In this chapter, you will learn about the functions, characteristics, and shapes of the sprue, runner, and gate.

Sprue
A sprue is a channel through which to transfer molten plastic injected from the nozzle of the injector into the mold. It is a part of sprue bush, which is a separate part from the mold.
Runner
A runner is a channel that guides molten plastic into the cavity of a mold.
Gate
A gate is an entrance through which molten plastic enters the cavity.

The sprue, the runner, and the gate will be discarded after a part is complete. However, the runner and the gate are important items that affect the quality or the cost of parts.

Gate

The gate is categorized into restrictive gate, which narrows the entrance, and nonrestrictive gate, which does not narrow the entrance.

• Restricts the flow and the direction of molten plastic.
• Simplifies cutting of a runner and moldings to simplify finishing of parts.
• Quickly cools and solidifies to avoid backflow after molten plastic has filled up in the cavity.

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Restrictive Gate

A restrictive gate has a narrow entrance to the cavity to restrict the amount of molten plastic in order to improve filling in the cavity.
The restrictive gate has the following characteristics.
• Generates shear heat by going through the narrow gate, raising the temperature of molten plastic and improving the filling in the cavity.
• Reduces residual stress, and thus reduces part defect such as warp.
• As the cooling solidification time is shortened, molding cycle is also shortened.
• As the gate trace is less, it is possible to complete finishing process in a short time.

The restrictive gate has the following types.

Side Gate	Submarine Gate �@�@(Tunnel Gate)
The most common gate. Put to the side of parts. The gate trace will be left. Often used for the structure with more than two cavities.	The gate will be automatically cut off during mold opening. The position is flexible (front, side, or back of parts). The gate needs to be thought about not to be left inside the cavity.

Pin Point Gate	Fan Gate
Suitable for molding multiple parts. The position is relatively flexible. The structure is complicated due to three plate method of die.	Suitable for large and flat plate parts. Finishing is difficult and cost is high due to the wide gate. The gate trace will be left.

Film Gate	Valve Gate
Suitable for thin plate parts. Finishing is difficult and cost is high due to the wide gate. The gate trace will be left.	The valve of the gate opens the gate according to injection timing. Sprue and runner will not be discharged due to hot runner method. Put to either the front or the back surface of parts.

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Nonrestrictive Gate

A nonrestrictive gate involves a method in which molten plastic fills up in the cavity directly from the sprue.
The nonrestrictive gate has the following characteristics.
• Reduces the loss of injecting pressure due to direct cavity filling from the sprue.
• Less molding material because there is no runner .
• The simple mold structure reduce the cost and produces a mold with less trouble.
• It is likely that residual stress is generated and parts have crack.

The nonrestrictive gate has the following type.

Direct Gate

The sprue plays the role as gate. Put on the front or back side of parts. The gate trace will be left.

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Determining Gate Position

Point 1 Set a gate position where molten plastic finished filling up in each cavity simultaneously. Same as multiple points gate. Point 2 Basically set a gate position to the thickest area of a part. This can avoid sink marks due to mold shrinkage . Point 3 Set a gate position to an unremarkable area of part or where finishing process can be easily done. Point 4 Avoid injecting from the direction where the air in the cavity or the gas generated from molten plastic is inclined to accumulate. Point 5 Fill up molten plastic using the wall surface in order not to generate jetting.