JPH0363587B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the shape of a reinforcing material kneaded into a molding material for plastic parts for watches, particularly plastic base plates (hereinafter abbreviated as plastic base plates).

Traditionally, wristwatch base plates are made of metal materials such as brass, but because watch base plates require complex processing shapes, many manufacturing processes are required to manufacture such metal base plates. The accompanying requirement for a large amount of machinery, jigs, tools, personnel, and installation space has been a major impediment to manufacturing rationalization.

However, streamlining the main plate manufacturing process has been a long-held dream in the watch industry, and as a means of achieving this dream, we have replaced the metal main plate with thermoplastic, which has high productivity, as the main plate material.
A so-called plastic base plate has been proposed in which the complex shape of the base plate is manufactured using a single injection molding process. This plastic base plate was developed several years ago as a base plate for mechanical watches equipped with barrels and balances, but the plastic base plates at that time did not meet the durability and reliability requirements for watch base plates that were required to be thin. It did not reach the point where it became popular.

However, today, the form of watches has changed from mechanical watches to quartz watches that are equipped with crystal oscillators and ICs, display digital displays using liquid crystal cells, and use pulse motors to move the hands intermittently every second. The strength requirements for watch base plates have been relaxed, and the plastic materials themselves and molding techniques have improved dramatically compared to those days, both from a technical perspective, and from a commercial perspective, such as the demand for lower costs for crystal watches. Demand for plastic base plates has once again increased expectations.

Next, the improved molding material forming the background of the present invention will be explained with reference to the drawings.

In general, the properties required of molding materials for forming plastic base plates, etc. are sufficient mechanical strength and stable dimensional accuracy of molded products. As a countermeasure, so-called engineering plastics, which are made by kneading plastic molding materials with inert fibrous materials such as glass fibers and carbon fibers as reinforcing materials, have been adopted. Glass fiber is mainly used, and the plastic base plate in this embodiment is also made of engineering plastic containing glass fiber.

Figure 1 shows the outer diameter of the glass fiber in the above-mentioned glass fiber-containing engineering plastic.
This is a characteristic diagram showing the relationship between aspect ratio and tensile strength PKg/cm ² of the molded product. Since it does not function as a reinforcing material and becomes a mere filler, it has no effect on the tensile strength, but it is known that when the aspect ratio is above a certain value, the aspect ratio and the tensile strength are in a substantially proportional relationship.

Engineering plastics, which are commonly used today from small watch parts to large home appliances and automobile parts, have an outer diameter of 10 to 12 μm and a length of l.
Glass fibers with a shape of about 300 μm are uniformly kneaded, and the aspect ratio of this glass fiber is 25 to 30, so the tensile strength at this time is approximately 1300 to 1500 kg/kg from the characteristic diagram shown in Figure 1. It turns out that cm ² . On the other hand, it has been confirmed that a tensile strength of 1200 kg/cm ² is sufficient for the current plastic base plate for crystal watches.

Next, a plastic base plate molded from conventional engineering plastic will be explained with reference to drawings.
FIG. 2 is a cross-sectional view of the vicinity of the gate part of a plastic base plate according to a conventional example. Generally, the molding method for a plastic base plate is a gate part A (in this example, productivity is taken into account) provided in a molding die (not shown). After injecting the molten molding material and molding the plastic base plate 1, when opening the mold, the molding material remaining inside the gate is removed from the plastic base plate 1. Since the cutting is performed by tearing, a protruding gate cutting mark 2 remains at a position corresponding to the gate part A on the plastic base plate 1 after the molding.
This cutting mark 2 consists of a resin part 2a and a forest of glass fibers 2b standing on top of this resin part 2a.Generally, the height of the resin part 2a is about 50 μm, and the length of the glass fibers kneaded on top of this resin part 2a ( For conventional glass fibers, the height is 300 μm).

As is well known, in a device such as a wristwatch that has a detailed structure, leaving the protruding gate cutting marks 2 on the main plate surface as described above will only hinder the installation of other parts (not shown). First, glass fibers 2b that are broken due to contact with other parts may remain as dust in the watch movement, causing the watch to stop, so a Post-processing had to be added.

Therefore, as another method that takes into consideration the above drawbacks, we introduce the third method.
The method of providing a sunken gate as shown in the figure has been conventionally used. That is, the gate part 6 is formed so that the position of the gate part A of the molding die for molding the plastic base plate 5 is sunk below the component mounting surface 5a of the plastic base plate 5. As a result, the gate cutting marks 2 remaining on the gate part A after molding can be sunk by the depth of the gate sunken part 6 from the component mounting surface 5a of the plastic base plate 5, and the depth of this gate sunken part 6 can be made sufficiently. If it can be made larger, it is possible to eliminate the protrusion of the gate cutting mark 2 onto the component mounting surface 5a.

However, in the case of quartz watches that require good design, the thickness of the main plate must be reduced in order to achieve a thin design, and in most cases, the thickness near gate A is less than 1 mm. . Let us consider the conventional molding conditions for a generally used plastic base plate having a thickness t of 900 μm near the gate portion in the conventional example shown in FIG.

In general, current injection molding conditions for engineering plastics are such that when molten plastic is injected from gate A, as the thickness _t2 directly below the gate becomes thinner, the flow of the plastic deteriorates, and the molding pressure is not transmitted sufficiently. It is known that defects occur due to insufficient filling, and it has been confirmed that the thickness t ₂ directly under the gate portion is required to be at least 700 μm in order to ensure quality as a plastic base plate.

Therefore, in the plastic base plate 5 whose thickness t is 900 μm, if the thickness t ₂ directly below the gate part is secured to be 700 μm, then
The depth t ₁ of the gate sunken portion 6 is 200 μm. By the way, since the maximum length of the glass fiber is 150 μm, when the minimum base plate thickness is 850 μm, t ₁ ≧0.18t is sufficient, and the thickness of the plastic base plate 5 is 1 mm or less, and the thickness t ₂ directly under the gate part needs to be 700 μm. Therefore, t ₁ ≦0.3t, which is 0.18
It is preferable that ≦t ₁ ≦0.3t, but it is preferable that the minimum depth t ₁ be equal to or longer than the length of the glass fiber.

However, as explained in Figure 2, the height
Since the gate cutting mark 2 of 350 μm remains protruding at the position corresponding to the gate part A, the tips of the glass fibers 2b still standing in this gate cutting mark 2
150 μm protrudes onto the component mounting surface 5a,
As mentioned above, the glass fibers 2b break and remain as dust in the movement, causing problems such as the clock stopping. Also, if the thickness t near the gate is 1 mm, the gate sunken part 6 is 300 μm.
However, even in this case, the gate cutting trace 2
Since the height is 350 .mu.m, 50 .mu.m still protrudes from the component mounting surface 5a, resulting in a similar problem.

The present invention is intended to solve the above-mentioned drawbacks, and its purpose is to use reinforcing glass fibers kneaded into molding materials in the production of plastic parts for watches, such as plastic base plates with a thickness of 1 mm or less. The object of the present invention is to provide plastic parts for watches that have sufficient strength and reliability by appropriately selecting the shape of the plastic parts, and that do not require extra post-processing.

Hereinafter, specific examples of the present invention will be explained with reference to the drawings.

FIG. 4 is a sectional view of the vicinity of the gate portion of a plastic base plate showing an embodiment of the present invention. Structurally, it is the same as the plastic base plate shown in Fig. 3, and the plastic base plate 10
Thickness t = 900 μm, thickness directly below the gate portion t ₂ = 700 μm
m, and the depth t ₁ of the gate sinking portion 11 is 200 μm. In this embodiment, the plastic base plate 10 is made of engineering plastic kneaded with glass fiber having a diameter of 6 μm and a length of 130 μm as a reinforcing material. The height of the gate cutting mark 12 is 50 μm of the resin part 12a and the glass fiber 12b.
The value obtained by adding the length of 130 μm, that is, 180 μm, sinks into the gate sunken portion 11 having a depth of 200 μm, so that the influence on the component mounting surface 10a can be eliminated.

In order to confirm that this example satisfies the required tensile strength, we first examined the shape of glass fiber as a reinforcing material to satisfy the conditions of this example.
Since it is 1200Kg/cm ² , it is necessary to obtain an aspect ratio of 20 from Figure 1, and considering that the diameter of glass fiber that can currently be mass-produced from a processing technology perspective is 5μm, it has a depth of 200μm. The shape of the glass fiber that can be sunk into the gate sinking part 11 and obtain an aspect ratio of 20 is 100 μm in length if the diameter is 5 μm, and the length is the limit value of 150 μm.
m, the diameter is 7 μm, and it can be seen that glass fibers having a diameter of 5 to 7 μm and a length of 100 to 150 μm as in this example satisfy the conditions.

In addition, in this embodiment, the gate portion thickness t=
900 μm was adopted, but as in this example, when t = 900 μm, the gate cutting trace could be made to sink into the gate sinking just as much as possible, but when t = 900 μm,
At 1 mm, the gate can be sunk with some margin, and some variation in the length of the glass fibers can be absorbed, allowing a so-called margin.

In the above example, the diameter is 6 μm and the length is
Sufficient strength can be obtained by injection molding a plastic board with a thickness of less than 1 mm using engineering plastic kneaded with glass fibers of 130 μm, or aspect ratio of 21.7, and by shortening the glass fibers. , all the gate cutting traces sink into the gate sinking part, so there is no need for extra post-processing to remove them, and the glass fibers are shortened and the plastic flows better, making it less than 1 mm. Sufficient molding pressure was obtained for the molded product, and a high quality plastic base plate including appearance quality could be obtained.

Therefore, the present invention has been developed by appropriately selecting the shape of the glass fibers that serve as reinforcing materials for engineering plastics, thereby preventing the glass fibers from protruding from the sunken gate portions and becoming trash in conventional injection molding using engineering plastics. However, with the present invention, there is no need to worry about the glass fibers protruding from the sunken part of the gate, and therefore, the clock will not stop due to the generation of dust. This has the effect of making it possible to manufacture thin plastic parts for watches at high quality and at low cost, contributing to thinner crystal wristwatches and lower prices.

[Brief explanation of drawings]

Figure 1 is a characteristic diagram showing the relationship between the aspect ratio K and tensile strength P of glass fiber kneaded into plastic, and Figure 2 is the first conventional example, a partial cross section showing the gate cut marks on the plastic base plate. Figure 3 shows a second conventional example, and is a partial cross-sectional view showing the gate cut marks formed on the gate sunken part, and Figure 4 shows an embodiment of the present invention, which shows the gate cut marks formed on the gate sunken part in the plastic base plate. FIG. 3 is a partial cross-sectional view showing the gate cut marks. 1, 5, 10... Plastic base plate, 5a, 10a...
Parts mounting surface, 2, 12...Gate cutting trace, 2
a, 12a...Resin part, 2b, 12b...Glass fiber, 6, 11...Gate sunken part, A...Gate part.

Claims (1)

[Claims] 1 In plastic parts that are injection molded with thermoplastic resin mixed with glass fiber and made of a thin plate with a gate part thickness of 1 mm or less during injection molding, glass kneaded with the thermoplastic resin The fiber shape is 5 to 7 μm in diameter and 100 to 150 μm in length.
A plastic part for a watch, characterized in that the gate part is formed with a gate sunken part, and the depth of the gate sunken part is equal to or deeper than the length of the glass fiber.