ETPL 1100 Chapter 11

1. ETPL 1100 Chapter 11 Extrusion and Blow Molding

2. Introduction • Extrusion derived from Latin Word “extrudere” • Ex – out • Trudere – to push • Central operation in plastic processing • Extruder’s job to melt, mix and pressurize system

3. Extruder Equipment

5. Extruder Equipment • Parts of machine • Feed hopper • Feed throat • Motor • Gear Box • Screw • Barrel • Heating/Cooling System • Control System

6. Extruder Equipment • Twin Screw • Co rotating • Counter rotating • Extruders are measured by screw diameter • Screws are characterized by Length/diameter ratio • Three sections to a screw • Feed • Transition • Metering

11. Extruder Equipment • Channel Depth is greatest in the feed section, permitting the screw to draw pellets into the machine. • The channel depth begins decreasing at the start of the transition section and continues decreasing until the metering section. • The transition section is where the pellets are compressed and melted and the pressure for the system is generated. • The metering section remains at a constant channel depth. The constant depth allows for a consistent output.

12. Extruder Equipment • Melting is accomplished in the transition section. • 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. • 75% of energy required comes form compression, 25% comes from barrel heaters. • After the extruder, the melted plastic goes through a die to form a specified shape

13. 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.

16. 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.

20. 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

22. 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

26. 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.

33. Blow Molding

34. 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

35. 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

37. Two basic blow molding methods • Injection blowing • Extrusion blowing • Major difference between the two is the way the hollow tube “parison” is produced

38. 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

41. 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.

44. 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

45. 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