The injection molding process can be broken into three phases, (1)Filling phase: During the filling phase plastic is injected into the cavity is just filled. When designing a part to be made by the injection molding process, the most important phase to as plastics flows into cavity, in contact with the mold wall freezes quickly. This creates a frozen layer of plastic between the mold and molten plastic. The flow fronts come into contact with the mold and freezes. The molecules in the frozen layer are therefore not highly oriented, and once frozen, the orientation will not change. The Frozen layer gains heat as more molten plastic floes through the cavity and loses heat to the mold. When the frozen layer reaches a certain thickness, equilibrium is reached. This normally happens early in the injection molding process, after a few tenths of seconds. (2) Pressurization phase, the pressurization phase begins after the cavity has just filled. Although all flow paths should be filled by this stage, the edge and corners of the cavity may not contain plastic. To completely fill out the geometry, extra plastic is pushed into the cavity during the pressurization phase. The polymer injection location I indicated by the yellow cone and the plastic is green. Some time the confidence of fill result does not predict a short shot, but still states that a good quality part cannot be molded. This is because the condition at the end of the filling phase is not suitable for the part to be adequately packed during the pressurization phase.(3) Compensating phases:Plastic have a high volumetric shrinkage, around 25% from average melt temperature to solid. Therefore more material must be injected into the cavity to compensate for the plastic shrinkage as it cools. This is the compensating phase.

4. Mould Die Design

When purchasing plastics mould steels, one looks, for Good machining properties, Good polish ability, Good photo – etching properties, Good spark erosion properties, Safe, uncomplicated heat treatment, Minimum inclusion level, Consistently high quality, Technical assistance and advice in tool in manufacturing and application.. Essential characteristic of injection mold steel like Uniform structure and freedom from internal, defects, Machinability Maximum freedom from distortion during heat treatment, Weld ability, Polish ability, Wear resistance and toughness, Steels for mould fall under two categories, Structural section of the mould, Medium carbon silicon killed forging quality steel with approximately 25% greater tensile strength then typical low carbon steel. It is annealed to approximately 165BHN. It is stress relieved for minimum deformation during machining.AISI 4130 type alloy steel is supplied pre-heated to approximately 300BHN to withstand the peening effect of flash. It has durability for heavy construction loads and long production runs. Component material is selected as NYLON-66, Nylon 66 is a polyamide made from adipic acid and hexamethylenediamine by polycondensation,NYLON-66 has Good toughness, High tensile strength, High elasticity, Good heat resistance, Excellent wear resistance, Wrinkle proof and excellent chemical resistance.Shrinkage is inherent in the injection moulding process. Shrinkage occurs because the density of polymer varies from the processing temperature to the ambient temperature. The shrinkage factor will depends on Plastic material, Processing condition, Product design, Mould design. Shrinkage allowance of Nylon 66 is 0.5% considered. The dimensions of the cam bush have been modified with considering shrinkage allowance as shown in Table 1. This total dimensions are incorporated in the mould designing of core and cavity plate.

Table.2 Dimensions of Cam bush with shrinkage allowance

S.

No Component Details Actual dimension in mm 0.5% shrinkage allowance in mm Total dimension in mm

1 Diameter 1 65 0.325 65.325

2 Diameter 2 35 0.175 35.175

3 Diameter 3 28 0.14 28.14

4 Diameter 4 20 0.10 20.10

5 Diameter 5 7 0.035 7.035