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Plastic Injection mold tooling

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The plastic injection mold (or plastic injection tooling, as it is sometimes referred to) determines what the final product will be.

An injection molding machine could use a mold the shape of a three prong cord. In this example, the operator would insert a terminated, multi-conductor cable. The injection molding machine and mold would then produce an appliance power cord.

If you change the plastic injection mold tooling, the same injection molding machine could mold antenna ends for cellular phones. For the second example, antenna ends, you would use the same injection molding machine, but a different mold (the shape of an antenna). The operator would insert coiled wire and the end bracket of the antenna. Then, if by “magic”, a second product, an antenna, is produced.


1. Sprue Bushing – This is the place where the injection unit seals up against the tooling and allows the transfer of material (plastic) at high pressure to flow from the injector unit into the mold halves. It should be made out of hardened tool steel and should be replaceable because this is where the injection unit is brought up against the mold, and it can see millions of operations.

2. The “Runner” or Runner System is how the material is “tunneled” through the mold to get from the Sprue Bushing (or Entrance) to the different parts or cavities. The runners connect the parts together, and the operator must snap them off to separate the parts. The runners can be reground and can be added back with new material, depending on product and material specifications.

3. Cavities are where the parts are molded. They are the shape of your finished product. Everything in a cavity is important because it affects the way your part will look: from its shape, to its surface finish, to its size. Cavities should be made of tool steel, hardened and chrome plated.

4. Wire shut-offs are the part of the mold that pinches off on what is leaving the mold (wires) so that the plastic does not flow out. The fit of these shut-offs is very critical. It must be tight enough not to allow the high pressure plastic to flow out of the cavity, but not so tight that it damages what it is shutting off on. These shut-offs should be removable because they have a high risk of being damaged. Also, when molding around wire, it may be important to have different size shut-offs, as you change wire from vendor to vendor, or if a particular vendor has a wide spec on their insulation size. You want to have the flexibility to make that change in the mold.

5. Loading bars are a section of the mold usually in one end of the cavity that is lifted up by the machine to allow the operator to insert a wire or terminated leads into it. Then it is retracted back into place and a new part is ready to be molded. Loading bars and parting line tooling are signatures of insert molders and wire harness molders because that is what is needed to mold around a terminated wire or an insert..

6. Knockouts are pins or rods that come up through the cavities to eject or push out the parts after they have been molded. When the parts are ejected, the operator removes them and loads in new inserts to be molded. Before the machine molds, Knockout Pins and Loading Bars are retracted and sit flush with the cavities. Knockout Pins usually leave telltale signs in plastic parts, usually in the shape of the round rod that did not line up perfectly with the cavity.

Plastic Injection Mold Tooling /injection unit

The injection unit can be mounted in two ways:

(A) Horizontally, to inject material from the back,
commonly called a Parting Line Injection Unit.

(B) Vertically, to inject material through the top of the mold,
commonly called “Through the Sprue Molding”.

Both of these layouts have advantages related to three factors:

1. Plastic Injection mold/ plastic injection tooling layout

A Typical plastic injection tooling Layout on a Horizontal Injection Unit of an Injection Molding Machine:

With a horizontally mounted injection unit or a parting line machine, the natural mold layout (and balancing of the mold) looks like a branching, family tree. This lines up the cavities in a row which makes it easier for an operator to load and allows the inserts with attached cables to be draped out of the front of the mold. Not all mold makers build parting line tooling. AmeriPlas designs and manu- facturers molds in-house and works with mold makers across the country who specialize in parting line molds in order to remain competitive and to be on the cutting edge of mold design.

A Typical Mold Layout on a Vertical Injection Unit of an Injection Molding Machine:

With a vertically mounted injection unit, the natural mold layout (and balancing of the mold) looks like a branching “H” pattern. This brings the cavities to the right and left and front and back. This does not lend itself to the molding of inserts with cables on them because the cables on the back side of the tool must be draped around the mold and this problem increases loading and handling time. The layout of the die lends itself to a more efficient use of space and works well in encapsulation, molding coils and contact bodies, where wire length is not such an issue. Unlike parting line tooling, not every mold maker understands how to design and build molds for vertical inject machines (or through the sprue molds).


An insert is anything that is placed in the mold so that plastic can be molded on it or around it. Any insert that is used must have dimensional consistency. This is especially true if the insert is being shut off on.

Variations in the size of the insert will change the amount of material needed to fill the mold. This can cause the cavities to be partially filled (known as short shot), or can cause too much material in the cavities (known as flashing). Variations of an insert on a shutoff area can cause plastic to squirt down the inserts (also, known as flashing) or can even push the inserts out of place.


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