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Write about different types of Runners and Gate used in Plastic Injection Molds with the help of diagrams.
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Runners

  • A Runner is a machined groove located between the Sprue Bushing and the Gate.
  • The function of a runner is to provide a passage for the material to flow from the Sprue Bushing to the Gate.
  • There are many types of Runner cross sectional shapes. Most common shapes are the Full Round, Half Round, and the Trapezoidal.
  • As the Runner branches and changes direction from the Sprue to the Gate the runner is reduced in its cross-sectional area (typically about 25%). The portion coming out from the Sprue is referred to as the Primary Runner, the next section is referred to as the Secondary Runner, and followed by the Tertiary Runner.
  • The length of a runner system should be kept to a minimum. Injection Pressure build-up due to long runner lengths can be reduced by increasing the runner diameter. However, larger runner diameters increase cycle time due to the added volume of material that needs to be chilled/ solidified.
  • Below the point where the bottom of the Sprue Bushing and runner meet, there is a small cold slug well, and some form of sprue puller.

Types of Runner

i. A Full Round Runner is the most efficient shape for reducing the cooling effect on the material as it flows in the runner.

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ii. The Half Round runner is simply a runner system machined with a ball nose cutter into one plate of the mold.

iii. Trapezoidal Runners are very common in three plate molds. While not as efficient in chilling effect of a full round runner, the ease of cutting the runner shape, and the elimination of the need to mate two runner plates together, makes the trapezoidal runner a good second choice of runner shape.

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Gate

  • A Gate is a small area between the runner and the part cavity. The type, size and location of a Gate in an injection mold is critical to efficiently producing quality parts.
  • The type of gate selected depends on many factors including:

    1. Gate witness marks,
  • Material Type,
  • Filler used (if any),
  • Tooling Costs,
  • Scrap Allowance and the Mold Plates used among others.

  • Small restrictive gates are preferred. Minimal sized gates provide a small witness mark, and detaching parts from the Gate/Runner is easier.

  • A single gate per cavity is desired, however, part size and the material used may dictate multiple gates to be used.
  • Locate the gate in an inconspicuous location of the part if possible. The Gate location should be at the thickest wall of the part (flow from thick, to thin) and the thickness of the gate should be approximately two thirds the size of the wall.
  • The gate location and the surrounding area is also the highest area of stress in the final part. Do not position the gate at a location that part function indicates bending or impact strength is required.

Types of Gates

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  1. Edge Gates are the most commonly used of all gating options.
    The height of the gate should equal 75-100% of the wall thickness up to .125 in.
    The width should equal 2 times the depth, as it would appear in a mold.

  2. Fan Gates deliver plastic to a wide area of the part. This minimizes backfilling, and provides for better part surfaces, and reduces stress as well as imperfections. A Fan Gate will leave a Scar the size of the cross section of the gate, and requires (typically) manual trimming from the runner.

  3. Pin Gates are used in three-plate molds. The actual gate diameter is from .030 - .100 diameter. A Pin Gate will leave a small Scar that is the size of the gate.

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  1. A Ring Gate will produce a Scar around the entire part, the height is equal to the gate height.

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  1. Sprue Gate
  • Sprue gate is used when single cavity cylindrical parts need to be balanced and concentric.
  • Sprue gated pars have very good weld-line strength (if any), and typically are lower stressed, and are of high strength. A Sprue Gate will leave a significant Scar equal to The the size of the sprue diameter at the point of contact of the part.

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  1. Sub Gate - (May Also called a Tunnel Gate, Cashew or Banana)
  • Gating away from the parting line can be accomplished by using a Sub Gate. The Sub Gate also provides for automatic De-Gating of the Runner and Part within the mold.
  • Cashew, Banana gates require split inserted steels. Split steels are required to facilitate machining. Standard inserts are readily available.
  • The diameter at the gate is .030-.090 for unfilled materials and .100-.125 for filled materials. The angle is typically at 30 to 45 degrees from vertical. Ejector Pins are required to ensure automatic de-gating.

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