ITP 330 Chapter 7- Polymer Processing & Converting

Extrusion blow molding

combines extrusion and blowing
- parting lines
- no preform, but parision is needed.
- moderate in cost
- PE, PP, PVC

Primary Processes

Extrusion
Injection molding
Blow molding
Rotational molding
Thermoforming

Polymer Extruder

Extrudes polymer and creates films.
Screw barrel functions
1.) pumping
2.) heating
3.) mixing
4.) pressurizing
5.) extruding

Single Screw Extruder

-Has one screw in the barrel
-Generate higher pressure and output than twin screw extruder
-Used for converting plastic into final products such as film or bottles

Twin Screw Extruder

two screws, used for mixing and compounding

Profile extrustion

shapes can be extruded by using a die at the exit (straws)

Cast Film

High speed and output. Used for printed package film, lamination web, and stretch wrap. (MD)

Uniaxial Orientation

Polymer molecule structure are in one direction, strength increases= fibers, rope, string, straps

Biaxial Orientation

Oriented in two directions. (2-5x stronger)
Involves stretching the film in both the machine and cross-machine directions.
Increased crystallinity.

Blown film extrusion

Extruding a tube of molten thermoplastic and continuously inflating it to several times the initial diameter to form a thin tubular product that can be used directly, or slit to form a flat film

Coextrusion

Both cast & blown film extrusion dies can be designed to be fed by more than one extruder. (combines performance advantage of 2 dissimilar materials)

Injection Molding

The process of forming a material by heating, forcing through barrel, and into a mold.

makes tubs, cups, caps. (dot at bottom)

Sprue

main channel molten plastic enters the mold

Runners

Distributes molten plastic to various cavities.

Gate

Small opening where molten plastic enters the cavities.

Pros of blow molding

- high production rates
- design flexibility
- repeatability
- can process a wide range of materials
- relatively low labor
- little to no finishing of parts
- minimum scrap losses

cons of injection molding

- high initial equipment investment
- high startup and running costs possible
- part must be designed for effective molding
- accurate cost prediction for molding job is difficult

Injection Blow Molding

combines injection molding and blow molding
- preform is injection molded then blown
- better control over material distibution
- 2 mold required (preform and blow mold)

Injection Stretch Blow Molding

Used for bottles with paneling and ridges
- uses a stretch rod

Thermoforming

Plastic sheets are heated over an open mold to a working temperature. Once workable, a vacuum is applied to the mold, forcing the plastic sheet to take the shape of the mold.
-tubs
-pressure assist thermoform

Rotational Molding

A closed mold is filled with a predetermined amount of plastic. The mold is heated, rotated, and then cooled to create a hollow plastic object with uniform wall thickness.
- big objects
- trash cans

Recycling

Recycling saves 2x as much energy as burning in an incinerator.