How to Design Injection Molding
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.
So 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.
Breaks
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.
Jetting
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
reida customer service