Basic Concept for Mold Design

In mold design, note the following points, some of which also apply to product design.

Ⅰ. Avoid uneven wall thicknesses whenever possible.

Ⅱ. Avoid designs with sharp corners.

Ⅲ. Do not demand stricter tolerances than necessary.

The processing accuracy of a mold must be within one-third of the tolerances. Stricter tolerances will present problems that the cost of the mold will be higher. Keep in mind that metal and resin differ in various aspects.

Ⅳ. Decide on the basic wall thickness of the product and so on according to the fluidity of the material.

If you have any similar products, refer to their trial results. If you do not have any molding examples available, it is necessary to examine the flow distance from a product drawing and set the basic wall thickness, sprues, runners, gate shape or the number of gates, and so on from the material fluidity data in Section 2.4.3. After the completion of the mold, check it by performing a trial and modify it, if necessary. The provision of ribs to compensate for the fluidity is also effective.

Ⅴ. Gate

1 Gate types and features

TORELINA™ is excellent in terms of fluidity, and can use the gates listed in Table. 4.1. In general, TORELINA™ is not suitable for submarine gates. If a submarine gate is required for design reasons, ensure that the angle of approach is large (30° to 35°).

Table. 4.1 Representative gate types and features

Gate type Features
Direct gate A sprue serves as a gate, so the fluidity is high, and it is easy to attain the required holding pressure. Gate processing is cumbersome, however.
Side gate Standard gate shape. Has a high degree of freedom in the gate position. An overlap gate has a similar structure.
Tab gate By providing tabs on some portions of the molded product, jetting and other appearance defects can be prevented.
Film gate/
fan gate
Can control the resin flow direction, so that warpage and deformation can be suppressed. Suitable for simple shapes such as flat sheets.
Disk gate Suitable for ring-shaped, cylindrical, and other molded products that require circularity.
Ring gate System in which ring-shaped secondary runners are provided for long, cylindrical molded products.
Controls the flow direction so that warpage and deformation can be prevented.
Submarine (tunnel) gate Method of providing long, conic gates that pass under the mold parting line from the runner end.
Enables automatic gate cutting when the product is extracted.
Pinpoint gate Enables automatic gate cutting when the product is extracted. Has a small gate diameter, so that the gate mark is not conspicuous. Lacks fluidity during molding, on the other hand. As the number of gates increases, prone to develop fragile portions such as at welds.

2 Gate size

If a gate is too small, the resistance at the gate will be high, which may result in defects due to increases in the shear heating of the resin, as well as problems (sink marks and voids) attributable to reductions in the gate seal time and so on. Depending on the wall thickness of the molded product, it is considered preferable for the gate diameter be at least 0.8, even for pinpoint gates.

3 Position

Decide on the position by keeping the following points in mind:

(1) Well-balanced position

For a multi-cavity mold, use CAE analysis and so on to optimize the mold, ensuring that the runner lengths (flow distances) are equal, so that each cavity is filled with molten resin from the sprue at as close to the same time as possible. It is preferable to create a layout that is well-balanced on the whole by considering the parting line (PL) and the slide core structure.

(2) For a product with an uneven wall thickness, provide a gate in a thick wall portion.

At a thick wall portion of a molded product, the resin is cooled quickly, so that the melt viscosity increases. Thus, if a gate is provided at a thin wall portion, the result will be insufficient fluidity. Also, solidification starts with the thin wall portion of the molded product, so that a sufficient holding pressure cannot be obtained, resulting in sink marks and voids in the thick wall portions.

(3) Provide a gate at a location where the appearance and strength requirements are not strict.

Near a gate, appearance defects such as silver streaks are likely to occur, and the flow pattern is likely to be turbulent. Thus, for glass fiber reinforced grades, for example, the portion near the gate may be more fragile than other portions. Also, excess molding pressure is applied to the gate, so that the residual strain may be large. Residual strain will result in poorer mechanical properties and so on. Thus, avoid providing a gate at any portion to which the principal stress carried by the product is applied.

(4) Gate position where appearance and strength requirements are not strict but where welds are expected to occur

At an opening of a molded product and for a multipoint gate, welds may occur. (For details, refer to Mechanical properties in the technical document.) Welds are more fragile than non-welds. Thus, provide a gate in such a way that welds are not formed at a portion to which the principal stress of the product is applied.

(5) Position at which no jetting occurs

For a standard side gate, for example, appearance defects like resin flow patterns, resembling a slithering snake (jetting), may occur immediately after the gate if, for example, the slug well in the runner is insufficient. To prevent jetting, provide a gate position in such a way that the flow direction of the molten resin changes immediately after the gate (the resin becomes turbulent).

(6) Shape that facilitates post-processing

From the perspective of productivity, it is preferable to provide a gate at a portion at which gate cutting is facilitated. By applying a pinpoint gate that enables automatic gate cutting when the product is extracted, productivity can be increased.

Ⅵ. Gas venting (vent)

Filling the cavity with resin requires that the air that originally occupies the cavity and the gas generated by the molten resin be discharged and replaced by the molten resin. If the injection speed is high, and discharge cannot catch up, the trapped air will be adiabatically compressed and become hot, causing burns and short shots. For this reason, the mold requires gas venting (vent) to discharge air and gas according to the resin filling speed. PPS resin tends to generate more gas than other resins when melted because it is molded at high temperatures. Thus, provide gas venting of about 5/1000 mm on the flow end and the runner of the product. If the gas venting is too great, this will lead to the occurrence of flash. It is necessary to decide on the size of the gas venting by considering an appropriate balance with the flash.

Ⅶ. Draft taper

To make it easy to extract the molded product from the mold, the mold must be provided with a draft taper. The mold shrinkage differs with the TORELINA™ grade, but as a rough guide, the draft taper should be about 2 to 3°. If only a small draft taper of 1° or less is permitted for design reasons, polish the mold sufficiently to reduce the mold release resistance.

Ⅷ. Material

The use of a steel material that has both abrasion resistance and corrosion resistance is recommended. At the gate, at the flow end, and at a location where the flow direction changes abruptly, in particular, the mold will wear considerably. Carefully select the steel to be used, and note whether it has been quenched. You are also advised to take appropriate measures, such as adopting a nesting system so that replacement is possible.
Table. 4.2 gives the relationship between injection molding mold materials and their properties. For the molding of TORELINA™, the use of SKD11, SKD61, SUS420, and other steels that have been quenched are recommended.

Table. 4.2  Mold materials and their properties

Mold steel Strength Abrasion
Processability Surface
SKD11 ◎ to ○ ○ to △
SKD61 ◎ to ○ ◎ to ○
SUS420 ◎ to ○ ◎ to ○ ◎ to ○
SUS630 ○ to △ ○ to △ △ to ×
SCM440 △ to × ○ to △
S55C × × × ×

◎High ⇔×Low

Ⅸ. Temperature control

For temperature control with a cartridge heater, use a heater with a heat capacity capable of increasing the temperature to 150℃ or greater. Place heaters so that there are no remarkable temperature variations between the different regions of the product. The stationary side of the mold comes in contact with the cylinder of the molding machine so that, as a result of heat transfer, it is likely to be hotter than the moving side. It may be necessary to place more heaters on the moving side to compensate. Nested portions, pins, and so on are likely to store heat. If it is necessary to consider cooling them, use a temperature controller based on oil or pressurized hot water. It is essential that heat loss due to transfer to the molding machine be prevented, using heat-insulating boards, so as to suppress mold temperature fluctuations.