MATERIALS USED IN MOULD MANUFACTURING

1) Mild Steel (C-45)

This steel is relatively cheap. The steel is soft and cannot be fully hardened. It is extensively used to manufacture plates like top plate, bottom plate, ejector plate, core and cavity housings, spacers etc. It has high tensile strength. This steel has good shock resisting and machining properties.

2) Case hardening Steel (17MnCr95)

Initially these steels do not have sufficient carbon content essential for heat treatment process. Carbon is induced into the outer layer of the steel to convert into high carbon steels with carbon content ranging from 0.9 to 1.2%. When heat-treated achieve a hardness up to 50 – 55 HRC on the surface and retains a soft tough core. This is used for finger cam, guide pillars, guide bushes, ejector guide pillars and bushes and push back pins, as they require adequate strength as well as wear and resistance properties.

Composition:
Carbon-0.17%, Silicon-0.25%, Manganese- 1.25%, Chromium-1.15%, Sulphur-0.035%


3) Hot Die Steel (T35Cr5MoV1)

This steel has alloying element like carbon, which increases strength, elasticity and hardness. Chromium improves corrosion resistance, toughness and hardenability. Molybdenum improves creep strength and vanadium, which oxidizes and promotes fine-grained structures which is very helpful in manufacturing core and cavity of the mould. This material is hardened up to 48 – 50 HRC. This type of steel is used for core and cavity inserts, core and cavity sub inserts and side core pins.

Composition:
Carbon-0.37%, Silicon-0.25%, Molybdenum-1.2%, Vanadium- 1.0%, Chromium-5.0%

4. Oil Hardened Non Shrinkable Steel (T110W2CR5)

These steels tend to have good hardening properties and have less dimensional changes during heat treatment. They are relatively inexpensive, readily available, have good machinability, good resistance to decarburization and have a high carbon content to provide good wear resistance. The depth of hardness is homogeneous. This material is hardened up to 48 – 50 HRC. This steel is used for guide rails, wear plates, wedges, side core holder, sprue and sprue puller bushes.

Composition:
Carbon-0.95%, Silicon-0.35%, Tungsten-0.5%, Manganese- 1.20%, Chromium-0.5%

GENERAL CONSIDERATIONS OF MOULD DESIGN

In the design of injection mold following machine specifications are to be considered. As frequently an existing machine or a certain machine size poses an important limitation, to which the design engineer has to submit.

1. Injection pressure:

This is the pressure by which thermo-plastic material is injected into the cavity.

2. Shot capacity:

The amount of melt in grams, which can be conveyed into the mold with one stroke of the screw.

3. Plasticizing rate:

This is the maximum amount of material that may be plasticized to an acceptable melt quality per unit time. It is expressed in terms of weight of polystyrene plasticized per hour.

4. Clamping force:

Clamping force is the force required to keep the mold halves together when injection takes place. Clamping force machine should be more than clamping tonnage required of the mold to prevent escape of the melt at parting surface. If the clamping force is insufficient, “flash” occurs at the mold joint or interface of the mold.

5. Distance between tie the bars:

The tool length and width should be within the distance between the tie bars to mount the tool easily without constraints. This decides whether the tool can be accommodated in the machine or not.

6. Max & Min daylight:

The minimum ‘day light’ is the distance between the mold platens when tool is in closed condition. The maximum daylight is the distance between the die platens when the tool is in open condition. The mold height is required to be larger than the minimum daylight and less than the maximum day light.

7. Ejection stroke:

The depth of the component decides the ejection stroke. Ejection stroke is indicative of the maximum ejection movement available.