ing Of Plastics 2pe2e

ING OF PLASTICS BY S.SUGUMAR CIPET CHENNAI

ING OF PLASTICS Methods used for ing of Plastics may be grouped according to principle on which the bond is made.

I.Cohesion Principle Involves an actual intermingling of molecules of the materials being ed. 

Some means is employed to cause two materials to flow together and fuse.

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Two techniques in this category - Solvent cementing - Thermal welding

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Cohesive bonding involves ing of similar pieces.

 different materials may be ed only when the flow properties are similar.

II. Adhesion Principle makes use of film which adheres to each other material to be ed. An adhesive, a substance different from either of the materials remains in the t.

III. Mechanical Linkage Involves production of a linkage between the two materials which simply holds the two together physically with neither flow of either material not “Sticking” (eg.) Screws for ing of plastics, Rivets, Nut & Bolt systems, etc.

SOLVENT CEMENTING Used commonly with amorphous polymers such as PS,ABS,PC,PMMA which are readily attacked by solvents. Not commonly used with crystalline thermoplastics. Not applicable for Thermosets.

Solvents for cementing of plastics ABS

MEK, Methyl Isobutyl Ketone, Methylene Chloride

Acrylic

Methylene Chloride, Ethylene dichloride

Cellulosics

MEK

Nylon

Aqueous Phenol, Solutions of Resorcinol in alcohol.

PP

Trichloro ethylene, ethylene dichloride chloroform and methylene chloride

PVC

Cyclohexane, Tetra hydrofurane Dichlorobenzene

PC

Mehtylene Chloride & Ethylene dichloride

PS

Styrene, Methylene Chloride, Ethylene Dichloride Acetone.

Polysulfone Methylene chloride

Dope Cement solvent + Small quantity of the same plastic which is to be bonded. (e.g) PVC + Cyclohexane or THF is the dope cement for PVC.

 Pre treatment of the substrate is essential before bonding.  Abrasion – using emery cloth-solvent wipe with ketene in the case of PVC.  MEK is the common dope cement for low/medium m.wt PVC.  Cyclohexane or THF for high m.wt PVC.

SPVC bonding typical formulation: PVC resin

- 100 parts by wt.

THF

- 100 parts by wt.

MEK

- 200 parts by wt.

Tin organic stabilize

-

1.5 parts by wt.

MIBK

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2.5 parts by wt.

Adhesives Elastomeric (e.g.) NR, Neoprene, Nitrile, etc., used for ing plastic to metals to metal, etc. Good bond strength and high degree of t flexibility could be achieved.

Thermoplastic  Solvent based  Hotmelt Adhesive Only one of this type cyanoacrylate cure through polymerization.

Thermosetting (E.G) PF,,epoxide, Polyester resin, Most versatile and durable

Methods of Adhesive Application  Brushing :- low cost, uniform application is difficult to achieve.

 Dipping :- masking is required for partial application

 Roll coating

 Spraying :- rapid method for large area application

WELDING  Process of ing similar or dissimilar materials with similar flow behaviour by thermal means.  ing of Thicker sections – welding, Thin films – sealing

Direct heat welding  Hot plate or Heated Tool Welding  Hot air welding / Hot gas welding  Resistance wire welding  Thermostacking

Indirect Thermal welding Ultrasonic welding Spin/ Friction welding Vibration welding Dielectric heating or High frequency sealing.

HOT plate WELDING: Plastic components to be welded are partially melted by being in with a high temperature thermal source or a hot plate, and then welded together before they become cold and solidify. As a method to obtain relatively firm welding with the relatively simple equipment, this method has long been used and is particularly suitable for large moldings.

Hot plate WELDING PROCESS The equipment consists of a hot plate to simultaneously heat two surfaces to be welded and a pressurizing mechanism. First step : Moldings to be ed are kept in with the hot plate to melt the welding areas. Second step : Two molten surfaces are kept in each other and held until they are cooled and solidified.

PROCESS OF HOT PLATE WELDING

Typical hot plate temperature to weld plastics ABS - 210 Deg-C - 230 Deg– C PP - 190 Deg-C – 210 Deg– C PVC - 210 Deg-C – 220 Deg– C

Pressure between a work piece and a hot plate

Minimum pressure to achieve close of the work piece with the hot plate

If the pressure is too high, the dimension can be smaller than the specification and flash may be generated.

If the pressure is too high, a flash is generated.

When the welding machine is not equipped with a pressure controller, some invention on the side of jigs is required, for example the installation of stoppers.

Measures to prevent sticking or the adhesion of molten resin to a hot plate. To use polytetrafluoroethylene (PTFE) sheets or to coat the hot plate with PTFE. Note : Mirror finish on the surface of the hot plate is required before the coating with PTFE.

Example of measures to prevent sticking

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Typical ts-Hot Plate Welding

FEATURES OF HOT PLATE WELDING:

merits

High weld strength and efficiency. The highest in all known welding processes. Relatively large degree of freedom in the total shape of the component and in the design of welding portion.

Demerits

Since the transfer of heat from the hot plate to the moldings is required, processing cycle time becomes relatively long. Measures to prevent the sticking of molten plastics to the hot plate is necessary, otherwise stringing may happen.

Hot gas welding / Hot air welding Applies only to thermoplastics Hot air/ Hot gas (Nitrogen) is used The principle of welding with hot air is to make plastic flow at the conjunction of two pieces (or two pieces and a welding rod of parent material) such that under a small amount of pressure the material will intermix and fuse together to give a homogeneous bond. Materials to be welded must be of the same type or similar flow behaviour. The temperature of the hot air should be sufficient to make the plastic begin to flow Temperature required varies with plastics, usually between 270 Deg-c – 350Deg-c under 3 to 4 lbs/sq in pressure.

Preparation Stages  Supply air to hot air gun should be free water or oil.  The material to be welded together and the welding rod must of the same grade of plastic.

Some plastic need special attention  PMMA, PC, ABS, easily absorb moisture and therefore they should be stored at warm and dry conditions prior to welding.  PE & PP may quickly oxidise. Therefore the surfaces to be welded, including the welding rod should either be mechanically scraped or cleaned with solvent such as Trichloro Ethylene or Acetone.

Thermostacking Thermostacking uses a system of hot air and cold stacking to form a stud that s plastic to plastic or plastic to metal, producing a tight assembly.

Process A holding fixture secures the plastic part to be stacked. Plastic studs protrude through holes in the material of the connection part. A heating manifold covers the protruding plastic studs and soften the plastic with finely controlled hot air. When plastic reaches a sufficiently molten stage, the heat ceases and a cold stake head descends. The cold stake head forms the plastic stud, solifides it and creates a permanent head.

Advantages :  Tight mechanical connection  Low production cost : can simultaneous stake multiple studs.  High speed production  No damaged to outside surface.

Resistance wire and Induction Welding  A wire of high electrical resistance (nichrome wire) is placed between the mating surfaces of the two thermoplastic parts.  Current flow in the wire generates heat to soften the adjacent plastic surfaces which can be pressed together to complete the t  The wire will remain in the t  High speed welding process.  Any thermoplastic could be welded.

Dielectric or High frequency heat sealing  Involves the use of electronic waves from an AC generator ing through a plastic sheet or film.  These waves causes high level molecular friction with in the plastic so that heat is generated with in the plastic and the plastic will melt at the point of .  By applying pressure at this point and allowing sufficient cooling time, a permanent bond is formed.  Plastics commonly used ABS, PVD, PMMA.  Biggest use vinyl films – raincoats, inflatable etc.

Spin Welding / Friction Welding  The generation of frictional heat by two rubbing surfaces is utilized to materials in the friction welding process.  The temperature at the rubbing surfaces raises to beyond the melting point of the plastic material in few seconds.  Final forging action ensures the weld is cooled in compression and forms a high integrity t.

Materials  Both amorphous and semi- crystalline thermoplastic can be ed by friction welding  The major welding variables are :- rotation speed, pressure and cooling time

Applications - Plastic floats

- Pipes

- Shafts

- Pressure vessels.

Vibration Welding  Also called linear friction welding.  Instead of rotary motion causing frictional heat in friction welding, in vibration welding the movement consists of linear oscillations.  Once molten material has been generated at the interface, vibration is stopped and the parts are aligned and weld consolidates on cooling.  Weld time – 1 to 5 sec. Vibrations applied – 100 – 240 Hz Pressure –1 to 4 n/mm 2

Main Advantages  Very large complex linear ts can be produced at high production rates.  Suitable to weld almost all thermoplastics.

Examples: Automobiles parts – Front and rear light assemblies, fuel filter, instrument , reservoirs for brake, etc.

ULTRASONIC WELDING

PROCESS Ultrasonic welding is a process in which strong frictional heat is generated at the portions of moldings to be welded by simultaneously applying pressure and ultrasonic vibration, to melt and weld these portions.

Ultrasonic Welding : Equipment consists of:Generator : - Converts electrical energy from main supply frequency of 50 HZ to an ultrasonic frequency of 20 KHz. Mechanical Resonance Unit:Converts electrical output from generator into mechanical vibrations which consists of transducer, booster and sonotrode. Transducer : uses a piezo electric material (lead zirconate titanate ) for conversion of electrical energy to mechanical vibrations. Transformer(Booster) : Transmits mechanical vibratory energy to the sonotrode and transforms the vibrations amplitude delivered by the transducer to the value required at the sonotrode.

Sonotrode:- is welding, stacking and insertion tool, performs the function of transmitting vibratory energy, and force and carrying on the forming operation. Welding with rigid PVC is good and SPVC is poor Metal insertion is also possible.