Device for injection-moulding a part

A tooling for injection-molding a part, the tooling including two cavity blocks each including a cavity formed therein of a shape corresponding to a shape of a part that is to be molded once the cavity blocks have been superposed. At least one of the cavity blocks includes a mechanism for positioning a core, which mechanism includes at least one bearing surface for bearing against the core and at least one presser screw screwed into a threaded passage of the cavity block for purpose of holding the core in position against the bearing surface. The tooling further includes a force-limiter limiting force applied to the core by the presser screw.

BACKGROUND

The present invention relates to a device for injection-molding a part, in particular a wax model.

Such a model is used in a lost wax molding method in a number of fields and for example for molding high pressure turbine blades for a turbine engine such as a turboprop or a turbojet.

In this method, one or more parts are injection-molded by using injection-molding tooling that has a cavity of shape corresponding to the shape of the model that is to be obtained. When the model has a cooling circuit, a ceramic core is used. In this event, the wax is injected into the cavity, around the core.

The wax models as obtained in this way are then mounted as a cluster on a support.

The cluster is then dipped in a bath of ceramic, referred to as a slip, and then dusted with a ceramic powder (stuccoworking). Dipping and stuccoworking are repeated several times until a layer of ceramic is obtained that is sufficiently thick and that forms a shell around the cluster.

The wax is then removed from the ceramic shell by passing the assembly in an autoclave where steam under pressure and at high temperature causes the wax to melt (dewaxing).

The shell is then baked in an oven in order to acquire sufficient mechanical strength for it to be used as a mold.

Metal, e.g. a nickel-based alloy, is then cast into the shell. After cooling, the shell is knocked out and then the various parts are removed from the cluster, i.e. they are separated from their common support.

When a core is used, it is removed chemically with the help of a bath.

The parts are then trimmed, ground, and then inspected.

As a general rule, tooling for injection-molding the wax model comprises two cavity blocks having a cavity formed therein of shape that matches the shape of the part that is to be molded once the cavity blocks are superposed. At least one of the cavity blocks is fitted with means for positioning the core, which means comprise at least one bearing surface for bearing against the core and at least one presser screw screwed into a threaded passage of the cavity block for the purpose of holding the core in position.

The core must be clamped strongly enough for it to be held properly in position and to ensure that it does not move while parts are being molded. However, if this clamping is too strong, that can deform, move, or damage the cores, such that the final parts made by molding no longer comply with their specifications.

A particular object of the invention is to provide a solution to this problem that is simple, effective, and inexpensive.

BRIEF SUMMARY

To this end, the invention provides tooling for injection-molding a part, the tooling comprising two cavity blocks each having a cavity formed therein of a shape corresponding to the shape of the part that is to be molded once the cavity blocks have been superposed, at least one of the cavity blocks being fitted with means for positioning a core, which means comprise at least one bearing surface for bearing against the core and at least one presser screw screwed into a threaded passage of the cavity block for the purpose of holding the core in position, the tooling being characterized in that it includes force-limiter means for limiting the force applied to the core by the presser screw.

The force-limiter means serve to avoid any damage to or movement of the core.

In a characteristic of the invention, the force-limiter means are torque-limiter means arranged between a tightening head and a threaded portion of the presser screw and designed to transmit clamping torque from the head to the threaded portion so long as said torque is less than a determined value, and to decouple the head from the threaded portion when said torque is greater than the determined value.

Advantageously, the tooling includes two surfaces for bearing against the core and two presser screws arranged facing the bearing surfaces and designed to hold the core (10) in position against the two bearing surfaces, the screws extending substantially perpendicularly to each other and each including torque-limiter means.

The core is thus properly held in position in the cavity of the bottom cavity block by the screws.

In one possibility of the invention, the determined value of the clamping torque beyond which the head is decoupled from the threaded portion of the presser screw lies in the range 0.1 newton-meters (Nm) to 5 Nm, and is preferably about 1 Nm.

Such a level of torque suffices to hold the core in position, while avoiding any degradation, deformation, or movement of the core.

In preferred manner, the tooling includes a stationary bottom cavity block and a movable top cavity block, the bearing surface being formed in the cavity of the stationary cavity block, the presser screw being mounted on the stationary cavity block.

In another characteristic of the invention, the two cavity blocks are mounted to pivot relative to each other about a pin between a molding position in which the two cavity blocks are superposed, and an un-molding position in which the two cavity blocks are spaced apart.

In advantageous manner, at least one of the cavity blocks is mounted to pivot about the pin via at least one rolling bearing, e.g. a ball bearing.

DETAILED DESCRIPTION

FIGS. 1 and 2show tooling of the invention for injection-molding a wax model, the tooling comprising a top portion1and a bottom portion2pivotally mounted relative to each other about a pin3.

More particularly, the bottom portion2comprises a cavity block4of generally rectangular shape having its top surface forming a join plane5in which there are provided a cavity6and an injection channel7(FIG. 2).

As can be seen better inFIGS. 3 and 4, the bottom cavity block4has a first bearing surface8(FIG. 3) extending transversely relative to the cavity block4, and a second bearing surface9(FIG. 4) extending longitudinally relative to the cavity block4, each of the bearing surfaces8,9serving to bear against a corresponding surface11,12of a core10(FIG. 5).

The bottom cavity block4is fitted with two presser screws13,14screwed into threaded passages of the cavity block4for the purpose of holding the core10in position against the bearing surfaces8,9, the screws13,14extending parallel to the join plane5.

The presser screws13,14are arranged facing the bearing surfaces8,9and they extend perpendicularly to these bearing surfaces. The screws13,14thus extend substantially perpendicularly relative to each other, and each of them includes torque-limiter means. These means are arranged between a camping head15and the threaded portion16of each presser screw13,14, and they are designed to transmit the clamping torque from the head15to the threaded portion16so long as said torque is less than a determined value, and to decouple the head15from the threaded portion16when said torque is greater than the determined value.

Such torque-limiter means are known for other applications, in particular from patent application FR 2 841 946, and their structure is not described in greater detail herein.

These torque-limiter means are calibrated in such a manner that the determined value for the clamping torque beyond which the head15is decoupled from the threaded portion16of the presser screw13,14lies in the range 0.1 Nm to 5 Nm, and is preferably about 1 Nm. Once the torque has been exceeded, a clicking noise may be produced so as to inform the operator.

The bottom cavity block4is mounted using screws17on an elongate and plane bottom plate18that is likewise rectangular in shape. The two opposite ends of the plate18are fitted with handles19. The plate18also has two lateral lugs20arranged on either side of a longitudinal midplane, and forming a clevis in which the pin3is mounted. More particularly, the pin3is guided by two ball bearings21mounted in the lugs20, and it extends perpendicularly to the above-mentioned midplane.

In the same manner, the top portion1of the injection-molding tooling includes a top cavity block22in which a cavity6is formed to face the cavity in the bottom cavity block4once the cavity blocks4,22are superposed.

The top cavity block22is fastened to a top plate23that is mounted to pivot relative to the bottom plate18about the pin3.

The end of the top plate that is remote from the pivot pin3is fitted with a handle24.

There follows a description in greater detail of the method of molding a wax model25of a blade.

When the model25includes a hollow portion, such as a cooling circuit, a ceramic core10is mounted in the cavity6of the bottom cavity block4.

The presser screws13,14are then tightened so that the surfaces11,12of the core10come to bear against the bearing surfaces8,9of the bottom cavity block4. For this purpose, the operator exerts torque on the screws that is greater than the maximum torque authorized by the torque-limiter means, and the operator is informed that the torque is indeed greater by a clicking sound, as mentioned above.

Once the core10has been correctly positioned and held in the bottom cavity block4, the injection-molding tooling is closed, i.e. the top and bottom portions1,2are pivoted so as to superpose the two cavity blocks4,22, and thus also the cavities6. The shape defined by the cavities6, when situated facing each other, corresponds to the shape to be given to the part25that is to be molded, as can be seen inFIG. 5. In particular, the cavities6define the shape of a blade25having an airfoil26and a platform27.

A press (not shown) then bears against each of the outside faces of the plates18,23and wax is injected, via the injection channel7, into the cavities6around the core10, and then allowed to cool so as to solidify and form a wax model25of the blade (FIG. 5).

The injection-molding tooling is then opened by pivoting of the top portion1about the pin3, and the model25together with its core10is then removed from the cavity6.