Plastics are materials based on organic natural, synthetic or organic
polymers, from which, after heating and applying pressure, products of complex
configuration can be molded. Polymers are high molecular weight compounds
made up of long molecules with a large number of identical groups of atoms
linked by chemical bonds. In addition to polymer, there may be some
additives in plastic.
Plastics processing is a set of technological processes that ensure the production of products - parts with a given configuration, accuracy and performance properties.
High quality of the product will be achieved if the selected material and technological process meet the specified operational requirements of the product: electrical, mechanical strength, chemical resistance, density, transparency, etc.
When processing plastics in conditions of mass production, to ensure high quality of products, materials science, technological, scientific-organizational and other problems are solved.
Materials science problems consist in the correct choice of the type and grade of polymer, so as to ensure the possibility of forming a product with a given configuration and operational properties.
Technological tasks include the whole set of issues of polymer processing technology that ensure product quality:
Preparation of polymers for processing,
Development-determination of technological parameters of the process,
Development of equipment,
· Choice of equipment.
The main stages of work on the use of plastics in products are as follows:
Polymers consist of repeating groups of atoms - units of the original
substance - a monomer, forming molecules thousands of times longer than the
length of non-polymeric compounds, such molecules are called
macromolecules. The more links there are in the polymer macromolecule (the
higher the degree of polymerization), the more durable the material and the
more resistant to heat and solvents. Due to the impossibility of efficient
processing of a low-melting and poorly soluble polymer, in some cases,
semi-finished products are first obtained - polymers with a relatively low
molecular weight - oligomers that are easily brought to a high molecular level
with additional heat treatment simultaneously with the manufacture of the
Depending on the composition, groups of polymeric compounds are distinguished: homopolymers - polymers consisting of identical monomer units; copolymers - polymers consisting of different starting monomer units; organoelement - compounds with introduced into the main chain or side chains of silicon atoms (organosilicon compounds), boron of aluminum, etc. These compounds have increased heat resistance.
The shape of the molecules can be:
In organic polymeric materials, the macrostructure is formed either by flexible macromolecules rolled into coils (globules), or by bundles of lamellas of more rigid macromolecules, arranged in parallel in several rows, since in this case they have a thermodynamically more favorable shape, in which a significant part of the lateral surface is adjacent to each other. to friend. Domains are formed in the folding areas, and the domains create fibrils connected by passage areas. Several domains, joining along the folding planes, form primary structural elements - crystals, from which plate-like structures - lamellas - arise when the melt is cooled. In the process of folding the lamellas, the ends of the molecules can be in different planes; sometimes these ends of the molecules partially return to the initial plane - in this case, they create loops.
Solid state polymers can be amorphous or crystalline.
When an amorphous polymer is heated, three physical states are observed: glassy, highly elastic, and viscous. These states are established based on the thermomechanical state curve. An amorphous polymer is below the glass transition temperature (Tg) in a solid state of aggregation. At temperatures above Tc, the polymer is in a highly elastic state; In this case, molecular mobility becomes so great that the structure in the short-range order has time to rearrange itself following a change in temperature, and macromolecules can bend under the action of external forces. In this case, the total deformation is composed of elastic and retarded highly elastic deformation. Under elastic deformation, the average center-to-center, intermolecular distances and bond angles in the polymer chain change,
A crystallizing polymer, depending on the cooling rate of the polymer melt, can exhibit two types of structures: amorphous and crystalline. Upon slow cooling of crystallizing polymers, the joint stacking of segments of macromolecules forms the structure of macromolecules. This makes it difficult for them to transition from one conformation to another, due to which there is no flexibility of macromolecules and there is no highly elastic state. With rapid cooling, the crystal structures do not have time to fully form, therefore, in the supercooled polymer, there is a "frozen" - amorphous structure between them. This amorphous structure, when reheated to a temperature above the melting point (Tm), creates a viscous state. The polymer structure is characterized by two states: crystalline (up to the melting point) and viscous (above the melting point).
The viscous-flow state, characteristic of the amorphous and crystalline state of the polymer, basically provides the necessary deformations during the flow of the polymer by the successive movement of the segments. The viscosity of the polymer increases with an increase in the molecular weight of the polymer, and the molding pressure of the articles also increases.
In conclusion, we note that with an increase in temperature to a certain value, the process of thermal destruction begins in the polymer material - the decomposition of the material.
Properties of polymers that determine quality in the processing process:
Viscosity properties of polymer melt.
The molding of products from polymers is carried out in the process of their viscous flow, accompanied by plastic deformation. In this case, a thin layer of material in contact with the stationary wall of the tool, due to adhesion to it, has a zero displacement velocity (stationary), the middle layer has the highest displacement velocity V; in the steady-state flow regime, the relationship between the shear stress t and the shear rate g is linear (Newton's law for viscous fluids): t = h * g, where h is the viscosity coefficient or viscosity. The nature of the dependence of the shear rate on the shear stress is represented by the flow curve, on which the sections are distinguished: 1 - a section of linear dependence, characteristic only for low shear stresses; 2 - a section with a nonlinear dependence, which is characterized by a decrease in viscosity with increasing shear stress; 3 - area with high shear stress.
The improvement of material flow is facilitated by: an increase in temperature, an increase in shear stress, an increase in the amount of moisture, a decrease in pressure and a decrease in the molecular weight of the melt.
Many properties of polymeric materials in products depend on the structure that the processing process forms. Depending on the polymer and processing conditions, an amorphous or crystalline structure occurs in products.
The structure of an article with an amorphous polymer is characterized by a certain degree of orientation of the sections of chain macromolecules and the arrangement of oriented regions along the section of the article along the direction of shear (flow) of the material. This leads to anisotropy of properties.
The structure of an article with a crystalline polymer is characterized by a certain degree of crystallinity (from 60 to 95%) and non-uniformity of crystalline regions over the cross section. The properties of such products obtained under different processing conditions, despite the morphological similarity of the structure, are different.
Quality indicators of products made of polymeric materials depend on the properties, conditions of preparation, processing and physical modification of the material. The appearance of products depends on the processing conditions, material purity, humidity.
Dielectric properties and chemical resistance depend on the chemical structure and modification of the polymer.
Mechanical properties- strength, impact resistance, deformation, rigidity, heat resistance - depend on the supramolecular structure, and the coefficient of friction and wear resistance, resistance to combustion depend on the chemical structure and modification.
Operational properties - dimensional accuracy and dimensional stability - depend both on the chemical structure, molecular characteristics, technological properties, and on the processing technology and manufacturability of the design.
Thermal stability of polymers . The main indicator in this case is destruction.
The destruction of polymers is a change in the structure of macromolecules. Destruction can proceed under the influence of heat, oxygen, chemical agents ( including water), light, high energy radiation, mechanical stresses, etc., both from an individual and from a set of parameters. It is accompanied by a decrease in molecular weight, the release of gaseous and low molecular weight products, a change in color and the appearance of odor.
Destruction can be accompanied not only by the destruction of macromolecules, but also by their crosslinking (structuring), which causes an increase in the mass and viscosity of the melt. The consequence of this is the violation of all material properties, a decrease in the stability of the properties of products.
During the processing of polymers, both thermal-oxidative and mechanical destruction can occur, and in hygroscopic materials, hydrolysis can also occur.
The signs of the classification of plastics are: purpose, type of filler,
performance properties and other signs.
Classification of plastics by service purpose:
According to the use of plastics, they are distinguished (rather conditionally):
According to the set of parameters of operational properties, plastics are divided into two large groups:
General technical plastics have lower performance parameters than
engineering plastics. Plastics for engineering and technical purposes
retain high values of mechanical properties not only at normal and elevated
temperatures, but can also operate under short-term loads at elevated
temperatures. This is not provided by plastics for general technical
purposes; they operate in an unloaded or lightly loaded state at normal
and medium temperatures (up to 55 C). Plastics for engineering and
technical purposes are divided into groups that provide certain properties in a
certain interval; there are five groups of plastics according to this
By the value of individual parameters of operational propertiesmake up a series of plastics for various parameters of performance properties. Classification parameters: mechanical properties, wear properties, linear thermal expansion and others.
Depending on the applicability of the filler and the degree of its grinding, all materials are divided into four groups:
The technological properties of plastics affect the choice of processing
method. The technological properties of plastics include: fluidity,
humidity, curing time, dispersity, shrinkage, tableting, volumetric
Fluidity characterizes the ability of a material to viscous flow of a polymer squeezed out for 10 minutes through a standard nozzle under the pressure of a certain weight at a given temperature. So for injection molding materials and processing modes are used, in which the melt fluidity is in the range of 2-20 g / 10 min, for blow molding into a mold - 1.5-7 g / 10 min, for the extrusion of pipes and profiles - 0, 3-1 g / 10 min, for film extrusion - 1-4 g / 10 min, for laminates - 7-12 g / 10 min. The fluidity of the thermosetting plastic is equal to the length of the rod in mm, pressed in a heated mold with a channel of decreasing cross-section. This indicator of fluidity, although it is a relative value, makes it possible to preset the processing method: with a Rashig fluidity of 90-180 mm, injection molding is used,
Shrinkage characterizes the change in dimensions during product molding and heat treatment:
Y = (Lf-Li) / Lf * 100%; Beat = (L-Lt) / Lf * 100%; where Y - shrinkage after molding and cooling; Ud - additional shrinkage after heat treatment; Lf, Li - shape size and product size after cooling; L, Lт - product size before heat treatment and after cooling.
Shrinkage of thermosetting plastics products depends on the method of forming the product and the type of crosslinking reaction: polymerization or polycondensation. Moreover, the latter is accompanied by the release of a by-product - water, which evaporates under the influence of high temperature. The shrinkage process takes place over time; the longer the holding time, the more complete the chemical reaction proceeds, and the shrinkage of the product after removal from the mold is less. However, after some time of exposureshrinkage with a further increase remains constant. Effect of temperature on shrinkage: shrinkage increases in direct proportion to temperature rise. Shrinkage after processing also depends on the moisture content of the material and the preheating time: with increasing moisture, shrinkage increases, and with increasing preheating time, it decreases.
The shrinkage of thermoplastic products after molding is associated with a decrease in density when the temperature is lowered to the operating temperature.
The shrinkage of the polymer in different directions with respect to the direction of flow is different for thermo- and thermosetting plastics, i.e. polymers have shrinkage anisotropy. Shrinkage of thermoplastics is greater than shrinkage of thermosets.
Moisture and volatile content. The moisture content in press materials and polymers increases during storage in an open container due to the hygroscopicity of the material or its condensation on the surface. The content of volatile substances in polymers depends on the content of residual monomer and low-boiling plasticizers, which can turn into a gaseous state during processing.
Optimum moisture content: thermosetting plastics 2.5 - 3.5%, thermoplastics - hundredths and thousandths of a percent.
Particle size distribution is evaluated by particle size and uniformity. This indicator determines the performance when feeding material from the hopper to the heating zones and the uniformity of heating the material during molding, which prevents swelling and unevenness of the product surface.
Volumetric characteristics of the material: bulk density, specific volume, compaction coefficient. (Specific volume is a value determined by the ratio of the volume of a material to its mass; bulk density is the reciprocal of the specific volume). This indicator determines the size of the loading chamber of the mold, the hopper and some dimensions of the equipment, and when processing press powders with a large specific volume, productivity decreases due to the poor thermal conductivity of such powders.
Tabletability is the ability to compress the press material under the influence of external forces and maintain the resulting shape after removing these forces.
The processes of plastics processing are based on physical and physicochemical processes of structure formation and shaping:
These processes can take place simultaneously and interconnected. Only
one process will prevail at a certain stage.
During the molding process, the polymer is heated to a high temperature, deformed by shear, stretching or compression, and then cooled. Depending on the parameters of these processes, it is possible to significantly change the structure, conformation of macromolecules, as well as physicomechanical, optical and other characteristics of polymers.
When a large number of polymers are cooled, the crystallization process takes place.
Crystallization, depending on the state of the melt, leads to different types of structure. Crystallization from a polymer melt in an equilibrium state without deformation leads to the formation of spherulite structures. The center of the formation of such structures is the embryo, from which ray-shaped fibrils are formed, consisting of many packed lamellas. Fibrils, growing in the radial direction and in width, form spherical structures - spherulites. Spherolites are formed simultaneously in a large number of crystallization centers. On the basis of this, spherulites form faces at the contact points and are polyhedra of arbitrary shape and size. Electron microscopic studies show that the spherulite fibril is composed of many lamellas stacked on top of each other and twisted around the spherulite radius.
Crystallization from a polymer melt occurs when crystallizers - nuclei are introduced into the polymer material.
If crystallization proceeds under high pressure (300 ... 500 MPa) and at high temperature, then a crystalline structure of straightened chains is formed; upon rapid cooling of the same melt, crystallization proceeds with the formation of complex chains, macromolecules in this case in the melt in the form of domains, and rapid cooling does not allow them to pass into a new conformation, i.e. acquire an elongated shape. It was also found that the crystallization temperature rises with increasing pressure. The practical significance of this property: the possibility of the transition of the polymer directly from the melt without cooling into a quasicrystalline state with increasing pressure; this eliminates the flow and slows down the relaxation processes. With increasing pressure, smaller spherulites are formed and therefore the mechanical strength of the products increases. Crystal sizes also depend on the cooling rate and temperature during the molding process. At a high cooling rate, a fine-crystalline structure is obtained, since there is not enough time to rearrange the crystals.
A coarser polymer structure can be obtained by increasing temperature, holding time and slow cooling, or by preheating the melt to a higher temperature before crystallizing. The shape of the crystals can be changed. So, using centers of crystallization and artificial nuclei (1 ... 2% of the mass), you can regulate the shape of the crystals. When a substrate-crystallizer is used, a large number of crystallization centers appear at its surface and a densely packed layer of crystals perpendicular to the surface is formed. Artificial nuclei are additional centers of crystallization, the shape of the crystal in this case depends on the shape of the nucleus of crystallization, spherulite structures grow on small crystals, and ribbon-like structures grow on long needle crystals.
Unsteady conditions of heat transfer and cooling rates during molding of articles from polymers contribute to the production of articles with an inhomogeneous structure (smaller crystals at the surface layers).
If necessary, uniform properties of the product can be ensured by annealing or subsequent heat treatment at a temperature below the melting point. During annealing, the volume of the product decreases and the density increases; and the higher the temperature and the longer the holding time, the higher the density of the product. Heat treatment is advisable in cases where increased hardness, elastic modulus, mechanical strength, heat resistance and resistance to cyclic loads are required; at the same time, the relative elongation and toughness decrease.
The completeness of these processes, in addition to destruction, largely determines the quality of the finished product, and the speed of these processes determines the productivity of the processing method. The quality of the product is largely influenced by the rate of polymer degradation, which is increased by thermal and mechanical action on the material from the working bodies of the tools during formation.
The shape of a thermoplastic product is obtained as a result of the development of plastic or highly elastic deformation in the polymer under the action of pressure when the polymer is heated. When processing thermosetting plastics, the formation of a product is ensured by a combination of physical processes of formation with chemical reactions of curing of polymers. In this case, the properties of products determine the speed and completeness of curing. Incomplete use of polymer reactivity during curing leads to instability of properties of thermosetting plastic products in time and destructive processes in finished products. The low viscosity of thermosetting plastics during formation leads to a decrease in the unevenness of properties, an increase in the stress relaxation rate and a lesser effect of destruction during processing on the quality of finished thermosetting plastics.
Depending on the processing method, curing is combined with the molding of the product (during pressing), it occurs after the product is formed in the mold cavity (injection molding and injection molding of thermosetting plastics) or during heat treatment of the formed workpiece (when molding large-sized products, for example, getinax sheets, fiberglass laminate and etc.). Full curing of thermosetting plastics requires in some cases several hours. To increase product removal from equipment, final curing can be performed outside the mold tooling, as shape stability is acquired long before this process is complete. For the same reason, the product is removed from the mold without refrigeration.
When processing polymers (especially thermoplastics), macromolecules are oriented in the direction of material flow. Along with the difference in orientation, structural inhomogeneity arises in different areas of products that are inhomogeneous in cross section and length, and internal stresses develop.
The presence of temperature differences over the section and length of the part leads to even greater structural inhomogeneity and the appearance of additional stresses associated with the difference in the rates of cooling, crystallization, relaxation, and different degrees of curing.
Inhomogeneity of material properties (for the reasons indicated) is not always acceptable and often leads to rejects (due to instability of physical properties, dimensions, warpage, cracking). Reducing the heterogeneity of the molecular structure and internal stresses can be achieved by heat treatment of the finished product. However, it is more effective to use methods of directed regulation of structures in the processing processes. For these purposes, additives are introduced into the polymer that influence the formation of supramolecular structures and contribute to the production of materials with the desired structure.
The branded assortment of polymers was created in order to quickly select
the type and grade of polymer for the manufacture of high-quality
products. The branded assortment includes grades that differ in viscosity
and performance properties.
The branded assortment by viscosity is divided into grades intended for processing by various methods (injection molding, pressing, etc.), with an increase in the grade number, the molecular weight increases and, as a result, the viscosity increases. These are brands of the basic assortment. Viscosity grades are modified to improve technological properties:
On the basis of the basic assortment of grades according to technological
properties, grades with improved properties are created by chemical or physical
modification. These grades are developed with such properties that, under
the recommended modes, to obtain high-quality products in all parameters
(accuracy, strength, appearance, etc.). Currently, polymeric materials are
produced in an assortment, and therefore for each product and molding method,
it is possible to select an appropriate base polymer grade and, if necessary, a
grade with improved processing properties.
Basic brands for the purpose of manufacturing quality products are divided into groups:
Choice signs. The main features of the choice of plastics are
operational and technological properties. To speed up the process of
choosing a material, special tables are used, each of which lists the grades of
materials in order of decreasing the average value of the presented performance
The procedure and algorithm for the selection of plastics. Plastics are selected based on the requirements for performance and geometric parameters of the product. Therefore, first, the type of plastic is selected based on the requirements for its performance properties, and then the base grade and the grade with improved technological properties, which can be efficiently processed by the selected method.
There are two methods for selecting the type of plastics:
The analogy method is used when it is impossible to accurately set the
parameters of the operational properties of the plastic; in this case, the
characteristic parameters of operational properties, purpose, advantages,
limitations, recommendations for use and processing methods are used to
select; in this case, recommendations for the use of plastics in other
types of products operating in similar conditions can also be used for
The procedure for choosing plastics by a quantitative method based on a set of set values of performance properties is as follows:
Injection molding is a method of molding products from polymeric materials, which consists in heating the material to a viscous-flow state and squeezing it into a closed injection mold, where the material acquires the configuration of the inner cavity of the mold and solidifies. This method produces products weighing from several grams to several kilograms with a wall thickness of 1–20 mm (usually 3–6 mm). For the implementation of injection molding, plunger or screw injection machines are used (Fig. 1), on which injection molds of various designs are installed (Fig. 2).
Fig. 1. Diagram of an injection molding machine with screw (a) and plunger (b) plasticization of the melt:
1 - hydraulic cylinder of the closing mechanism; 2 - piston of the hydraulic cylinder of the closing mechanism; 3 - movable plate; 4 - half molds; 5 - fixed plate; 6 - plasticizing cylinder, 7 - auger; 8 - loading window of the plasticizing cylinder; 9 - bunker; 10 - screw drive; 11 - body of the hydraulic cylinder of the injection mechanism; 12 - the piston of the injection hydraulic cylinder; 13 - auger hydraulic cylinder; 14 - torpedo - melt flow divider; 15 - dispenser; 16 - plunger
Fig. 2. Injection mold:
1 - movable half-form; 2 - pusher; 3 - ejection plate, 4 - ejectors; 5 - channels of the mold thermostating system; 6 - gate bushing; 7 - central sprue; 8 - centering sleeve; 9 - centering column; 10 - motionless half-form; 11 - nozzle of the injection molding machine; 12 - spreading sprue; 13 - inlet sprue; 14 - shaping cavity
The main process parameters are injection molding the melt temperature T p , mold temperature T F , the pressure casting P L , in the form of pressure P f , the holding time under pressure t PEPs , the cooling time t OHL or curing time t in the form of holesfor thermosetting materials. Both thermoplastic and thermosetting materials are processed by injection molding, but the type of material determines the specifics of the physicochemical processes accompanying the heating and solidification of these types of plastics. The process flow diagram is shown in Fig. 3. The analysis of the injection molding process can be carried out according to the following components: transfer of the material into a viscoplastic state -> its supply to the dosing zone -> melt accumulation -> melt flow in the "nozzle-mold" system -> melt flow in the mold channels and the forming cavities -> shaping the structure of the product.
Fig. 3. Technological diagram of injection molding:
1 - carriage (gondola car, tank car); 2 - suspended crane beam; 3 - material warehouse; 4 - vacuum dryer; 5 - injection molding machine; 6 - conveyor; 7 - machine tool; 8 - packing table; 9 - crusher; 10 - extruder; 11 - cooling bath; 12 - granulator