How do you select a parting line?

Table of Contents

Mold designers are often challenged to create a part that is both aesthetically pleasing and functional.

Aesthetics can be defined as the “look factor”, which means how something looks on paper; while function determines whether it will achieve what you want in reality. For these reasons alone, we need some guidance when it comes to mold design!

This article provides information about parting lines for injection molded products and how to choose them when designing a product so that there are no potential surprises when it comes to production.

How important is the proper selection of parting lines?

Injection molding is a manufacturing process in which parts are made by injecting molten material into a mold. The mold, usually made of metal, contains a cavity that provides the desired shape for the final product.

One of the most important aspects of injection molding is the design of the mold parting line. The parting line is the line where the two halves of the mold come together.

It is also the line where the molten material is injected into the mold. The position of the parting line affects the strength, appearance, and function of the final product.

Therefore, it is important to carefully consider the location of the parting line when designing injection molded parts. Choosing the correct parting line is important because it affects the appearance of the part, the structural integrity of the part, and the cycle time.

When done well, it can produce a strong, attractive, and functionally perfect product. However, if done poorly, it can result in a weak, ugly, or malfunctioning product. Therefore, it is clear that the parting line plays a key role in determining the quality of an injection molded part.

What is the parting line of injection molding?

The parting line is the junction line between the two halves of the mold and is a closed curve. It is also the location of the injection point. They are used to create the parting surfaces and to separate the surfaces.

For a successful injection, the molten plastic must be injected at a very high temperature. The plastic cools quickly after entering the mold, so it must be injected quickly and evenly to prevent any defects. If the plastic is not injected evenly, then the mold making will not be filled properly and voids or uneven thicknesses will appear in the final product.

The parting line is also the location of the gate (funnel opening) and the runner (the path from the gate to the cavity). The runner system helps to distribute the molten plastic evenly throughout the mold.

The parting line can be straight or curved, depending on the mold design. This form is based on the needs of the structure of the plastic part, sometimes combining steeped and curved parting lines, or beveled parting lines and curved parting lines and integrated parting lines to form a comprehensive parting line.

In some cases, the parting line can be hidden so that it is not visible in the final product. However, in most cases, the parting line is visible and needs to be carefully designed to avoid any problems with the injection molding machine.

Types of mold parting lines for injection molds

The parting line is the line that separates the two halves of the mold. Many factors determine the location of the parting line, including the type of product being produced, the production volume, and the molding process.

The most common types of parting lines are curved parting line, beveled parting line, stepped parting line. The straight parting line is the simplest and most efficient parting method, but it can only be used for simple products with well-defined geometry.

Beveled parting lines are more complex, but they allow for greater flexibility in product design. Stepped parting lines are typically used for large or complex products that require multiple levels of accuracy. Regardless of which type of parting line is used, it is important to consider all relevant factors before making a decision.

How does the parting line affect the final injection molded product?

The success of an injection molded part lies in the design of its mold structure. A properly designed and built mold will result in a nearly flawless final product that is consistent in size and appearance from piece to piece.

One of the most important aspects of mold design is the creation of the parting line. Parting lines are the lines on the molded part where the two mold halves meet.

They are also referred to as mold seams or joint lines. Properly designed parting lines are critical to ensure that the mold can be opened and closed without damaging the milled detail or leaving marks on the part’s smooth surface. In addition, parting lines should be located in areas that are inconspicuous or hurt the functionality of the final product.

For these reasons, it is important to work with an experienced mold designer who understands the intricacies of designing and making quality injection molds.

Can I control the parting line?

The parting line is the line where the two mold halves meet. The relative position of the parting line has a significant impact on the design considerations of the moldable product and therefore on the manufacturability and cost of the product.

To control the parting lines, it is important to understand how they are created. Parting lines are created when the molten plastic flows forward to meet the mold wall at an angle. The result is a line of plastic parts that is not bonded to the mold wall.

This can be caused by a variety of factors, including the geometry of the mold, wedge shaped insert, the material properties of the plastic, and the process conditions of injection molding. By understanding how parting lines are created, it is possible to control them to some extent.

However, due to these factors, there will always be some degree of variation in the position of the parting line. Therefore, it is important to take this variability into account in product design.

For example, parting lines can be hidden by using mating surfaces or designing features that do not require an exact fit. In addition, special coatings can be applied to the mold surface to improve the release and minimize variation in parting line locations.

Ultimately, parting lines are an inherent part of the injection molded products and cannot totally eliminated.

How to choose the correct parting line for injection molding?

Generally, the mold structure is roughly divided into two main complex parts: fixed half and moving half. When the mold body is closed, the contact surface of the cavity and core plates makes a parting line. Usually, the parting line of the mold is perpendicular to the mold opening direction of the injection molding machine.

Parting lines are one of the most important features of an injection molded product. They represent the intersection of the two halves of the mold, a closed circle of curves that must be carefully designed to ensure that the finished product meets all dimensional tolerances.

There are many factors to consider when selecting parting lines for a particular product:

  • Flow direction
  • Parting surface finish
  • Draft angle

In addition, it is important to consider the aesthetic and functional impact of the parting line. With careful planning, it is possible to create a parting line that meets all of these requirements.

The aesthetics of the vertical parting line are important for the visible part. The parting line should be perpendicular to any major surfaces that will be seen. Otherwise, the parting line will be visible on the finished part. The structural integrity of the part will be affected by the location of the parting line.

The wall thickness should be uniform over the entire circumference of the part. If the parting line is located on a thin wall, it may cause warpage or distortion during the injection molding process.

Cycle time is influenced by the number and arrangement of cavities in the mold. If the parting line is located in one cavity, it will take longer to fill and will result in a longer cycle time.

To select the correct parting line, you need to consider all of these factors. Choose a parting line that will result in a structurally sound part with a good appearance and a reasonable cycle time.


When doing product and mold design, there is some 3D design software that can recommend the location of the parting line, but when making a injection molding project, experienced designers will choose the location of the best parting line by considering the cost of the mold, the aesthetics of the product and other aspects.

If you have injection molds and injection products to be made but are not very professional in injection product design and mold design, don’t worry, just leave it to ZetarMold‘s engineers, we will make a DFM report and confirm the location of the parting line with you before doing the mold design, there will be a detailed report on it.

Latest Posts
Share on facebook
Share on twitter
Share on linkedin
Share on pinterest
Tell us your request right now and contact us today about getting started on your next project together!


Or Fill Out The Contact Form Below:

Ask For A Quick Quote

We will contact you within one working day, please pay attention to the email with the suffix “”.

Ask For A Quick Quote

We will contact you within one working day, please pay attention to the email with the suffix “”.

Ask For A Quick Quote

We will contact you within one working day, please pay attention to the email with the suffix “”.