Weld Line: Its Causes, Defects, and How to Avoid It

Weld Line- Its Causes, Defects, and How to Avoid It

While defects in product manufacturing are quite inevitable, it could have a considerable effect on product value and the operations costs of a production company, most specially for plastic products.

One of the most common blemishes or defects in production are weld lines.

In this post, we focus on weld lines, specifically its main causes, other effects which cause defects and how to avoid it. If you are a manufacturing company, you can benefit from the itemization in this post as to weld lines. Hence, read on.

How Do Weld Lines Appear?

Weld lines have a significant effect on manufactured products. It is one of the most common blemishes for manufactured products. To briefly describe, weld lines are also called knit lines. It emerges and becomes visible when the flow fronts cannot mold or knit during the molding process.

Another name given to weld lines is meld line. This term is commonly used by machinists. However, experts say that weld lines and meld lines have one stark difference: their meeting angles.

Weld lines have a meeting angle of 135 degrees and while meld lines have meeting angles more than 135 degrees. This is important information for you to better mitigate if it is indeed a meld line or weld line.

Now, for you to have a clearer view of how weld lines appear, here is the process. It all starts with plastic flows in the manufacturing process. Ideally, plastic flow is a smooth process.

However, obstruction is inevitable during this phase. As such, the plastic flow might be split in various parts and might splatter around the object.

When this happens, two split flows occur at two opposing ends. When they finally rejoin, there will be visible line dents in the surface. These are your weld lines.

The good news is, once the plastic flows rejoin, it will suddenly create an uninterrupted flow. Thus, when this goes on in a cycle, the weld lines will eventually disappear.

Causes of Weld Lines

Now that it is clear where weld lines come from and how it emerges, the next thing to do is to identify the main causes of weld lines. While the list can go on, the following are the most common causes of weld lines.



It is one of the most common reasons as to why weld lines appear. The rule is that, the lower the pressure, the harder it is for the machine to push the plastic flow and have the opposite flows meld together.

As is the case, a wider and more pronounced weld line shall appear on the surface. The two main causes of low pressure are mold design issues and faulty machines.


The risk with insufficient heat is that the solidification process would be tampered. When this happens, there occurs an uneven solidification process where one side of the plastic flow is solidified while the other is prematurely solidified.  

Mold Design:

Mold Design

You might not know it, but even mold designs contribute to the emergence of weld lines. The common reason for this is usually improperly positioned gates and unnecessarily thickened walls.

This is primarily the reason why experts encourage that you consult with professionals when it comes to design before production.


Slow speed plastic flow is detrimental because the material will also flow very slowly into the mold. When this happens, uneven solidification may also take place. We have already mentioned that such occurrences may lead to wide and very pronounced weld lines.


One of the meticulous processes that a producer must employ is to check that the material is smooth-surfaced and pure.

When there are a lot of impurities in the material, the more the plastic flow shall be prone to obstructions and an obstruction means having weld lines.

Excess Mold Release:

An excess mold release is a major cause of weld line due to two variables: high pressure and speed. The rule is, a higher pressure shall be required to push the material in the machine when there is excess mold.

When the same pressure is maintained or lower than that, the drop in speed shall create the weld lines.

Why You Should Avoid Weld Lines

Why You Should Avoid Weld Lines

As have been said, weld lines do not only affect the design, aesthetic, and product value but generally, the overall cost of production.

From this alone, you should already see the need to avoid weld lines. But specifically, the following are some of the major reasons why you should avoid weld lines.


Technically, weld lines are considered as the weakest area in the product. This means that in this part, the material is brittle and shall easily break off when pressed with shock and pressure.

When such happens, you would not expect the same value return for the product. As a matter of fact, the whole production process might even be halted.

And if you still continue with it and go on with the post-pressing assembly, best believe that the product will break with the slightest drill or handle.

Deforms Surface Appearance:

Regardless of how much effort and conscious aesthetics you put in a mold design or product design, one weld line will change the product value.

How to Fix the Injection Molding Welding Line

How to Fix the Injection Molding Welding Line

The good news is, welding lines can be fixed. This might be a technical section so the simplest presentation of the interventions you can avail shall be laid down here.

Thus, here are the best interventions in fixing the injection molding welding line.

A. Material Selection

The first and major intervention should be proper material selection and adjustment. Out of a multitude of material selection interventions, the following are the most important.

Adjustment of Rheology, Molecular Weight, and Melt Viscosity:

Melt viscosity measures the melt flow of a material. In technical terms, it is inversely connected to molecular weight. On the other hand, rheology determines the resistance of a material to deformities.

Taking it all in, you need to select materials with high rheology, low viscosity for faster flow and draining and high molecular weight for impact resistance.

Colorants or Additives:

While colorants and additives indeed increase the aesthetic value of products, you must know that some colorants and additives have components that are incompatible with plastics and resins.

Incompatibility here means that some of the chemical properties of colorants and additives affect the impact resistance of plastics. As is the case, you must consult with professionals if you ever want to improve your product by using additives.

Proper material selection is a key tool in lessening lead time and overall production costs. Knowing what would avoid weld lines would further lead to profits and gains when it comes to molding.

B. Mold Design Optimization

Mold Design Optimization

Optimizing mold design does not only prevent mold lines. It also lessens the cost of product manufacturing. The most common interventions in mold design optimization are the following:

Gate Location and Type:

The main principle to keep in mind is that the cavities where the molten plastic is injected is made possible through the gates. Hence, note that the position of the gates shall consequently impact where the weld lines shall appear.

The proper remedy here is to position the gates where the weld lines shall become less visible. Also, choose the proper type of gate so that weld lines shall be eliminated entirely. Most of all, use the proper runner as well as the gate size.

Exhaust and Ejection:

The phase of exhaust and ejection denotes the process of plastic removal from the mold. In this sense, you must choose an injection mold with an optimized ejection and exhaustion system.

When the ejection and exhaustion pins are not in good condition or do not have the proper design, the air in the gates can directly affect the mold material in the cavity.

When there is too much air, the solidifying process can become uneven. This consequently produces weld lines.

Injection molding is nothing without molds. As such, optimizing mold designs should always be incorporated in your production model.

Again, consult with professionals as to how you could improve mold designs. Remember, by avoiding weld lines, you can lessen operations cost.

C. Processing Optimization

Processing Optimization

Aside from optimizing the equipment, the process of production itself must also be optimized. In doing so, the following processing optimization are interventions which you could also consider:

Injection Molding Parameters:

In this aspect, all the main molding processes are taken into consideration. This is done to prevent molding defects and upend molding quality.

As is the case, the injection molding parameters which should be optimized are the following:

  • Gate size and type
  • Number of gates
  • Resin/plastic temperature
  • Molding temperature
  • Switch over in terms curing time, volume filled, and injection pressure

Process Control:

Of course, upon optimizing the injection molding parameters, process control should also be subsequently optimized.

The most important process control and monitoring should be along the following lines:

  • Injection pressure
  • Melt, mold, and air temperatures
  • Filling timeCooling time
  • Screw and gate positions
  • Holding time

Production approaches may vary depending on clients, and main products, among others. Nevertheless, in the injection molding industry, there are generic processes and parameters that must always guide production. Optimizing each of these in your own production lines are very vital.

Tools for Identifying and Evaluating Weld Lines

Tools for Identifying and Evaluating Weld Lines

Injection molding is a high-precision process. In between, identification and evaluation of defects such as weld lines, are in place to ensure quality control and process optimization.

When it comes to molding, the following tools are the most common tests to identify and evaluate weld lines.

Visual Inspection:

This test is done primarily to thoroughly examine the quality, impact resistance, strength, and other value points of the molded material.

Here, a team of specialists and quality control personnel shall conduct a quality assurance evaluation before, during, and after molding and shall ensure that the molded materials are intact, safe, secure, and of good quality.

Tensile or Flexural Strength Test:

As the term implies, flexural strength test is done to evaluate the strength and impact resistance of the molded product. It also measures resistance to bending and deformities. This is not only a common test in the injection molding industry but is also used widely in engineering and architecture.

Thermal Characteristics:

Identifying the thermal characteristics of various materials used in injection molding is a very imperative tool in assessing the probability of weld line emergence.

Note that plastics, resins, polymers, and thermoplastics come in variants and each would have different melting and cooling points. As such, the thermal test is done to identify which materials go best and which ones should never be combined at all.


As what we have outlined in this post, weld lines should not be considered as an inevitable occurrence in the production process. As a matter of fact, it can be avoided using specific interventions.

In avoiding it, the major causes must be identified. Good thing, all the needed information in assessing the weld line and how to amend it are laid down here.

Weld lines, at all costs, should be avoided because it has a direct impact on the aesthetics and value of the product. Hence, if you want to get the target value of the manufactured products, you should know what a weld line is and how to avoid it.

Kemal: Your Leading Injection Molding Supplier

Injection Molding Supplier

Injection molding is a very competitive industry especially at a time when mass production is the key to profit leverage. As you can glean from this post, it is not as easy as you think.

A lot of considerations should be kept in mind and it requires a certain degree of skill, and technical know-how.

Hence, if you are looking for a dependable injection molding supplier, Kemal Manufacturing is one of the companies which you should check out. Kemal offers high-precision injection molding using a wide variety of materials.

Kemal also offers instant quotations for your inquiries about injection molding, other services, logistics, design, and delivery.



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