Piston mould

Abstract: A piston-casting mould for casting all pistons (2) in a single series with little weight variation one from the next, in which spacers (54) of fixed dimensions are employed to locate a dome-forming core (44) of the mould a fixed distance from a stationary portion (22) of the mould from one casting to the next. Increasing or decreasing the dimensions of the spacer (54) permits increasing or decreasing the as-cast weight of the piston (2).

Description: This invention relates to a permanent mould for gravity-casting pistons for internal combustion engines as specified in the preamble of claim 1. More particularly, it concerns a mould which casts such pistons with little weight variation from one piston to the next in a series, and which is readily converted to casting pistons having a different weight in a different series of castings. An internal combustion engine requires that all of its pistons have substantially the same weight. That is to say, that the variation in weight from one piston to the next is less than 0.8%. Hence, for example, engines having pistons weighing about 500 grams will typically require that there be no more than 4 grams weight difference from one piston to the next. Owing in part to significant variations in the as-cast weight of pistons leaving conventional piston moulds, it has heretofore been common practice in the industry to cast extra metal onto the piston in a region thereof known as a "balance pad". The balance pad often contains as much as 20-25 grams of extra metal, much of which is subsequently machined away in a weight-control station located downstream of the casting operation where the weight of the piston is brought within acceptable limits. The weight-control operation involves weighing the piston before removing metal from the balance pad, machining metal from the balance pad and then re-weighing the piston to check its final weight. This weight-control procedure is time-consuming and costly and, if eliminated, would improve the productivity of the piston manufacturing operation and reduce the cost of pistons produced thereby. It would be desirable to eliminate the machining for weight-control step and to shift the weight-control procedure to the casting station if possible. Heretofore that has not been possible. One of the reasons for significant as-cast weight variations is the design of the mould used to shape the piston. Heretofore the mould design, the tolerances between the several components of the movable mould, and the stack-up of the tolerances of those components has resulted in wide weight variations from one piston to the next, all cast from the same mould. In this regard heretofore, the core forming the firing face of the piston (hereafter termed the "dome") is typically located against a surface or surfaces on the mould segments which shape the sidewalls of the piston, which, in turn, is located against a stationary mould part. Since each of the several mould parts has its own manufacturing tolerance and allowances for clearances within the mould, the stack-up of the mould components from one casting to the next allows for wide variations of spacing between the core used to shape the firing face of the piston and the stationary base of the mould. This type of location system simply does not control the location of the dome-forming core accurately enough to control piston weight. Some clearances must be provided between the several components of the mould and these clearances reveal themselves as locational error of the dome core relative to the piston interior as the sidewall segments of the mould randomly move up and down in this clearance. Hence, each time the several mould components come together to form the mould cavity, the components are positioned in a slightly different position from the previous casting. This particularly affects the location of the dome-forming core relative to the stationary base of the mould. A permanent mould according to the present invention is characterised by the features specified in the characterising portion of claim 1. It is an object of the present invention to provide a permanent mould for the gravity-casting of internal combustion engine pistons, which mould (1) produces a series of cast pistons having little weight variation from one piston to the next, (2) can be readily converted to cast a different series of pistons having a different weight than the previous series of pistons, and (3) obviates the need for a downstream station for machining each cast piston to adjust its weight. It is a further object of the present invention to provide a permanent mould for the gravity casting of pistons wherein spacer means are provided between a dome-forming core and a stationary base component of the mould to establish a fixed distance therebetween from one casting to the next and thereby ensure that all of the pistons cast from that mould using the spacer have substantially the same weight. This and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows. In accordance with the present invention, a spacer of controlled dimension is provided to locate the dome-forming core at a controlled distance from a stationary component of the mould (i.e., the mould base), which distance will not change from casting one piston to the next in a particular series of castings, and until such time as the spacer may be replaced with another spacer adapted to increase or decrease the desired weight of the piston. In this latter regard, a slightly longer spacer may be used to increase the distance between the dome-forming core and the mould base and thereby add weight to the piston, or a slightly shorter spacer may be used to decrease the distance between the dome-forming core and the mould base and thereby reduce the weight of the piston. More specifically, the invention comprehends a permanent mould for sequentially casting a series of internal combustion engine pistons (e.g., out of aluminium) with little weight variation from one piston to the next in that series. The pistons each have a top wall, a firing face (i.e., the dome) formed on the exterior surface of the top wall, and a side wall depending from the top wall and including a ring band portion adjacent the top wall and a skirt portion more remote from the top wall than the ring band portion. Appropriate grooves for receiving compression rings and oil rings are formed into the ring band portion when the pistons are machined. The mould itself comprises a stationary base, a first core movable to and fro within the base for shaping the interior of the piston, a set of mould segments movable laterally with respect to the first core so as to converge upon the first core during mould closing to define a mould cavity for forming the side wall of the piston when the mould is fully closed and which diverge from the first core when the mould is opening to release the cast piston from the mould cavity. A second core is movable into axial alignment with, and opposing relation to, the first core when the mould is in the mould-closed position and serves to close off the mould cavity and to shape the dome on the top wall of the piston. At least one (preferably four) spacer(s) is provided which extends between the stationary base and the second core when the mould is in the mould-closed position so as to locate the second core a fixed distance from the base from one casting to the next which, in turn, serves to consistently mould one piston after the other with substantially same top wall thickness. This, in turn, substantially reduces any variation in piston weight from one piston to the next in a given series of pistons cast from the mould. In the event it becomes necessary to change the weight of the pistons being cast, the spacer(s) will simply be replaced with another set of spacer(s) which are either slightly longer or slightly shorter than the original set so as to increase or decrease the piston weight respectively. This may be necessitated, for example, when different batches of metal having slightly different densities are used, or some other variable (e.g., thickness of the mould coating or wear of the mould components) in the casting operation changes and causes variations in the piston weight. In accordance with a preferred embodiment of the present invention, the dome-forming core is pivotally connected to one of the laterally-moving mould segments for arcuate movement into alignment with the first core in the mould-closed position. In this preferred embodiment, the laterally-moving segments include through-holes which extend between the dome-forming core and the stationary base and the spacer(s) comprises a pin(s) extending through the through-holes so as to engage the dome-forming core and the base on opposite ends of the pins. The invention will better be understood when considered in the light of the following detailed description of a specific embodiment thereof which is provided hereafter in conjunction with the accompanying drawings, in which: Figure 1 is a side, sectioned view taken through the centre of a piston mould according to the present invention, shown in a mould-closed position, including a portion A taken at a different elevation to show a spacer in elevation; Figure 2 is a cross-sectional view taken in the direction 2-2 of Figure 1; and Figure 3 is a sectioned view like that of Figure 1, but with the mould in a mould-open position. The drawings show a piston 2 having a top wall 4 and a depending side wall 6 including a skirt portion 8. Wrist-pin holes 10 are formed in the side wall 6 during the casting operation. The top wall 4 includes a dome 14 and is defined by a peripheral ring belt portion 12 of the side wall 6 into which compression ring and oil ring grooves are subsequently machined. A hollow cavity 16 is formed in the centre of the piston 2 by a first core 18 which reciprocates to and fro within an opening 20 in a stationary base ring 22 of the mould. The core 18 is carried on a core shaft 24 which is sheathed with semi-circular wear plates 26 which slide against the interior surface of the opening 20. The stationary base ring 22 includes a mesa 28 which, in turn, includes an upstanding plateau portion 30. A flat upper annular surface 32 on top of the mesa 28 lies outwards of the plateau 30 and provides a seat for spacer pins 54 (to be described hereinafter). A sloping surface 34 adjacent the top edge of the mesa 28 serves to guide the spacer pins 54 into position on upper surface 32 as the mould closes. Mould segments (preferably halves) 36 and 38 converge radially (preferably diametrically) upon the core 18 during mould closing and diverge therefrom during mould opening to release the cast piston from the mould cavity C . The mould segments 36 and 38, along with the core 18, serve to define that portion of the mould cavity C which forms the side wall 6 including the ring belt 12 of the piston. The mould segments 36 and 38 also serve to carry wrist-pin-forming cores 40 and 42 which reciprocate with respect to their respective mould segments 36 and 38 to form the respective wrist-pin openings 10. As best shown in Figure 1, the wrist-pin-forming cores 40 and 42 extend into the mould cavity C when the mould is in the mould-closed position (see Figure 1), and, as best shown in Figure 3, are retracted therefrom when the mould is in the mould-open position. A core 44 is pivotally connected to mould segment 36 via a hinge member 46 and associated fastening means 48. The core 44 has a moulding face 50 which serves to shape the dome 14 in the top wall 4 of the piston 2. In the mould-closed position, the core 44 pivots downwards into axial alignment with the first core 18 to close-off the mould cavity C . The core 44 has surfaces 52 thereon (see Figure 1) adapted to engage upper ends of spacer pins 54, which extend through through-holes 56 in the mould segments 36 and 38. Upper ends 58 of the through-holes 56 are larger than the remainder of the through-holes 56 and are each adapted to receive a head portion 60 on the upper end of a respective spacer pin 54 to prevent the pin 54 from falling out of the through-hole 56 when the mould is in the mould-open position. In the mould-closed position, the upper end 60 of each spacer pin 54 directly engages the surface 52 of the core 44 whilst the other end 62 of the pin 54 engages the surface 32 of the base ring 22 thereby locating the core 44 a fixed distance from the base 22, which distance will be constant from one piston to the next whilst casting pistons in a given series of pistons, regardless of the manufacturing tolerances of the mould segments 36 and 38, the base ring 22, the core 44 or clearances therebetween. The net effect of locating the core 44 at a fixed distance from the base ring 22 is to consistently provide pistons 2 with upper walls 4 which are consistently of the same thickness as one another, and correspondingly pistons which are consistently about the same weight as one another. If, for some reason, it is desirable to increase the as-cast weight of the piston 2, it is a simple matter to replace the spacer pins 54 with slightly longer pins to slightly increase the distance of the core 44 from the base 22. Similarly if, for some reason, it is desirable to reduce the weight of the piston 2 in the as-cast condition, it is a simple matter to replace the pins 54 with slightly shorter pins 54. In the mould-closed position, the dome-forming core 44 is held tightly in position by a piston 64 which presses against a pad 66 on the backside of the core 44. The piston 64 is biased against the pad 66 by a compression spring 68 held in place in a bore 72 by an anchoring plate 70. The spring 68 and piston 64 are movable in bore 72 formed in an overhanging portion 74 of the mould segment 38 opposite the hinge-bearing mould segment 36. Whilst the invention has been disclosed primarily in terms of a specific embodiment thereof it is not intended to be limited solely thereto, but rather only to the extent set forth hereafter in the scope of the claims which follow.

Claims: A permanent mould for sequentially casting IC-engine pistons (2) with little weight variation from one piston to the next in a series, said pistons (2) each having a top wall (4) and a sidewall (6) depending from said top wall (4), said mould comprising: a stationary base (22); a first core (18) moveable to and fro within said base (22) from a first mould-closed position for shaping the interior of each of said pistons (2) and a second mould-open position for releasing each of said pistons (2) from said mould; a set of mould segments (36,38) moveable with respect to said first core (18) so as to converge upon said first core (18) and to engage said base (22) in the mould-closed position so as to define a mould cavity (C) for shaping said sidewall (6) of each piston (2) and to diverge from said first core (18) to release said piston (2) from said cavity (C) in the mould-open position; and a second core (44) moveable into axial alignment with, and in opposing relation to, said first core (18) when said mould is in the mould-closed position for closing-off said cavity (C) and shaping a firing face on said top wall (6) of said piston (2); characterised in that the mould includes spacer means (54) extending between said base (22) and said second core (44) in the mould-closed position at a location outside of the mould cavity to space said second core (44) a fixed distance from said base (22) so as to consistently cast pistons having substantially the same top wall thickness from one piston to the next in said series. A permanent mould according to claim 1, in which said spacer means (54) are interchangeable with other spacer means for changing the thickness of the top wall (4) of each cast piston (2) from one series of pistons to the next. A permanent mould according to claim 1, in which said second core (44) is pivotally connected to a first (36) of said mould segments (36,38) for arcuate movement into axial alignment with, and in opposing relation to, said first core (18) when said mould is in the mould-closed position; through hole means (56) in said segments (36,38) extend between said second core (44) and said base (22) when said mould is in the mould-closed position; and said spacer means (54) extend through said through hole means (56), each of said spacer means (54) having a first end (62) engaging said base (22) and a second end (60) engaging said second core (44) when said mould is in the mould-closed position. A mould according to claim 3, in which said spacer means comprises at least one pin (54). A mould according to claim 4, in which said spacer means comprises a plurality of pins (54) positioned radially outwards of said first core (18). A mould according to claim 3, in which said segments comprise a pair of mould halves (36,38). A mould according to claim 6, in which a pair of said spacer means (54) extends through each of said mould halves (36,38). A mould according to claim 3, in which said base (22) has a seat portion (32) engaging said first end (62) of each of said spacer means (54), and ramp means (34) located radially outwards of said seat portion (32) for engaging said first end (62) during mould closure to guide said first end (62) onto said seat portion (32). A mould according to claim 3, in which each of the mould segments (36,38) includes a reciprocating core pin (40,42) which extends into said cavity (C) in the mould-closed position to form a wrist-pin bore and retracts from said cavity (C) in the mould-open position to release said piston (2) from said mould. A mould according to claim 3, in which the mould includes clamping means (64,66,68) secured to a second (38) of said segments (36,38) for engaging said second core (44) and pressing it firmly against said spacer means (54) when said mould is in the mould-closed position so as to prevent displacement of said second core (44) during the casting of each piston. A mould according to claim 10, in which said clamping means (64,66,68) includes a spring-biased piston (64) engaging said second core (44). A mould according to claim 11, in which said clamping means (64,66,68) engages said second core (44) adjacent a centreline of said first and second cores (18,44). A mould according to claim 3, in which said spacer means (54) is interchangeable with longer or shorter spacer means for changing the thickness of the top wall (4) of each cast piston (2) from one series of pistons to the next.

Inventors: PARSLEY CLOVIS LEE, PREISER ROLAND MARTIN, PARSLEY, CLOVIS LEE, PREISER, ROLAND MARTIN
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1993
Main classification: B22D15
IPC codes: B22D15, F02F3