UNIT 7 WELDING AND PLASTICS

Transcription

1 UNIT 7 WELDING AND PLASTICS Welding and Plastics Structure 7.1 Introduction Objectives 7.2 Properties of Plastics 7.3 Industrial Uses of Plastics 7.4 Injection Molding 7.5 Classifications and Important Features of Injection Molding Machine 7.6 Some Special Injection Molding Processes 7.7 Defects and Precautions in Injection Molding Process 7.8 Compression Molding Process 7.9 Principle of Compression Molding 7.10 Advantages of Compression Molding Process 7.11 Plastics and Environmental Issues 7.12 Summary 7.13 Answers to SAQs 7.1 INTRODUCTION Plastics can be used to make different types of parts of molding them to different shapes, extruding them to different sections, insulation coatings on electrically conducting wires. Plastics are also used as ingredient material for other materials like paints, varnishes, to improve their adhering capability, various polymer matrix composites. The present unit describes the technologies at their introductory level, which are used for plastics processing with their relative advantages, disadvantages and industrial applications. In industrial applications plastics are being considered as a suitable replacement of metals due to the following properties : (d) (e) (f) Their easy availability. Low processing cost in case of mass production. Low material cost as compared to cost of metal. Long product life. Resistance to corrosion and water proofing capability. Better surface finish, etc. Normally plastics are divided into two broader categories thermoplastics and thermo sets. The major difference between these two type is thermo sets under a curing process during heating and shaping, which cause a permanent chemical change in their molecular structured once, these plastics are processed, these cannot be melted again by reheating. Thermoplastics are just opposite to this fact. Thermoplastics do not cure, their chemical structure remains unchanged upon reheating even though they transform solid to fluid. Due to the above difference thermoplastics contributes to more than 80% of the total plastic tonnage. Plastic is processed by plastic shaping processes which can be classified according to the resulting product geometry as describe below : Continuous extruded products with constant cross-section other than sheets, films and filaments. Continuous sheets and films. 91

2 Manufacturing Processes-III Solid molded parts. (d) (e) (f) (g) (h) Hollow molded parts with thin sectioned walls. Discrete parts made of formed sheets and films. Casted parts made with the help of special costing processes. Plastic parts made by forming operations. Foamed products of plastics. Different processing and shaping processes used for plastics are described in later sections of this unit. Objectives After studying this unit, you should be able to understand introduction of plastics and their classification, favourable properties of plastics, reasons for suitability of plastics for industrial uses, industrial applications of plastics, process of different types of injection molding, compression molding process, and plastics and its related environmental issues. 7.2 PROPERTIES OF PLASTICS Before discussing more about the processing methods of plastics, it is important to describe the important properties of plastics. (d) (e) (f) (g) Density of plastics is very low (1 to 2 gm/cm 3 ) as compared to metals. Strength of plastics is sufficient to make low weight, high strength machine parts. Antifriction and self lubricating properties of plastics enables it to be a good replacement of metal parts. Plastics components offer noiseless operation of moving parts, corrosion resistance, water proofing, leak proof joints, etc. Their costing is very easy due to low melting point and excellent flow ability in liquid state into the mold cavity. Low cost of manufacturing in case of plastics. Plastics exhibits insulation for heat and electricity. Plastics are chemically stable when subjected to solvents, oxidizing agents, gases, etc. 7.3 INDUSTRIAL USES OF PLASTICS 92 Above mentioned exclusive properties of plastics and other advancements are making plastics very much popular for industrial uses. Plastics are replacing metal in many engineering industries. These are replacing sheet metal parts like, zinc and aluminium alloy castings and cast iron parts. Plastics are being used in automobile industries, where weight reduction is one of the most important considerations due to fuel economy. Earlier plastics were used in automobiles to make fiscal panels, interiors and other non-load bearing components. Now-a-days, it is gaining popularity to make load bearing

3 parts and other components like bumpers, brake, fluid tanks and body parts. In addition to the above other industrial and commercial applications of plastics are listed below. (d) (e) (f) Polysters are also a type of plastic fibers and it is used in textile industry. Polyethylene is used to make wide variety of different size begs used for packaging purpose. A type of plastic (PET) is used for making containers and bottles used in pharmaceutical, food processing, cold drinks, washing power industries as packaging of material. Computer hardware industry uses plastics for making various hardware parts. Plastics are used for surface coating of metal parts to protect them against rusting. There are some special purpose plastics and in medical implant applications, aerospace industries and bearing industries. Normally plastics are used in situations determined on the basis of their advantageous properties as compared to metals. Their relative low cost, ease of manufacturing, imperviousness to water, low weight make them a good substitute of metals. However, plastics may create some environment related problems which are discussed in later sections of this unit. Welding and Plastics 7.4 INJECTION MOLDING Most of the plastic articles are produced by injection molding method. Thermoplastic is used as raw material in this process. Working Principle of Injection Molding In this process the thermoplastic polymer is heated to a plastic state. The hot plastic in plastic state is forced to flow under high pressure into a prepared mold cavity, where it is allowed to solidify. The solidified plastic is the final product which is removed form the mold cavity. The process of injection molding is very fast, the product cycle time is very short, it is of the order of 10 to 30 seconds only. A single mold may have more than one mold cavities so molding of more than one component is possible in one cycle time. By this process complex and intricate shapes are easily produced. Size of the products produced by this way may vary largely from very small to very large. Major limitations of injection are described below. Equipment for Injection Molding Capability to fabricate the mold cavity. Accuracy and quality of the product largely depend on the dimensional accuracy and quality of the mold cavity produced for injection molding. The mold cavity should be the replica of the product to be produced. Proper provisions should be provided with the mold cavity to take out the solidified final product after its molding. The process of product removal should not damage the mold cavity as same mold cavity has to be used many number of times. Molds are normally made with ultra care so a mold of large size for complex parts can cost more than five lakh rupees. So injection molding is economical for mass production. Equipment for injection molding resembles with the equipment for metal die casting. This equipment is named injection molding machine. It consists of two principal parts (i) molten plastic injection unit and (ii) mold clamping unit. 93

4 Manufacturing Processes-III Molten Plastic Injection Unit Molten plastic injection unit consists of a barrel having a feed hopper at its one end. The hopper supplies the plastic pellets. Inside the barrel, there is a screw like extruder screw. This screw can be rotated to mix the prepared material in the barrel and it also acts as a ram used to inject molten plastic into the mold. A non-return valve mounted near the screw prevents the molten plastic from flowing backward along the screw threads. The screw retracts to its former position without making the flow of molten plastics back. So due to the double action of Rota table screw it is called reciprocating screw. The injection unit of the molding machine, does three functions : Mold Clamping Unit melts the plastics (polymer), homogenize the molten plastics, and then properly injects the plastics to the mold cavity. Other important unit of injection molding machine is the clamping unit. It is concerned with the operation of mold. It does three functions are discuss below : It holds the two halves of the mold in proper alignment with respect to each other. It keeps the mold closed during the process of injection by applying clamping force. The clamping force resists the injection force. It opens and closes the mold at the appropriate timings in the molding cycle. The clamping unit consists of two platens, a fixed platen and other movable platen along with a mechanism for translating the latter. The mechanism is operated with the help of hydraulically operated power press, capable to apply a clamping force of several thousand tons. The clamping force value depends on the size of molding machine. A typical molding machine with its cycle by cycle operation is shown in Figure 7.1. Feed hopper Heaters Reciprocating screw Mold cutting Nozzle 94 Figure 7.1 : Line Diagram of Injection Molding Machine Working of Injection Molding Machine Working of injection molding machine is explained sequentially cycle by cycle as given below. The first action is mold open and machine is ready to start fresh molding. The first portion of molding cycle starts when mold is closed and clamped. Raw material, plastic or polymer is fed through hopper to the barrel, which is heated and brought to right temperature and viscosity. It is injected under pressure into the mold cavity by the ram. The process of injection is the second part of the molding cycle

5 when the screw (ram) moves towards the mold cavity. The injected plastic coals and begins to solidify when it comes in contact with the cold surface of mold cavity. Ram pressure is maintained to pack the molten plastic into the cavity that compensate contraction of plastic during solidification. Third part of molding cycle is the screw is rotated and retrated back with the non-return valve open to permit fresh polymer to flow into the forward portion of the barrel. Till the completion of third part the polymer in the mold cavity has completely solidified. The fourth part of the molding cycle is mold is opened and product is taken out of the mold. Welding and Plastics 7.5 CLASSIFICATIONS AND IMPORTANT FEATURES OF INJECTION MOLDING MACHINE Classification of injection molding machines into different categories depends on its two major components, the injection unit and clamping unit. On the basis of injection molding unit these machines are classified as discussed below. Reciprocating Screw Machines This is designed in such a manner so that same barrel can be used for melting, mixing and injection of the molten plastics. This type of molding machines are preferred where low intensity of mixing of different additives are required to make injectable molten plastic mixture to the mold cavity. Screw Pre-plasticizer Machines This is also called two stage molding machine. In this machine two different barrels are used, one for heating and mixing then prepared plastic mix is transferred to another barrel which is used for injection of the mix to mold cavity. The first barrel is screw driven which prepares the plastic mix. The second barrel is plunger driven it maintains the temperature of prepared mix and injects it to the mold cavity. This machine is also named plunger type injection molding machine. On the basis of clamping unit there can be three types of different design of molding machine as described below. Toggle Type In this case an actuator moves the crosshead forward, extending the toggle links to push the moving platens towards a closed position. The open and closed positions are shown in Figure 7.2. In this mechanism, at the beginning of stroke speed is high but mechanical advantage is low, when the stroke is likely to complete, the mechanical advantage is high but speed is very low so speed and force are maintained as these are desirable in the molding cycle. The way the machine can give lower values of cycle time. Actuating cylinder Crosshead links Fixed platen Moved platen Direction of movement Open position head Cross head Moving platen Closed position Figure 7.2 : Working of Toggle Clamping Unit 95

6 Manufacturing Processes-III Hydraulic Type Clamping Unit Hydraulic type clamping works similar to toggle type clamping unit, here the moving platen is moved by hydraulic pressure. These clamping units are used for high tonnage injection molding machines. These clamping units are relatively flexible as compared to toggle type clamping units with reference to adjustment of tonnage applied and length of stroke. Hydro-mechanical Clamping Unit This clamping unit explores advantages of both of the above categories. These clamping units are designed for large tonnage values. Movements of pushing plastic to mold are faster. Locking of the position of hydraulic cylinder is done by mechanical means. This maintains the tonnage value by building high pressure in the locked position. There are some special injection molding processes which are described below. 7.6 SOME SPEICAL INJECTION MOLDING PROCESSES Thermoplastic Foam Injection Molding This is one of the special purpose injection molding process. It involves the molding of thermoplastic parts that posses a dense or thick outer skin surrounding a lightweight foam core. These parts are recommended for structural applications. These produced either by introducing gas with molten plastic into the injection unit or by mixing a gas producing ingredient with the starting pellets. During injection, insufficient amount of molten plastic is filled to the cavity, where it expends as foams to fill the mold completely. This is used to make causing of machines, furniture, tanks, etc. Multi Injection Molding Processes In this process different polymers are injected sequentially. These polymers may be injected into same mold cavity to have different layers of polymers on the same produce or in to different mold cavities mutual connected. So the part produced by this may consists of two integrally connected plastics. The first method is called sand witch molding and later one is called Bi injection molding method. Injection Molding of Thermosets The difference between injection molding of thermoplastics and thermosets is, thermosets barrel length is relatively shorter to avoid premature curing and so solidification of molten plastic. Sufficient amount of molten plastic is accumulated ahead of the screw before its injection to the mold cavity. Thermosets are used to make composite with the reinforcement of glass fibres and wood fibres. Reaction Injection Molding 96 This process involves mixing of two highly reactive liquid ingredients (plastics) and the mixture is quickly injected to the mold cavity, where a chemical reaction takes place and solidification occurs. Normally the ingredients used are thermoset plastics. Urethanes, epoxies and urea formaldehyde are the examples of ingredients. This process is used to produce larger automotive components such as bumpers, etc. There is a need to control the process very precisely as quantity of ingredients to be mixed together is determined very precisely relative to the mold cavity. Once the process of reaction injection molding is designed and established it is found cheaper in long run (mass production).

7 7.7 DEFECTS AND PRECAUTIONS IN INJECTION MOLDING PROCESS Welding and Plastics Some of the common defects that can occur in an injection molding process their remedy and related precautions are described below. Pour Short Flashing This defect resembles with costing defect, it is the result of incomplete filling of the mold cavity. This can be corrected by increasing temperature and filling pressure of the molten plastic. The pressure and temperature suitably can be determined at the time of process design. The barrel of molding machine should large enough to accommodate slightly more volume of molten mix then required. Flashing is the result of squeezing of molten polymer between the mold plates. If injection pressure is very high or clearance between the mold plates is large then this defect appears. Sink Marks and Voids Sink marks and voids are two different molding defects but these are being discussed here simultaneously as their cause is same. A sink mark occurs when the outer surface of the mold solidifies but contraction of internal material causes the skin to the depressed below its intended profile. Sometimes void is formed due to contraction of internal material during cooling of mold. The chances of these defects are more in case of thicker sections. It is recommended to design sections of uniform thickness to avoid the occurrence of sink marks and voids. Weld Lines Shrinkage When flowing molten plastic flow around a convex shaped portion of a mold cavity, the two different streams of flowing material meet in opposite direction and a boundary form that is called weld line. Mechanical properties of weld line may be different (inferior) from rest of the molded product. The defect can be checked by maintaining high melting temperature, high injection pressure and proper ventilation. Plastics and polymers have high value of thermal expansion coefficients so appreciable shrinkage takes place on cooling of molded plastic parts. To compensate the shrinkage, the dimension of mold cavity is made larger than the desired dimensions of the processed plastic part. The amount of enlargement of mold cavity can be worked out on the basis of type of plastic, its injection temperature molding size, etc. The above mentioned data should be considered while designing the process and so designed and established procedure can be used for mass production. 7.8 COMPRESSION MOLDING PROCESS Compression molding is used for molding process of thermosetting plastics. The raw material used for compression molding can be in any form like powder, pellets, liquid or perform. The amount of polymer must be precisely controlled to obtain repeatable consistency in the molded product. Normally the charge is preheated prior to its placement into the mold. Preheating of raw material softens the plastics and shortens the production cycle time. Preheating is done with the help of infrared heaters, ovens. Reheating is also done by the use of hot rotating screw in the barrel of molding machine. The process of compression along with its working principle is described below. 97

8 Manufacturing Processes-III 7.9 PRINCIPLE OF COMPRESSION MOLDING The process of compression molding is explained with the help of line diagram shown in Figure 7.3. The process in normally carried out in three sequential steps. Hot chores loaded to mold Loading of the charge (raw material or plastic) into the lower half of mold. As we know that mold of compression molding process is designed into two parts. This is important as the mold has to be opened after solidification of molded parts. The amount of the charge is calculated very precisely so that it should be fully consumed during the operation of molding. If the charge kept there in, fall short the molded part will be of rejected quality. So precise determination of quantity of charge is done at the time of designing and established the process. The lower half of the mold is first heated to maintain the temperature of charge to a particular point during the process of molding. Punch Cavity Upper mold half Movement of upper molding half to compress the charge Knockout pin Preheated lower mold Knockout pin Compressed charge to the required shape Compression 98 (d) Figure 7.3 : Process of Compression Molding The placed charge to lower half of mold is compressed by bringing both halves of the mold close together. This pressing of charge is done under pressure which enable the charge to flow and acquire the shape of the mold cavity with ultra precision and dimensional accuracy. The precision and dimensional accuracy depends upon the quality of mold design and exact calculation of the amount of charge to be molded. The step is executed along with the previous step. The charge is heated by means of the hot mold to polymerize and cure it into a solidified desired shaped molded plastic component. Last step of compression molding is opening of molding halves and removal of the molded plastic part by pressing the knockout pin towards inside. This is done after molding, solidification and cooling of the molded part. Compression molding machine is a type of press which is oriented vertically and contains two platens molding halves are fasten to these platens. The operation of applying pressure is possible by two types of actuations : upstroke of bottom platen, or by downstroke of top platen. Pressing is normally hydraulically powered. A pressure upto few hundred tons can be applied by hydraulic power. In addition to pressing pressure a clamping pressure of the order of 100 ton can be used for proper clamping to avoid flushing. Molds for compression molding are simpler than the molds used for injection molding. As the material is not powered to the cavity so required getting system is not needed. There is almost no scope of appearing defects like powered short, etc. Mold cavities

9 should be designed to consider the flowing capabilities of charge on pressing. Any design interfering the flow of charge should be avoided. Common raw materials (plastics) used for compression molding are phenolics, melamine, urea, formaldehyde, epoxies, elastomers, etc. Major applications of compression molding using thermosetting plastics are done in making plugs, sockets, switches, handles, dinner plates. Welding and Plastics 7.10 ADVANTAGES OF COMPRESSION MOLDING PROCESS Main advantages of compression molding process are listed below : Less expending process, low maintenance cost. Negligible scrap. No residual stress into the molded part. Its major limitation is the time of compression molding is slightly higher than that of other similar processes so it gives slightly more cycle time. The process is recommended for flat thin parts. In addition to the above processes some other processes are also used for making plastic components like transfer molding, blow molding, rotational molding, thermoforming casting and molding. The detailed description of these process is not included in this unit PLASTICS AND ENVIRONMENTAL ISSUES Some of the environmental issues related to use of plastics are discussed below : Toxicity Due to their insolubility in water and inertness pure plastic generally have low toxicity in their finished state. Plastics pass through the digestive system with no ill effects. However, large pieces of plastics can do mechanical damages and obstruction to digestive system. Plastic components are manufactured by adding a variety of toxic additives to pure plastics. Traces of these chemicals can be pass on when it comes with contact of food items. Non-degradability Plastics are very durable and degrade very slowly, the molecular bonds that make plastic so durable make it very much resistant to natural process of degradation. Billions of ton of plastic discarded in last 75 years may persist for another thousand years. Sometimes plastic is burnt to dispose off which is also dangerous and not recommended because burning plastic can release toxic and environmentally unfriendly gases. These gases may cause ozone layer depletion and other environmental damages. Manufacturing process involved in making plastic articles often creates large quantities of chemical pollutants. Use of plastic begs created a problem of their disposal and begs disposed off improper way have started problem of chocking drainage system of cities. It is advised to use biodegradable plastics at the place of commonly used plastics. Starch powder is mixed with plastic as a filler material to make it easily degradable. There is a great necessarily to work more in this area to make easily degradable plastic or to recycle it. Manufacturing processes involved in making plastic articles should also be modified to make them environment safe. 99

10 Manufacturing Processes-III 7.12 SUMMARY Now-a-days plastic is considered as one of the commonly used engineering material as a suitable replacement of metal. This is due to its easy availability, low cost and low manufacturing cost and compatible mechanical properties. Normally plastics are divided into two categories thermoplastic and thermosetting depending upon their capability of being reused. Plastics are widely being used for commercial and industrial applications due to the exclusive advantages over other materials. Two major processes used for making plastic components are injection molding and compression molding processes. These processes resembles with casting and metal forming respectively which are used for metal processing. In case of injection molding the molten plastic with additives is injected into a specially designed mold cavity by the injection molding machine where it is allowed to cool down and solidify. This gives the plastic molded part according to the pre-designed mold cavity. Injection of molten plastic is done under heavy pressure to fill the mold cavity completely. Equipment of injection molding is divided into two parts injection unit and clamping unit. The clamping unit basically constitutes the accurately and precisely designed mold cavity. On the basis of these two units injection molding machine is classified into different categories. The details of injection molding machine and its categorization are explained in the unit in details. Like costing defects, there are chances of appearance some defects into injection molded products like pour short, flashing, weld lines and voids. These defects can be predicted in advance and necessary care can be taken to avoid them from the molded products. In case of compression molding process the hot charge premixed with all additives is kept in the lower half of molding box and it compressed by the upper half of the molding box under high pressure. The compressed part is cooled and allowed to solidify to give molded plastic part. High pressure application provides proper flow ability to the charge to fill all parts of the mold cavity and to get a good quality product. The whole process is designed and established by predicting all possible defects which may appear in a molded component and precautionary measures are incorporated in the design of the process itself. Use of plastics is not very much encouraged from environment point of view, because there is a problem of disposal of plastic waste as plastic is not a degradable item. Disposal difficulties may give different type of problems. In addition to this plastic manufacturing processes may produce harmful chemical pollutants. It is suggested to work more to develop biodegradable type of plastic and to modify the related manufacturing processes to make them friendly for mankind in future ANSWERS TO SAQs Refer the preceding text for all the Answers to SAQs. 100