Chapter 15
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Chapter 15. Processes Used to Form Plastic Materials. Objectives. Closed molding processes: injection molding, compression molding, rotational molding, blow molding, and extrusion. Open molding processes: boat hulls and shower enclosures.

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Chapter 15

Chapter 15

Processes Used to Form Plastic Materials


Objectives

Objectives

  • Closed molding processes: injection molding, compression molding, rotational molding, blow molding, and extrusion.

  • Open molding processes: boat hulls and shower enclosures.

  • Blow molding processes: high volume production of bottles and other containers.

  • Laminations and calendaring: sheet products.


Introduction

Introduction

  • Plastic stock can be purchased in many forms: sheets, pellets, powder, granules, and rods.

  • With plastics there is normally no scrap generated.

  • Trimmed material is usually ground up and reused.

  • Eight basic forming processes: compression molding, injection molding, rotational molding, blow molding, thermoforming, extrusion, hand layup, and casting.

  • Plastics are used extensively to form composites.

  • Casting: a liquid resin is poured into a mold and solidifies.

  • Molding: Softened semi-solid material is formed in the desired shape in a mold or die using pressure and sometimes heat.


Closed molding

Closed Molding

  • Many molding processes use a two piece closed mold.

  • The closed mold halves are normally held in a press which provides pressure on the part during cooling or curing.

  • The five major closed molding processes are : compression molding, injection molding, rotational molding, blow molding and extrusion.

  • Each type of stock has its advantages and weaknesses and must be properly selected for use with a particular manufacturing process.


Compression molding

Compression Molding

  • There are two types of compression molding – hot and cold.

  • In cold compression molding the unheated mold closes on the molding compound to form it in the desired shape.

  • Curing is sometimes done by moving the still closed mold into an oven.

  • In hot compression molding the molding compound is placed between die halves that are heated between 225 and 3250F.

  • Pressure (100 to 2000 psi) and heat improve the flow into small channels in complex parts.

  • Compression molding is used to make car body parts, appliance components, truck liner panels, tote boxes, dinnerware, etc.


Injection molding

Injection Molding

  • Pellets flow from a funnel shaped feed hopper into a heated compression cylinder.

  • The plunger forces a controlled quantity of material through the injection molding nozzle into a closed mold.

  • Often the mold will be cooled by water running through cooling channels.

  • After the part cools (seconds) the mold is opened and the part is ejected.

  • Injection molding is used to manufacture electrical equipment housing, cell phone cases, appliance parts, automotive components, microwavable dishes , etc.


Rotational molding

Rotational Molding

  • Rotational molding (www.rotomolding.org) is primarily used to make seamless hollow products such as balls, containers, picnic coolers, floats, and toys.

  • Fastest growing in the U.S. plastics industry.

  • Powder or liquid coats the mold’s inside surface where it forms a thick skin or uniform layer.

  • It is then cooled.

  • Some machines are capable of producing tanks as large as 500 gallons (1893 liters) in size.

  • One innovative boat manufacturer, Triumph Boats of Durham, NC is making boats that will not chip or crack.


Blow molding

Blow Molding

  • Used to make household bottles and containers for cosmetics, pharmaceuticals, foods, and toiletries.

  • Flexible containers for juice, milk, and other beverages are now made by most manufacturers.

  • The process can produce 60,000 half liter water bottles per hour (Sidel, Inc.).

  • Extrusion blow molding is the most common.


Injection blow molding

Injection Blow Molding

  • One of the major advantages of injection blow molding is that parts are produced with no flash (rough edges).

  • This is not the case with injection molding or blow molding – flash must ne trimmed off.

  • Cost of injection blow molding machines range from $ 100,000 to $ 400,000.

  • Today 60% of all plastic bottles up to 16 oz. capacity are produced by injection blow molding machines.

  • A disadvantage is that the process is not practical for making bottles with handles.


Thermoforming

Thermoforming

  • Thermoforming involves heating a thermoplastic sheet to its softening point and forcing the material into or over the contours of a one piece mold.

  • Vacuum forming and drape forming are variations of thermo forming.


Extrusion

Extrusion

  • Extrusion is used to produce more plastic products than any other process except injection molding.

  • Extrusion is similar to squeezing toothpaste out of a tube.

  • The extruder converts thermoplastic powder, pellets, or granules into a continuous melt.

  • The melt is then forced through a die opening to produce long shapes.

  • Typical products include auto trim, house siding, garden hoses, and soft drink straws.


Pultrusion

Pultrusion

  • Pultrusion consists of pulling continuous roving through three processing stages.

  • Bundles are prepared by pulling continuous roving through a resin bath or impregnator.

  • Next the bundles are pulled into preforming fixtures where they are partially shaped and excess resin and air are removed.

  • Finally the bundles are pulled through heated dies for forming and then into an oven for curing.

  • The manufacturing work cell used for pultrusion consists of a pultrusion machine, the forming dies, a pulling system, and a cut off saw.

  • Fishing rods, hockey sticks, tent poles arrows and golf club shafts are pultruded.


Extrusion1

Extrusion

  • Film is either produced in tubular (blown) form (most frequently used) or in cast form (extruded through a linear slot die).

  • Wire coating: Extrusion is also used to coat wire. Liquid plastic surrounds the wire. It is then cooled and wound with a coil winding machine.

  • Extrusion blow molding: Most extrusion blow molded containers have flash. After the product is molded and cooled, the flash (excess material) is trimmed.


Open molding

Open Molding

  • Open molding is an important process for constructing composite material.

  • Glass fiber reinforcement roving is encapsulated in resin.

  • Open molding is used to make prototypes, pools, tanks, boats, etc.

  • Virtually any size product can be open molded.

  • The major drawback is that it is time consuming, resulting in high per unit labor cost.


Casting

Casting

  • The advantage of casting is that intricate shapes can be made using flexible molds.

  • Casting is used to make skylights, greenhouses, bus shelters, and boat parts.

  • Centrifugal casting: Pipes, tubing, and other round objects can be produced using the centrifugal casting process.

  • Centrifugal casting can even be used to manufacture long cylinders with external threads.

  • It requires minimum labor and can be automated if high volumes are desired.


Summary

Summary

  • Plastic stock can be purchased in many forms: sheets, pellets, powder, granules, and rods.

  • With plastics there is normally no scrap generated.

  • Trimmed material is usually ground up and reused.

  • Eight basic forming processes: compression molding, injection molding, rotational molding, blow molding, thermoforming, extrusion, hand layup, and casting.

  • Casting: a liquid resin is poured into a mold and solidifies.

  • Molding: Softened semi-solid material is formed in the desired shape in a mold or die using pressure and sometimes heat.

  • The five major closed molding processes are : compression molding, injection molding, rotational molding, blow molding and extrusion.

  • Rotational molding (www.rotomolding.org) is primarily used to make seamless hollow products such as balls, containers, picnic coolers, floats, and toys.

  • Fastest growing in the U.S. plastics industry.

  • One of the major advantages of injection blow molding is that parts are produced with no flash (rough edges).


Home work

Home Work

  • 1. What is casting and what is molding?

  • 2. Where is rotational molding used?

  • 3. What is a major advantage of injection blow molding?


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