What is Mold core and cavity inserts

Mold core and cavity inserts are components of a mold used in the manufacturing of plastic and metal parts.

A mold core (sometime small inserts we call core inserts) is the internal part of the mold which is normally located at the core side (moveable side),  this creates the internal features of the part being molded. The core is typically made out of steel such as 1.2344, S136 etc. and is typically removable from the mold.

A cavity insert same function as core inserts, but located at fix side, it is the external part of the mold that creates the external shape of the part being molded. The cavity insert is typically made out of steel such as NAK 80, S136, 1.2344, H13 and is typically removable from the mold.

Both the mold core and cavity insert are typically made using the process of EDM (electrical discharge machining) or CNC (computer numerical control) machining. The core and cavity inserts are then assembled into a mold base to form the complete plastic mold. The mold is then used in the injection molding or casting process to produce the desired parts.

EDM:

• First, a mold design is created in a 3D modeling software.
• The design is then loaded into the EDM machine, which uses electrical discharges to erode the surface of a conductive material (such as steel or aluminum) to create the desired shape of the core or cavity insert.
• The finished product is then heat-treated to increase its strength and durability.

CNC:

• The process is similar, but instead of using electrical discharges, a CNC machine uses cutting tools (such as end mills or drills) to remove material from a block of steel or aluminum to create the desired shape of the core or cavity insert.
• The finished product is then heat-treated to increase its strength and durability.

Both EDM and CNC can produce very precise and accurate parts and are widely used in the manufacturing of mold cores and cavity inserts.

Some guideline to manufacture mold core and cavity inserts

When we manufacture the mold core and cavity inserts during the mold manufacturing, there are some dimensions that we need to manufacture with some tolerances so that it fits well in the pocket of the insert plate or mold cavity/core.

And this will save some work when we will fit the mould cavity & core, high tolerance, and good manufacturing technology will save lots of time for the mold-master, below are some manufacturing mold core /cavity insert Guideline that we need to pay attention to make high quality plastic mold.

When we make the corner radius In the manual milling machine, make a 0.2mm big step there, after hardening, we don’t have to grind on the corner when we fit the insert.

For the outside dimension of the mold cavity/core Inserts, the tolerance should be +0/-0.015mm, this will be easy fit into the pocket, smooth and good quality.

The tolerance for the hole to the sprue bushing should be H7 (0/+0.015mm).

The depth of the treading should be 1.5-2 multiply by the diameter of the tread. In this case for M8 screw, the depth of the treading should be 1.5 to 2 multiply by 8, equal to 12-16mm.

The guidance of the ejector pin should be about 15-20mm long, this will make sure that the ejector pins ejecting smoothly with long lifetime.

The clearance area for the ejector pins should be 1mm bigger than the diameter of the ejector pins., please see below illustration with explaining.