Category Archives: Mold Design

Mold tool design

Injection Mold Sprue and Runner

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sprue-runner-gate

This will describe the sprue and runner system of molds. More comprehensive reference can be found in Runner and Gating Design Handbook: Tools for Successful Injection Molding

Related posts:

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:

conventional-runner

Improved 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.

balanced-runner

Hot Runners or sometimes called runnerless, can be more economical in high volume injection molding.

Try Pro/E Wildfire 3.0 for 30 days

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This post is old. New version of ProE Wildfire 5.0 is coming out soon.

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ProE-free

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Functions of Mold Base Parts

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moldpartsFixed Clamping Plate
Holds The fixed side of the mold to the fixed platen of the injection machine.

Locating Ring
It is used to locate the center of the injection machine so that the sprue bushing and the nozzle are aligned. Commonly fitted into the counterbore in the fixed clamping plate.

Fixed Cavity Plate
Used to hold the fixed cavity block, leader pin/bushing, and sprue bushing.

Movable Cavity Plate
Used to hold the movable cavity block, leader pin/bushing.

Movable Clamping Plate
Holds The movable side of the mold to the movale platen of the injection machine.

Spacer Block
Mounted between the movable clamping plate and the movable cavity plate to give space and allow the ejector plate to move when ejecting the
part.

Ejector Retainer Plate
Holds the ejector pins and the return pins in place.

Ejector Plate
Pushes the ejector pins and return pins at the same time. Mounted to the ejector retainer plate to form the ejector unit.

moldparts1Support Pillars
Bars placed between the spacer blocks to give additional support to the movable cavity plate.

Sprue Bushing
Has a tapered hole through which the material is forced into the runner. It is butted up against the nozzle of the injection machine.

Return Pins
Same as ejector return pins. Make sure that the ejector unit is back in its original position when the mold closes.

Leader Pins and Bushings
Precisely align the two halves of the mold base (fixed and movable).

Different types of gate for injection molding

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

Runner and Gating Design Handbook: Tools for Successful Injection MoldingIndustrial Design Books)

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.

side-gate

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.

submarine-gate

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.

fan-gate

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

film-gate

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.

pin-gate

Runner and Gating Design Handbook: Tools for Successful Injection Molding

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.

banana-gate

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.

direct-gate