What is core and cavity in injection molding?

Table of Contents

The core and cavity are the two halves of the mould where the hot molten plastic material is injected into the core and cavity and the gap between the core and cavity is filled with the hot molten material during injection molding process and then hardened into final shape.

The mold design of the core and cavity is critical to the proper molding of the product. The core is the raised part of the mold and the cavity is the recessed part of the mold.

The cavity and core in a typical plastic injection molding machine of an injection mold are the molding parts of the mold and play a key role in the shape of the molded part.

For some complex molds, the cavities or cores are often inserted, i.e., a part of them is removed from the cavity or core to form an insert structure, which is similar to the decomposition of cavities and cores.

Other words for core and cavity in injection molding

The male mold, i.e., the rear mold, is also called the upper convex mold (usually the mold core plate with an ejector pin), and the female mold, i.e., the front mold, is also called the concave mold.

A set of injection molds is divided into upper and lower molds, depending on how the molds are installed, some upper molds are called convex molds when they are convex, and lower molds are called concave molds when they are concave.

Some are called convex dies when the upper die is concave and the lower die is concave; some are called convex dies when the upper die is concave and the lower die is convex.

So anyway, the convex mold is called convex mold shut, and the concave mold is called concave mold.

How to distinguish the core and cavity of the mold

Distinguish according to shape and form

Molds are designed as two-plate or three-plate molds, depending on the type of component to be manufactured. The two plate mold requires a single day in the light, while the three plate mold requires two days.

Observe the concave and convex shape of the mold directly. In general, the shape of the concave is the cavity (there is no ejector pins), generally, the convex is the core (there is an ejector plate pin).

The cavities and cores in injection molds are generally called concave and convex molds, where the cores and cavities can be in the fixed half mold or the dynamic mold.

In other words, the cavity and core can be in the fixed mold, the cavity plate and core can be in the dynamic mold, and the core of the dynamic mold is the core of the dynamic model, and the core of the fixed mold is the core of the fixed model.

According to the composition of the structure to distinguish

The cavity is the working part of the outer surface of the molded plastic part, according to its structure, it can be divided into two categories: integral and combined.

The cavity and the core are the product-forming part of the mold, the cavity, and the core is fixed on the A and B plates respectively, and the part in the middle of the core and the cavity are injected into the product after the mold is closed.

Types of mold cavities and cores

According to their structure, mold cavities and cores can be divided into two types: integral type and combined type.

Integral type

This type of cavity is made of a whole piece of metal material, such as straight pull mold. It is characterized by simple structure, high strength, good rigidity, not easy to deformation, and no trace of stitching in plastic component parts.

Combination type

When the shape of plastic parts is more complex, the combination type cavity is often used to improve the processability, reduce heat treatment deformation and save quality steel. There are more structural forms of combined cavities.

For large and complex molds, the four sidewalls and the bottom are processed, heat treatment, grinding, polishing, and pressed into the mold sleeve, the four walls are connected in the form of latches, to make the inner joints tight.

The outside of the joint should be left 0.3 ~ 0.4mm gap, in the four corners of the embedded parts of the corner radius R should be larger than the radius of the mold sleeve corner r.

The whole embedded is commonly used in multi-cavity mold or shape more complex plastic parts, such as plastic gears, the whole insert is commonly used in cold extrusion, electroforming or mechanical processing, and other methods of processing, and then embedded, it is not only easy to process, and can save high-quality steel.

Injection mold cavity and core size calculation

Calculation of the cavity size

Calculation of the cavity size of the injection mold, taking into account the use of wear and tear of the size of the larger tolerance to take negative values.

In the calculation of the cavity size, the selection of plastic shrinkage rate is often not easy to choose very accurately, the size should be selected smaller, leaving so much room, or a large mold will be scrapped.

A = (A1 + A1Q – 0.75Z) + K

A – cavity size (big end)

A1 – the corresponding upper size of the part

Q – shrinkage rate

Z – part tolerance

K – molded part

Calculation of core size

Calculation of core size, taking into account that the size becomes smaller after use and wear, the tolerance is taken as a positive value.

In the calculation of the core size, the size should be selected larger, leaving a margin for reduction, to correct.

B=(B1+B1Q+0.75Z)-K

B – core size (small end)

B1 – the corresponding lower limit size of the part

Q – shrinkage rate

Z – part tolerance

K – molded part

Calculation of cavity and core depth dimensions of injection molds

Calculation of cavity depth dimensions

In the calculation of the cavity depth size, the size should be selected smaller, leaving room for deepening, to adjust the depth of the mold.

C = (C1 + C1Q – 0.75Z) + K

C – the actual size of the depth of the cavity

C1 – the corresponding upper size of the part

Q – shrinkage rate

Z – tolerance of the part

K – molded part

Calculation of core height

When calculating the height of the core, the size should be chosen larger to allow room for shortening to adjust the length of the core.

D=(D1+D1Q+0.75Z)-K

D – the actual size of the height of the core

D1 – the corresponding lower limit size of the part

Q – shrinkage rate

Z – tolerance of the part

K – molded part

Selection of mould cavities and core steel

According to the type of plastic injection molding, the shape of the product, dimensional accuracy, product appearance quality and use requirements, quantity, taking into account the cutting, polishing, welding, etching, deformation, wear resistance and other properties of the plastic materials, while taking into account the economy and the manufacturing conditions of the mold and processing methods, in order to choose different types of steel.

For the molding of transparent plastic products mold

The cavity and core are required to choose a high mirror polishing performance of high-grade imported mold steel, such as 718 (P20 + Ni class), NAK80 (P21 class), S136 (420 class), H13 class steel, etc., of which 718, NAK80 for the pre-hardened state, without heat treatment; S136 and H13 class steel are annealed state.

The hardness is generally HB160-200. The hardness of S136 is generally HRC40-50, and the hardness of H13 is generally HRC45-55 (can be determined according to the specific grade).

Peal for the product appearance quality requirements

Long life, mass production of moulds, the mould cavity and parts with mould steel selection as follows.

a) Cavities need to use high-grade imported steel with high mirror polishing performance, such as 718 (P20 + Ni class), NAK80 (P21 class), etc., are pre-hardened state, no need for heat treatment.

b) The cores can be made of low-grade imported P20 or P20+Ni steel, such as 618, 738, 2738, 638, 718, etc., all in pre-hardened condition; for moulds with small production quantities, domestic plastic mould steel or S50C, S55C and other imported high-quality carbon steel can also be used.

For the appearance of the product quality requirements of the general mold

The mould cavities and parts of the mould steel are selected as follows.

a) Small, precision mould cavities and cores are used in the middle-grade imported P20 or P20 + Ni type steel.

b) Medium and large moulds, the injection moulded plastic no special requirements for steel, the cavity can choose low-grade imported P20 or P20 + Ni class steel; core can choose low-grade imported P20 class steel or imported high-quality carbon steel S50C, S55C, etc., can also choose domestic plastic mould steel.

c) For etched cavities, when etched pear pattern should strive to avoid the use of P20 + Ni class 2738 (738) grade.

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