A Design Guide on Optimizing Parting Lines in Injection Molding

A Design Guide on Optimizing Parting Lines in Injection Molding

Parting lines in injection molding are more than simply a design element; they are vital to overall efficiency, aesthetics, and product quality.

Parting lines are an unavoidable element of the injection molding process. These seemingly small lines can substantially influence the finished product’s appearance and functioning, resulting in higher production costs and wasted resources.

If not properly adjusted, parting lines can cause defects like flash, mismatch, and apparent seams, reducing product quality and increasing post-production effort.

Manufacturers may improve product quality, minimize waste, and optimize their manufacturing process by following this guidance, enhancing customer happiness and profitability.

In this blog article, we will present realistic, easy-to-implement solutions for improving parting lines in injection molding.

How Are Parting Lines Formed?

How Are Parting Lines Formed?

Parting lines result from the injection molding process of filling the molds. As the material cools, it adheres to the mold’s shape, including where the two halves meet. This forms the parting line.

Here are the factors that influence the formation of parting lines:

  • Mold Design: Complex mold designs may require multiple parting lines or parting lines in less-than-ideal locations. The surface finish of the mold can also affect the visibility of the parting line on the final product.
  • Viscosity: Materials with a higher viscosity may not flow as smoothly, potentially leading to more visible parting lines.
  • Injection Pressure: Higher pressures will force the material into the tiniest crevices of the mold, including the area where the two mold halves meet, leading to more visible parting lines.
  • Mold Temperature: If the mold is too cold, the material may solidify too quickly, leading to more visible parting lines. Conversely, if the mold is too hot, the material may not solidify quickly enough, potentially leading to issues like sagging or warping that can affect the parting lines.
  • Cooling Rate: If the material cools unevenly, it can lead to shrinkage or warping, making parting lines more visible.
  • Ejection Mechanism: If the part is not ejected evenly, it can cause stress on the part, potentially leading to deformation along the parting lines, making them more visible.

Understanding the formation of parting lines and their influencing factors is crucial for optimizing their placement and minimizing their impact on the final product.

Let’s discuss how parting lines can make or break your injection molding product.

How Can a Parting Line Affect Your Product Quality?

The parting line in injection molding is a critical aspect that can significantly affect the final product’s quality, smoothness, strength, and aesthetics.

Plastic Distribution and Cooling

Plastic Distribution and Cooling

When molten plastic is injected into the mold, the shape and placement of the parting lines affect how the plastic flows and distributes across the mold cavity. The geometry of the mold can influence factors such as filling patterns, flow rates, and cooling rates.

Improper parting line design may result in uneven distribution of plastic, leading to variations in wall thickness, air traps, or incomplete filling. These issues can compromise the structural integrity and functionality of the final part.

Aesthetic Considerations

After the molding process is complete, separating the mold halves and the ejection of the part can leave traces on the surface of the finished product. These traces are known as parting line marks.

The location and design of the parting lines need to be strategically planned to minimize the visibility and impact of these marks. By optimizing the mold shape and ejection process, manufacturers can reduce aesthetic damage and enhance the overall visual appeal of the part.

Production Cost

Production Cost

The location of the parting line can also influence the production cost of the molded part. The complexity of the mold design and the chosen split line placement affect the ease of manufacturing and subsequent finishing processes.

A well-designed parting line can minimize the need for additional post-molding operations, such as trimming or sanding, thus reducing production time and cost. Balancing aesthetics with part quality and cost is a crucial consideration in determining the optimal placement of the parting line.

Types of Parting Lines in Injection Molding

Types of Parting Lines in Injection Molding

Here are some common types of parting lines:

1. Vertical Parting Line

A vertical parting line runs vertically along the mold, dividing it into two halves. This is the most common and straightforward type of parting line.

It is often used for molds with simple geometries and symmetrical parts.

2. Beveled Parting Line

A beveled parting line is an inclined parting line that is angled or chamfered. This type of parting line is used when there is a need to minimize flash or visible parting line marks on the final product.

The beveled design helps create a smooth transition between the mold halves, reducing the chances of aesthetic damage.

3. Curved Parting Line

Curved Parting Line

A curved parting line deviates from a straight line and follows a curved path along the mold. It is commonly used for molds with intricate or organic shapes where a straight parting line would result in sharp corners or edges on the final part.

4. Stepped Parting Line

A stepped parting line has multiple levels or steps along the mold surface. This type of parting line is used when molding parts with varying wall thicknesses or when there is a need to accommodate inserts or over-molding.

Stepped parting lines allow for precise control over material flow and can help ensure uniform filling and solidification of the part.

5. Comprehensive Parting Line

A complex parting line involves the integration of multiple parting line configurations within a single mold. This approach is employed when molding parts with intricate geometries, varying wall thicknesses, or the need for inserts or over-molding.

Design Considerations for Parting Lines

Design Considerations for Parting Lines

When designing an injection mold, the placement and management of the parting line are critical considerations.

Here are some key factors to keep in mind:

1. Parting Line Placement

The placement of the parting line can significantly affect the quality and aesthetics of the final product. It should be placed in a location that minimizes its visibility and impact on the product’s functionality.

2. Mold Complexity and Cost

The complexity of the mold and the parting line’s position can influence the production cost. Balancing aesthetics with part quality can be a challenge, but it’s essential for cost-effective production.

3. Avoiding Steep Parting Lines

The parting line should not be too steep. A steep parting line can lead to molded-in stresses from resin flow. A rounded or tapered parting line is preferable, ideally about 60% of the thickness of the joining wall.

4. Consideration of Plastic Shrinkage

4. Consideration of Plastic Shrinkage

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.

To prevent challenging ejection, the engineer should ensure the shrinking portion doesn’t shrink too much.

Kemal:  Your Plastic Prototype Manufacturing Service Provider

Navigating the complexities of injection molding and parting line optimization can be challenging. But you don’t have to do it alone. Kemal is here to help.

As your trusted plastic prototype manufacturing service provider, we bring years of expertise and cutting-edge technology.

With our years of experience and advanced technology, we offer comprehensive manufacturing solutions tailored to meet your specific needs. Whether you require rapid prototyping or small-batch production, Kemal is dedicated to delivering top-notch results.

Don’t let the complexities of injection molding slow down your production process. Contact Kemal today and let us help you turn your design into a high-quality, injection-molded product.


The parting line is the edge where the two halves of an injection mold meet, while the parting surface refers to the entire contact area between the two halves. Essentially, the parting line is a line on the parting surface.

The parting line in an injection molded product can be identified as the line or edge where the two halves of the mold meet. It’s often visible as a slight ridge or seam on the surface of the final product.

When the two halves of an injection molding mold do not match properly, it creates a discrepancy known as a parting line mismatch.

The outcome of this misalignment can be seen in the final product as irregular surfaces or distinguishable seams, which will impact its quality in appearance and function.

In injection molding, flashing can result from excessive plastic material being pushed out through small crevices or gaps between the mold parts.

This can happen due to various factors, such as high injection pressure, inadequate clamping force, poor mold design, and the mold is worn out or damaged.


To ensure the quality, durability, and visual appeal of injection molded products, optimizing the placement of parting lines is essential.

Have you considered the impact of parting line placement on your manufacturing process?

Let Kemal guide you in achieving the best possible design for your plastic prototypes. Contact us now to explore how we can help you unlock the full potential of your injection molding ventures.

Put your parts into production today

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