The present invention relates to a die casting method for in- casting, in an object, a cylindrical liner in a cylindrical opening in the object, whereby a casting insert is placed inside the cylindrical liner, and a metal melt is pressurized against the outside of the liner. The invention is particu¬ larly but not exclusively directed to a method of die casting a cylinder block of aluminum in which cylinder liners of another material, such as cast iron or sintered metal, are cast in the cylinder openings.

Today's aluminum engines of aluminum with cast-in cylinder liners are manufactured by die casting, which means that the aluminum melt is pressurized during the injection and harden¬ ing stages. In order to prevent the melt from penetrating into the cylinder liners, upper casting inserts in the form of cylindrical bodies are used, which fill out the liner, as well as lower casting inserts, which abut the lower edges of the liners and the upper casting inserts, filling out what is to become the cylinder block crank case.

The upper casting insert is dimensioned so that a gap of ca. 0.2 mm is formed between the liner and the insert when the liner is cold. This gap increases by ca. 0.5 mm to a total of ca. 0.7 mm upon heating to ca. 500°C upon contact with the melt. The pressure in the melt subjects the liners, however, to great pressure from the outside, and the liners are de¬ formed so much that they come into contact with the insert. The result is that the liners are subjected to great stresses during the casting process. These stresses are not desirable since they can give rise to out-of-roundness at a subsequent stage of the manufacturing process, and this in turn can require extra machining of the liners themselves.

The purpose of the present invention is to develop the die casting method described above by way of introduction so that

the risk of out-of-roundness due to stresses in the liners can be completely eliminated.

This is achieved according to the invention by virtue of the fact that a casting insert is used which leaves a gap between itself and the liner, and that the pressurized melt is allow¬ ed to penetrate into the gap to create equal pressure from the pressurized melt on both sides of the liner.

The invention minimizes in the casting stage external stres¬ ses on the liner, and this results in the liner being rounder after casting than is the case when previously known die casting methods are used. Shrink stresses which arise later are required to keep the liner in place and for heat transfer when the engine is running.

The invention will be described in more detail with reference to an example shown in the accompanying drawing, where Fig. 1 shows a cross section through a cylinder block with upper and lower casting inserts, which are used in previously known die casting methods. Fig. 2 shows schematically the pressure load on the liner in the known die casting methods. Fig. 3 shows a cross section corresponding to Fig. 1 with upper and lower casting inserts, which are used in the die casting method according to the invention, and Fig. 4 is a schematic illu¬ stration corresponding to Fig. 2 of the die casting method according to the invention.

In Fig. 1, 1 and 2 designate opposite sides of a die cast aluminum blank for a cylinder block 3 after the pressurized melt has hardened and after the die components (not shown) in contact with the sides 1 and 2 have been removed, but before the upper and lower inserts 4 and 5 of the die have been removed. The upper insert 4 of the die is a cylindrical body for each cylinder in the cylinder block, while the lower insert 5 is a body 7 conforming to the shape of the crank case 6. The diameter of the cylinder 4 is so adapted to the inner diameter of a cylinder liner 8 that, when the liner is cold, there is a gap "a" of ca. 0.2 mm between the external

surface of the cylindrical insert 4 and the interior surface of the liner 8. When the liner is heated upon contact with the melt, the gap widens to ca. 0.7 mm.

As is evident from Fig. 1, the lower edge of the liner 8 is in direct contact with an upper surface 9 of the insert 5, and this means that no melt can flow into the gap "a" from below. The upper insert 4 is made with a flange portion 10 in contact with the upper edge of the liner, said flange portion preventing the melt from flowing from above into the gap "a". The result is that there will be a pressure difference be¬ tween the inside and outside of the liner as illustrated in Fig. 2, and this in turn results in plastic deformation of the liner.

Fig. 3 shows the cylinder block 3 in a corresponding manner, but with somewhat modified upper and lower inserts 14 and 15, respectively, which are used in carrying out the method according to the present invention. Insert 14 is slightly conical to obtain a draft angle of about 2° relative to the inside of the liner 8, and is so dimensioned that a gap "a" of between 5 mm and 10 mm is obtained between the outside of the insert 14 and the inside of the liner 8. The insert 14 is provided on its outside with at least three peripherally spaced guide heels (one shown) for centering in the cylind¬ rical opening in the liner 8, and it has an upper flange 17 lying above the upper edge of the liner 17 so that an annular gap 18, interrupted by the guide heels 16 is formed. The lower insert 15 has an annular depression 19 in the support- ing surface for the upper insert 14. In the depression 19 there are at least three peripherally spaced supporting heels 21 (one shown) which support the lower edge of the liner 8. This design provides a lower passage 22, interrupted by the supporting heels 21, between the lower edge of the liner 8 and the upper surface of the lower insert 15. As an alterna¬ tive to the guide heels 16, axial ribs (not shown) can be used which extend along a certain portion of or the entire axial length of the insert 14. The ribs can become downwardly narrower.

The embodiment described of the upper and lower inserts 14 and 15 means that the melt under pressure, which is injected between the outsides of the liners 8 and the insides of the die components not shown, can flow through the gap 22 between the lower edge of each liner and the surface of the depres¬ sion 19 and up into the gap "a" between the inside of the liners 8 and the insert 14. The gap 18 thus serves as a venting channel to let out air which is forced out by the incoming melt. The result is that the pressure of the melt against the outside of the liner is balanced by the same pressure from the melt in the gap "a", as is illustrated in Fig. 4, and this leads to a minimal external stress on the liner. After the melt has hardened and the inserts 14 have been removed, the aluminum material on the insides of the liners is removed in a subsequent removal and machining operation.