WO1982003030A1

WO1982003030A1 - Impregnation of porous articles

Info

Publication number: WO1982003030A1
Application number: PCT/GB1982/000083
Authority: WO
Inventors: Int Ltd Ultraseal
Original assignee: Young Peter David
Priority date: 1981-03-11
Filing date: 1982-03-11
Publication date: 1982-09-16
Also published as: JPS58500240A; IT8220107A0; GB2094674A; EP0073783A1; IT1150274B; AU8150082A

Abstract

Apparatus for impregnating a porous article comprising a first support (16) for the article, a first cover (10) arranged to seal against the support to form an impregnation chamber, means for applying reduced pressure to the upper part of the first cover, means (4) for raising and lowering the first cover, passage means in the support for supplying impregnant to the impregnation chamber and removing it therefrom, a second support (15) adjacent the first support, conveyor means for moving the article from the first support to the second support, a second cover (11) arranged to cooperate with the second support to form a second treatment chamber, means (5) for raising and lowering the second cover, and means for treating the article with fluid in the second treatment chamber to effect washing of the article and/or curing of the impregnant in the pores of the article. Preferably the apparatus includes a third support (16) to form a third treatment chamber and means (6) for raising and lowering the third cover (12), the conveyor means being arranged to move the article consecutively from the first support to the second support and then form the-from the second support to the third support, and the second chamber being a washing chamber and the third chamber being a curing chamber. The invention also provides a method of impregnating porous articles in which excess impregnant is removed by centrifugal force and/or gas blast and in which curing of impregnant in the pores is effected by gas at a temperature in the range 60-130 C.

Description

Impregnation of porous articles

The invention relates to the sealing of pores in porous rigid articles, particularly metal articles such as metal castings and sintered metal products but also materials such as wood or inorganic materials such as brick, stone, or concrete. It is however important that the pores in the material are not so big as to permit leaching out of the impregnant. In a modification of the invention, the impregnant of the invention is used to impregnate materials such as paper and cloth.

The impregnation of metal castings with resins to seal porosity is a process which is well established. Porosity in castings is invariably inconsistent in size and shape, thus rendering the components unusable due to leakage. This is especially true where they are subjected to pressure. Apart from this well known cause for rejection of castings used in the motor industry there are many other problems caused by porosity including plating failures, "blow out" during stove enamelling, and sites for corrosion, entrapment of organic material and possible bacteria growth.

Generally, porosity can be divided into three types: through poro sity, blind porosity and enclosed porosity. Through porosity causes leakage and is the type with which founders are primarily concerned. Blind porosity, having one entrance to the surface only, will not produce a leakage but can cause surface finishing problems through absorption of treatment fluids. Enclosed porosity causes no problems unless present in excess where it can cause structural failure. Other similar defects often encountered in the foundry include cold laps, cracks, blow holes, and inclusions ail of which are often referred to, incorrectly, as forms of porosity. Frequently examination of a casting rejected for leakage and marked by the inspector as porous, reveals that it actually has a cold lap, crack or a blow hole.

Various method of sealing porosity employed over the years include plugging, coating with epoxy resin, and welding. Ail of these techniques are highly labour intensive and therefore expensive, and there is no guarantee that the treatment will be successful. A straightforward method in common use employs a solution of a high molecular weight polymer. The cleaned components are simply dipped into the solution for several minutes and on removal and subsequent evaporation of the solvent sealing is accomplished. This technique is not suitable for use on highly machined components that are to meet tight dimensional tolerances due to the thin film of polymer left on the surfaces. In addition any surface treatments such as conversion coating, anodic or chemical, that are called for, must be carried out prior to dipping. As penetration is limited and incomplete filling of the pores with resin is an inescapable consequence of employing solvent, more than one treatment may be required and tightness of the casting at high pressures must not be expected.

The process of vacuum impregnation came into use some twenty five years ago and over this period many types of sealants have been used, ail with varying degrees of success, from "Bakelite" (Trade Mark) varnish to sodium silicate and a wide range of polyesters. Modern processes of this type employ specially tailored unsaturated polyesters that have high penetration power in combination with low viscosity monomers such as styrene. Typically, the unsaturated polyesters are reaction products of phthalic anhydride and maleic anhydride with propylene glycol. A combination of inhibitors and catalysts is chosen to stablise the viscosity of the impregnant during production runs and to give suitable curing at temperatures in the region of 130°C. Cross linking of the linear polyester by styrene results in a hard, solvent resistant resin which completely fills the pores. A typical manufacturing process involves placing the cleaned, cold castings in an autoclave and subjecting them to a vacuum of not less than 12.7 mms of Hg for a minimum of 20 minutes. At this point the impregnant is admitted to the autoclave and brought to a level approximately two inches above the castings. Pressure of 90 to 100 lbs. per sq. in. is then applied to the autoclave for 30 minutes or more.

The impregnant fluid is then returned to the storage tank to allow the removal of the castings and their transfer to the wash tank for the removal of the surface film left by the sealant. After a short draining period, the parts are submerged in an oil for 45-60 minutes at 130°C or alternatively placed in a hot air recirculation oven for 1½ hours. The final steps are to transfer the parts to a fresh water rinse to remove surface contamination. With this existing method of treatment pressure tightness of up to 12,000 lb. per sq. in. can be obtained within a temperature range of -40°C to +250°C.

Other systems include anaerobically curing impregnants e.g. of the type described in U.K. Patent Specification 1 297 103.

The present invention has been particularly developed for use with impregnants of the type described in U.K. Patent Specification No. 1547801 but is not limited thereto. Such impregnants cure thermally at a temperature of about 90°C and are thus conveniently cured by contacting the casting with hot water, either in the form of a bath or in the form of a spray.

Impregnation processes previously used have been batch processes but these are not altogether convenient e.g. in the treatment of cylinder blocks for motor vehicles. It is however possible to treat metal castings individually. This generally necessitates blocking off of the casting and pressurising with impregnant from within, or drawing a vacuum and submerging the casting in the impregnant. Neither of these two ways is highly desirable because of the inherent problems of sealing up the openings in the casting.

According to our U.K. Patent Application 2072231, published 30th September 1981 , an article to be impregnated is placed on a base plate or other support member, a cover such as a cylinder is placed over it in sealing relation with the base plate (in the manner of a bell jar), vacuum (i.e. subatmospheric pressure) is applied to the upper part of the cylinder to remove air from the pores (e.g. for 30 seconds or more), impregnant is admitted (preferably through the base plate) under the action of the vacuum until it at least covers the pores, the vacuum is broken, excess impregnant is removed from the cover (preferably by gravity drainage), and the cover is removed from the base plate. The casting may then be removed from the base plate and placed on a conveyor for further treatment e.g. a cold water wash and a not water spray in the case of the impregnant described in Specification No. 1547801.

In our published U.K. Patent Application 2 072 231 washing and curing after impregnation take place in a tunnel through which castings pass on a conveyor. However, such an arrangement takes up a great deal of floor space and it has been found to be desirable to make the treatment conditions, particularly time and teperature, more readily controllable.

According to one aspect of the present invention, cleaning and/or curing is effected within one or more further treatment chambers formed by a cover engaging a support for the article. Conveyor means are provided to move the article from one support to the next or the support can be formed, at least in part, by a portion of the conveyor as described in the above-mentioned U.K. Patent Application.

According to another aspect of the invention, curing of impregnant in the pores is effected by contact with gas (preferably air) at a temperature in the range 60-130°C. Particularly with impregnants as described in U.K. Patent Specification No. 1 547 801 this not only effects a very rapid cure but "burns off" unwanted impregnant from the surface of the casting before it has a chance to cure. This means that a preliminary thorough cleaning and washing step is not required. If draining is insufficient, or insufficiently rapid, impregnant can be removed by mechanical means such as centrifuging or a gas blast- In other words, after impregnation, the whole treatment can be "dry".

The hot air may be static but is preferably blown onto the article; this may be done in a tunnel (e.g. with the air current transverse to the direction of movement) or in a static oven. It is desirable that the article is rotated during the hot air treatment.

It is found that a hot air treatment of this type not only heats the article to the imp regnant-curing temperature but vaporises off excess impregnant on the article surface while leaving impregnant in the pores substantially unaffected (apart from the heat curing). Effluent gases may in some cases be burnt so avoiding pollution problems.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:-

Fig. 1 is a side elevation of apparatus according to the invention;

Fig. 2 is an end elevation of the apparatus of Fig. 1,

Fig. 3 is similar to Fig. 2 but shows a modification for simultaneously treating two articles; and

Fig. 4 is a section through a modified curing station.

Referring now to Figs. 1 and 2, the apparatus comprises a frame 1 having a gantry 2 which supports three air cylinders 4, 5, 6. The respective rods of these cylinders carry bell jar covers 10, 1 1, 12 for movement between an upper position shown in full lines and a lower position shown in ghost lines. In their lower positions, the covers 10, 11, 12 cooperate with supports 14 , 15, 16 to provide first, second and third treatment chambers (from right to left in Fig. 1).

The first treatment chamber is an impregnation chamber and it operates in substantially the same manner as the impregnation chamber described in Figs. 1 - 3 of our above-mentioned U.K. application. The cover 10 makes sealing contact with the support 14, a vacuum is drawn through line 18 and impregnant is admitted through the support 14, preferably by releasing vacuum in a simultaneously evacuated impregnant reservoir. After impregnation the vacuum is released in the treatment chamber and impregnant drains back through the support 14.

The second treatment chamber has a support 15 with an inner area which is rotatable (not shown). This permits the article to be cleaned by centrifuging off impregnant still on its surface. The cover 11 has an axial rotary cap (not shown) inside it for engaging the top of the article and keeping it on the turntable provided by the inner area of the support. Air jets 20 (three are shown in Fig. 1) are arranged to direct cold air on to the article and blow off excess impregnant. It is however possible to effect cleaning by centrifuging alone or air jets alone. Alternatively it is possible to use a conventional cold water wash with or without jets. The third treatment chamber, as shown in Fig. 1, has a pipe 22 for supplying hot water to the cover 12, the water passing out through the support 16 after use and being recycled. Hot air or steam can also be used.

The transfer system comprises a fixed rail 25 secured to the frame 1 and running the full, length thereof. The rail 25 has four rollers 26 on which are mounted a movable rail 27 which can be shuttled to and fro by a bracket 28 attached to the rod of a pneumatic cylinder (not shown). On the rail 27 is mounted a carrier rail 29 which carries four spaced arms 30. At its outer end each arm 30 has a gripper means 31. In the particular case in question, the article is a carburettor with an opening at the top and the gripper means 31 comprises an air cylinder which is arranged to expand a rubber ring within the opening. Two parallel links 32 are used to mount the carrier rail 28 on the rail 27 at each end and a further air cylinder 3* is used to raise and' lower it. The four arms 30 thus move up and down at the same time.

It will be seen that, with the covers raised, the arms 30 and their gripper means 31 can move articles forward and deposit them on the supports. The arms then move to a position between the covers which are then lowered. After the treatments have been completed the arms 30 reengage the articles and move them forwards again.

As shown in Fig. 3 each arm 30 has two gripper means 31 for simultaneously transferring two articles in side-by-side relation.

As shown in Fig. 4, the support 14 comprises a grille 35 beneath which is disposed a fan 36 having a motor 37 which can rotate it about a vertical axis. A sleeve 38 is mounted within the cover 10 by radial arms (not shown) and contains the porous article. Black heater elements 39 are disposed in the annular space 41 so formed and in practice air is forced by the fan 36 upwards through the space 41 and down over the article for recirculation. Such recirculation is economical and serves to burn any gases and vapours given off by the article 20 so avoiding air pollution problems. In this case the cover 10 is of thin stainless steel. In operation, the transfer system lifts the articles from one conveyor and places them on another conveyor after treatment. It has been found in practice that the total cycle time can often be reduced to about one minute.

In a modified form of the invention, the article is cleaned in the second chamber with an organic solvent and the impregnant recovered therefrom. Any solvent remaining on the article in the third chamber is evaporated.

If desired, a final cure can be effected by placing the articles leaving the apparatus in a muffler oven.

Claims

1. A method of impregnating porous articles in which excess impregnant is removed by centrifugal force and/or gas blast and in which curing of impregnant in the pores is effected by gas a temperature in the range 60-130°C.

2. A method according to Claim 1, wherein the curing gas temperature is in the range 100 - 130°C.

3. A method according to Claim 1 or 2, wherein hot gas produced by a heater element is recirculated to the heater element after contacting the article.

4. Apparatus for carrying out the method of Claim 1 comprising a chamber for removing excess impregnant from the article and a chamber for curing the impregnant in the pores, wherein the excess impregnant removal chamber has a turntable for spinning the article and/or gas jets directed towards the article.

5. Apparatus according to Claim 4 wherein the curing chamber has a heater element and means for effecting recirculating gas flow.

6. Apparatus for impregnating a porous article comprising a first support for the article, a first cover arranged to seal against the support to form an impregnation chamber, means for applying reduced pressure to the upper part of the first cover, means for raising and lowering the first cover, passage means in the support for supplying impregnant to the impregnation chamber and removing it therefrom, a second support adjacent the first support, conveyor means for moving the article from the first support to the second support, a second cover arranged to cooperate with the second support to form a second treatment chamber, means for raising and lowering the second cover, and means for treating the article in the second treatment chamber to effect cleaning of the article and/or curing of the impregnant in the pores of the article.

7. Apparatus according to Claim 6 wherein which includes a third support adjacent the second support, a third cover arranged to cooperate with the third support to form a third treatment chamber and means for raising and lowering the third cover, the conveyor means being arranged to move the article consecutively from the first support to the second support and then from the second support to the third support, and the second chamber being a cleaning chamber and the third chamber being a curing chamber.

8. Apparatus according to Claim 7 wherein the conveyor means comprises an arm mounted on a carrier arranged to move along beside the supports, the arm being arranged to engage the article at its free end, when the cover is raised, and to carry it forward to the next support.

9. Apparatus according to Claim 8, wherein the arm is arranged to be moved to a position where it is not aligned with the covers when the covers are to be lowered.

10. Apparatus according to Claim 8 or 9, wherein the carrier is pivotally mounted so that the arms can lift the article from the support.

11. Apparatus according to Claim 7 wherein the three supports comprise a part of the conveyor means which is arranged to move forward when the covers are raised.

12. Apparatus according to any one of Claims 6 - 10 wherein the second support has rotary turntable means for centrifuging excess impregnant from the article.

13. Apparatus according to any one of Claims 6 - 10 and 12 wherein the second cover has cold air jets for blowing excess impregnant from the article.

14. Apparatus according to any one of Claims 6 - 10, 12 and 13 wherein the second chamber has means for the supply of a liquid cleaning fluid.

15. Apparatus according to any one of Claims 6 to 14, wherein the third chamber has means for the supply of hot gas, hot water or steam to effect curing.

16. Apparatus according to any one of Claims 6 to 14, wherein the third chamber has a heater element and fan means are provided to circulate the resulting hot air within the chamber.

17. Apparatus according to Claim 16, wherein the cover has an inner sleeve for receiving the article, the heater element is disposed in the annular space between the sleeve and the cover, the support is apertured and the fan means is below the support and arranged to move air through a closed path such that the air contacts the heater element and then the article.