants within the mold can also contribute to burn marks. For example, if the mold is not properly cleaned and maintained, it can accumulate contaminants that can cause discoloration or blackening on the surface of the part.

To prevent burn marks, it is important to carefully monitor and control the processing parameters, such as melt temperature, mold temperature, injection pressure, and injection speed, to ensure that the material is processed within the optimal range. Proper mold design, venting, and maintenance can also help to minimize the risk of burn marks by ensuring that the mold is properly sealed, vented, and free of contaminants.

Injection molding tooling

Solutions to prevent burn marks

there are several solutions to prevent burn marks in plastic injection molding, including:

Adjusting the processing parameters: By carefully monitoring and adjusting the melt temperature, mold temperature, injection pressure, and injection speed, it is possible to prevent overheating and reduce the risk of burn marks. This may involve optimizing the processing conditions for the specific material being used, as well as the geometry and complexity of the part.
Optimizing the venting system: Proper venting of the mold can help to prevent trapped air or gases that can cause localized overheating and burn marks. This may involve adjusting the location or number of vents, as well as the size and shape of the venting channels to ensure that air and gases can escape from the mold efficiently.
Improving mold design: Proper mold design can also help to prevent burn marks by ensuring that the mold is properly sealed, vented, and designed to accommodate the specific material being used. This may involve adjusting the gate location or size, adding cooling channels to the mold, or modifying the geometry of the part to improve flow and reduce the risk of overheating.
Cleaning and maintaining the mold: Regular cleaning and maintenance of the mold can help to prevent the buildup of contaminants that can contribute to burn marks. This may involve using specialized cleaning agents or techniques to remove contaminants, as well as inspecting the mold for signs of wear or damage that could impact its performance.

By implementing these solutions, it is possible to prevent burn marks and ensure that the final part meets the necessary quality and aesthetic standards.

Conclusion

Injection molding material

In summary, the top 5 common defects in plastic injection molding are:

Sink Marks, caused by improper cooling or insufficient packing pressure.
Warpage, caused by uneven cooling or residual stress.
Short Shots, caused by insufficient material or air traps.
Flash, caused by improper mold clamping or excessive injection pressure.
Burn Marks, caused by overheating or poor venting.

Addressing these defects is crucial for producing high-quality, defect-free parts, as they can impact the aesthetics, functionality, and durability of the final product. These defects can also increase production costs, waste materials, and cause delays in the production process.

Designers and product development personnel should pay closer attention to injection molding and its potential defects to ensure that the final product meets the necessary quality standards. This may involve working closely with injection molding experts, conducting thorough testing and analysis, and implementing best practices for mold design, processing, and maintenance. By addressing these defects and striving for continuous improvement, manufacturers can improve the quality, reliability, and performance of their plastic injection molded products.