Mould for manufacturing disc-like objects

Details Mould for manufacturing disc-like objects Mould for manufacturing disc-like objects

1998-03-10

2001-11-27

Vargot, Mathieu D. (Department: 1732)

Plastic article or earthenware shaping or treating: apparatus

Female mold and charger to supply fluent stock under...

With means to heat or cool

C249S079000, C249S080000, C249S135000, C264S001330, C425S552000, C425S810000

Reexamination Certificate

active

06322350

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention lies in the field of manufacturing disc-like plastic objects by means of a mould. The manufacture of disc-like objects by means of a mould is known. Diverse known techniques are, among others, injection moulding, injection pressing, injection compression and coining. The invention is applicable to all of these as well as other suitable techniques.
The invention relates to a mould for manufacturing disc-like objects, which mould comprises:
at least two mould parts which are mutually movable between a closed position in which they bound a mould cavity, which mould cavity is connected to an injection conduit for admitting heated, plasticized plastic under pressure into said mould cavity, and an opened position in which a disc-like object can be removed; and
removing means for removing a central portion from a disc-like object to form a central hole present therein, which removing means comprise a cylindrical part which is movable in axial direction in the closed situation of the mould cavity.
Such a mould is known generally and in many embodiments.
The disc-like objects are in particular transparent information carriers such as CDs and DVDs. For a correct scanning of the digital information arranged thereon, such information carriers must comply with very specific technical requirements. In addition to flatness and transparency, such technical requirements also include properties relating to birefringence, which must meet a required standard within close tolerances and must generally be as low as possible.
The birefringence is an optical measurement value for internal material stresses. These internal stresses are introduced during manufacture by injection moulding or injection pressing of polycarbonate substrates. They are caused by shrinkage, flow lines and/or inclusions of air or other contaminants. The stresses result in the light of the laser being additionally diffused during scanning, whereby phase differences occur when a substrate is scanned. This diffusion of light rays results in a decrease in the effective intensity of the laser beam. Thereafter, light rays are also deflected and diffusion occurs when radiation transfers from the one medium to another medium with a different density.
The wavelength of a scanning laser is roughly 780 nm in air and about 500 nm in polycarbonate. The light diffusion index is 1.55. In the specification for the birefringence of polycarbonate substrates is stated that this may vary by a maximum of 20% from the given value and may thus amount to a maximum of 100 nm. A larger variation will certainly result in problems. It is remarked here that in the case of production processes for mass-produced articles such as those under consideration, it must be ensured under all circumstances that the relevant norm is achieved, even in the most unfavourable conditions. It is therefore desirable to design a mould such that the birefringence is thus as low as possible in all conditions and is nominally considerably lower than the stated tolerance limits.
The birefringence is measured in terms of phase differences and decrease in light intensity between a &khgr;-component and the &ggr;-component of polarized light. The birefringence is expressed in nanometres (nm).
A further explanation concerning the described concepts can be found in “Kunststoffe German Platics” 1989/1, reprint, “Birefringence in Optical Information Storage Discs made of Polycarbonate”.
For manufacture of information carriers of the described type an optionally replaceable wall of the mould cavity is provided with a negative impression of information to be arranged on the disc-like object. Such information must be transferred with very great reliability and accuracy onto the disc-like object.
It is further necessary to prevent the occurrence of a phenomenon known as “dishing”. This is the phenomenon where a disc-like object is not flat but has a shape which curves in substantially rotation symmetrical manner. In this same context the phenomenon of “warpage” often occurs, wherein the disc-like object acquires a saddle shape. It may even occur that a disc-like object produces so-called “click-clack”. This is the phenomenon where an object has two mutually differing stable shapes which can transpose into each other by exerting a force on the central zone.
It is an object of the invention to essentially prevent to a considerable extent the described undesired phenomena and generally improve the quality of a disc-like object.
It a further object of the invention to embody a mould of the described type such that the time required to manufacture a disc-like object is reduced.
Summarizing very briefly the objectives of the invention as specified above, a main purpose of the invention is to improve the quality of a manufactured disc-like object while reducing the cycle time.
With a view to these objectives the invention aims to considerably increase the “operating window”. This operating window is understood to mean the total range in which combinations of parameter values are possible wherein the set requirements can be fulfilled.
SUMMARY OF THE INVENTION
The invention provides a mould which has the feature that at least one of the two mould parts is provided with first tempering means which are active substantially only in the middle zone of the mould cavity for tempering the inflowing plastic during the inflow thereof via the pressure line and thus the filling of the mould cavity.
It is noted that an injection mould is known, of which the so-called mirrors, i.e. the substantially plane walls of the mould cavity, the so-called punch and the injection conduit or injection bush are provided with cooling means. However, such cooling means have not been found effective in realizing the above stated objectives of the invention.
In a specific embodiment this mould has the feature that the ratio of the diameters of the middle zone and of the mould cavity lies in the range of (0.30±0.15).
It should be appreciated that the plasticized plastic flowing in via the central zone is subjected at that position to a temperature change. At the location of this central zone there is a flow in substantially radial direction in order to fill the mould cavity. By cooling the inflowing plastic in this zone by means of the first tempering means a certain “freezing” of the relevant material takes place, whereby the effective passage of the material is limited. This passage therefore varies during the injection of plasticized plastic. The smaller the passage becomes, the higher the pressure required to keep constant the flow rate of the inflowing plastic. If the pressure were to be kept the same, the flow rate of the inflowing plastic would then decrease. It will be apparent that it is possible to experiment with the given parameters until the quality of an obtained disc-like object is as high as possible, or at least lies within for instance a required norm.
In a further embodiment the mould has the feature that at least one of the two mould parts is provided with second tempering means which are active substantially only in the peripheral zone of the mould cavity for tempering the inflowing plastic during the inflow thereof via the injection conduit and thus the filling of the mould cavity.
This variant is particular can have the special feature that the second tempering means co-act directly with the peripheral wall surface of the mould cavity.
As will be described hereinbelow, the second tempering means can be arranged in a moving, so-called venting ring. The advantage of this second tempering means is that the index of refraction can hereby be influenced in terms of the birefringence of a transparent information carrier. The birefringence displays a sharp rise on the outside of the disc-like information carrier and, as will be described hereinbelow, reaches a critical limit of about ±50 mm. By embodying the second tempering means such that the relevant peripheral zone has a higher temperature than the substantially plane end walls of the mould cavity, the co

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