Building a Plastic Injection Mold - Lesson 1 of 10 Mold Making Basics

Introduction: Building a Plastic Injection Mold - Lesson 1 of 10 Mold Making Basics

Basics: One of the most important parts of building a mold that is capable of making millions of part, is the mold design. In this instructable we will take look at the pig picture when designing and building molds. We will break down 10 future lessons into the following lessons.

  1. Part Design
  2. Plastic Selection
  3. Molding Machines - Plastic Processing
  4. Mold Styles & Costing
  5. Mold Cavity Splits & Shrinkage
  6. Cutters Methods Used Fabricate the Mold
  7. Mold Design
  8. CNC Programming Setting Up The Job
  9. Polishing & Fitting
  10. Debugging the Mold

Plastics are essentially squeezed into a chamber with a machine that takes small pellets and melts them before it is pushed into the cavity. Cooling is routed thru the mold to make the process run super fast. If you find that you need lots of parts, you can add multiple cavities or a pattern of cavities. One can imaging when plastic molecules are cooling, then tend to shrink from a liquid state to a solid state. Molds must accommodate for these changes, so the cavities and cores are scaled up so the plastic will shrink back to size. We will refer to this subject as shrinkage.

These cavities and cores are mounted in a mold base where small pins are used to knock the plastic parts out of a cavity or core. The core is traditionally on the "B" side of the mold. This is also refereed to as the ejector side of the mold. The cavity is on the "A" side of the mold or the hot side. That is the fixed and stationary in the molding machine when it is bolted down. We traditionally inject the plastic into this "A" side of the mold.

Step 1: Identify How Many Parts You Will Need Then Size the Mold and the Press

Here is an 16 cavity mold starting up. This mold will make 3.5M parts/month and run at a 12 second cycle time. We need to first understand how many parts are needed then we can figure out how many cavities and how big the mold and the molding press will need to be to satisfy those requirements.

Here is a Screencast on Mold Making Basics:

Here is a Screencast on Mold Making Workflow:

Step 2: Mold Cutting

When CAMing in Fusion 360 you need to start with large cutters, then work towards small cutters. It is important to not skip a step, or you will start breaking cutters. 1/2" Dia to 3/8" Dia. to 1/4" Dia to 1/8" Cutter to 1/16 Dia. " Cutter to 1/32" Dia. cutter. Remember, the deeper you cut with the smallest cutter it tends to break. So try to design your parts with the biggest radi in each corner.

Here is a screencast on CAMing:

Step 3:

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    Paul Parmar
    Paul Parmar

    3 years ago

    sorry, i could not download the contents?

    do i have to become paid member for this?



    4 years ago

    good and interesting hope u

    Many idea mold part dision


    4 years ago

    Thanks I want made plastic mold at home

    Please help me


    5 years ago

    Great tutorial, I hope to see more parts!

    I'm interested to see how in depth you go with the tutorial. It could be a great way to introduce people to how much work it actually takes to make their polymer widgets. I'm also interested in what perspective you will be designing from (i.e. molder, part designer, or tool builder).

    One thing i would like to point out though; it is a common misconception about needing cold slugs at the end of each runner branch. Polymers exhibit "fountain-like" flow behavior during injection, in which the flow front is pushed out against the walls as new material flows in through the center. For those who would like a visual example, a similar behavior can be seen with "pillow" lava ( The only time a cold slug is actually pushed in front of the material is upon exiting the nozzle. Resin freezes in the nozzle tip (or drools out) between cycles forming a plug. This plug should be caught by the sprue puller. Adding additional wells provides no real benefit, and only increases runner weight.