Plastic Injection Mold Design


The plastic injection mold design process is a complex engineering project that involves a great deal of 3-D spatial perception, mathematics, mechanics, physics, creativity, ingenuity and common sense. No wonder there are so few qualified plastic injection mold design professionals!


One of the essential components of any mold design is how to design gates and runners. This has been the topic of many discussions and a great deal of research has been done to try and manage this aspect of the tool design.

What is a mold gate?

Just as the name implies, it is an opening that leads to a different place. In the plastic molding process, a gate is an opening that allows molten plastic to be squeezed (injected, hence the name plastic injection molding), into a cavity of a mold. The cavity, just as the name implies, is an opening that needs to be filled in order to make the part.

A gate is a lot like the nozzle on a tube of toothpaste, only it goes in a mold, not your mouth! Every plastic injection mold design has to have some type of gate.

Basic types of gates

  • Direct sprue gate
  • Tunnel gate, aka submarine gate or hook gate
  • Edge gate, or fan gate
  • Three plate drop gate
  • Hot runner gate
  • Cashew gate or banana gate

Direct sprue gate

In this type of plastic injection mold design gate, the molten plastic comes directly from the molding machine into the part to be molded. The sprue is a sort of channel that connects the mold to the molding machine.

The direct sprue gate has many advantages and is quite commonly used. If you need to fill a large part, and are not concerned with gate vestige, this is a good choice. Gate vestige is the rather ugly rough looking remains of where the sprue meets the part.

If you look on the back of many plastic parts you can see evidence of the direct sprue, usually right in the center of the part. This is one of the simpler types of plastic injection mold design.

Tunnel gate

The tunnel, sub gate, submarine gate is preferred for parts that cannot have gate vestige, and for a higher level of automation. Because of the plastic injection mold design, the molded plastic part is separated from the runner and gate by a shearing action.

The placement of tunnel gates is critical and can be a bit tricky to machine. Very often the tunnel is truncated and results in what is referred to as a smiley gate. This is due to the appearance of the opening in the mold cavity: it resembles a smile!

Edge gate

The edge gate is one of the simplest gates. The plastic enters the part at the parting line, at the edge and is molded together with the part and runner. This entire solid part must then be separated by hand or machine.

Cashew or banana gate

Called this due to the cross section shape of the geometry. Very similar to a tunnel gate, but with the additional ability to access a wider range of the part. For example: a cashew or banana can sort of hook underneath the rim of a part and gate in an area where a slight vestige is acceptable.

A more advanced plastic injection mold design might incorporate a banana or cashew gate.

Three plate drop gate

The three plate drop gate is similar to the edge gate in that it molds a solid runner that must be separated from the part. This is done automatically, however, by the mechanics of the movable three plated molding system.

Typically, the detached runner falls into a bin or is picked up by a robotic arm. This process also leaves some gate vestige. Look on the bottom of most plastic glasses or cups and you will see a good example of this type of plastic injection mold design.

Hot runner gate

The hot runner gate is very popular, especially in high production applications. By far the most complex, expensive and most prone to problems, it is also the most efficient type of plastic injection mold design for gates.

The plastic is kept molten in a heated manifold and injected precisely into the cavities. This has many advantages, especially in the case of multiple cavities in the mold. It is not uncommon to have as many as 64 cavities making plastic parts in one shot!


Choosing a gate for a plastic injection mold design can be somewhat difficult. By determining the demands of the plastic part, the budget, the molding capabilities, the mold making level of expertise and the type of resin the process becomes manageable.

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