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1、2.4 Compression and Transfer Molds2.4.1 Compression MoldingCompression molding is the basic forming process where an appropriate amount of material is introduced into a heated mold, which is subsequently closed under pressure. The molding material, softened by heat, is formed into a continuous mass

2、having the geometrical configuration of the mold cavity. Further heating (thermosetting plastics) results in hardening of the molding material. If thermoplastics are the molding material, hardening is accomplished by cooling the mold.Fig. 2-6 illustrates types of compression molding. Here the moldin

3、g compound is placed in the heated mold. After the plastic compound softens and becomes plastic,the punch moves downand compresses the material to the required density by a pressure. Some excess material will flow (vertical flash) from the mold as the mold closes to its final position.Continued heat

4、 and pressure produce the chemical reaction which hardens the compound. The time required for polymerization or curing depends principally upon the largest cross section of the product and the type of molding compound. The time may be less than a minute, or it may take several minutes before the par

5、t is ejected from the cavity.Since the plastic material is placed directly into the mold cavity, the mold itself can be simpler than those used for other molding processes. Gates and sprues are unnecessary. This also results in a saving in material, because trimmed-off gates and sprues would be a co

6、mplete loss of the thermosetting plastic. The press used for compression molding is usually a vertical hydraulic press. Large presses may 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

7、 can easily move from one to the next. By the time he gets around to a particular press again, that mold will be ready to open.Heating elements(司(b)(c)(d)Fig. 2-6 Types of Compression Molding (a) positive (b) semipositive (c) flash(d) die design for making a compression-molded part with undercutsThe

8、 thermosetti ng plastics which harde n un der heat and pressure are suitable for compressi on molding and transfer molding. It is not practical to mold thermoplastic materials by these methods, since the molds would have to be alter nately heated and cooled. In order to harde n and eject thermoplast

9、ic parts from the mold, cooli ng would be n ecessary.2.4.2 Tran sfer Moldi ngThe tran sfer moldi ng process con sists of placi ng a charge of material (extrudate or preheated preform) into the chamber, referred to as the pot. The press is activated and travels upward mak-ing con tact with the float

10、ing plate, which closes the two halves of the mold.Pot or Sprue Type Transfer MoldingTranferPlunger Molding Powder0PlungerMoldingCompoundMold Closed and FilledKnock Out PinMold ClosedCavities FilledFig. 2-7 The illustrates the pot or sprue type transfer molding of thermosetsMold OpenMolded PartsFig.

11、 2-8 The illustrates the plunger transfer molding of thermosetsKnock Out PinFurther travel of both plates causes con tact of the plun ger with the material in the pot. Material is the n forced through a sprue or sprues directly in to the closed cavity. When the cavity is completely filled, the exces

12、s material forms a cull in the pot (excess waste material). After the part is cured, the press is opened and the floating plate and bottom plate separate from the top plate, exposing the plunger and cull. As the press travel continues, the floating plate motion is stopped by straps faste ned to the

13、top plate.This separates the two halves of the mold, and the part rema ins in the lower half un til kno ck- out pins extract it. Since the process requires that the sin gle charge (shot) of material be transferred from the pot to the cavities, it is known as pot-type transfer (Fig.2-7). An operator

14、is n eeded to remove the cull from the pot plun ger, remove the part or parts, clea n the mold, charge a sin gle shot of preheated material into the pot area, and activate the press.A relatively short time after the pate nti ng of the tran sfer mold; tran sfer presses were developed. These con sist

15、of a main clamp ing ram located at either the top or the bottom of the press, with one or more auxiliary rams moun ted opposite the clampi ng ram. The clampi ng rams activate the movable plate n. The auxiliary rams are faste ned to the stati onary plate n and are used to activate a plunger, which mo

16、ves within a transfer sleeve or cylinder. For the plunger in the bottom half of the mold, the process consists of placing preheated preforms or extrudates in the transfer sleeve or cylinder, closing the two halves of the mold, and activating the plunger, which forces material out through channels, k

17、nown as runners, and through the restricted gate area into the mold halves. When the cavities are completely filled, the excess material rema ins as a cull at the face of the plunger. After the material is cured, the press is opened at the parting line, parts are removed and the gate, runner and cul

18、l. This moldi ng process is com monly called the plung- er-tra nsfer method. A typical mold con struct ion is show n in Fig.2-8. If the bottom plunger-transfer mold is constructed, the operation may be automated, since auxiliary devices may load the preheated preforms, and unioading trays may be uti

19、lized to receive and separate the parts, runner, gates, and plun ger culls. In all other eases an operator is required for each press.The two-stage plun ger tran sfer process requires a conven ti on ally desig ned hydraulic or toggle top clamp press, with a bottom transfer cylinder and plunger. A re

20、ciprocal screw within a heated barrel is moun ted horiz on tally n ext to the press. The gra nu lar material charge is preheated in the barrel and is discharged into the tran sfer cyli nder or sleeve through an ope ning in its side. The material flow from the same way as described in the plun ger-tr

21、a nsfer moldi ng process.The two-stage plun ger-tra nsfer molds are similar in con structi on to the plun ger tran sfer, except that a special tran sfer cyli nder or sleeve and plun ger are required.2.4.3 Compressio n MoldsThermosetti ng compressio n-m oldi ng compo unds can be molded into articles

22、of excelle nt rigidity and shape retention by supplying heat and pressure. Apart from the molding material, themold itself is of great importa nee.Compression molds nowadays are heated electrically exclusively. The mold is loaded with molding compound, by hand, with the aid of a filling device, or w

23、ith pellets.A con struct ion draw ing should be man datory for every mold to be n ewly produced. Any new ideas concerning the mold, such as stability of the mold con struct ion, optimum heati ng, aids to demolding and ejection, e.g., slides, split cavities, cores, etc., can be included in advanee an

24、d given due consideration. The cost of such drawings will be more than justified as a rule by the ensuing efficie nt mold product ion and by fewer alterati ons and less finishing work on the completed mold. The more accurately details are incorporated in the design, the more finishing work is avoide

25、d, e.g., specification of the draft angle required and dimensional tolerances.Because alterations to compression molds are always very expensive, it is of particular importa nee that the detail draw ings be completely clear.The mold must be of sufficie ntly solid and rigid con struct ion to en able

26、it to withsta nd the high pressures required with compression molding. The outer walls should be only slightly flexible. If the mold is too flexible, the result could be jamming of the two mold halves on opening, or troublesome ejection. High-walled parts may well exert the total compression a press

27、ure on the side walls. The bottom of the mold must be well supported to absorb the pressure exerted on it and to avoid deflect ion.As the material costs are comparatively low compared to wages, one can afford to have the mold solidly con structed without in curri ng any sig ni fica nt in crease in c

28、ost. The higher steel content ensures a more uniform temperature distribution and temperature control, apart from the greater rigidity. A good polish of the molding areas is absolutely essential for trouble-free ejection and to give a satisfactory surface to the molded article. The mold surface shou

29、ld be glass-hard so that it can withsta nd the weari ng effect the moldi ng material exerts whe n flow ing un der pressure and so that it reta ins its polish. On the other hand, the tool steel n eeds to possess a tough core, as a slight distorti on of the mold walls and the ribs is un avoidable. It

30、is recomme nded to use a carburiz ing steel for the shape-givi ng mold parts. This has already proved itself in the con structi on of molds for the plastics-process ing in dustry. The mold surface must be resista nt to con sta nt attack by chemicals, which is particularly prevale nt with certa in ty

31、pes of compres- sion-m old ing compo un ds. A mold can be protected from chemical attack and frictional wear by chrome plati ng of the moldi ng surfaces.A further importa nt requireme nt is that the mold con sists of as few in terlock ing parts as possible. The fitt ing of several parts into each ot

32、her is always fraught with dan ger because of the possible distort ion caused by the high compressi on pressures employed. Should it not be possible to avoid working with inserts, it is then essential that the inserts always be fitted into the compres-sion mold in line with the pressure and n ever a

33、cross it.A compressi on mold basically con sists of an upper and a lower part. I n no rmal cases the lower half is fitted to the table of the press and the upper half to the ram. Both mold halves are guided by harde ned dowels. Asymmetrical parts cause large on e-sided pressure loads to be exerted o

34、n the mold. They require compe nsati on through special guides.Ejection usually calls for special equipment. Parts such as flat dishes or plates are easily ejected by compressed air, which is already available on the machi ne for clea ning flash and material residue from the molds. In all other case

35、s, ejection by ejector pins or ribs is feasible. For parts with a multitude of fibs and openings, ejector pins are essential because of the material shri nkage.2.4.4 Tran sfer MoldsThermosett ing moldi ng compo unds can be processed by the tran sfer mold ing process. The hot injectio n cyli nder, ho

36、wever, should not con tain material reserves for several parts si nee the material would, only cure in the heat. Thermosets can only be transfer molded if the material volume corresp onds to the volume of the part to be produced plus the sprue. It would be expedie nt to mold with material that has b

37、een predried in a high-frequency oven, to be taken out of the oven only just before it is metered into the tran sfer cyli nder.The most favorable and most accurate type of metering in this case-as with conventional compressi on moldi ng-is also achieved with precompressed pellets. As they are alread

38、y of a certain den sity, greater leeway can be give n to the dime nsions of the injectio n cyli nder, which has a decisive in flue nee on the injectio n pressure required. Because the material is injected through a small nozzle bore very uniform heat permeation is achieved. Whereas in compression mo

39、ldin g-eve n with well prewarmed pellets-the material does n ot flow very easily, thoroughly plasticized material en ters the cavities in tran sfer moldi ng. The material is additi on ally warmed by the heated mold walls. Heat permeation is therefore better than with compression molding. A considera

40、bly shorter cure time is n eeded for the tran sfer moldi ng process tha n for the compressi on molding method. The transfer molding process also is of particular advantage when long cores have to be employed due to the n ature of the parts. In this in sta nee, their guida nee and support aga inst un

41、 ilateral pressure is con siderably easier to desig n tha n for compressi on mold ing. This is also the reas on why inject ion around sen sitive metal parts is possible. The cores and the in serts must be adva ntageously positi oned in the flow path of the material by arranging the runners accord in

42、 gly.The requireme nts to be met by a tran sfer mold are basically the same as those for a compres- sio n mold. Due to the injectio n pressure required, which lays around 1000 to 1800 bar in the injection cyli nder but is somewhat lower in side the mold cavity, although still higher tha n with ordin

43、ary compressi on moldi ng, the mold must be more solidly con structed. Particular care must be taken with the venting of the shape-giving cavities as the mold is already fully clamped during inject ion. If this is not observed, voids and in complete parts will result in the same manner as can be exp

44、erie need whe n injectio n moldi ng thermoplastics material. However, as ven ti ng of the mold is not possible in the same way as it is done on sta ndard compressi on molds, air vents have to be positioned and dimensioned so that they permit the gases to escape from the material without allowing the

45、 latter to clog up the venting channels.The con struct ion of a tran sfer mold differs from that of a compressi on mold in that the charging chamber does not exist. This has bee n replaced by an inject ion cyli nder and pist on positi oned in the center of the mold. One differentiates between the two basic types of transfer mold as follows: transfer mold with top injection cylinder and piston. These molds can be operated on sta ndard presses, in which case the restricti on in the ope ning stroke has of course to be take n into con sideratio n.Transfer mold with bottom injection