April 10, 2024

Injection Molding Defects Identifying, Troubleshooting & Preventing

injection molding defects

Introduction to Injection Molding

Injection molding is a cornerstone of the manufacturing industry, transforming plastic resins into myriad shapes and sizes for every imaginable purpose. At its core, this process involves melting plastic material until it’s in a liquid state and then injecting it into a mold cavity under high pressure. Once it cools, you have a molded part that should, in theory, perfectly match the mold’s design. This technique cranks out everything from car parts to containers at an astounding rate, making it indispensable. However, like every manufacturing method, it’s not without its quirks — namely, injection molding defects. Understanding the common causes of these imperfections is your first step toward mastering the injection molding process.

What are the common causes of injection molding defects?

Why do these pesky defects in injection molding occur? Well, several factors are usually at play. Variables such as incorrect injection pressure, inconsistent wall thickness, or even the wrong mold temperature can throw a wrench into the works. The science of injection molding requires a Goldilocks approach: everything needs to be just right. Too much or too little of any ingredient in this delicately balanced recipe, and you’ll find yourself facing a slew of common injection molding defects, affecting everything from the cosmetic appearance to the structural integrity of your plastic parts.

Types of Injection Molding Defects

Injection molding defects can really affect how plastic parts look and work. These issues come from a mix of things like the materials used, how the mold is designed, and the steps in the process. We often see defects like sink marks, weld lines, short shots, and warping. Each of these problems makes it tough to make perfect parts.

Sink marks

Sink marks are dents in a molded part, commonly observed in thicker sections when the outside cools and solidifies faster than the inside. On the other hand, as the core cools, it contracts, pulling the surface inward towards a more solid state. It is a common rule in the industry recommending to maintain the wall thickness consistency to reduce the risk of the sink marks formation, however when dealing with different wall thickness it is essential to make sure that the cooling is sufficient to prevent the formation of sink

Cooling process is so vital in the making of sink marks. To make them go away, it is necessary to pay much attention to the design of the mold and the cooling system. Approaches like cooling lines installation close to thicker sections can help to attain an even temperature, which in turn will reduce the instances of sink marks. Furthermore, increasing the material temperature is another method that can be used to eliminate the defect by giving the plastic in the thicker areas more time to cool down gradually, resulting in a uniformed surface of the part without depressions.

These strategies when implemented can really help in reducing the frequency of sink marks and enhance the product quality. This quality consciousness not only enhances the good looks of the final product but also its strength. Thus, the market value of injection molding services is kept high in the competitive world of injection molding industry.

Flow lines

Mold lines are a common issue in the injection mold industry. They are presented as waves or lines on the surface of the plastic part. Such lines are commonly formed when the molten plastic enters the mold at different rates. This leads to formation of lines around the part. The flow lines are not just an aesthetic problem. In addition, they may also point to the underlying problems of the way the injection is done. This kind of problem may lead to the weakness of the part.

To stop flow lines, you could do the following actions. It is the key that the mold is designed correctly. This is to ensure that the melted plastic flows smoothly and evenly. The gate shape and location must be adjusted. This guarantees that the molten materials are evenly distributed inside the mold. The right kind of plastic also matters a lot. Selecting the right thickness of plastic will avoid the flow interruptions, minimizing the chance of flow lines occurrence. Moreover, adjusting the speed and magnitude of the plastic injection can help in filling the mold with more uniformity. This makes the part not have the uneven look given by flow lines.

With the mold design in mind, the plastic choice and the injection molding process, makers can really minimize the flow lines. Therefore, the recycling process results in the production of better plastic parts. These parts are aesthetically pleasing and functional as well.

injection molding defects

Surface delamination

This problem of delamination is a big issue that is faced in injection molding. It is visible when the thin layers of plastic start coming off and peeling from the main body of the plastic. This issue occurs if the plastic is too dirty or if the heat is not applied the right way during the injection process. It is mainly used for complex parts and part that need a good quality finish.

A few surfaces can be delaminated. For example, if other stuff is added to the plastic, the bag might not stick together as well. In the event that the mold is not smooth or the plastic is not heated evenly, the layers begin to separate and cracks appear. In order to stop this, producers must ensure that the plastic is clean and the mold is in excellent condition. They also should ensure well how they use heat and the way they inject the plastic. It is the effectiveness of the two activities that can ensure that the chances of surface problems are reduced. In the end, there are the products that look and perform the way they are supposed to.

Weld lines

Weld lines are one of the most essential issues in the molding process. They are the result of the flowing molten plastic to meet and solidify within the mold, but not completely bonding together. These sorts of flaws are similar to lines or seams that can be seen on the surface of the molded part. It is frequently seen where the flowing molten plastic travels around obstacles such as cores or inserts in the mold. Components with complex designs or those having more than one injection point stand a greater chance of developing weld lines.

The production process can be improved by several means, which can be taken to tackle this problem. First of all, improving the mold design will better direct the flow of molten plastic and thereby, decrease the possibility of weld lines in crucial areas. The temperature of the molten plastic and the mold can also be adjusted; this helps in the fusion of the plastic with ease. In addition, we might be able to change the injection speed to improve the bonding of the plastic streams. Through the control of these factors—the mold design, the material temperature settings, and the injection speed—manufacturers have the potential to greatly decrease the frequency of weld lines, which in turn increases the aesthetic and structural integrity of the molded parts. These adjustments will allow the product to retain its high quality and be able to meet the demands of both manufacturers and consumers; at the same time, they will help solve one of the most common defects in injection molding.

Short shots

The short shots in injection molding come about when the melted base material does not fill the mold cavity completely. In this case, the flaw manifests itself in parts with complex designs or that are distant from the injection spot. The cases of plastics with high viscosity or those used in complex molds are more likely to have short-shots because of the issues associated with the material flowing through narrow spaces.

The effect of short shots on the functionality of injection-molded parts can be quite different. As an example, elements may not be strong enough or have compromised seal integrity, which is critical for components designed to fit together or to contain fluids. The defect destroys the entire system and makes it impossible to apply for the intended purpose.

Correcting short shots can be done by taking some remedial measures, as given below. Modifying the mold design to have smooth flow paths and strategically placing gates could be the keys to improving the quality of the product. Furthermore, the injection pressure can be increased in order to ensure that the molten plastic reaches all the necessary regions of the part. Using a base material with a lower viscosity as well can also help to improve flow, therefore reducing the possible appearance of this type of defect. By tackling the causes of short shots—mold design, material selection, and processing conditions—manufacturers can reduce the incidence of this issue, improve the quality and performance of their products, and continue to adhere to the high standards of injection molding.

injection molding defects


Injection molding warping is a condition where parts will tend to twist or bend and not be in the desired original shape. This problem causes the parts that are supposed to fit together to look bad and don’t actually fit well. It’s a big problem for products that must have very high accuracy levels. The uneven cooling process of plastics makes warping more of an issue. If the heat and cooling of the part are not consistent, it could become distorted or misshapen.

There are two main kinds of warping: flattening, or bending known as planar warping, and twisting. During the planar warping, the parts bend in one direction. Twisting creates more complications and can eventually lead to the whole part being out of alignment. Both of these challenges are concerning because they affect appearance as well as functionality.

Remedy of warping can be by varying the mold design, the materials used and the way the part is made. Ensuring that the part cools down evenly is one of the main things to keep in mind. This entails the modification of a mold and a cooling system. Also, you can pick up plastics that can cool down at the same time all over. Also, the way it works on the injection molding machine may be modified to minimize the possibility of warping. By doing so, the defects are reduced and the quality of the parts produced is improved substantially. This shows that the whole injection molding process will work better.


Jetting is an issue that takes place when the plastic entering the mold is done too fast. This is because more pressure is put on them. The plastic comes against the mold surface, and the parts that were not molded properly get stuck on the mold. This may result in an undesirable pattern on the final product. The spots, which will make the part look bad, are the results of these marks. They may also damage the efficacy of the component. Discuss the main aspects of social media marketing and its significance for businesses in the digital age. This is, however, the case if the marks are on crucial points.

Jettings, however, are an imperfection different from others, such as flow lines and weld lines. It is because of the fact that plastic is poured into the mold at the beginning. Flow lines and weld lines are, in a sense, the path the plastic takes as well as the cooling process once it’s in the mold. But what gives that first rush of color and shape is the plastic that is poured into the mold. This makes the problem less complex and, therefore, it becomes easier to find and rectify.

To prevent jetting, you can try different techniques, as follows:. One way is to feed the plastic in progressively, Thus, it prevents the pressure from exceeding. Besides this, you can modify the mold or the part itself in its shape. This allows the bottle to be squeezed out easily. Through the examination of the mold design, the part, and how the plastic is injected, makers can increase the probability of a jetting-free process. As a result, products turn out to be more attractive and functional.

Burn marks

Burn marks on injection-molded parts are the dark or black discolorations that may appear on the surface of the part that has come out of a mold. These traces are usually due to the extreme heat or high temperatures occurring during molding. Similarly, the bubbles may also be caused by the air that gets trapped inside the mold. The burn marks can mar the appearance of the part, and in some cases, the performance of the part might be affected if the burn occurs on its important components.

To avoid these marks, you should follow the temperatures throughout the process carefully. It is therefore important that the mold temperature is kept at a suitable level. Apart from that, you can also have air escape from the mold by eliminating trapped air.

The practical method to prevent burn marks is to slow down the injection speed since less air will be trapped air and the risk of overheating will be reduced. A good way to reduce CO2 emissions is to use heat-resistant materials. To achieve this, manufacturers will need to concentrate on these three aspects—accurate temperature control, air pocket avoidance, and the right material choice—in order to diminish the appearance of burn marks on the molded parts and obtain an appealing surface.

injection molding defects

Vacuum voids

Vacuum voids refer to air pockets or bubbles that are caught either inside or on the surface of injection molded parts. Such holes may produce an irregular-looking surface or a bumpy finish. Sometimes it is just a surface scratch. On the one hand, coatings can improve part performance, but on the other hand, they can affect it. To illustrate, if the part is expected to be completely solid, and is meant for strength, then the voids created by the vacuum can destroy it. Moreover, there is a possibility of voids in those parts that require airtightness, such as seals, which can cause leaks.

Injection-molded part performance is often constrained by the presence of vacuum voids. They might decrease the strength of the part or, at most, affect its sealing ability. This is of great importance in the cases of industries where high performance and reliability are a life-and-death matter, such as in the automotive or medical device industries.

In order to reduce the appearance of voids, several approaches can be used. There are ways to perfect mold design to allow air to escape during the injection process. The choice of materials and control of injection parameters is other measures that minimize the occurrence of air being caught inside the injection mold. The factors that are to be given due consideration in this case are the mold design, material selection, and processing methods. These factors help in the production of superior quality molded parts with fewer air pockets, which helps in the parts’ proper and intended performance.


Flash, commonly referred to as excess, is a fine plastic layer developing on the parting line of an item made by injection molding. I have heard it so often. While a little bit of shine could be fine, too much can make the part the wrong way or functionally. This surplus material can even be taken in the adjacent part of the piece that makes it look messy and does not allow the parts to fit together.

The causes of a flash could be varied, from problems with mold design to the actual details of the injection heating. If the mold is not tightly sealed along the parting line, flash could get out by the leakage of plastic. On top of that, if the plastic is overheated, it gets more fluid and it is more likely to be pushed out of the mold, resulting in more flash. This is only two of many probable reasons.

Various methods can be used to solve the issue of flash. An action of the mold design improvement is the one that should lead to a tighter seal at the parting line. Appropriate material selection that include having the flow properties that are best suited for the part design can also help. In conclusion, the necessity of precision in the settings of heating and injection to prevent the plastic from turning too runny can minimize the chances of flash. The manufacturers can do so by concentrating on these factors and hence significantly cut down on the wasteful excess material and enhancing the quality of the injection-molded parts.

Given these challenges, it’s clear that understanding and preventing injection molding defects is crucial for anyone looking to produce high-quality plastic parts. In the next section, we’ll delve into the role of material properties and how they influence these defects.

Defect TypeOverviewRemedial Measures
Sink MarksDents observed in thicker sections due to uneven cooling.– Ensure wall thickness consistency.
– Install cooling lines close to thicker sections.
– Increase material temperature for better cooling.
Flow LinesWaves or lines on the surface when molten plastic enters at different rates.– Design mold for even plastic flow.
– Adjust gate shape/location.
– Choose the right plastic thickness.
– Adjust injection speed.
Surface DelaminationThin layers peeling from the main body of the plastic due to impurities or incorrect heating.– Ensure plastic and mold cleanliness.
– Utilize even heating and correct injection methods.
Weld LinesLines or seams from molten plastic meeting but not bonding. Seen around obstacles in the mold.– Improve mold design for better plastic flow.
– Adjust molten plastic and mold temperature.
– Modify injection speed for improved bonding.
Short ShotsWhen melted material does not completely fill the mold cavity, affecting parts with complex designs or far from the injection spot.– Modify mold design for smoother flow.
– Increase injection pressure.
– Use material with lower viscosity.
WarpingParts twist or bend, causing fit issues. Occurs due to uneven cooling.– Adjust mold design and cooling system for even cooling.
– Select plastics that cool uniformly.
– Modify injection molding machine settings.
JettingOccurs when plastic enters the mold too quickly, leading to undesirable patterns.– Reduce injection speed to prevent excessive pressure
– Modify mold or part design for better flow.
Burn MarksDark discolorations from excessive heat or trapped air.– Closely monitor and control mold temperature.
– Reduce injection speed to minimize trapped air.
– Use heat-resistant materials.
Vacuum VoidsAir pockets or bubbles caught inside or on the surface, leading to irregular look or compromised functionality.– Improve mold design for better air escape.
– Adjust injection parameters and material choices to minimize trapped air.
FlashExcess material along the parting line of a molded item, causing appearance or fit issues.– Enhance mold design for tighter seal.
– Select materials with appropriate flow properties.
– Fine-tune heating and injection settings to prevent excessive molten plastic.

The Role of Material Properties in Injection Molding Defects

When it comes to injection molding, not all plastic is created equal. The choice of plastic material can either pave the way for smooth sailing or rough waters ahead. Material properties such as moisture content, resin temperature, and flow rate are like the characters in a drama; each plays a crucial role in the narrative of molding a perfect part.

Moisture content is the sneaky antagonist, causing issues like surface delamination when too high. It’s like trying to bake a cake with wet flour—expect a flop. Resin temperature and flow rate are the dynamic duo, ensuring the molten plastic waltzes smoothly into every nook and cranny of the mold cavity. Get these parameters wrong, and you might end up with a variety of defects, from short shots to weld lines. It’s a delicate balance, akin to cooking a gourmet meal where timing and temperature must be just right.

injection molding defects

How to Choose the Right Mold Design for Your Product

Choosing the right mold design for your molded part is crucial for achieving the best possible outcomes in injection molding. It involves careful consideration of the part’s features, including dimensions, tolerances, and desired appearance. Yet, designing perfect molds requires expertise and detailed knowledge of the molding process. That’s where professional injection molding manufacturers come into play.

ACO Mold stands out by providing one-stop manufacturing services, from the initial mold design to the final production. We provide a kick-off meeting and a comprehensive Design for Manufacturability (DFM) report. This report will show part design optimization, which is essential for perfect molds. ACO Mold ensures that every molded part meets the highest standards of quality and precision. Contact us now for your injection molding needs.

Preventing Injection Molding Defects

Best practices for mold design

Uniform wall thickness is like the holy grail of mold design. It ensures even cooling, reducing the chances of warping and sink marks. Think of it as the balanced diet of the plastic injection molding world—essential for good health. Additionally, the right gate size can make or break the success of a molded part. Too small, and you risk short shots; too large, and you might end up with flashes from the edges of the part.

Process optimization

Optimizing the injection molding process is of great importance; every adjustment must be in harmony. The injection speed, pressure, and temperature need to be dialed in precisely to avoid common molding defects such as jetting or burn marks. It’s a symphony of parameters where each must hit the perfect note.

Quality control

Last but not least, rigorous quality control is the final guard of flawless injection molding. Regular checks can catch potential defects early, saving time, material, and, most importantly, your brand’s credit. Implementing a thorough quality control protocol ensures the final product is polished and defect-free.


Mastering the art of injection molding is no small feat. It demands a keen eye for detail, a solid understanding of material properties, and a meticulous approach to mold design and process optimization. Yet, the rewards of overcoming injection molding defects are substantial, leading to the production of immaculate plastic parts that meet, or even exceed, expectations.

Remember, each molded part tells a story about the choices made in materials, design, and processing. By paying close attention to the common injection molding defects and arming yourself with strategies to prevent them, you become the author of a success story in the world of injection molding.

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