SCREW AND SCREW CONNECTION FOR COMPONENTS MADE OF FOAMED PLASTIC

Abstract

The present disclosure relates to a screw connection comprising a support component, an additional component and a screw to fasten the support component to the additional component. The support component has a tube made of thermoplastic foam to accommodate the screw in a friction and positive fit. The screw has a blunt-edged thread, enabling the screw to compress the foam in the tube as it is driven in. The ratio of the inner diameter of the tube to the diameter of the screw depends on the degree of foaming of the tube and decreases as the degree of foaming increases. In comparison to conventional screws, the threads of the screw of the present disclosure penetrate less deeply into the foam or they displace it, preventing damage to the tube while enabling a tight fit.

Claims

1-23. (canceled)

24. A screw for a screw connection with a support component made of thermoplastic foam and comprising a tube with a predetermined degree of foaming of at least 2%, the screw comprising: a screw head; and a screw shaft comprising a blunt-edged thread for compacting the foam, the thread having flank angles (w) of 30°-90°; wherein: the ratio of an inner diameter of the tube to an outer diameter of the screw shaft is based on the degree of foaming of the tube, the ratio for a tube having a first degree of foaming being greater than the ratio for a tube having a second degree of foaming, the second degree of foaming being greater than the first degree of foaming.

25. The screw according to claim 24, wherein the ratio is one of: greater than 0.8 with a degree of foaming of 2-5%; between 0.7 and 0.9 with a degree of foaming of 5-10%; or between 0.5 and 0.8 with a degree of foaming of greater than 10%.

26. The screw according to claim 24, wherein the thread has flank angles (w) of approximately 60°-90°.

27. The screw according to claim 24, wherein the screw shaft comprises a tip configured to puncture a bottom section of the tube.

28. The screw according to claim 27, wherein the thread comprises rounded crests at least in sections at a distance from the tip of the screw shaft.

29. The screw according to claim 24, wherein the screw shaft comprises thread pitches greater than 2 millimeters.

30. The screw according to claim 29, comprising a thread-cutting thread restricted to a segment near a tip of the screw shaft.

31. The screw according to claim 24, wherein the screw shaft comprises a blunt tip configured to be tightly fixed at a bottom of the tube when screwed in.

32. The screw according to claim 24, wherein the screw shaft comprises a tip and thread pitches, the thread pitches increasing in a direction along the thread toward the tip.

33. The screw according to claim 24, wherein the screw shaft comprises a tip, and a distance of a crest of the thread from a center axis of the screw decreases in a direction toward the tip.

34. The screw according to claim 24, wherein a profile of the thread varies in its geometrical form such that an angularity of the thread increases in a direction toward the screw head.

35. The screw according to claim 24, comprising one of a fixed or freely rotating washer having a diameter greater than a diameter of the screw head.

36. The screw according to claim 24, wherein the screw shaft has an outer diameter of 4 to 6 millimeters and a length of 12 to 20 millimeters.

37. The screw according to claim 24, wherein the thread has a height and a width, a ratio of the height to the width being less than 1.

38. The screw according to claim 24, wherein the thread comprises flanks, a thread root, and a rounded transition between the flanks and the thread root.

39. The screw according to claim 38, wherein the transition between the flanks and the thread root comprises segments.

40. The screw according to claim 39, wherein the transition comprises: a first segment comprising an outer thread area having flank angles of 30°-90°; a second segment between a first flank and the thread root, the second segment having a tangential transition with angles of 145°-170°; and a third segment at the thread root between the first flank and a second flank, the third segment having angles of 170°-180°.

41. A screw connection comprising: a support component made of thermoplastic foam, the support component comprising a tube with a predetermined degree of foaming of at least 2%; and a screw for connecting the support component to an additional component, the screw comprising a screw shaft having a blunt-edged thread for compacting the foam, the threads having flank angles (w) of 30°-90°; wherein the ratio of an inner diameter of the tube to an outer diameter of the screw shaft is based on the degree of foaming of the tube, the ratio for a tube having a first degree of foaming being greater than the ratio for a tube having a second degree of foaming, the second degree of foaming being greater than the first degree of foaming.

42. The screw connection according to claim 41, wherein the thermoplastic foam comprises one of a MuCell-processed foam or an integral foam injection molding processed foam.

43. The screw connection according to claim 42, wherein at least the tube comprises a compact outer layer with one of an integral foam or a microcellular foam.

44. The screw connection according to claim 42, wherein the tube comprises a compact layer having a side-wall thickness of 0.1 to 3 millimeters, and a bottom thickness of 0.1 to 3 millimeters.

45. The screw connection according to claim 42, wherein the tube comprises a compact layer having a side-wall thickness of 0.7 to 1.5 millimeters, and a bottom thickness of 0.7 to 1.5 millimeters.

46. The screw connection according to claim 42, wherein: the screw comprises a screw head having a base; the screw shaft comprises a tip; and the tube has an inner height smaller than a length of the screw from the tip to the base.

47. The screw connection according to claim 42, wherein the tube comprises a base, a mouth, and a central axis perpendicular to the mouth, the tube widening towards the mouth such that an angle perpendicular to the central axis lies between 0.5 and 5°.

48. A method of connecting a support component to an additional component, the method including the following steps: forming, using one of a chemical or physical foaming process, the support component with a tube made of thermoplastic foam; providing a screw having multiple blunt-edged threads on a shaft of the screw, the threads having flank angles (w) of 30°-90°, wherein: an outer diameter of the shaft is larger at least in parts than an inner diameter of the tube; and a ratio of the inner diameter of the tube to the outer diameter of the shaft is based on the degree of foaming, the ratio for a tube having a first degree of foaming being greater than the ratio for a tube having a second degree of foaming, the second degree of foaming being greater than the first degree of foaming; placing an additional component onto the tube; and screwing in the screw in a non-thread-rolling manner through the additional component and into the tube, wherein the foam of the tube is compacted as the screw is driven in.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG. 1 shows a cross-sectional view through a conventional prior art plastic tube of compact injection molded material with a thread-rolling screw;

[0037] FIG. 2 shows a cross-sectional view through an exemplary plastic tube produced using a MuCell® process;

[0038] FIG. 3 shows a cross-sectional view through an exemplary thread providing information on the profile of the thread;

[0039] FIG. 4 shows a cross-sectional view through an exemplary plastic tube produced using a MuCell® process;

[0040] FIGS. 5 and 6 show additional embodiments of the screw according to the present disclosure; and

[0041] FIGS. 7 to 9 show the interaction of the tube with various screws of the present disclosure.

DETAILED DESCRIPTION

[0042] FIG. 1 shows a lateral view of a section through a plastic tube of compact injection molding material, with a conventional thread-rolling screw. The screw includes a major diameter d.sub.1, a minor diameter d.sub.2, a pitch P, and a distance X.sub.max from the bottom of the screw to a first thread. When used in plastics, the known, sharp-edged screw damages the plastic on being screwed in, by rupturing the screw-in openings or by stress cracking. Additionally, relatively high torque is required to drive the screw.

[0043] In screw connections using MuCell® injection molded parts, the compact layer is destroyed by this type of screw. As a result, a conventional screw completely loses its hold in the remnants of the foam layer and can hardly be screwed in a second time.

[0044] FIG. 2 shows a lateral view of a section through a plastic tube 4 produced using a MuCell® or other similar process, with a screw 3 according to the present disclosure. The outer shape of the screw 3 may be conical. Typically, the screw has a major diameter measuring 4, 4.5, 5, 5.5 or 6 mm, and a screw length of 12 to 20 mm.

[0045] The tube 4 consists of an integral foam or a microcellular foam with a compact outer layer (i.e. compact layer 8). Giving the screw 3 a profile with blunt thread crests 13 and rounded thread roots (such as profile shown in FIG. 4) enables the compact outer layer of thermoplastic foam to closely adapt its shape, by plastic deformation and friction, to the screw as it is being screwed in, without the outer layer being cut. The material displacement or compaction is illustrated in FIG. 2. The compact layer 8 is not ruptured, but rather is merely pressed into the softer inner layers of the tube 4. These layers, with a proportion of foam of 2 to 5%, possess a certain degree of elasticity, such that after the screw 3 is removed the “thread” reverts to its original form. Cracking or rupturing of the tube 4 may therefore be avoided. This is aided by a thread angle (w) of the thread 5 being obtuse, for example 110°.

[0046] For example, the transition between the flanks and the thread root here takes place section by section. As the first segment (flank 23), the outer thread area in FIG. 3 has flank angles of 30°-90°, or flank angles greater than 60°. The transition to the second section or segment 25 can be a clean break. The second segment 25 in the transition between the flank 23 and the thread root 24 represents a tangential transition with angles w2 of 145°-170°. The third segment encompasses the thread root 24 between two flanks 23 and represents a segment of a circle with angles w3 of 170°-180°.

[0047] FIG. 4 shows another embodiment, a section through a support component 2 with a plastic tube 4 produced using a MuCell® process or a similar process, during the screw-in operation. The tube 4 shown represents a stress-relief tube that is fundamentally smaller than a compact tube. For the screw 3 of the present disclosure, the ratio of the inner diameter 26 (as shown in FIG. 7) of the tube 4 to the outer diameter 15 (as shown in FIG. 5) of the screw 3 at the selected degree of foaming of 2-5% amounts to 0.85 to 0.95 and is thus considerably larger than that of conventional screws with a ratio of 0.8. If the degree of foaming is increased, the ratio becomes smaller. The material of the tube 4 situated “behind” the compact layer 8 is softer and generates a low counteracting force. Therefore, the screw 3 will have to penetrate farther into the tube 4 in order to create equal torque. If glass fibers or other reinforcing elements are injected into the plastic of the tube 4 or support component 2 with the same degree of foaming, then the linkage increases, the tube 4 becomes more stable (in spite of the higher degree of foaming), and the ratio may be enlarged again. A large ratio (approaching 1) enables less material to be required for the same screw.

[0048] When it is screwed in, the screw 3 presses itself into the outer layer of the tube 4 and penetrates the tube bottom 33. The tube 4 is slightly open such that the tube side walls are angled. The opening angle a of the tube sidewalls is 3-5° in relation to a central axis of the tube (where the central axis is perpendicular to the tube bottom 33). Similar to an insertion guide this makes it easier to set the screw 3 in the tube 4. The torque exerted on driving in the screw increases through the angle a.

[0049] FIGS. 5 and 6 are schematic views of two exemplary variants of the screw 3, each having a screw head 22 connected to a screw shaft. The screw 3 according to FIG. 5 illustrates the segmentation of the threads. The flank rises in two steps 23, 25 from a thread root 24. As shown in FIG. 5, the pitch 14 of the threads may vary. The pitch 14 in FIG. 5, for example, decreases in a direction towards the screw head 22, with the result that the torque increases as the screwing-in operation nears completion.

[0050] In contrast, the screw in the embodiment shown in FIG. 6 shows that the distance 18 between the crest 13 of the thread and a center axis m of the screw shaft may vary. Increasing the distance 18 as the thread approaches the screw head 22 enables an increase in torque as the screwing-in operation nears completion.

[0051] FIG. 7 shows the tube 4 of the support component 2 prior to the screwing-in operation. An additional component 6 is placed on the tube 4 and is intended to be connected to the support component 2 to form a screw connection. The tube 4 has a height 30 reaching from its mouth 32 to its bottom 33. The mouth 32 is larger than the inner diameter 26 of the tube 4 (as measured approximately midway of the height 30). This is based on an opening angle a in the range of 3-5°, as explained above.

[0052] FIGS. 8 and 9 show screws 3 with different lengths 34 and tips 20, 21. The tip of the screw may be blunt or sharply pointed. The tip 20 of the screw in FIG. 8 can have a short, pointed cutting portion that then transitions into a blunt shape. The transition may be smooth or abrupt. This tip 20, with a thread-cutting section, is adapted to puncture the bottom 33 (as shown in FIG. 7) of the tube 4 and to roll a thread into this bottom 33. For this purpose the screw 3 is longer (length 34 from the screw head 22 to the tip 20) than the height 30 of the tube 4.

[0053] The screw 3 of FIG. 9 may be shorter, but still slightly longer than the height 30 of the tube 4. When this screw 3 is driven in it will jam at the bottom 33 of the tube 4 to form a tight fit. This means that the tip 21 may be blunt and does not need to puncture the bottom. Nevertheless, the torque increases near the end of the screwing-in operation due to the jamming. FIG. 9 further shows that a freely rotating washer 7 is added at the screw head 22 to distribute the force over a larger area when the additional component 2 is fixed in place.

TABLE-US-00001 LIST OF REFERENCE NUMBERS screw connection 1 carrier/support component 2 screw 3 tube 4 threads 5 additional component 6 washer 7 compact/outer layer 8 crest of the thread 13 pitch 14 outer diameter of the screw 15 distance, crest to center axis 18 screw tip 20, 21 screw head 22 flank, first segment 23 thread root 24 second segment 25 inner diameter of the tube 26 height of the tube 30 bottom thickness 31 tube mouth 32 tube bottom 33 length of screw 34 angle of the tube side walls a thread width b thread height h center axis of the screw m thread flank angle w angles at thread segments w1, w2, w3