ETPL 1100 Chapter 11

Extrusion and Blow Molding

Introduction

a. Extrusion derived from Latin Word “extrudere”

i. Ex – out

ii. Trudere – to push

b. Central operation in plastic processing

c. Extruder’s job to melt, mix and pressurize system

Extruder Equipment

Extruder Equipment

a. Parts of machine i. Feed hopperii. Feed throatiii. Motoriv. Gear Boxv. Screwvi. Barrelvii. Heating/Cooling Systemviii. Control System

Extruder Equipment

b. Twin Screwi. Co rotating

ii. Counter rotating

c. Extruders are measured by screw diameter

d. Screws are characterized by Length/diameter ratio

i. Three sections to a screwi. Feed

ii. Transition

iii. Metering

Extruder Equipment

f. Channel Depth is greatest in the feed section, permitting the screw to draw pellets into the machine.

g. The channel depth begins decreasing at the start of the transition section and continues decreasing until the metering section.

h. The transition section is where the pellets are compressed and melted and the pressure for the system is generated.

i. The metering section remains at a constant channel depth. The constant depth allows for a consistent output.

Extruder Equipment

g. Melting is accomplished in the transition section.

h. Heat is transferred from the barrel walls to the plastic material, Also energy is transferred from the motor to the material by compression of the screw.

i. 75% of energy required comes form compression, 25% comes from barrel heaters.

j. After the extruder, the melted plastic goes through a die to form a specified shape

Compounding

• The process of blending basic plastics with Plasticizers, fillers, colorants, etc.

• Most compounders use twin screw extruders – Co Rotating

– Material is forced through the gap between the screws (nip), Where intensive mixing takes place. Requires less power and creates less degradation.

– Counter Rotating– Material is transferred from one screw to the other at the point

of intermeshing, creating a long figure 8 pattern which yields more mixing. Require more power and causes more degradation.

Major Types of Extrusion Products

– Profile• Extruding the plastic through a die, then cooling,

makes the final shape of the product. • The die shape forms the final shape of the

product. • Cooling comes from air jets, water troughs, water

sprays or cooling sleeves.

Major Types of Extrusion Products

– Pipe• Tubular Pipes are formed by the outside

dimension of the die orifice, and on the inside by a mandrel.

• The mandrel is held in place by thin pieces of metal called spider

Major Types of Extrusion Products

– Sheet Extrusion• Materials thicker than 0.25mm• Produces stock for most thermoforming

operations• The sheet is formed by extruding plastics

through dies with long horizontal slots• Two major types of die

– T shape– Coat Hanger

• The extruded sheet goes through a set of roller to provide the desired surface finish and texture and to accurately size the thickness

Major Types of Extrusion Products

– Film Extrusion• Materials thinner than 0.25mm• Similar to sheet extrusion• Blow Film

– The film is produced by forcing molten material through a die and around a mandrel.

– It leaves the orifice in tube form. – The tube is expanded into a bubble by blowing air

through the center of the mandrel until the desired film thickness is reached.

– The film is cooled by air blowing on the outside.

Blow Molding

Blow Molding

• Technique adopted from the glass industry

• Developed for thermoplastics in the late 1950’s

• Used to make one piece containers

• Used to produce containers, toys, packaging units, automobile parts and appliance housings

Principles of blow molding

• A hollow tube “parison” is placed in a female mold and the mold is closed.

• The parison is then forced, blown, by air pressure against the walls of the mold

• After a cooling cycle, the mold opens and ejects the finished product

Two basic blow molding methods

• Injection blowing

• Extrusion blowing

• Major difference between the two is the way the hollow tube “parison” is produced

Injection blow molding

• Two molds are required• One for perform• One for blowing

• First the perform is made in a mold, then it is transferred to blowing mold, where the perform is expanded

• Gives better shaping, but is more expensive

Extrusion blow molding

• Procedure• A hot tubular parison is extruded continuously• The mold halves close sealing off the open end of the

parison• Air is then injected and the hot parison expands against

the mold walls• After cooling the product is ejected

• Controlling wall thickness is a major disadvantage

• Many different methods of forming blow-molded products have been developed and each has an advantage in molding a given product.

Advantages of Blow molding

– Most thermoplastics and many thermosets can be used

– Die costs are lower the injection molding– Extruders compound and blend materials

well– Extruders plasticates material efficiently– Extruders are basic to many molding

processes– Extrudates may be any practical length

Disadvantages of blow molding

– Costly secondary operations are sometimes needed

– Machine costs are high– Purging and trimming produces waste– Limited shapes and die configurations are

available– Screw Design must match material melt and

flow characteristics