Understanding Splay in Injection Molding

Table of Contents

Defects such as splay in injection molding are one of the reasons plastic products don’t do well in the market. 

splay in injection molding

Such flaws can cause both functional and aesthetic anomalies that often result in unpleasant experiences for end users. The bad thing about splay is that its presence on even the smallest part of the product can negatively affect the visual appeal of the product leading to its rejection by end users. 

But what exactly causes splay and how do injection mold designers guide against it? You’ll get the full details about this plastic impairment in this piece plus much more. So, fasten your seat belt as we unravel the mystery of splay in injection molding.  

1. Understanding Splay in Injection Molding

1.1 Overview of Splay Defect

Splay affects the quality of plastic products. However, the extent of the damage from splays on plastic often requires some explanation to most plastic production enthusiasts. 

You’ve baked a delicious cake, perfectly mixed and blended. But when you take it out of the oven, you notice something odd – a splotch of uneven color on the icing. 

That’s a bit like what splay is in the world of injection molding – an unexpected flaw that can mess up the whole masterpiece.

Now, in the world of molding, we’re not dealing with cakes, but with plastic parts. Splay is like a surprise guest at a party you didn’t invite. 

It’s a defect that sneaks into the process and leaves its mark – tiny streaks or spots on the surface of the part. It’s a bit like painting a wall and finding unexpected droplets ruining your smooth canvas.

Still, the first natural point of call in explaining the effects of splays in plastic injection molding is to understand its causes and how they develop. 

1.2 Causes of Splay

During the injection molding process, the plastic resin material is often heated to achieve a pliable consistency. 

However, when this heating process goes above the required temperature the introduction gas into the system occurs. These gases often end up trapped in molten plastic. 

Consequently, when these plastic products solidify, the trapped gases show up as surface blemishes with the appearance of a squeezed fabric.  

It can be compared to building sandcastles on the beach but with wet sand. The result is usually a castle that holds poorly. 

Just like those damp patches can mess up your sandcastle’s structure, splay can affect the surface quality and strength of your injection-molded parts. This is usually not a problem for production methods like high-quality CNC machining. So, understanding splay is like knowing how to keep your sandcastle dry and solid, ensuring your parts come out flawless.

Again, the choice of materials is critical in the formation of splays considering that some plastic resins are hygroscopic which means they absorb moisture around them. So, when you employ these plastic resins, they tend to absorb moisture from the environment which ends up trapped in the molten plastic. The result is usually this defect known as splay.  

But it’s not just moisture that can cause trouble. 

Sometimes, if the plastic resin isn’t stored properly or if it’s exposed to high temperatures for too long, it can start to degrade. This can release gases that create those same pesky bubbles, leading to splay defects.

To prevent these issues, manufacturers need to ensure their materials are dry and in good condition. It’s a bit like making sure your cake batter is just right before you pop it into the oven. By keeping a close eye on the resin and the injection process, you can keep those unwanted bubbles at bay and create flawless parts.

2. Detecting and Analyzing Splay

2.1 Splay Inspection Techniques

Much like solving a mystery case as a detective where specific tools like UV rays, magnifiers, and adhesives help to lift fingerprints, specific tools are also required by inspectors to identify splays. These tools also include certain techniques and methods to reveal this anomaly. 

The natural first on this list of tools is visual inspection. This involves the use of the eye to examine every bit of the molded part to see if any telltale mark that indicates the presence of splay will show up. But sometimes, just looking can be inadequate to identify some signs. Fortunately, there are advanced tools that can perform surface analysis to identify hidden clues of splay. Such equipment as a Scanning Electron Microscope (SEM) acts like a super-powered magnifying glass to uncover any trace of splay no matter how tiny. 

For the most hidden clues, non-destructive testing methods are employed. These are like using fingerprint analysis to catch a criminal without damaging the evidence. Techniques like X-rays and ultrasounds can reveal what’s happening inside the part without actually taking it apart. This is crucial because splay defects might not always be visible on the surface.

By using these inspection techniques, manufacturers can identify splay defects and determine their extent. It’s a bit like piecing together the clues of a mystery to solve the case and prevent further defects in the future.

2.2 Root Cause Analysis

Identifying impairments in finished products is one thing but determining their root cause often helps to avoid such defects for future productions. Root cause analysis in injection molding is like being a detective-scientist. It’s about digging deep to find out why splay defects happened in the first place.

When splay defects occur, it’s not enough to just know they’re there. Manufacturers need to understand why they happened. Was it because of the material? Or perhaps the machine settings? This is where root cause analysis comes in.

Think of root cause analysis as peeling back layers to get to the heart of the issue. Inspectors evaluate every production stage, setting, and equipment including plastic moisture and temperature to determine the cause of splay. 

Just like scientists might use data and experiments to understand a natural event, manufacturers use data from the injection molding process. They might change one variable at a time, like adjusting the temperature or pressure, to see how it affects the outcome. It’s all about connecting the dots to uncover the real reason behind the splay defects.

A perfectly executed root cause analysis will always assist manufacturers in resolving both existing problems and the ones that can potentially happen in the distant future. 

3. Prevention and Mitigation of Splay

3.1 Material Handling and Drying

Two of the most helpful activities that can assist manufacturers in avoiding splay defects are proper material handling and using suitable drying techniques. 

Imagine you accidentally add a wet ingredient to the batter while baking a cake. It might not turn out as tasty as expected. Likewise in injection molding, using damp or moist material can lead to splay defects. That’s why appropriate material handling and drying are crucial.

Think of the plastic pellets used in injection molding as the ingredients for your cake. If these pellets absorb moisture, just like flour might, they won’t mix well with the other ingredients – in this case, the molten plastic. This can result in splay defects, ruining the “flavor” of your molded parts.

To avoid this, manufacturers need to make sure their plastic pellets are bone dry before molding. They use special machines to dry the pellets thoroughly, just like you might use an oven or a dehydrator in your kitchen. This ensures that no unwanted moisture is sneaking into the mix.

So, just like a chef carefully prepares each ingredient for a perfect cake, manufacturers in the high-quality mold manufacturing industry take the time to handle and dry their plastic pellets. This extra effort ensures that their injection molding process is smooth and free of splay defects, resulting in high-quality finished parts.

3.2 Process Optimization

While material handling and drying techniques are two of the most useful activities to avoid splays, it is critical to also evaluate a bunch of activities that make up the whole injection molding process. This assessment often helps manufacturers to optimize their production techniques and parameters. 

Process optimization is usually like adjusting the gas pedal, steering wheel, and brakes while driving a car to make the ride smooth and safe. In injection molding, it’s a bit like tuning a car – optimizing the process settings ensures a smooth production journey and avoids bumps like splay defects.

Imagine you’re in a race, and the track is the mold where your part is formed. If you go too fast, you might skid off the track – that’s similar to what happens in injection molding. If the injection speed is too high, the molten plastic might not flow evenly, causing splay defects.

Now, think about how you’d adjust the air conditioning in your car to keep it comfy inside. Similarly, adjusting the temperature in the injection molding process ensures that the plastic flows smoothly without splay. The right pressure is like the right amount of pressure on the gas pedal – too much or too little can lead to splay.

Just as a skilled driver knows their car’s sweet spots, experienced manufacturers know the perfect process settings for each plastic and mold. This optimization prevents splay and other defects, ensuring that the parts that come out of the “race” are flawless and ready to perform their roles.

4. Design Considerations for Splay Prevention

4.1 Gate Design and Placement

The design stage is usually the best point to arrest any anomalies including splays injection molding. It saves time, effort, and cost comparable to using prototype technologies like 3D printing technology but on higher levels. One critical design consideration for injection molding is the gate design and placement. 

In injection molding, the gate – where the molten plastic enters the mold – plays a crucial role in preventing splay defects.

Think of the gate as the doorway to your mold. If it’s too small, the plastic might not flow smoothly, causing splay. Imagine trying to fit through a tiny door – it would be a struggle. Likewise, if the gate is too big, too much plastic might rush in, also leading to splay defects.

4.2 Mold Venting and Cooling

The access through the mold for cooling the hot molten plastic material is the vent and this also requires careful consideration during design. 

Think of mold venting as providing “breathing room” for the molten plastic. Just as your cake batter from the baking analogy expands as it bakes, plastic also expands as it fills the mold. If there’s no way for the air to escape, it can cause splay defects, similar to how your cake might collapse without proper ventilation.

Now, consider mold cooling as the cooling rack for your cake. After baking, you cool the cake, so it sets properly. In injection molding, cooling the mold helps the plastic solidify smoothly, reducing the chances of splay defects. For instance, if you took your cake out of the oven too early – it might collapse. Likewise, premature removal of plastic from a hot mold can lead to splay defects.

Like a professional baker baking that perfect cake – you make sure it’s not only well mixed but also baked and cooled just right. In like manner, skilled manufacturers ensure that they employ high quality mold manufacturing with well-designed vents to prevent splay defects. This attention to detail ensures that the finished parts are flawless and ready to serve their purpose.

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5. Quality Control and Continuous Improvement

5.1 Quality Assurance Measures

Taking quality assurance measures in injection molding is like solving a mystery as a detective. You would want to know how many stones are there and which ones to leave unturned to crack the case. 

In comparison, injection molding quality control is like being a detective for splay defects. Quality inspectors engage appropriate tools and methods to assess every corner to ensure they don’t miss any hidden impairment.

Think of these tools as zoom lenses that help us see even the tiniest of defects. Just as a detective needs sharp eyes to spot clues, manufacturers use inspections to catch any signs of splay defects. This ensures that only high-quality parts move forward, just like how a detective only solves cases with solid evidence.

5.2 Ongoing Improvement Strategies

Imagine you’re practicing a sport – at first, you might not be perfect, but with practice, you get better. Similarly, manufacturers continuously work to improve their molding processes to prevent splay defects. They’re like athletes always striving for a new personal best.

Think of it as a race against splay defects. Manufacturers don’t just want to cross the finish line; they want to beat their own records. By learning from every production run, they find ways to make their process more efficient and effective, just as an athlete analyzes their performance to do better next time.

Now, imagine having a coach to guide you in your sport. Manufacturers have experts who provide training and advice on preventing defects like splay. These coaches ensure that everyone is on the same page, just like a sports team following a game plan.

In the world of injection molding, every part is a new opportunity to excel. Just as athletes aim for gold medals, manufacturers aim for flawless parts. With ongoing training, smart strategies, and teamwork, they ensure that splay defects don’t stand a chance.


Splay defects in injection molding are like unexpected party crashers – they can ruin the whole experience. However, this guide has exposed their root causes including when moisture or gases decide to join the plastic party, causing unsightly blemishes. These defects not only affect a product’s looks but also how it works, making them unwelcome visitors in the world of plastic manufacturing.

Just as a party host takes steps to ensure a smooth gathering, manufacturers take preventive actions to keep splay defects away. By controlling factors like moisture and perfecting the molding process, they ensure the products turn out as planned. It’s all about making sure every part is a star of the party, free from imperfections.

But never forget that in injection molding, attention to detail is key to producing quality parts.


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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