Chapter 15

1. Chapter 15 Processes Used to Form Plastic Materials

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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).

17. 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?