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Chapter: Mechanical - Manufacturing Technology - Manufacturing of Plastic Components

Manufacturing of Plastic Components

Plastics are the best alternatives used in the areas where the component size is very small and weight reduction is required in order to minimize the cost of material.


MANUFACTURING OF PLASTIC COMPONENTS

 

 

Plastics are the best alternatives used in the areas where the component size is very small and weight reduction is required in order to minimize the cost of material. Hence knowledge about various types of plastics, its properties, production method etc is very important.

 

 

TYPES OF PLASTICS

 

Plastics are of two types 1.Thermo plastics 2.Thermosetting Plastics

 

Common plastics USED in molding are

 

      HDPE (stiff bottle, toys, cases, drum)

 

      LDPE (flexible bottle)

 

      PP (higher temperature bottle)

 

      PVC (clear bottle, oil resistant containers)

 

      PET (soda pop bottle)

 

      Nylon (automotive coolant bottle, power steering reservoir)

 

 

INJECTION MOULDING


Injection Molding

 

Most widely used process. Suitable for high production of thermoplastics. Charge fed from a hopper is heated in a barrel and forced under high pressure into a mold cavity. Several types. Variety of parts can be made.

Basic components:

 

mold pieces (define the geometry of the part), and sprue, gates, runners, vents, ejection pins, cooling system

 

There are two types of injection moulding.

1.     Plunger type injection moulding.

2.     Screw type injection moulding.

 

In screw type injection moulding machine the plunger is replaced by a screw.A receiprocating screw now forces the material into the mould.since the screw action generally helps to pack the materials better , a given plunger travel will push more material into the cavity. Finally the action of the screw , as it rotates and mixes, adds energy to the melt.

 

Band heaters are still needed to fully heat the melt. All of this results in a much better and more consistent part.

 

Virtually all industrial presses are screw type presses.

 

 

BLOW MOULDING



used to make thermoplastic bottles and hollow sections. Starting material is a round heated solid-bottom hollow tube perform.

 

Perform inserted into two die halves and air is blown inside to complete the process

 

General steps

 

     Melting the resin- done in extruder

 

     Form the molten resin into a cylinder or tube (this tube is called parison)

 

     The parison is placed inside a mold, and inflated so that the plastic is pushed outward

 

against the cavity wall

 

     The part is allowed to cool in the mold and is then ejected

 

     The part is trimmed

 

The parison can be formed by

 

A)Extrusion process

B)Injection molding process

(A)Extrusion blow molding

–  Parison is formed from by forcing the plastic through an extrusion

 

die.

 

Material enters the die, flow around the mandrel so that extrudate would be cylindrical

 

The die would have a hole at the center so that air could be blown into the cylinder

 

In some blow molding operations, the air is introduced from the bottom through an inlet

 

This process can be;

–  continuous extrusion blow molding

 

     During  the  process,  the  extrusion  runs  continuously,  thus  making  a

 

continuous parison.

 

     using multiple mold to match the mold cycle to the extrusion speed

 

–  Intermittent extrusion blow molding

     During  the  process,  the  extruder  is  stopped  during  the  time  that  the

 

molding occur

 

     use either reciprocating screw or an accumulator system

 

     In this system, the output of the extruder is matched by having multiple molds which seal and blow the parison and then move away from extruder to cool and eject

 

     In practical case, the mold cycle is longer than time required to extrude a new parison

 

     If the mold cycle is twice than time needed for creating a parison, a two mold system can be used

 

     The method is sometimes called rising mold system - system of which two or more molds are used to mold parts from one extruder during continuous process

 

b)Injection Blow Molding

 

     The parison is formed by the injection of molten resin into a mold cavity and around a core pin

 

     The parison is not a finished product, but it is subjected to subsequent step to form the final shape

 

     Second step, blowing of the intermediate part in a second mold

 

     Because of distinct separation of the two steps, the parison made by injection molding is called a perform

 

Process

 

     The mold is closed

 

     Resin is then injected to form a cylindrical part

 

     The mold is opened and perform is ejected

The perform can be stored until the finished blow molded is needed.

 

The flexibility of separating the two cycles has proven useful in manufacture of soda pop bottle.

Comparison of extrusion and injection blow molding


 

 

ROTATIONAL MOULDING


It is also known as Rotomoulding, rotocasting or spin casting.

 

The thin walled metal mould is a split female mould made of two pieces and is designed to be rotated about two perpendicular axes. The steps followed in rotational moulding are.

 

STEP-1

 

A predetermined amount of plastic , powder or liquid form , is deposited in one half of a mould.

 

STEP 2

 

The mould is closed.

 

STEP -3

 

The mould is rotated biaxially inside an oven. The hollow part should be rotated through two or more axes, rotating at different speeds, in order to avoid the accumulatiuon of polymer powder.

 

STEP 4

 

The plastic melts and forms a coating  over the inside surface of the mould.

 

STEP -5

 

The mould is removed from the oven and cooled usually by fan. The polymer must be cooled so that it solidifies and can be handled safely by the operator.The part will shrink on cooling, coming away from the mould and facilitating easy removal of the part.

STEP-6

 

The part is removed from the mould.

 

Advantages of rotational moulding

 

1. Moulds are relatively inexpensive.

 

2. Different parts can be moulded at same time. 3.Very large hollow parts can be made.

 

4.Parts are stress free.

 

5.Very little scrap is produced.

 

Limitations of rotational moulding.

 

1.Moulding Cycles are long 10-20 mins

 

2.It is not possible to make some sharp threads. 3.Cannot make parts with tight tolerance.

 

 

 

TRANSFER MOULDING


Transfer Molding

A process of forming articles by fusing a plastic material in a chamber then forcing the whole mass into a hot mold to solidify.

 

Used to make products such as electrical wall receptacles and circuit breakers

 

Similar to compression molding except thermosetting charge is forced into a heated mold cavity using a ram or plunger.

Examples: electrical switchgear, structural parts

Process Variables

 

      Amount of charge

 

      Molding pressure

 

      Closing speed

 

      Mold temperature

 

      Charge temperature

 

      Cycle time

Advantages

 

      Little waste (no gates, sprues, or runners in many molds)

 

      Lower tooling cost than injection molding

 

      Good surface finish

 

      Less damage to fibers

 

      Process may be automated or hand-operated

 

      Material flow is short, less

 

Disadvantages

 

      High initial capital investment

 

      Labor intensive

 

      Secondary operations maybe required

 

      Long molding cycles may be needed.

 

 

 

COMPRESSION MOULDING.



Compression Molding

The process of molding a material in a confined shape by applying pressure and usually heat.

 

      Almost exclusively for thermoset materials

 

      Used to produce mainly electrical products

 

Thermoset granules are compressed in a heated mold to shape required.

 

Examples: plugs, pot handles, dishware

 

 

Applications of compression moulding.

1.Dishes , Handles , container caps, fittings, electrical and electronic  components.

 

2.Scoops, spoilers, hoods, fenders.

 

3.Polyester fiber glass resin systems (SMC/BMC)


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