Method of manufacturing a composite part comprising a core and UHMWPE skin regions

Abstract

A method of manufacturing a composite part comprising a core and at least one skin region formed of a low friction UHMWPE skin polymer attached thereto, by: a) providing a mold with a heatable mold cavity; b) loading into the mold cavity UHMWPE powder followed by a core element having a surface with at least one contacting region provided with a plurality of anchoring sites, loading onto the core element, a layer of UHMWPE in powder form adjacent the contacting region, and applying a heat pressing step to melt the skin polymer powder to form a molten skin polymer matrix, and cooling to solidify the skin polymer matrix forming a skin region mechanically engaged into anchoring sites of the core element.

Claims

1. A method of manufacturing a composite part comprising a core, and two skins formed of a low-friction UHMWPE thermoplastic polymer with a single elevated temperature high pressure consolidation, the method comprising the following steps: a) providing a mold having upper and lower surfaces; b) producing a consolidatable fiber reinforced layup by the steps of b)i) introducing UHMWPE powder onto the lower surface of the mold; b)ii) introducing a lower nonwoven fleece core element with surfaces each having at least one contacting region comprising a plurality of anchoring sites, onto the UHMWPE powder introduced in step b)i); b)iii) optionally introducing one or more woven or non-woven fiber reinforcing layers containing glass fibers, basalt fibers, carbon fibers, aramid fibers, and mixtures thereof, as reinforcing fibers, wherein the one or more woven or non-woven fiber reinforcing layers further optionally comprise one or more thermoplastic fibers which melt and penetrate recesses and cavities of the one or more woven or non-woven reinforcing layers during consolidation in step c); b)iv) if at least one woven or non-woven fiber reinforcing layers has been added, selecting the one or more woven or non-woven fiber reinforcing layers such that an uppermost non-woven fleece with anchoring sites is present; b)v) introducing UHMWPE powder onto the nonwoven core element; c) consolidating the consolidatable fiber reinforced layup in the mold at an elevated temperature and a pressure of 20 to 60 bar to produce a consolidated fiber reinforced layup by causing UHMWPE to intermix with the anchoring sites; and d) cooling the consolidated fiber reinforced layup to solidify the skin polymer matrix to form a skin region mechanically engaged into the anchoring sites of the core element forming a composite part, and removing the composite part having a UHMWPE bottom skin and a UHMWPE top skin, wherein the lower non-woven fleece of step b) ii) and non-woven fleece with anchoring sites of step b)iv) both comprise thermoplastic fibers.

2. The method of claim 1, wherein the consolidatable layup is subjected to vibration prior to step c) to cause UHMWPE powder to migrate into cavities proximate anchoring sites.

3. The method of claim 1, wherein the UHMWPE skin polymer powder is present patchwise on at least one surface of the composite part.

4. The method of claim 1, wherein the nonwoven core element comprises a nonwoven fleece of thermoplastic fibers and reinforcement fibers, wherein the nonwoven fleece has cavernous surface structures which act as the anchoring sites.

5. The method of claim 4, wherein the nonwoven fleece comprises nonwoven thermoplastic fibers as reinforcing fibers.

6. The method of claim 5, wherein the thermoplastic fibers are selected from the group consisting of polyphenylene sulfide fibers, polyetherimide fibers, polyetherether ketone fibers, polypropylene fibers, and mixtures thereof.

7. The method of claim 4, wherein the nonwoven fleece comprises inorganic reinforcing fibers and thermoplastic fibers which become molten at the elevated temperature of the consolidation step c).

8. The method of claim 7, wherein the inorganic reinforcing fibers are selected from the group consisting of basalt fibers, carbon fibers, glass fibers, and mixtures thereof.

9. The method according to claim 1, wherein consolidation is carried out at a temperature of 190 to 230 C.

10. The method of claim 1, wherein at least one optional woven or non-woven fiber reinforcing layer is present, and comprises a woven or unidirectional fiber reinforcement layer.

11. The method of claim 10, where two woven or unidirectional fiber reinforcement layers are present, and a further nonwoven fleece layer lies between the two woven or unidirectional reinforcement layers.

12. The method of claim 10, wherein at least one reinforcement layer is selected from fabric, multiaxial stitch or unidirectional reinforcement.

13. A method of manufacturing a composite part comprising a core and two skin regions formed of a low-friction UHMWPE thermoplastic polymer, the method comprising the following steps: a) providing a mold with a heatable mold cavity; b) loading into the mold cavity UHMWPE powder, followed by loading a core element which is a solid or hollow body having a surface with at least one contacting region provided with a plurality of anchoring sites which are recesses, rims or under-cuts in the solid or hollow body, and loading into the mold cavity UHMWPE powder over the core element, embedding the anchoring sites; c) applying a heat pressing step whereby the UHMWPE powder is melted to form a molten skin polymer matrix; d) cooling following heat pressing, whereby the melted UHMWPE solidifies to form a skin region mechanically engaged into the anchoring sites of the core element forming the core.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above mentioned and other features and objects of this invention and the manner of achieving them will become more apparent and this invention itself will be better understood by reference to the following description of various embodiments of this invention taken in conjunction with the accompanying drawings, wherein:

(2) FIGS. 1 to 4 show various embodiments of the invention as schematic vertical section views through the composite part, wherein in each figure, a) shows the situation before applying the skin polymer to the core element, b) shows the situation after applying the skin polymer and distributing the same within surface structures of the core element, and c) shows the situation after the heat pressing step, including an enlarged portion of a segment at the upper left of the composite part;

(3) FIGS. 5 to 7 shows vertical section views of further embodiments of the composite part;

(4) FIG. 8 shows a photographic reproduction of one embodiment of the composite part; and

(5) FIG. 9 shows an enlarged section (micrograph) of an edge region of the composite part of FIG. 8.

DETAILED DESCRIPTION

(6) It will be understood that the figures are not necessarily drawn to scale. In some instances, relative dimensions are substantially distorted for ease of visualization. Identical or corresponding features in the various figures will generally be denoted with the same reference numerals.

(7) In the following examples, UHMWPE was used as skin polymer, polypropylene was used as core thermoplastic polymer, and glass fibers were used as reinforcement fibers. As will be understood from the above, other suitable materials may be chosen. Depending on material choice, the temperature and possibly also the pressure applied in the heat pressing step may have to be adapted. For this purpose it may be appropriate to conduct a small series of test runs systematically changing temperature and pressure.

(8) In the example of FIG. 1, a first layer 2 of approximately 2 mm of UHMWPE in powder form is uniformly distributed on a lower mold plate 4 of a heatable mold cavity (not shown). Subsequently, a core element 6 formed of a lofty mat comprising a thermoplastic fleece 8 containing reinforcement fibers 10, e.g. with 20 to 80% by weight of randomly oriented polypropylene (PP) fibers and 80 to 20% by weight of glass fibers is placed onto the first layer 2 of skin polymer powder. Thereafter second layer 12 of approximately 2 mm of UHMWPE in powder form is uniformly distributed onto the top of the core element 6. As seen from FIG. 1b, at this point a fraction of the skin polymer powder has penetrated into interstitial regions between the various PP and glass fibers in the regions near the surface of the core element.

(9) The stacked arrangement thus prepared is then subjected to a heat pressing step by driving an upper mold plate 14 against lower mold plate 4 under appropriate heating. During this process, the UHMWPE powder is driven further into the core element and is caused to melt; simultaneously, the PP fibers of the core element are caused to also melt and the lofty fiber arrangement is compressed to some extent. After cooling down, the resulting composite part comprises a central core made up of glass fibers 10 and a solid PP core matrix 18, which is stacked be-tween a lower skin 20 and an upper skin 22 formed of a solid matrix of UWHMPE. As particularly seen from the enlarged section in FIG. 1c. the boundary region be-tween skin and core regions comprises an arrangement of geometrically inter-twined regions of solidified PP, solidified UHMWPE and glass fiber portions, thus providing an strong mechanical coupling of the various layers.

(10) The same principle is applied in the examples of FIGS. 2 and 3, and therefore only the respective difference will be discussed.

(11) In the example of FIG. 2, only an upper skin layer 12 is applied. Moreover, the core element 6 contains, in addition of PP fibers 8 and glass fibers 10, two internal reinforcement layers 24a and 24b. These reinforcement layers can be formed, e.g., of a reinforcement fabric or a multi-axial or unidirectional reinforcement layer. Such reinforcement layers are well known in the field of fiber reinforced thermoplastics. Alternatively, one could also use a plate-like solid element for reinforcement.

(12) Analogously to the first example, the heat pressing step leads to formation of a composite part, wherein the core 16 now contains the reinforcement layers 24a and 24b. Depending on the type of reinforcement element, molten PP is driven into recesses and cavities of the reinforcement element and provides mechanical connection to the core matrix.

(13) The example of FIG. 3 only differs from the example of FIG. 3 in that the upper skin 22 does not cover the entire uppers surface of core 16. Accordingly, the upper layer of UHMWPE powder is only applied as a patch 12a covering a part of the core element, and correspondingly, the upper skin is formed as a skin patch 22a. A substantially different embodiment is shown in FIG. 4. In this case, the core element 6 is constituted of a plate element 26 provided with a plurality of vertical passages 28 and a plurality of horizontal recesses 30 disposed in the lateral plate surface. After applying a first layer 2 of approximately 10 mm of UHMWPE powder, the core element 6 is placed thereon and then further UHMWPE powder is applied taking care to fill the vertical passages 28 and the horizontal recesses 30 and also forming an upper UHMWPE layer 12. After heat pressing and cooling, a composite part as shown if FIG. 5c is formed. A UHMWPE skin 32 completely sur-rounds the solid core 16, which is essentially identical to the originally inserted core element 6. Anchoring of the UHWMPE skin 32 to the core 16 is provided by the geometric structure formed by the vertical passages 28 and horizontal recesses 30.

(14) As already mentioned, various embodiments of the method and of the composite part are possible. FIG. 5 just shows one example of a composite part with a patchy UHMWPE skin, which can be shaped e.g. to provide low friction at selected, exposed regions 22a of the composite part. As shown in FIG. 6, a substantially solid core 16 with appropriate horizontal recesses 30 and vertical undercuts 34 can optionally contain an additional functional element 36. As shown in FIG. 7, the core 16 can be formed of a plurality of tubular elements 38 arranged substantially along a principal plane of the composite part. Mechanical anchoring between the skin matrix and the core is provided to some extent by the geometric arrangement of tubular elements 38. Nevertheless, it may be appropriate to provide further anchoring by applying a suitable adhesive on the surface of the tubular elements.

(15) The composite part illustrated in FIGS. 8 and 9 comprises an upper UHMWPE skin layer and a bottom UHMWPE skin layer each consolidated to a skin thickness of about 2 mm. The core layer comprises an upper and a lower PP-GF fleece with approximately 50% GF content and each about 1 mm thick, and a stack of reinforcement PP-GF fabric layers sandwiched therebetween. A heat pressing was carried out starting from 25 C. and rising up to 200 C. under 20 bar pressure, After reaching 200 C. the pressure and temperature were maintained for 10 min, followed by a cooling from 200 C. to 70 C. still under 20 bar pressure. As seen from FIG. 8 and particularly from FIG. 9, the UHMWPE skin (bottom part of FIG. 9) intimately embeds various protruding structures of the PP-GF fleece, which in turn is anchored into the PP-GF fabric layer acting as reinforcement.