Injection Mold Making

This will describe the sprue and runner system of molds.

Related posts:

• Gate system
• Product/Part design

Sprue

A sprue is a channel through which a molten plastic material is being injected from the nozzle of the injection machine into the mold. It has a smooth, round, tapered wall to allow smooth material flow.

Runner System

Runners are channels where material flows from the sprue to the cavities.

Conventional Runner:

Improved Runner:

Balanced Runner: Resin is evenly distributed throughout the cavities. Volume and condition is the same. There is more material used because the total runner length is longer compared to the other two.

Hot Runners:

The narrow and shallow portion of the runner as it enters the cavity is called the gate.

Listed here are the different types of gate with their characteristics. Sprue and runner are described here.

Side gate: This is the most commonly used gate type and is commonly used for mold structures with 2 or more cavities. It is placed at the side of the plastic product. The gate has to be cut manually by a cutter.

Submarine gate: The positioning of this gate is flexible thoughout the sides of the plastic product. It can be placed on the fixed or movable side of the mold but the design has to be thought about carefully so that the product will not be left inside the fixed cavity. The gate automatically cuts itself as the mold opens.

Fan gate: It is commonly used for large and flat plate products. It is placed at the side of the product – same as the side gate. The gate has to be cut manually by a cutter.

Film gate: Similar to fan gate except that it is commonly used for thin and flat plate products.

Pin gate: This is possible for molding multiple cavities or parts. The gate positioning is relatively flexible at the top side of the product. The runner layout is very flexible as well. The mold base structure is complicated because it uses a 3-plate method.

Banana gate: This is not a very common gate. It is used when the visible surface of the product requires no trace of the gate. The gate automatically cuts itself as the mold opens.

Direct gate: The sprue serves as the gate. It is placed on the top side of the product. The gate has to be cut manually by a cutter. This gate type can only produce one part per shot. The molding cost is low because the material from the runner was ellimenated. The injection pressure was reduced due to direct cavity filling. The simple mold structure makes the mold cost lower.