Question

nowi the desinm et Plasticati ouble Shoot diheront preblems dori

this question is from my subject " polymer processing design"" please give me answer of this .. .
the book which I am following is polymer processing principle and design by Donald G Baird

Answer 1

The extrusion process converts a solid plastic feed stock material into a molten viscous fluid, and then to a finished solid or flexible film product for practical use. Plasticating extruder components mainly are hopper, plasticizing extruder (extruder barrel and rotating screw), die assembly and a cooling assembly. Below are the problems which occur during processing the polymers via extrusion method and there remedies based on the knowledge of design of plasticating extruder components.

1) Die Swell :

Die swell is the most prominent polymer process defect. In die swelling the profile of extruded material grows in size, reflecting its tendency to return to its previously larger cross section in the extruder barrel immediately before being squeezed through the smaller die opening.

Die swell occurs because the sudden release of pressure causes the polymer chains to relax.

Remedies include :-

• Decreasing the extrusion rate by means of decreasing screw speed.
• By increasing the length of die end(land).

Land length: The length of the steel (of die) that runs parallel to the polymer flow. The land-length ratio is the ratio of the land length to the die gap (wall thickness).

• By improving design factors in order to increase drawdown ratio.

Drawdown: The size of the designed die dimensions relative to the final part dimensions. The dimensional ratio of the die to final part is used to offset the “swell” of the thermoplastic as it exits the die.

Following figure shows the effect of die geometry on the final cooled down extrude.

Required section Getrically square Correct die form to die gives barrel effect produce a geometrically correct shape

2) Bubbles in extrudate:-

Most prominent causes for bubbles in extrudate include-

• Moisture Absorption

Polymers have high moisture absorption.When the polymer pellets are heated, the absorbed moisture is also heated and turned into steam. It passes through the extruder and boils when the pressure is relieved at the die lips.The result is a pattern of long bubbles, and pits.

Remedies-

1. To remove moisture, the material must be pre-dry in a hot over before loading into the hopper.
2. Also a vent must be designed in extruder to remove trapped steam.
3. Dehymidifying dryers can be used.

• Trapped Air

If the starting material is powder rather than pallets then this problem is more prominent, also if the extruder barrel is short this defect may occur.

Remedies-

1. By avoiding over speed of extrusion
2. Incorporating vents and vacuum hoppers will eliminate trapped air and are essential for powdered raw material.

• Polymer Degradation

Overheating may produce degradation of the polymer material that is used. The degradation varies from one polymer to the other.This degradation might cause the polymer to produce gases that might become trapped inside the material.

Remedies-

1. Since this defect is mainly caused by overheating of polymers the most effective remedy will be to control heating temperatures of polymers. And designing with needed temperature controls.

• Depolymerization of the Polymer

It is the process of converting a polymer into a monomer or a mixture of monomers. All polymers depolymerize at high temperatures driven by increase in entropy. Some of the products of the depolymerization could be volatile monomers,
that could be entrapped inside the material.

Remedies-

1. Since this defect is mainly caused by overheating of polymers the most effective remedy will be to control heating temperatures of polymers. And designing with needed temperature controls.

3) Blistering defect:-

Blistering may be caused by water either absorbed into the granules or lying on the surface. The amount of steam produced to give blisters will depend on:

• the amount of water present
• the extrusion temperature.

Polymers are hygroscopic and /or require a high process temperature are normally carefully dried and stored in sealed tins.

Reasons for blistering are:-

1. SCREW ROTATION TOO FAST

A screw rotation speed that is too high will tend to ``whip'' air into the molten plastic. This excessive air may not be drawn out of the material during the molding process and pockets of the air may be forced to the surface of the molded part, forming blisters

2.LOW BACK PRESSURE

The back pressure setting controls the density of the melt. A low setting results in a melt that is not dense enough to push out excessive gases.

Low back pressure can be controlled by designing the hopper as per required pressure.