機(jī)械制造類(lèi)外文資料

1、2.4 Compression and Transfer MoldsCompression 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 mate-rial, softened by heat, is formed into a continuous mass having the geometrical c

2、onfiguration of' the mold cavity. Further heating (thermosetting plastics) results in hardening of the molding ma-terial. If thermoplastics are the molding material, hardening is accomplished by cooling the mold.Fig. 2-6 illustrates types of compression molding. Here the molding compound is plac

3、ed in theheated 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 and pressure produ

4、ce 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 part is ejected from t

5、he 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 complete loss of the

6、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 can easily move fr

7、om one to the next. By the time he gets around to a particular press again, that mold will be ready to open.2模具設(shè)計(jì)與制造專(zhuān)業(yè)英語(yǔ)Heating elementsPunchChargeOpenEjector pinMoldLandOverlapFlashClosed(a)(b)(c)PartPlug(d)Fig. 2-6 Types of Compression Molding(a) positive(b) semipositive(c) flash(d) die design for

8、 making a compression-molded part with undercutsThe thermosetting plastics which harden under heat and pressure are suitable for compression molding and transfer molding. It is not practical to mold thermoplastic materials by these meth-ods, since the molds would have to be alternately heated and co

9、oled. In order to harden and eject thermoplastic parts from the mold, cooling would be necessary.The transfer molding process consists of placing a charge of material (extrudate or preheated preform) into the chamber, referred to as the pot. The press is activated and travels upward mak-Chapter 2 Pl

10、astics Molds3ing contact with the floating plate, which closes the two halves of the mold.Pot or Sprue Type Transfer MoldingTranferPlungerMoldingPowderMold Closedand FilledKnock Out PinMold OpenSpruePunchMolded PartsCavityFig. 2-7 The illustrates the pot or sprue type transfer molding of thermosetsP

11、lungerMoldingCompoundMold ClosedCavities FilledKnock Out PinMold OpenMolded PartsFig. 2-8 The illustrates the plunger transfermolding of thermosetsFurther travel of both plates causes contact of the plunger with the material in the pot. Materi-al is then forced through a sprue or sprues directly int

12、o the closed cavity. When the cavity is completely filled, the excess material forms a cull in the pot (excess waste material). After the4模具設(shè)計(jì)與制造專(zhuān)業(yè)英語(yǔ)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 tra

13、vel continues, the floating plate motion is stopped by straps fastened to the top plate.This separates the two halves of the mold, and the part remains in the lower half until knock-out pins extract it. Since the process requires that the single charge (shot) of material be trans-ferred from the pot

14、 to the cavities, it is known as pot-type transfer (Fig.2-7). An operator is needed to remove the cull from the pot plunger, remove the part or parts, clean the mold, chargea single shot of preheated material into the pot area, and activate the press.A relatively short time after the patenting of th

15、e transfer mold; transfer presses were devel-oped. These consist of a main clamping ram located at either the top or the bottom of the press,with one or more auxiliary rams mounted opposite the clamping ram. The clamping rams acti-vate the movable platen. The auxiliary rams are fastened to the stati

16、onary platen and are used toactivate a plunger, which moves within a transfer sleeve or cylinder. For the plunger in the bot-tom half of the mold, the process consists of placing preheated preforms or extrudates in thetransfer sleeve or cylinder, closing the two halves of the mold, and activating th

17、e plunger, whichforces material out through channels, known as runners, and through the restricted gate area intothe mold halves. When the cavities are completely filled, the excess material remains as a cull atthe face of the plunger. After the material is cured, the press is opened at the parting

18、line, partsare removed and the gate, runner and cull. This molding process is commonly called the plung-er-transfer method. A typicalmold construction is shown in Fig.2-8. Ifthe bottom plung-er-transfer mold is constructed, the operation may be automated, since auxiliary devices mayload the preheate

19、d preforms, and unloading trays may be utilized to receive and separate theparts, runner, gates, and plunger culls. In all other eases an operator is required for each press.The two-stage plunger transfer process requires a conventionally designed hydraulic or toggle top clamp press, with a bottom t

20、ransfer cylinder and plunger. A reciprocal screw within a heated barrel is mounted horizontally next to the press. The granular material charge is preheated in the barrel and is discharged into the transfer cylinder or sleeve through an opening in its side. The material flow from the same way as des

21、cribed in the plunger-transfer molding process.The two-stage plunger-transfer molds are similar in construction to the plunger transfer, ex-cept that a special transfer cylinder or sleeve and plunger are required.Thermosetting compression-molding compounds can be molded into articles of excellent ri

22、-gidity and shape retention by supplying heat and pressure. Apart from the molding material, theChapter 2 Plastics Molds5mold itself is of great importance.Compression molds nowadays are heated electrically exclusively. The mold is loaded with molding compound, by hand, with the aid of a filling dev

23、ice, or with pellets.A construction drawing should be mandatory for every mold to be newly produced. Any new ideas concerning the mold, such as stability of the mold construction, optimum heating, aids to demolding and ejection, e.g., slides, split cavities, cores, etc., can be included in advance a

24、nd given due consideration. The cost of such drawings will be more than justified as a rule by the ensuing efficient mold production and by fewer alterations and less finishing work on the com-pleted mold. The more accurately details are incorporated in the design, the more finishing work is avoided

25、, e.g., specification of the draft angle required and dimensional tolerances.Because alterations to compression molds are always very expensive, it is of particular im-portance that the detail drawings be completely clear.The mold must be of sufficiently solid and rigid construction to enable it to

26、withstand 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 trou-blesome ejection. High-walled parts may well exert the total compression a pressure on

27、 the side walls. The bottom of the mold must be well supported to absorb the pressure exerted on it and to avoid deflection.As the material costs are comparatively low compared to wages, one can afford to have themold solidly constructed without incurring any significant increase in cost. The higher

28、 steel con-tent ensures a more uniformtemperature distributionand temperature control, apart from thegreater rigidity. A good polish of the molding areas is absolutely essential for trouble-free ejec-tionand to give a satisfactory surface to the moldedarticle. The mold surface should beglass-hard so

29、 that it can withstand the wearing effect the molding material exerts when flowingunder pressure and so that it retains its polish. On the other hand, the tool steel needs to possess atough core, as a slight distortion of the mold walls and the ribs is unavoidable. It is recommendedto use a carburiz

30、ing steel for the shape-giving mold parts. This has already proved itself in theconstruction of molds for the plastics-processing industry. The mold surface must be resistant toconstant attack by chemicals, whichis particularlyprevalent withcertain types of compres-sion-molding compounds. A mold can

31、 be protected from chemical attack and frictional wear bychrome plating of the molding surfaces.A further important requirement is that the mold consists of as few interlocking parts as possi-ble. The fitting of several parts into each other is always fraught with danger because of the pos-sible dis

32、tortion caused by the high compression 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-6模具設(shè)計(jì)與制造專(zhuān)業(yè)英語(yǔ)sion mold in line with the pressure and never across it.A compression mold basically consists of an

33、upper and a lower part. In normal 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 hardened dowels. Asymmetrical parts cause large one-sided pressure loads to be exerted on the mold. They require compensation through special guide

34、s.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 machine for cleaning flash and ma-terial residue from the molds. In all other cases, ejection by ejector pins or ribs is feasible. For parts wit

35、h a multitude of fibs and openings, ejector pins are essential because of the material shrinkage.Thermosetting molding compounds can be processed by the transfer molding process. The hot injection cylinder, however, should not contain material reserves for several parts since the mate-rial would, on

36、ly cure in the heat. Thermosets can only be transfer molded if the material volume corresponds to the volume of the part to be produced plus the sprue. It would be expedient to mold with material that has been predried in a high-frequency oven, to be taken out of the oven only just before it is mete

37、red into the transfer cylinder.The most favorable and most accurate type of metering in this case-as with conventional compression molding-is also achieved with precompressed pellets. As they are already of a cer-tain density, greater leeway can be given to the dimensions of the injection cylinder,

38、which has a decisive influence on the injection pressure required. Because the material is injected through a small nozzle bore very uniform heat permeation is achieved. Whereas in compression mold-ing-even with well prewarmed pellets-the material does not flow very easily, thoroughly plasti-cized m

39、aterial enters the cavities in transfer molding. The material is additionally warmed by the heated mold walls. Heat permeation is therefore better than with compression molding. A con-siderably shorter cure time is needed for the transfer molding process than for the compression molding method. The

40、transfer molding process also is of particular advantage when long cores have to be employed due to the nature of the parts. In this instance, their guidance and support against unilateral pressure is considerably easier to design than for compression molding. This is also the reason why injection a

41、round sensitive metal parts is possible. The cores and the inserts must be advantageously positioned in the flow path of the material by arranging the runners ac-cordingly.The requirements to be met by a transfer mold are basically the same as those for a compres-Chapter 2 Plastics Molds7sion mold.

42、Due to the injection pressure required, which lays around 1000 to 1800 bar in the in-jection cylinder but is somewhat lower inside the mold cavity, although still higher than with ordinary compression molding, the mold must be more solidly constructed. Particular care must be taken with the venting

43、of the shape-giving cavities as the mold is already fully clamped during injection. If this is not observed, voids and incomplete parts will result in the same manner as can be experienced when injection molding thermoplastics material. However, as venting of the mold is not possible in the same way

44、 as it is done on standard compression molds, air vents have to be positioned and dimensioned so that they permit the gases to escape from the material without allowing the latter to clog up the venting channels.The construction of a transfer mold differs from that of a compression mold in that the

45、charg-ing chamber does not exist. This has been replaced by an injection cylinder and piston positioned in the center of the mold. One differentiates between the two basic types of transfer mold as fol-lows: transfer mold with top injection cylinder and piston. These molds can be operated on standard presses, in which case the restriction in the opening stroke has of course to be taken into c