PLASTIC PROCESSING polymer

INTRODUCTION Polymer are long chain giant organic molecules assembled from many small molecules called monomers . More monomer molecules joined in units of long polymer. Example; polyethylene

POLYMER MATERIALS The polymers are a diverse group of engineering materials. They are the main components of plastics, rubbers, resins, adhesives and paints. These materials have distinctive microstructures built from macromolecular chains and networks of carbon and other light elements.

CLASSIFICATIONS OF POLYMERS Thermoplastics Thermosets Long chain Polymers Natural polymers Homopolymer Copolymer

THERMOPLASTICS A type of plastic that can be softened by heat, hardened by cooling, and then softened by heat over and over again. Thermoplastics are not cross-linked. Thermoplastics such as polyethylene, Nylon, Polyvinyl chloride which soften on heating. Example; Polyvinyl chloride

POLYVINYL CHLORIDE Pipes Electric cables Clothing and furniture Unplasticized polyvinyl chloride (uPVC) for construction Healthcare(Components of dialysis installations and temporary blood storage devices) Flooring( buildings covering the home, hospitals, offices, schools, etc.)

THERMOSET Thermoset - having the property of becoming permanently hard and rigid when heated or cured; "the phenol resins and plastics were the original synthetic thermosetting materials" They are hard and rigid at room temperature and do not soften on heating. Examples and uses of thermoset plastic materials: Epoxy resins - used as coating materials, caulks, manufacture of insulating materials, etc ... Phenolic resins - tool handles, billiard balls, sprockets, insulation, etc ... Unsaturated polyester resins - manufacture of plastics reinforced fiberglass commonly known as polyester, fillers, etc ...

Properties of Polymers The physical properties of a polymer, such as its strength and flexibility depend on: Chain length - in general, the longer the chains the stronger the polymer; Side groups - polar side groups give stronger attraction between polymer chains, making the polymer stronger; Branching - straight, un branched chains can pack together more closely than highly branched chains, giving polymers that are more crystalline and therefore stronger; Cross-linking - if polymer chains are linked together extensively by covalent bonds, the polymer is harder and more difficult to melt.

Characteristics of Polymers Low Density. Low coefficient of friction. Good corrosion resistance. Good mould ability. Excellent surface finish can be obtained. Can be produced with close dimensional tolerances. Economical. Poor tensile strength. Low mechanical properties.  Poor temperature resistance.  Can be produced transparent or in different colours

Applications of Polymers Polymeric materials are used in and on soil to improve aeration, provide mulch, and promote plant growth and health. Medicine Many biomaterials, especially heart valve replacements and blood vessels, are made of polymers like Dacron, Teflon and polyurethane. Consumer Science Plastic containers of all shapes and sizes are light weight and economically less expensive than the more traditional containers. Clothing, floor coverings, garbage disposal bags, and packaging are other polymer applications.

Industry Automobile parts, windshields for fighter planes, pipes, tanks, packing materials, insulation, wood substitutes, adhesives, matrix for composites, and elastomers are all polymer applications used in the industrial market. Sports Playground equipment, various balls, golf clubs, swimming pools, and protective helmets are often produced from polymers.

Compression and Transfer Molding Process Processing of Thermosetting Plastics Compression and Transfer Molding Process

Compression Molding

Compression Molding Process The process of molding a material in a confined shape by applying pressure and usually heat. Compression molding process is followed by two step first one preheating and pressurizing.

Process Description

Process Description Charge is placed in cavity of matched mold in the open position, Mold is closed by bring the two halves together, Pressure is exerted to squeeze the resin so that it fills the mold cavity, While under pressure the mold is heated which cures the resin.

Process Description

Stages in Compression molding cycle

Stages in Compression molding cycle The various stages of the compression molding cycle time can be represented as a function of the force required to close the old at a constant rate. In the “plastication” stage the force increases rapidly as the polymer feed is compressed and heated.

Stages in Compression molding cycle The second stage flow commences, once the yield stress of the elastomer is exceeded. tc is the point at which the mold fills and compression of the melt occurs. Mold filling the majority of chemical reaction should take place after tc.

Parameters 1.The quantity of molding material  2.Pressure of the molding process Range of pressure 2000-3000 psi(13.8-20.7 MPa) 3.Mold temperature Temperature range 300ºF to 375ºF (149⁰C- 191⁰C) 4.Cure time variables. The period required to harden thermosetting material to partial and complete polymerization is called cure time

Applications & products Radio Cases Automotive ash trays & electrical parts

Transfer Molding Process In the transfer molding, polymer charge is transferred from the transfer pot to the mold. The mold is cooled and molded part is ejected. Generally, thermoset plastics (such as epoxy, polyester, phenol-formaldehyde, vinyl ester, silicone) are processed by transfer molding process, but certain thermoplastic materials can also be processed. This process is widely used to encapsulate items such as integrated circuits, plugs, connectors, pins, coils etc.

Steps In Transfer Molding The required amount of polymer charge is weighted and inserted into the transfer pot. The transfer pot is heated by the heating element above the melting temperature of the polymer charge. The plunger is used to push the liquid polymer charge from the transfer pot into the mold cavity. The mold cavity is held closed until the resin gets cured. The mold cavity is opened and the molded part can be removed. Process Parameters  Heating time  Melting temperature of the charge  Applied pressure  Cooling time

Pros Cons Fast setup time and lower setup costs Wastage of material Low maintenance cost Plastic parts with metal inserts can be made Design flexibility Dimensionally stable Wastage of material Production rate lower than injection molding Air can be trapped in the mold

Processing of Thermoplastics 1. INJECTION MOLDING 2. BLOW MOLDING 3. THERMOFORMING 4. EXTRUSION MOULDING

INJECTION MOLDING Injection molding is one of the most commonly used processing method for the plastic components It is defined as “ A manufacturing process for forming objects, as of plastic or metal, by heating the molding material to a fluid state and injecting it into a mold “.

INJECTION MOLDING (CONTINUED) It is used to process thin walled plastic parts for a wide variety of shapes and products . BASIC PRINCIPLE Plastic material is melted in a heated chamber and then injected in the mold , where it cools and finally the finished plastic part is ejected out of the cavity .

INJECTION MOLDING MACHINE The raw material in the form of powder/pellets is introduced through hopper into the cylinder

Machine parts Hopper is a tapered container working with a hopping motion to transfer grains into cylinder Inside the cylinder there is a piston /plunger which is used to move the material from cylinder into the mold Heating arrangement is placed above and below the cylinder . The mold is in two halves , it has a mold cavity and the mold has two ejector pins.

Process As soon as the plastic is melted it will be pushed by the piston into the sprue through which the material would go into the mold cavity . The cavity will be filled by the pressure of the piston and the mold should not be opened until the plastic solidifies. When the plastic solidifies the ejector pins are hydraulically moved backwards .

Stages of Injection molding Clamping : The two halves of mold are tightly closed before the molten plastic is injected. Injection : Injection of raw material from hopper into cylinder to the cavity is called the injection stage. Cooling : The material is cooled for at least 30-60 sec Ejection : The 2nd mold plate moves back by ejector pins and the product is tapped out

IMPORTANT POINTS PRODUCTION RATE : Production rate depends upon the clamping and de-clamping time of the mold halves. INJECTION TIME: The filling process should be optimal not too slow in order to stop the production of cracks. SHOT VOLUME : The amount of material that is injected into the mold is referred to as shot volume PRESSURE and CYCLE TIME : The pressure applied is in the range of 35-140 MPa while the cycle completes in 2-60 sec

MATERIALS USED Acetate Acrylonitrile butadiene styrene (ABS) Cellulose acetate PVC ( poly vinyl chloride) Polycarbonate Polyesters e.t.c.

DISADVANTAGES ADVANTAGES Higher production rate Minimum wastage of material Complex geometry can easily be produced DISADVANTAGES Tooling cost higher High setup cost Large undercuts can not be formed

APPLICATIONS It can be used to produce thin walled plastic products of varying size and dimensions e.g. Medical devices such as surgical syringes Open container like buckets Plastic housings of different tools Daily use products such as tooth brushes

Parts manufactured: Table Chair

BLOW MOLDING “ It is a plastic processing technique which is used to produce hollow plastic parts by inflating a heated plastic until it fills a mold and forms the desired shape” The raw material used for processing in blow molding is called parison Parison is a preform tube that is hollow and it has a hole at one end

PROCEDURE First the parison is obtained by means of extrusion or injection molding process Above the preform there is a blower and the preform is positioned between two split halves of the mold. The mold closes over the tube and the hot air is injected from the blower into the hollow tube . The air expands the tube against the sides of the mold After sufficient cooling , the mold opens to release the product.

Important Process Parameters Cooling temperature of plastic material Air pressure required ( usually 2-4 Mpa) Cooling time Pressure is exerted both axially and radially , both cavities are pressurized with a non-reactive gas such as argon.

Types of Blow molding Extrusion blow molding Injection blow molding Stretch blow molding MAJOR DIFFERENCE: All the types have one primary difference that is making of parison by extrusion , injection and stretching .

INJECTION BLOW MOLDING The injection blow molding is used for the production of hollow objects in large quantities . The main applications are bottles, jars and other containers. The process is divided into three sections Injection Blowing Ejection

EXTRUSION BLOW MOLDING In this process the blow molding machine is based on a standard extruder barrel and screw assembly to plasticize the polymer. The polymer is heated inside the barrel , the molten polymer is led through a right angle and through a die to emerge as a hollow section. When the parison has moved enough length it is cut through by a knife and fits inside the bottle mold . The bottle mold moves toward the jet where the air blows and the product is obtained after cooling .

Stretch blow molding Stretch blow molding is also carried out by injection . Used the production of high quality containers. This process is divided into four steps Injection : The material is injected as discussed before. Stretching : One conditioned to correct temperature , the parison is stretched by the blow pin/stretch rod that stretches it longitudinally . Blowing : Using two levels of air pressure , the preform is blown circumferentially. Discharge : After a set time for cooling the molds open and the product is removed .

Limited to hollow parts . ADVANTAGES Low tooling costs . Fast production rates . Ability to mold complex parts . Little scrap generated . Large hollow shape can be produced . Produced parts can be recycled . DISADVANTAGES Limited to hollow parts . Thick parts cannot be manufactured.

Applications Different types of plastic products can be manufactured by this process such as: Bottles in different shapes and sizes Jars and containers Fluid oil tanks Mugs Toys etc

THERMOFORMING “ Thermoforming is a plastic processing technique in which the thermoplastic sheets are formed by applying temperature and pressure inside the mold .” The raw material here is usually plastic sheet Different types of this process are Vacuum forming Pressure forming Matched die forming

VACUUM FORMING The vacuum pressure is used to form the heated thermoplastic sheet into the desired shape. Thermoplastic sheet is placed on the mold surface and fixed with the help of clamping unit. The sheet is heated until it softens and there after , vacuum needs to be applied quickly. There is tank with nozzles that suck air and creates vacuum in between sheet and mold.

Few important parameters How much heat is required ? Amount of vacuum needed . These parameters solely depends upon the type of thermoforming and the thickness needed.

PRESSURE FORMING The forming process is closely related to the vacuum forming . The air pressure required is much higher as compared to the pressure required in the vacuum forming The preheated plastic sheet is placed on the mold surface and then air pressure is applied quickly above the sheet. The high pressure is developed in between the softened sheet and pressure box Due to the high pressure, the preheated plastic sheet can be deformed into the mold cavity in a fraction of second (means high production rate).

The formed sheet is held in the mold cavity for cooling for few seconds The formed parts thereby solidifies and is ejected.

MATCHED DIE FORMING A matched die forming is also called mechanical forming . Mold consists of two parts i.e. (i) die and (ii) punch Thermoplastic sheet is heated with the application of heat until it softens Preheated sheet is placed into the die and through the punch , pressure is applied on sheet Air between the die and sheet is evacuated by using vacuum pump , and sheet conforms to the mold shape. Formed part is cooled and ejected from the mold.

Materials used ABS (Acrylonutrile butadiene styrene) Celloluse acetate LDPE (Low density polyethylene) HDPE (High density polyethylene) PVC etc

ADVANTAGES Design flexibility Rapid prototype development Low initial setup costs Low production cost Less thermal stresses Good dimensional stability DISADVANTAGES Poor surface finish Parts may have non-uniform thickness All parts needs to be trimmed Limited materials can be made

Applications Thermoforming has many applications like Food packaging Automotive parts Aircraft windscreens Vehicle doors

Extrusion Moulding Continuous process  used to produce both solid and hollow products that have a constant cross-section. Ex: - Window frames, hose pipe, curtain track, garden trellis. Thermoplastic granules are fed from a hopper by a rotating screw through a heated cylinder. The tapered screw compacts the plastic as it becomes elasticised. The die which is fitted to the end of the extruder barrel  determines the cross-section  of the extrusion. Thicker cross-sections are extruded more slowly as more time is required  for the initial heating and subsequent cooling of the larger quantities of material which are involved. As the extrusion leaves the die it is cooled  by passing through a cooling trough containing cold water.

Extrusion Moulding

End of UNIT III