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Injection Molding Weld Line: Its Causes and How to Avoid It 

Discover the causes, effects, and solutions for weld lines in injection molding. Optimize mold design, material selection, and process for defect-free parts.

Weld lines are a nightmare in the world of injection molding. Do you know why? Well, the answer stands clearly behind the reason that they impact the quality and structural integrity of molded parts. Now, you must be wondering why these lines occur and what the overall impact they can have on the design is. These are common doubts, and we have this article here for you to tell you how you can avoid them.

But before jumping straight to the solution, we will explore the root cause and meaning of this concept. 

What are weld lines in injection molding?

Weld lines are also known as knit lines. These lines occur when two or more melt fonts meet, and solidification happens. Thus, a bond is formed. You will be able to see these lines on the surface of the molded parts. What usually happens is that a faint line or a slightly irregular line forms on the surface of the molded object.

In the science language, you can say that lines occur due to the merging of molten plastic flow fronts during the melting process. So, wherever a joint has occurred and a bond has formed, that area is thought of as a weak and compromised area. This not only impacts the aesthetic appeal of the object but also reduces its mechanical strength.

Overall, you can observe that there is an increased risk of failure, and the performance of the object is reduced. Moreover, the shelf life of the product is now shorter due to these weld lines. We are sure you are eager to find out the cause of this problem and then the solution to get more quality & defect-free components.  

Causes of weld lines

Firstly, you should know what causes these weld lines to occur during welding.

Flow obstruction

So, what happens is that whenever the molten plastic phases an obstacle or goes through any change in the speed due to varying geometrics, there is an obstruction in the flow. Eventually, this leads to the formation of weld lines because the interruption causes the material to meet and form an unwanted bond. 

Material factors

Another reason for the formation of weld lines is the kind of material you have chosen for molding. Usually, materials with high viscosity experience resistance to flow. This causes obstruction in the flow and weld lines form. Other than that, the additives & fillers can also alter the flow of the material. Thus leading to the formation of weld lines.

You’d be surprised to see how different cooling systems can cause welding lines. Whenever there is a slight difference in the cooling rates, different materials can solidify at different times. So, wherever two separate materials meet at different thermal rates, welding lines can form easily. 

Design related factors

When the parts have lots of twists and turns or different thicknesses, the molten plastic doesn’t flow evenly. That uneven flow makes those lines where the hot plastic meets and sticks together. Imagine sharp corners or edges in the design. They kind of mess up the flow of the molten plastic, causing those lines to form.

If a part isn’t the same thickness all over, it cools at different speeds, and that makes those lines appear where the plastic solidifies at different rates. Another reason is when the openings where the plastic enters the mold are too small or in the wrong place; it messes with how the mold gets filled up. That can create disruptions in the flow and make those lines.

Sometimes, the entry points for the plastic can harden too quickly. It’s like a sudden stop in the flow, causing lines where it solidifies. If the mold doesn’t have good vents, it traps air or gas. That trapped air messes with how the plastic flows and boom weld lines happen.

Machine Parameters

Always fill the molten plastic at a constant speed to avoid weld lines. If the filling speed is too fast or too slow, it can mess with how it flows inside the mold. For temperature, Just like cooking, if it is off, things don’t mix well. If the plastic’s too hot or too cold, it messes with how it moves inside the mold and hardens, making those lines.

Put enough pressure or give the plastic enough time to settle. Otherwise, the parts might not get filled up properly. Another possible reason is insufficient cooling, leading to improper solidification of plastic. That unevenness causes those lines to pop up because of differences in cooling temperature rates. Also, if you try to kick the part out before it’s ready, it disrupts how the plastic flows, making those lines.

Effects of weld lines

Now that you know what causes the weld lines to appear, let’s know what their impact is.

Reduced mechanical strength

You already know what weld lines are and how they meet at points where the hot plastic joints up. Well, these spots are often weaker than the rest because when the plastic cools and sticks together there, its arrangement gets a bit messed up. That makes those spots not as strong as the surrounding material.

Aesthetic defects

You might have noticed those faint lines or marks on some plastic stuff. These are the weld lines, and they are not-so-nice lines that show up on the surface of molded parts. Well, these lines bother you the same way when your favorites get a scratch, especially to things like fancy gadgets or shiny car parts. They mess with the smoothness and look of the surface, making the whole thing look less polished and nice.

Functional impairment

Sometimes, these weld lines mess with how big or small parts turn out, especially in those spots where precision matters most. So, if a part’s a bit off in the important areas, it might not fit or work like it’s supposed to. Imagine a container holding liquid or gas. Weld lines could mess with the seals or connections, making it a bit shaky. If these lines affect how tightly everything sticks together, it might end up leaking or not holding the pressure it should.

Material performance

Think of parts having these lines as not being as strong as they could be, especially in places where the part needs to stay strong and reliable, like in planes or medical devices. These lines might make the part weaker and not as long-lasting. Imagine those weld lines altering how the material behaves in certain spots. They can make the material a bit less stiff or change how heat or electricity moves through it in those areas.

How to avoid weld lines?

After knowing the impact of weld lines on a product, let’s find out how to avoid them. 

Mold design optimization

When you design the molds, making sure the walls are all the same thickness and the parts flow smoothly from one section to another helps the plastic move evenly. Also, have the gates in the right spot. These gates where the plastic gets in should be put in smart places. If they’re in the right spots, they help the plastic spread out evenly and avoid those annoying lines.

Material selection & preparation

When they pick the plastic, they gotta make sure it’s just right for the job. They need ones that move easily and fit well for what they’re making so they don’t end up with those lines. Sometimes, they add stuff to the plastic to make it better. But they gotta be careful! Some things they add might make the plastic act up and cause those lines. So, it’s about choosing things that actually help the plastic flow nicely.

Process optimization

Injection Molding, Weld Lines, Process optimization

While optimizing the process, adjust the injection speed, mold temperature, filling pressure, etc., to get better results from the different types of raw material you are using. There is a need to have a balance so that the plastic is flowing smoothly within the mold. Proper pressing and cooling of plastic is also important to have the best settling process. This way, there are no mold lines on the final product. 

Run a mold flow analysis.

Injection Molding, Weld Lines, Mold Flow

Before starting with mold design, you should do the mold flow analysis. Begin with the simulation test with the help of special software and check the behavior of molten plastic inside the mold cavity. You can identify issues with the material you are using, and then you can optimize the mold design. 

Parting thoughts!

You do understand now that by paying close attention to filling speed, temperature, pressure, etc, you can have control over the appearance of weld lines. If you are still facing any doubts, by contacting us, you can make sure whatever product you are making is awesome and reliable for everyone out there.

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