April 2010 Archives

Among the alloy tool steels, the so called hot work die steel (JIS name is SKD61) is being used as the material for the cavity or the core. It is also considered precious as a material for thin core pins because it has a relatively high hardness, withstands wear, and also has relatively high resistance to shock.

In order to bring out its excellent characteristics, SKD61 needs quenching, and after quenching, it is tempered to stabilize the metallographic texture and to improve its toughness. However, since it is known that depending on the conditions of tempering SKD61 causes change of dimensions and reduction of hardness, unforeseen failures can result if the tempering is done without understanding this trend. Therefore, in this course, the tempering characteristics of SKD61 are explained below.

(1)Reduction in the hardness due to tempering

The hardness of SKD61 decreases in various cases by tempering after it is quenched. For example, if a material quenched at 1030°C is tempered, the following changes occur depending on the tempering temperature. (The following data is an actual example, and there will be changes depending on the size of the work and the plate thickness.)

Example

Therefore, since the hardness decreases suddenly if the tempering is done at an unnecessarily high temperature, care should be taken when resistance to wear is necessary.

(2)Changes in the dimensions due to tempering

The external dimensions of the work change, when SKD61 is tempered after it is quenched. Depending on the tempering temperature, the external dimensions can become larger or smaller. If additional machining is not to be done after tempering, it is necessary to carry out the machining work before quenching taking into considerations the change in the dimensions after tempering.

Example: Quenching at 1030°C

Mold base is a comprehensive name used for the parts for containing the cavity for plastic injection mold, and also has the role of directly installing the mold to the plastic injection molding machine.

Mold base is a set of parts that constitute the outer periphery part of a plastic injection mold, and is constituted mainly from the following parts.

Although previously the constituent parts of a mold base were all designed and manufactured as required, standard mold bases have recently come into wider use and are being used all over the world. In the case of large sized molds or small sized molds, even at present, they are being designed individually for each mold.

Although the standards for mold bases have been prepared in Japan in metric units, they are still being prepared in inch units in the U.S.A. In Europe, as in Japan, it is common to prepare them in metric units.

The following two types of structures are the most commonly used ones for the structure of a mold base.

The selection between these two structures is determined by the method of the gate used. When adopting a pin point gate structure, always the structure (2) is used. In the case of a side gate or a tunnel gate, the structure (1) is used normally.

The material for the constituent parts of the mold base is generally the carbon steel for machine construction (S55C, 220C, etc.,) and is used most often in the non-hardened condition. In special applications, pre-hardened steel, or stainless steel, or an aluminum alloy is used some times. A mold based used in combination with accessory parts like guide pins, guide bushes, return pins, etc.

Hot runners are a method of carrying out molding without generating scraps by heating and melting the runner part during plastic injection molding. Various types of structures have been realized for hot runners such as the method of heating or injecting. The major hot runner structures and their characteristics are as follows.

Open gate structure:

On-Off Control structure:

Hot edge gate structure:

Valve gate structure:

As mass production is continued using molds for plastic injection molding, the gas components generated from the plastic, soot, or moisture in the atmosphere gets accumulated on the surfaces of the metal parts or in the gaps between the nested division parts thereby becoming the causes of molding defects. Therefore, it is necessary to dismantle and clean the mold periodically. The disassembling and cleaning of molds is generally done according to the following procedure.

In order to make maintenance easy, it is wise to incorporate techniques at the time of designing the mold. Some ideas for this are given below.