MACHINING & JOINING OF PLASTICS:
1. High precision items must be machined from standard shapes by Drilling, grinding etc., to meet close tolerances.
2. It is desirable to make parts by machining when production volume is less.
3. To supplement moulding operations. (e.g) Cutting of sheets
4. Drilling a hole.
ABRASIVE CUTTING METHODS
1. ABRASIVE WHEEL CUTTING
High quality of 0.01 mm roughness could be achieved No further finishing operation. Economical since tool cost is less.
2. Use of air or high speed rotation to impinge a stream of abrasive particles on line of work to remove material.
SHEAR CUTTING METHOD
Punching and simple shearing specially for cutting sheets. Nibbling tool used oscillates 40 / 60 times/second.
Accurate and high speed cutting device
THERMAL CUTTING METHODS
Hot wire and Hot gas flame torch to melt plastic and serve it.
Hot wire techniques used for PMMA / EPS
DRILLING OF PLASTICS
Point angle of 80 degree is recommended for plastics.
As drill penetrates in to work piece heat dissipation becomes increasingly difficult.
Drill heat is the cause of gumming and burring
Because of the elastic recovery of many thermoplastics drilled holes are often smaller than drill diameter.
LAMINATED PLASTICS PRESENT DIFFICULTIES IN DRILLING
Drilling at right angles to the lamination causes
swelling and cracking around
Drilling parallel to lamination tends to separate
them.
Molded thermoplastics some times have large residual stresses.Such internal stressed parts may crack when drilled.
JOINING OF PLASTICS
Methods used for Joining 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 joined.
Some means is employed to cause two materials to flow together and fuse.
Two techniques in this category
1. Solvent cementing
2. Thermal welding
Cohesive bonding involves joining of similar pieces.
Different materials may be joined only when the flow properties are similar.
II. Adhesion Principle
Makes use of film which adheres to each other material to be joined.
An adhesive, a substance different from either of the materials remains in the joint.
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 joining 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 keten e 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 - 2.5 parts by wt.
ADHESIVES
Elastomeric: (e.g.) NR, Neoprene, Nitrile, etc., used for joining plastic to metals to metal, etc.
Good bond strength and high degree of joint 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
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