Experimental study of the effect of different heat treatment parameters on the mechanical strength of thermoplastic parts welded by hot plate method

Dissertation for receiving the master's degree "M.Sc". First, a brief discussion about polymers, plastics and their sub-branches and heat treatment is discussed on them, and then we will discuss the effect of annealing heat treatment on heavy polyethylene sheets that have been welded by the hot plate method.

First, the polyethylene sheet, which was prepared in a roll with a thickness of 2.3 mm, was cut into smaller dimensions so that it could be used by the welding machine. The Elumatec® hotplate welded them. In order to achieve a higher speed of operation and more assurance of the same welding conditions, fixtures made of wood fiber are used so that several samples can be welded together in each step, and in addition, the sheets do not get distorted. In this way, all parts are welded under the same condition and then annealed under different conditions. Sheet parts were welded at 7 bar clamping pressure, plate temperature 230 degrees Celsius and holding time 15 seconds and then annealed in different conditions in terms of temperature, pressure, holding time and cooling rate of heat treatment. To ensure the results, three samples are considered for each annealing mode, and the final result for each mode is determined by averaging these three samples. A drying oven is also used for annealing the parts, and the fan in it causes uniform circulation of the air inside the oven, resulting in uniform heat transfer. After annealing, the samples are cut and prepared for the impact test according to the ASTM standard and the test is performed. The samples required to perform the split isod impact strength test were cut according to the ASTM D256 standard and their impact resistance was measured by the impact resistance determination device with a 5 Joule pendulum. The results show that some annealed samples have significantly higher impact resistance than the non-heat treated samples. To compare the results with the non-annealed state, we also test a sample without annealing. Finally, the effects of various annealing parameters on the strength of the parts have been identified and extracted. Keywords: HDPE - extrusion - heat treatment - annealing. Definition of plastics. The word plastic is of Greek origin and derived from the word Plastikos, which means "forming or placing in a mold for molding". is The SPI Plastics Industry Association has provided a more detailed and specific explanation as follows:

Plastics are a group of materials that fully or partially contain compounds of carbon with oxygen, nitrogen and hydrogen or other organic and inorganic elements. These materials, in their final state, turn into a solid state. In several stages of their manufacturing and production process, they take a liquid form and as a result, they are able to form three-dimensional objects in various shapes. The process of forming them is by using heat and pressure.

In fact, plastics are solid and stable materials with the origin of oil and gas, which today are a very suitable substitute for wood, metal, glass, and ceramics. Plastics are part of a larger family of materials called polymers.

2-1: Polymers

The structure of polymers consists of large molecules that are formed by sticking together many smaller molecules. These smaller molecules are called monomers, and the act of connecting them is called monomer insertion.

If the constituent units of a polymer (monomer) are of the same type, it is called a homopolymer, and if the monomers that make up a polymer are different, it is called a copolymer.

Figure 1-1 Amber) ). It is called the fossilized sap of a tree, which is usually valued for its beautiful color.. This material is usually used to make decorative objects and jewelry.

1-2-1: Classification of polymers

In the most important classification, polymers are divided into two groups:

A) Natural polymers: which are the result of natural interactions. Such as starch, cellulose, natural rubber (latex), proteins (such as silk thread) and all kinds of natural gums and resins such as amber (Figure 1-1), turpentine, turpentine, petroleum materials such as bitumen or polysaccharides such as sugar.

b) Synthetic polymers: that is, compounds created by humans. Such as elastomers, plastic and synthetic fibers, coatings and adhesives, etc.

4-1: Types of plastics

Plastics are divided into two major groups: thermosetting or thermoset and thermoplastic or thermoplastic. Thermoset or thermoset plastics are hardened by chemical reaction and thermal or chemical treatment, they become permanent and cannot be softened again. Thermosets have high hardness, stiffness, resistance to heat and chemical solvents, and high electrical resistance. Thermosets, unlike thermoplastics, are not chemically stable and active, and with the passage of time, transverse connections are created in them. Usually, additives to thermosets include: sawdust, clay, They add porcelain soil and cotton fibers. Although rubber is a thermoset due to the presence of crosslinks in the chain, which is called "vulcanization", it is not fragile and has the ability to move. In fact, its most important properties are elasticity, flexibility, and return to the original state. Sulfur is combined with rubber in the double joints and creates special and very important properties in rubber, such as: resistance to heat, resistance to atmospheric and chemical factors, wear and elasticity. Also, in addition to sulfur, which is the most important additive, softener (paraffin) and colored grains (pigment) and reinforcement (soot) and fillers such as talcum powder are also added to rubber.

Thermoplastic or thermoplastic plastics are materials that harden during cooling and can be softened and molded by heating again.

1-5: Modification of properties in plastics

Another feature of plastics is that their properties can be improved. This action is possible by using additives and performing heat treatment.

1-5-1: Additives

With the help of additives, the life of materials and parts is increased, their physical and mechanical characteristics are improved, their process is significantly simplified, their quality loss is controlled, and The vulnerability of materials against various waves is prevented. It is also possible to make them conductive or non-conductive to electricity and heat according to the requirements and protect them from microbiological attacks. These materials include stabilizers, fillers, pigments, mold release materials, anti-microbial materials, impact modifiers, process aids, anti-static materials, anti-fire and smoke materials, organic peroxides, anti-oxidants, softeners, foaming materials, surface activators, stabilizers, each of which is added to plastics to create desirable properties.

1-5-2: Heat treatment

Another method of improving the physical and mechanical properties of plastics is the use of heat treatment. In the operations of molding, machining, polishing and other production operations that are performed on plastics and composites, internal tensions arise in them. The use of chemicals (such as adhesives) in this operation will also cause parts to become sensitive and crack.

Rapid cooling of molded plastic parts after exiting the mold or after annealing (in the production of parts from thermoset materials) also creates internal tensions in them.