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How Are Plastic Injection Molds Made

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Plastic injection molds are made by complex steel cavities called molds are meticulously crafted and machined to shape molten plastic into desired parts through injection molding. Here’s a video where you can see exactly how it’s made!

Injection molding relies on custom-designed molds to create plastic parts. Here’s a breakdown of the mold-making process:

1. Mold Base:

  • Chromium steel bars are used for their durability during high-pressure plastic injection.
  • The bars are assembled and precisely milled to the desired dimensions.
  • A typical mold splits into two halves, each constructed from various components.

2. Machining:

  • Holes are drilled for guide pins and bushings that ensure proper alignment when the mold closes.
  • Grinding smooths surfaces for subsequent machining.
  • CNC machines precisely carve the mold components from the steel base. This can be time-consuming, taking hours for intricate parts.

3. Detailing and Finishing:

  • For very fine features, an Electrical Discharge Machining (EDM) process uses a shaped electrode to “burn” the design into the mold cavity.
  • Coolant channels are drilled to accelerate the cooling and hardening of molten plastic within the mold.
  • Lettering is engraved in reverse on the mold surface to appear raised on the final plastic part.
  • The mold is polished for a smooth surface and optimal casting.

4. Molding Process:

  • The precisely machined halves are clamped together, forming a sealed cavity.
  • Molten plastic is injected under high pressure, filling the entire mold cavity.
  • Once cooled and solidified, the mold opens, and the finished plastic part is ejected.

5. Multi-Step Molding:

  • Some parts may require a two-step process. For instance, jet ski button handles might involve molding a hard plastic base followed by injecting a softer, rubber-like plastic around it in a separate mold.

6. Beyond Plastic:

  • The core principles of mold making apply to other materials like aluminum and rubber. The specific type of metal used for the mold may differ, but the overall process remains similar.

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