August 2010 Archives

An embedded split structure is used very frequently as the cavity and core structure. This splitting has the following purposes.

1.A part that is difficult to form by machining is divided as a different part, thereby making its machining easy. The machining cost can be reduced and it is possible to increase the machining quality.
2.It is possible to provide the air vent function of discharging air or gas from the parted surface.
3.A part that can easily break is divided in advance so that it can be replaced easily.

The following patterns can be considered as the patterns for embedded splitting.

1.Round core pin shape splitting
2.Square core shape splitting
3.Odd shape splitting

Among these, a little know-how is described here related to assembling when the splitting is made in the shape of a square core.

The following figure shows an example of splitting a square core shape. In order to insert a square core, it is necessary to machine a square shaped hole in the main core.
The commonly used machining methods are - wire cutting electric discharge machining, shape carving electric discharge machining, or further dividing the main core and carrying out grinding operations.

Whatever be the machining method used, a very small rounding will remain in the corner part. The size of rounding is about R0.15mm to R0.05mm. If even such a fine rounding remains, if assembly is attempted with the corner of the core having a sharp edge, the assembling does not progress smoothly. If hitting with a copper hammer is done by force the assembly, the edge of the corner is likely to get crushed in an unstable manner.

In order to carry out smooth assembling, fine chamfering is done of the sharp edges of the core by machining or by hand finishing so as to make the edges match with the corner radius R of the square hole. If this is done, the assembling can be done smoothly. Since the magnitudes of R and C are very slightly different, although a clearance may be generated, if the clearance is about 0.03mm, it can be said to be too small for any possibility of the molten plastic flowing in and creating burrs. On the contrary, it may also have the effect of discharging air or gas.

However, in the case of special environments such as a material like PPS having very good fluidity, it is recommended that the management of the clearance is made based on prototyping data.

The runner is a path for making the molten plastic flow from the sprue to the molded product. The cross-sectional shape of the runner is selected depending on the size of the molded product, the type of plastic, the estimated molding conditions, etc.
The basic standards for selecting the runner cross-sectional shape are explained here.

The figure below shows some typical runner cross-sectional shapes.

The runner is selected from the following three types.
1)The type carved on the moving half
2)The type carved on the fixed half
3)The type carved on both the fixed half and the moving half.

The selection among these is made appropriately based on the restrictions imposed by the shape of the molded product, the mold parting position, etc.

The symbols in the figure have these meanings: : Most suitable, : Suitable, : Not suitable

The most important role of the runner is to make the molten plastic flow in a condition in which no pressure loss is present. An unnecessarily large volume runner has the bad effects of increasing the scrap, worsening the material cost, lengthening the molding cycle, increasing the waste material, etc.

An index of the efficiency of a runner is the diameter and area of the inscribed circle in the cross-sectional shape. If the area of the inscribed circle increases, the area for hot plastic to flow becomes wide, and hence the molten plastic can flow easily.

Consequently, a runner with a circular cross section is the most ideal one. However, since it will be necessary to carve the runner in both the fixed side and the movable side, the cost of manufacturing the mold increases. In order to solve these problems, a rectangular cross-sectional shape runner or a runner with a semicircular cross-sectional shape that is made still deeper is used.

It is desirable to provide an angle at the side surface of the runner for separating from the mold. In addition, the inner surface of the runner is polished to a smooth surface using a rubber grinder or by carrying out lapping, thereby preventing pressure loss.