Abrasion Resistance – the ability to withstand the effects of repeated wearing, rubbing, scraping, etc., that tend to remove material from its surface.
Actuation – to put into mechanical action or motion.
Amine – any of a class of organic compounds derived from ammonia by replacement of hydrogen with one or more alkyl groups.
Amorphous – Latin meaning without form. Noncrystalline structure.
Anneal – to prevent the formation of or remove stresses in plastics by cooling from a suitable temperature.
Aramid – any of a group of lightweight but very strong heat-resistant synthetic aromatic polyamide materials that are fashioned into fibers, filaments or sheets.
Arc – 1) a luminous glow formed by the flow of electric current through ionized air, gas or vapor between separated electrodes or contacts; 2) a portion of the circumference of a circle.
Arc Resistance – 1) the resistance to the flow of current offered by the voltaic arc (i.e., if the carbons of an arc lamp are 1/32" apart, the arc resistance will be 1½ ohms); 2) the resistance of a material to the effects of an arc passing across its surface stated as a measure of the total elapsed time taken to form a conducting path (of material carbonizing by the arc flame) across the surface under prescribed conditions of applications of a high voltage, low current arc (as across an insulator).
Atactic – a polymer exhibiting no stereochemical regularity of structure.
AWG – acronym for American Wire Gauge, a standard system for designating wire diameter.
Band Saws and/or Hand Power Saws – see Sawing
Base – the material woven (such as paper, woven cotton, glass fabric or glass fiber mat, felted asbestos, aramid fibers, graphite and nylon fabrics) in the form of sheets or rolls which can be impregnated with resin to form laminated plastics.
Binder – the organic or inorganic material which encapsulates and holds together the base in reinforced or otherwise heterogeneous composites.
Bond Strength – 1) the measure of the force required to separate objects or materials bonded together; 2) the strength of the bond between fiber and matrix; 3) the degree of attraction between adjacent atoms within a molecule, usually expressed in J/mol.
Brake Bending – see Forming
Breakdown – the disruptive discharge through insulation due to failure under electrostatic stress.
British Thermal Unit (B.T.U.) – the quantity of heat required to raise the temperature of one pound of water 1°F from 58.5 to 59.5°F (0.5°C from 14.7° to 15.2°C) —its point of maximum density.
Buff Polishing – see Finishing
Butt Welding – see Welding
Canvas – a cotton fabric weighing more than four ounces per square yard. (Used as the base material for NEMA grades C, CE and some L grade laminates.)
Capacitance – the property of an electric nonconductor that permits the storage of energy as a result of the separation of charge that occurs when opposite surfaces of the nonconductor are maintained at a difference of potential.
Carbonyl – organic functional group occurring in aldehydes, ketones, carboxylic acids, esters and their derivatives.
Celsius – also referred to as Centigrade, is equal to the difference between the temperature in Fahrenheit less 32, divided by 1.8.°C = (°F - 32) ÷ 1.8.
Cement – a dispersion of “solution” of unvulcanized rubber or a plastic in a volatile solution. This meaning is peculiar to plastics and rubber industries and may not be an adhesive composition.
Contact Welding – see Welding
CNC Machine – see Routing
CNC/NC Panel Saws – see Sawing
Coefficient of Friction – static: the ration of the limiting friction developed to the corresponding normal pressure, if two surfaces move relative to each other.
Coefficient of Thermal Expansion – the unit change in dimension of a material for a unit change in temperature.
Cold Forming – see Forming
Composite – 1) a structure or an entity made up of distinct components; 2) a complex material, such as fiberglass, in which two or more distinct, structurally complementary substances, especially glasses and polymers, combine to produce structural or functional properties not present in any individual component; 3) reinforced laminates (i.e., canvas phenolic, glass epoxy, etc.).
Compressive Strength – crushing a load at failure divided by the original sectional area of the specimen.
Conductivity – the reciprocal of volume resistivity. It is the conductance of a unit cube of any material.
Copolymer – a polymer formed through the interpolymerization of two (or more) chemically different monomers with each other.
Copper-Clad Laminate – laminates (i.e., FR4) having copper foil bonded to one or both surfaces and intended primarily for use in printed circuits.
Corrosion – chemical action which causes destruction of the surface of a material by oxidation or chemical combination. Also caused by reduction of the electrical efficiency between a metal and a contiguous substance or to the disintegrating effects of strong electrical currents or ground return currents in electrical systems. The latter is known as electrolytic corrosion.
Crazing – minute lines appearing in or near the surface of materials such as plastics, usually resulting as a response to environment. Crazing cannot be felt by running a fingernail across it. (If the fingernail catches, it is a crack.)
Creep – the dimensional change with time of a material under load. At room temperature it is also called cold flow.
Cross-Linking – the setting-up of chemical valence links between the molecular chains of polymer molecules, leading to the formation of a three-dimensional network of polymer chains that is infusible and insoluble. This usually reduces the thermoplasticity of the material.
Crystallinity – a molecular structure resulting from the formation of solid crystals with a geometric pattern.
Cure – to change the physical, chemical or electrical properties of a material by chemical reaction, by the action of heat and catalysts alone or in combination, with or without pressure. Specifically to convert a low molecular weight polymer or resin to an insoluble, infusible state.
- Die Cutting – process of cutting shapes from sheets of plastic by pressing a shaped knife edge into one or several layers of sheeting. The dies are often called steel rule dies, and pressure is applied by hydraulic or mechanical presses.
- Laser Cutting – uses a laser to cut materials into virtually any shape, and is often used in industrial manufacturing. Laser cutting works by directing the output of a high power laser, by computer, at the material to be cut. The material then either melts, burns, vaporizes away, or is blown away by a jet of gas, leaving an edge with a high quality surface finish. Industrial laser cutters are used to cut flat-sheet material, as well as structural and piping materials.
- Punching – the process of stamping out shapes from thin sheets of plastics. Similar to die cutting, but used for smaller parts.
- Shearing – process of cutting sheets to size out of a larger stock. Shears are used as the preliminary step in preparing stock for other fabrication processes or smaller blanks.
- Water Jet Cutting – uses a high-pressure water to cut materials into virtually any shape, and is often used in industrial manufacturing. Water jets are used to cut flat-sheet material.
Delamination – the separation of a laminate along the plane of its layers. Also the separation of bonded insulation within the adhesive layer or at the adhesive interface.
Density – weight per unit volume of a given substance.
Diamond Edge Polishing – see Finishing
Die Cutting – see Cutting
Dielectric – 1) any insulating medium which intervenes between two conduits and permits electrostatic attraction or repulsion to take place across it; 2) a material having the property that energy required to establish an electric field is recoverable in whole or in part, as electric energy (see insulation for clarification).
Dielectric Constant (Permittivity or Specific Inductive Capacity) – the specific inductive capacity of a dielectric. That property of a dielectric which determines the electrostatic energy stored per unit volume for unit potential gradient.
Dielectric Strength – the voltage which an insulating material can withstand before breakdown occurs, usually expressed as a voltage gradient (such as volts per mil).
Dimensional Stability – ability to retain precise shape and size. Dissipation – unusable or lost energy, as the production of heat in a circuit.
Dissipation Factor (loss tangent, tans, approximate power factor) — the tangent of the loss angle of the insulating material.
Drape Forming – see Forming
Drilling – the process of creating singular or multiple holes in plastic shapes. Can be done manually or with a CNC.
Elongation – the fractional increase in length of a material stressed in tension.
Epoxy Resins – straight-chain thermosetting resins containing at least one three-membered ring consisting of two carbon atoms and one oxygenation.
Extrusion – the method of processing plastic by forcing heat softened plastic through an opening of the desired shape of the cross-section of the finished product.
Extrusion Welding – see Welding
Fabric-Base Laminate – laminated insulating material formed by bonding woven cloth (of fiberglass, cotton or synthetic fibers) with resin under heat and pressure.
Fahrenheit – equals 1.8 multiplied to the sum of the temperature in Celsius and 32. °F = 1.8 x (°C + 32).
Fiber – a thread or threadlike structure such as cellulose, wool, silk or glass yarn.
Fibre – 1) a specific form of chemically jelled fibrous materials manufactured in sheets, rods and tubes; 2) commonly used interchangeably with fiber.
Filament – 1) fiber characterized by extreme length; 2) the resistance wire through which filament current is sent in a thermionic tube to produce the heat required for electron emission.
Filament Winding – resin-impregnated roving or single strands of glass or other reinforcement wound in a predetermined pattern onto a suitable form or mandrel and then cured.
- Buff Polishing – a power-driven buffer used to restore edges and surfaces to its original high luster. A stationary machine with polishing wheels is preferred.
- Diamond Edge Polishing – a milling machine head equipped with a diamond cutter to maximize finish quality.
- Flame Polishing – a hydrogen-oxygen torch gently melts the sanded or machined edges providing a smooth glossy look.
- Hot/Cold Stamping – the use of heated or cold stamps to decorate or identify the finished product.
- Jointer-Planer – a standard woodworking machine, the jointer-planer is a rotating knife that removes material to produce the desired edge or surface finish.
- Painting – applying paint to a plastic substrate using conventional spray, spray masked, roller coat or brushed application methods.
- Sanding – removing material using an abrasive medium to alter the surface finish.
- Screen Printing – using standard silk screening equipment to apply ink to a plastic substrate, producing a high quality image.
- Solvent Polishing – improving edge quality, using solvents such as methylethyl ketone (MEK) or methylene dichloride.
Fishpaper – 1) a type of vulcanized fibre paper treated chemically for insulating purposes where high mechanical and electrical strength and flexibility are required; 2) a vulcanized fibre in thin cross-section.
Flame Polishing – see Finishing
Flange – a rib or rim for strength, for guiding, or for attachment to a pipe.
Flash Point – the lowest temperature at which a flammable liquid will produce a combustible vapor that will burn in the presence of a flame, under certain prescribed conditions of test.
Flexural Strength – the strength of a material in bending, expressed as the tensile stress of the outermost fibers of a bent test specimen at the instant of failure.
- Brake Bending – bending plastic to an angle using a brake press. Can be manual or NC controlled.
- Cold Forming – a bend that can be accomplished without heating the material.
- Drape Forming – the process by which a flat thermoplastic sheet is heated and stretched over a male mold. Because the sheet is heated and the mold is not, the lower temperature of the latter causes the sheet to begin cooling on contact thereby preventing the sheet from stretching and becoming too thin.
- Line Bending – a method of forming a sharp bend in the material. The radius of the bend can be controlled by adjusting the width of the heated area.
- Membrane Pressing – uses an inflated bladder or “membrane” to physically press heated plastic sheet onto a wood or composite substrate or “core” which eliminates the need for molds.
- Pressure Forming – a version of thermoforming which results in parts with very high definition. With pressure forming, a hot plastic sheet is forced against a mold. Usually, this mold is a female mold, which means it is concave. Compressed air is then added to the back of the heated sheet in order to form the plastic with the mold.
- Thermoforming – a manufacturing process where plastic sheet is heated to a pliable forming temperature, formed to a specific part shaped in a mold, and trimmed to create a usable product. The sheet, or “film” when referring to thinner gauges and certain material types, is heated in an oven to a high-enough temperature so that it can be stretched into or onto a mold and cooled to a finished shape.
- Vacuum Forming – a version of thermoforming, whereby a sheet of plastic is heated to a forming temperature, stretched onto or into a single-surface mold, and held against the mold by applying vacuum between the mold surface and the sheet.
Formulation – 1) a combination of ingredients before processing or made into a finished product. Also used as a synonym for a material or compound; 2) a selection of components of a product formula or mixture to provide optimum specific properties for the end use desired.
Friction Stir Welding – see Welding
Gasket – piece used to make a joint fluid-tight.
Glass Cloth – fabric used as insulating material base formed by weaving yarns comprising glass filaments and possessing high strength, heat resistance and dielectric properties.
Glass Fiber – glass in fibrous form that has cooled to a rigid condition without crystallizing.
Hand or Pin Routing – see Routing
Heat Distortion Point – the temperature in degrees Celsius at which a standard test bar (ASTM D648) deflects .010 in. under a stated load of either 66 or 264 psi, when the temperature is raised at a specific rate of increase.
Heat Loss – power dissipated as heat.
High Frequency (RF) Welding – see Welding
High-Pressure Laminates (molding pressure high) – laminates molded and cured at pressures not lower than 1,000 psi (4.8 MPa). Pressures of 1,000 to 2,500 psi (4.8 to 17 MPa) are not uncommon.
Hot Air/Wedge Welding – see Welding
Hot/Cold Stamping – see Finishing
Hot Gas Welding – see Welding
Hot Plate Welding – see Welding
Impact Resistance – relative susceptibility of material to fracture by stress at high speeds.
Impact Strength – ability to withstand physical shock loading or work required to fracture under shock loading a specified test specimen in a specified manner.
Impregnate – 1) to fill the voids and interstices of material with a compound (this does not imply complete fill or complete coating of the surfaces by a hole-free film); 2) the process of thoroughly soaking a material of an open or porous nature with a resin.
Inert – deficient in active properties; not affecting other substances when in contact with them such as inert gases not participating in any fashion in chemical reactions.
Infrared – the band of light in the electromagnetic spectrum that lies between the visible light range and the radar range.
Infrared Welding – see Welding
Injection Welding – see Welding
Insulation – material having a high resistance to the flow of electric current, to prevent leakage of current from a conductor.
Insulation Resistance – the ratio of the applied voltage to the total current between two electrodes in contact with a specific conductor under prescribed conditions of test.
Isomer – one of two or more compounds, radicals or ions that contain the same number of atoms of the same elements but differ in structural arrangement and properties.
Isotactic – having a stereochemical regularity of structure in the repeating units of a polymer.
Jointer-Planer – see Finishing
Kelvin – the absolute temperature scale (metric). K = °C + 273.
Kraft Paper – 1) relatively heavy, high strength sulfate paper used for electrical insulating material; 2) paper made from sulfate wood pulp, chiefly pinewood chips by digestion with a mixture of caustic soda.
Laminate – 1) (v) to build up to desired shape or thickness; 2) (n) – a material composed of successive layers of material, usually bonded together under heat and pressure.
Laser Cutting – see Cutting
Laser Welding – see Welding
Light Transmission – the amount of light that a plastic will allow to pass.
Line Bending – see Forming
Loss Factor – the product of the power factor and the dielectric constant of a dielectric material.
Low-Pressure Laminates – laminates molded and cured in the range of pressures up to 400 psi.
Mat – 1) a randomly distributed felt of fibers, usually glass, used in reinforced plastics; 2) a nonwoven fabric of fibrous material used as a plastic reinforcement.
Membrane Pressing – see Forming
Mica – a transparent, flaky mineral which splits into thin sheets and has excellent insulating and heat resisting properties, consisting of orthosilicates of aluminum or potassium; occurs naturally.
Microwave Welding – see Welding
Moisture Resistance – the ability of a material to resist absorbing ambient moisture.
Monomer – the simple, unpolymerized form of a compound which is the building block of a polymer.
Ohm – practical unit of electric resistance.
Ohm’s Law – the strength of a direct current is directly proportional to the potential difference and inversely proportional to the resistance of the circuit.
Orbital Welding – see Welding
O-ring – a ring used as a gasket.
Outgassing – to remove occluded gases by heating.
Painting – see Finishing
Panel Saws – see Sawing
Permeability – 1) the passage or diffusion (or rate of passage) of a gas, vapor, liquid or solid through a barrier without physically or chemically affecting it; 2) the ability of a material to carry magnetism as compared to air which has a permeability of one.
Permittivity – preferred term for dielectric constant. It is that property of a dielectric material that determines how much electrostatic energy can be stored per unit of volume when unit voltage is applied; the relative permittivity of most materials varies from 2 to 10, air having 1.
Phenolic Resin – 1) a synthetic resin produced by the condensation of phenol with an aldehyde (usually formaldehyde); 2) any of several types of thermoset plastics obtained by the condensation of phenol or substituted phenols.
Plastic – 1) high polymeric substances, including both natural and synthetic products, but excluding the rubbers that are capable in their manufacture of flowing under heat and pressure; 2) a material that contains an organic substance of large molecular weight, solid in finished state.
Plasticizer – chemical agent added to plastics to make them softer and more flexible.
Pneumatic – moved or worked by air pressure.
Polymer – 1) a compound formed by the chemical union of two or more monomers of the same kind; 2) a synthetic or natural compound of high molecular weight.
Pressure Forming – see Forming
Punching – see Cutting
Reinforcement – material used to reinforce, strengthen or give dimensional stability to another material; can be chopped, woven or braided.
Resin – 1) a substance that is polymeric in structure and predominantly amorphous; 2) an alternative term, like polymer, to the predominately used name of plastic.
Resistance – property of a conductor that opposed the current flow produced by a given difference of potential. The ohm is the practical unit of resistance.
Rockwell Hardness – a test for hardness (resistance to indentation) in which a hardened steel ball or diamond point is pressed into the material under test.
Routing – routers are the most versatile pieces of equipment and produce a high-quality machined edge, ready for finishing.
- CNC Machine – CNC routing is the cutting, shaping, drilling, milling and surfacing of materials using a computer-controlled router or milling machine. Used for high-volume or precision parts.
- Hand or Pin Routing – used for trimming operations and pattern cutting in low-volume parts.
Sanding – see Finishing
- Band Saws and/or Hand Power Saws – useful for trimming formed parts or cutting irregular shapes.
- CNC/NC Panel Saws – used for high-volume, intricate and precise parts. The part is designed on a CAD system and geometrically programmed directly into the CNC machine.
- Panel Saws – commonly used for trimming or blanking a sheet for further fabrication in small volumes. Saws can be horizontal or vertical.
Screen Printing – see Finishing
Shear Strength – 1) ability of a material to withstand shear stress or stress at which a material fails in shear; 2) the maximum shear strength stress that a material is capable of substaining.
Shearing – see Cutting
Slurry – a watery mixture of insoluble matter.
Solvent Polishing – see Finishing
Solvent Welding – see Welding
Specific Gravity – the density of any material divided by that of water at a standard temperature.
Speed Tip Welding – see Welding
Spherulites – spherical crystalline body of radiating crystal fibers.
Spin Welding – see Welding
Surface Resistivity – the resistance of a material between two opposite sides of a unit square of its surface.
Syndiotactic – having a stereochemical regularity where the molecules can be described in terms of alternation of configurational base units that are mirror images of one another.
Tear Strength – 1) force required to initiate or continue a tear in a material under specified conditions; 2) the force acting substantially parallel to the major axis of the test specimen.
Tensile Strength – the longitudinal stress required to break a prescribed specimen divided by the original cross-sectional area at the point of rupture (usually expressed in pounds per square inch), within the gauge boundaries sustained by the specimen during the test.
Thermal Conductivity – the ability of a material to conduct heat; physical constant for quantity of heat that passes through volume of a substance in unit of time for unit difference in temperature.
Thermal Stir Welding – see Welding
Thermionic – relating to, using or being the emission of charged particles (as electrons) by an incandescent material.
Thermoforming – see Forming
Thermoplastic – plastics capable of being repeatedly softened by increases in temperature and hardened by decreases in temperature. These changes are physical rather than chemical.
Thermoset – a classification of plastic resin that cures by chemical reaction when heated and, once cured, cannot be resoftened by heating.
Ultrasonic Welding – see Welding
Vacuum Forming – see Forming
Vibration or Friction Welding – see Welding
Volume Resistivity – resistance between opposite faces of 1 cm cube of material, usually in ohm-cms.
Water Jet Cutting – see Cutting
Weir – a dam to divert water flow.
- Butt Welding – plastic pieces are positioned with the ends a certain distance to the heating element until the material is melted. The heating element is then removed and the molten surfaces of the plastic pieces are pressed together. The pieces cool down under pressure until the melt is resolidified.
- Contact Welding – the same as spot welding except that heat is supplied with convection of the pincher tips instead of electrical conduction. Two plastic parts are brought together where heated tips pinch them, melting and joining the parts in the process.
- Extrusion Welding – allows the application of bigger welds in a single weld pass. It is the preferred technique for joining material over 6 mm thick. Welding rod is drawn into a miniature hand-held plastic extruder, plasticized and forced out of the extruder against the parts being joined, which are softened with a jet of hot air to allow bonding to take place.
- Friction Stir Welding – a nonconsumable rotating tool is pushed into the materials to be welded and then the central pin, or probe, followed by the shoulder, is brought into contact with the two parts to be joined. The rotation of the tool heats up and plasticizes the materials it is in contact with and, as the tool moves along the joint line, material from the front of the tool is swept around this plasticized annulus to the rear, eliminating the interface.
- High Frequency (RF) Welding – also known as dielectric sealing, R.F. (radio frequency) heat sealing. Radio frequency welding is a very mature technology that has been around since the 1940s. Two pieces of material are placed on a table press that applies pressure to both surface areas. Dies are used to direct the welding process. When the press comes together, high frequency waves (usually 27.12 MHz) are passed through the small area between the die and the table where the weld takes place. This high frequency (radio frequency) field causes the molecules in certain materials to move and get hot, and the combination of this heat under pressure causes the weld to take the shape of the die.
- Hot Air/Wedge Welding – uses hot air or a wedge to heat the coating on the fabric where it is to be bonded together. A nozzle or heated wedge is positioned between two rollers that pull the material through the machine. As the material is pulled through the machine, hot air is applied to the surfaces to be fused together. Pressure from the rollers and heat from the hot air cause the plastic to fuse as the plastic cools.
- Hot Gas Welding – a plastic-welding technique analogous to gas welding of metals, though the specific techniques are different. A specially designed heat gun (hot air welder) produces a jet of hot air that softens both the parts to be joined and a plastic filler rod, all of which must be of the same or a very similar plastic. - Hot Plate Welding – related to contact welding, this technique is used to weld larger parts, or parts that have a complex weld joint geometry. The two parts to be welded are placed in the tooling attached to the two opposing platens of a press. A hot plate, with a shape that matches the weld joint geometry of the parts to be welded, is moved in position between the two parts. The two opposing platens move the parts into contact with the hot plate until the heat softens the interfaces to the melting point of the plastic.
- Infrared Welding – plastic pieces are positioned with the ends a certain distance to the heating element until the material is melted. The input of heat is performed through infrared radiation and is not limited by the maximum temperature of a nonstick coating. The heating element is then removed and the molten surfaces of the plastic pieces are pressed together. The pieces cool down under pressure until the melt is resolidified. No weld fillers are needed.
- Injection Welding – uses a heated tip and injection pressure to form welds. The hot (interchangeable) tip melts the surface of the plastic and creates a weld zone into which molten plastic is injected. There is an actual physical mixing of the weld bead and the plastic. Injection welding is said to produce the strongest welds in this group. Because the tip cleans the plastic surface prior to welding, preparation is unnecessary unless there’s heavy contamination.
- Laser Welding – requires one part to be transmissive to a laser beam and either the other part absorptive or a coating at the interface to be absorptive to the beam. The two parts are put under pressure while the laser beam moves along the joining line. The beam passes through the first part and is absorbed by the other one or the coating to generate enough heat to soften the interface creating a permanent weld.
- Microwave Welding – is based on the heating of the material by electromagnetic irradiation in the microwave frequency range. The physical interactions between the material and the electromagnetic radiation cause the material to melt and the parts are welded together.
- Orbital Welding – a specialized area of welding whereby the arc is rotated mechanically 360° around a static work piece (around an object such as a pipe) in a continuous process.
- Solvent Welding – a solvent is applied to temporarily dissolve the polymer at room temperature. When this occurs, the polymer chains are free to move in the liquid and can mingle with other similarly dissolved chains in the other component. Given sufficient time, the solvent will permeate through the polymer and out into the environment, causing the chains to lose their mobility. This leaves a solid mass of entangled polymer chains, which constitutes a solvent weld.
- Speed Tip Welding – the plastic welder, similar to a soldering iron in appearance and wattage, is fitted with a feed tube for the plastic weld rod. The speed tip heats the rod and the substrate, while at the same time it presses the molten weld rod into position. A bead of softened plastic is laid into the joint, and the parts and weld rod fuse.
- Spin Welding – another form of frictional welding. With this process, one part is held stationary, while the other one is rotated at high velocity. The rotating part is then pressed against the fixed part with significant force. This welding process is related to vibration welding.
- Thermal Stir Welding – a welding method and apparatus that separately plasticizes or melts the surfaces to be joined followed by a subsequent weld matrix stirring process.
- Ultrasonic Welding – high frequency (15 kHz to 40 kHz ) low amplitude vibration is used to create heat by way of friction between the materials to be joined. The interface of the two parts is specially designed to concentrate the energy for the maximum weld strength.
- Vibration or Friction Welding – the two parts to be assembled are rubbed together at a lower frequency (typically 100-300 Hz) and higher amplitude (typically 1-2 mm) than ultrasonic welding. The friction caused by the vibration motion, combined with the clamping pressure between the two parts, creates the heat which begins to melt the contact areas between the two parts. At this point, the plasticized materials begin to form layers that intertwine with one another, resulting in a strong weld.