Técnicas e instrumentos de recolección de datos, validez y confiabilidad

There are three ways of filling the mould,

The first is by having a screw in full forward position. The screw starts to rotate after the mould is closed and fills the mould; the pressure is to the injection ram to apply the final injection pressure.

Secondly is to fill the previously plasticized material. The screw acting as a ram comes forward slowly and when it reaches its full forward position it starts to turn until the mould is filled. Then a small cushion is maintained and hydraulic pressure is applied to complete the cycle

Lastly, is to start with a full barrel and continue until the point where the screw comes forward and a small cushion is left behind. This method allows for a rapid filling cycle.

METHODS AND REMIDES FOR RECTIFICATION OF COMPONENT DEFECTS:

Injection moulding faults may appear during starting a new model, when

changing to a new material, or during the regular operation of mould. Major moulding faults are short shot, parts flashing, brittleness, and discolourisation, degrading and burning of material etc…

Let’s discuss some them

1. Short shot:

A short shot is the case where, usually the material solidifies before it is completely fills the cavity and also due to insufficient effective pressure on the material in the cavity.

The short shots can be over come by increasing the temperature and pressure of the material and also by increasing the nozzle-sprue-runner gate system to the required effectiveness. Other times it may require improvements in the mould and part design.

Apart from these short shot can be rectified by,

Reference books for further reading: Injection molds by R.G.W pye, injection molding handbook, 108 proven designs,

SUBJECT: MOULD THEORY

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DIPLOMA IN TOOL AND DIE MAKING

Checking the material in the hopper.

Checking for blockages in the hopper’s throat.

Increase feed if it low

Increasing the plasticizing capacity of the machine.

Check for loss of injection pressure during cycle Providing uninterrupted flow of material in the cavity/s

2.Parts flashing:

When the parting surface in the mould is not flat, there is an unbalanced force at certain instances. The plastic material when, under pressure with in the impression will exert a force which tends to open the mould in the lateral direction. If this happens some flashings may appear on the angled face.

Parts flashings may be over come by Increasing the gate system if required.

Equalizing the filling rate.

Increasing the wall thickness. Thin walls may cause premature freezing.

Check for loss of pressure in cylinder.

Uninterrupted filling.

3.Brittlens:

Due to degradation of the material during moulding the component may loose its property of with standing external force and impact. This disorder is known as

brittleness.

Brittleness can be over come by,

If the material is thermally degrading then lower cylinder and nozzle temperature.

Increase injection speed, pressure, and forward time.

Increase mould temperature if low.

Reduce the screw speed.

4. Material discolourisation:

Material in the mould often looses its original colour due to burning and degradation that is caused by excessive temperatures, material hang-ups in the system. The discolourisation of component be solved by,

Providing a clean nozzle.

Checking for colorant and additives degrading Proper functioning of thermo couple

Proper working of temperature control

Reference books for further reading: Injection molds by R.G.W pye, injection molding handbook, 108 proven designs,

SUBJECT: MOULD THEORY

page 54 of 96

DIPLOMA IN TOOL AND DIE MAKING

Proper removal of lubricant and unwanted oil from mould Providing better mould lubricants.

5. Surface defects:

Surface defects are faults like splay marks caused by wetness of hygroscopic materials and water condensed on no hydroscopic pellets, contamination etc…

Splay marks and surface disturbance at the gate are probably the hardest moulding faults to over come. Some of the remedies are,

I) If the moulding condition do not help it is usually necessary to change the gating system and mould temperature control.

Heating at the gates with a propane torch will determine the advisability of adding a heat sink or a heating cartridge in the gating area.

Localizing gate heating.

5. Wrapage and Excessive Shrinkage:

Wrapage and excessive shrinkage are the faults caused by the faulty design of the part, misallocation of the gate, irregular-moulding conditions. These problems could de over come by

Increasing the cycle time.

Increasing the pressure without excessive packing.

Increasing the injection boost without excessive packing.

Keeping the packing to the minimum for wrapage Keeping the packing to the maximum for shrinkage.

Increasing the wall thickness.

Providing sufficient venting.

Adding ribs and fillets.

6. Poor welds (flow marks):

Flow marks the results of welding cooling material around the projection, such as pins, which show up as broken parts, and their visibility depends upon the material, colour and surface. Poor welds may be prevented by venting at the weld, adding a run-off from the weld section, increasing the wall thickness of the part at the weld,

Reference books for further reading: Injection molds by R.G.W pye, injection molding handbook, 108 proven designs,

SUBJECT: MOULD THEORY

page 55 of 96

DIPLOMA IN TOOL AND DIE MAKING

lubricating or local heating the mould