WO2020229698A1

WO2020229698A1 - Method of producing composite springs, and of a mold core for such method

Info

Publication number: WO2020229698A1
Application number: PCT/EP2020/063780
Authority: WO
Inventors: Gregor Daun; Harald Roedel; Jan Wucherpfennig; Christian Siemensen; Dirk Meckelnburg; Martin Moeller; Andreas SACHS; Wolfgang Schrof
Original assignee: Basf Polyurethanes Gmbh
Priority date: 2019-05-16
Filing date: 2020-05-18
Publication date: 2020-11-19

Abstract

The invention comprises to a method of producing a mold core (19) having at least one external groove for making composite springs, in particular composite coil springs, comprising the steps of providing a master die (1), the master die (1) having an inner shape that defines a negative shape of the mold core (19), providing a gas tight flexible shell (9) having at least one feed opening (11), expanding the shell by introducing unbonded particulate matter (13) into the shell (9) through the feed opening (11), assembling the master die (1) and shell (9) such that the master die (1) encloses the shell (9) and the particulate matter (13) causes the shell (9) to assume a negative shape of the master die (1), and solidifying the shape of the shell (9) by applying a negative pressure to the inside of the shell (9), such that the mold core (19) is obtained.

Description

Method of producing composite springs, and of a mold core for such method

The invention relates to a method of producing a mold core having at least one external groove for making composite springs, in particular composite coil springs.

When producing composite springs, there is a general requirement of forming a composite material such as a composite wire into the shape of the desired composite spring and then solidifying the composite material. This is typically achieved by encasing the composite material at least partially with a mold that defines the shape of the composite spring to be produced, and by supplying energy to achieve a solidification, e.g. curing of the matrix material of the composite material.

In conventional applications, an internal mold core has been used to define at least the inner portion of the composite spring, in particular when producing composite coil springs. After the composite spring has been produced, challenges were usually present in finding efficient ways to unform the inner mold from the composite spring. Conventionally, lost core methods were employed using casting sand or low melting alloys as mold material. Alternatively, segmented inner support structures have been experimented with which would hold the composite wire in a radially expanded state and the segments of which could be retracted away from the composite wire, i.e. composite spring after solidification. While the quality of composite springs produced in conventional methods has been found to meet the requirements set by the industry, the manufacturing process still leaves room for improvement in terms of cost efficiency and production time.

It was thus an object of the invention to suggest a method of the initially mentioned type which mitigates the problems of the prior art as far as possible. In particular, it was an object of the invention to suggest a method which allows production of composite springs in a more cost efficient and less time consuming manner, preferably without sacrificing quality of the composite spring.

The invention achieves its object by suggesting a method of producing a mold core having at least one external groove for making composite springs, in particular composite coil springs, comprising the steps of:

Providing a master die,

Providing a gas tight flexible shell having at least one feed opening,

Expanding the shell by introducing unbonded particulate matter into the shell through the feed opening,

Assembling the master die and shell such that the master die encloses the shell and the particulate matter causes the shell to assume a negative shape of the master die, and

solidifying the shape of the shell by applying a negative pressure to the inside of the shell, such that the mold core is obtained.

The invention is based upon the realization that by placing the particulate matter in a shell and using the filled shell as a mold core, the material which constitutes the mold core is not lost during manufacturing. Also, the solidification of the mold core by way of applying the negative pressure has the beneficial effect that the mold core is maintained in its solidified state precisely as long as the negative pressure is maintained inside the shell. Once the negative pressure is released from the shell, the mold becomes soft and deformable again, allowing for easy and fast removal of the mold from the composite spring, that is, after the mold produced in the inventive method has been provided to and used in a method of producing the coil spring itself, which is another aspect of this invention. The process according to the invention provides furthermore for great flexibility regarding the type and amount of particulate matter used for forming the mold core. Still further, it is an advantage of the invention that prior to solidification of the mold core by applying the negative pressure, it is conveniently possible to create a mold core having various shapes, depending on the application needed. In a preferred embodiment, the step of assembling the master die and the shell is conducted prior to introducing the particulate matter into the shell. Alternatively, it would also be possible to fill the shell fully or partially prior to its assembly with the master die.

In a further preferred embodiment, the method comprises the step of applying a positive pressure differential between the inside of the shell and the outside of the shell, the pressure outside the shell being lower than inside the shell. By providing this pressure differential, the shell is made to conform very closely to the shape of the master die such that any residual cavities in between the outside of the shell and the shape of the master die are eliminated. The pressure differential is preferably achieved by applying a negative pressure to the outside of the shell, in between the master die and the shell, after inserting the shell into the master die. Alternatively, it would also be possible to apply a positive pressure to the inside of the shell after inserting the shell into the master die.

In a further preferred embodiment, the pressure differential is produced by applying a negative pressure on the outside of the shell, and said negative pressure on the outside of the shell is a first negative pressure, wherein the negative pressure on the inside of the shell is a second negative pressure, and furthermore, the second negative pressure is lower than the first negative pressure. As a consequence of this, the shell reliably separates from the master die when the negative pressure inside the shell is applied and has reached the level predetermined as the second negative pressure level. Preferably, the first negative pressure is applied before the second negative pressure is applied and is released when the pressure inside the shell reaches the second negative pressure, or at least is lower than the first negative pressure. Waiting to release the first negative pressure until at least some measure of negative pressure has been provided in the shell ensures that the particulate matter is solidified sufficiently so that the particulate matter retains the desired shape conforming to the shape of the master die even if the shell no longer is made to adhere to the master die by a way of the outer (first) negative pressure.

In a further preferred embodiment, the step of introducing the particulate matter into the shell further comprises compacting the particulate matter by using at least one of gravity, vibration or mechanical force. Additionally, compacting the particulate matter inside the shell provides for an optimized shaping of the mold core, as the particulate matter is being firmly pushed against the shape of the master die. According to a further preferred embodiment, the step of compacting is effected by applying a mechanical compression force onto the particulate matter, preferably with a push rod from the side of the feed hole, and further preferably maintaining the compression force while the evacuation of the shell commences. In a first preferred alternative of the method, the particulate matter is substantially dry, meaning that no fluid is added to the particulate matter inside the shell. In a second preferred alternative, however, the particulate matter is at least partially mixed with a liquid prior to or after inserting the particulate matter into the shell. The liquid preferably is configured to enhance the structural solidity of the mold core after the negative pressure has been applied.

In a further preferred embodiment, the master die comprises a plurality of die segments that together define the master die, and the shape that is impressed onto the shell. In a preferred embodiment, the master die is composed of two, three, four, or more segments. A multipart master die provides the advantage of easier unforming from the mold core after applying the (second) negative pressure into the shell, in particular in cases where the shape of the mold core contains undercuts which make the mold core cling to the master die.

In a further preferred embodiment, the master die comprises or consists of a polymer material, preferably produced by 3D printing. While 3D printing becomes more cost efficient and thus competitive in the industry on its own already, a particular benefit of using a 3D printed master die in the inventive application is that it obviates the need invest in master dies made from metal (which is both time consuming and cost intensive). In addition, the technology eliminates or decreases the contact of the die with chemicals or heat associated with the solidification of the composite wire. Composite wires that are wound around a mandrel tend to develop an axially stretched elliptical cross section. In contrast to this naturally forming shape a cross section with the shape of an axially flattened ellipse (or“egg” shape) is preferred as it tends to generate lower shear stress on the inside of the spring.

Preferably, the core groove has a cross-sectional design that will shape the cross section of the soft spring wire into an axially flattened ellipse or slight egg shape with the sharp tip of the ellipse (or“egg”) on the inside of the coil spring. In a further preferred embodiment, the master die comprises an inside wall and at least one protrusion extending away from the wall (or several protrusions) wherein the (respective) protrusion defines the corresponding external groove in the mold core such as to give the groove a shape which produces a composite wire having an axially flattened cross section.

If for example, the protrusion is desired to produce a groove representative of half the thickness of the spring to be produced in the radial direction, i.e. if the groove is supposed to hold half the radial thickness of the composite wire, it would be preferred to dimension protrusion ,such that it has a height (i.e. radially with respect to a longitudinal axis of the spring to be produced) with respect to the inside wall and a width (i.e. axially with respect to the longitudinal axis of the spring to be produced), wherein preferably, the height is more than half the width The higher the protrusion extends away from the inside wall with respect to the width, the“flatter” the cross section of the composite spring will be once it is produced.

The shell is configured to contain the particles in an air tight containment that is strong enough to hold the particles on the one hand side, while being flexible enough to adhere very closely to the shape of the master die on the other hand side. In a further preferred embodiment, the shell comprises at least one layer of a polymeric foil, said foil having a thickness in a range of 5pm to 1 mm, preferably being a thermoplastic foil having a thickness in a range of 5 pm to 50 pm or an elastomeric foil having a thickness in a range of 50 pm to 1 mm. Preferably, the foil has an elongation at break in a range of 10% to 200%, and further preferably a thermal stability of more than 120°C, in particular 150°C or more in hot oil. The elongation at break may preferably be measured as defined in DIN ISO 1798 or alternatively in ISO 527-3.

Preferably, the shell comprises two or more layers of polymeric foil. Advantageously, the layers are positioned adjacent to one another and may glide along each other. This allows the shell to be more durable even when using foil from the lower end of the spectrum of elongation capability, since stress inside the foil layers is reduced.

The particulate matter is configured to move into the cavity defined by the shell, fill it up to the perimeter and, preferably through a combination of form closure and force closure, form a rigid body under application of pressure. It has been found that while very small globular particles of identical size would be ideal for the particle movement whereas larger, edged particles of non-uniform size and/or shape would be ideal for forming a rigid body. In view of these aspects, the particulate matter preferably comprises or consists of particles that have a bulk angle of 10° or more, preferably 20° or more, further preferred of 30° or more, in particular as defined in DIN ISO 4324 :1983-12, and/or have an aspherical surface with a particle size of 0,5 to 5 mm, preferably as defined in ISO 13320:2009-10 and/or have an uneven size distribution, and/or have a density of 3,0 g/cm³ or less, in particular 2,9 g/cm³ to 0,7 g/cm³, preferably respectively as defined in DIN EN ISO 845:2009-10, and/or have a melting temperature according to DIN 53765:1994-03 or ISO 1 1357-3:201 1 -05 of 200 °C or more; and/or have a thermal stability of more than 120°C, preferably as defined in DIN EN 1856-1 :2009- 09; and/or have a thermal conductivity of 0,02 W/(m K) or greater, further preferably 0,3 W/(m K) or greater, particularly preferred 200 W/(m K) or greater, preferably as defined in DIN 4108- 4:2013-02.

One preferred material that meets the above parameters is aluminum or an aluminum alloy. Aluminum has te advantage of having a rather high thermal conductivity in a range of more than 200 W/(m K) which benefits the curing of the composite around the mold core. Other preferred materials include: polystyrene granules, in particular bulk sand, polyamide granules, for example commercially available as Ultramid^® B33 or Ultramid^® T KR 4350, or granules made of thermoplastics such as amorphous thermoplastics, for example on the basis of polyethersulfone, polysulfone and/or polyphenylsulfone, commercially available as Ultrason^®. The invention has hereinabove been described in a first aspect with regard to the method of producing the mold core itself. In a second aspect the method also relates to a method for producing a composite spring, in particular a composite coil spring, the method comprising the steps of: Providing a mold core having an external groove, providing a composite wire for the composite spring to be produced, assembling the mold core and composite wire such that the wire is disposed in the external groove of the mold core, solidifying the composite wire such that the composite spring is produced, and removing the composite spring from the mold core after a predetermined amount, preferably all, of the composite wire has solidified; wherein the step of providing the mold core encompasses the method of producing the mold core according to any one of the preferred embodiments described hereinabove under the first aspect. The benefits and preferred embodiments of the first aspect are at the same time benefits and preferred embodiments of the method under the second aspect, which is reference is made to the explanations given hereinabove to avoid unnecessary repetition.

Preferably, the composite wire is fixed to the particle filed shell core by means of an additional foil which is wound around the composite wire and the core to temporarily fix the wire to the core.

In a further preferred embodiment, the method further comprises the steps of

Providing a mold core having an external groove,

Providing a composite wire for the composite spring to be produced,

Assembling the mold core and composite wire such that the wire is disposed in the external groove of the mold core,

Solidifying the composite wire such that the composite spring is produced, and Removing the composite spring from the mold core after a predetermined amount, preferably all, of the composite wire has solidified; wherein

The step of providing the mold core encompasses the method of producing the mold core according to any one of the preceding claims.

In a further preferred embodiment, the assembly is effected as follows: i) the composite wire is assembled to the mold core;

ii) thereafter, the external mold is assembled around the mold core and composite wire, and

iii) thereafter, the mold core is removed from the assembly prior to the step of solidifying the composite wire. In a further preferred embodiment, prior to removing the composite spring, the method comprises the step of:

fully or partially releasing the negative pressure inside the shell, and

separating the mold core from the assembly.

In a further preferred embodiment of the invention, the method comprises the step of retaining the particulate matter in the shell after releasing the negative pressure.

In a further preferred embodiment, the method comprises the step of reusing the mold core after production of the composite spring in the method of producing the mold core to any one of the preferred embodiments describe herein above to create a new mold core, wherein the step of introducing particulate matter into the shell preferably is omitted.

In a further preferred embodiment, the method comprises the step of catching the particulate matter removed from the shell, preferably in a second complimentary shell, the interior which communicates with the interior of the first shell, in particular through a shell connection device that comprises at least one pressure port for applying and releasing pressure to and from the shell, respectively.

The invention will hereinafter be described in more detail with respect to the accompanying drawings. Herein:

Fig. 1 shows a schematic representation of a method of producing a mold core according to a preferred embodiment, Fig. 2 shows a schematic representation of a method of producing a composite spring according to a preferred embodiment, and

Fig. 3 shows a schematic representation of a method of producing a composite spring according to a further preferred embodiment.

Fig. 1 schematically depicts a method 100 of producing a mold core 19 for subsequent use in a method 200, 200’ of producing a composite spring (ef. Fig. 2 and 3). The method 100 comprises a method step 101 wherein a master die 1 is provided. The master die 1 comprises a number of protrusions 5 inside a cavity 3. The cavity 3 defines a negative shape of the mold core 19 to be produced. Preferably, the master die 1 is formed as a multipart die and comprises a first die segment 1 a and at least a second die segment 1 b. In further preferred embodiments, the master die 1 may comprise more than two segments. Additionally, the master die 1 comprises at least one evacuation port 7 for withdrawing air from the cavity 3.

In step 103, a shell 9 is provided and introduced into the master die 1 . The shell 9 comprises a feed hole 1 1 for introduction of particular material 13. In step 105, a first negative pressure NP1 is applied in between the outside of the shell 9 and the master die 1 , as a consequence of which the shell 9 adopts the shape of the cavity 3.

In step 107, the particulate matter 13 is introduced into the shell 9 through the feed hole 1 1 . Steps 105 and 107 may also be conducted in reverse order, in which in step 105’ the shell would be pre-filled with particulate matter 13 and then introduced into the master die 1 , or be filled with particulate matter 13 after having been introduced into the master die 1 in step 103. The negative pressure NP1 would be applied afterwards in step 107’.

After the particulate matter 13 has been introduced into the shell 9, and the first negative pressure NP1 has been applied, the particulate matter 13 preferably is compacted in a next method step 109 by applying mechanical force onto the particulate matter 13 with a push rod 15.

Preferably, the push rod 15 comprises an evacuation port 17 for withdrawing air from the inside of the shell 9.

In step 1 1 1 , a second negative pressure NP2 is applied inside the shell 9, leading to a solidification of the particulate matter 13. As soon as the predetermined negative pressure NP2 is reached, or is at least lower than the first negative pressure NP1 , the first negative pressure NP1 may be released. The particulate matter 13 will maintain its shape due to the application of the second negative pressure NP2 and the mold core 19 has been produced.

While maintaining the second negative pressure NP2, the master die 1 is then removed in a next method step 1 13, and the mold core 19 may be provided in a method 200, 200’ of producing the composite spring, as is exemplarily described in the ensuing figures 2 and 3.

Fig. 2 exemplarily shows a first preferred embodiment of producing a composite spring with a mold core 19 as obtained in the method 100 shown in Fig. 1 . In step 201 , the mold core 19 obtained in method 100 is provided.

In step 203, a composite wire 21 for producing the composite spring is provided.

In step 205, the composite wire 21 is wound around the mold core 19.

In step 207, the composite wire 21 is solidified, for example by applying energy with an energy source 23. After the composite wire 21 has solidified to a predetermined sufficient rate, the particulate matter 13 inside the shell 9 is softened by releasing the negative pressure NP2 maintained previously inside the shell 9, and what previously was the mold core 19 can easily be removed from the assembly. This is shown in step 209.

In method step 21 1 , the composite spring is obtained. Optionally, a step 213, the still particle-filled shell 9 is reused in the method 100 for producing the mold core 19 anew by placing the shell 9 inside the master die 1 again. In the manner described hereinabove for Fig. 1 , the shell 9 is made to adopt the shape of the master die 1 , and the particulate matter 13 is solidified to obtain once again the mold core 19 which can then be provided for another round of producing a composite spring in the method 200.

The method shown in Fig. 2 makes do without resorting to an external mold for solidifying the composite wire 21 . However, depending on the shape requirements and complexity of the composite spring to-be-produced, an external mold 25 may be employed, which is shown in Fig. 3 with exemplary preferred method 200’. Method 200’ is largely similar to method 200. In addition to method 200, in method step 202, an external mold 25 is provided. The pre-assembly of mold core 19 and composite wire 21 is assembled in step 205” with the external mold 25 such that the composite wire 21 is disposed within a volume between the mold core 19 and the external mold 25.

In a first preferred variant of method 200’, the solidification of the composite wire 21 is carried out in step 207’ with both the mold core 19 and the external mold 25 being assembled.

In step 208, the negative pressure inside the shell 9 is released at the particulate matter 13 softened. In step 209’, the shell 9, still filled with particulate matter 13, can easily be removed from the composite wire 21 and external mold 25. If the solidification process according to step 207’ has already taken place, the composite spring would now be obtained.

In a second preferred alternative of method 200’, it is also possible to remove the shell 9 and particulate matter 13 prior to the solidification step 207” and solidify the composite wire 21 while being held only by the external mold 25.

After removing the composite wire 21 from the external mold 25, in method step 21 1 , the composite spring can be withdrawn from the procedure, while in method step 215, the external mold 215 can be reused in the method 200’ for producing the next composite spring.

Like in method 200, the shell 9 filled with particulate matter 13 may be reused in method step 213 to reproduce the mold core 19. One advantage of removing the particulate matter from the external mold 25 prior to the solidification step 207” is that the particulate matter 13 is not subjected to the solidification energy, e.g. high temperatures, which increases the longevity of the shell 9 and the particulate matter 13.

List of reference signs:

I master die

1 a, b first and second segment 3 cavity

5 protrusions

9 shell

I I feed hole

13 particulate matter 15 push rod

17 evacuation port

19 mold core

21 composite wire

23 energy source

25 external mould

100 method

101 method step

103 method step

105, 105’ method steps 107, 107’ method steps

I I I method step

1 13 method step

200’ method

201 method step

202 method step

203 method step

205 method step

207” method step

208 method step

209 method step

209’ method step

21 1 method step

213 method step

215 external mold

217 method step

NP1 (first) negative pressure

NP2 second negative pressure

Claims

Claims:

1. A method of producing a mold core having at least one external groove for making composite springs, in particular composite coil springs, comprising the steps of:

Providing a master die, the master die having an inner shape that defines a negative shape of the mold core,

Providing a gas tight flexible shell having at least one feed opening,

2. The method of claim 1 ,

wherein the step of assembling the die and shell is conducted prior to introducing the particulate matter in to the shell.

3. The method of claim 1 or 2,

comprising the step of applying a positive pressure differential between the inside of the shell and the outside of the shell, the pressure outside the shell being lower than inside the shell, preferably, wherein the pressure differential is produced by applying a negative pressure on the outside of the shell, said negative pressure on the outside of the shell is a first negative pressure, and the negative pressure on the inside of the shell is a second negative pressure, and the second negative pressure is lower than the first negative pressure, wherein further preferably, the first negative pressure is applied before the second negative pressure is applied, and released when the pressure inside the shell reaches the second negative pressure.

4. The method of any one of the preceding claims,

wherein the step of introducing the particulate matter into the shell further comprises compacting the particulate matter, preferably by using at least one of gravity, vibration, or mechanical force, wherein preferably, the compacting is effected by applying a mechanical compression force onto the particulate matter, preferably with a push rod from the side of the feed hole, and further preferably maintaining the compression force until the evacuation of the shell commences.

5. The method of any one of the preceding claims,

wherein liquid material is introduced together with the particulate matter into the shell.

6. The method of any one of the preceding claims,

wherein the master die comprises a plurality of die segments that together define the master die, and the shape that is impressed onto the shell.

7. The method of any one of the preceding claims,

wherein the master die comprises or consists of a polymer material, preferably produced by 3D printing.

8. The method of any one of the preceding claims,

wherein the master die comprises an inside wall and a helical protrusion extending away from the wall, wherein the protrusion defines the corresponding helical external groove in the mold core, and said protrusion preferably has a height with respect to the inside wall and a width, wherein further preferably, the height is more than half the width such as to give the composite wire an axially flattened cross section.

9. The method of any one of the preceding claims,

wherein the shell comprises at least one layer of a polymeric foil, said foil having a thickness in a range of 5 pm to 1 mm, and preferably an elongation at break in a range of 10% to 200% and further preferably a thermal stability of more than 120°C.

10. The method of any one of the preceding claims,

wherein the particulate matter comprises or consists of particles that

have a bulk angle of 10 ° or more, and/or

have an aspherical surface, with a particle size of 0,5 to 5 mm, and/or

have an uneven size distribution, and/or

have a density of 3,0 g/cm³ or less, and/or

have a thermal stability of more than 120°C, and/or

have a thermal conductivity > 0,02 W/(m K).

1 1 . A method for producing a composite spring, in particular a composite coil spring, the method comprising the steps of:

Providing a mold core having an external groove,

- Providing a composite wire for the composite spring to be produced, Assembling the, mold core and composite wire such that the wire is disposed in the external groove of the mold core,

The step of providing the mold core encompasses the method of producing the mold core according to any one of the preceding claims,

wherein preferably, the composite wire is fixed to the particle filled shell core by means of an additional foil which is wound around the composite wire and the core to temporarily fix the wire to the core.

12. The method of claim 1 1 ,

further comprising the steps of

Providing an external mold, wherein the external mold and the mold core together define a volume in between them, said volume having the shape of the composite spring to be produced, and

Assembling the external mold, the mold core and the composite wire such that the wire is disposed in the volume between the external mold and the mold core; wherein the step of solidifying the composite wire is conducted after the assembly step, wherein the assembly is preferably effected as follows:

i) the composite wire is assembled to the mold core;

iii) thereafter, the mold core is removed from the assembly prior to the step of solidifying the composite wire.

13. The method of any one of claims 1 1 or 12,

wherein prior to removing the composite spring, the method comprises the step of:

fully or partially releasing the negative pressure inside the shell, and

separating the mold core from the assembly.

14. The method of any one of claims 1 1 or 13,

comprising the step of:

retaining the particulate matter in the shell after releasing the negative pressure.

15. The method of claim 14,

comprising the step of:

reusing the mold core after production of the composite spring in the method of producing a mold core of any one of claims 1 to 13 to create a new mold core, wherein the step of introducing particulate matter into the shell is omitted.