The introduction of ejector pins injection molding to manufacturing processes has resulted in automated operations, improved production speed, and ensured more efficient products. It has also improved consistency in the designs of products.
Despite the improvement of manufacturing processes by injection molding, there are, however, many flaws in its design that need to be optimized. This is to guarantee better and more effective products.
Thus, this article looks at ejector pins injection molding, and how to optimize its design to have more effective products.
What Are Injection Molding Ejector Pins?
Ejector pins are vital in creating parts. They are an integral component of the ejection system in mold, which determines the final outcome of products in an injection molding process.
Injection molding is a manufacturing process that involves injecting molten plastic in a metal mold to assume the shape of the mold. Therefore, ejector pins injection molding involves the removal of completed parts from the die molds. The metal mold is made of two parts: A and B sides. Upon cooling of the molten material in the mold, both parts of the metal mold are separated to allow for the removal of the solid plastic. Injection molds are built such that when they are opened, the A-side half is lifted, leaving the formed part and the B-side.
Ejector pins are located on the B-side half of a mold, from which they push the formed part out of the mold. The pin mark of an ejector mold is commonly imprinted on finished products as a dent.
Types of Ejector Pins
There are many types of ejector pins used in product manufacturing. Below are the commonest types you will find ideal for the process.
Through-Hard Ejector Pins
These ejector pins are heat-treated to ensure consistency in the hardness through the diameter of the pin. Through-hard pins can withstand working temperatures up to 200°C, and it is mostly suitable for plastic ejection system in mold.
Case Hardened Ejector Pins
They are also known as Nitride H13 pins, are much harder pins than the through hard pins, and are suitable for die casting ejection systems in mold. Case hardened pins are nitrated to 65 – 70 HRC and can withstand temperatures above 200°C.
Black Ejector Pins
Manufacturers developed these ejector pins because of the inability of the Nitride H13 pins to be employed in working temperatures above 600°C. The black ejector pin is coated with a black surface treatment which allows it to be self-lubricating and withstand high temperatures up to 1000°C. It is an expensive ejector pin and is suitable for a metal ejection system in the mold for automobiles.
Injection Molding Design Considerations
An injection mold design must be such that it functions as planned. An error in the design may result in the cracking or shrinking of part, which can prove too costly or too tasking to remedy.
Hence, the need to execute a well-structured design is important, and there are certain factors to consider.
● Create Draft Angles
Draft angle is a slant in shape that is applied to both sides of an injection mold. This slight distortion in the shape of the mold allows for the easy removal of the plastic from the mold.
Draft mold must be present to provide resistance against friction while removing the part from the mold. Allowing more draft angles would ensure the easy release of parts from the ejection system in mold.
The absence of draft angles would result in large ejector pin marks on the part during removal and scrapes on the wall of the mold.
● Uniform Wall Thickness
When molten material is poured into a mold with uniform wall thickness, it flows freely without restriction, filling up the wall’s cavity and assuming its defined shape.
Non-uniform mold walls will result in the cooling of the thinner section of the molten material. Therefore, as the thicker section cools, it will result in the shrinking of the material, stress concentration, and eventually cracking during removal.
Nevertheless, if your design does not allow uniform wall thickness, it can be remedied by coring and adding gussets.
(Coring is a process of removing the molten plastic from the wide area to ensure uniformity along the wall. Gussets are support structures that you add to the wall as reinforcements to reduce the wall’s thickness)
● Ensure Round Edges
Having round corners on the inside and outside of the part has several advantages. It reduces the stress concentration and prevents the part from cracking.
Sharp corners limit the flow of molten plastic in the die, and upon cooling the plastic pulls against the sharp corners and is difficult to remove.
Round-edged parts are easy to produce, more economical, and allow better formation and removal of products.
● Reduce Undercuts
Undercuts are protruding features in the mold design that obstruct the removal of either side of the mold. Undercuts are necessary and unavoidable in a mold design as they prevent the part from direct ejection from the mold.
However, undercuts can be remedied by creating interlocks or latches that allow for easy removal or assembly. As much as possible, the design team must keep the number of undercuts in an ejection system in mold at a minimum.
● Gate Locations
Gates are the entry points of the molten plastic to the mold. However, upon cooling of the part, the gate leaves a pin mark, which is most times, still visible even after dent removal.
The design team can use an edge gate to remedy this, where the resulting dents would be less noticeable. The molten material is also injectable through an extension located on the ejection pin.
Upon cooling the part, the ejector pin can push off the resulting pin mark from the gate during the removal of the part from the mold.
● Nature of Material
The type of material used should depend on the function of the product
Some materials are thick, some are flexible, while others are hard or brittle. The type of material chosen would determine what purpose the part is for and its design.
While you can thicken some materials it is also possible to bend them into shapes. Also, you should consider these before choosing the type of material.
Common Defects and Solutions
Injection molding using ejector pins comes with some defects. Below are the possible defects, and the necessary solutions.
The main reason for the breaking of ejector pins is the difference in the required force to eject the part from the mold and the strength of the pin.
Ejection of parts from the mold requires force. Sometimes the required force exceeds the strength of the pin due to its unsupported length, thereby leading to breaking.
Therefore, the most efficient way to remedy ejector pin breaks is to employ large amounts of ejector pins with larger diameters. This way, the force required has even distribution across various pins, thereby reducing breakage.
Ejector Pin Marks
These are are “dents” left on the part by ejector pins during removal from the mold. This pin mark can result in the cracking of the products while in use. Hence, it is important to design an ejection system in the mold to prevent ejector pin marks.
- Arrange the ejector pins such that the force of ejection across the part is constant.
- Place the ejector pins on hard parts such as metal inserts, pillars, and ribs to avoid ejector pin marks defect.
- Design the position of the ejector pins on the flat surface of the part rather than on slopes.
Jetting occurs as a result of small gate size, or there is fast speed injection of molten material into the mold resulting in a distorted shape.
This can be remedied by:
- Increasing the size of the gate
- Controlling the flow of the molten material to the mold
Other Types of Ejectors
While they might not be common with many machinists, there are also other types of ejectors. Below are some examples.
These are simply hollow ejector pins. It consists of a hard-surface sleeve pin with a hole and a core pin that fits in the hole. The holes in the ejector sleeves are to guide and protect the pin.
In an ejection system in mold, ejector plates function alongside the ejector pins. It holds the head of the pins to prevent them from coming out during the ejector pins injection molding process.
They have lubricating hollow grooves and are applied to the surface of thin products that require a high surface finish but would be marred by ejector pin marks defect.
They limit ejector pin marks and are also applicable for use on the surface of products with high warpage.
Design experts cannot ignore the importance of ejector pins in the injection molding process. This is because the formed product in the mold depends on the efficiency of the ejector pins to avoid ejector pin marks defects.
Therefore, it is important to trust a company with knowledge of Design for Manufacturing, how to use ejector pins, and an excellent record of handling ejector pins injection molding design.
We at RapidDirect can guarantee optimized designs for your ejector pins injection molding process. Consult our support team for manufacturing advice and if you already have a CAD file it remains a little step. Upload it now to get an instant quote for your design.
In design cases, where the surface area is limited, such that there are no points on the part that allows for ejection from the mold, you can optimize your design by:
– Adding bosses to act as ejected pads
– Replacing ejector pins with liquid silicone rubber parts, such that upon cooling, parts are manually ejected from the molds via the liquid silicone rubber.
When choosing a material, it is best to consider the function of your product and the properties it should possess. Cheaper alternatives would fail in the long run. Therefore, it is best to use high-performance materials that satisfy our product needs.