A Design Guide on Optimizing Parting Lines in Injection Molding

A parting line is an inevitable part of injection molded products. It is the line of separation between where two halves of a mold meet. It manifests itself on injection molded parts as a line that runs the entire perimeter of the part. In most parts, it is just a line that runs straight down the middle. However, in more complex parts, its location varies.

While a parting line might seem harmless, it is very important in determining the usefulness of a part. This is because its position is influential in helping designers and molders to decide where to insert other features of the part. If a machinist places an important feature on the wrong side of the parting line, it could lead to serious issues with the whole injection molding process.

This article will fully explain the formation process of parting lines, the different types of parting line in injection molding and how to consider them while designing your part. Stay with us till the end to find out the best guide to optimize parting lines.

How are Parting Lines Formed?

The formation of parting lines is a result of the injection molding process itself rather than any error. Generally, the mold machinists use for producing injection molded parts is usually divided into two halves (known as the fixed half and the moving half). When the machinists close the mold body, the mold halves (known as the core plates) and the surface of the cavity make a parting line between both halves of the part. 

How Parting Lines are Formed

On most occasions, the parting line of a molded product is perpendicular to the opening direction of the mold used to make the product.  When the machinist opens the mold and removes the cooled and solidified part, the moving half of the mold moves and separates from the fixed half (which is stationary). This explains the full process of the formation of the parting line.

However, machinists sometimes make the structure of the mold be parted several times from various directions. This procedure is known as multi-step parting.

Types of Parting Lines

The type of parting line in injection molding generally depends on the specific function and structure of the plastic part.  However, there are five main types which are: vertical, stepped, inclined, curved, and integrated parting lines.

1. Vertical Parting Line

injection molded product with vertical parting line

The vertical parting is the most popular of all parting lines in plastic design. Machinists form the vertical parting line in a perpendicular direction from the mold opening direction.

2. Beveled Parting Line

The plastic parting line is in a beveled shape here

3. Curved Parting Line

injection molded product with curved parting line

Here, machinists build the mold to create a curved parting line.

4. Stepped Parting Line

Here, machinists make the plastic parting line in a stepped shape. For this type of parting line, there’s usually a large force on one side of the cavity. This causes a relative possibility of sliding between the fixed and moving halves of the mold.

This force can then be canceled out by generating an injection force on both sides of the cavity. There’s also a possibility of the generated force being too large or too small. If the force is too small, machinists compensate for it with a guide pin. Here, the effect of the smaller unbalanced injection force might not be significant on the final product.

However, if the force is too large, a couple of solutions can be applied. Here are a few examples:

  1. Make a wedge-shaped insert on one side of the cavity. This way, the insert offloads some of the injection force. Hence, the molds can maintain the relative position between the cavity and the core.
  2. Lay out the molds in a symmetrical shape. This helps to make the injection force applied to both sides balanced. The structure of the mold should also be made as compact as possible to support the symmetrical arrangement.

5. Comprehensive Parting Line

As the name suggests, this is one of the most carefully done parting lines in plastic design. Machinists and designers work on this parting line based on the structure of the plastic part. They could combine all other parting lines to make a comprehensive plastic parting line.

Design Considerations for Parting Lines

The first place to know the best parting line to use for an injection molded product is the mold design. In some parts, the choice is obvious while it might not be so clear in some places. This section will address the importance of parting lines in plastic design.

The first consideration is to determine the opening direction of the mold relative to the part. Machinists call this factor “the line of draw.” It is very important in determining how the design of the part will look like. It also helps to know which sides to add the features of the product. Apart from this, it also helps to determine how the vestiges left by the two halves of the injection molded product will look like on the final product.

Another factor that is important when determining the parting line is where to place components on the part. This is because the shrinking of plastic while it cools might shift the position of the part in the mold. This could displace the functional features of the part, rendering it useless. Also, the machinists should ensure the shrinking part doesn’t shrink too hard to avoid difficult ejection.

One way to ensure the components stay in the right place is to draft the walls of the injection molded part away from the parting line. The more draft, the less the probability of the features falling apart.

Another good way to determine the part line in injection molding is with checking the Design for Manufacturability (DfM) of your product. Apart from suggesting the best positions for your part line, it also helps to check for defects and optimize your part for manufacture. This will help provide economic options to manufacturing your part.

At RapidDirect, we provide free DfM for injection molded products on our instant quoting platform. However, you’ll have to request a manual evaluation for this feature if desired.


Taking all these part line considerations into the design of your injection molded product is important, but that’s not all. There are various important injection molding guides and rules that you have to follow before your injection molding process can be a success.

This is why you should let RapidDirect handle your injection molding process. With a wide team of engineers at your disposal, we provide various options and ideas to improve your part design and manufacture. With us, you can bring your custom product or design to life within a short period of time.

Talk to our engineers today to get the best injection molding services at your fingertips!

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