Plastic Injection Mold Making
Plastic injection molds are typically constructed from hardened or pre-hardened steel, aluminum, and/or beryllium-copper alloy. Steel molds cost more, but are often preferred because of their high durability. Hardened steel molds are heat treated after machining, and they are by far superior in terms of wear resistance and lifespan.
Many steel molds are designed to process well over a million parts during their lifetime. For lower volumes, pre-hardened steel molds provide a less wear-resistant and less expensive option.
Aluminum molds, on the other hand can cost substantially less, but they typically are ill-suited for high-volume production or parts with narrow dimensional tolerances. Nevertheless, aluminum molds can economically produce tens of thousands to hundreds of thousands of parts, when designed and built using computer numerical control (CNC) machines or Electrical Discharge Machining (EDM) processes.
Beryllium copper is used in areas of the mold that require fast heat removal or areas that see the most shear heat generated.
How Are Plastic Injection Molds Made?
Molds are constructed through two main methods: standard machining and Electrical Discharge Machining (EDM).
In its conventional form, standard machining requires the manual use of lathes, milling machines and drill presses. With advanced technology, CNC machining has become the predominant means of creating more complex and accurate molds, while still using standard machining methods. With CNC, computers are used to control the movement and operation of the mills, lathes, and other cutting machines.
In modern CNC systems, the mold design and manufacturing processes are both highly automated. The mold’s mechanical dimensions are defined using computer-aided design (CAD) software, and then translated into manufacturing instructions by computer-aided manufacturing (CAM) software. “Post processor” software then transforms these instructions into the specific commands necessary for each machine used in creating the mold. The resulting commands are then loaded into the CNC machine.
One potential drawback to standard and CNC machining is that any changes to a hardened mold require annealing to soften the mold, followed by heat treatment to harden it again.
Electrical Discharge Machining
Even more recently, Electrical Discharge Machining (EDM) has become widely used in mold making. Also known as “spark machining,” EDM is a simple process in which a desired shape is obtained using electrical discharges or sparks.
For this process, a shaped electrode (usually made of copper or graphite) is very slowly lowered onto the mold surface, which is immersed in kerosene. This process is done over a period of many hours. A voltage applied between the tool and the mold causes spark erosion of the mold surface in the inverse shape of the electrode.
There are several advantages to EDM processes. For instance, EDM allows the formation of shapes that are otherwise difficult to machine. The process also allows pre-hardened molds to be shaped so that no heat treatment is required. It’s also very useful for when working with extremely hard metals or those that are particularly difficult to machine with traditional techniques.
The plastic injection mold consists of two primary components, the injection mold (A plate) and the ejector mold (B plate). These plates are designed to work in conjunction as follows:
- Plastic resin from the molding machine enters through a “sprue” or “gate” on the A plate.
- A sprue bushing seals tightly against the nozzle of the injection barrel of the molding machine. This allows the molten plastic to flow from the barrel into the mold (or “cavity”).
- The sprue bushing directs the molten plastic through channels (called “runners”) that are machined into the faces of the A and B plates.
- The molten plastic flows through the runner and enters one or more specialized gates and into the cavity to form the desired part.
A mold is usually designed so that the molded part reliably remains on the B plate of the mold when it opens. The runner and the sprue are drawn out of the A side. The molded part then falls freely when ejected from the B plate.
A single plastic injection mold can have multiple cavities. In fact, some extremely high-production molds (like those for bottle caps) often have about 130 cavities.