COMPRESSION MOLDING

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A method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity typically is around 200 degree Celsius.

The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, and heat and pressure are maintained until the molding material has cured.

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• A widely used molding process for thermosetting plastics.e.g epoxies, phenolics , polyurethanes .

• Also used for making rubber tires or

boots. • Molding compound available in several

forms: powders , pellets, or liquid.

• There is a flash formed which subsequently

removed by trimming.

COMPRESSION MOLDING (1) CHARGE IS LOADED;(2) AND (3) CHARGE IS COMPRESSED AND CURED

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ADVANTAGES There are a number of advantages of compression

molding. These are:

1. Molds are simpler2. Require low maintenance3. Less scrap4. Low residual stresses in the molded part

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Transfer molding consists of melting the thermosetting plastic resin in a separate chamber and then injecting it into heated closed mold.

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Transfer Molding

(1)CHARGE IS LOADED INTO POT, (2) SOFTENED POLYMER IS PRESSED INTO MOLD CAVITY AND CURED, AND (3) PART IS EJECTED

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PROCESS CAPABILITIES

• Typical parts made by transfer molding are:

1. Electrical connector.2. Electronic components.

Notes : The molds tend to be more expensive than those for

compresion molding.

Suitable for intricate shapes with varying thickness.

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Plastics Forming

ThermoformingCold Forming and Solid Phase

Forming.

THERMOFORMING

Thermoforming is a process for forming thermoplastic sheet or films over a mold by means of application of heat and force.

Heating is usually accomplished by radiant electric heaters, located on one or both sides of the starting plastic sheet at a distance of roughly 125 mm.

Forced application is in term of air pressure or vacuum pressure.

The sheets used in thermoforming process usually are made by sheet extrusion and are available as a coiled strip.

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The methods by which the forming step is accomplished can be classified into two basic categories.

1. Vacuum thermoforming

2. Pressure thermoforming

Vacuum thermoforming – The earliest method (developed in 1950s) , in which negative pressure is used to draw a preheated sheet into a mold cavity .

Pressure thermoforming – An alternative to vacuum forming involves positive pressure to force the heated plastic into the mold cavity.

The process sequence is similar to vacuum thermoforming but the different is the sheet is pressurized from above into the mold cavity.

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VACUUM THERMOFORMING (1) A FLAT PLASTIC SHEET IS SOFTED BY HEATING; (2) THE SOFTED SHEET IS PLACED OVER A CONCAVE MOLD CAVITY; (3) A VACUUM DRAWS THE SHEET INTO THE CAVITY AND (4) THE PLASTIC HARDENS ON CONTACT WITH THE COLD MOLD SURFACE, AND THE PART IS REMOVED AND SUBSEQUENTLY TRIMMED FORM THE WEB.

PRESSURE THERMOFORMING. THE SEQUENCE IS SIMILAR TO VACUUM THERMOFORMING, THE DIFFERENCE BEING (2) SHEET IS PLACED OVER A MOLD CAVITY (3) POSITIVE PRESSURE FORCES THE SHEET INTO THE CAVITY

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Thermoforming Products

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Cold forming process is carried out at room temperature.

Plastics such as polypropylene (PP), polycarbanate (PC) and ABS are easily cold formed as they are sufficient ductile at room temperature.

Process that have been used in the cold working metals such as rolling,forging, coining and deep drawing also can be used to form thermoplastics.

Cold Forming

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This forming process is carried out at temperature of 10 or 20 degree celcius below the melting temperature of the plastic.

The forming operation takes place while the polymer is still in a solid state.

These two processes(cold & solid-phase forming) are not used as widely as hot processing methods and generally are restricted to special application.

Solid-Phase Forming

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Rotational MoldingIn this process, a thin walled

metal mold is made in two pieces and is designed to be rotated about two perpendicular axes.

A pre-measured quantity of powdered plastic (thermoplastic or thermosetting) material is placed inside the warm mold.

The mold is heated usually in

a large oven and is rotated continuously about the two principal axes.

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Rotational Molding Products

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Foam Molding Products such as styroform cups, food container

are made by foam Molding. The raw material used is polystyrene beads. A common method of foam molding is to use pre-

expanded polystyrene beads in which the beads are expended partially by steam in an open-top chamber.

The beads then are placed in a storage bin and allow to stabilize for a period of 3 to 12 hours.

Then they can be molded into desired shapes .

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Casting Some thermoplastics (such as nylons and acrylics)

and thermosetting plastics (epoxies, phenolics, polyurethanes and polyester) can be cast into variety of shapes.

In the basic conventional casting of thermoplastic, a mixture of monomer,catalyst, and various additives is heated to above its melting point and poured into the mold.

Potting and encapsulation is a part of plastic casting that is important to electronics industry.

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Potting and encapsulation involve casting the plastic material (typically a liquid resin e.g expoxy) around an electrical components such as transformer to embed it in the plastic.

Potting is carried out in housing or case, which becomes an integral part of the component and fixes it in position.

In Encapsulation, the component is coated with a layer of the plastic,surrounding it completely and then solidfying.

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In electronics, potting is a process of filling a complete electronic assembly with a solid compound for resistance to shock and vibration, and for exclusion of moisture and corrosive agents. Thermosetting plastics are often used.

A small transformer potted in epoxy. The surface visible on the right is formed by the potting compound that has been poured into the plastic box.

Potting

24 Solenoid from Amisco (Cinisello Balsamo, Italy)

encapsulated with ZYTEL® 74G20.

Warner Electric’s self-supporting clutch coil for automotiveair conditioners encapsulated in ZYTEL® 70G33 L.Interchangeable mould permits use of the same toolto encapsulate coils terminated with either leadsor connectors.

Automotive engine temperature sensor encapsulatedwith ZYTEL® glass-reinforced PA66.

Encapsulation

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Extrusion

Injection molding

Foam molding

Blow molding

Rotational molding

Thermoforming

Process Characteristics

Continuous, uniformly solid or hollow, and complex cross section; high production rates; relatives low tooling costs; wide tolerance.

Complex shapes of various sizes; thin walls; very high production rates; costly tooling; good dimensional accuracy.

Large parts with high stiffness-to –weight ratio; less expensive tooling than injection moulding; low production rate.

Hollow, thin-walled parts and bottles of various sizes; high production rates; relatively low tooling costs.

Large, hollow items of relatively simple shape; relatively low tooling costs; relatively low production rate.

Shallow or relatively deep cavities; low tooling costs; medium production rates.

SUMMARY

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Part similar to impression ; expensive tooling; medium production rate.

More complex parts than compression moulding; higher production rates;

Simple or intricate shapes made and inexpensive process; widely used in electrical and electronic industry.

Process Characteristics

Compression molding

Transfer molding

Casting