Cavity and Core in mold

In this chapter, you will learn about the cavity and the core. You will learn the types of inserts and their processing methods, position, material and different usage depending on production quantity per molding cycle.

In previous chapters, we learned that how molten plastics was injected into the mold. The mold is composed of a cavity and a core. The cavity is a concave cavity plate. The core is a convex core plate. We previously stated that molten plastics is injected into the cavity. Actually, it is injected into the space between the cavity and the core.

Cavity and Core

Cavity and core are categorized into integral method and insert method. The method is determined by the part quantity and the molding shape.

Integral method

The cavity and the core have the same shape as the part.

Insert method

Another part is inserted into the mold to get the final shape of the part.

Using a different material from that of the mold to improve abrasion resistance or make temperature control easier.
It is difficult to directly machine a mold when complicated shapes or multiple shapes are aligned, whereas attaching an insert as a separate part makes the process easier.
It is possible to change the material of the part that is subject to damage due to repeated molding with a special material or replace only the damaged part.

An insert attached to the cavity side is called a cavity insert. An insert that is attached to the core side is called a core insert.

The following factors determine whether an insert is used.

Mold Condition	Molding Condition
Mold Material	Single part	Multiple part	Special shape
S55C	Intergral	Core insert	Core insert, Cavity insert
Special steel	Core insert, cavity insert

 

The shape of a part determines whether an insert is attached to the cavity, the core, or both.

Also, the insert method is used to process the cavity and the core with special materials, depending on the molding material.

An Insert is categorized into the following types:

Flange insert

If the shape of the part is round, a flange insert is used. The insert does not need a baffle, so only a flange is needed to prevent an insert from lifting. A round shape is easy to process when making an insert or a mold.

Also, a flange is attached only to the long side when using a square flange insert.
As in the figure, if H >L., attach a flange to the L direction, and if H < L, to the H direction.

Pocket Insert

A pocket insert is generally used when the shape of the part is square. A pocket insert is attached to the mold by an attaching bolt, as shown below..

Making Inserts

When making an insert or a mold, be careful with the accuracy of measurement so that the insert and the mold can be attached without a gap. If the measurement is not accurate, an insert can not be attached to a mold, or a flash may result due to the big gap, and the part will not look good. Attach an insert and a mold by processing as follows.

 

For a flange insert: As the insert is mounted through the mold, chamfers are applied to the mold side whose corner will be convex.

For a pocket insert: As the insert is mounted in the mold, chamfers are applied to the mold side whose corner will be convex.

When making an insert or a mold, chamfers are applied to its corners. The corners can not be processed at a right angle due to the edge R of cutware or relief of cutware, even when an insert or a mold is made accurately as measured. If an insert is attached to a mold without chamfering, the corner will interfere and prevent the insert from being attached at the proper position. Also, an excessive gap between an insert and a mold will cause part defects. Therefore, be sure to fulfill the matching requirement (positional tolerance of hole and axis) that is regulated by the Japanese Industrial Standard (JIS).

The chart below shows the typical matching tolerances.

	Hole tolerance (f D )	Axis tolerance (f d )
Regular Process	H7	h6
Precise Process	H6	h5

In the chart above, the hole tolerance (Ø D ) refers to the tolerance of the hole size (Ø D ) of the mold in the above figure, and the axis tolerance (Ød ) refers to the tolerance of the axis size (Ø d ) of the insert in the above figure. As the matching tolerance varies depending on outer diameter of hole or axis or types of matching (gap matching, midpoint matching, or draw matching), refer to the JIS reference chart.

Position of Insert

The position of an insert is determined by part quantity or type of insert. The positions of insert are as follows:

When a flange insert is used: Process The insert is attached directly to the mold, Both the insert and the mold need to be made precisely. Installation It is easy to position the insert to a mold because the insert is made precisely.

When a pocket insert is used: Process The mold is made with the measurement of an insert + a runner. Even if there is an error in the mold measurement, the mold is easily processed because a runner (made by a separate part) is used for adjustment. Installation It is difficult to position the insert because it is attached with the runner inserted in-between and a measurement error can easily occur.

When a pocket insert is used: Process The mold can be processed easily because a wedge measurement is used to adjust, even if there is an error in a mold measurement. Installation Because an insert is attached set out, an error by attachment occurs less frequently and insert positioning is easy.

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