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Mold Core & Mold Cavity: Differences And Design Consideration

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In injection molding, the “mold core and mold cavity” refers to the precision parts used at the heart of the mold for its key operations. The structure of the core and cavity is typically extremely complex, making their fabrication challenging and costly, often with labor expenses far exceeding the material costs. The choice of material for the core and cavity directly affects the mold’s cost and its service life.

mold cores and mold cavities at FirstMold

For product designers or those requiring mold manufacturing services or plastic parts mass production services, the purchase includes mold design, the fabrication of mold parts (mainly the core and cavity), mold assembly, feasibility of the mold for injection production, quality control, and production efficiency, among other aspects.

For mold manufacturers, after completing the mold design, the main focus of the manufacturing department is on the core and cavity. This highlights the importance of the core and cavity in the composition of the mold.

Today, let’s explore the most crucial part of mold composition — the core and cavity.

What is the Core and Cavity?

The core and cavity include the mold cavity and the mold core, which are tightly combined to form the overall shape and internal structure of the mold. The mold cavity is primarily responsible for forming the product’s internal shape and structure and controlling the product’s precision. Conversely, the mold core mainly forms the product’s external shape and structure, completing the overall molding of the product.

Differences Between the Core and Cavity

Besides the primary functions mentioned above, the differences between the cavity and core are reflected in the following aspects:

mold cavity of a hairdryer product
Mold cavity
Mold core with molded plastic in it
Mold core with molded plastic in it

Procedural Differences During Injection

During the injection process, there are some differences in the procedural steps between the core and cavity. Plastic granules enter a closed space in the mold through a cavity (note that this closed space is formed by two parts, but most often, this space is considered part of the cavity, hence also referred to by many as the mold cavity).

At this time, the mold core is tightly fitted to the cavity. When the molten plastic cools into a solid state, the core is pulled open, and the product on the core is ejected through the ejection system. Note, throughout this process, the cavity remains stationary, while the core moves.

Differential Pressure Endurance

Due to the differences in shape and function, the core and cavity withstand different pressures. The cavity usually has to withstand higher pressures because it needs to fully fill the mold cavity with the product material during the molding process and ensure the integrity of the product’s internal structure. The core withstands lower pressure, mainly to give the product a perfect appearance.

Material Selection for Cavity and Core

There are also some differences in material selection for the core and cavity. Because the cavity has to withstand higher pressure and wear, it typically requires more wear-resistant materials, such as alloy steel or carbide. The core can use relatively softer materials, such as aluminum alloys, to better fill and cool during the product molding process.

Usage Cycle and Maintenance Variability

The usage cycle and maintenance of the core and cavity also differ. Since the cavity undergoes greater pressure and wear, its service life is usually shorter. Manufacturing companies often replace the cavity regularly to ensure product quality and production efficiency. In contrast, the core is subject to less wear and pressure, so its service life is relatively longer.

How to Differentiate Between the Core and Cavity?

The methods to differentiate between the core and cavity primarily include:

Observing the Appearance:

The mold cavity usually appears smoother, with distinct protrusions and recesses, whereas the mold core tends to have a rougher appearance, with holes and cutting marks. The appearance of the cavity and core may not have noticeable differences, depending on the type and purpose of the mold.

Detection and Testing Method:

Determine the roles and positions of the cavity and core through detection and testing, including positioning, clamping, and pressure tests.

Observing the Structure:

The structure of the core and cavity differs; the cavity typically contains components such as buttons or clips for easy disassembly and assembly, while the core includes critical parts like part joints and mold sleeves.

Checking the Parting Line and Dowel Pin:

The core and cavity have different shapes for parting lines and dowel pins, which helps confirm their positions.

Observing Weight and Material:

The cavity is generally lighter than the core, and it usually uses materials with higher wear resistance and hardness requirements, such as tool steel or alloy steel, while the core commonly uses cast iron, steel, or alloys.

Checking the Markings:

After disassembling the mold, pay attention to the markings on the core and cavity parts. For example, the cavity may be marked with “F” or “1,” while the core may have “R” or “2.”

Observing the Product Shape and Positioning:

By observing the shape of the product and the mating surfaces and positioning pins of the cavity and core, one can preliminarily judge their positions.

These methods can be used individually or in combination to more accurately differentiate between the core and cavity.

Manufacturing of Mold Cavity and Mold Core

The process of making and processing the mold cavity and mold core shares similarities but also exhibits some differences, detailed as follows:

1. Material Preparation

Before starting the machining process, it is necessary to prepare the required materials. The type and size of the mold core and cavity material depend on the mold’s type and specifications. Additionally, various tools, fixtures, and auxiliary materials needed for machining must be prepared.

2. Traditional Milling

Initial machining is done on a milling machine, including drilling cooling holes, threading holes, drilling and tapping screw holes, drilling and reaming ejector pin holes, mold numbering, datum angles, etc. These steps must be carried out with precision according to the drawing requirements to ensure the shape and size of the core and cavity meet the design.

3. CNC Machining

Rough machining is performed on a CNC machining center, working on the shape and features. CNC machining efficiently completes complex shape cutting while ensuring accuracy and surface quality.

4. Heat Treatment Processing

The cavity and core undergo heat treatment processing as per design requirements to achieve the desired hardness and wear resistance. During the heat treatment process, it’s important to control parameters such as temperature, time, and cooling speed to ensure the results meet the specifications.

5. Grinding

Finishing is conducted on a grinding machine, including grinding six-face angles to precision dimensions to fit the frame size, etc. The precision requirements for external dimensions are stringent, necessitating precise grinding and finishing. If the core or cavity is a single piece, its external dimensions should be 0.03mm-0.05mm less than the drawing dimensions. If it is in two pieces, the combined external dimensions of the two pieces should be 0.03mm-0.05mm less than the drawing dimensions, with perpendicularity of 0.01 and parallelism of 0.01.

6. CNC Finishing

For mold cores and cavities requiring higher precision, CNC finishing is necessary. A CNC machining center completes high-precision cutting work, such as engraving text and mold numbers.

7. Wire Cutting

Wire-cutting technology is used to process insert holes, lifter holes, ejector pin holes, sprue holes, etc.

8. Electrical Discharge Machining (EDM)

EDM technology is utilized for core and cavity, such as fine grooves and narrow slots.

9. Mold Polishing

The surface of the mold core and cavity is polished to obtain better surface quality and meet roughness requirements. During polishing, you must pay attention to the direction and area of polishing to avoid damaging other areas. For areas requiring a mirror finish, rough polishing should be followed by fine polishing after the trial mold.

10. Assembly

All parts are assembled to form a complete mold. During assembly, attention must be paid to fitting accuracy and fastening requirements to ensure the mold is stable and reliable in use.

11. Trial Molding:

Finally, a trial molding test is conducted to verify the machining quality and performance of the mold. The trial mold checks whether the mold’s gating system is clear, the cooling waterway is working correctly, and the molded product meets design requirements. Any issues discovered during the trial mold must be adjusted and repaired promptly to ensure the mold can be put into production use.

Design Considerations for Core and Cavity in Mold Design

The design of the core and cavity in mold manufacturing is complex and forms one of the core technologies of every mold manufacturer. While it’s challenging to describe the intricacies of core and cavity design in detail within this text, here are some reference suggestions for fellow professionals:

Design Basics

The determination of the thickness for the core and cavity should consider the basic thickness required after adding cooling channels and the thickness needed to maintain the strength of the core and cavity. The main consideration for core and cavity strength is their ability to withstand injection pressure without causing mold deformation, which could lead to product flash.

The design of core and cavity strength should take into account:

a. Injection pressure

b. Product projected area

c. Product height

d. Product thickness

e. Product shape

f. Product projection shape

g. Total number of mold cavities.

Setting Safety Dimensions

When designing a mold, it’s essential first to estimate the dimensions of the core and then estimate the size of the mold base. The key to estimating dimensions lies in how to determine the safety distance. The choice of safety distance significantly affects mold costs! Determine the cavity and core’s thickness and size based on the product’s characteristics, ensuring its width fits between two mold feet. The length of the core or cavity should not surpass the distance to the return pin.

Product Dimensions(mm)Safety Distance(mm)
<2025
20-6030
60-8035
80-30040
300-60045
Core and cavity safety size reference table

The above datas serve as standard considerations, but in reality, each industry and even each part may have its specific design standards. If you’re seeking more detailed design information, visiting our mold design page is recommended.

Material Selection

The choice of material for the mold core and cavity directly affects the lifespan and price of the mold. From the perspective of the products being manufactured, if the product demand is low, choosing high-quality mold core and cavity materials increases the mold cost, leading to increased expenses for the company—a waste if the demand is small. 

Conversely, suppose the product demand is high and inferior mold cavity and core materials are selected. In that case, the mold may fail to meet production requirements, necessitating a new mold and reducing production efficiency, similarly increasing mold costs. To make a rational choice of mold core material, it’s crucial to understand the commonly used materials and their characteristics, and it’s very important to select different materials based on actual work experience.

Common Materials: P20, 738, 738H, 718, 718H, NAK80, 2316, 2316A, S136, etc. Materials used for hardened molds include 2344, 8407, SKD11, SKD61, etc.

Material Characteristics:

P20

Generally suitable for plastic molds requiring high hardness, toughness, and wear resistance. Nitriding can improve the lifespan of the mold. It is applicable for thermoplastic plastics like PS, PE, PP, and ABS without added fire retardants and is a commonly used mold core material with a relatively low price.

738

Offers improvements in various properties compared to P20, with 738H offering even better effects and correspondingly longer mold life.

NAK80

Has excellent electrical discharge machining and polishing properties, commonly used in the production of plastic products requiring high transparency.

2316

Suitable for molds processing high-acidity plastics, offering high polishability and acid resistance, commonly used in molds for forming PVC plastic materials.

2344

Typically used in hard plastic molds, its mold life can reach up to 1 million cycles. However, due to the high cost of the mold, it is suitable for plastic products with high production volumes. The same applies to other hard mold materials like 8407, SKD11, etc.

Wrapping Up

The content related to the core and cavity is intimately connected with those who work in the mold industry. Given their importance in mold structure, even if you’re not involved in mold making or injection molding, having a basic understanding of both can be somewhat beneficial in your future product designs.

The core and cavity are fundamental components that dictate the shape, structure, and quality of the final molded product. Understanding how they work and their design considerations can enhance your ability to design products that are not only functional and aesthetically pleasing but also manufacturable and cost-effective. This knowledge can help bridge the gap between design concepts and the practicalities of production, leading to more successful projects and innovations.

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