An Optimizing Guide For Design of Injection Molding Parting Lines

A huge volume of identical plastic parts designs for injection molding are being produced at a low price per sample. It has enabled rapid tooling services for prototyping and short-run production. But,  a separating line is required for items made by injection molding. It is the boundary between the two mold halves.

It appears as a line that encircles the entire injection-molded object. In essence, it consists of a line that parallels the center continuously. Additionally, its positioning varies in more complicated components.

You might not spend much time considering the injection molding part design and placement of the separating line. But, while creating a 3D model using CAD software that will be injection molded. it is important to take into account the location, which might have a variety of effects on your product. Although it may not be as evident on more complicated parts, the separating line on some pieces is clearly located directly down the center.

The parting lines formation, their various forms in part design for injection molding, and how you take them into consideration while designing your part are all clearly explained in this blog. Continue reading to learn the best suggestion for optimizing parting lines.

An Optimizing Guide For Design of Injection Molding Parting Lines
An Optimizing Guide For Design of Injection Molding Parting Lines Source:Unknown

How are Parting Lines of Injection Molding Part Design Made?

The mold design is often split roughly into two parts the fixed part and the movable part. The parting line is formed by the cavity and core plates which is the contact surface between them when the mold is sealed.

The boundary of the die’s two parts makes the major dividing line. Additionally, as the pressure from the injected metal tries to separate the die halves, a flash will form near the parting line.

When constructing plastic part design for injection molding, choosing a separating line is crucial. The separating line is the fundamental building block for creating and designing molds. The fabrication of molds is simplified and the caliber of plastic components is improved with a properly chosen separating line. Otherwise, it will make manufacturing molds difficult.

The mold’s parting line typically moves vertically to the mold opening direction to make a product. The parting line is chosen based on the product’s design and appearance criteria, as well as the processing efficiency and mold layout.

The mold’s structure may occasionally be split multiple times in different directions. This process refers to multi-step separation.

Plastic mold design for injection molding
Plastic mold design for injection molding

                               Plastic mold design for injection molding Source:Unknown

Classification of Parting Lines

The structure and purpose of the plastic object are typically taken into account when choosing the sort of parting line for an injection molding design. Although, the five most common categories are, comprehensive, curved, stepped, Inclined, and planer parting lines.

1. Planer Parting Line

The most widely used parting line in plastic design is the perpendicular one. The planer parting line has been created that is perpendicular to the mold opening direction.

2. Inclined Parting Line

In this, the injection molding parting line is shaped like a bevel, as the name suggests.

3. Curve Parting Line

The dividing line has a curved curvature, as shown in the figure below.

4. Stepped Parting Line

The parting line is made stepped for various plastic pieces based on the design for manufacturing injection molding parts and their unique circumstances, as illustrated in the image below.

Due to the significant pressure on one end of the body, the injection eccentric force is generated on both edges of the body, which results in a relative tendency for the molds of the fixed half and the moving half to slide past one another. The graphic below depicts its sliding direction.

A guiding pin can be used to support the eccentric unbalance if it is not too high. But if the eccentric pressure is too high, the guiding pin and guide bush will wear out too quickly and the quality of the plastic component will suffer. The following solutions may be employed to make up for the flaw.

(1) Place an insert in the shape of a wedge on one edge of the hollow space, as illustrated in the image below. By doing this, it is possible to counteract the injection eccentric force, keep the cavity’s location in relation to the core, and modify it easily while wearing.

(2) Set up the two mold cavities such that they are symmetrical. Make the mold structure compact and the injection forces that are applied to each edge of the pattern body equal. The flow of part materials is greatly improved by the use of side gates.

Parting line types (a) vertical (b) Stepped (c) Curved
Parting line types (a) vertical (b) Stepped (c) Curved

                         Parting line types (a) vertical (b) Stepped (c) Curved Source:Unknown

5. Comprehensive Parting Line

According to the requirements of the product’s construction, this form may occasionally combine steeped and curved, or inclined and curved parting lines, to create a comprehensive separation line.

Factors of Parting Lines for Injection Molding Part Design

Companies are realizing that early detection of manufacturing issues reduces the overall cost of the product, enhances the product’s quality, and speeds up its entry into the market.

 You’ll get better components faster if you check your design for the problems listed in this guide before sending the design for manufacturing injection molding.

  • By creating a connection between part geometry and mold design, the part designer is able to make choices that will not negatively impact mold design.
  • The placement of the separating line and draw direction are two factors that significantly affect the price of molds and products.
  • The part’s position in the mold could be altered by the plastic’s tendency to shrink as it cools. As a result, the part can lose its ability to perform its intended functions. Additionally, in order to prevent challenging ejection, the engineer should make sure the shrinking portion doesn’t shrink too.
  • Drafting the walls of the item for injection molding away from the splitting line is one method to guarantee the components will remain in the proper location. The risk of the features coming apart reduces as there is more drafting.
  • Examining your product’s design for manufacturing injection molding is another effective method for determining the part line for injection molding. It aids in fault detection and manufacturing optimization in addition to providing the ideal places for your part line. This will make it possible to manufacture your component economically.
Parting design for injection molding
Parting design for injection molding Source:Unknown

                                       Parting design for injection molding


The parting line is very essential for plastic part design for injection molding. This makes the fabrication of molds simplified and the quality of injection molded products is increased with a properly chosen parting line.

It’s crucial to incorporate all of these part line factors into the injection molding part design. Just to make sure your injection molding process is successful, you must adhere to a number of significant injection molding guidelines.

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