An ejector pin is an ejector system that pushes the mold out of the mold cavity after injection. In addition to ejector systems such as ejector blades and ejector blocks, the ejector allows efficient removal of parts after molding.
There are different types of ejector pins. Selecting the right post is challenging due to its impact on impression functionality and certain aesthetic properties. This article deals with the definition of the ejector and its functions and shortcomings. Keep reading!
What is ejector pin injection molding?
Ejector pin injection molding is an ejector system mechanism that controls the removal of cooled/molded parts from a mold cavity. This helps increase the efficiency of the injection molding process.
The mechanism can be understood for both sides of the mold (Sides A and B) because each side plays a specific role in completing a cycle in the injection molding system. Side A is movable while side B contains the ejector system. As it cools, the mold cavity opens and separates, and the ejector pin pushes the now hardened plastic out of the mold.
Note: When ejecting, the ejection force creates a dent in the molded part.
Types of Ejector Pins Used in Injection Molding
There are several types of ejector pin systems suitable for injection molding. As a parts manufacturer, you should consider the following points when using quick tools:
Ejector Pin #1: Continuous Rigid Ejector
Consistently hard ejector pins must be heat treated to ensure the pin diameter can withstand high temperatures. Therefore, they are suitable for working with plastic systems below 200 °C.
Ejector Pin #2: H13 Nitride Ejector
H13 nitride pins or hardened ejector pins withstand temperatures up to 600°C. They can be drilled or tapped, but they are also relatively soft and can chip easily compared to other types of ejector pins.
Ejector Pin #3: Black Ejector
Black ejector pins replaced the H13 nitride pins because the H13 nitride pins could not withstand temperatures above 600°C. Therefore, the pins can withstand high temperatures. They are manufactured with a special self-lubricant to withstand temperatures of 1000°C and are suitable for working with automotive parts. Although they are expensive, they meet the necessary conditions to withstand high temperatures.
Other types of ejection systems
The pins discussed previously are the most common in injection molded parts. However, there are other ejector systems that may not be well known but are useful.
– Ejector sleeves are hollow pins consisting of a hard shell pin. The pin has a hole that protects and guides the pin, as well as a corresponding central pin
– Ejector plates Work in conjunction with the ejector pins, holding the pin head in place to prevent them from coming out during the molding process
– Ejector blocks feature lubricating hollow grooves and are used in thin products to reduce ejector pin marks. They are therefore suitable for products that require high surface quality.
How does an ejector pin work?
Knowing how an ejector pin works will help you choose the right machine and avoid ejector pin marks and other defects. To better understand the process, we divide the injection molding process into three phases: injection, cooling and releasing the part.
• In the injection phase, the operator introduces the molten plastic into the die mold cavity.
• The cooling phase allows the molten plastic to solidify into a hardened plastic.
• After cooling, in the part release phase, a special mechanism called an ejector system is used to remove the molded part from the mold cavity. There are two general methods: manual and automatic.
On the one hand, the manual part release mechanism is time-consuming and inefficient. This can result in a non-functional/aesthetically unattractive product if release is not done correctly.
Automatic ejector pins and other ejector systems, on the other hand, are faster and more efficient. They apply force to the finished parts and push them outward in an axial movement. The part does not fall freely because it is guided by the fixing plates and the ejector box.
Below is a brief description of how the ejector pin system works:
- After the cast part has cooled, the support is moved out of the way.
- The mold is pushed towards the pins;
- The pins fit into the holes in the mold half;
- The pins then press on the frame to eject it.
Causes and solutions for ejector pin marks
During the ejection process, the ejector pins leave an impression (also called an ejector pin mark) on the molded part. This means they are necessary. However, under certain circumstances, they can worsen and have adverse functional and aesthetic effects. Here are the most common causes of ejector pin marks and their corresponding solutions.
Shorter cooling time
If the part does not cool before the ejection process begins, the pin may penetrate the part. This can have catastrophic consequences, both functionally and aesthetically. To resolve this issue, it is necessary to ensure sufficient cooling time for the impression to cure properly.
Fine product and other features
The design of the part also determines whether there are ejector pin marks. These marks are most visible on very thin pieces. Therefore, it is generally accepted that the injection molded product must have a wall thickness of at least 2.5 mm. Additionally, an ejector pin should not be used on plastics that require a good surface finish. Instead, other ejector systems, such as the ejector blade, must be used.
Residence time and residence temperature
A long dwell time can also increase the likelihood of marks forming on the ejector pin. Residence time is the time the plastic spends in the mold, while residence temperature is the temperature maintained in the mold. In general, the residence temperature is higher than the glass transition temperature (the temperature at which an amorphous polymer develops glass-like properties).
Residence time can increase the glass transition temperature, which can lead to premature ejection. Premature ejection means the product is probably not hard, which could result in marks on the ejector pins. You can avoid this by shortening the dwell time.
Placement and arrangement of ejector pins
Ejector pin placement can also result in pin marks. However, its placement depends on the complexity of the mold and the mold design. Below are some positioning tips you can use:
- The more complex the part, the more ejector pins you will need.
- Place the ejector pin on parts such as steps, metal inserts, columns and thick parts.
- Always place the ejector pin on a flat surface as its ability to support the load on the frames will be limited if you place it on steps or slopes.
- Pay attention to the distance between the pin and the cooling channel to avoid disturbing the cooling process.
- Place the pin where the ejection force is greatest, e.g. B. in the brain and rib.
Types of machines
Choosing the right injection molding machine can also help reduce marks on the molded part. For example, large-capacity machines are unsuitable for working with small molds in low-volume injection molding due to internal stress caused by increased pressure.
holding pressure
Marks on the ejector pins can be reduced by changing the holding pressure, which is the pressure that holds the molten plastic in place as it cools.
Note: The greater the pressure, the more difficult it will be to remove the product.
Other Methods for Reducing Ejector Pin Marks
- Use abrasion-resistant plastics such as nylon, polyoxyethylene and ultra-high molecular weight plastics as they can resist stains.
- Use a release agent for an easier and smoother release process.
- Incorporate a tilt angle to reduce the part release process and the work done by the ejector pin.
How do you choose the correct ejector pins during injection molding?
To choose the right pens, you need to consider the material, size, etc. Below is an explanation of these and other factors you may need to keep in mind during the injection molding cycle.
Choose a large ejector pin diameter
In general, it is best to use a large ejector pin for a large molding because a large diameter reduces the pin's penetration into the molding, reducing penetration force. Additionally, you must make sure that the diameter is a whole number (avoid decimals and non-standard values).
Ejector pin size
Although the pins must be large in diameter, they must also be correspondingly small in size. The adjustment should depend on the size of the frame. Reducing the size of the ejector pin reduces penetration into the molding.
Strength Requirement
The pin must be strong enough to withstand the high injection pressure. You can avoid bending the pin by ensuring the diameter is at least 2.5mm. Otherwise, you must use a shoulder ejector pin. You must also ensure that the thickness can counteract the undercut during injection molding.
Other factors include the type of materials. In addition, the price of the ejector system must be considered, especially for replacement during maintenance, as the price of the ejector system may increase the cost of the mold.
Concluding
Ejector pins allow removal of a molded part from the mold cavity. There are different types of pens and considerations to keep in mind when working with them. Therefore, choosing the right pen can be quite a challenge. This article discusses the ejector pin system and other considerations associated with using the system in injection molding.
Common questions
What is a pin mark?
Ejector pin markings, also known as pin push, are bright or white stamps. These marks are left by an ejector pin and are visible on the Class A surface of the component. Ejector pin marks should be avoided as they may crack.
How to score points with ejector pins?
Knowing where the pins are will help you find the ejector pin markings faster. Ejector pin marks appear on a frame as shiny or whitish-looking dents. These marks occur in the areas where the ejector pins contact the part.
What material is the ejector pin made of?
Ejector pins are generally made of steel, H-13, M-2, and 420 stainless steel. These materials can also undergo heat treatment and other processes to improve their mechanical properties.
