core pull injection molding

Everything You Need to Know About Core Pull Injection Molding

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In plastic injection molding, core pull injection molding plays a significant role in achieving precision and versatility. It makes the manufacturers develop new design possibilities and produce high-quality parts. In this comprehensive guide, we will delve into the world of core pull injection molding, exploring its significance and working principles. We will also discuss the advantages, limitations, and various types of core pull systems, providing valuable insights for optimizing your injection molding processes. Whether you are a seasoned molding professional or a newcomer to the field, understanding core pull injection molding is essential for the full potential of plastic part design and production. Let’s dive in!

Understanding Core Pull Injection Molding

Injection mold core pull is to make the parts with side holes or undercuts into active cores. When the plastic part is demolded, the movable core is pulled out first, and then the plastic part is ejected from the mold. The mechanism that completes the pulling out and reset of the movable core is called the core pulling mechanism. When the plastic part has internal and external holes or undercuts that are different from the mold opening direction, which hinders the direct demoulding of the plastic part, an oblique slider core-pulling mechanism must be used.

The core-pulling mechanism is divided by function and generally consists of five parts: forming components, moving components, transmission components, locking components, and limit components. There are two situations in which the core-pulling mechanism ejects the product from the injection mold: one is to complete the lateral core-pulling first when the mold is opened, and then push out the product.

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Core Pull Injection Molding Work Principles

Core pull injection molding works on the principle of controlled movement of a core inside the mold during the injection molding process. The core is a moveable part of the mold that creates internal features and undercuts in the molded part. Here’s how the core pull system operates:

  • Mold Closure: During the mold closing phase, the core and cavity come together to form a closed mold.
  • Plastic Injection: Molten plastic is injected into the mold cavity under high pressure.
  • Cooling Stage: The plastic inside the mold cools and solidifies to the desired shape.
  • Ejection Phase: Once the plastic has solidified, the mold opens, and the core is pulled away from the cavity by a hydraulic or mechanical system.
  • Part Ejection: The molded part is then released from the mold cavity without distortion or damage, thanks to the retraction of the core.
  • Mold Reset: The core returns to its original position, and the mold is ready for the next cycle.

By employing core pull mechanisms, manufacturers can produce intricate and complex parts with smooth surfaces and internal features that enhance the functionality and aesthetics of the final product.

Advantages and Limitations of Core Pull

Advantages of Core Pull Injection Molding:

  • Complex Part Design: Core pull enables the production of intricate parts with undercuts, threads, and internal features, expanding design possibilities.
  • Enhanced Aesthetics: Smooth surface finishes and precise detailing are achievable with core pull, enhancing the visual appeal of the molded parts.
  • Functionality Improvement: Core pull allows for the creation of parts with superior functionality, such as snap-fit assemblies and interlocking components.
  • Cost Efficiency: Core pull eliminates the need for secondary operations, reducing production costs and lead times.

Limitations of Core Pull in Injection Molding:

  • Increased Complexity: Implementing core pull adds complexity to the mold design and molding process, requiring skilled expertise.
  • Cycle Time: Core pull can extend cycle times due to the need for additional movement and retraction.
  • Maintenance: Core pull mechanisms require regular maintenance to ensure smooth operation and prevent downtime.
  • Cost Consideration: The initial investment in core pull systems may be higher than conventional molds, impacting the project’s budget.

Design Considerations for Core Pull Systems:

Designing core pull systems in injection molding requires careful consideration to ensure the effectiveness, reliability, and safety of the system. Here are some key design considerations for core pull systems:

  • Part Design: The part design should incorporate features that require core pull. Identify areas with undercuts or internal features that necessitate the use of core pull to achieve the desired part geometry.
  • Core Placement and Number: Determine the locations and number of cores required for the part design. Position the cores appropriately to create the desired internal features.
  • Core Pull Direction: Decide on the direction in which the cores need to move to release the part from the mold. Core pull can be vertical, horizontal, or angled depending on the part’s features.
  • Mechanical or Hydraulic Actuation: Choose between mechanical or hydraulic actuation for the core pull system. Mechanical systems are simpler and cost-effective, while hydraulic systems offer more precise control.
  • Actuation Mechanism: Select the type of actuation mechanism, such as cams, levers, or hydraulic cylinders, based on the required force and movement.
  • Actuation Timing: Ensure that the core pull system’s actuation timing aligns with the mold opening and closing sequence to avoid interference with other mold components.
  • Core Pull Sequence: Plan the sequence in which the cores will move during mold opening and closing. Consider the interaction between different core pull movements.
  • Ejection Mechanism: Coordinate the core pull system with the ejection mechanism to ensure smooth and efficient part ejection.
  • Lubrication and Maintenance: Incorporate proper lubrication points and design the core pull system for easy maintenance and longevity.
  • Safety Mechanisms: Implement safety features to prevent improper operation or damage to the mold due to incorrect core pull sequencing.
  • Material and Coatings: Select appropriate materials and coatings for the core pull components to ensure durability and minimize wear.
  • Interlocks and Sensors: Consider using interlocks and sensors to prevent mold closing if the cores are not fully retracted or if any obstruction is present.
  • Testing and Validation: Conduct thorough testing and validation of the core pull system during mold trials to ensure its functionality and reliability.

By addressing these design considerations, the core pull system can be successfully integrated into the injection mold to create parts with complex internal features or undercuts, expanding the design possibilities and capabilities of the injection molding process.

Types of Core Pull Injection Molding and Their Applications:

There are many types of core pull injection molding. The choice of core pull type depends on the specific requirements of the injection molding project, including part design complexity, force needs, cycle time considerations, and budget constraints. Proper selection and implementation of the appropriate core pull system are vital to achieving successful injection molding outcomes. Here are the common types of core pull injection molding as below.

Hydraulic Core Pull:

This type of core pull system utilizes hydraulic cylinders to actuate the movement of cores in the mold. It is suitable for large and heavy molds or applications requiring high force.

Applications: Automotive parts, large-scale consumer products, industrial components.

Mechanical Core Pull:

Mechanical core pull uses mechanical levers, gears, or cams to move the cores. It is a cost-effective option for moderate force requirements.

Applications: Small to medium-sized parts with undercuts, electronic enclosures, and medical devices.

Pneumatic Core Pull:

Pneumatic core pull employs compressed air to actuate the movement of cores. It offers rapid cycling times and is ideal for small to medium-sized molds.

Applications: Thin-walled parts, consumer electronics, packaging components.

Hydraulic-Mechanical Hybrid Core Pull:

This type combines hydraulic and mechanical systems, offering a balance of force and cost-effectiveness.

Applications: Medium-sized parts with complex features, household appliances, and furniture components.

Electric Core Pull:

Electric core pull utilizes electric motors to drive the core movement, offering precise control and energy efficiency.

Applications: Electronics, optics, miniature components.

Indexing Core Pull:

Indexing core pull is used when multiple cores need to move independently at different angles during the molding process.

Applications: Gears, camshafts, rotating components.

Examples of Parts Made Possible Through Core Pull Injection Molding

Core pull injection molding opens up the possibility of manufacturing intricate and complex parts that would be challenging or impossible to produce using conventional molding techniques. Some examples of parts made possible through core pull injection molding include:

  • Threaded Components: Core pull allows for the creation of threaded parts with internal threads, such as screw caps, bottle closures, and threaded connectors.
  • Undercut Designs: Parts with undercuts, like hooks, snaps, and latches, can be efficiently molded using core pull mechanisms.
  • Hollow Parts: Core pull enables the production of hollow components, like containers, tubes, and vials, with internal voids or cavities.
  • Gears and Cogs: Complex gear shapes, including internal gears and cogs, can be achieved through core pull injection molding.
  • Nested Parts: Core pull facilitates molding nested parts, where one part fits precisely within another, like nesting containers or compartments.
  • Handles and Grips: Ergonomic handles and grips with intricate shapes can be molded using core pull technology.

By leveraging core pull capabilities, manufacturers can produce innovative and sophisticated parts that meet precise design requirements, enhance functionality, and elevate the overall aesthetics of the final products.


In conclusion, core pull injection molding offers significant advantages, enabling the production of intricate parts with undercuts and internal features. Its capabilities include creating threaded components, hollow parts, gears, and more. Proper design considerations are essential for successful implementation. As a professional custom injection molding manufacturer, Zhongde is committed to delivering high-quality plastic parts that cater to your specific needs. By embracing core pull technology, we enable precision, versatility, and innovation in plastic part manufacturing, meeting the demands of modern applications and ensuring exceptional quality in the final products.

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