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二、The injection and Compression Molding
Injection molding si principally used for the production of the thermoplastic parts, although some progress has been made in developing a method for injection molding some thermosetting materials. The problem of injecting a melted plastic into a mold cavity form a reservoir of melted material has been extremely difficult to solve for thermosetting plastics which cure and harden under such conditions within a few minutes. The principle of injection molding is quite similar to that of die-casting. The process consists of feeding a plastic compound in powdered or granular form from a hopper through metering and melting stages and then injecting it into a mold. After a brief coolling period, the mold is opened and the solidified part ejected. Injection-molding machines can be arranged for manual operation, automatic single-cucle operation, and full automatic operation. The advantage of injection molding are:
(i) a high molding speed adapted for mass production is possible;
(ii) there is a wide choice of thermoplastic materials providing a variety of usefull properties;
(iii) it is possible to mold threads, undercuts, side holes, and large thin sections.
Several methods are used to force or inject the melted plastic into the mold. The most commonly used system in the larger machines is the in-line reciprocating screw.The screw acts as a combination and plasticizing unit.As the plastic is fed to the rotating screw,it passes through three zones as shown: feed,compression, and metering. After the feed zone, the screw-flight depth is gradually reduced,forcing the plastic to compress. The work is converted to heat by shearing the plastic, making it a semifluid mass. In the metering zone, additional heat is applied by conduction from the barrel surface. As the chamber in front of the screw becomes filled, it forces the screw back, tripping a limit switch that activates a hydraulic cylinder that forces the screw forward and injects the fluid plastic into the closed mold.An antiflowback valve prevents plastic under pressure from escaping back into the screw flights.
The clamping force that a machine is capable of exerting is part of the size designation and is measured in tons. A rule-of-thumb can be used to determine the tonnage required for a particular job. It is based on two tons of clamp force per square inch of projected area. If the flow pattern is difficult and the parts are thin,this may have to go to three or four tons.
Many reciprocating - screw machines are capable of handing thermosetting plastic materials.Previously these materials were handled by compression or transfer molding.Thermosetting materials cure or polymerize in the mold and are ejected hot in the range of 375℃～410℃.Thermoplastic parts must be allowed to cool in the mold in order to remove them without distortion.Thus thermosetting cycles can be faster.Of course the mold must be heated rather than chilled,as with thermoplastics.
The importance of Injecting the mold are :
⑴、Plastics have the density small, the quality light, the specific tenacity big, the insulating property good, the dielectric loss low, the chemical stability strong, the formation productivity high and the price inexpensive and so on the merits, obtained day by day the widespread application in the national economy and people's daily life each domain, as early as in the beginning of 1990s, the plastic annual output already surpassed the steel and iron and the non-ferrous metal annual output sum total according to the volume computation.In mechanical and electrical (for example so-called black electrical appliances), domains and so on measuring appliance, chemical, the automobile and astronautics aviation, the plastic has become the metal the good substitution material, had the metal material plastic tendency.
⑵、Take the automobile industry as the example , as a result of the automobile lightweight, the low energy consumption development request, the automobile spare part material constitution occurred obviously has modelled the band steel the change, at present our country automobile plastic accounts for 5% which the automobile was self-possessed to 6%, but overseas has reached 13%, forecast according to the expert, the automobile plastic bicycle amount used will also be able further to increase.On modern vehicles, regardless of is outside installs the assorted items, the internal installation assorted items, the function and the structural element, all may use the plastic material, outside installs the assorted items to have the bumper, the fender, the wheel hub cap, the air deflector and so on; After the internal installation assorted items have in the display board, the vehicle door the board, the vice-display board, the sundry goods box lid, the chair, the guard shield and so on; The function and the structural element have the fuel tank, the radiator header, the spatial filter hood, the fan blade and so on.Statistics have indicated, our country in 2000 automobile output more than 200 ten thousand, the vehicle amounted to 1,380,000 tons with the plastic.Looked from the domestic and foreign automobile plastic application situation that, the automobile plastic amount used already became one of weight automobile production technical level symbols.
⑶、Injection of a molding formation as plastic workpiece most effective formation methods because may by one time take shape each kind of structure complex, the size precise and has the metal to inlay a product, and the formation cycle is short, may by mold multi-cavities, the productivity be high, when mass productions the cost isvery inexpensive, easy to realize the automated production, therefore holds the extremely important status in the plastic processing profession.Statistics have indicated, plastic mold composition all molds (including metal pattern) 38.2%, the plastic product gross weight about 32% is uses in injecting the formation, 80% above engineering plastics product all must use the injection formation way production. 4. counts according to the customs, our country in 2000 altogether imported mold 977,000,000 US dollars, in which plastic molding forms altogether 550,000,000 US dollars, occupied for 56.3%,2001 years altogether to import mold 1,112,000,000 US dollars, in which plastic molding forms altogether 616,000,000 US dollars, accounted for 55.4%.From the variety, the import volume biggest is the plastic molding forms.
⑷、Counts according to the customs, our country in 2000 altogether imported mold 977,000,000 US dollars, in which plastic molding forms altogether 550,000,000 US dollars, occupied for 56.3%, 2001 years altogether to import mold 1,112,000,000 US dollars, in which plastic molding forms altogether 616,000,000 US dollars, accounted for 55.4%.From the variety, the import volume biggest is the plastic molding forms.
In compression molding the palstic material as powder or preforms is placed into a heated steel mold cavity,Since the parting surface is in a horizontal plane ,the upper half of the mold descends vertically.It closes the mold cavity and pressures for a predetermined period.A pressure of from 2 to 3 tons square inch and a temperaure at approximately 350F converts the plastic to a semiliquid which flows to all parts of the mold cavity.Usually from 1 to 15 minutes is required for curing,altough a recently developed alkyd plastic will cure in less than 25 secends. The mold is then opended and the molded part removed.If metal insers are desired in the parts,they should be placed in the mold cavity on pins or in the holes before the plastic is loaded.Also, the preforms should be preheated before loading into the mold cavity to eliminate gases,inprove flow,and decrease curing time.Dieletric heating is a convenient method of heating the preforms.
Since the plastic material is placed directly into the mold cavity,the mold itself can be simpler than those used for other molding precesses.Gates and sprues are unnecessary.This also results in a saving in material,because trimmed-off gates and sprues would be a complete loss of the thermosetting plastic.The press require the full attention of one operator.However,several smaller presses can be operated by one operator. The presses are conveniently located so the operator can easilymove from one to the next.By the time he gets around to a particular press again,that mold will be ready to open.the thermosetting plastics which harden under heat and pressure are suitable for compression molding and transfer molding.It is not practical to moid shermoplastic materials by these methods,since the molds would have to be alternately heated and cooled.In order to harden and eject thermoplastic parts form the mold,cooling would be necessary.
Types of molds for compression molding.The molds used for compression molding are classified into four basic types, namely ,positive molds,landed positive mold,flash-type molds,and semipositive molds.In a positive mold the plunger on the upper mold enters the lower mold cavity.since there are no lands or stops on the lower die ,the plunger completely trap the plastic material and descends with full pressure on the charge.A dense part with good electrical and physical properties is produced.The amount of plastic placed in the die cavity must be accurately measured,since it determines the thickness of the part .A landed positive mold is similar to a positive mold except that lands are added to stop the travel of the plunger at predetermined point.In this case,the lands absorb some of the pressure that should be exerted on the parts.The thickness of the parts will be accurately controlled,but the density may vary cansideraby.In a flash-type mold,flash redges are added ti the top and bottom molds.As the upper mold exerts pressure on the plastic,excess material is forced out between the flash ridges where it forms flash.This flash is further compressed.becomes hardened,and finally stops the downard thavel of the upper mold.A slight excess of the plastic material is always chared to ensure sufficient pressurs to produce a dense molded part.This type of mold is widely used because it is comparatively easy to construct and it controls thickness and density within colse limits.The semipositive mold is a combination od the flash type and landed posive molds.In addition to the flash ridges,a land is employed to restrict the travel of the upper mold.