question: Why does plastic when bring stretched or ripped get warm?
comment: Friction between the molecules making up the object, this is not unique to plastic, any material will warm up when being bent or stretched because you are causing movement and therefore friction
passages: High Frequency welding, also known as Dielectric Sealing or Radio Frequency (R.F.) Heat Sealing is a very mature technology that has been around since the 1940s. High frequency electromagnetic waves in the range of Radio Frequencies can heat certain polymers up to softening the plastics for joining. Heated plastics, under pressure weld together. Heat is generated within the polymer by the rapid reorientation of some chemicals dipoles of the polymer, which means that the heating can be localized, and the process can be continuous.
   There are many synonyms for the term high-performance plastics, such as: high temperature plastics, high-performance polymers, high performance thermoplastics or high-tech plastics. The name high temperature plastics is in use due to their continuous service temperature (CST), which is always higher than 150 °C by definition (although this is not their only feature, as it can be seen above).
   Section::::Welding techniques.:Contact welding.
  
  This is the same as spot welding except that heat is supplied with thermal conduction 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.
  
  Section::::Welding techniques.:Hot plate welding.
   Heat sealing is the process of sealing one thermoplastic to another similar thermoplastic using heat and pressure. The direct contact method of heat sealing utilizes a constantly heated die or sealing bar to apply heat to a specific contact area or path to seal or weld the thermoplastics together. Heat sealing is used for many applications, including heat seal connectors, thermally activated adhesives and film or foil sealing.
   Plastic welding
  
  Plastic welding is welding for semi-finished plastic materials, and is described in ISO 472 as a process of uniting softened surfaces of materials, generally with the aid of heat (except solvent welding). Welding of thermoplastics is accomplished in three sequential stages, namely surface preparation, application of heat and pressure, and cooling. Numerous welding methods have been developed for the joining of semi-finished plastic materials. Based on the mechanism of heat generation at the welding interface, welding methods for thermoplastics can be classified as external and internal heating methods, as shown in Fig 1.
   In a ferromagnetic workpiece, plastics can be induction-welded by formulating them with metallic or ferromagnetic compounds, called susceptors. These susceptors absorb electromagnetic energy from an induction coil, become hot, and lose their heat energy to the surrounding material by thermal conduction.
  
  Section::::Welding techniques.:Injection welding.
   Section::::Description of the thermally induced shape-memory effect.
   Extra heat is contributed by the intense pressure and friction taking place inside the barrel. In fact, if an extrusion line is running certain materials fast enough, the heaters can be shut off and the melt temperature maintained by pressure and friction alone inside the barrel. In most extruders, cooling fans are present to keep the temperature below a set value if too much heat is generated. If forced air cooling proves insufficient then cast-in cooling jackets are employed.
   Heat fusion
  
  Heat fusion (sometimes called heat welding, butt welding or simply fusion) is a welding process used to join two different pieces of a thermoplastic. This process involves heating both pieces simultaneously and pressing them together. The two pieces then cool together and form a permanent bond. When done properly, the two pieces become indistinguishable from each other. Dissimilar plastics can result in improper bonding.
  
  Section::::Applications.
   Plastics can also be classified by: their various physical properties, such as: hardness, density, tensile strength, resistance to heat and glass transition temperature, and by their chemical properties, such as the organic chemistry of the polymer and its resistance and reaction to various chemical products and processes, such as: organic solvents, oxidation, and ionizing radiation. In particular, most plastics will melt upon heating to a few hundred degrees celsius.
   Welding thermoplastic is very similar to welding glass. The plastic first must be cleaned and then heated through the glass transition, turning the weld-interface into a thick, viscous liquid. Two heated interfaces can then be pressed together, allowing the molecules to mix through intermolecular diffusion, joining them as one. Then the plastic is cooled through the glass transition, allowing the weld to solidify. A filler rod may often be used for certain types of joints. The main differences between welding glass and plastic are the types of heating methods, the much lower melting temperatures, and the fact that plastics will burn if overheated. Many different methods have been devised for heating plastic to a weldable temperature without burning it. Ovens or electric heating tools can be used to melt the plastic. Ultrasonic, laser, or friction heating are other methods. Resistive metals may be implanted in the plastic, which respond to induction heating. Some plastics will begin to burn at temperatures lower than their glass transition, so welding can be performed by blowing a heated, inert gas onto the plastic, melting it while, at the same time, shielding it from oxygen.
   Yield strength anomaly
  
  In materials science, the yield strength anomaly refers to unusual materials wherein the yield strength (i.e., the stress necessary to initiate plastic yielding) increases with temperature. 
  
  For the vast majority of materials the yield strength decreases with increasing temperature.
   Over jacketing extrusion allows for the application of an outer layer of plastic onto an existing wire or cable. This is the typical process for insulating wires.
   Common applications for the heat sealing process: Heat seal connectors are used to join LCDs to PCBs in many consumer electronics, as well as in medical and telecommunication devices. Heat sealing of products with thermal adhesives is used to hold clear display screens onto consumer electronic products and for other sealed thermo-plastic assemblies or devices where heat staking or ultrasonic welding is not an option due to part design requirements or other assembly considerations. Heat sealing also is used in the manufacturing of bloodtest film and filter media for the blood, virus and many other test strip devices used in the medical field today. Laminate foils and films often are heat sealed over the top of thermoplastic medical trays, Microtiter (microwell) plates, bottles and containers to seal and/or prevent contamination for medical test devices, sample collection trays and containers used for food products. Medical and the Food Industries manufacturing Bag or flexible containers use heat sealing for either perimeter welding of the plastic material of the bags and/or for sealing ports and tubes into the bags.
   Overjacketing extrusion is a coating process, in which individual bare wires or bundles of pre-coated wires are coated with a layer of insulating polymer. A wide variety of materials may be used, depending on the specific application. For many applications, such as insulated cables, the polymer should be a good insulator, flexible, and wear resistant.
   Hot plate welding can be used to join some combinations of dissimilar thermoplastics. Typically, semi-crystalline plastics are only compatible with semi-crystalline plastics, and amorphous plastics are only compatible with amorphous plastics. If the plastics have the same melting point and melt viscosity, conventional or high-temperature hot plate welding can be used. With different melting points or different viscosities, dual hot plates should be used, with each hot plate set to a different temperature. Common thermoplastic combinations include:
  
  BULLET::::- ABS – PMMA
  
  BULLET::::- ABS – PC
  
  BULLET::::- ABS – SAN
  
  BULLET::::- PMMA – PC + ABS
   Secondly, because soft robots are made of highly compliant materials, one must consider temperature effects. The yield stress of a material tends to decrease with temperature, and in polymeric materials this effect is even more extreme. At room temperature and higher temperatures, the long chains in many polymers can stretch and slide past each other, preventing the local concentration of stress in one area and making the material ductile. But most polymers undergo a ductile-to-brittle transition temperature below which there is not enough thermal energy for the long chains to respond in that ductile manner, and fracture is much more likely. The tendency of polymeric materials to turn brittle at cooler temperatures is in fact thought to be responsible for the Space Shuttle Challenger disaster, and must be taken very seriously, especially for soft robots that will be implemented in medicine. A ductile-to-brittle transition temperature need not be what one might consider "cold," and is in fact characteristic of the material itself, depending on its crystallinity, toughness, side-group size (in the case of polymers), and other factors. 
   For products such as plastic sheets or films, cooling is achieved by pulling through a set of cooling rolls (also known as calender or chill rolls), usually 3 or 4 in number. In sheet extrusion, these rolls not only deliver the necessary cooling but also help determine sheet thickness and surface texture (in case of structured rolls; i.e. smooth, levant, haircell, etc.). A common processing defect known as nerve may occur when contact time between the rollers and extrudate is too brief, resulting in insufficient cooling time.
   It is necessary to include some basics of plastic deformation to understand the link between heat produced and the plastic work done. If we carry out a compression test on a cylindrical specimen to, say, 50% of its original height, the stress of the work material will increase usually significantly with reduction. This is called ‘work hardening’. During work hardening, the micro-structure, distortion of grain structure and the generation and glide of dislocations all occur. The remainder of the plastic work done – which can be as much as 90% of the total, is dissipated as heat.
   Heat deflection temperature
  
  The heat deflection temperature or heat distortion temperature (HDT, HDTUL, or DTUL) is the temperature at which a polymer or plastic sample deforms under a specified load. This property of a given plastic material is applied in many aspects of product design, engineering and manufacture of products using thermoplastic components.
  
  Section::::Determination.
   The term "polymers" is often used instead of "plastics" because both terms are used as synonyms in the field of engineering. If the term "high-performance thermoplastics" is used, it is because both standard and technical as well as high-performance plastics are always thermoplastics. Thermosets and elastomers are outside of this classification and form their own classes.
  
  However, the differentiation from less powerful plastics has varied over time; while nylon and poly(ethylene terephthalate) were initially considered powerful plastics, they are now ordinary.
  
  Section::::History.
   There are two different types of die tooling used for coating over a wire, tubing (or jacketing) and pressure. In jacketing tooling, the polymer melt does not touch the inner wire until immediately before the die lips. In pressure tooling, the melt contacts the inner wire long before it reaches the die lips; this is done at a high pressure to ensure good adhesion of the melt. If intimate contact or adhesion is required between the new layer and existing wire, pressure tooling is used. If adhesion is not desired/necessary, jacketing tooling is used instead.
  
  Section::::Die types.:Coextrusion.
   Section::::Welding process.:Temperature during welding.
  
  Large temperature gradients exist in the electrofusion joint during the fusion cycle. The low thermal conductivity of polymers is the main cause of these large gradients. Recent efforts to model the thermal history at various locations using finite element modeling have been successful. 
  
  Section::::Welding process.:Pressure during welding.
   Generally, chasers pitch must become soft or even liquid when heated at moderate temperatures, in order to fit the object and fill all its nooks and crannies. For many uses, it is desirable that the pitch is soft like glazing putty while it is still cool enough to be molded with the fingers. On the other hand, at room temperature it must be hard enough to hold the object. It must strongly adhere to the metal while it is cooling.
   However, some plastics are partially crystalline and partially amorphous in molecular structure, giving them both a melting point, the temperature at which the attractive intermolecular forces are overcome, and also one or more glass transitions, the temperatures above which the extent of localized molecular flexibility is substantially increased. These so-called semi-crystalline plastics include: polyethylene, polypropylene, polyvinyl chloride, polyamides (nylons), polyesters and some polyurethanes.
  
  Section::::Properties and classifications.:Conductive polymers.
  