(Last Updated On: July 8, 2021)

Selecting a parting line is an important part when designing plastic injection molds. The parting line is the basic element for mold design and manufacturing. A reasonably selected parting line makes the mold manufacturing process easier and quality of plastic parts higher. Otherwise, it will bring trouble to mold making.

In order to remove the plastic part from the mold, and to load the movable core or inserts into the mold body, the mold must be divided into two or more parts. Generally, the mold structure is roughly divided into two main parts: fixed half and moving half. When the mold body is closed, the contact surface of the cavity and core plates makes a parting line.

Usually the parting line of the mold is perpendicular to the mold opening direction of the injection molding machine. When opening the mold, eject or remove the cooled and solidified plastic parts from the mold (or automatically drop), and then remove the debris in the mold cavity, or place the insert to movable core in the mold cavity. The picture shows the basic form of the parting line. The mold opens from A. The fixed half of mold is not moved and the moving half moves and separates. Usually we call the parting line of the moving and fixed molds the main parting line.

However, sometimes the structures of mold requires it to be parted several times from several directions, which is the case of multi-step parting. The following are the basic forms of multi-step parting.

In the below figure, in order to take out the condensed material at the point gate, the mold is first parted from A. After the condensed material is pulled off and taken out, the part is parted from B to eject and be removed.

In the below figure, the mold is first parted from A. Then under the action of the ejector, the mold opens from B, and the plastic part is removed.

According to the sequence of the parting, the letters A and B are used at the parting lines.

Basic forms of parting line

The form of parting line depends on the specific situation of the plastic part, but generally there are several types: vertical stepped, inclined, curved and integrated parting lines.

1. Vertical parting line

The vertical parting line is commonly used. It is a plane perpendicular to the mold opening direction, as shown in the below figure.

2. Stepped parting line

According to the specific situation of some plastic parts, the parting line is made stepped, as shown in the below figure.

For the stepped parting line, due to the large force on one side of the cavity, the injection eccentric force is generated on both sides of the cavity, which causes the relative sliding tendency between the fixed half and the moving half molds. Its sliding direction is shown in the below figure.

If this unbalanced eccentric force is small, a guide pin can be used to support it. But if the eccentric force is too large, it will cause excessive wear of the guide pin and the guide bush, cause the decline in the quality of the plastic part. To compensate for the shortcoming, the following solutions can be used.

(1). Set a wedge-shaped insert on one side of the cavity, as shown in the below figure. In this way, the eccentric force of the injection can be offset, the relative position between the cavity and the core can be maintained, and it can be easily changed when worn.

(2). Arrange the two mold cavities to be symmetrical, as shown in below 2 figures. Make the injection forces received on both sides of the mold body balanced, and make the mold structure compact. The use of side gates is also very beneficial to the flow of part materials.


3. Beveled parting line

As in the below figure, the shape of the parting line is beveled.

4. Curved parting line

As in the below figure, the shape of the parting line is curved.

5. Comprehensive parting line

This form is based on the needs of the structure of the plastic part, sometimes combining steeped and curved parting lines, or beveled and curved parting line to form a comprehensive parting line, as shown in the below figure.

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