BLOW molding: In straight intermittent extrusion, extruder screw

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Last updated: September 9, 2019

BLOW MOLDING: Blow molding is a process by which hollow plastic parts are manufactured.

This process is also known as blow forming.  PROCEDURE:                                                                                                                            In very basic terms, plastic is melted down, clamped into a mold, and then air is blown into the heated plastic until it fills a mold and forms the desired shape to produce hollow parts. All types of hollow plastic parts can be made through this process such as plastics bottles. PROCESS MATERIALS: A variety of thermoplastic materials can be used to form blow molded parts, including the following:  Low Density Polyethylene (LDPE)  High Density Polyethylene (HDPE)  Polyethylene Terephthalate (PET)  Polypropylene (PP)  Polyvinyl Chloride (PVC) TYPES OF BLOW MOLDING: Following are the three main types of blow molding: Extrusion Blow molding Injection Blow molding Stretch Blow molding EXTRUSION BLOW MOLDING: This type of blow molding process begins with melting down the thermoplastic resin by extruder and forming it into a parison. The parison is a tube like piece of plastic with a hole in one end in which compressed air can pass through. Now the parison is clamped into a mold and air is pumped into it.

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The air pressure then pushes the plastic out until it fills the mold and forms the desired shape to produce hollow part. Now the plastic is allowed to cool down, once the plastic has cooled and hardened, the mold opens up and the part is ejected. All types of hollow plastic parts can be made through this process such as plastic bottles.  TYPES OF EXTRUSION BLOW MOLDING: Continuous Extrusion molding: In this extrusion process, continuous extrusion of plastics takes place and the individual parts are cut off by knife. Intermittent extrusion molding: In straight intermittent extrusion, extruder screw run, stops and pushes melt out. Here, accumulator is used to gather the mold.   INJECTION BLOW MOLDING:  This type of molding is carried out for producing large quantities of hollow objects. The main product of this process are jars, PET bottles etc.

 The main steps of the injection blow molding are, Injection: The polymer is melted by extruder barrel and screw assembly and then injected through heated cavity and core pin to form preform. Blowing: Here the preform is passed to the hollow chilled mold. Then the compressed air is blown into preform through core rod. Ejection: Now the final product is cooled, ejected from blow mold and stripped off the core rods. This product then passes to the quality control section for leakage testing.     STRETCH BLOW MOLDING: Stretch blow molding is the common method for producing soda bottles. In SBM process, the plastic is first molded into a preform using the injection molding process. These preforms produced are with bottle necks on one end including threads on the neck.

 They are then cooled and packaged. After cooling these preforms are later fed into a reheat stretch blow molding machine. In this process, infrared heaters are used to heat the preforms and then high pressure air is blown into bottles using metal blow molds.    FACTORS IN BLOW MOLDING: Parison characteristics Size, shape, straightness, wall thickness, mechanical strength. Stretch blow molding pressure is less needed. Inflation pressure is set in accordance to the process.  Mold design.

  Blow ratio=Mold diameter/Parison diameter. Here, the blow ratio must be 1.5 to 3. ADVANTAGES OF BLOW MOLDING: Production rate is high. Quick product revisions for increased flexibility.

 Tooling cost is low than other molding. Cheaper part price than Rotational Molding. Recycle trim and flash into finished parts. Time consumption on the mold-creation process is less. Less scrap generated.  DISADVANTAGES OF BLOW MOLDING: Suitable for hollow parts only.

 Defect may be visible. Cannot manufacture thick parts. Environmental Disadvantages. Process and Material Limitations.

 Bottles with calibrated neck finished are not produced. Trimming required BLOW MOLDING APPLICATION: Milk bottles Pharmaceutical bottles Antifreeze bottles Polypropylene bottles Coliseum seats One-piece chair Ice chests and coolers Double-wall player case Garbage cans Drums Fuel tanks  

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