Introduction: The parting line is a line that appears on injection molded parts. It is the line where the two halves of the mold meet. It runs around the entire part. Sometimes it is just a straight line down the middle. Sometimes it is in a different place.

The parting line might seem like a small detail, but it’s actually a big deal when it comes to how useful the part is. That’s because where the parting line is determines where the designer and the mold maker can put other features on the part. If the mold maker puts an important feature on the wrong side of the parting line, it can cause big problems for the whole injection molding process.

In this article, we will fully explain the process of parting line formation, the different types of parting lines in injection molding, and how to consider them when designing a part.

How is the Parting Line Formed?

The parting line is formed by the injection molding process itself, not by any mistake. Typically, the mold that mold machinists use to produce injection molded parts is divided into two halves (called the fixed half and the moving half). When the machinist closes the mold body, the mold half (called the core plate) and the cavity surface form a parting line between the two halves of the part.

Usually, the parting line of the molded product is perpendicular to the opening direction of the mold used to make the product. When the mold is opened and the cooled and solidified parts are taken out by the machinist, the moving half of the mold moves and separates from the fixed half (fixed). This is the whole process of the parting line formation.

But machinists sometimes separate the mold structure from all directions multiple times. This is called multi-step separation.The Injection molding machine completes the parting line injection molding by joining the mold halves to form a parting line.

What are the Types of Parting Lines?

The type of parting line in plastic injection molding usually depends on the specific function and structure of the plastic part. However, The type of parting line in injection molding usually depends on the specific function and structure of the plastic part.

There are various types of parting lines, including curved parting line, beveled parting line, vertical parting line, mold parting line, curved parting lines , injection molding parting line, and so on.

Straight Parting Line

The straight parting line is the most common type in mold design. It has a simple shape and is easy to process. It is suitable for mold design of most products. The straight parting line can effectively reduce the processing difficulty of the product and improve production efficiency.

Arc-Shaped Parting Lin

The arc-shaped parting line is developed from the straight parting line. It is more complicated, but it is also more beautiful.

The arc-shaped parting line is suitable for some mold designs that have high requirements on product appearance, such as mold design of automobiles, mobile phones, and other products.

Step Parting Line

Here, the machinists make the plastic parting line into a step shape. For this type of parting line, one side of the cavity usually gets a big force. This makes it possible for the fixed half and the moving half of the mold to slide against each other.

The force can be balanced by creating injection force on both sides of the cavity. The force can also be too big or too small. If the force is too small, the machinist will use a guide pin. In this case, the effect of a small unbalanced injection force on the final product may not be a big deal.

Comprehensive Parting Line

Like the name says, this is one of the most complex parting lines in plastic design. Machinists and designers work on this parting line based on the structure of the plastic part. They can combine all other parting lines to make a complete plastic parting line.

What are the Design Considerations for the Parting Line?

First thing you need to do is figure out which way the mold opens in relation to the part. Machinists call this the “draft line.” This is important because it determines how the part will look. It also helps you know what features to add to the part to make it work. It also helps you know how the parting lines will look on the final part.

Another important thing to consider when deciding where to put the parting line is where to put the part on the mold. This is because the plastic shrinks as it cools and that can move the part in the mold. That can mess up the way the part works and make it not work. Also, you don’t want the part to shrink so much that it’s hard to get it out of the mold.

One way to make sure the part stays where it’s supposed to is to keep the walls of the injection molded part away from the parting line. The more draft you have, the less likely the thing is to fall apart.

Understand Product Features

Before designing the parting line, you must first fully understand the product’s structure, size, material, and other characteristics. This will help you determine the reasonable demoulding direction and parting line position.

Follow the Principle of Aesthetics

The design of the parting line should be aesthetic, making the product look more beautiful. The parting line should be hidden as far as possible in a place where the product is not easily noticed, or designed as a line that is coordinated with the product’s appearance.

Consider the Production Process

When designing the parting line, you should consider the production process to make it smoother. For example, the parting line should not conflict with the internal structure of the product, so as not to affect the molding and demolding of the product.

Optimize Mold Structure

The design of the parting line should be coordinated with the mold structure to make it easier to make, process, and maintain the mold. If the parting line is set reasonably, it can reduce the cost of making the mold and increase the service life of the mold.

Consider the Production Process

When designing the parting line, you should consider the production process to make it smoother. For example, the parting line should not conflict with the internal structure of the product, so as not to affect the molding and demolding of the product.

What is the Importance of the Design of the Parting Line?

Affect Product Appearance

The design of the parting line is directly related to how good the product looks. If you design the parting line right, the product will look better.

Impact on Production Efficiency

Good parting line design can improve production efficiency and reduce production costs. The design of the parting line involves various links such as mold manufacturing, processing, and maintenance, which has a profound impact on the production process.

Impact on Product Accuracy

How you design the parting line will affect how accurate the dimensions and positions of your parts are. If you design your parting line right, you can make sure your parts come out the right size and in the right place.

What are the Principles of Parting Line Design?

Mold the part with coaxiality requirements on the core side. If it is molded on the cavity and core side, the coaxiality is difficult to guarantee because of inaccurate clamping.

Distance between plates: The parting line should be determined to make sure the mold cavity and core opening have the shortest stroke.

Try to avoid using the core pulling mechanism as much as possible. If you have to use it, try to keep the core pulling distance as short as possible and keep the slider on the side where the core is being pulled.

Since the locking force is small when the inclined slider is closed, for large plastic parts with large projection areas, the parting line with large projection area can be placed on the main plane after the cavity and core are clamped, and the smaller projection area is used as the horizontal parting surface,

otherwise the locking mechanism of the inclined slider must be very large, otherwise burrs will be generated due to poor locking.

The overall structure of the injection mold should be simplified, minimize the number of parting lines, and use straight parting lines as much as possible. Also consider whether the mold is easy to process and process, whether it is easy to remove the finished plastic parts, and select the parting surface on the edge of the plastic part as much as possible.

Avoid sharp corners on the parting line. Sharp corners will affect the life of the injection mold. So, the mold designer should check very carefully. If it cannot be avoided, suggest to the customer whether the product can be modified.

What is the Formation Process of the Parting Line?

‌Mold Design

When designing a mold, you need to consider where the parting line will be. The parting line is where two halves of the mold come together to make the product. You want to put the parting line in a place where it won’t show on the finished product. That way, it won’t mess up the way the product looks.

‌Mold Manufacturing

The mold is made into two or more parts according to the designed parting line position, so that it can be separated when the product is molded. There will be a certain gap between these parts at the junction, forming the basis of the parting line.

‌Product Manufacturing

When you make something with a mold, you have to put the two parts of the mold together and then pour the stuff into the mold. But you can’t make the place where the two parts of the mold come together completely smooth, so there’s a line on the thing you made. That line is called the parting line.

‌Post-Processing

Sometimes, when you’re using easy-to-peel paint technology, you need to form the parting line. You can use masking tape on a specific part of the mold, apply the easy-to-peel paint, and then form the parting line by sticking, coating, peeling, and so on. This method is mainly used to form a specific parting line during the coating process.

‌Operation in SOLIDWORKS

When you’re using SOLIDWORKS for mold design, the process of generating parting lines includes defining the draft direction and draft angle, selecting the parting line to generate the core/cavity division, performing draft analysis, and making manual adjustments when necessary to ensure the accuracy and effectiveness of the parting line.

Conclusion

The parting line is a key part of the design of injection molded products. It marks the junction of the mold and has a direct impact on the function and appearance of the part. This article discusses the formation process and main types of parting lines (such as straight lines, arcs, verticals, etc.).

And emphasizes the factors that need to be considered in the design, including mold opening direction, part shrinkage and aesthetics. Reasonable parting line design not only improves product quality, but also improves production efficiency and reduces costs, so it is crucial in the injection molding process.