SELECTION OF TOOL STEEL IN INJECTION MOULDING

The selection of tool steel is very important for any tool upon which the development and economical operation of the process depends. Care should be taken especially while injection moulding plastics, which may affect the factors like mould life, quality, cost of mould and machine etc.

The steel used in the manufacture of mould varies depending on the applications. Proper material selection and proper combination of alloys in varying percentages are required for finished moulds.

Following properties are desired from the steels used in mold making

· Characteristics permitting economical machining

· Capacity for heat treatment without problems

· Ease of polishing

· High Wear resistance

· High Thermal conductivity

· Good Corrosion resistance

· Good Toughness

GENERAL CONSIDERATIONS OF MOULD DESIGN IN INJECTION MOULDING

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.

EJECTION SYSTEM in injection moulding

After the moulding has solidified and cooled down, it has to be removed from the mould. It would be ideal if gravity could separate the part from cavity or core after mould opening. The moulding is kept in place, however, by undercuts, adhesion and internal stresses and, therefore, has to be separated and removed from the mould by ejection system.

Ejection system is usually actuated mechanically by the opening stroke of the moulding machine. If this simple arrangement is not sufficient ejection can be performed pneumatically or hydraulically.

Ejection system is normally housed in the moving half of the mold. Mold opening causes the mechanically actuated ejector system to be moved towards the parting line thus ejecting the component.

Four factors should be considered in designing the ejection mechanism:

· Shape and geometry of the part.

· Type of material and wall thickness.

· Projected production volume.

· Component position relative to the parting line.

The basic ejection techniques are:

· Pin ejection.

· Blade ejection.

· Sleeve ejection.

· Valve ejection.

· Air ejection.

· Stripper plate ejection.

According to shape, material wall thickness, component position relative to parting plane and aesthetic considerations the pin ejection technique is chosen for the part ejection.

Factors to be considered while designing the cooling

1) Wall thickness of the component.

2) The active area of cooling channel.

3) Position of gate and location of runners.

4) Temperature of the mould.

5) Length of cooling hole.

6) Type of cooling.

FEED SYSTEM in injection moulding

It is necessary to provide a flow-way in an injection mould to connect the nozzle (of the injection machine) to the each impression. This flow-way is termed as feed system. Normally feed system comprises of

1) Sprue.

2) Runner.

3) Gate.

1) Sprue

The sprue is the channel along which the molten plastic first enters the mould. It delivers the melt from the nozzle to the runner system. The sprue is incorporated in a hardened steel bush which has a seat designed to provide a good seal with the nozzle.

The sprue has to demold easily and reliably and therefore has to be tapered. The taper is generally 2° in most cases.

2) Runner

The runner is the flow path by which the molten plastic travels from the sprue (i.e. the moulding machine) to the gates (i.e. the cavity). To prevent the runner freezing off prematurely, its surface area should be small so as to minimize heat transfer to the mould. However, the cross sectional area of the runner should be large so that it presents little resistance to the flow of the plastic but not so large that the cycle time needs to be extended to allow the runner to solidify for ejection.

The following factors must be considered while designing a runner system.

Cross section of runner.

Size of the runner.

Runner layout.

The different types of runner profiles widely used in mould are

1) Half round.

2) Fully round.

3) Rectangular.

4) Hexagonal.

5) Trapezoidal.

6) Modified trapezoidal.

3) Gate

Gate is the small orifice which connects the runner to the cavity. It has a number of functions. Firstly, it provides a convenient weak link by which the moulding can be broken off from the runner system. In some moulds the degating may be automatic when the mould opens. The gate also acts like a valve in that it allows molten plastic to fill the mould but being small it usually freezes off first. The cavity is thus sealed off from the runner system which prevents material being sucked out of the cavity during screw-back.

Gate locations should ensure following conditions:

· Ensure a balanced flow (rapid and uniform filling) in the cavity so that certain areas of the part are not over packed.

· Ensure mold fills under realistic temperatures and pressures.

· Minimize weld lines as much as possible, or position them in non critical areas.

· Prevent "jetting" by positioning the gate so that the material flow is smooth and uniform.

· Avoid air entrapment.

· Gate into the thickest section and direct material flow.

Gates are classified in two categories based on the method of de-gating.

1. Manually Trimmed Gates: Examples are Direct or sprue Gate, Ring Gate, Edge Gate, Fan Gate, etc.
2. Automatically Trimmed Gates: Examples are Pinpoint Gate, Submarine or Tunnel Gate, etc.
   

PARTING SURFACE

The parting surface of a mould are those surfaces of both mould plates adjacent to the impressions which butt together to form a seal and prevent the loss of plastic material from the impression.

The various factors to be considered while selecting the parting surface are:

Shape of the Components.

Type of Mould.

Method of Ejection.

Method of Manufacture.

Location and Type of Gate.

Aesthetics of moulding.

In general the parting surface is classified as,

Flat parting surface.

Non-flat parting surface.

Flat parting surface

Parting surface having only one plane is termed as Flat parting surface. The position of the parting surface will therefore be at the top of the moulding.

Non-flat parting surface

Parting line which lies on a non-planar or curved surface termed as non-flat parting surface.

DESIGN CONSIDERATIONS IN INJECTION MOULD

The critical items to be considered while designing a injection mould tool are

Shrinkage.

Draft angle.

Selection of Parting Surface.

Number of Cavities.

Feed System.

Cooling System.

Ejection System.

Venting.