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What is a mold?

By Ron Pleasant on Jun 12, 2015 1:59:00 PM

When discussing a plastic injection mold, what exactly are we describing? Simply put, a plastic injection mold is a tool, custom designed using Computer Aided Design (CAD, in engineering) or Computer Aided Machining (CAM, in manufacturing utilizing CNC machining tools) to produce a specific plastic part. Using thermoplastics, these molds can produce anything from helmet visors to pipes to bottles to parts for cars and many, many other everyday items one can imagine. 


Mold designs come in either Dedicated or Modular designs.

Dedicated designs are:

  1. All inclusive, which means they include the entire mold base;
  2. Have very high production levels that will be generally dedicated to a molding machine;
  3. Use various materials;
  4. Can be dictated by part size.

Modular designs are:

    1. Cavity Insert set only – “master frame” usually owned by molder;
    2. Frame stays in press for faster changeover times;
    3. Customer is only paying for the custom steel used for his project;
    4. High production – high cavitation;
    5. Prototyping;
    6. Various steel types available.

The Process

1)     The cold sprue is the first point of plastic entry into the mold. The melt stops at the nozzle (interfaces plastic delivery of molten material from the molding machine to the mold).The travel of the plastic through the tool is what is called “injection.” That entire process looks like this:

2)     The runner system

    • Gating: The Tab Gate is removed manually with side cuts;Screen_Shot_2015-06-12_at_1.50.18_PM-1
  • Subgate: Automatic de-gating and separation of parts and runners can be automated;
  • Fan gates: Specific purpose for part cosmetics; manu
    ally removed;
  • Diaphragm gates (Round tubular parts): Usually on single cavity molds; plastic is fed to the inside of the part; thin entrance of plastic to entire ID of part; maintains a very round uniform part; deformation is minimized;
  •  3-Plate Gates: Part design requires the part to be gated in the center of the part, not on an edge; uses a Cold Runner Design.

3) Hot Sprue: The melt is carried into the mold to the “parting line” and can be direct gated to the part on a single cavity mold at this point or to the same cold runner system and gating as above. This has less waste and requires less of a parting-line opening (movement of press).

4)     Hot Manifold: The melt is carried all the way to the part; there’s no waste or regrind; there’s a faster cycle time.

5)     All of this happens at pressures as high as 20,000 psi.

  • Consider that your home water pressure is 40-60psi;
  • Shop air pressure is 80-120psi
  • Cavity steel must seal to hold this pressure
  • Must allow for air to displace from cavity as it is filled with plastic;
  • Vents – appropriate to m
    aterial type – associated with viscosity


6)    The accuracy must be within .0002-.0003 for this to happen. Consider that this is the thickness of paper split 10 times. This requires extremely accurate machining practices.

7)    Cooling of the plastic is called the “Cure.”

  • Cooling is required to solidify the molten plastic;
  • Pack and hold awaiting “Gate freeze;”
  • Part cools and stabilizes to acceptable temperature;
  • Cooling lines are drilled, milled or turned to allow circulation;
  • Proximity to the part is directly related to cycle time;
  • Must be able to recover before next cycle;
  • Different steel types have different thermal conductivity;
  • Affects part size, warpage, and cosmetic appearance;
  • Parts shrink a predetermined amount – reverse engineered;
  • Not always perfect – may need steel safe conditions on very critical tolerances.

8)    The mold opens and the part(s) ejects:

  • Ejector pins – most common way to push part out of mold;
  • Sleeves – around pins;
  • Blades – thin areas that restrict the use of pins due to size;
  • Stripper plates

9)    Mold closes and the inject cycle starts over.


We hope this article helped to explain what a mold is and the injection-mold cycle. For more information on mold-making be sure to check out more of our blogs or signup for our online newsletter today.


Topics: injection molding

Author: Ron Pleasant

Copyright Pleasant Precision, Inc.