What is a Parting Line? | Parting Line in Injection Molding | Importance of Parting Line

Introduction To Parting Line 

Manufacturing a mold is a lengthy process. There are so many factors to consider that need to be taken care of. One of the most important parts of any mold is the parting line.

Now you must know what the parting line is, but if you don’t, I must tell you that it’s a mold’s separation and collision point.

Every mold is divided into two halves, and deciding where one part will meet the other is crucial. Sometimes the choice is effortless, depending on the application, but sometimes it’s not easy.

Let’s briefly discuss choosing parting lines and their significant role in the injection molding process.

First of all, we start by selecting the “Line of Draw,” which means finding out in which direction the mold will open out parallel to the molded part. For some products, there’s a potential need for using actions like lifters and cams. The usage of action is avoided as much as possible.

However, when it comes to more complicated designs, a critical decision has to be made, like which part of the component will be forged into the tooling face and which should be part of the action.

Parting lines In Injection Molding 

Parting Line in injection molding poses similar kind of challenges as discussed above.

Sometimes the parting line is right in the middle, but sometimes finding a parting line is a drag on its own.

A parting line varies by product geometry. Let’s take a simple example of a plastic cup.

The outer surface will be designed by the first half of the mold ( A-Side ), while the inner surface will be developed by the other half ( B-Side ). The parting line will take shape on the outside edge of the cup’s brim. 

If you’re in the profession of plastic engineering and when you send an undrafted 3D CAD model of such a part to any company, make sure they figure out where the parting line should be.

As a professional, one fact should be obvious in your mind Molders, and designers think differently from each other.

Molders think like you; they want to produce the best possible part. However, designers think about what that part will be useful for after it comes out of the mold. Quite a distinction in the mindset!

The location of parting affects both in most cases.

Let me share an example to understand the concept clearly.

Interesting Read – Plastic Mold: Information about Male Vs. Female Molds

In the above image, there is a part with various features. You can see that Most of the features are at 90 degrees to the flattest sides of the vertical in the image.

This setup is much better than making that direction the line of the draw of the mold.


The above image shows us how the three holes are perpendicular to the draw line. The holes at the right side of the image are situated inside an arced ledge.

For that side of the part, cam action would be perfect.

The other hole in the middle of the left side of the part is located at an extension above the area, so cam action here wouldn’t be possible. IA lifter assembly fits the best in a scenario like this,

Additionally, a very prominent part of selecting a parting line in injection molding is the components designed on a particular side of the mold.

Plastic’s most fundamental properties include shrinking when cooled. It shrinks onto surfaces, which is both a boon and curse for designers and molders.

Shrinking is handy as it helps ensure that the part stays on one side of the mold as it opens.

However, as the plastic will stick and shrink too hard, the part will eject hard from the mold or get stuck in it.

Overcoming that is tricky but quite interesting in the application; angle the molded part’s walls away from the parting line. That makes it easy to remove the part.

We have to ensure that the part is shrinking onto one side of the mold opposite the other. Things might not go that smoothly, but depending on the scenario, using more or fewer features can help a lot.

Also Read – Types of Gates for Injection Molding | Applications of Gate Designs

The Importance of Parting Lines 

Having an awareness of the parting line is the most crucial part. As mentioned above, parting challenges can efficiently address if the right actions are taken at the right time.

Figuring out the parting line is very obvious in most cases. When dealing with more complex parts, cam action or lifter assembly can come in handy.

Evaluating split lines and draft walls are other feasible options, as mentioned in the article above.

You will find many CAD packages online and others associated with different companies. All those CAD packages definitely work in most cases, but sometimes they don’t.

Focus on building your knowledge and practical skills to manually apply methods like drafting walls and lifter assembly.

Final Thoughts 

Thus, was my knowledge and insights on parting lines and their importance. I have tried my level best to keep the article as neat and brief as possible. Feel free to share your reviews in the comment box.

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