JP2006316818A - Self-tapping boss for resin molded housing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a synthetic resin self-tapping boss constructed into such a shape as to effectively prevent cracking caused by screwing without increasing a thickness and making the diameter of a lower hole of the self-tapping boss larger than the outer diameter of a self-tapping screw. <P>SOLUTION: The synthetic resin self-tapping boss wherein a male screw is screwed into a small-diameter lower hole by fastening the screw having the male screw has a reinforced shape at the front end. The reinforced shape at the front end of the self-tapping boss is concentrically cylindrical with respect to a cylindrical hole whose diameter is larger than that of the lower hole. As shown in Fig.11, the inner diameter of the concentrically cylindrical reinforced shape is larger than the outer diameter (d) of the self-tapping screw and smaller than the outer diameter (e) of a screw head. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a housing provided with a self-tapping boss shape that is screwed in by tightening a screw. Resin cracking (boss part cracking) when a self-tapping screw is screwed into a prepared hole of a resin housing boss. This relates to a resin crack prevention structure that can be prevented.

  The screw head 2 of the self-tapping screw 1 is tightened, and the self-tapping screw 1 is tightened while screwing into the inner surface of the pilot hole 11 of the self-tapping boss 10 of the housing H. Because the strength of the joint part (usually called the weld line) is lower than other parts, stress concentrates on the joint part of the resin flow during screw fastening, and the inner diameter part (the inner surface of the small hole) ) Begins to crack, and the crack progresses to the outer diameter portion (outer surface) and the self-tapping boss breaks and breaks.

  As countermeasures against resin cracks in the self-tapping boss, the difference between the outer diameter of the self-tapping screw and the pilot hole diameter of the self-tapping boss is reduced, and the depth of penetration into the inner surface of the screw hole is reduced to reduce the stress. Therefore, a measure is taken to increase the pilot hole diameter (boss inner diameter) (see FIG. 5), or the strength of the self-tapping boss 10 is increased, and therefore the resin wall thickness t is increased. Changes in the structure of the self-tapping boss have been made, such as increasing the outer diameter of the self-tapping boss.

  However, increasing the pilot hole diameter of the self-tapping boss reduces the engagement of the thread formed on the inner surface of the self-tapping boss (see FIG. 5), and as a result, the boss This is not an effective measure because the internal thread will be damaged.

  Further, when the thickness t of the self-tapping boss is increased to increase the outer diameter of the boss, as the thickness increases, sink marks (recesses due to shrinkage when the resin is cooled) are formed on the back side of the self-tapping boss. It tends to occur (see FIG. 6). When the back side of the self-tapping boss 10 is an appearance portion, the occurrence of this sinking greatly damages the appearance. Therefore, this measure is not an appropriate measure because it causes another serious problem.

  Further, as a means for complementing (or reinforcing) the inner surface of the boss for self-tapping when thread breakage occurs due to the enlargement of the pilot hole diameter, the screw is tightened after applying an adhesive to the self-tapping screw, and the screw And a self-tapping boss are adhered to each other to reinforce the connection (Japanese Patent Laid-Open No. 2004-122511). In the case of this method, the work man-hours and the material cost and management man-hours of the adhesive increase.

Further, as a means for complementing the thread breakage due to the expansion of the inner diameter, a so-called C surface (conical surface with an inclination angle of 45 degrees) or counterbore is provided at the tip of the prepared hole of the self-tapping boss, and the self-tapping screw is screwed in. There is a method for preventing stress from occurring at the tip of the self-tapping boss, thereby preventing resin cracking. However, since there is a restriction on the shape of the ejector sleeve 21 used when forming the shape of the upper end portion of the self-tap boss with the mold (the thickness of the ejector sleeve is reduced, which causes a decrease in the mold strength). The C-plane or counterbore cannot be taken too large (see FIG. 7), and thus a significant effect cannot be obtained.
That is, as shown in FIG. 7, the inner diameter pin 20 is thickened, the ejector sleeve 21 is thinned, a counterbore 10a is formed at the tip of the self-tapping boss 10, and the thickness of the outer end 10e is set to the ejector sleeve. The outer end 10e cannot be made too thin as described above, and as a result, the height of the counterbore cannot be made too high.

Further, with only the counterbore, the bottom becomes flat, so that there is no guide shape for guiding the tip of the self-tap screw 1, and a collision with the counterbore portion occurs when the self-tap screw is screwed in (see FIG. 8). No. 2002-118371).
In addition, the size of the self-tapping boss 10 is limited due to spatial restrictions due to the dense arrangement of parts and devices, and restrictions due to other requirements, such as the boss outer diameter and inner diameter, the C surface dimension, and the counterbore dimension. Since the thickness is naturally determined by the thickness of the parts to be installed, the dimensions of each part cannot be freely changed to increase the strength.

Further, when the self-tapping screw is screwed into the pilot hole of the self-tapping boss, the inner surface thereof is scraped off by the thread of the self-tapping screw to generate chips. When the self-tapping screw is repeatedly screwed and removed, chips are interposed in the screwing surface and damage the screwing surface. In order to reduce such damage to the inner surface of the pilot hole as much as possible, a vertical groove is provided in the inner surface of the lower hole of the self-tap boss, and the vertical groove is used to capture and collect chips on the inner surface of the boss. There is also a conventional technique for reducing damage to the screwed surface by chips without interposing chips on the surface (Japanese Patent Laid-Open No. 11-93928).
JP 2004-122511 A JP 2002-118371 A Japanese Patent Laid-Open No. 11-93928

In order to prevent resin cracking and screw thread damage, after confirming the strength of the joint of the resin flow due to the finished product, evaluate the shape to satisfy the restrictions on the inner and outer diameter dimensions of the self-tapping boss. Need to be performed. Also, the addition of additional work such as bonding increases the work and management man-hours.
Accordingly, the present invention provides a self-tapping boss that is not cracked by screw tightening without increasing its thickness and without increasing the pilot hole diameter of the self-tapping boss with respect to the outer diameter of the self-tapping screw. It is an object of the present invention to devise a shape structure of a self-tap boss that can effectively prevent the above.

[Solution 1] (corresponding to claim 1)
Means 1 for solving the above-mentioned problem is that a self-tapping boss made of synthetic resin in which the male screw part is screwed into a pilot hole having a small diameter by tightening a screw having a male screw part, has a reinforcing shape at the tip thereof. It is to have.
The above-mentioned “reinforcement shape” means a shape structure in which the tip of the self-tapping boss is not screwed with the self-tapping screw, and stress due to screwing with the self-tapping screw does not occur at the tip of the self-tapping boss. To do.
[Solution 2] (corresponding to claim 2)
Means 2 for solving the above-mentioned problem is that the reinforcing shape of the tip of the resin-molded self-tapping boss has a cylindrical hole having a diameter larger than that of the pilot hole. It is a concentric cylindrical reinforcing shape.
[Solution 3] (Corresponding to Claim 3)
Means 3 for solving the above-mentioned problem is that the inner diameter of the concentric cylindrical reinforcing shape of the self-tapping boss of the solving means 2 is larger than the outer diameter (d) of the self-tapping screw, and the outer diameter of the screw head. It is smaller than (e).

The self-tapping bosses of Solution 1 (Claim 1), Solution 2 (Claim 2), and Solution 3 (Claim 3) are greatly improved in strength by adding a simple reinforcing shape. It is. The principle and reason are as follows.
When the self-tapping screw is screwed into the self-tapping boss, the internal stress in the expansion direction acts on the self-tapping boss. On the other hand, the tip of the boss against the internal stress in the expansion direction acting on the self-tapping boss. The part is the weakest and the stress concentrates on this part and breaks, but the tip is provided with a reinforcing shape, and the self-tapping screw is not screwed into this part. Stress does not work.
As described above, since the tip of the self-tapping boss is unlikely to crack when the self-tapping screw is screwed in, the self-tapping boss of the present invention having the reinforcing shape as a whole is resistant to breakage due to cracking. And extremely strong.
And since the said simple reinforcement shape is shape | molded by adding a simple remodeling change to the injection mold of the self-tap boss | hub, the said reinforcement shape can be added by a simple change of a metal mold | die.
The mold structure according to the sixth aspect forms the reinforcement shape by the inner diameter pin and the ejector sleeve, and the reinforcement shape can be formed by a very simple change of the mold structure without impairing the mold strength. In addition, modification of the mold for forming the reinforcing shape can be dealt with by a simple method in which the ejector sleeve is cut to be a little thinner and a part thereof is cut to be a little shorter.

Next, Example 1 and Example 2 will be described with reference to the drawings.
Since the strength of the self-tapping boss due to the mechanical strength of the material is inversely proportional to the size of the cross-sectional area of the stressed portion, increasing the cross-sectional area of the target portion leads to an increase in strength. A portion with an inner diameter larger than the outer diameter of the self-tapping screw is provided at the tip of the prepared hole for the self-tapping boss made of synthetic resin formed integrally with the housing, and the longitudinal cross-sectional area of this portion is increased to increase the screw. It becomes the reinforcing shape of the self-tap boss at the time of fastening, and the occurrence of cracks can be prevented. In addition, even after the lack of structural strength at the resin flow joint (usually referred to as a weld line) has been confirmed, it is easy to make a self-tapping boss by simply changing the shape of the injection mold. It can be reinforced by adding a reinforcing shape to greatly improve its strength.

[Example 1]
FIG. 9 shows the self-tapping boss having the reinforcing shape of the present invention, but the basic structure and material thereof are the same as the conventional one, and only the point that the boss is provided with the reinforcing shape of the present invention. Is different from the conventional one. This example is an injection-molded product made of a synthetic resin such as PS resin or ABS resin.
In the structure shown in FIG. 9, A is the height of the reinforcing shape, B is the inner diameter of the self-tapping boss, C is the inner diameter of the reinforcing shape, D is the outer diameter of the self-tapping boss, and E is the self-tapping boss. This is the thickness of the part.
This boss was designed with a plan that a self-tapping screw having an outer diameter of dmm is screwed into a pilot hole of the inner diameter B. Desirable dimensional relationships of the respective parts are as follows.
B: outer diameter d- (0.5 to 0.7) mm of self-tapping screw
C: Self-tapping screw outer diameter d + (0.1-0.2) mm
D: About 0.5 to 0.7 times of E (where E is the thickness of the resin wall of the housing H from which the boss protrudes).

In the above dimensional range, the stress at the threaded portion of the self-tap screw on the inner surface of the boss correlates with the height A of the reinforcing shape, and the correlation is as shown in FIG.
The horizontal axis of the graph in FIG. 10 represents the dimension A, and the vertical axis represents the stress at the threaded portion of the self-tap screw on the inner surface of the boss. When the dimension A is in the range of 0 to 1.0 mm, the stress decreases almost linearly. When the dimension A is 1.0 mm or more, the stress becomes almost constant. From this, the following can be understood.
That is, the length of the self-tapping boss is preferably the minimum that can accommodate the length of the self-tapping screw to be used, and the length of the reinforcing shape (dimension A) is the limit that ensures a sufficient reinforcing effect. It is desirable to stop.
From the above, the height h of the reinforcing shape is x2 to x3, and may be at most x4 even if safety is expected. However, x2 is 0.5 mm, x3 is 1.0 mm, and x4 is 1.5 mm.

FIG. 11 shows an example in which components such as a printed circuit board are detachably attached to a self-tapping boss of a housing of an image forming apparatus such as a printer using a self-tapping screw.
The self-tapping boss 10 of this example is made of ABS resin, and its outer diameter is 6.0 mm. The height h of the reinforcing shape 12 of the large-diameter hole (corresponding to the height A in FIG. 9) is 1.0 mm. is there. Specific dimensions of other parts shown in the drawings are as follows.
a: 4.0 mm
b: 3.5 mm
c: 2.5 mm
d: 3.0 mm
e: 7.0 mm
The stress of this specific example (stress value on the vertical axis in FIG. 10) is about 1.5 times the stress of the comparative example (without the reinforcing shape in FIG. 11).

[Example 2]
The second embodiment is a modification of the above solution and embodiment, but there is no difference in the principle of solving the problem, and the reinforcing shape 30 is a conical hole (tapered hole), and the pilot hole 11 ( The same as the pilot hole 11 in the example of FIG. 11), in which a self-tapping screw having an outer diameter of dmm is screwed, the tip of the pilot hole 11 is a conical hole. The tapered hole has an inner diameter of bmm and a height k of approximately 1.5 mm. In the range of k / 2 from the tip of the tapered surface 31, the stress due to screwing of the thread of the self-tapping screw into the tapered surface 31 is almost zero.

These are sectional drawings which show the reinforcement shape of the boss | hub for self taps of this invention. These are sectional drawings which show the principal part of the metal mold | die which shape | molds the reinforcement structure of the Example of FIG. FIG. 3 is a cross-sectional view of a conventional self-tapping boss. These are sectional drawings which show the general attachment structure by the boss | hub for the conventional self tap, and a self tap screw. These are sectional drawings which show the attachment structure by the boss | hub for conventional self taps, and the small diameter self tap screw. These are sectional drawings which show the attachment structure by the boss | hub for self-tapping and the self-tapping screw. FIG. 3 is a cross-sectional view showing a main part of a mold for forming a conventional self-tapping boss upper portion in which a so-called C-plane (conical surface with an inclination angle of 45 degrees) or counterbore is provided at the tip of a pilot hole for a self-tapping boss. . FIG. 5 is a cross-sectional view exaggerating inconveniences that occur when a self-tapping screw is screwed into a self-tapping boss having only a counterbore provided at the tip. These are sectional drawings which show the division | segmentation relationship of a reinforcement shape to the boss | hub for self taps of this invention. These are the figures which show the correlation with the magnitude | size of the height dimension A of the reinforcement shape, and the stress in the self tap screw screwing part of the inner surface of the boss | hub for self taps. These are sectional drawings which show the attachment structure by the boss | hub for self taps, and a self tap screw. These are sectional drawings of Example 2. FIG.

Explanation of symbols

1: Self-tapping screw 2: Screw head 10: Self-tapping boss 11: Pilot hole 12, 30: Reinforcing shape 20: Inner pin 21: Ejector sleeve t: Self-tapping boss thickness A: Reinforcing shape height B : Inner diameter C of the prepared hole C: Diameter of the reinforced large diameter hole D: Outer diameter of the self-tapping boss H: Housing E: Wall thickness of the housing

Claims (8)

  A self-tapping boss made of a rigid resin, wherein the male screw part is screwed into a pilot hole having a small diameter by tightening a screw having a male screw part, and has a reinforcing shape at a tip part thereof.   The self-tapping reinforced shape according to claim 1, wherein the reinforcing shape of the tip of the resin-molded self-tapping boss is a concentric cylindrical reinforcing shape having a cylindrical hole larger in diameter than the pilot hole. boss.   The self-tapping boss according to claim 2, wherein an inner diameter of the concentric cylindrical reinforcing shape is larger than an outer diameter (d) of the self-tapping screw and smaller than an outer diameter (e) of the screw head.   The self-tapping boss according to claim 2, wherein the height of the reinforcing shape is 1.0 to 1.5 mm.   5. An electronic component mounting structure in which a self-tapping screw is screwed into the self-tapping boss of claim 2 and the electronic component is detachably fixed to a casing.   A mold structure comprising an inner diameter pin and an ejector sleeve for forming the boss shape.   The reinforcing shape of the tip of the resin-molded self-tapping boss is a reinforcing shape with a conical inner surface whose tip is larger than the outer diameter (d) of the self-tapping screw, and the height of the reinforcing shape is 1.0 to The self-tapping boss according to claim 2, wherein the boss is 1.5 mm.   The boss for self-tapping according to claim 2, wherein a conical portion made of counterbore is provided between a cylindrical hole having a diameter larger than that of the prepared hole and the prepared hole.

Images