Chapter 3 Plastics, Mold Materials and Management 3.1 Plastics 3.1.1 Condition for Injection and Molding The large molecular structure of polymer and the features of molecular thermodynamic movement have determined that polymer has such states of aggregation as the glassy state, highly-elastic state as well as viscous flow state etc. The transformation of aggregative state is nfluenced by chemical composition, molecular structure, the stress applied whereon and the environment temperature, and is mainly related with temperature when the composition of Membrane blow molding Calendering forming Extrusion forming Processing and molding temperature njection molding 加工与型方法的适应 Flexibility of processing (或Tm)7 and molding 熔纺 Vacuum and pressure molding Heat stretch of membrane and fiber Melting spinning 延成型 空吹塑成型 Deformation rate Cold stretch of the membrane and 空和压力成型 fiber 膜和纤维热拉伸 薄膜和纤维冷拉伸 Hollow molding Glassy state 玻璃态尚弹态黏 固体移皮状弹性体数娃 Hard solid Highly-elastic state Rubber-elastomer viscous liquid Viscous flow state Fig 3-1: relationship between polymer state and plastic molding As indicated in the Fig 3-1, when the temperature is below the vitrification temperature(T.), the polymer is under glassy state, which is hard solid, wherein energy from the molecular thermodynamic movement is low, the elastic modulus is large, and the deformation is reversible Under such aggregative state, it is not suitable for molding with large-scale deformation. When the temperature is above the vitrification temperature(T,), the plastics will become softer, and
Chapter 3 Plastics, Mold Materials and Management 3.1 Plastics 3.1.1 Condition for Injection and Molding The large molecular structure of polymer and the features of molecular thermodynamic movement have determined that polymer has such states of aggregation as the glassy state, highly-elastic state as well as viscous flow state etc. The transformation of aggregative state is influenced by chemical composition, molecular structure, the stress applied whereon and the environment temperature, and is mainly related with temperature when the composition of polymer is certain. Fig.3-1: relationship between polymer state and plastic molding As indicated in the Fig.3-1, when the temperature is below the vitrification temperature (Tg ), the polymer is under glassy state, which is hard solid, wherein energy from the molecular thermodynamic movement is low, the elastic modulus is large, and the deformation is reversible. Under such aggregative state, it is not suitable for molding with large-scale deformation. When the temperature is above the vitrification temperature (Tg ), the plastics will become softer, and Hollow molding Hard solid Highly-elastic state Rubber-elastomer Viscous liquid Viscous flow state Processing and molding temperature Deformation rate Heat stretch of membrane and fiber Vacuum and pressure molding Cold stretch of the membrane and fiber Extrusion forming Injection molding Membrane blow molding Calendering forming Glassy state Flexibility of processing and molding Melting spinning
therefore, the vitrification temperature(Tg)is the maximum temperature that can be used for plastics. The lower limit temperature applied to plastics is called the brittle temperature, below which the plastics will easily get cracked and damaged under stress When the temperature is between vitrification temperature and viscous flow temperature (T), the polymer shall be under highly-elastic state, wherein energy of the molecular thermodynamic movement increases and the elastic modulus greatly reduces. For amorphous polymer, certain materials can be molded through pressurization, bending, hollow molding or vacuum molding For crystalline polymer, stretch and drawing of the membrane and fiber can be made between vitrification temperature and melting point (T-Tm) When the temperature is above viscous flow temperature(or melting point ) viscous flow will happen to the polymer under external force; calendering molding, certain extrusion and blow molding are usually adopted when the temperature is within a range slightly higher than the viscous flow temperature. When at a temperature much higher than the viscous flow temperature flow deformation of the fusant will occur under small external force. The deformation herein is mainly nonreversible viscous deformation which can be permanently preserved upon cooling of the fusant. Hence, this range of temperature is usually used for injection molding. Excessively high temperature, however, will greatly reduce the viscosity of polymer, thereby bringing difficulty to molding and worsening the quality of products. Moreover, when the temperature is higher than the decomposition temperature (Td), decomposition and metamorphism of the polymer will occur. Moldability refers to the plastics'ability of deforming under the action of temperature and pressure as well as their molding ability in the mold cavity. Plastics with moldability can be made into plastic products of various shapes through such molding methods as injection, mold pressing and extrusion etc. The moldability mainly depends on such factors as the rheology, thermal property, physical mechanical property of materials, the chemical property of thermoset plastics, technical factors(such as temperature, pressure and molding period etc) as well as the structural dimension of molds. As indicated in the figure is the relationship between moldability temperature and pressure. Exceedingly high temperature will produce sound fluidity yet will also result in high shrinkage and even decomposition; excessively low temperature will lead to low fluidity and formability, and the performance of products may be reduced; excessively high pressure will cause overflow and increase the internal stress of products; excessively low pressure will instead lead to insufficient filling in the mold and subsequent material shortage. The area formed by the four curves in the figure is the optimal molding area
therefore, the vitrification temperature (Tg ) is the maximum temperature that can be used for plastics. The lower limit temperature applied to plastics is called the brittle temperature, below which the plastics will easily get cracked and damaged under stress. When the temperature is between vitrification temperature and viscous flow temperature ( Tf ), the polymer shall be under highly-elastic state, wherein energy of the molecular thermodynamic movement increases and the elastic modulus greatly reduces. For amorphous polymer, certain materials can be molded through pressurization, bending, hollow molding or vacuum molding. For crystalline polymer, stretch and drawing of the membrane and fiber can be made between vitrification temperature and melting point （Tf ~ Tm ）. When the temperature is above viscous flow temperature (or melting point), viscous flow will happen to the polymer under external force; calendering molding, certain extrusion and blow molding are usually adopted when the temperature is within a range slightly higher than the viscous flow temperature. When at a temperature much higher than the viscous flow temperature, flow deformation of the fusant will occur under small external force. The deformation herein is mainly nonreversible viscous deformation which can be permanently preserved upon cooling of the fusant. Hence, this range of temperature is usually used for injection molding. Excessively high temperature, however, will greatly reduce the viscosity of polymer, thereby bringing difficulty to molding and worsening the quality of products. Moreover, when the temperature is higher than the decomposition temperature (Td ), decomposition and metamorphism of the polymer will occur. Moldability refers to the plastics’ ability of deforming under the action of temperature and pressure as well as their molding ability in the mold cavity. Plastics with moldability can be made into plastic products of various shapes through such molding methods as injection, mold pressing and extrusion etc. The moldability mainly depends on such factors as the rheology, thermal property, physical mechanical property of materials, the chemical property of thermoset plastics, technical factors (such as temperature, pressure and molding period etc) as well as the structural dimension of molds. As indicated in the figure is the relationship between moldability, temperature and pressure. Exceedingly high temperature will produce sound fluidity yet will also result in high shrinkage and even decomposition; excessively low temperature will lead to low fluidity and formability, and the performance of products may be reduced; excessively high pressure will cause overflow and increase the internal stress of products; excessively low pressure will instead lead to insufficient filling in the mold and subsequent material shortage. The area formed by the four curves in the figure is the optimal molding area
Viscoelastic limit Overflow and distortion 黏弹性极限 Decomposition line Pressure Material overflow line Material shortage line 缺料丝 Difficulty to mold 模不足 Temperature Insufficient filling in the mold Fig 3-2: relationship between plastic molding pressure and temperature A-molding area: abad line of surface: b-material overflow line c-Decomposition line: d--Material shortage line 3.1.2 Definition and Classification of Plastics According to the chemical property, plastics roughly fall into such two types lastics and thermoplastic plastics 1. Thermoplastic Plastics 1)Polyvinyl chloride(Pvc)resin 2) Polystyrene(PS)resin 3)ABS resin and As resin 4)Acrylic resins(PMMA) 5)Polyethylene(PE) 6) Polypropylene(PP) 7)Flurocarbon resins 8)Polyamide resins(nylon, PA) 9)Acetal resins(POM 10)Polycarbonate resin(PC) 1) Cellulose plastics 2. Thermoset plastics 1)Phenol resins(bakelite) 2)Urea resins. 3)Melamine resins 4)Unsaturated polyester resins(FRP) 5)Epoxy resins 6) Polyurethanes resin 3.1.3 Properties of Thermoplastic Plastics 1. Polyvinyl Chloride(Pvc) resin The PVC monomer arises from the reaction between ethyne and hydrochloric acid, and the monomer thereafter is aggregated into pvc through various methods. pvc is devoid of
Fig.3-2: relationship between plastic molding pressure and temperature A—molding area；a—bad line of surface；b—material overflow line； c—Decomposition line；d—Material shortage line 3.1.2 Definition and Classification of Plastics According to the chemical property, plastics roughly fall into such two types as thermoset plastics and thermoplastic plastics. 1. Thermoplastic Plastics 1) Polyvinyl chloride (PVC) resin. 2) Polystyrene (PS) resin. 3) ABS resin and AS resin . 4) Acrylic resins (PMMA). 5) Polyethylene (PE). 6) Polypropylene (PP). 7) Flurocarbon resins. 8) Polyamide resins (nylon, PA). 9) Acetal resins (POM). 10) Polycarbonate resin (PC). 11) Cellulose plastics. 2. Thermoset Plastics 1) Phenol resins (bakelite) 2) Urea resins. 3) Melamine resins. 4) Unsaturated polyester resins (FRP). 5) Epoxy resins. 6) Polyurethanes resin. 3.1.3 Properties of Thermoplastic Plastics 1. Polyvinyl Chloride (PVC) Resin • The PVC monomer arises from the reaction between ethyne and hydrochloric acid, and the monomer thereafter is aggregated into PVC through various methods. PVC is devoid of Temperature Difficulty to mold Pressure Viscoelastic limit Overflow and distortion Material overflow line Material shortage line Decomposition line Insufficient filling in the mold
stability against heat and light, and therefore, stabilizers shall be added therein to avoid discoloration or decomposition upon the materials' being processed into finished products. Various stabilizers are available, including metal soap and organic tin compounds and they fall into such types as transparent, opaque, soft and hard ones. The processing temperature of soft PVC is 70C-100 C. Hard PVC requires less time for processing, yet the temperature therewith must rise to approximately 150C. Due to its low fluidity during fusion, hard PVC tends to be more difficult to be injected and molded than polystyrene(PS). However, even without intensifier, hard PvC still boasts a pretty high mechanical intensity; it is also sound in electric insulation, acid resistance kali resistance as well as water resistance; moreover, it is transparent and can be colored freely and processed effectively. Soft PVC is endowed with sound plasticity and hence is inferior to hard PVC in the aforementioned properties, yet it can be made into soft membranes or thin plates. The softening temperature whereof is approximately 80C and the decomposition temperature is round273℃ PVC, with such shortcoming as weak heat resistance capacity, will soften promptly under the temperature of 65C-80C and can decompose easily under high temperature PVC can be used to make tablecloth, packing membranes, vehicle curtains, briefcases handbags and chemical shoes etc. Its glue film can absorb ultraviolet radiation and therefore embodies sound thermal insulation property. Hard PVC can be applied to such fields watercourse piping, chemical plant piping, building materials, signboards, telephones, electric parts as well as drug-proof apparatus etc 2. Polystyrene(PS)Resin The composite ethylene gas made from benzene(adopted from coal tar or petroleum) and ethylene gas, is dehydrogenated by metal oxide catalyzer heated to 600C into styrene monomer which is then catalyzed and aggregated into polystyrene resin through such catalyzer as inorganic or organic peroxide The polystyrene resin in the market is colorless and transparent, hard and slightly crispy, with an average molecular weight of 70 thousand -100 thousand. It is excellent in water resistance and electric insulation and can fight against erosion from strong acid or alkali, yet it is devoid of resistance against organic solvent and oil and is weak in heat resistance. Another important feature of polystyrene resin is that it is endowed with sound thermal stability and fluidity in fusing resin, and thereby can be molded very well. Though a bit crispy, the PS can be colored freely. High impact polystyrene(HIPS)resin is obtained by adding butadiene and styrene copolymerized rubber in styrene resin, wherein the two substances are in several hierarchies respectively; in addition, ultraviolet radiation absorbent or antioxidant can be added therein to improve lightfastness and also fiberglass is sometimes added into the basal material to increase PS caters to the transparency and color tone of modern visual beauty. It can be extensivel applied to the manufacturing of table articles like bread boxes and beef oil cans, commodity containers, toys, fruit dishes, toothbrushes, handles of brushes, soapboxes as well as flavoring containers etc 3. ABS Resin and As resin The aggregation of styrene monomer, acrylonitrile monomer and butadiene rubber can produce a polymer(ABS)constituted by acrylonitrile(A), butadiene rubber(B)and styrene(s)in
stability against heat and light, and therefore, stabilizers shall be added therein to avoid discoloration or decomposition upon the materials’ being processed into finished products. Various stabilizers are available, including metal soap and organic tin compounds and they fall into such types as transparent, opaque, soft and hard ones. The processing temperature of soft PVC is 70℃-100 ℃. Hard PVC requires less time for processing, yet the temperature therewith must rise to approximately 150℃. Due to its low fluidity during fusion, hard PVC tends to be more difficult to be injected and molded than polystyrene (PS). However, even without intensifier, hard PVC still boasts a pretty high mechanical intensity; it is also sound in electric insulation, acid resistance, alkali resistance as well as water resistance; moreover, it is transparent and can be colored freely and processed effectively. Soft PVC is endowed with sound plasticity and hence is inferior to hard PVC in the aforementioned properties, yet it can be made into soft membranes or thin plates. The softening temperature whereof is approximately 80 and ℃ the decomposition temperature is around 273 . ℃ PVC, with such shortcoming as weak heat resistance capacity, will soften promptly under the temperature of 65℃~80℃ and can decompose easily under high temperature. PVC can be used to make tablecloth, packing membranes, vehicle curtains, briefcases, handbags and chemical shoes etc. Its glue film can absorb ultraviolet radiation and therefore embodies sound thermal insulation property. Hard PVC can be applied to such fields as watercourse piping, chemical plant piping, building materials, signboards, telephones, electric parts as well as drug-proof apparatus etc. 2. Polystyrene (PS) Resin The composite ethylene gas made from benzene (adopted from coal tar or petroleum) and ethylene gas, is dehydrogenated by metal oxide catalyzer heated to 600℃ into styrene monomer, which is then catalyzed and aggregated into polystyrene resin through such catalyzer as inorganic or organic peroxide。The polystyrene resin in the market is colorless and transparent, hard and slightly crispy, with an average molecular weight of 70 thousand ~100 thousand. It is excellent in water resistance and electric insulation and can fight against erosion from strong acid or alkali, yet it is devoid of resistance against organic solvent and oil and is weak in heat resistance. Another important feature of polystyrene resin is that it is endowed with sound thermal stability and fluidity in fusing resin, and thereby can be molded very well. Though a bit crispy, the PS can be colored freely. High impact polystyrene (HIPS) resin is obtained by adding butadiene and styrene copolymerized rubber in styrene resin, wherein the two substances are in several hierarchies respectively; in addition, ultraviolet radiation absorbent or antioxidant can be added therein to improve lightfastness and also fiberglass is sometimes added into the basal material to increase intensity. PS caters to the transparency and color tone of modern visual beauty. It can be extensively applied to the manufacturing of table articles like bread boxes and beef oil cans, commodity containers, toys, fruit dishes, toothbrushes, handles of brushes, soapboxes as well as flavoring containers etc. 3. ABS Resin and AS Resin The aggregation of styrene monomer, acrylonitrile monomer and butadiene rubber can produce a polymer (ABS) constituted by acrylonitrile (A), butadiene rubber (B) and styrene (S) in
various proportions. The characteristic thereof, due to the ingenious combination by the three components, shows such features as high impact resistance, large tensile strength and high rigidity which will not alter even under low temperature. Above all, the polymer possesses strong heat al and oil resistance, which endows it with such outstanding quali devoid of in ps and hips as resin is a butadiene free resin in abs resin it retains the transparency of styrene resin and is greatly improved in such properties as heat resistance, chemical resistance, oil resistance as well as mechanical intensity etc ABs resin boasts such quali th and mechanical intensity, sound emical resistance and heat resistance, stable dimension and easy processing etc and is relatively cheaper industrial material. It is mainly applied to the manufacturing of shells for radios and televisions, air conditioning and warming machines, juice machines, hair driers and shavers basal body of electric fans, vehicle meters, front protective grillages as well as air conditioning carrla AS resin can be used in such fields which require high mechanical intensity, chemical resistance and transparency such as battery compartments, meter boxes, blades of electric fans and beef oil case et 4. Acrylic Resins(PMMA) Acetone works together with prussic acid to form cyaniding propyl alcohol which is dehydrated with oil of vitriol, the substance is further combined into methacrylates through reaction with alcohol ester, and finally through polymerization with catalyzer, PMMA is thereby PMMA, like polystyrene(PS) he optimal transparency among all plas more difficult to be cut than polystyrene resin. It can be tabular polymethyl methacrylate and can be heated and bent into curved surface, the relative density thereof is also light. PMMA can be tinged into gallant colors and is extensively applied in building materials and furniture PMMA can be used to manufacture vehicle parts. Provided with sound lightfastness and ceilings. In addition, it embodies sound mechanical processing and bonding properties and can be applied in the manufacturing of delicate transparent models, optic lens, false teeth, contact lenses 5. Polyethylene(Pe) The commonest plastic that can be found in daily life is polyethylene(PE)which lymerized from ethylene industrially fractionated from petroleum Polyethylene of low density added with a small quantity of oxygen is heated to 200C under high pressure of approximately 2000 air pressure, white wax like polyethylene can thereby be obtained, and the method is called high pressure ethylene polymerization. With special organic metal catalyzer, the ethylene can be polymerized under normal temperature into polyethylene of high density, and such method is called low pressure ethylene polymerization. Polyethylene is generally a milk white, translucent or opaque substance which is lighter than water and can fall drop by drop like olefin when being burnt. It has sound water resistance, electric insulation, acid and alkali resistance. The substance can dissolve in hot toluene yet is quite stable against most chemicals and therefore it can be easily molded. The melt index indicates the fusion viscosity of
various proportions. The characteristic thereof, due to the ingenious combination by the three components, shows such features as high impact resistance, large tensile strength and high rigidity, which will not alter even under low temperature. Above all, the polymer possesses strong heat resistance as well as chemical and oil resistance, which endows it with such outstanding qualities devoid of in PS and HIPS. AS resin is a butadiene-free resin in ABS resin. It retains the transparency of styrene resin and is greatly improved in such properties as heat resistance, chemical resistance, oil resistance as well as mechanical intensity etc. ABS resin boasts such qualities as high impact strength and mechanical intensity, sound chemical resistance and heat resistance, stable dimension and easy processing etc and is a relatively cheaper industrial material. It is mainly applied to the manufacturing of shells for radios and televisions, air conditioning and warming machines, juice machines, hair driers and shavers, basal body of electric fans, vehicle meters, front protective grillages as well as air conditioning machines in the carriage etc. AS resin can be used in such fields which require high mechanical intensity, chemical resistance and transparency such as battery compartments, meter boxes, blades of electric fans and beef oil case etc. 4. Acrylic Resins (PMMA) Acetone works together with prussic acid to form cyaniding propyl alcohol which is dehydrated with oil of vitriol, the substance is further combined into methacrylates through reaction with alcohol ester, and finally through polymerization with catalyzer, PMMA is thereby got. PMMA, like polystyrene (PS), possesses the optimal transparency among all plastics and is more difficult to be cut than polystyrene resin. It can be tabular polymethyl methacrylate and can be heated and bent into curved surface; the relative density thereof is also light. PMMA can be tinged into gallant colors and is extensively applied in building materials and furniture. PMMA can be used to manufacture vehicle parts. Provided with sound lightfastness and transmittance, it is most suitable for making signboards, transparent covers and transparent ceilings. In addition, it embodies sound mechanical processing and bonding properties and can be applied in the manufacturing of delicate transparent models, optic lens, false teeth, contact lenses as well as machine parts. 5. Polyethylene (PE) The commonest plastic that can be found in daily life is polyethylene (PE) which is polymerized from ethylene industrially fractionated from petroleum. Polyethylene of low density added with a small quantity of oxygen is heated to 200℃ under a high pressure of approximately 2000 air pressure, white wax like polyethylene can thereby be obtained, and the method is called high pressure ethylene polymerization. With special organic metal catalyzer, the ethylene can be polymerized under normal temperature into polyethylene of high density, and such method is called low pressure ethylene polymerization. Polyethylene is generally a milk white, translucent or opaque substance which is lighter than water and can fall drop by drop like olefin when being burnt. It has sound water resistance, electric insulation, acid and alkali resistance. The substance can dissolve in hot toluene yet is quite stable against most chemicals and therefore it can be easily molded. The melt index indicates the fusion viscosity of