Mastering Core and Cavity Placement in Injection Molding

Table of Contents

Injection Mold cavity
Injection Mold cavity

I. Introduction:

Injection molding is never complete without the essential tools and equipment that make production possible. Chief among these tools is the mold which is a combination of two halves – the mold cavity and the core.

These two halves continue to serve as the critical tool required to achieve precision and the required quality for plastic parts. Nevertheless, it is essential to know that these two halves of the mold are distinct parts though they serve the same purpose together and anomaly in their placement can be detrimental to the quality of the part.

Thankfully, this comprehensive post will ensure that you understand the significance of these mold halves and how to successfully navigate their correct placement for a high-quality part production.

Without further ado, let’s dive in.

II. Understanding Cores and Cavities:

Mold Core and Cavity
Mold Core and Cavity

Essentially, the injection molding process liquifies plastic resins under specified temperature. It is this molten plastic material that is injected into the mold using adequate pressure, which once cooled, is ejected from the mold resulting in a part with the desired shape and finish. However, the product might have defects and flaws if there is a placement error with the mold cavity or core.

In its basic form, the mold has two sides with one half held in place by the injection molding machine while the other half has the ability to move since it is attached to the machine’s moving clamp.

Consequently, the moveable side of the mold, which we’ll refer to as the B-side, has an ejector also attached to the machine’s clamp. It is this ejector that helps detach the plastic part from the mold as the mold opens up.

While the principle of mold separation and part ejection is the same for different plastic parts, the mold design is however, different, accounting for the uniqueness of different plastic parts.

For instance, the mold design for plastic parts used in beverage packaging takes into cognizance the ease of removal of the mostly cylindrical parts. This is why its mold is designed with the cavity side forming the outside of the mold.

Such designs not only ensure that the part is retained in the mold on activating the ejector but also guarantees a non-defective product with the easy removal of the part which is critical.

III. Factors Influencing Core and Cavity Placement:

Mold part design
Mold part design

To ensure you get a plastic part with the expected quality, so many considerations and processes go into the mold design, especially the placement of the core and cavity. We’ve highlighted the critical ones every mold designer needs to thoroughly evaluate before deciding on the placements of these mold divides.

1. Part Design

The complexity of the part geometry which constitutes its part design is a fundamental consideration when deciding the most suitable placement for mold core and cavity. Fortunately, several plastics have simple and regular shapes, making the placement of mold cores and cavities a relatively easy task.

Yet, there are several complex part geometries that require careful planning to position mold components including cooling channels, ejector pins, and gates along with the mold’s halves.

Chief among this part design considerations include maintaining uniform wall thickness for the mold considering that too much variation in the wall thickness only results in production issues. These production hitches can include ejection difficulties and cooling irregularities which are at the roots of several flaws in injection molding.

Additionally, incorporating draft angles and ensuring symmetry in part design can go a long way in ensuring that both the core and cavity experiences less friction when parts are released from the mold.

Incorporating draft angles along with its symmetry helps release the vacuum in the mold while providing ample space for part ejection.

Fundamentally, all of these factors help ensure the strategic location of the mold’s core and cavity during the mold design while also ensuring that the part stays on the correct side of the mold.

Material Characteristics
Material Characteristics

2. Material Characteristics

Your choice of material is another crucial factor that significantly influences the correct placement of mold’s core and cavity. The reason is not far-fetched.

Different materials often possess different characteristics, especially critical ones like flow rates, injection speed, shrinkage, heating, and cooling temperatures. These factors are largely responsible for the success of any injection molding project.

For instance, the design flow rate of the injected resins may work perfectly when the cavity is placed on the moving clamp but might be problematic if placed as a stationary side of the injection machine.

Consequently, employing simulation software that can analyze the flow and indicate a suitable location for the mold cavity and core becomes an important part of the design process.  Moreover, this software can also simulate other factors like shrinkage, temperature differences, and distribution of molten plastic both in the core and cavity of the mold for a given runner system.

3. Mold Design

From material selection to part design what is left now is the mold design which determines the shape, dimension, and quality of the parts including its aesthetics.

Typically, having a base mold is the first point of call ensuring that critical features including the mold cavity and core are well designed to meet part specifications. Then other features like ejector pins and mold plates play their part in achieving a perfect mold design for the product. Nevertheless, the complexity, size, and intricate details of the mold required equally play critical roles in achieving success.

Still, the mold base which serves as the foundation for the final mold must also meet the production volume requirement of the project.

But that’s not all.

Understanding the pivotal role of mold elements such as sprues, gates, and runner systems is vital to ensuring that a mold which guarantees quality part production emerges.

For instance, understanding that from the gate through the sprue of the mold the resin can either have a free or restricted flow which can significantly impact the quality of the final part.

Also, understanding that the runner system chosen can limit or improve the resin delivery to the mold and this can help in deciding the best location for both the core and cavity of the mold.

IV. Choosing the Side for the Core and Cavity:

 Mold Core and Cavity sider
Mold Core and Cavity sider selection

Specific guidelines and rules are required to successfully determine the intricate details of mold design such as the correct placement of the mold cavity and core. However, one rule is fundamental – one half of the mold containing the ejector pin must retain the part when the mold is separated after production.

Consider a typical injection molding machine which holds the mold in place during part manufacture. You’ll easily notice that one half of the mold is often attached to the machine’s moving clamp. Also, this mold half usually contains the ejector and the pins which means that the plastic part must stay within this mold section when the ejector is activated. 

During the injection molding process, the mold’s two halves are clamped tightly together as the molten plastic flows into the mold. On cooling, the moveable side – usually the cavity of the mold, moves away from the machine leaving the part stuck in the other half – usually the core of the mold.

As an example, if the part is cylindrical in shape, the cavity will be separated from the rest of the mold after cooling while the core will retain this part as the ejector is then used to push it out from the core.

You must remember, however, that plastic shrinks and as a result clings tighter to the core. Incidentally, this core is often associated with creating the hollow portion of the parts.

Again, the part also shrinks away from the mold cavity often responsible for creating the exterior of the plastic part. Both the core and cavity of the mold have their individual and collective functions in the proper formation of plastic parts.

Nonetheless, understanding these functions helps designers allocate and place them appropriately during design. It’s no wonder mold designers depend on software applications and advanced years of experience to accurately establish the best placement for each side as this task can be difficult.

V. Technical Aspects of Core and Cavity Placement:

Ejector Pins for Demold molds
Ejector Pins for Demold molds

Certain technical considerations are involved in deciding the correct mold cavity and core placement. However, the major technicalities to consider in mold design include the ejector systems, gates, runner systems, and sprues. Let’s help you understand the function of these elements.  

1. Ejector System Configuration

Once you recognize that a standard injection molding process relies heavily on a functional ejection system, then configuring a working ejector system for the mold becomes very important. This single consideration can be the difference between a flawless injection molding process and a disastrous one, especially with mold cavity and core placement in a mold.

As earlier stated, the core often holds the ejector pins and since the plastic part sticks to the core, it is only natural that the core remains stationary while the cavity is designed as the moving part during ejection. However, in some cases where there is an inadequate draft angle or release agent for the mold, the part may become difficult to dislodge from the core after cooling.

These inadequacies can result in excessive friction between the mold core and the plastic part leading to defects on the part. Hence, the need for an effective ejection system. To avoid difficulties in part removal, the ejection system must be such that adequate draft angle is designed into the mold as well as specifying adequate release agent for the mold. Also, the part shape should undergo critical evaluation before deciding the best suited placement for both the mold cavity and its core. The shape complexity, dimensions, and part size are all essential considerations to achieve repeatable ejection.

2. Gates, Runners, and Sprues

Other important mold components that require technical consideration due to their significant influence on the mold cavity and core placement include the gate and runner systems. Considering that the runner systems directly feed the mold with molten plastic, ensuring that they are designed for efficient delivery to the cavity and core is vital.

Also, since the sprues form part of the molten plastic delivery system to the mold, ensuring their compatibility with the cavity and core arrangement is always in order.

However, the gate, which is in direct contact with the mold cavity and core often takes center stage in determining the right placements for the two halves of the mold especially since it impacts the flow and fill of molten plastic.

In all, the gate and the runner system require that they are the right size and dimensions to ensure that they meet the demands of delivering an adequate amount of molten plastics to the mold. This will help achieve a quality and flawless plastic part.

VI. Practical Tips for Core and Cavity Placement:

parting line for  Injection mold
Parting line in Injection mold

With all that has been said, it is often vital to remember some practical tips and rules of thumb to help you successfully navigate the mold design waters, especially with mold cavity and core placement. These tips often help you avoid mistakes that result in defective and below-quality parts. We’ve highlighted the major ones below.

Firstly, ensuring that corners and edges are designed as rounded rather than sharp ends often leads to difficulty in demolding plastic parts. Also, the addition of bosses for threaded inserts and ribs to help strengthen parts, especially the areas with thin walls is another useful tip that can enhance the placement of core and cavity in a mold.

Again, ensuring that walls are not perpendicular to the parting line of the mold helps mold designers incorporate easy part removal systems into their designs. More importantly, designing mold wall thickness as uniform dimensions gives the mold more structural integrity as well as ensuring easy removal of the parts from the mold.

Additionally, ensuring that hollows are created in the thick sections of the cavity is another vital consideration you should never overlook in your mold designs.

This tip and all the others highlighted here are necessary to avoid all the common pitfalls that often lead to manufacturing issues while deciding on the most suitable locations for mold cavity and core.


At the end of the day, achieving a flawless plastic part largely depends on your understanding of suitably positioning your mold cavity and core during its design. The impact of their placement significantly impacts the output of any plastic part manufacturing, especially with the quality and precision of part finish. Consequently, you need to critically consider factors like part design, material selection, and mold design in choosing a suitable for both mold cavity and core. However, some practical tips such as introduction of ribs and bosses, uniform wall thickness, and draft angles for easy part removal always come in handy when designing your mold.

Ultimately, giving the core and cavity a careful and thorough evaluation before manufacturing the mold can help manufacturers achieve high performance while ensuring a suitable mold core and cavity location. 


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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