US3728429A

US3728429A - Slush molding process

Info

Publication number: US3728429A
Application number: US00067631A
Authority: US
Inventors: F Sliwinski; D Colby; P Rogers
Original assignee: McCord Corp
Current assignee: McCord Corp
Priority date: 1968-08-29
Filing date: 1970-08-27
Publication date: 1973-04-17
Anticipated expiration: 1990-04-17

Abstract

A process for slush molding hollow thin-walled flexible plastisol articles which begins with the application of an initial thin coating of liquid plastisol to the interior surface of a cooled open mold. This initial coating is then gelled by impinging pressurized hot gas directly onto the exterior surface of the mold, preferably while the mold is inverted and rocking about a horizontal axis. Thereafter, the thickness of this initially gelled coating is increased by applying additional liquid plastisol to the interior surface of the mold, which additional plastisol is then gelled by the latent heat stored in the mold wall during the previous heating step. The entire gelled plastisol coating is finally fused by a second impingement of pressurized hot gas directly onto both the exterior mold surface and the plastisol coating on the interior mold surface, while the mold is rotating about a horizontal axis.

Description

United States Patent 1 Colby et al.

[ Apr. 17, 1973 SLUSH MOLDING PROCESS [75] Inventors: Daniel E. Colby, Somersworth; Philip E. Rogers, Dover; Frederick J. Sliwinski, Rochester, all of N H.

[73] Assignee: McCord Mich.

[221 Filed: Aug. 27, 1970 [21] Appl. No.: 67,631

Corporation, Detroit,

Related US. Application Data [63] Continuation of Ser. No. 756,178, July 29, 1968 abandoned.

[52] US. Cl. ..264/302, 264/303, 264/310, 264/D1G. 6O [51] Int. Cl "B290 5/12 [58] Field of Search 264/302-303, 236, 342, 310, 268, DIG. 60

[56] References Cited UNITED STATES PATENTS 3,002,230 10/1961 Stewart ....264/244 2,588,571 3/1952 Porter ...264/302 2,939,180 6/1960 Hickler et al.-... ...264/302 3, l 34,831 5/1964 De Fusco et al .264/302 PLASTISOL RESERVOIR II -lIlI 3,315,016 4/1967 Wersosky et a1. ..264/302 Primary ExaminerRobert F. White Assistant Examiner-Gene Auville Attorney--Maurice E. Gauthier and McGlynn, Reising, Milton & Ethington [5 7 ABSTRACT A process for slush molding hollow thin-walled flexible plastisol articles which begins with the application of an initial thin coating of liquid plastisol ,to the interior surface of a cooled open mold. This initial coating is then gelled by impinging pressurized hot gas directly onto the exterior surface of the mold, preferably while the mold is inverted and rocking about a horizontal .axis. Thereafter, the thickness of this initially gelled coating is increased by applying additional liquid plastisol to the interior surface of the mold, which additional plastisol is then gelled by the latent heat stored in the mold wall during the previous heating step. The entire gelled plastisol coating is finally fused by a second impingement of pressurized hot gas directly onto both the exterior mold surface and the plastisol coating on the interior mold surface,

while the mold is rotating about a horizontal axis.

9 Claims, 10 Drawing Figures PATENTED APR 1 H975 SHEET 1 [IF 3 Izwezzioa'w.- Daniel E 60221 J Slizwizmidt,

PATENTEDAPRIYW 3728,1129

SHEET 2 0F 3 47 I I sea Daniel E. 002 3 3,

03121,?) 80932 15, 33 M M fl fliiofinegys PATENTEUAPRITIQB 3,728,429

' sum 3 OF 3 l FILL ll GELLATION AND ROCKING Ill REFILL WITH OWELL TIME FOR ADDITIONAL GELLATION |v FUSION WITH. ROTATION V COOLING SLUSH MOLDING PROCESS CROSS-REFERENCES TO RELATED APPLICATIONS BACKGROUND OF THE INVENTION This invention relates generally to the molding or I casting of hollow articles from a flowable material which solidifies when subjected to heat, and more particularly to a process and apparatus for slush molding plastisol skins for padded automobile components such as arm rests, crash pads and the like.

A number of prior art slush molding operations have been developed in the past. A typical example of one such method is disclosed in U.S. Pat. No. 2,588,571 (Porter). The Porter method includes the steps of applying a coating to the interior surface of the mold by a process of filling and dumping, and thereafter running the mold into a radiant heat oven for the purpose of gelling or preliminarily curing the initial coating. This step may be repeated again on selected portions of the mold:where a thicker coating is desired. Thereafter, the. mold is placed in another radiant heat oven for the final cure.

Experience has indicated that a number of rather serious problems are associated with this type of molding operation. To begin with, the procedure of simply I filling and dumpingto coat the interior mold surfaces prior to gellation may result inthin and/or bare spots in the finished product. Frequently, this defect is caused by air bubbles which become trapped between the liquidplastisol and the mold wall in mold recesses and undercuts. The seriousness-of this problem naturally increases as parts take on more complex-designs. The

heating of molds in a large or openoven is the-source of still further problems. For example, where complicated parts are being produced, open oven heating may fail to supply adequate heat to mold recesses and undercuts within the time alloted for gellation, the result being a skin with inadequately gelled or cured-areas. Also, where the molds are held in-a fixed inverted position during the gelling cycle, excess plastisol may run down the mold walls and accumulate and/or remaintrapped in corners, undercuts and the like. Such'excess accuproperly mate each mold with the induction heating coils will often produce rejects, either due to underheating or scorching. Another problem with induction heating lies in the fact that once the mold is surrounded by the induction coil, further relative movement between the mold and coil is not possible until the gelling or fusing cycle has been completed. Where a complicated part is being produced, mold immobility during heating may result in an inability to properly gel or fuse the plastisol coating in difficult areas such as remote mold recesses and undercuts. The seriousness of this problem may in some cases entirely preclude the possibility of molding certain articles on an induction heated slush molding line.

SUMMARY OF THE INVENTION vention to be. hereinafter described in greater detail, a

mulations of plastisol are-frequently objectionable, particularly in situations where metal inserts must subsequently be fully inserted into the corners and undercuts of the plastisol skin.

A different prior art slush molding operation involving induction heating is disclosedin ,U.S. Pat.- No. 3,315,016 (Wersosky, et, al.). The induction heating method is a distinct improvement over open oven heating, primarily because it is much faster and moreefficient. Here again, however, experience has indicated that other problems are associated with induction heating. For example, because of their bimetallic plated.

construction, the induction heating molds are more difficult and expensive tomake. Also, the induction heating coils usually must be specially designed to accommodate each particular mold This limits the flexibility of any one production line because a mold change will in manyinstances involve a corresponding change in the heating coils; Furthermore, failure to, properlyarrangethepla'ted bimetallic layers of the mold or. to

plurality of open molds are carried by a chain conveyor through a series of-statio'ns at'which separate operations are sequentially performed on each mold. Beginning at the first station, an empty'cooled mold is initially filled with a plastic material, such as for exampleliquid plastisol. The filled mold is then carried to a second station where itis inverted and the plastisol contained therein durriped into'anunderlying reservoir. After dumping, the interior surface of the mold remains coated with a thin layer of ungelled plastisol. At this point, it should be noted that someareas of the interior mold surface may not have' receivedan even coating clue to air. bubbles having been trapped between the 1 plastisol and mold wall during the initial filling operation. Also,aexcess puddles of plastisol'may begin to accumulate in recesses, undercuts and'other areas of the inverted mold. Therefore, to'insure an even distribution of the liquid plastisol over the entire inner surface of the mold, the inverted mold is then rocked about a fixed axis. This rocking action encourages plastisol flow into recesses where air had previously been trapped, and also promotes plastisol flow out of recesses where an excess of liquid plastisol is accumulating. While the mold is rocking-in an inverted position, a heating 5 chamber oroven is moved into an'operative position surrounding ;the mold. The oven, which is suitably dimensioned to accommodate-continued rocking of the inverted mold,'.is.zprovided with a series'of orifices which directistreams of pressurized hot' gas directly onto the exterior mold surface. Heating in this'manner, with the mold.in=motion relative to the'streams of hot gas impingingon its exterior surface, continues until the liquid plastisol coating is gelled; Experience has indicated that the resulting gelled coating extremely uniform .and-zwithout bare s'pots'or excess deposits in recesses'and undercuts. This is due to a combination of factors. By rocking the-mold during gellation, the liquid plastisol,which incidentally undergoes a marked drop in viscosity during heating and prior to gellation, is

deprived of the opportunity' to accumulate and gel in mold recesses and undercuts. Also, because the initial,

liquid plastisolcoating is'extre'mely thin, usually on the order of about 15 to 25 mils, air bubbles trapped I between the plastisol and mold wall will expand and The mold then continues on to a fourth station, where,

after the expiration of a predetermined dwell time, it is again inverted and its contents dumped into a second underlying reservoir. At this point, the latent heat stored in the mold wall during the previous gellation step has caused additional plastisol to gel on the interior mold surface, thus increasing the thickness of the initially gelled coating. The inverted mold is again rocked about the same axis and a second oven of similar construction to the first moved into an operative position surrounding the mold. Heating now continues, again by virtue of high temperature streams of hot gas impinging directly onto the exterior mold surfaces until complete gellation of the entire coating is insured. Thereafter, the mold is rotated about the same axis as the impingement of hot gas continues. Rotation of the mold exposes both the exterior mold'surface and the plastisol coating on the interior mold surface to direct impingement of hot gas until the plastisol coating has been completely fused. This continued motion of the mold during fusion again insures more uniform heat distribution to all parts of the mold, without an excess accumulation of plastisol in recesses and undercuts. Direct impingement of hot gas on the plastisol coating also markedly decreases the time required to complete fu The mold is then moved through a cooling zone where it may be sprayed with an appropriate cooling medium. Following cooling, the skin is stripped from the mold cavity, and the mold returned by the chain conveyor to the initial filling station.

Apart from the advantages mentioned above, the present invention embodies another important feature which contributes substantially to its adaptability to high speed commercial operations. More particularly, the ovens employed to gel and fuse the plastisol coatings may be dimensioned to accommodate a wide range of mold sizes and designs. This feature obviates the necessityfor changing ovens each time a new product is being runthrough the slush molding line and thus markedly increases the adaptability and flexibility of the equipment being employed.

These and other objects and advantages of the present invention will become more apparent as the description proceeds with the aid of the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of a preferred embodiment of an automated slush molding line employed in the practice of the present invention;

FIG. 2.is a perspective-view of one of the vertically movable ovens, with portions broken away;

FIG. 3 is a sectional view taken along line 33 of FIG. 2 showing the oven lowered to the operative position in order to apply heat to a rocking mold at station ll;

inches, a surface of approximately 1.52 square feet,

and a volume of about 144.4 cubic inches.

. Referring initially to FIG. 1, one embodiment of an automated slush molding line constructed in accordance with the principles of the present invention is schematically shown as including a sprocket-driven chain conveyor 10 suitably adapted to support a plurality of molds indicated typically by the reference numetal 12. Conveyor 10 is arranged to run horizontally through a series of stations (numbered l-VI in the drawings). Beginning at station I, a given mold 12, which had previously been cooled and stripped, is filled with a metered amount of liquid plastisol. At this stage in the operation, the mold surface temperature is preferably between l00l50F. Liquid plastisol 13 at a temperature of about l20l 25F and of a composition similar to that disclosed in US. Pat. No. 3,315,016, but without the filler (Crystalline CaCO is pumped by means of a pump 22 into the mold from a central reservoir 14 through a main manifold line 16, metering valve 18 and fill pipe 20.

After filling, the mold is'carried by conveyor 10 to station II, at which point the mold is inverted and its.

tively thin plastisol coating 28 (See FIG. 5B) of about I 20 mils thickness is applied 'to the interior mold surface. At this point, the initial coating 28 is still in a liquidstate, thus exhibiting a tendency to flow downwardly along the interior surfaces of the inverted mold. To avoid a build-up of excess plastisol in mold undercuts and recesses, and to encourage flow of plastisol into other areas which had not been coated due to air having been trapped between the plastisol and mold wall, the inverted mold is then rocked in both a clockwise and counterclockwise direction about an axis 30 passing through its geometric center.

At this point, it should be understood. that the procedure of filling and dumping is but one of a number of different methods which may be employed to apply initial coating 28. As an alternative a lesser amount of plastisol might be deposited in an upright mold and the mold thereafter rocked to wash plastisol up onto the upper interior mold surfaces. Thereafter,

tive position indicated in dotted lines at 32a. As can be best seen in FIGS. 2 and 3, oven 32 is comprised basi cally of a box-like container open at the bottom, with an insulated roof 34, side walls 36 and end plates 39. The interior of the oven is subdivided by a perforated metal baffle member 38 into a plenum chamber 42 and a heating area 44. One or more gas burners B are mounted on the roof 34 of oven 32. The burners are supplied by lines 46 with a mixture of gas and air, and by lines 47 with compressed air. The gas is ignited and the products of combustion fed directly into plenum chamber 42. When the oven 32 is lowered to the operative positionindicated at 32a in FIG. 1, the inverted mold 12 is located within the heating area, there being ample room between the mold and the interior surfaces of baffle member 38 to permit continued rocking of the mold as diagrammatically indicated in FIG. 3.

As the mold continues to rock, the gaseous products of combustion, heated to a temperature of about 800900F, exit through perforations 48 in baffle member 38 to impinge directly onto the exterior surfaces of the inverted mold 12 in the form of a plurality of high velocity jets indicated diagrammatically in the drawings by arrows 50. Heating in this manner continues for a period of -25 seconds until the initial plastisol coating 28 on the interior mold surface has gelled. By continuously rocking the mold during heat- 2 ing, a more uniform impingement of the hot gases on the mold surface is achieved, with less chance of localized scorching. As soon as gellation is completed, at which point the mold has been heated to a surface temperature of about 250F, the oven 32 is returned to its raised inoperative position. Thereafter, the mold 12 is rotated to the upright position and moved by conveyor 10 to station lll.

At station Ill, the mold 12 is refilled from manifold 16 through a second metering valve 52 and depending fill pipe 54. The mold is then moved on to Station IV. The mold remains filled for a predetermined dwell time, say about 10 seconds, during which the latent heat stored in the mold wall as a result of the previous heating step at station ll gels additional plastisol 56 (See FlG. 5C). The net effect of this second gellation is to increase the thickness of the initially gelled layer, thus producing a thicker coating 28 (See FlG. 5D) having a total thickness of between 40-80 mils.

At the expiration of the dwell time, the mold 12 is again inverted and its ungelled contents dumped into tray 58. This ungelled plastisol is drained from tray 58 back through pipe 60 to central plastisol reservoir 14. Immediately upon inverting the mold 12 at station lV, rocking again commences about axis 30. As rocking continues, a second oven 62 of the same or similar construction as oven 32 is lowered to an operative position indicated at 62a. Products of combustion, again at a temperature of about 800900F, are fed into plenum chamber 42 from burner B, the latter being supplied with fuel and air by lines 46' and 47'. The gaseous products of combustion escape from the plenum chamber through perforations in baffle plate 38' to impinge directly onto the exterior mold surfaces. Heating in this manner continues for a brief interval, until complete gellation of the plastisol coating 28' of theinterior mold surface is assured. During this period, any excess plastisol which might otherwise accumulate in mold recesses and undercuts will again drain downwardly into tray 58. Thereafter (See FIG. 4), the mold is rotated a full 360 about axis 30, with the result that both the exterior mold surface and the gelled plastisol coating 28' on the interior surface is exposed to the direct impingement of high velocity streams of hot gas. Direct impingement of hot gas on the plastisol 5 coating 28' enables fusion'to be completed in a relatively short time, usually within -30 seconds. Because the hot gas enters the mold, the heating of remote recesses and undercuts is greatly facilitated. Also, continued motion of the mold relative to the impinging streams of hot gas, first by rocking and then by rotation, again produces a more uniform distribution of heat, with much less chance of localized scorching.

As soon as the coating 28' is completely fused, the oven is returned to its raised inoperative position. At this point, the mold surface temperature is approximately 350F. The mold is then moved'on to Station V where it is cooled by any conventional means, such as 20 for example by water spray nozzles 64. The cooled mold is then moved on to station VI, at which point the completely fused skin is stripped. The empty mold is then returned by conveyor 10 to Station I.

The advantages to be gained from the practice of the 5 foregoing process should now be apparent to those skilled in the art. These advantages include the initial application of a relatively thin plastisol coating 28 on the interior of the mold. This thin coating enables air bubbles trapped between the plastisol and mold wall to expand and burst more easily during gellation, thus producing a smoother and more uniform surface on the resulting skin. By inverting and rocking or rotating the mold during initial gellation and subsequent fusion, ample opportunity is provided for the removal of excess plastisol from mold recesses and undercuts. Flow l -of plastisol into uncovered areas is also encouraged. Uniform heating of the plastisol skin is made possible [by subjecting the moving mold to direct impingement I ;of hot gas on the exterior mold surface during gellation, and on both the exterior and coated interior mold surfaces during fusion.

While the invention has been described with respect to an apparatus which includes a horizontal conveyor carrying molds through a series' of six consecutive stations, it is apparent that this represents only one of the means which may be employed to practice the process described above. For example, instead of employing a conveyor, the molds may be manually transported from one station to the next. The number of stations at which different operations are performed may be varied.

Furthermore, it will be apparent that the

ovens

32 and 62 may take any of a wide variety of forms. It may also be advisable under certain circumstances to use means other than the

ovens

32a and 62 for impinging hot gases on the mold surfaces during gellation and fusion.

The above-described process and apparatus are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency of said claims are intended to be embraced thereby.

Weclaim:

l. A process for molding thin-walled plastisol articles comprising:

a. applying a relatively thin liquid plastisol coating to the interior surface of an open mold;

l l l I b. inverting the mold and imparting motion to the inverted mold about an axis while the plastisol coating remains in a liquid state, the motion imparted to the mold about said axis I being such as to promote drainage of excess liquid plastisol from mold undercuts and recesses; l

. initially gelling said plastisol coating by impinging a flow of hot gas directly onto the exterior mold surface, while the mold remains inverted and also while the mold remains in motion about said axis thereby to cause said exterior mold surface to be substantially uniformly exposed to the hot gas;

. increasing the thickness of said initially gelled .coating' by applying additional liquid plastisol to the previously gelled plastisol and employing the latent heat developed in the mold w'all during the prior heating step to gel said additional liquid plastisol;

. fusing said gelled plastisol coating by direct impingement of a pressurized flow of hot gas on the exterior mold surface, the said fusion being accompanied by rotation of the mold about said axis relative to said impinging gas such that said flow of hot gases also impinges directly onto the gelled plastisol coating on the interior of the mold; f. cooling the mold following fusion; and, g. thereafter stripping the resulting thin-walled plastisol article from said mold. 2. A process for slush molding hollow thin-walled articles comprising the steps of: v

a. applying a relatively thin liquid plastisol coating to .the interior surfaces of an open mold, the application of said coating being accomplished by at least partially filling the mold with plastisol and thereafter inverting the mold to drain any excess liquid plastisol from the mold; a

b. imparting a rocking motion to the inverted mold about a given axis while the plastisol coating remains in a liquid state, the motion imparted to i the mold about said axis being such as to promote impinging hot gas directly onto the exterior mold surfaces while the mold remains inverted and in rocking motion about said axis;

(1. increasing the thickness of the previously gelled plastisol coating by refilling the mold with liquid plastisol, allowing the latent heat stored in the mold wall as a result of-the previous heating step to gel additional plastisol, and thereafter emptying the mold of any remaining liquid plastisol;

. fusing the gelled plastisol coating by impinging hot gaseous products of combustion onto both the exterior mold surfaces and the gelled plastisol coating, while the mold is in rotating motion; and,

thereafter removing the resulting hollow thinwalled molded article from said mold.

3. The process claimed in claim 2 wherein the application of said relatively thin coating of liquid plastisol is accomplished while the temperature of the interior mold surface remains within a range of between 100-l50F.

4. The process as claimed in claim 2 wherein the thickness of said relatively thin coating is approximately 15 to 25 mils. 5. The process as claimed in claim 2 wherein the said hot gas constitutes the gaseous products of combustion, the temperature of the hot gaseous products of combustion impinging on the exterior surfaces of the mold being between 800900F.

continued drainage of excess liquid plastisol from the mold while preventing the same from accumulatingin mold undercuts and recesses; c. gelling said relatively thin plastisol coating by 6. The process as claimed in claim 5 further characterized by initial gellation being accomplished within 15 to 40 seconds.

7. The process as claimed in claim 6 wherein the additional gellation occasioned by the latent heat in the mold wall continues for a dwell period of approximately 10 seconds.

8. The process as claimed in claim 5 further characterized by the fusion of said gelled plastisol coating being accomplished while the mold is rotating about said axis.

9. The process as claimed in claim 8 wherein the temperature of the gaseous products of combustion impinging on the exterior mold surfaces and the gelled plastisol coating is between 800900F.

Claims

2. A process for slush molding hollow thin-walled articles comprising the steps of: a. applying a relatively thin liquid plastisol coating to the interior surfaces of an open mold, the application of said coating being accomplished by at least partially filling the mold with plastisol and thereafter inverting the mold to drain any excess liquid plastisol from the mold; b. imparting a rocking motion to the inverted mold about a given axis while the plastisol coating remains in a liquid state, the motion imparted to the mold about said axis being such as to promote continued dRainage of excess liquid plastisol from the mold while preventing the same from accumulating in mold undercuts and recesses; c. gelling said relatively thin plastisol coating by impinging hot gas directly onto the exterior mold surfaces while the mold remains inverted and in rocking motion about said axis; d. increasing the thickness of the previously gelled plastisol coating by refilling the mold with liquid plastisol, allowing the latent heat stored in the mold wall as a result of the previous heating step to gel additional plastisol, and thereafter emptying the mold of any remaining liquid plastisol; e. fusing the gelled plastisol coating by impinging hot gaseous products of combustion onto both the exterior mold surfaces and the gelled plastisol coating, while the mold is in rotating motion; and, f. thereafter removing the resulting hollow thin-walled molded article from said mold.
3. The process claimed in claim 2 wherein the application of said relatively thin coating of liquid plastisol is accomplished while the temperature of the interior mold surface remains within a range of between 100*-150*F.
4. The process as claimed in claim 2 wherein the thickness of said relatively thin coating is approximately 15 to 25 mils.
5. The process as claimed in claim 2 wherein the said hot gas constitutes the gaseous products of combustion, the temperature of the hot gaseous products of combustion impinging on the exterior surfaces of the mold being between 800*-900*F.
6. The process as claimed in claim 5 further characterized by initial gellation being accomplished within 15 to 40 seconds.
7. The process as claimed in claim 6 wherein the additional gellation occasioned by the latent heat in the mold wall continues for a dwell period of approximately 10 seconds.
8. The process as claimed in claim 5 further characterized by the fusion of said gelled plastisol coating being accomplished while the mold is rotating about said axis.
9. The process as claimed in claim 8 wherein the temperature of the gaseous products of combustion impinging on the exterior mold surfaces and the gelled plastisol coating is between 800*-900*F.