Parting Line in Injection Molding: Design Considerations

Table of Contents

Parting lines for water outlet assembly
Parting lines for water outlet assembly

Paying careful attention to details is why big manufacturing giants always score high with customers regarding quality, aesthetic appeal, and parting lines in injection molding. This point is definitely not lost on plastic injection molding manufacturers seeing that they consider seemingly minute details as very important in their designs. 

One of these intricate considerations is the adjoining portion of each half of the mold used in their injection molding process. Consequently, this guide explores the effects of parting lines on the quality of molding injection products. It examines what it is, how it works, and the contributing factors that influence it in manufacturing simple and complex parts. So, let’s get right to it.  

1. Understanding the Parting Line

1.1 Definition and Function

While manufacturing technologies like 3D printing technology and injection molding continue to advance at almost neck-breaking speed, it becomes easier to take simple things for granted. For injection molding, overlooking a minor detail, such as mold linings, can cause a major setback for any injection molding process. 

Parting lines are what you see on the edges of the two separate parts that make up the molds. These lines give the operator a reference point to ensure the individual mold halves properly align while coupling them. 

Think of parting lines as dress seams that, if not carefully considered, can misalign the dress edges, giving you an awkward look when wearing such an outfit. Ensuring that these parting lines fit perfectly together guarantees that the eventual product is void of anomalies and irregular bulges that can give it an unpleasant look. 

The effect of parting lines on injection molded parts far exceeds just coupling two mold parts. An experienced injection molding operator will tell you that alignment is everything when it comes to getting a quality plastic product. It ensures that the product achieves the dimensions and shape intended during its design. 

Furthermore, it guarantees the consistency of the product promoted by a perfect closure of the mold while coupling. More importantly, ejecting the product upon cooling becomes more effective if the parting lines are given the required priority. 

1.2 Factors Affecting Parting Line Placement

With the definition and functions of parting lines out of the way, it is only natural to examine influencing factors that manufacturers and designers in high-quality mold manufacturing should never forget. These factors determine in no little way the placement and configuration of parting lines for any injection molding project. 

  • Geometry: This component is perhaps the most significant of all the injection molding considerations for parting lines. The reason is that parts molded often come in different shapes, sizes, and features, which will warrant a simple or complex parting line as determined by the intricacy of these parameters. 
  • Draft angles: While shapes may be simple or complicated, the nature of the product’s surface often requires critical evaluations when determining the location and type of parting lines required. The product’s surface is often governed by the presence of draft angles which are essentially tapered finishes on the mold. It ensures efficient ejection of the product from the molds with the introduction of a specific angled curve on the mold. Consequently, parting lines must take these curves into cognizance if they must be placed appropriately.
  • Undercuts and complex features: In the high-quality mold manufacturing industry, design is often critical as this is the stage where a product’s buildability is considered. Undercuts and features that ensure effective functionality are also included at this stage. However, the complexity of these undercuts and features often determine the type and placement of parting lines for injection molding. Internal depressions in the mold, also known as undercuts, if too deep, can become problematic if not carefully examined when determining the parting line type and position. 
parting line for lego Injection mold
parting line for lego Injection mold

2. Design Considerations for Parting Line Placement

The optimal location of parting lines on molds is an essential parameter that needs to be properly embodied in any mold design process to ensure high-quality injection mold part production. You’ll think that most professionals already know this, and you may be right, but not always. Also, the tendency to omit seemingly unimportant factors by designers is not impossible. Let’s delve deeper into these factors. 

2.1 Parting Line Location Determination

A balance between aesthetics and functionality of a product is the main focus of designers and manufacturers when it comes to placing parting lines of molds. Therefore, positioning parting lines in a mold is considered an essential skill for high-quality mold manufacturing. 

For certain products, their functionality is often considered second to aesthetics seeing that the beauty or design is often the point of attraction for customers. In this case, mold designers and manufacturers have a duty to put aesthetics as a critical factor when deciding on the most suitable point to place parting lines. Consequently, the appeal of such products will greatly depend on the invisibility of the parting line marks on the final product. 

Nevertheless, functionality, for most parts, is still the most important criterion for placing parting lines on a mold. The reason is that many end users worry about the perfect working condition of a product before they worry about if it is visually pleasing to them or not. Hence, manufacturers need to consider the positioning of the parting lines does not adversely affect the functioning of the product

Furthermore, unique products with specific criteria for integrating parts alignment and dimension often require that designers pay close attention to these details when placing parting lines. A misalignment of these integrating parts might be catastrophic to both the functionality and visual appeal of such products. 

2.2 Draft Angle Requirements

Frankly, the consideration of aesthetics and functionality for situating parting lines is basic. However, there are more critical factors to consider if you want to avoid irregularities that the wrong positioning of parting lines may cause. 

One such critical but often overlooked factor is the draft angle on a mold. Draft angle is the small indentation, often tapered surface finish you see on a mold. The essence of this is to ensure that the product removes easily on cooling. More importantly, the effect draft angle may have on the placement of parting lines is crucial to the effective design of a mold. 

Think of the draft angle as the cart behind the horse, considering that it depends on the material and dimension of the molds, which are the horses driving the cart. In essence, a poorly dimensioned draft angle can cause issues during the removal of the mold and can potentially damage the product or leave unsightly marks on the product. 

In the end, integrating the right dimension of draft angles parting line consideration is a prerequisite to achieving a flawless production of plastic injection molding parts. Manufacturers and designers will do well to put these factors into consideration to meet both in-house and global manufacturing quality standards. 

2.3 Managing Undercuts and Complex Features

One other critical but often forgotten parameter in position parting lines on molds is the presence of undercuts. Undercuts are somewhat similar to draft angles in that they are both depression points on the mold. But while the draft angle is often a slight indentation, undercuts are more pronounced depression on the surface of the mold to accommodate complex geometries on special products. 

However, these recesses can cause difficulty for manufacturers during the demolding of the product if not adequately considered. Thankfully, there are supporting features that help in easily removing the molds. These supporting devices include lifters and slides, which though helpful for demolding, can make placing parting lines a bit challenging.

Designers need to adequately consider undercuts and their supporting devices to ensure that parting lines fit the mold well. Moreover, other complexities, including geometry, will also need to be considered to make the necessary adjustment needed for a snug and correctly fitting parting line. 

Parameters, including alignment, verticality, and correct dimensioning, will form part of the checklist items when designing or installing parting lines on molds. In any case, an economical design is essential after putting the above-mentioned parameters. In the end, achieving a high-quality product remains the priority of every manufacturer, especially in tight-tolerance industries where injection moldings are used. 

Industrial mold maker
Industrial mold maker

3. Best Practices for Parting Line Design

After considering critical factors for placing parting lines on a mold, you should understand that knowing these parameters on their own will be insufficient for achieving quality injection mold products. However, combining them with best design practices for parting lines is the first step in getting quality products. 

3.1 Communication with Mold Makers

Proper interfacing with mold makers is a crucial factor that can singularly make or mar your design for parting lines. Proper alignment with mold makers, including adequate communication to clear out issues like design intent and feasibility, is crucial for achieving an optimally functional product that is also visually appealing. 

More importantly, experienced mold makers can help identify design issues that can lead to inefficient production and high cost of production. Hence, liaising with mold makers will help designers develop a suitable design that is buildable and cost-effective without sacrificing quality. 

Parting line placement in such designs is often effective, given that experienced mold makers understand the best techniques and procedures to achieve the intended design and aesthetic features of the products. 

3.2 Ensuring Proper Mold Alignment

Even if your mold is manufactured using very precise manufacturing methods like high-quality CNC Machining, it will still be prone to errors if adequate attention is not given to its alignment. Proper mold alignment is one important criterion that must not be missed in the list of considerations for placing parting lines. 

Consider mold alignment as the buttons that hold the two sides of your shirt in place. Any mismatch between buttons and their respective buttonholes only makes the shirt look awkward on you. So, misaligning the parting lines on a mold only leads to defective products with flaws that make them unsightly and less functional. 

Ultimately, injection molding processes rely on the accuracy of design inputs like bushings and even alignment pins to ensure a high-quality product. 

3.3 Minimizing Flash and Defects

Defects and flashes are typically found in a misaligned mold during production. Such misalignment often leads to molten plastic material escaping through the opening offered by this misfitting error of the mold edges in the case of flash. And flash can look very horrible to the sight of an onlooker especially end users of the product. 

Additionally, several other defects can occur when there are gashes between the two halves of the mold as a result of this misalignment, further causing unpleasant looks and even certain dysfunction for the products. Fortunately, there are preventive maintenance practices that ensure that all these problems are totally avoided. 

One such practice is maintaining the injection molding tools. Practices like oiling, proper cleaning, and upholding prescribed machine allowances for these tools go a long way in averting the challenges associated with misalignment. 

Furthermore, employing mold venting ensures that air entrapment that results in flash is eradicated. Features such as screws and pins that enhance the venting of the mold can be integrated at the mold design stage to ensure that flash formation is canceled out absolutely. 

In the end, parting line placement is more effective and serves its optimal function when all these practices are imbibed by injection molding manufacturers.  

4. Case Studies and Examples

Scenarios exist in the real world in several industries of the usefulness of parting line placement in mold design and manufacturing. Such examples give more insight into the application, practicality, and need for a well-designed parting line. Let’s take a look at these instances. 

Automotive Interior Panel

With the complex and multiplicity of different parts that make up the final product in the automotive industry, it’s no wonder why there is a very tight tolerance for errors in this industry. One way to ensure automakers ensure they meet up these standards is by incorporating parting line design considerations into their manufacturing process. 

Especially for interior panel manufacturing, where plastic parts are required to line perfectly with their metal counterparts, giving careful thought to the placement of parting lines helps achieve a high level of quality. What’s more, experienced machinists and shop fabricators know well to place these parting lines out of sight for a more pleasing and functional product. 

Medical Device Housing

The medical industry is another sector with a very tight margin for errors, and the reason for this is obvious – lives are involved. Hence, medical device manufacturers often utilize parting lines with strategic placement to achieve the industry-required quality. 

Usually, housing for medical devices often poses challenges, especially with locking feature misalignment. When designing these features, bioengineers ensure that they put parting line location as part of their priority list to ensure that there is no disparity between part assembly and the device functionality. 

Consequently, strategic parting line placement ensures more efficient and cost-effective production of medical device housing while also ensuring the quality of the product

Consumer Electronics Enclosure

It is so easy to take for granted the several parts that play significant roles in the production of consumer electronics because they are ubiquitous. However, carefully considering these parts will show that, in fact, we won’t have consumer electronics as we have them today with their ever-so-useful functions. 

One such part, as you may have guessed, is parting lines. If wrongly placed, their location on the device enclosure can spell doom for the production and the end user. With several complex geometries and features like undercuts, knowing the most suitable location to place parting lines on the mold becomes a very helpful and indispensable skill. 

This will help in the neat removal of the product from the mold and help achieve the required quality upon cooling. More importantly, placing parting lines correctly helps save a lot of money that may be required for post production reworks. 


It might look like overstating the facts of the matter, but the truth is this: without properly, adequately, and strategically thinking about the right placement of parting lines, the functionality, aesthetics, and efficiency of most products will be greatly diminished. 

Parting lines are the reason doctors can prescribe a medical device with great confidence that it will achieve its purpose without becoming a source of discomfort to patients. 

Automakers with close collaboration and constant communication with mold makers also ensure auto parts come out functionally and visually pleasing. And without carefully considering where will be the best position for parting lines on a mold, manufacturers set themselves up to lose money while ending up with a product quality that is below par. 

So, what’s stopping you from integrating this best practice into injection molding processes?  


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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