Ejection & Reset Mechanisms in Injection Molds
Comprehensive guide to the most effective ejection systems for injection molds, ensuring optimal performance and product quality.
The method of product ejection in injection molds is influenced by factors such as product material and shape. Generally, ejector pins (straight pins, stepped pins), sleeves, ejector plates, and air are used. These ejection methods can be used alone or in combination, depending on the mold life and the difficulty of mold processing. In modern injection molds, selecting the appropriate ejection system is crucial for maintaining production efficiency and ensuring product integrity.
Ejection systems in injection molds play a vital role in the manufacturing process, as they are responsible for safely and efficiently removing the finished product from the mold cavity. A well-designed ejection mechanism prevents damage to both the product and the mold, while ensuring consistent quality in mass production environments. This guide explores the various ejection systems used in injection molds, their advantages and disadvantages, and the key principles for effective implementation.
Figure 1: Overview of typical ejection components in injection molds
I. Requirements for Ejection Mechanisms in Injection Molds
The ejection system of injection molds should ensure that the product is ejected smoothly, completely, and without deformation. The mechanism should be as simple and reliable as possible, with stable ejection and complete reset. In high-volume production environments, the reliability of the ejection system directly impacts overall productivity and cost-effectiveness.
In general, a spring reset method is adopted. When the product structure causes unbalanced脱模力, injection molds must be equipped with ejection balance and guiding devices. These balancing mechanisms are critical for maintaining uniform pressure distribution across the product surface during ejection, preventing warping or damage.
The plastic part should remain on the mold half with the ejection mechanism, typically on the core side (male mold) in injection molds. This design consideration ensures that the ejection force is applied from the most effective direction, minimizing the risk of product damage during the ejection process.
The plastic part must not be deformed or damaged. Ejection force should be applied to the parts of the product with the greatest rigidity and strength, such as ribs, flanges, and shell sidewalls, with the largest possible contact surface. This principle is fundamental in the design of ejection systems for injection molds, as improper force distribution can lead to costly defects and production delays.
For good product appearance, ejection mechanism positions should be设在产品内部 in injection molds. Concealing ejection points on less visible surfaces enhances the aesthetic quality of the finished product, reducing the need for post-processing and improving overall product value.
The ejection mechanism structure in injection molds must be reliable. Given the high cycle counts typical in injection molding operations, ejection systems must withstand repeated stress without failure. This reliability is achieved through careful material selection, precise manufacturing, and robust design principles.
Figure 2: Proper ejection force distribution on plastic components in injection molds
II. Ejector Pin Type Ejection Mechanisms in Injection Molds
1. Advantages and Disadvantages of Ejector Pin Mechanisms
Advantages
- Ejector pins are relatively easy to machine. Even when hardness requirements exist, processes like quenching and grinding are easier compared to other methods used in injection molds. They can be positioned at any location on the product, making them the most commonly used ejection method.
- Ejector pin holes are also easy to machine with sufficient precision. They offer minimal sliding resistance, and jamming rarely occurs in well-maintained injection molds.
- Damaged pins have good interchangeability, making maintenance straightforward in injection molds. This reduces downtime and maintenance costs in production environments.
- The most commonly used ejection components include round ejector pins, headed ejector pins, flat ejector pins, and sleeves. Ejection positions should be located where脱模力 is greatest, not at the thinnest parts of the product. The force distribution can be improved by increasing the ejection area in injection molds.
Disadvantages
- Ejection over a small area concentrates stress on local areas of the product. For cup-shaped and box-shaped products with small draft angles and high脱模力, defects such as indentations and piercing may occur, making ejector pins unsuitable for such applications in injection molds.
- In thin-walled sections, ejector pins can cause visible marks or deformation, affecting the aesthetic quality of the finished product in injection molds.
- They may not provide sufficient ejection force for large or complex parts, requiring additional ejection mechanisms to be used in conjunction in injection molds.
Figure 3: Different types of ejector pins commonly used in injection molds
2. Principles for Ejector Pin Arrangement in Injection Molds
① Ejector pin arrangement should balance the ejection force as much as possible. Areas with complex structures requiring greater脱模力 should have a corresponding increase in the number of ejector pins. This balance is crucial in preventing product deformation during ejection in injection molds.
Uneven force distribution is one of the primary causes of product defects in injection molding, making proper ejector pin spacing and positioning essential design considerations.
② Ejector pins should be placed at effective positions, such as ribs, pillars, steps, metal inserts, and local thickened areas with complex structures. Ejector pins on both sides of ribs and pillars should be arranged symmetrically as much as possible.
The edge distance between ejector pins and ribs or pillars is generally D=1.5mm, as shown in Figure 7-16. Additionally, the center line of the ejector pins on both sides of a pillar should ideally pass through the center of the pillar.
Symmetrical Arrangement
Proper Alignment
Incorrect Alignment
Figure 4: Proper and improper ejector pin arrangements around pillars in injection molds
③ Avoid placing ejector pins across steps or on inclined surfaces. The top surface of ejector pins should be as flat as possible, and pins should be arranged at structural positions where the plastic part can withstand force well, as shown in Figure 7-17.
Proper Positioning
Incorrect Positioning
Figure 5: Comparison of proper and improper ejector pin positions on stepped or inclined surfaces in injection molds
④ For deep ribs (depth > 20mm) in plastic parts or where it is difficult to arrange round ejector pins, flat ejector pins should be used. When flat ejector pins are required in injection molds, insert forms should be used as much as possible at the flat ejector pin locations to facilitate machining, as shown in Figure 7-18(a).
⑤ Avoid sharp steel and thin steel, especially the top surface of ejector pins should not touch the front mold surface in injection molds, as shown in Figure 7-18(b). This prevents damage to the mold surface and ensures proper functioning of the ejection system.
⑥ The arrangement of ejector pins should consider the edge distance between ejector pins and water channels, avoiding interference with the machining of water channels and water flow in injection molds. Proper spacing ensures effective cooling while maintaining ejection functionality.
⑦ Consider the排气 function of ejector pins. For排气 during ejection, ejector pins should be arranged in areas prone to vacuum formation in injection molds. For example, on large flat surfaces of the mold cavity, although the clamping force of the part is small, vacuum can easily form, increasing脱模力. Strategic placement of ejector pins helps alleviate this issue by allowing air to enter as the product is ejected.
Figure 6: Optimized ejector pin arrangement in a complex injection mold design
III. Stripper Plate Type Ejection Mechanisms in Injection Molds
Stripper plate ejection is a very commonly used ejection mechanism in mold design. Stripper plate ejection mechanisms are suitable for demolding large cylindrical plastic parts, thin-walled containers, and various types of cover-shaped plastic parts in injection molds. They are not suitable for plastic parts with complex peripheral shapes on the parting surface where machining the stripper plate core hole is difficult.
The characteristics of stripper plate demolding include uniform ejection, high force, stable movement, and minimal risk of plastic part deformation. These advantages make stripper plate systems a popular choice in many injection molds applications where product integrity is paramount.
Figure 7: Stripper plate ejection system in injection molds
Design Points for Stripper Plate Ejection Mechanisms in Injection Molds
① The matching structure between the stripper plate and the core should be tapered, as shown in Figure 7-20. This design reduces movement abrasion and plays an auxiliary guiding role. The taper angle should be 3' to 10°.
This tapered design is particularly important in high-volume production injection molds, where repeated movement can lead to premature wear without proper engineering.
② The stripper plate should have a tapered fit with the core. The inner hole of the stripper plate should be 0.2~0.3mm larger than the forming part of the core, as shown in Figure 7-21. This small clearance ensures proper movement while maintaining the necessary precision for product formation in injection molds.
This precise clearance is critical for preventing binding while ensuring that the stripper plate can effectively eject the product without causing damage. In injection molds, maintaining this tolerance throughout the mold's life requires careful material selection and heat treatment processes.
Figure 8: Tapered配合结构 between stripper plate and core
Figure 9: Proper clearance specifications for stripper plates in injection molds
Summary of Ejection Systems in Injection Molds
Both ejector pin and stripper plate systems have their unique advantages in injection molds. The selection between them depends on the specific product design, material characteristics, and production requirements. While ejector pins offer flexibility and ease of maintenance, stripper plates provide superior force distribution for delicate or large components. In many advanced injection molds, hybrid systems combining multiple ejection methods are used to achieve optimal results for complex parts.