INJECTION MOLD WITH A SELF-CENTERED CORE

Abstract

INJECTION MOLD WITH A SELF-CENTERED CORE The injection mold (1) according to the invention comprises an outer part (10) and an inner part (11) made of two materials having different coefficients of expansion, the inner part (11) having a higher coefficient of expansion than the outer part (10). It is characterized in that the outer part (10) is connected to the inner part (11) by a system of spring-loaded pushers (3). The spring-loaded pusher system allows, at ambient temperature (20°C), the inner part of the mold to remain in place relative to the outer part despite the existing clearance between the two parts of the mold. Upon heating, the inner part expands more than the outer part at the same temperature, the clearance between them decreases, and the pushers are forced to compress until the clearance between the two parts of the mold is zero. Figure to be published with the abbreviation: Figure 5

Description

Title of the invention: INJECTION MOLD WITH A SELF-CENTERED CORE TECHNICAL FIELD OF THE INVENTION

[0001] The technical field of the invention is that of injection molds comprising an outer part and an inner part constituting the core of the mold. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Some injection molds include a core for manufacturing a part requiring molding elements inside and outside the part to be manufactured.

[0003] The major problem with this type of mold is demolding the part. The difficulty is even greater for parts of revolution such as a housing because its diameter may be reduced in places, thus making it difficult to remove the parts of the mold located in its center.

[0004] To solve this problem, a mold with metallic parts is used to withstand various mechanical stresses at molding temperatures approaching 200°C, and their coefficient of thermal expansion is greater than that of the molded part. Consequently, upon cooling, their dimensional reduction is greater than that of the molded part. The metallic parts then clamp onto the molded part, which poses a problem for demolding.

[0005] It was therefore proposed to use steel for the outer parts of the mold. Since their coefficient of thermal expansion is not too far removed from that of the molded part, particularly composite parts, their dimensional reduction compared to that of the molded part (known as differential expansion) is small, thus limiting the clamping effect. For the parts of the mold located inside the molded part, aluminum was chosen. Its coefficient of thermal expansion is twice that of steel. Consequently, upon cooling from 200°C to 20°C, the aluminum parts shrink more than the molded part. They therefore detach naturally. A gap is created between the molded part and these aluminum parts, which facilitates demolding and removal from the inside of the molded part.

[0006] However, these solutions are not satisfactory because the mold design is such that its hot geometry (expanded at approximately 200°C) freezes (by polymerization) the necessary geometry of the hot part so that it has the correct geometry once it returns to room temperature (20°C). The mold must also be airtight when hot to prevent resin leakage. Therefore, certain parts must be in contact to compress sealing gaskets. These requirements Respect for geometry and sealing requires that the external parts of the mold are always correctly positioned relative to the internal parts, and vice versa.

[0007] However, the internal aluminum parts, due to their higher coefficient of expansion, shrink more than the external steel parts when returning to ambient temperature. Therefore, at ambient temperature, there is some play between certain aluminum parts and other steel parts, particularly where they must be in contact when hot. This play allows for movement, and thus the relative positions of the different parts of the mold are not controlled.

[0008] During the heating process for injection preparation, the higher the mold temperature rises, the smaller the gaps between the parts of the mold become, until they disappear at the operating temperature close to 200°C. Summary of the invention

[0009] The invention provides a solution to the problems mentioned above by preventing the internal parts of the mold from assuming an undesired position relative to the external parts, ensuring that all seals are formed simultaneously, and that their position remains constantly centered and balanced from the initial temperature of 20°C until the curing temperature stabilizes. For composite parts, the invention also guarantees the position of the fiber preform to be injected.

[0010] The injection mold according to the invention comprises an outer part and an inner part made of two materials having different coefficients of expansion, the inner part having a coefficient of expansion greater than that of the outer part, it is characterized in that the outer part is connected to the inner part by a system of spring pushers disposed on a contact surface between the outer part and the inner part.

[0011] The spring-loaded plunger system allows, at room temperature (20°C), the inner part of the mold to remain in place relative to the outer part despite the existing gap between the two parts of the mold. When heated, the inner part expands more than the outer part at the same temperature, the gap between them decreases, and the plungers are forced to compress until the gap between the two parts of the mold is zero.

[0012] Advantageously, at least one of the spring-loaded pushrods comprises Belleville washers. Belleville washers are springs that are commercially available standard components and therefore low cost.

[0013] Advantageously, the spring-loaded plungers are fixed to the inner part of the mold. The plungers are thus integral with the inner part. This inner part has a greater thickness of material to allow for the machining necessary for integrating the spring-loaded plungers than the outer part.

[0014] Advantageously, all spring-loaded pushbuttons are identical. The structure of the pushbuttons is identical.

[0015] Advantageously, the spring-loaded plungers are positioned on an upper and a lower surface of the inner part of the mold. This allows the inner part to be centered within the outer part of the mold.

[0016] Advantageously, the spring-loaded pushers are oriented perpendicularly to the contact surface between the outer and inner parts. In this way, the force exerted by the pushers is optimum.

[0017] Advantageously, the mold has a vertical X-axis and the spring-loaded plungers are distributed symmetrically axially on the upper and lower surfaces of the inner part of the mold. The upper and lower plungers are arranged opposite each other in a plane perpendicular to the X-axis, thus ensuring that the inner part maintains its orientation during the heating of the mold and the cooling of the workpiece and the mold.

[0018] Advantageously, the outer part is made of steel and the inner part of aluminum. Steel has a coefficient of thermal expansion close to that of molded parts, whether composite or not, and its dimensional reduction compared to that of a molded part (referred to as differential expansion) is small, which limits the clamping effect. Aluminum has a coefficient of thermal expansion twice that of steel; therefore, upon cooling from 200°C to 20°C, the aluminum part shrinks more than the molded part. It thus naturally separates from the molded part. This gap created between the composite part and the aluminum part facilitates its demolding and removal from the inside of the composite part.

[0019] Advantageously, the outer part includes housings for receiving the spring-loaded plungers. These housings allow for proper lateral positioning of the inner part of the mold within the outer part.

[0020] Advantageously, the inner and outer parts of the mold are separated by a distance d at ambient temperature and the spring plunger has a travel d0 greater than or equal to the distance d. Thus the spring plunger is always under tension and allows better holding of the inner part of the mold in the outer part regardless of the temperature.

[0021] The manufacturing process for the part comprises the following steps: • heating of the mold and expansion of the inner and outer parts, and compression of the spring-loaded pushers until the inner part is in contact with the outer part of the mold, in order to create a seal, • Insertion of the resin into the mold to create a part, • Heating of the mold to a polymerization temperature of the resin, for example, approximately 200°C • Cooling of the mold to ambient temperature, dimensional reduction of the outer and inner parts, release of the spring-loaded plungers and separation of the part from the inner part, • extraction of the part from the mold.

[0022] Other advantages may become apparent to those skilled in the art upon reading the examples below, illustrated by the accompanying figures, which are given for illustrative purposes. BRIEF DESCRIPTION OF THE FIGURES

[0023] The figures are presented by way of example and in no way limit the invention.

[0024] [Fig.1] is a perspective view of a cross-sectional mold according to the invention with a molded part;

[0025] [Fig.2] is detail A of [Fig.1];

[0026] [Fig.3] is detail B of [Fig.1];

[0027] [Fig.4] is a perspective view of the inner part of the mold according to the invention;

[0028] [Fig.5] is the detail of a spring-loaded pusher according to the invention. DETAILED DESCRIPTION

[0029] Unless otherwise specified, the same element appearing on different figures has a unique reference.

[0030] Throughout this description, the upper part of the figures will be referred to as "top" or "upper," and the lower part of the figures will be referred to as "bottom" or "lower," defining the vertical line corresponding to gravity. The part corresponding to the center of the mold or directed towards it will be referred to as "interior," and the part corresponding to the body of the mold or the part extending away from the center of the mold will be referred to as "exterior."

[0031] The injection mold 1 according to the invention is visible in cross-section [Fig. 1]. It comprises an outer part 10 and an inner part 11. The molded part 2 is obtained by injecting resin into a space 12 between the outer part 10 and the inner part 11 of the mold 1. Both the outer part 10 and the inner part 11 may be composed of one or more elements. The space 12 in which the part 2 is molded is the space remaining free when the mold 1 is heated. In this example, the molded part is a housing with axis X.

[0032] In the illustrated example, the injection mold 1 is cylindrical in shape with vertical axis X, the inner part 11 comprises an upper surface 110 and a lower surface 111 opposite respectively an upper surface 100 and a lower surface 101 of the outer part 10. The part to be produced can, for example, be a part of revolution with the same axis X.

[0033] Spring-loaded pushers 3 are arranged on a contact surface 13 between the upper surface 110 of the inner part 11 and the upper surface 100 of the outer part 10, and between the lower surface 111 of the inner part 11 and the lower surface 101 of the outer part 10. In this example, the pushers 3 are axially oriented because the contact surface 13 between the inner part 11 and the outer part 10 is axially oriented. The spring pushers 3 are divided into a series of lower pushers 33 and a series of upper pushers 34.

[0034] The spring-loaded plungers 3, shown in detail in [Fig. 5], comprise a stack of Belleville washers 30 positioned between a base 31 and a cover 32. A retaining screw 33 is screwed onto the base 31 and passes through the stack of Belleville washers 30. The retaining screw 33 has a head 330 bearing against a rim 320 of an opening 321 in the cover 32. The screw 33 is long enough to allow movement of the cover 32 towards the base 31. At rest, at room temperature (20°C), the base 31 and the cover 32 of the spring-loaded plunger 3 are separated by a distance d0, corresponding to the travel of the plunger 3.

[0035] At room temperature, the upper surface 100 of the outer part 10 and the upper surface 110 of the inner part 11 of the mold 1 as well as the lower surface 111 of the inner part 11 and the lower surface 101 of the outer part 10 are separated by a length d when the inner part 11 is properly positioned. When the temperature rises to 200°C, the different surfaces 100 and 110, 111 and 101 come into contact due to the greater expansion of the inner part 11 compared to the outer part 10, this differential expansion thus compressing the spring plungers 3. During the temperature decrease and the contraction of the inner part 11 and outer part 12, the spring plungers 3 relax until the inner part 11 of the mold 1 is repositioned in its initial position relative to the outer part 10 of the mold 1.The travel d0 of the spring plunger is greater than or equal to the length d so that the set of plungers is always in contact with both the outer part 10 and the inner part 11 of the mold 1. The length d corresponds to half the clearance between the inner part 11 and the outer part 10 of the mold.

[0036] In the illustrated example, the spring-loaded plungers 3 are placed in recesses 112 in the inner part 11, opening into the upper 110 and lower 111 surfaces. The base 31 is fixed to the inner part 11, in particular the base 31 of the lower plunger 33 to prevent it from falling out of the inner part 11. The lower plunger 33 must have a greater thrust force than the upper plunger 34 due to the weight of the inner part 11 of the mold 1, because in addition to managing the balancing of clearances, it must also support the mass of the entire inner part 11. The spring-loaded plungers 3 can be fixed to the outer part 10 instead of the inner part 11 without departing from the scope of the present invention.

[0037] The outer part 10 of the mold 1 also has recesses 102 opening into the upper 100 and lower 101 surfaces to receive the cover 32 spring pushers 3. These housings 112 and 102 ensure the lateral position of the inner part 11 relative to the outer part 10 throughout the manufacturing process of the molded part 2.

[0038] The spring-loaded pushers 3 are distributed evenly over the lower 111 and upper 110 surfaces of the contact surface 13 to distribute the pressure homogeneously. They are preferably arranged symmetrically axially between the top and bottom of the mold 1.

[0039] We will now describe the process of producing a molded part using the mold according to the invention. The molded part can be a composite part.

[0040] When cold or at room temperature, approximately 20°C, the inner part 11 of the mold 1 is not in direct contact with the outer part 10 but is held there by the spring-loaded plungers 3. The mold 1 is heated to approximately 200°C, the inner part 11 and outer part 10 expand, but due to the different coefficient of expansion, the outer part 10 expands less than the inner part 10. During this expansion, the spring-loaded plungers 3 will compress until the inner part 11 and the outer part 10 come into contact, thus making the mold watertight.

[0041] The resin intended to form the part is then injected. The resin polymerizes thanks to the temperature of the mold and thus solidifies. The mold is then cooled, its temperature dropping from 200°C to 20°C. This temperature decrease causes the inner part 11 and outer part 10 to contract. The inner part, having a lower coefficient of expansion than the outer part, shrinks more and detaches from the outer part, but the spring-loaded pushers, which were compressed, gradually relax and thus hold the two parts of the mold in position. During this time, the molded part, which contracts less than the inner part, detaches from the latter, thus facilitating demolding.

Claims

Demands

1. Injection mold (1) comprising an outer part (10) and an inner part (11) made of two materials having different coefficients of expansion, the inner part (11) having a coefficient of expansion greater than that of the outer part (10), characterized in that the outer part (10) is connected to the inner part (11) by a system of spring-loaded pushers (3) disposed on a contact surface (13) between the outer part (10) and the inner part (11).

2. Mold (1) according to claim 1, characterized in that at least one of the spring-loaded pushers (3) comprises Belleville washers (30).

3. Mold (1) according to any one of the preceding claims characterized in that the spring-loaded pushers (3) are fixed to the inner part (11) of the mold (1).

4. Mold (1) according to any one of the preceding claims characterized in that all spring-loaded pushers (3) are identical.

5. Mold (1) according to any one of the preceding claims characterized in that the spring-loaded pushers (3) are positioned on an upper surface (110) and a lower surface (111) of the inner part (11) of the mold (1).

6. Mold (1) according to the two preceding claims characterized in that the spring-loaded pushers (3) are oriented perpendicular to the contact surface (13) between the outer part (10) and the inner part (11).

7. Mold (1) according to both claims 4 and 5, or according to claim 6, characterized in that the mold comprises a vertical axis X and that the spring-loaded pushers (3) are distributed symmetrically axially on the upper surface (110) and the lower surface (111) of the inner part (11) of the mold (1).

8. Mold (1) according to any one of the preceding claims characterized in that the outer part (10) is made of steel and the inner part (11) is made of aluminum.

9. Mold (1) according to any one of the preceding claims characterized in that the outer part (10) comprises housings (102) for receiving spring-loaded pushers (3).

10. Mold (1) according to any one of the preceding claims characterized in that the inner part (11) and the outer part (10) of the mold (1) are separated by a distance d at room temperature and the spring-loaded pusher (3) has a travel d0 greater than or equal to the distance d.

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