Cassette housing with integral tape guide

Housing for a magnetic tape cassette, and method of molding such a cassette. The housing is made from a molded plastic lower part and a molded plastic cover, the lower part including a tape guide unit extending between two true cylindrical tape guides and integral with the cassette locating surfaces.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to cassettes for magnetic tape used for recording and 
playback of information; and more particularly, to such cassettes having 
guides for maintaining accurate angular alignment of tape passing an open 
front region through which a magnetic head passes to contact to the tape, 
and to a method of manufacturing such a cassette. 
Cassettes of this general type are used on a large scale in apparatus for 
many uses, both in the consumer goods sector and for professional and 
business activity. A particular style of cassette with which the invention 
is especially useful is commonly known as the "compact cassette," 
"standard cassette" or "Philips cassette;" external dimensions and 
preformance characteristics of these cassettes are standardized through 
adherence to International Electrotechnical Commission publication no. 
94A. This cassette has established "support planes" defined by locating 
surfaces, from which critical dimensions for tape alignment are measured. 
Therefore, when designing high quality cassette apparatus, such as a 
high-fidelity audio recording and/or playback machine, customarily 
referred to as a tape recorder, for optimum results the recording/playback 
head is located accurately with respect to mounting surfaces which engage 
these locating surfaces. 
When relatively high frequencies are to be handled by a tape recorder, it 
is critical that the tape runs exactly perpendicular to the gap in the 
magnetic head. Deviation from perfect perpendicularity produces what is 
called, in professional jargon, azimuth error. In the case of the compact 
cassette, this error is observed as the result of misalignment between 
tape guides located inside the cassette, over which tape is stretched as 
it passes the openings of the front wall region, and locating surfaces on 
the lower exterior of the cassette housing. Of course, when the cassette 
is turned over to play or record the other tracks on the same run of tape, 
the locating surfaces for what had been the top of the cassette now become 
the reference points for establishing the support planes. 
Because of the vast quantities of compact cassettes which are made and 
sold, economy of construction is maximized wherever possible. This goal 
has dictated, for example, the use of relatively inexpensive low rigidity 
plastic materials for the cassette housing, which materials can usually be 
molded to produce interlocking shapes. As a result of this material 
choice, and the preference for simplest possible molds, azimuth error is 
an important source of limitation on the high frequency response of the 
cassette. 
2. Description of the Prior Art 
The original compact cassette is described in U.S. Pat. No. 3,394,899 and 
uses a housing made of two identical halves, each including one pair of 
locating surfaces and half of the guide surfaces. Each half is box-shaped 
and has a bottom with openings for the passage of reel drive spindles, 
cassette positioning pins, and a drive capstan. Each housing half has 
half-height side and rear walls. Along the front edge or wall a number of 
openings are produced, for the heads and pressure rollers to pass through. 
To guide a tape passing by the front openings, a tape guide unit must be 
provided. In the conventional two-part cassette housing, half-height 
guides project substantially transversely to the bottom (and the cover, 
which is simply "the bottom" of another half turned upside down). However, 
when molding such a housing half it is necessary to provide a taper or 
"draft" in those parts which extends substantially in the direction in 
which the mold is opened, so that the housings can be removed easily. 
Because of this draft the tape guides are not true cylindrical surfaces 
about axes perpendicular to the support planes of the cassette, but have 
more of a conical shape. Azimuth errors of plus or minus three minutes of 
arc are inevitable with this style of construction. 
In contradistinction to the conventional construction described above, U.S. 
Pat. No. 3,934,842 teaches a three-part housing, also made of moldable 
plastic parts. In this cassette the tape guide unit is a separate molding 
which can be made from a plastic having desirable low-friction properties. 
Further the tape guide unit is molded with the mold interface 
perpendicular to the bottom cassette wall, so that the sliding surfaces 
provided for tape guiding can be theoretically perpendicular to the 
support planes. This separate guide unit is then mounted in the box-shaped 
cassette part. However, the accuracy depends on the cumulative dimensional 
tolerances of the box-shaped outer part and the tape guide unit, and also 
on the tolerances observed in assembly per se. Moreover, the top and the 
bottom of the guide unit each are tapered to provide draft, so that these 
surfaces by which the tape guide unit is mounted in the cassette may again 
provide an oblique relationship of the tape guides relative to the bottom 
of the box-shaped part. This construction also has the disadvantage that 
rigidity of the cassette housing is necessarily reduced by making the 
assembly from three parts. 
A further feature of the cassette described in U.S. Pat. No. 3,934,842 is 
that all the surfaces in the region projecting forward from the tape guide 
depend from the cover. This prevents use of identical upper and lower 
halves, but enables easy threading of the tape onto the lower housing when 
the cassette is being assembled, the cover being attached last and 
partially enclosing the front. 
Still another construction and related manufacturing method for cassettes 
is known in the field of so-called professional or digital cassettes. 
These cassettes utilize a metal cassette frame, which constitutes the tape 
guide unit and the side walls of the cassette housing; and two thin 
plastic covers which are mounted on the upper and lower sides of the metal 
frame. Such cassettes have been available since approximately 1971; for 
example, under the designation Philips model LGH 6003 or LGH 6005. The 
metal frame of this three-part cassette is cast in a composite mold having 
a movable third mold section which can form cassette guiding surfaces 
which are true cylinders perpendicular to the support planes. As a result 
of this high quality and expensive construction these professional 
cassettes provide correct tape guidance with only a very small tolerance 
on the azimuth angle, and are quite rigid. This rigidity is obtained 
almost entirely from the rigidity of the metal frame, the covers being 
comparatively thin and readily deformable. These professional cassettes 
are not readily suited for use in consumer equipment, however, because 
there is no provision for the break-away tabs in the rear wall (opposite 
to the front wall opening through which heads enter) which are used to 
code the type of tape used in the cassette, or whether or not the cassette 
is pre-recorded. Many consumer-type cassette apparatus contain sensors for 
detecting the presence or absence of such tabs, so as to inhibit erasing 
of pre-recorded tape, or to change the bias current used for recording to 
match most closely the properties of the magnetic tape with which the 
cassette has been loaded. Further, such code openings or tabs cannot be 
readily provided in the rear wall of a cassette housing utilizing the 
professional cassette style metal frame. Because the frame is essentially 
a hollow ring, the provision of such an opening would lead to an 
unacceptable localized reduction in strength, and might even seriously 
impair its structural integrity. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a magnetic tape cassette whose 
housing can be molded entirely from plastic materials and yet maintain 
azimuth angular error less than approximately one minute of arc, so that 
accurate recording and playback of information at frequencies between 15 
and 20 kHz is possible. 
A further object of the invention is to provide a housing for a magnetic 
tape cassette having only two major parts and having high rigidity. 
Yet another object of the invention is to provide tape guides in a cassette 
perpendicular to the support planes at the same time that the cassette has 
provision for coding tabs and openings at the rear without an increase of 
overall dimensions. 
Still another object of the invention is to provide an economical method of 
producing high quality cassettes by plastic molding. 
According to the invention, a housing for a magnetic tape cassette consists 
of a lower part and a cover only, a tape guide unit of the cassette being 
a continuous section of material extending between two true cylindrical 
guides, formed as a unitary element with the cassette locating surfaces 
and the lower part. 
In a preferred embodiment of the invention, the tape guides of the unitary 
element have upper and lower shoulders for vertical positioning of tape 
passing over the guides, these shoulders lying in planes parallel to the 
support planes for the cassette. 
According to yet another aspect of the invention, in a method of 
manufacturing a housing for a magnetic tape cassette a unitary part having 
a box-shaped portion and a tape guide unit is formed from plastic material 
introduced into a mold having first, second and third sections, the first 
and second sections forming the external bottom surfaces and opposed 
internal surfaces of the lower parts respectively, these sections being 
movable with respect to each other in a first direction generally 
perpendicular to the bottom surface, the third section forming true 
cylindrical tape guiding surfaces on the guide unit perpendicular to 
planes of the cassette locating surfaces, the third section being movable 
in a direction perpendicular to the first direction. 
The invention will be described in greater detail hereinafter with 
reference to a preferred embodiment shown in the drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1 an assembled cassette housing is shown, but without tape, tape 
reels or the pressure pad assembly that normally are installed before the 
housing is closed. The housing 1 is made from a box-shaped lower part 2 
and a cover 3 which fits over the lower part and is secured to it by a 
number of screws 4. The lower part 2 is shown in more detail in FIG. 2, 
and includes a bottom 5 substantially flat on its interior and having 
openings 6 formed in the bottom for reel drive spindles to pass, openings 
7 for cassette guide pins or other positioning devices, and openings 8 for 
a tape drive capstan to pass through. Extending upwardly from the bottom 
are a right side wall 9 and left side wall 10, a rear wall 11 and a front 
wall 12 which is almost completely open for the passage of magnetic heads 
and a pressure roller. 
In FIG. 2 the cover 3 is also shown turned over with its rear edge adjacent 
the rear wall 11 of the lower part. Openings having functions 
corresponding to those described for the lower part are located at mirror 
image positions as shown in FIG. 2; for example, the reel drive spindle 
openings 13 which correspond to the lower part openings 6, and the capstan 
openings 14 corresponding to the openings 8. Openings 15 are also provided 
at locations generally corresponding to the cassette guide pin openings 7 
in the lower part 2 but having, as will be described hereinafter, a 
different shape. 
Also extending upward from the bottom of the lower part 2 near the front 
wall 12 is a tape guide unit 16 generally parallel to the front wall and 
having a height corresponding to the interior distance between the bottom 
5 and the cassette cover 3 when the housing is assembled, for example as 
shown in FIG. 1. The tape guide unit 16 has a comparatively complex 
structure and performs several functions. Two projecting tape guides 17 
are located at the extreme ends of the tape guide unit, and other 
symmetrically located tape guides 18, 19 and 26 are located at 
intermediate positions between the guides 17. The far guides 17 have a 
generally rounded shape perpendicular to the bottom 5, and upper shoulders 
41 and lower shoulders 42 generally parallel to the bottom 5; and are 
symmetrically located about and adjacent a central recess 20 into which in 
normal operation a recording/playback head of a recorder apparatus passes. 
The recess 20 is designed to accommodate, in the usual fashion known in 
the art, means for holding the tape properly and resiliently against the 
head, such as a resilient bracket having a pressure pad and a metal 
screening bracket. For clarity in description of the lower part itself, 
these well-known components are not shown in the drawing. 
Also projecting perpendicularly upward from the bottom are two pins 21 
which function as bearing spindles for tape guide rollers commonly used to 
reduce friction and the variation of friction as a function of the amount 
of tape being wound on or off the adjacent reel. Also projecting upward 
from the bottom 5, in line with the rear wall 11, are two break-away tabs 
22 at the standardized locations used to indicate whether or not a 
cassette contains pre-recorded tape . 
The cover 3 also includes front lugs 23, 24 and 25 symmetrically disposed 
about the front to rear center line of the cover, so located that when the 
cover is mounted as shown in FIG. 1 the lugs partly close off the 
otherwise largely open front wall 12 so as to protect magnetic tape 
passing over the tape guides behind the front wall opening. As will be 
clear to one of ordinary skill in the art, when a complete cassette using 
this housing is assembled, the magnetic tape would be contained on reels 
concentric with the openings 6 and 13, a run of the tape extending from 
one reel to the other and passing in front of the tape guide unit 16. 
Mirror image rectangular openings 27 and 28 are formed in the bottom 5 of 
the lower part and in the cover 3 respectively, in which transparent 
windows can be glued so that the edges of the magnetic tape contained on 
reels in the cassette can be observed. 
The external dimensions and details of the assembled housing are 
conventional and so are not further described here. The details will 
correspond, for example, to IEC publication no. 94A referred to 
previously. However, unlike the cassette described in U.S. Pat. No. 
3,934,842 the guiding surfaces of the tape guides 17, 18, 19 and 26 are 
cylindrical surfaces and are exactly perpendicular to the adjacent 
portions of the bottom 5, this perpendicularity dependent entirely on the 
molds used for forming the lower part 2 and not adversely affected by 
subsequent mounting and assembly operations. 
To stiffen the housing generally reinforcement ridges 29 are provided in 
the otherwise unused space at each of the rear corners, and a 
reinforcement member 30 projects upwardly from the bottom and forwardly 
from the rear wall generally between the spaces that would subsequently be 
occupied by the two tape reels. Further, to indicate the use of special 
tape in a housing, for example chromium dioxide magnetic tape, additional 
openings can be formed in the rear wall 11 at the locations 31 without 
significantly affecting the rigidity and structural integrity of the 
box-shaped lower part 2 because the entire rear wall 11, ridge 29 and 
reinforcement 30 form part of one unitary molding with the bottom 5. This 
basic rigidity combined with the special rigidity provided by the integral 
tape guide unit 16 leads to another reduction of tolerance build-up. The 
external surface of the bottom 5 immediately adjoining the guide pin 
openings 7 serves as a cassette locating surface when the cassette is 
installed with the cover uppermost on a conventional apparatus. The guide 
unit 16 also includes cassette positioning members 32 facing away from the 
bottom 5, and having guide pin openings 33 corresponding to the openings 7 
at the bottom surface. The members 32 are so arranged that, when the 
housing is fully assembled, the top surfaces of the members 32 project 
through the openings 15 of the cassette cover to provide cassette locating 
surfaces used when the cassette is being played in the inverted, or cover 
down position. 
As a result of this configuration, the tape guide unit 16 which extends 
continuously between the two far tape guides 17 functions like a beam 
which reinforces the bottom 5 so that the lower part 2 has a very high 
rigidity even though its front wall is almost completely open. Further, 
both the upper and lower locating surfaces and guide pin openings are 
formed in the tape guide unit or surfaces of the bottom which are integral 
with and immediately below the tape guide unit. 
The pair of far tape guides 17 are formed with upper and lower shoulders 41 
and 42 lying in planes parallel to each other and to planes passing 
through the cassette locating surfaces of the bottom 5 adjoining the 
openings 7 and of the positioning members 32. Therefore azimuth error is 
minimized because the critical tape positioning is performed by surfaces 
within the cassette housing that are a portion of the most rigid part of 
the housing and are close to and rigidly spaced with respect to the 
cassette locating surfaces, although the cassette housing has only 
required two elements. 
A mold used in the method according to the invention is shown schematically 
in FIG. 3. The mold consists essentially of a first or lower mold section 
34, a second or upper mold section (not shown) movable with respect to the 
first mold section in the direction of the arrow A, and a third or sliding 
mold section 35 mounted in the lower mold section 34 so as to be slidable 
between the molding position shown in FIG. 3 and a retracted or release 
position by movement in the direction of the double arrow B perpendicular 
to the direction of the arrow A. A lower part 2 is shown still in the 
lower section 34, exposing the internal surfaces of the bottom of the 
cassette and the co-planar cassette upper locating surfaces which were 
formed by complementary surfaces of the second mold section. The third, 
sliding mold section 35 contains the complementary surfaces for forming 
the cylindrical tape guide surfaces as true cylindrical surfaces 
perpendicular to the planes of the locating surfaces, and the shoulders of 
the far tape guides 17, removal of the lower part being possible only 
after the second section 35 has been slid away from the molded lower part. 
By the use of such a mold a lower cassette part can be obtained which, in 
the assembled cassette, will provide an azimuth error of less than one 
minute of arc. 
It will be clear to those of ordinary skill in the art that many other 
variations and embodiments are possible within the spirit of the 
invention. The cover and lower part need not be molded from the same 
homogenous material, although it would be desirable that they have similar 
temperature coefficients of expansion. By selection of the relative 
locations of the tape guides, so that some upwardly extending front lugs 
can be part of the lower part, yet permit withdrawing the third mold 
section, the cover can be made as a substantially flat unit. Also, it may 
be possible to simplify the pressure pad arrangement by molding resilient 
elements into the front central recess. These and other variations are 
made possible by the basic invention without losing the advantage or 
rigidity, low azimuth error, and relatively low cost. 
Further, the invention may be practiced using any moldable material. For 
example instead of the usual polystyrol, an especially stable plastic may 
be chosen such as an acrylonitrile reinforced with glass fibres or glass 
pellets.