Making a Mold


One of the main components of the injection molding press is the actual mold. The mold encompasses many different mechanisms, but it can easily be broken down and analyzed.

Learning the Basics

A mold is the tooling required for injection molding. The mold is created from two halves, half A and half B. These two halves each house separate components. These molds are generally made from steel, aluminum, or beryllium copper. However, the material selection is not limited to just those and is more reliant on what sorts of materials will be injected into the mold.

The First Half of the Puzzle

Half A of the injection mold is also referred to as the injection half. This half of the mold is fixed and it’s the side that contains the mold cavity.  It also houses the sprue, sprue bushing, and runners. Below is a detailed diagram showing each of these components.

  • The mold cavity is the area of the mold that the plastic fills to form the desired product. This cavity has been machined using a process that’s either CNC or EDM. Mold cavities are also known as cavities, or cavity images.

  • The sprue is the plastic material that sets in the sprue bushing.

  • A sprue bushing is a part of the mold that seals tightly against the nozzle of the injection barrel. By doing this, it allows the molten plastic to enter the mold and fill the cavity.

  • The runner allows the molten plastic to flow into the cavities through a series of channels. These channels have also been machined into the mold.

The Other Half of the Puzzle

Half B of the injection mold is also known as the ejection half. The ejection half of the mold is movable and it’s the core of the mold. This half of the mold contains the ejector pins, ejector bar plate, ejector retainer plate, and the knockout rod. Below are a few detailed diagrams showing these instruments.

  • The ejector pinsare what pushes the finished plastic part out of the mold. Ejector pins are made up of a face, body, and a head. These pins come in various sizes and they impact the design. By using the proper size and arrangement, the impact of the mold can be minimized. Ejector pins are also known as knockout rods/knockout pins.

  • An ejector retainer plate holds the heads of ejector pins in place. The main function of this plate is to lock the heads of the pins so they don’t come out during the injection molding process.

  • The ejector bar plate has almost the same role as an ejector retainer plate. Ejector bar plates are bolted to the retainer plate to ensure the heads stay in place. It’s main function is the same as the ejector retainer plate.

Molding the Cost

Why is it so important to know the basic mold components? Well, the mold and mold design directly affect the cost of your project. The number of cavities the mold will have and the total volume of the mold helps determine the costs. Remember the cost breakdown of injection molding? This is where that forty percent of your costs are coming from. A general rule of thumb to stick to is the more required of the mold the higher the costs will be and the longer it’ll take to be created.


"DME Mold Bases." DME Mold Bases. N.p., n.d. Web. 05 May 2016. <>.

Bryce, Douglas M. Plastic Injection Molding...material selection and product design fundamentals. Volume 2: Fundamentals of Injection Molding series. Michigan: Society of manufacturing Engineers, 1997. Print.

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