GB2171979A

GB2171979A - Tape guide pole for cassettes

Info

Publication number: GB2171979A
Application number: GB08501074A
Authority: GB
Inventors: Cheuk Fai Ho
Original assignee: CASTFAST MAGNETICS MOULDING LI
Current assignee: CASTFAST MAGNETICS MOULDING LI
Priority date: 1985-01-16
Filing date: 1985-01-16
Publication date: 1986-09-10
Also published as: GB2171979B; GB8501074D0; DE3601007A1

Abstract

A tape guide pole (11), of plastics material and for use in a magnetic tape cassette, has an elongate curved surface portion (13) for contacting the tape, which curved surface portion (13) extends around the periphery of the pole through an arc at least as great as the included angle (sic) through which the path of the tape is guided. <IMAGE>

Description

SPECIFICATION Tape guide pole for cassettes The present invention relates to tape guide poles for use in cassettes for guiding the movement of the magnetic tape inside the cassette shell. More particularly but not exclusively, the invention relates to tape guide poles for video cassettes.

Known video cassettes generally make use of four tape guiding elements inside the cassette shell.

In order to obtain a satisfactory play back from from the video cassette, the surface finish of these tape guiding elements must be of a high quality.

Guide elements with rough surfaces not only increase the tape running friction which will cause unsteady tape feeding, but they will also damage the tape by scraping the magnetic particles off the surface. This happens in the case of tape guiding elements which are in contact with the coated side of the magnetic tape.

Tape guiding elements commonly used are of four types; a steel pole, a plastics pole, a steel pole with a plastics roller and a plastics pole with a plastics roller, These suffer from various disadvantages as follows.

Steel tape guide pole.

A well polished steel tape guide pole is a good element for tape guiding. However, this pole has to go through a series of expensive production processes. These are, cutting of a steel rod to the desired length, centreless grinding to remove rough surface flaws and give the required diameter and superfinishing to the required surface finish.

Furthermore, superfinished steel tape guide poles need extremely careful handling, as the mirror finish surface is vulnerable to scraping damage.

All these factors lead to a high production cost.

Plastics tape guide pole or steel tape guide poles with plastics roller.

With this design, the finish of the tape guide pole is not crucial to cassette performance. If a steel tape guide pole is used, expensive processes such as centreless grinding and superfinishing can be avoided. If a plastics tape guide pole is used, it can be integrated to the plastics cassette shell thus reducing the cost of this pole to zero. The plastics roller is produced by injection moulding. Using this design, the overall coast is lower than using steel tape guide poles.

In order to reduce the friction between the tape and the plastics roller, the roller is allowed to rotate on the tape guide pole while the magnetic tape is running. Due to rubbing of the roller with the tape guide pole and the plastics cassette shell, a squeaking noise is generated during fast forward and rewinding of the tape. Because of this noise, this type of tape guiding element is not acceptable in production of high grade video cassette tapes.

Round plastics tape guide pole.

Round plastics tape guide poles are produced by injection moulding. The material cost is low. Because all the round surface of the tape guide pole must be smooth, the cavity of the injection mould must be made as a deep hole. The dimensions of this hole are: Dia. 2.5mm and depth 20.5mm. To polish such a small deep hole is very difficult and only a highly equipped mould making workshop can fabricate it.

Where an injection moulding process is used, a rough mark will be generated at the gate where the molten plastics material enters the cavity in which the material is allowed to cool down and set to the final shape.

Because the whole round surface of the tape guide pole must be smooth, the gate must be located at either end of the pole. This causes a problem during injection moulding, for, if molten plastics material is injected directly into an open cavity in form of a jet or spiral, then surface marks or flaws tend to be produced in the final moulding.

This phenomenon is referred as jetting. As surface flaws caused by jetting are not acceptable on tape guide poles, more sophisticated injection moulding machines and process control equipment are necessary to eliminate this effect.

According to the present invention, there is provided a tape guide pole of plastics materials for use in a magnetic tape cassette which pole has an elongate curved surface portion for contacting the tape, and means for locating the pole in a predetermined orientation in a cassette such that, in use, the tape contacts only said elongate curved surface portion.

Preferably the curved surface extends around the periphery of the pole through an arc the angle of which is at least as great as the included angle through which the path of the tape is guided but not more than 180 degrees.

The invention further provides an irregular cross section plastic pole adapted to be used in a magnetic tape cassette for guiding the direction of movement of the magnetic tape inside the cassette shell the cross section of which comprises a round edge which makes an included angle greater than 58 degrees with the centre of the curve and an irregular edge which can be used for locating the pole in a definite position inside a magnetic tape cassette shell so that its round edge is always in contact with the magnetic tape.

Preferably the pole has an elongate hollow extending along at least a part of its length which hollow opens away from the curved surface portion.

The invention further provides a method of manufacturing a pole according to the invention comprising the steps of injection moulding plastics material into an elongate mould having a parting line extending longitudinally along the mould, one mould portion having a concave surface for shaping the curved surface of the pole, separating the mould portions, and removing the pole from the mould portion having said concave surface.

A preferred embodiment of a tape guide element according to the invention is now described by way of example with reference to the drawings in which: Figure 1 shows a typical video tape cassette with guide elements according to the prior art. A side panel is removed to reveal the spools, tape path and guide elements; Figure 2 shows another typical video tape cassette with guide elements according to the prior art; Figure 3 is-a side view of a tape guide element according to the invention; Figure 4 is a rear view of the element of Figure 3; and Figure 5 is a cross-section view on the lines A-A in Figure 3.

Referring to Figure 1, a video tape cassette for the "VHS" system is shown with four guide elements 1, 2, 3 and 4. These elements are of one or more of the type described above as prior art. An enlarged view of element 1 is shown in which the included angle through which tape 5 is diverted, is 57 degrees.

Figure 2 shows a video tape cassette for the "BETA" system with guide elements 6, 7, 8 and 9 of one or more of the type described above as prior art. The included angle for element 6 is 58 degrees. It is elements 1, 4, 6 and 9 which are contacted by the coated side of the magnetic tape.

Referring now to Figures 3, 4 and 5, a guide element according to a preferred embodiment of the invention is shown in the form of a pole 11 having a rounded surface or edge 13 extending along its length, about which surface 13 is smooth to 0.2mm maximum. The rounded surface 13 extends around an arc of 174 degrees-. The element is produced with a hollow 15 extending longitudinally and opening to a rear face 17 away from the smooth, rounded surface 13.

In use, the pole 11 is substituted in the cassette for elements 1, 4 contacting the magnetic coated side of the tape 5 in Figure 1, and for elements 6 and 9 in the embodiment of Figure 2. A suitable socket or locating device (not shown) is provided in the moulding of the cassette so as to orientate the pole 11 with its rounded surface 13 in contact with the tape 5. The surfaces other than the rounded surface 13 need not be totally free of flaws or roughnesses as these surfaces do not contact the tape. Thus the element can be injection moulded in a mould which parts along the edges of the rear face 17 and the only part of the mould that needs polishing is that which forms the rounded surface 13.

The pole 11 is made of POM acetal resin which has good frictional properties and high wear resistance.

The tape guide pole is made hollow for two reasons. First, the thickness can be kept uniform throughout the pole Uniform cooling down of the material causes less shrinkage distortion to the final moulding. Thus a higher degree of dimensional stability can be achieved.

Second, by making it hollow, a core pin is available inside the cavity to break the flow of the incoming material. In this way jetting can be eliminated.

The advantage of this embodiment of a guide pole according to the invention is that it overcomes the defect and weakness of the round plastic tape guide pole. When a round tape guide pole is fixed to a cassette shell, only a portion of its sur- face is in contact with the magnetic tape. The maximum included angle is 58 degrees#forVHSand 57 degrees for Beta. By making a tape guide pole with an irregular cross section which has a round edge making an included angle greater than 58 degrees and by providing a locating device which enables this tape guide pole to be fixed in a definite position in a cassette shell so that only its round edge is in contact with the magnetic tape, the function of this irregular cross section tape guide pole will be the same as a round tape guide pole.

This modification in shape greatly reduces the difficulties encountered both in making an injection mould and in the injection moulding process.

Since only the round portion of the tape guide pole need be free of marks and the# included angle made by the round edge is less than 180 degrees, it is possible to make the injection mould parting line parallel to the pole axis. By so doing, the portion of the injection mould needing polishing is in form of a shallow groove instead of a small deep hole. Thus mould fabrication cost is greatly reduced. This design also allows the gate to be located anywhere along the parting line of the mould. By directing the molten material against some obstruction, such as core pin, to break the flow, jetting can be eliminated. Perfect quality control is easier to achieve and reject rate can be kept to a low level.

It will of course be understood that the present invention has been described above purely by way of example and modifications of detail can be made within the scope of the invention as defined by the following Claims. It will be appreciated for example, that the angles of 57 degrees and 58 degrees mentioned arise from specific embodiments oof cassettes described; other cassettes may em- ploy angles of a different magnitude.

Claims

1. A tape guide pole of plastics material for use in a magnetic tape cassette which pole has an elongate curved surface portion for contacting the tape, and means for locating the pole in a-prede- termined orientation in a cassette such that in use, the tape contacts only said elongate curved surface portion.

2. A pole according to Claim 1 wherein the curved surface portion extends around the periphery of the pole through an are the angle of which is at least as great as the included-angle through which the path of the tape is guided but not more than 180 degrees.

3. An irregular cross section plastic pole adapted to be used in a magnetic tape cassette for guiding the direction of movement of the magnetic tape inside the cassette shell the cross section ofwhich comprises a round edge which makes an included angle greater than 58 degrees with the centre of the curve and an irregular edge which can be used for locating the-pole in a definite position inside a magnetic tape cassette shell so that its round edge is always in contact with the magnetic tape.

4. A pole according to any one of Claims 1 to 3 having an elongate hollow extending along at least a part of its length which hollow opens away from the curved surface portion.

5. A pole according to Claim 4 wherein the thickness of the plastics material is substantially uniform throughout the pole.

6. A pole according to any of Claims 1 to 5 wherein the plastics material is acetal resin.

7. A method of manufacturing a pole according to any one of Claims 1 to 6 comprises the steps of injection moulding plastics material into an elongate mould having a parting line extending longitudinally along the mould, one mould portion having a concave surface for shaping the curved surface of the pole, separating the mould portions, and removing the pole from the mould portion having said concave surface.

8. A method according to Claim 7 comprising the step of including a core pin in the mould to break the flow of the incoming material.

9. A method of manufacturing a tape guide pole substantially as hereinbefore described with reference to Figures 3,4 and 5 of the accompanying drawings.

9. A tape guide pole for use in magnetic tape cassettes substantially as hereinbefore described with reference to and as shown in Figures 3, 4 and 5 of the accompanying drawings.

10. A method of manufacturing a tape guide pole substantially as hereinbefore described with reference to Figures 3, 4 and 5 of the accompanying drawings.

Amendments to the claims have been filed, and have the following effect: (a) Claims 1 & 2 above have been deleted or textually amended.

(b) New or textually amended claims have been filed as foilows: (c) Claims 3 - 10 above have been re-numbered as 2 - 9 and their appendancies corrected.

1. A tape guide pole of plastics material for use in a magnetic tape cassette which poles has an elongate curved surface portion for contacting the tape, which curved surface portion extends around the periphery of the pole through an arc the angle of which is at least as great as the included angle through which the path of the tape is guided but not more than 180 degrees, and means for locating the pole in a predetermined orientation in a cassette such that, in use, the tape contacts only said elongate curved surface portion.

2. An irregular cross section plastic pole adapted to be used in a magnetic tape cassette for guiding the direction of movement of the magnetic tape inside the cassette shell the cross section of which comprises a round edge which makes an included angle greater than 57 degrees with the centre of the curve and an irrgular edge which can be used for locating the pole in a definite position inside a magnetic tape cassette shell so that its round edge is always in contact with the magnetic tape.

3. A pole according to either of Claims 1 or 2 having an elongate hollow extending along at least a part of its length which hollow opens away from the curved surface portion.

4. A pole according to Claim 3 wherein the thickness of the plastics material is substantially uniform throughout the pole.

5. A pole according to any of Claims 1 to 4 wherein the plastics material is acetal resin.

6. A method of manufacturing a pole according to any one of Claims 1 to 5 comprising the steps of injection moulding plastics material into an elongate mould having a parting line extending longitudinally along the mould, one mould portion having a concave surface for shaping the curved surface of the pole, separating the mould portions, and removing the pole from the mould portion having said concave surface.

7. A method according to Claim 6 comprising the step of including a core pin in the mould to break the flow of the incoming material.

8. A tape guide pole for use in magnetic tape cassettes substantially as hereinbefore described with reference to and as shown in Figures 3,4 and 5 of the accompanying drawings.