Molds and Tooling: Turning Molten Plastic Into Millions of Parts
A single steel injection mold can cost $50,000 to build -- and then stamp out 500,000 identical plastic bottle caps before it wears out. That trade-off, huge upfront cost for near-zero cost per part, is why tooling is one of the most important decisions in industrial design.
What You'll Learn
By the end of this lesson, you will be able to: Explain why manufacturers invest tens of thousands of dollars in tooling before producing a single sellable part. Identify the core, cavity, and other major components of an injection mold. Describe how draft angles and wall thickness affect whether a part can be molded at all. Evaluate a simple product design for mold-friendliness.
What Is Tooling?
Tooling is the set of custom hardware -- molds, dies, jigs, and fixtures -- built specifically to mass-produce one part. For injection-molded plastics, the tool is a two-piece steel or aluminum mold machined to the exact negative shape of the part. Cutting that mold on a CNC machine can take weeks and cost anywhere from $5,000 for a simple aluminum prototype tool to over $100,000 for a complex, high-volume steel mold for something like a car dashboard panel. Once built, though, the mold can be bolted into an injection-molding machine and cycle open-inject-cool-eject every 15 to 60 seconds, essentially forever -- which is how a single mold can produce hundreds of thousands or even millions of identical parts.
Inside an Injection Mold
An injection mold has two halves. The cavity side shapes the outside surface of the part; the core side shapes the inside surface and usually holds the ejector pins. Molten plastic, heated to around 400-550 degrees F depending on the material, is forced through a sprue (the main entry channel), down runners (branching channels), and into the part itself through a narrow gate. Once the plastic cools and solidifies -- often in just a few seconds for thin parts -- the mold opens and ejector pins push the finished part free so the cycle can repeat.
LEGO molds are held to tolerances of about 0.002mm -- roughly 1/50th the width of a human hair -- which is why bricks made in 1958 still snap together with bricks made today. That precision comes entirely from the tooling, not from the plastic itself.
Designing for Moldability: Draft Angles and Wall Thickness
A part can't just be any shape -- it has to actually release from the mold. Designers add a draft angle, typically 1 to 3 degrees, to any vertical wall so the part slides out of the cavity instead of scraping and sticking. Wall thickness matters too: thick sections cool more slowly than thin ones, which causes sink marks (visible dimples) and warping. That's why designers hollow out thick areas into a honeycomb of thinner ribs -- you can see this if you look inside the back of almost any plastic remote control or appliance housing.
Match each injection mold part to what it does.
Terms
Definitions
Drag terms onto their definitions, or click a term then click a definition to match.
Why would a manufacturer choose an aluminum prototype mold over a steel production mold for a new product?
A designer skips draft angles and makes a part's walls perfectly vertical (0 degrees). What is the most likely result?
Redesign a Part for Moldability
Sketch a simple object (a phone stand, a small tray, a toy figure). Mark every wall on your sketch and label whether it has a draft angle. Identify any thick solid sections and redraw them as thinner ribbed sections instead. Write two sentences explaining how your redesign would reduce sink marks and make the part easier to eject from a mold.
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