Plastic Processing

Plastic processing

introduction •

Those methods used to convert plastics materials in the form of pellets, granules, powders, sheets, fluids, or preforms into formed shapes or parts. The plastics materials may contain a variety of additives which influence the properties as well as the process ability of the plastics. After forming, the part may be subjected to a variety of ancillary operations such as welding, adhesive bonding, machining, and surface decorating (painting, metalizing).

plastics •

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Plastics(polymers) are formed when small organic molecules are stitched together to form a long chain. This  process is called polymerization. Those organic molecules suitable for polymerization are called monomers. A monomer is the single building block used in creating the  polymer. It is necessary for a monomer to be at least  bifunctional(capable of forming two covalent bond) to  polymerize. Some monomers are polyfunctional, capable of forming three or more bonds. These can form three-dimensional arrangements of chain called a network.

Polymerization  polymerization is generally of two types: Addition polymerization Condensation polymerization Addition polymerization: It consists of breaking the double carbon bond (C=C) in  bifunctional polymers so that the chain can be formed. Addition polymerization to produce thermoplastic materials. Some plastic materials made by API. Poly- styrene(CH2=CH-C6H5) II. Poly -vinyl chloride(CH2=CHCl) III. Ploy- propylene(CH3CH=CH2) IV. Poly-methyle methacrylate(CH2=CC3O2H4) V. Poly-vinyl acetate(CH3COOCH=CH2) VI. Poly-ethylene(CH2=CH2) •

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Polymeric materials

 plastics

thermoplastic

elastomers

thermosetting

Commodity  plastics

Engineering  plastics

Commodity  plastic

Polyethylene

polyamides

phenolics

Polypropylene

acetals

Polystyrene,PVC

cellulosics

unsaturated polyester ureas

Engineering  plastics silicones urethanes melamines

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Thermoplastics: these are the plastic that can be softened & melted by heat and can then be formed into the required shape when they are hot.

These materials can be melted a number of times. that means it is possible to recycle thermoplastic.

Frequent remelting is avoided in industry since some chemical degradation occurs during remelting. Thermoplastic materials tend to consist of long polymer chains with little breadth, akin to a two-dimensional structure. The fabrication processes used for thermoplastics such as blow moulding,& injection moulding etc.

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Thermosetting materials: these are the plastics that can be melted once they are solidified.

The raw materials for thermosetting materials are usually called resins. They are mixed and placed in the mould, and heated and compressed during which process the materials achieve the strength and hardness.  polymerization occurs by strong network bonds (cross-linking) with the application of heat, pressure and/or time. The manufacturing process(compression moulding, transfer moulding) used are more expensive compared to those of thermoplastic materials. These materials are characterized by a three-dimensional networks of molecules. These materials can not be recycled., when heated these materials burn and char.

Extrusion of plastic 1.

It is the process of confining the material in a closed cavity and then allowing it to flow from only one opening so that the metal will take the shape of the opening. The operation is identical to the squeezing of toothpaste out of a toothpaste tube. 2. Extrusion can be used to process most thermoplastics. 3. It is possible to combine a variety of resins to gain special physical,  biological or chemical properties. 4. The plastic that is generally in the form of pellets or granules is fed into the extruding machine through a hopper.

Conti…. There are no. of plastic extrusion processes used in the industry: Solid extrusion: this produces a part with a uniform cross section. Hollow extrusion: this process is used to extrude part that have hollow cross-sections such as pipes and tubes. Co-extrusion: this is the process of extruding two or more materials through a single die with two or more orifices arranged so that the extruded from the multiple openings merge & weld together into a laminar structure before chilling. Advantages:  practically any cross-section can be easily produced. Overall cost of the part produced by extrusion is low. The tooling costs are relatively low. The equipment used is simple and relatively inexpensive. •

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Injection moulding •

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Injection moulding is similar to pressure die casting. In this process, plastic material in a highly softened state is forced to flow at high  pressure through a nozzle into the mould cavity, the plastic solidifies in the die and then is ejected by opening the die. Injection moulding pressure usually ranges from 70MPa to 200MPa. Injection moulding is the most widely used plastic processing method. It can be used to produce a wide variety of  products. Complex shape may be produced & size may ranges from very small (50gm) to very large(25kg). Most polymer may be injection moulded, including thermoplastic, fibre-reinforced thermoplastics, thermosetting plastic & elastomers.

Injection moulds

Reaction injection moulding •

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Reaction injection moulding is the  process used for moulding thermosetting materials such as  polyurethane and epoxy, which exist in liquid form before they polymerize. The process was originally developed to mould very large automobile parts such as bumpers, interiors trim panels & spoilers. Because of the low pressure used, the cost of the mould is much less compared to the conventional injection moulding.

Liquid injection moulding •

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Liquid injection moulding is used for moulding silicone products. In this  process, pumping systems deliver a two  part-liquid silicone (catalyst & cross linker) directly into a mixer for homogenization. The mix is then injected directly into heated mould cavities in as little as3 to 10 sec using a relatively low pressure. Moulding & vulcanization (curing) occur inside the mould cavities within 10 to 90 sec due to the high mould temp.

Co-injection moulding •

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This is the process that uses two materials to mould a part. the two materials have different quality with one being hard that form the skin, while the softer one forms the core through the injection moulding  process. It requires two injection units for the two different plastic being used. The skin materials is injected first into the mould cavity, & is immediately followed by a core material.

Blow moulding •

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Blow moulding is the process of inflating a hot ,hollow thermoplastic  preform or parison inside a closed mould so that its shape conforms to that of the mould cavity. A wide variety of hollow parts, including plastic bottles, can be  produced from many different thermoplastic materials using this  process. Typical parts made are bottles, toys, air ducts for automobile, chemical & gasoline tanks, & a no. of household goods.

Extrusion blow moulding An extrusion blow-moulding machine consists of an extruder similar to that used with plastic extrusion, which softens the plastic & forms it into a tube (called a parison or preform) through a conventional type die & a split body mould. Steps involved in this process: The die closed around the parison, sealing both end. A blow pin is inserted into the parison to inflate it, causing it to expand & confirm the shape of the mould cavity. The mould is cooled & once the part has solidified, the mould opens & the  part is removed. It is usually to make items of weight greater than 350g such as containers for food, laundry or waste. •

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Injection blow moulding •

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Injection blow moulding is a twostage process with the parison being  produced in the first stage which needs to be transferred to the blow mould. Thus the first operation is identical to the injection moulding. The air is injected into the plastic at a pressure between .5 to 1 MPa. Injection blow moulding is used to achieve very accurate wall thickness, high quality neck finish, wide mouth openings & to process  polymers that can not be extruded. The usual applications include  pharmaceutical, cosmetic, singleserving liquor bottles that weight less than 350g

 Stretch blow moulding •

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Stretch blow moulding is best known for producing PET (polyethylene terephthalate) bottles commonly used for water, juice & a variety of other products. It produces a part with the required  properties for the work material by  producing desirable molecular orientation. In this process, a preform or parison is elongated mechanically in the mould & then expanded radially in the blowing process. Stretch blow moulding is only used for difficult-to-blow crystalline & crystalizable polymers such as  polypropylene & PET.

 thermoforming •

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In this process, a thermoplastic sheet can be formed into a 3-D shape  by the application of heat & differential pressures. First, the plastic sheet is clamped to a frame & uniformly heated to make it soft & flowable. Then a differential pressure (either vacuum or pressure or both) is applied to make the sheet conform to the shape of a mould or die  positioned below the frame. It is possible to use most of the thermoplastic materials. It is used for making such parts as covers, displays, blister  packaging, trays, drinking cup & food packaging.

 Vacuum thermoforming •

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Vacuum forming is thermoforming process that forms thermoplastic sheets into 3-D shapes through the application of heat & vacuum. During this process, plastic material is heated (170-220 deg C) until it becomes  pliable, & then is placed over a mould of the requisite shape & drawn in by a vacuum until takes on the desired shape. Typical value for the vacuum developed by the vacuum  pump should be about 35 torr.

Pressure thermoforming •

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Pressure thermoforming is an improvement over vacuum forming, in that it utilizes both vacuum and compressed air to forced the  plastic sheet against the mould. Vacuum pulls on one side of the sheet & compressed air  pushes on the other. Reasonably high pressure approaching 3.5 MPa are used for forming the requisite shapes.

Compression moulding •

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Compression moulding is the oldest  plastic-processing method. A compression mould is made of two halves with one each being connected to the platens of the press. The mould is electrically heated to maintain the required temp. The pressure maintained on the materials is of the order of 14 to 40 MPa of moulding area. The most widely used plastic is  phenol formaldehyde’  commonly known as ‘Bakelite’

 Transfer moulding •

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Transfer moulding is very similar to compression moulding and is developed to avoid the disadvantages found in that process. In this method, thermosetting charge is heated & compressed in a separate chamber & then injected into the closed mould where it is allowed to cool and solidify.

Plastic production design •

The designer should first determine the most desirable moulding process that is to be used for the manufacture. Then the designer should design the part taking the best aspects of that process into account.

A few of the general guidelines for designing the plastic parts are given: •

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Wall thickness: walls of plastic parts should be maintained as uniform as practically possible. Draft: the part needs to be released & ejected from the mould. For this purpose, a draft or taper needs to be provided on all surfaces that are normal to the parting plane. Radii and fillets: it is necessary to provide generous radii & fillets from the strength point of view as well as for the flow of  plastic melt through the mould cavity.

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Holes:

The holes sizes & their length need to be considered properly in order to provide the necessary rigidity to the core pins. •

Bosses:

The function of the bosses is for locating, mounting & assembly. •

Ribs:

Ribs are often the practical way to increase the stiffness of large surfaces. They also help as feeders to allow the material flow for isolated bosses.rib thickness should be smaller than the wall thickness