GB1591380A

GB1591380A - Casting process by vacuum moulding

Info

Publication number: GB1591380A
Application number: GB3799277A
Authority: GB
Original assignee: Nippon Gakki Co Ltd
Current assignee: Nippon Gakki Co Ltd
Priority date: 1977-09-12
Filing date: 1977-09-12
Publication date: 1981-06-24

Description

(54) IMPROVED CASTING PROCESS BY VACUUM MOULDING (71) We, NIPPON GAKKI SEIZO KABUSHIKI KAISHA, a joint stock company organised under the laws of Japan of No. 10-1, Nakazawacho, Hamamatsu-shi, Shizuokaken, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to improved casting process by vacuum moulding.

In vacuum moulding, the surface of the refractory filler such as moulding sand to be exposed to the mould cavity is tightly covered with a covering film such as a thermoplastic synthetic resin film through the application of pneumatic suction to the film via the refractory filler. Thus, the wall of the mould cavity is covered by the covering film, i.e. a synthetic resin film in general.

At casting, molten metal at a very high temperature is charged into the mould cavity and the molten metal melts a part of the covering film, which is pneumatically sucked into the refractory filler. Concurrently with this, the other part of the covering film is burnt due to the heat given off by the molten metal to vanish. Thus, the molten metal comes into direct contact with the refractory filler, is pneumatically sucked into the refractory filler and solidified in order effectively to prevent undesirable crumbling of the mould.

However, in the actual casting process, it is often experienced with the upper half of the mould, especially of a mould provided with a large-sized inner cavity, that the mould accidentially crumbles during charging of the molten metal. This is due to the fact that the molten metal firstly spreads in the mould cavity flowing over the cavity wall of the lower half of the mould before coming into direct contact with the cavity wall of the upper half of the mould, the film covering the cavity wall of the upper half of the mould evaporates due to the extremely high radiation heat given off by the molten metal in advance of the direct contact with the molten metal, and the exposed refractory filler is apt to crumble due to the vanishing of the covering film.

This trouble may start with the lower half of the mould also depending upon the magnitude of the pneumatic suction pressure, the grain Size of the refractory filler and the spreading speed of the molten metal in the mould cavity. That is, as the molten metal spreads over the cavity wall of the lower half of the mould, the portion of the covering film close to the leading end of the molten metal is molten and sucked into the refractory filler so that the refractory filler is exposed to the cavity. Direct contact of the exposed refractory filler with the spreading molten metal inevitably causes development of casting defects such as sand marks and blow holes.

The above-described portion of the covering film which is molten and sucked into the refractory filler forms a shell layer together with the refractory filler granules. However,'the relatively high permeation speed of the molten covering' film leads to large diffusion thereof into the refractory filler. In other words, the density of the molten covering film so diffused into the refractory filler is very low. So, the obtained shell layer lacks glutinosity and strength and tends to develop casting defects such as sand marks at charging of the molten metal.

In order to obviate this trouble it has been proposed to coat the surface of the covering film which comes into contact with the refractory filler with fine granules mixed with a suitable bonding agent. However; in this case, there is poor penetration of the molten covering film into the refractory filler. Thusj the greater part of the covering film is then burnt within the mould-cavity and the gas generated by burning of the covering film cannot bye discharged out of the mould cavity through the refractory filler, so that gas holes are formed in the mould.

We have sought to provide a novel casting process in which casting can be carried out with considerably reduced danger of mould crumbling, in which the incidence of casting defects such as sand marks and blow holes on the products obtained is reduced or obviated and which yields products of high casting surface quality.

In accordance with the present invention, we provide a process of casting by vacuum moulding characterized by tightly covering a pattern surface of a pattern with a covering film by suction applied to said pattern surface; coating the exposed surface of said covering film with a solution of an initial condensate of a thermosetting resin to form a thermosetting resin layer or granular dispersion on said covering film; setting a moulding box in position relative to said pattern; filling the center cavity of said moulding box with a fluid refractory filler; covering the exposed surface of said refractory filler; compacting said refractory filler by the application of pneumatic suction; removing said pattern from the remainder in order to form one mould half; preparing the other mould half in similar manner; coupling said mould halves to each other in order to form a complete mould having an inner cavity; and casting molten metal into said cavity of said complete mould.

The covering film is generally thermoplastic and is caused to cover the pattern surface when in a heated state.

Preferably, the covering film is composed of a thermoplastic resin and has a thickness in the range from 20 to 100 microns. In general, the thickness of the solidified thermosetting resin layer is in the range of from 2 to 100 microns. The thermosetting resin may be a phenolic, furan or urea resin but novolak type phenolic resins are preferred. The thickness of the novolak type phenolic resin layer is preferably in the range of from 5 to 40 microns.

The preferred solvent for this initial condensate of thermosetting resin is alcohol, moulding sand is generally used as the refractory filler and a thin synthetic resin film is generally used to cover the exposed surface of the refractory filler.

Further features and advantages of the present invention will be made clearer from the following description, reference being made to the embodiment shown in the accompanying drawings in which: Figures 1 to 3 are explanatory side sectional views for showing the process in accordance with the present invention; Figure 4 is an enlarged side sectional view of the cavity wall configuration in the mould prepared by the process shown in Figures 1 to 3.

Figure 5 is an enlarged side sectional view of the cavity wall configuration shown in Figure 4 during molten metal casting; and Figures 6 to 8 are photographs showing the development of casting defects on products obtained in the later-described examples.

The vacuum moulding process in accordance with the present invention is shown in sequence in Figures 1 to 3.

To begin with, a pattern 1 such as that shown in Figure 1 is prepared. This has a top pattern surface 3 of a predetermined shape and a suction chamber 2 confined therein. This suction chamber 2, on the one hand, is connected to a given suction source such as a vacuum pump (not shown) via a conduit 5 and, on the other hand, is pneumatically connected to the pattern surface 3 via a number of suction holes 4 opening in the latter.

A covering film 6, which, in general, is provided as a thin thermoplastic synthetic resin film, is heated and placed over the pattern surface 3 of the pattern 2 in the stretched state. Suction is then applied to bring the covering member 6 into tight surface contact with the pattern surface 3. The thickness of the covering film 6 is preferably in the range of from 20 to 100 microns.

Next, the covering film 6 is coated with an alcohol solution of an initial condensate of a thermosetting resin such as a phenolic resin, a furan resin or an urea resin, in order to form a thermosetting resin layer 7.

The term "initial condensate" refers to a condensate in the so-called A-stage, i.e. the stage in which the condensate is still meltable by the application of heat and soluble in a solvent.

The thickness of the coating is preferably in the range of from 2 to 100 microns in terms of the solid component.

When the thickness of the coating falls short of 2 microns, a shell layer, hereinafter explained in detail, cannot be regularly formed thick enough to prevent undesirable crumbling of a fluid refractory filler such as moulding sand when molten metal is cast into the mold.

At the upper limit of the range, when the thickness of the coating exceeds 100 microns, insufficient suction of the thermosetting resin into the refractory filler leaves part of the thermosetting resin between the molten metal and the above-described shell layer. The presence of such residual thermosetting resin inevitably causes the development of foundry defects on the casting surface.

Alcohol is advantageously used for the solvent of the initial condensate of the thermoset ting resin in the process of the present invention. This is due to the fact that the critical surface tension of the synthetic resin film used for the covering film is in general 40 dynes/cm or below, and that alcohol optimally dissolves the initial condensate of the thermosetting resin while providing excellent wettability even under the critical surface tension value. However, various types of solvents other than alcohol, e.g. organic solvents, water or mixtures thereof, may be used for the initial condensate of the thermosetting resin in accordance with the present invention without having an undesirable influence on the smooth and successful prosecution of the process.

In addition, when the thickness of the resin coating falls within the above-described range, it is not always necessary for the resin coating uniformly to cover the entire surface of the covering film. Thus, a dispersion of the resin on the surface of the film in granular form also leads to successful results.

When preparation of the pattern 1 is complete, a moulding box 8 is set in position relative to the pattern 1 in known manner, the pattern surface 3 being covered with the covering film -6 and the thermosetting resin layer 7.

As shown in Figure 2, the moulding box 8 has a cavity construction open at both vertical ends. The center cavity of the box 8 is encompassed by an elongated suction chamber 9 which has a number of suction holes 11 formed through the inner side walls 10 thereof. A filter such as a wire net is disposed on the outsides of the suction holes 11 in order to prevent undesirable invation of the refractory filler granules into the pneumatic system which might cause fatal malfunction of the pneumatic system. A plurality of porous suction pipes 14 are spanned horizontally across the center cavity of the box 8 and connect one side portion of the suction chamber 9 to the other side portion. A mesh filter is wound around each of the suction pipes 14 in order to protect the pneumatic system for suction from the invasion of the refractory filler granules.The suction chamber 9 is connected to a given suction source such as a vacuum pump (not shown) via a conduit 16.

After the moulding box 8 has been set up in position relative to the pattern 1, the center cavity of the box 8 is filled with fluid refractory filler 15 such as moulding sand. The top opening of the box 8 is then closed with a suitable synthetic resin film 17, and pneumatic suction is started via the pneumatic suction system 9, 11, 14 and 16 in order to compact the refractory filler 15 under pressure.

When the compaction is complete, suction working on the pattern 1 is stopped while suction working on the moulding box 8 is retained. When the moulding box 8 is lifted in this state, the box 8 is separated from the pattern 1 and a mould half such as that shown in Figure 3 is obtained.

The outer mold half to be coupled with this mould half in order to form a complete mould is prepared in substantially the same way.

Figure 4 depicts in detail a part of the mould thus obtained, which defines the cavity of the mould. At the stage when preparation of the mould half is complete, the solvent for the initial condensate of the thermosetting resin has already volatilized. Thus, the surface of the compact refractory filler 15 is covered with the thermosetting resin layer 7, preferably 2 to 100 microns thick, and the covering film 6 such as a synthetic resin film, preferably 20 to 100 microns thick the latter being exposed to the cavity of the mould.

After the mould halves have been coupled to each other in order to form a complete mould, the mould is subjected to casting, which process is shown in detail in Figure 5. During the casting, molten metal 19 spreads over the wall of the mould cavity. Following this spread of molten metal, the portion of the covering film 6 in contact with the molten metal 19 and the portion of the covering film 6 close to the leading end of the spreading molten metal 19 are evaporated due to heat given off by the molten metal 19, and are sucked into the compact refractory filler 15 in order to be fluidized therein and to form a glutinous layer 20.

Concurrently with this procedure, the thermosetting resin layer 7 sandwiched by the covering film 6 and the refractory filler 15 becomes molten due to the heat given off by the molten metal 19, permeates into the refractory filler 15 and solidifies therein in order to form a solidified layer 18. The formation of this solidified layer 18 from the thermosetting resin effectively hinders excessive permeation and diffusion of the above-described molten covering film into the refractory filler 15, thereby affording proper thickness, or depth, td the glutinous layer 20. Thus, the solidified layer is firmly combined with the glutinous layer 20 in order to form a fortified shell layer 18 covering the refractory filler 15.This shell layer 18 excellently retains its strength until the cavity of the mould is completely filled with the molten metal and the molten metal is solidified in the mould cavity. Friction working on the cavity wall upon spreading of the molten metal causes no damage on the cavity wall fortified by the shell layer 18. Thus, undesirable mixing of the refractory filler into the molten metal can be effectively avoided. The presence of such a fortifying shell layer is especially advantageous in the case of the upper half of the mould in which evaporation of the covering film tends to take place due to the radiative heat given off by the high temperature molten metal.

The following examples are illustrative of the present invention, but are not to be constructed as limiting the invention.

Example 1.

A mould having a cavity of a rectangular solid shape was used for casting. The length of the cavity was 300 mm., the width 100 mm. and the height 16mm. The pressure of the suction in the mould was 360 Torr. and grey cast iron was used as the casting metal at 1,4000C. An ionomer thermoplastic synthetic resin film of 50 microns thickness, "Surlyn" (trade mark) produced by Du Pont, was used at about 100"C as the covering film. Methanol was used as the solvent for the thermosetting resins and the weight content of the resin in the solution was 17%.

Results in the casting were evaluated by the following criteria.

In order to evaluate the sand mark grade and the blow hole grade, three pieces of cast pieces were prepared under a common processing condition and the surface of each piece was cut off by 2mm. in thickness. The grades are shown in the form of averages of the number of defects observed on the three pieces.

The thicknesses of the resin coating are shown after evaporation of the solvent. However, except for novolak type phenolic resin, the thicknesses of the coating are shown with the resins in the glutinous state.

The results of the evaluation are shown in Table 1, as follows: Table 1 No. Thermo- Coating Casting Sand mark Blow hole setting thickness surface defect defect resin in microns 1 Novolak 1 Somewhat 10 0 phenol rough 2 ditto 2 good 1 0 3 ditto 5 good 0 0 4 ditto 10 good 0 0 5 ditto 15 good 0 0 6 ditto 20 good 0 0 7 ditto 40 good 0 0 8 ditto 50 good 0 1 9 ditto 75 good 0 1 10 ditto 100 good 0 1 11 ditto 110 Tortoise 0 2 shell like stripes 12 ditto 120 ditto 0 .2.

13 Resol 1 Considerably 20 0 phenol rough 14 ditto 2 Somewhat 5 0 rough 15 ditto 5 ditto 5 0 16 ditto 10 ditto 4 Q 17 ditto 15 good 2 0 18 ditto 20 good 1 0 19 ditto 40 good 1 1 20 ditto 70 good 1 2 21 ditto 100 good 0 5 22 ditto 110 Tortoise 0 5 shell like stripes 23 ditto 120 ditto 0 5 24 Furan 1 Considerably 15 0 rough Table I (continued) 25 ditto 2 Somewhat 4 0 rough 26 ditto 5 ditto 2 0 27 ditto 10 ditto 2 0 28 ditto 15 good 1 0, 29 ditto 20 good 0 0 30 ditto 40 good 0 1 31 ditto 70 good 0 1 32 ditto 100 good 0 2 33 ditto 110 Tortoise 0 5 shell like stripes 34 ditto 120 ditto 0 5 35 Urea 1 Considerably - 30 0 rough 36 ditto 2 ditto 20 0 37 ditto -5 Somewhat 10 0 rough 38 ditto 10 ditto 5 0 39 ditto 15 ditto 3 0 40 ditto 20 ditto 3 0 41 ditto 40 ditto 2 - 0 42 ditto 75 ditto 1 Q 43 ditto 100 ditto 1 0 44 ditto 110 ditto 1 -2 45 ditto 120 ditto 1 2 46 None 0 Considerably 35 - 0 rough It will be well understood that, regardless of the difference in the resin used, the use of the thermosetting resin layer ensures excellent results in the casting process particularly when the thickness is in the range of from 2 to 100 microns.

More advantageous results are obtained through the use of the novolak type phenolic resin as the thermosetting resin. Further, the best results are obtained through the use of the novolak type phenolic resin at a thickness in a range from 5 to 40 microns.

Example 2.

The process was almost similar to that used in the foregoing example. An ethylene-vinyl acetate copolymer film of 100 microns thickness was used for the covering film. The initial condensate of phenolic resin containing 6% by weight of hexamethylene tetramine as a hardening agent was used for the thermosetting resin with methyl alcohol as the solvent. The content by weight of the resin in the solution was 16%. The solution was sprayed on to the exposed surface of the covering film and the thickness of the thermosetting resin layers on the test pieces are listed in Table 2 below.

The results obtained in the casting were evaluated in a manner substantially similar to that employed in the foregoing example, and the numbers of the casting defects observed on the test pieces are also listed in Table 2.

Table 2

No. Number Wrinkles Coating Number ofcasting~ per 100 cm 2 thickness defects per 100cm2 in microns (*) 6 4 2 0 0 2 4 6 8 47 1 0 48 1 x 49 2 K 50 D( 5 x 51 0/ D 2 0 kO ~10 0 52 13= 20 0 53 ~~ D 50 OX 54 > 100 gx 55 P 150 = 56 ~ 12 > 200 0:( I (*) 0; upper mould half X; lower mould half In connection with the above table, the right-hand point of a triangular symbol for a particular sample indicates the number of wrinkles for that sample. For example, sample No.

52 has no wrinkles whereas sample No. 55 includes about 3 wrinkles, both per 100 cm".

The centre of a cross and a circle for a particular sample indicate the number of casting defects for that sample. Taking sample No. 50 for example, the lower mould half includes about 0.5 defect and the upper mould half includes 4 defects, both per 100 cm2.

The conditions of the test pieces regarding the development of casting defects are shown in Figures 6 to 8. The test piece shown in Figure 6 contains no thermosetting resin coating. It will be clear that this test piece contains a significant number of refractory filler granules mixing in the metal configuration. The test piece shown in Figure 7 contains a thermosetting resin:layer 20 microns thickness. Substantially no refractory filler granules are observed in the metal configuration. The test piece shown in Figure 8 contains a thermosetting resin layer 150 microns thickness. It will be clear that excessive coating by the thermosetting resin develops an increased number of wrinkles on the surface of the test piece.

As is clear from the foregoing description, the use of the present invention ensures successful metal casting with an enhanced casting surface condition and a reduced development of casting defects such as sand marks and blow holes.

WHAT WE CLAIM IS: 1. A process of casting by vacuum moulding characterized by tightly covering a pattern surface of a pattern with a covering film by suction applied to said pattern surface; coating the exposed surface of said covering film with a solution of an initial condensate of a thermosetting resin to form a thermosetting resin layer or granular dispersion on said covering film; setting a moulding box in position relative to said pattern; filing the center cavity of said moulding box with a fluid refractory filler; covering the exposed surface of said refractory filler; compacting said refractory filler by the application of pneumatic suction; removing said pattern from the remainder in order to form one mould half; preparing the other mould half in similar manner; coupling said mould halves to each other in order to form a complete mould having an inner cavity; and casting molten metal into said cavity of said complete mould.

2. A casting process as claimed in claim 1, in which said covering film is thermoplastic and is caused to cover the pattern surface when in a heated state.

3. A casting process as claimed in claim 2, in which said covering film is a thermoplastic synthetic resin.

4. A casting process as claimed in any of claims 1 to 3, in which said covering film has a thickness in the range of 20 to 100 microns.

5. A casting process as claimed in any of claims 1 to 4, in which the thickness of the thermosetting resin layer is in the range from 2 to 100 microns, in terms of the solid component.

6. A casting process as claimed in any one of claims 1 to 5, in which said thermosetting resin is a phenolic resin, a furan resin or an urea resin.

7. A casting process as claimed in claim 6, in which said phenolic resin is a novolak type phenolic resin.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

Table 2

No. Number Wrinkles Coating Number ofcasting~ per 100 cm 2 thickness defects per 100cm2 in microns (*) 6 4 2 0 0 2 4 6 8 47 1 0 48 1 x 49 2 K 50 D( 5 x 51 0/ D 2 0 kO ~10 0 52 13= 20 0 53 ~~ D 50 OX 54 > 100 gx 55 P 150 = 56 ~ 12 > 200 0:( I (*) 0; upper mould half X; lower mould half In connection with the above table, the right-hand point of a triangular symbol for a particular sample indicates the number of wrinkles for that sample. For example, sample No.

52 has no wrinkles whereas sample No. 55 includes about 3 wrinkles, both per 100 cm".

The centre of a cross and a circle for a particular sample indicate the number of casting defects for that sample. Taking sample No. 50 for example, the lower mould half includes about 0.5 defect and the upper mould half includes 4 defects, both per 100 cm2.

The conditions of the test pieces regarding the development of casting defects are shown in Figures 6 to 8. The test piece shown in Figure 6 contains no thermosetting resin coating. It will be clear that this test piece contains a significant number of refractory filler granules mixing in the metal configuration. The test piece shown in Figure 7 contains a thermosetting resin:layer 20 microns thickness. Substantially no refractory filler granules are observed in the metal configuration. The test piece shown in Figure 8 contains a thermosetting resin layer 150 microns thickness. It will be clear that excessive coating by the thermosetting resin develops an increased number of wrinkles on the surface of the test piece.

As is clear from the foregoing description, the use of the present invention ensures successful metal casting with an enhanced casting surface condition and a reduced development of casting defects such as sand marks and blow holes.

WHAT WE CLAIM IS: 1. A process of casting by vacuum moulding characterized by tightly covering a pattern surface of a pattern with a covering film by suction applied to said pattern surface; coating the exposed surface of said covering film with a solution of an initial condensate of a thermosetting resin to form a thermosetting resin layer or granular dispersion on said covering film; setting a moulding box in position relative to said pattern; filing the center cavity of said moulding box with a fluid refractory filler; covering the exposed surface of said refractory filler; compacting said refractory filler by the application of pneumatic suction; removing said pattern from the remainder in order to form one mould half; preparing the other mould half in similar manner; coupling said mould halves to each other in order to form a complete mould having an inner cavity; and casting molten metal into said cavity of said complete mould.
2. A casting process as claimed in claim 1, in which said covering film is thermoplastic and is caused to cover the pattern surface when in a heated state.
3. A casting process as claimed in claim 2, in which said covering film is a thermoplastic synthetic resin.
4. A casting process as claimed in any of claims 1 to 3, in which said covering film has a thickness in the range of 20 to 100 microns.
5. A casting process as claimed in any of claims 1 to 4, in which the thickness of the thermosetting resin layer is in the range from 2 to 100 microns, in terms of the solid component.
6. A casting process as claimed in any one of claims 1 to 5, in which said thermosetting resin is a phenolic resin, a furan resin or an urea resin.
7. A casting process as claimed in claim 6, in which said phenolic resin is a novolak type phenolic resin.
8. A casting process as claimed in claim 7, in which the thickness of the novolak type

phenolic resin layer is in the range of from 5 to 40 microns.
9. A casting process as claimed in any one of claims 1 to 8, in which alcohol is used as a solvent for said initial condensate of said thermosetting resin.
10. A casting process as claimed in any one of claims 1 to 9, in which moulding said is used for said refractory filler.
11. A casting process as claimed in any one of claims 1 to 10, in which a thin synthetic resin film is used for covering said exposed surface of said refractory filler.
12. A process of casting by vacuum moulding substantially as herein described with reference to the accompanying drawings and/or either of the specific examples.