GB2122531A - Method and apparatus for making dental prostheses - Google Patents

Abstract

To provide for simple and errorproof mixing of materials for pressure injection moulding of dentures of other dental prostheses, a liquid and powder mixture of methyl- methacrylate base plastic is intimately mixed to form an essentially bubble-free mixture which is poured into an injection vessel 2, permitted to swell, and then injected under predetermined pressure, within a narrow tolerance range, into a plaster mould retained within separable halves 301, 302, of an injection mould 3. The process is automatically controlled by a sequencing control unit 8, for example a micro- processor unit, which applies first closing pressure to the mould halves by a pressure element 4, and then injection pressure by a piston-cylinder unit 1, after a suitable swelling period has elapsed. Preferably, the pressures are applied hydraulically and the pressures are monitored by a pressure sensor 6 which, through the control unit 8 controls a pump P for maintenance of the requisite pressure. <IMAGE>

Description

SPECIFICATION Method and apparatus for making dental prostheses The present invention relates to a method of and apparatus for making dental prostheses such as dentures or false teeth, and more particularly to a method and apparatus which permits practically automated, programmed sequencing of the method steps, so that the human intervention in the manufacture of the prostheses is reduced to command the variables of the process.

Reference is made to German Patent Specification No. 26 37 826 the disclosure of which is hereby incorporated by reference, and which describes a process and an apparatus to carry out the process for making dental prostheses in which plastic material, which is polymerizable, is introduced into a vessel which retains the mould of the dental prosthesis. A fluid acts on a compression injection piston, operating within an injection cylinder, the piston acting on a punch or presser element which, in turn, can act on the polymerizable material, to inject, under pressure of the punch or pressing element, plastic material through a nozzle into the hollow form of the prosthesis.

The vessel, in accordance with the German Patent Specification has an overflow opening, so that plastic material of dough-like consistency can be used. The fluid pressure which acts on the injection piston is so controlled that it moves in steps, and supplies plastic material until at least some of it exits from the overflow opening.

The injection process, as described in the aforementioned specification, is carried out in intervals.

It is an object of the invention to provide an injection process to make dental prostheses which permits continuous injection, and is suitable for plastic materials which have a still higher degree of viscosity than heretofore possible.

From one aspect the invention consists in a method of making a dental prosthesis of polymerizable, temporarily flowable plastics material, which comprises the following steps (a) mixing the plastics material in the form of a powder and in the form of a liquid to form a powder-liquid slurry, suspension or solution, in a common vessel, to obtain a homogeneous, essentially bubble-free mixture; (b) filling the powder-liquid mixture into an injection vessel; (c) tightly closing the injection vessel and introducing the injection vessel into an injector press; (d) permitting the mixture to swell during a pre-determined time interval (e) providing a separable mould form, positioned in communication with the injection vessel, and applying injection pressure to the injection vessel to inject the swollen mixture into the mould form under a predetermined pressure;; (f) controlling and maintaining the predetermined pressure within a given pressure tolerance range during the injection step; and (g) opening the mould form and removing the finished injected dental prosthesis.

From another aspect the invention consists in apparatus for manufacturing a dental prosthesis, of a polymerizable, at least temporarily flowable plastics mixture consisting of liquid and solid components comprising a separable mould structure adapted to retain a prosthesis mould form, and having separable mould elements; an injection vessel for temporarily storing the mix mixture, and permitting the mixture to swell; means for applying a closing pressure to the seperable mould elements of the mould structure; a fluid powered piston-cylinder unit positioned for applying injection pressure to said injection vessel and injecting the mixture therein into the mould form in the mould structure; control means controlling application of closing pressure of the closing pressure application means, and injection pressure of the fluid powered piston-cylinder unit;; sensing means sensing the applied closing pressure and the applied injection pressure, and signalling the actual pressures to the control means, said control means controlling the application of the pressures to result in actual pressures being applied within a predetermined tolerance range, and permit release of application of the pressures only upon termination upon injection of the material and mouldinggof the prosthesis.

A typical material for dental prostheses is a plastic material based on the methylethacrylate (MMA). In accordance with a feature of the invention, the starting material-typically MMA-plastics-is introduced into a common vessel in the form of powder and liquid, and mixed to obtain a homogeneous, essentially bubble-free mixture. The powder-liquid mixture is then filled into an injection vessel which is then tightly closed and introduced into an injector apparatus or device. The mixture is then permitted to swell for some time and then injected into the hollow dental mould which, preferably, is in the form of a separable mould form. The injection pressure is predetermined and the swollen mixture is, thus, injected under predetermined pressure which may vary only within a given tolerance range during the injection step.Thereafter, the prosthesis mould can be removed from the injection vessel, and the prostheses then removed therefrom.

The apparatus, in accordance with a feature of the invention, has a separable mould structure adapted to retain the prosthesis mould form which, in turn, has separable mould elements. The injector comprises a fluid-powered piston-cylinder unit coupled to the injection vessel to apply injection pressure thereto and inject the swollen mixture into the mould form. To prevent separation of the separable mould elements, a structure is provided which applies closing pressure against the separable mould elements thereof. A control unit is provided to control application of the closing pressure against the mould elements and the injection pressure.The applied closure pressure is sensed, and the injection pressure likewise is sensed, to provide feedback actual pressure signals to the control element to maintain both said pressures within given tolerance ranges and permit release of the pressures only upon termination of injection of the prosthesis material and moulding thereof in the mould.

The apparatus and the method have the advantages that the sequencing can be carried out essentially automatically. The separable mould elements, within the mould housing, are securely pressed together and the sequencing of pressing-together the mould elements and injection is automatically controllable, with suitable interlocks to prevent inadvertent erroneous operation.

In order that the invention may be more clearly understood, reference will now be made to the accompanying drawings which illustrate one embodiment thereof by way of example and in which: Figure 1 is a highly schematic side view of a moulding apparatus for dental prostheses in accordance with the invention, Figure 2 is a detailed sequencing diagram of the steps of the process and the sequencing of operating steps of the apparatus, and Figure 3 is a schematic diagram of the filling cylinder with a plastic material container inserted therein.

Referring now to the drawings a pistoncylinder unit 1 (Fig. 1) is coupled to a plunger or piston 1 a. Fig. 3 illustrates the cylinder 2, and coupling for the vessel 3 in greater detail.

A clamping apparatus 4 is coupled to the vessel 3, since the vessel 3 contains separable elements. The vessel 3 is provided with feedback sensors, for example to sense operating pressures and temperatures, the pressure sensor being shown at 6 and the temperature sensor 6a in Fig. 1. The cylinder 2, adapted to retain the polymerizable plastic substance, can be moved between two positions, one of which is shown in Fig. 1, that is, when it is ready for operation and injection of plastic material. In another position-not shown-it can be removed from the operating position.

An interlock 7 is provided which prevents operation of the plunger or piston 1 a if the cylinder 2 is not in proper operating position, as schematically indicated by the broken line 7a. This interlock may also be coupled to the vessel 3, and further can be arranged to sense if fill materal, and/or a container therefor, is properly positioned within the cylinder 2 (see Fig. 3).

An electronic control unit 8, which includes a sequencing switching arrangement, is provided, the electronic control unit 8 including a plurality of printed circuit (PC) boards, which provide for sequencing and control of the process, and which can be programmed for proper sequencing of the processing steps, in accordance with well-known sequencing programming technology. Rather than using electronic control, electro-mechanical control can also be provided, although electronic control, due to versatility, and low space requirements are preferred. The respective printed circuit boards are schematically shown at 8a, 8b ...

8n. The electronic control unit is additionally coupled to a switch box 9 which may include one or more switches 9a, 9b for control of process parameters and/or variables. The switches, shown only schematically, preferably are pushbutton switches which may be suitably labelled.

The control unit 8 and the switch unit 9 can be combined as a single control assembly, and, further, additionally combined with one or more indicators which indicate the then-pertaining process step, the condition of the overall apparatus, for example ON/OFF, digital, or analog indicators for process variables, such as pressures, temperature or the like which arise and/or other indications, as well known in process and process control applications.

A pressure storage vessel 10 retains a pressure medium, such as a pressurized hydraulic fluid, compressed air or the like. It can be supplied with sensors to sense pressure, tern- perature or the like, such as pressure sensor 6 and temperature sensor 6a, for display on the control unit 8. Accessory apparatus, such as pumps P to provide pressurized liquid or gas in the pressure fluid storage vessel 10, and control circuitry therefor are shown schematically in the drawing since such structures and their control are well known in standard articles of commerce.

The piston-cylinder unit 1 preferably is operated by hydraulic pressure and, for example, may be as shown in detail in the referenced German Patent Specification No. 26 37 826, although other structures are also suitable. In a preferred form, a pressure fluid, typically a pressurized hydraulic fluid, acts on a piston which is operative within a compression cylinder. The piston carries a piston rod which is coupled to a pre-head or plunger I a which, in turn, engages a packing ring 1 b (Fig, 3) which covers the contents of polymerizable plastic material retained within the holding vessel 2. The holding vessel 2 includes therein a liner and flangible membrane combination 15, into which the polymerizable material can be filled. The packing ring 1 b, preferably, is made of POM (polyoctylmethacrylate) plastics material.This packing ring is seated in the cylinder 2 and, upon rupturing of the membrane 15, can inject, under pressure, plastics material in the dental prosthesis mould through the outlet spout thereof. Plaster is the preferred form for the dental prosthesis mould, since it uniformly transfers heat from the vessel 3. The vessel 3 is formed in two parts, that is, in two half-shell elements 301, 302. The vessel 3 is preferably made of a high-quality brass alloy and is held by the clamping arrangement 4 in a rigid frame on the base plate 11 of the apparatus housing 1 2. Suitable holding framing members have been omitted from the drawing for clarity and can be introduced and built in any suitable manner. An interposed wrought steel block 3a supports the vessel 3 on the base plate 11.

The pressure sensor 6, in the simplest form, can be a pressure-sensitive switch. The clamping arrangement 4 presses the two half-shells of the vessel 3 against each other, for example by pressing a movable half-shell against a fixed one. Pressure sensor 4 can be constructed to provide a signal when the engagement pressure between the two halfshell elements has reached a predetermined value. A suitable closing pressure for the halfshell elements is in the order of 3,000 Kp.

Preferably, the closing pressure apparatus 4 includes a hydraulic piston-cylinder unit which, like the hydraulic unit 1, is coupled to the pressure fluid storage vessel 10 by suitable hydraulic lines (not shown), and further including a suitable pressure control circuit.

The control circuits for the pressure lines from the vessel 10 to the pressure arrangement 4 and to the cylinder-piston unit 1 are preferably separate.

The piston unit of the compression arrangement 4 is shown schematically at 4b and is movable along the axis 4a. Movement of the piston need extend for only a few millimeters along the axis 4a. The vessel 3 is split into parts 301, 302 along a centre line 3b, to form the two half-shells, the half-shell 302, for example, being securely connected to the base post 3a, whereas the half-shell 301 is slightly movable, for compressive engagement by the pressure arrangement 4, 4b, The pressure fluid storage vessel 10 is connected, hydraulically, and through an independent control loop 1 Od or control circuit to the hydraulic cylinder 1. A logical interconnection and interlock between the hydraulic connection to the cylinder/piston arrangement 4 and the cylinder/piston arrangement 1 can be connected to and in the control unit 8.The special connection lines 4ctherefore have been shown schematically in the drawing, and can be placed and connected as well known.

The hydraulic system preferably also includes an electric drive motor coupled to a pump P overflow sump for hydraulic oil and suitable valves, all interconnected via pressure hoses 1 0a in accordance with well-known hydraulic control technology.

The operating pressure of the hydraulic unit is preferably, within a working range of between 1 50 and 200 Bar, with a supply capability of between about 0.2 to 0.3 cm3/min.

The valves of the hydraulic control units are preferably electrically controlled magnetic valves, operable by low-voltage control signals. Chokes are preferably included within the valves.

The hydraulic cylinder 1, forming the injection pressure cylinder, preferably has a piston surface of about 7 cm2, and a stroke of up to about 5 cm. The piston injection speed can be controlled and, for example, may vary between 1 and 5 mm/sec. The pressure force on the injection cylinder is about 1 ,200 kg. A hydraulic amplification factor of 1:2 or more can be obtained when considering the usual surface of the vessel, and the mould located therein. Of course, the surfaces and diameters 9f the respective vessel 2 and the vessel 3 are known.

The filling cylinder 2 is preferably coupled to its own drive unit 5, and may be a pivoting drive, for example utilizing right-angle gears or the like, and is coupled thereby to the filling cylinder to be moved, within a horizontal plane 5a between the working position, shown in Fig. 1, and the filling position, in which the cylinder 5a is tilted or pivoted out of alignment with the pressing piston or plunger 1 a, so that access to the open end of the cylinder 2 is readily available. In accordance with the preferred form, the cylinder 2 can be pivoted between the operating position, shown in Fig. 1, and the quiescent, or filling position, for example 90 out of plane of the drawing. When in working position, the central axis of the filling cylinder 2 is in alignment with the axis 3b of the vessel 3.

The interlock 7, which is preferably also coupled to the control unit 8, which controls the automatic sequencing of the processing steps, ensures that the programme can run only if the cylinder 1 and the vessel 2 are in alignment, the vessel 2 is filled, and in appropriate working condition, and accurately aligned with the axis 3b. Likewise, the pressure sensor 6 is coupled to the control unit 8 via lines 6e to ensure that the two halves or portions 301, 302 of the vessel 3 are securely engaged with each other and are accurately vertically symmetrical with respect to the central axis 3b. The pressure sensor 6, sensing the engagement pressure of the elements 301, 302 against each other, other sensors, operating switches and the like, and if desired, further gauges and sensors, pushbutton and other switches can be provided.

One interlock element which is desirable is to provide a sensor if a sealing element 14 (Fig.

3) is properly seated in alignment with the outlet spout and its liner 1 3 (Fig. 3) of the filling vessel 2. Simple positioning sensors, or limit switches, are suitable; in view of the pressures involved, however, strain gauges or piezo electrical elements can be used. Sensors can be connected not only externally of the vessel 3-as shown in connection with sensor 6, which is to sense the engagement pressure of the two halves 301, 302-but additionally within the pressure vessel, to sense injection pressure, and, preferably, to provide corresponding pressure signals to the control unit 8, for the processing therein and, if desired, indication or display.

The sidewall 3c of the vessel 3 preferably includes a temperature sensor 6a thereon, located within the region of the plaster mould.

Sensor 6a, like sensor 6, are coupled to the control unit, shown schematically only by connecting arrows 6c. The control unit 8 includes the respective PC boards 8a, 8b . . 8n which are preferably arranged as insertion units, accessible from the front plate of the control unit. Preferably, the housing for the entire structure is made of glass fibre reinforced polyester with rapid connecting holding locks to connect the housing to the base plate 11.

The switching unit 9 includes the main power switch which can connect the apparatus to a power network, for example 380 V threephase current or 110/220 V network supplies.

The electronic control unit includes a turnon delay element for the injection press. The temperature of the vessel is sensed by the temperature sensor 6a, that is, in the region of the plaster mould. The desired temperature to which the vessel 3 can be heated, for example electrically, and as well known, can be pre-selected, so that the cylinder 1 can be operated only after the plastic mixture is introduced in the liner 1 5 of the vessel 2 has been suitably mixed and a certain time had elapsed to permit it to swell, as will appear below.

Such a suitable time is between about three and six minutes. A temperature-time curve can be provided, which is manually controllable, for example by a potentiometer 801, forming part of the control unit so that the time-temperature relationship of the process can be matched to the characteristics of the plastic material which is to be injected, to provide for close and accurate match.

Additionally, or in lieu of the temperature sensor, a viscosity sensor can be provided to sense the viscosity within the vessel 2 when it is in operating position.

The control unit 8 controls the injection time, compression time and the overall run of the process. Injection and compression time, for usual dental materials, is in the order of between 5 to 1 5 minutes, preferably between about 7 to 1 2 minutes, particularly when using highly viscous plastic materials on an MMA basis, derived from a mixture of powder and liquid in the ratio of 2:1, by weight for example 30g powder to 159 liquid.

Fig. 2 gives a flow diagram of the operating sequences of the programme in which the legends on the drawing are annotated below.

A Connect power, 380V B1 turn ON B2 green monitor lamp lights B3 pump starts B4 pressure builds up B5 apparatus ready to operate C1 apply sealing disc to mould vessel 3 C2 red programme cancel key D1 insert mould vessel D2 operate compression cylinder D3 mould halves compressed D4 blue monitor lamp lights E insert filler liner F mix powder-liquid G X operate starter G2 green monitor lamp lights G3 start delay operates H1 stir powder/liquid H2 manually or automatically J pour mixture in injection cylinder K place top element in injection cylinder L insert Delrin disc and press snugly on mixture M1 introduce injection cylinder in align ment with plunger M2 yellow lamp lights N injection programme runs 01 end of injection programme 02 all three monitor lamps extinguish 03 hydraulic cylinder 4 to start position P mould form 3 and injection cylinder 2 may be removed Operating sequence of injection and method of making a dental prosthesis, with reference to Fig. 2: A power connection line-not shown-is connected to a power network, see step A, Fig. 2. Then, an ON/OFF switch is operated-for example switch 9a-as schematically illustrated by step B1. As a result, a control lamp will light-see block B2, the pump will start-block B3 causing pressure in the pressure fluid storage vessel 10 to build up-block B4 and, when a pressure of about 200 bar has been reached, this pressure will be indicated, showing, also, that the unit is ready for operation-block B5.

Let it be assumed that the dental prosthesis mould has already been introduced into the vessel 3. This, of course, is the necessary step which is primary to making a dental prosthesis. The seal 14 (Fig. 3) must then be located on the vessel 3 - step C1 - and the mould vessel 3 is introduced into the apparatus, for example to a fixed position defined by a stop, properly located and centered on the support socket 3a. In this position, the axis of the vessel halves 301,302 will be precisely symmetrical above the line 3b. Only a few millimeters will separate the left wall 3d of the vessel from the plunger piston 4b. Introduction of the vessel 3 is shown in step Dl (Fig.

2).

In accordance with the feature of the invention, the control unit 8 includes a red "danger" or "warning" lamp which can be extinguished manually, but only if the vessel 3 is properly seated on the socket 3a. Extinction of the red warning lamp 802 (Fig. 1) is commanded by a suitable switch and schematically shown as substep C2 in Fig. 2.

After introduction of the vessel 3 in working position, and, for example, consequent upon operation of the warning elimination button 802, with proper seating of the vessel 3, the compression cylinder 4 will have hydraulic fluid applied thereto from the pressure storage vessel 10, as indicated by the consequent step D2. Consequently-see block D3-the two halves 301, 302 of vessel 3 will be pressed together, and, when the appropriate compression force is indicated and sensed by pressure sensor 6, a blue monitoring lamp 803 (Fig. 1) will light-see block D4. Upon pressurization of the cylinder-piston arrangement 4, the plunger 4b will move towards the right in the direction of the double arrows 4' about the axis 4a, so that the two halves 301, 302 of the vessel 3 are tightly engaged.The pressure, preferably, is in the range of 3,000 Kp, and this pressure is continuously maintained until the injection step is terminated. The pressure is provided by the pressurized fluid from storage vessel 10.

Next, and with the vessel 2, for example, removed from alignment of the apparatus, a plastic container 1 5 with a fracturing membrane, for single-use, is introduced in the cylinder 2. Plastic pulverized injection material, as well as liquid, are poured either into the liner 15, and there intimately mixed, or poured in a suitable mixing vessel, and intimately mixed remote from the unit 2, to be then poured into the lined cylinder unit 2.

Insertion of liner 15, as schematically indicated by step E, Fig. 2, mixing of plastic powder and liquid, and introduction into the liner 1 5 by step F. The process starter switch, for example switch 9b, can than be operated-step G 1-which causes a second green control lamp to light, see block G2. Additionally, a start-time delay is energized-see block G3, and started to commence a timing period.

The time-delay is provided to delay the energization of the pressure cylinder, piston combination 1, and may, for example, provide for a delay between three and six minutes, while, simultaneously, causing heating of the vessel.

The time delay is shown by block G3, and a green control lamp, block G2 is lit.

While the starter has been operated, and externally of the unit, the powder and liquid of the plastic composition are thoroughly stirred and mixed-step H1-which can be carried out manually or automatically, see block H2. The mixture can then be poured into the liner vessel 15, remote from the cylinder 2-step J-and the filled liner vessel then introduced into the cylinder 2-step K. Thereafter, the plunger or piston 1 a and the packing ring 1 b is seated on the liner, so that it closely fits on the upper edge of the filling cylinder 2, for tight engagement therewith, see step L. The cylinder 2 is then placed in accurate alignment with the central axis 3b-step M1, causing a yellow control lamp 804 to light see block M2.From this point on, the programme proceeds in automatic sequence, for example, controlled by a stepping motor or stepping programme under control of a clock-part of every microprocessor-of the control unit 8.

The automatic programme can start as soon as the inter-lock 7 has signalled to the control unit 8 that: (1) the cylinder 2 has plastic material therein; (2) the liner 1 5 and the cylinder 2 are properly aligned with the axis 3b of the mouldholding vessel 3; and (3) desirably, not necessarily, that all other desired operating conditions pertain, namely that the pressure of the cylinder-piston unit 4 continues to be at proper level that the pressure in the fluid storage vessel is maintained, that temperatures are proper and the like.

When all the interlocking conditions have been satisfied, and the cylinder 2 properly positioned by the positioning drive 5, for example, which will then be deenergized, the injection programme can start automatically, as indicated by block N (Fig. 2).

When a suitable and desired pressure is reached, the pump of the pressure fluid, connected to the pressure fluid storage vessel 10, can stop, so that a suitable injection pressure applied on the plunger 1 a will remain at a pre-determined level. This pressure is provided to fill the dental prosthesis hollow mould and is maintained for a predetermined period of time. If the pressure should drop below a given level, for example below a given tolerance value, the pump of the hydraulic pressure system providing pressure fluid to vessel 10 is again started until the desired pressure is obtained. A suitable location for a pressure sensor is indicated in Fig.

1 by the broken rectangular outline associated with reference numeral 6.

At the end of the pressure maintenance period, that is, the compression or post-compression time, all operating control lamps and all hydraulic cylinders will revert to OFF or quiescent position. This is schematically indi cated by step 01 and block 02, indicating turn-off of the control lamps and block 03 indicating reversion of the hydraulic pistoncylinder units 1,4 to their starting or quiescent position. Thus, piston-cylinder unit 4 will move in the direction of the arrow 4' towards the left, and the piston or plunger 1 a or the piston-cylinder unit 1 will move upwardly in the direction of the arrow 1'. All interlocks will, also, unlock and the pivoting movement control element 5 can pivot the cylinder 2 outwardly, schematically indicated by arrow 5b.Of course, rather than pivoting, a translatory shifting movement can be used for the cylinder 2 which, for example, can be hydraulically controlled, for example by pressure fluid from the storage vessel 10. Conversely, pressure cylinder 1 and the piston-cylinder unit 4 can be replaced by electrically driven spindles; combined hydraulic-pneumatic or hydraulic-electric operating units can also be used to move respective plungers by compressed air or any other gas, or mechanically by spindle drives, directly or indirectly coupled to the respective moved elements, for example by heavy-duty compression springs.

The last step, see step P, Fig. 2, is then the removal of the liner 15, which will have burst in the region of the spout 13, to permit injection of the mixture of powder and liquid plastic material, from the cylinder 2, and removal of the mould 3, including opening of the mould halves or shells 301, 302 to remove the plaster cast mould, with the injected dental prosthesis therein from the injection vessel 3.

Rather than using position sensors or pressure sensors, photoelectric sensors can be used, for example coupled by light guides to the control unit 8, and logically inter-connected with suitable sequencing and control units therein. The vessel 3, and the filling cylinder 2, as well as the containers therein, are released at the end of the programme from locked connection in the apparatus, so that they can be removed, separately or together, and exchanged for different moulding vessels, with different moulding forms, or the mould forms replaced, to prepare and initiate a new mixing and injection programme.

The programming control unit 8 can be used to also control mixing of powder and liquid of the plastic material for injection in the apparatus, and, further, to introduce a stirrer or a centrifugal mixer, particularly if a timed sequence is desired. This is particularly simple if the powder and liquid are directly mixed within the container 15. It is only necessary to retain the container 1 5 in a position which is out-of-alignment with the axis 3b and, then, for example automatically, lowering a stirrer element S into the vessel, and operating the stirrer element for a predetermined time interval, which can readily be determined by observation of the mixture, for example for between 5-30 seconds.Alternatively, the mixture can be stirred and intermixed by seating the cylinder 2 on a support and mixing from below, for example by vibration and shaking of the support. The external mixing and pouring-in of the mixed powder and liquid, then, can be eliminated as a separate processing step.

Some plastic materials require mixing remote from the injection process. Yet, in the operation of the time sequence can still be included in the programming of the programming control unit 8. The programming control unit 8 and the respective interlocks are provided to ensure that the respective steps are carried out, for example by less than apprentice personnel, by monitoring proper introduction of the filled mixed unit into the liner 15, presence of the liner 1 5 and/or position of the filling cylinder 2 and its tight connection in alignment with the axis 3b (Fig. 1) prior to application of injection pressure.

The spatial arrangement of the major portions of the injection apparatus is best seen in Fig. 1. Fig. 1. is schematic, in that holding frame units have been omitted. Such holding frame units can be secured to the base plate 11, as well known, for example by welding.

The entire unit is light enough to be readily transportable, together with the base plate 11. The housing 12, preferably formed with a handle, can then be used to carry the entire unit. Electrical components not shown, for example current supply, transformers, relays and circuit breakers and the like, as well as components associated with the hydraulic pressure fluid system have been omitted from the drawing for clarity. The powder-liquid mixture in vessel 2 usually will form a slurry, suspension, or solution mixture of dough-like consistency. The step of pouring the powderliquid mixture into the liner 1 remote from the cylinder 2, or when already inserted-is preferably carried out by pouring the mixture in a thin stream into the container, to ensure that the mixture is not lumpy. The time delay -block G3 (Fig. 2)-is also used to permit the mixture to swell and to further ensure intimate association of the powder and liquid of the mixture. This time, of course, can also be used to preheat the mould. The injection pressure should be maintained at a fixed level and the pressure sensor 6 is provided to monitor this pressure and, if it should approach a lower tolerance level, provide a "low pressure" signal to the control unit 8 which, then, can respond to energize the pressure fluid pump P coupled to the pressure fluid storage vessel to raise the pressure again above the tolerance limit and maintain the pressure at the given level, within a predetermined tolerance band.

The volume of injection, particularly with respect to injection volume per unit time, that is, the injection rate which is effected by the compression of the piston or plunger 1 a against the backing ring 1 b (Fig. 3) is preferably controllable, for example by a control setting on the control unit 8. This control is easiest effected by controlling the speed of operation of the piston or plunger 1 a. A suitable hydraulic valve in the hydraulic connection between the pressure fluid storage vessel 10 and the hydraulic piston within the piston-cylinder unit 1 provides a suitable control function.

The timing period of the time delay after operation of a starter-step G1 and block G3 (Fig. 2), preferably can be preset by a suitable command control unit in the control unit 8, which, typically, can be a timer circuit, operated mechanically in synchronism with the frequency of a power network, or electronically from the clock generator which is a standard part of a customary microprocessor unit which may be used for the control unit 8.

Feedback sensors, feeding back the actual position of the respective structure elements, for example the piston or plunger 1 a, the cylinder 2, the mould vessel 3, and the like, are preferably provided and connected to the control unit in customary control loop technology to automatically control the operation of the respective elements; otherwise, a timing unit may be used for timing of the movement of respective elements. Connecting lines 1 c, 4c, 5c, 6c, Pc + Scare provided.

The respective printed circuit (PC) boards 8a, 8b ... 8n are preferably so arranged that the respective control functions of the various elements are separately controlled thereby, so that repair and exchange is readily possible.

The pump P, connected by hydraulic lines schematically indicated by the line bundle 1 0a to the pressure fluid storage vessel 10 is, then, controlled by a single appropriate PC board, or separable tray, or electronic assembly unit within the control unit 8, for example the PC board 8b, or an equivalent insert element or panel within the control unit 8.

The system readily permits versatility and matching of the automatic control function to desired use. Thus, for some applications, the timing of the respective functions, for example the swelling time, e.g., time delay, block G3 (Fig. 2), can be entirely manually controllable, or automatically controllable; likwise, flexibility is provided since as many of the individual PC boards or control functions can be provided as are desired. For example, the feature of automatic mixing and stirring of the contents within the cylinder 2, and automatic introduction in line with the axis 3b (Fig. 1) can be controlled; in other installations, which may be less costly, these functions can be carried out manually and, consequently, the respective control panels or PC boards within the control unit 8 need not be provided.The control unit 8 will then only respond to the respective signals from the interlock switches to prevent operation of the plunger 2, under the high pressure used therein, without presence of the cylinder 2 and the filling therein.

The interlock functions, thus, should always be maintained, although the automatic operating functions can be selectively employed, or not. If automatic insertion of the cylinder 2 is desired, then, preferably, the cylinder 2 is permanently connected to the drive 5 for pivoting movement in a horizontal plane, or translatory movement, see arrow 5b. The drive and the cylinder 2 are then, preferably, permanently connected.

Various changes and modifications may be made, and various features described can be used separately or conjointly, within the scope of the inventive concept.

A suitable material for the packing ring 1 b is a POM plastic, commercially known under the trade name "Delrin".

Claims (22)

1. A method of making a dental prosthesis of polymerizable, temporarily flowable plastics material, which comprises the following steps (a) mixing the plastics material in the form of a powder and in the form of a liquid to form a powder-liquid slurry, suspension or solution, in a common vessel, to obtain a homogeneous, essentially bubble-free mixture; (b) filling the powder-liquid mixture into an injection vessel; (c) tightly closing the injection vessel and introducing the injection vessel into an injector press; (d) permitting the mixture to swell during a pre-determined time interval; (e) providing a separable mould form, positioned in communication with the injection vessel, and applying the injection pressure to the injection vessel to inject the swollen mixture into the mould form under a predetermined pressure;; (f) controlling and maintaining the predetermined pressure with a given pressure tolerance range during the injection step; and (g) opening the mould form and removing the finished injected dental prosthesis.

2. A method according to claim 1, wherein the step of applying injection pressure comprises applying hydraulic pressure from a hydraulic pressure source to a hydraulic piston or plunger acting on the homogeneous, swollen mixture in the injection vessel while providing hydraulic pressure during the entire injection step.

3. A method according to claim 2, wherein the step of providing hydraulic pressure comprises storing pressurized hydraulic fluid in a pressure fluid storage vessel; monitoring the injection pressure; and controlling the pressure within the pressure fluid storage vessel to remain within said given tolerance range.

4. A method according to claim 1, 2 or 3, wherein the step of mixing the polymerizable material comprises mixing a powder and liquid of methyl-methacrylate base plastics material.

5. A method according to claim 4, wherein the step of filling the powder-liquid methyl-methacrylate base mixture comprises introducing said mixture in a fine stream into the injection vessel.

6. A method according to claim 4, wherein the step of mixing the powder and liquid comprises introducing the powder and liquid methyl-methacrylate base plastics material into the injection vessel; mixing the powder and liquid to a homogeneous, essentially bubble-free mixture and permitting such mixture to swell to thereby combine steps (b) and (d).

7. Apparatus for manufacturing a dental prosthesis of a polymerizable, at least temporarily flowable plastics mixture consisting of liquid and solid components comprising a separable mould structure adapted to retain a prosthesis mould form, and having separable mould elements; an injection vessel for temporarily storing the mixture, and permitting the mixture to swell; means for applying a closing pressure to the separable mould elements of the mould structure; a fluid powered piston-cylinder unit positioned for applying injection pressure to said injection vessel and injecting the mixture therein into the mould form in the mould structure; control means controlling the application of closing pressure of the closing pressure application means, and injection pressure of the fluid powered piston-cylinder unit;; sensing means sensing the applied closing pressure and the applied injection pressure, and signalling the actual pressures to the control means, said control means controlling the application of the pressures to result in actual pressures being applied within a predetermined tolerance range, and permit release of application of the pressures only upon termination upon injection of the material and moulding of the prosthesis.

8. Apparatus as claimed in claim 7, wherein the control means includes means for manually controlling or commanding the injection pressure being applied to the pistoncylinder unit to thereby vary the injection speed.

9. Apparatus as claimed in claim 7 or 8, further including a pressure fluid storage vessel and a pressure fluid pump, said pump being connected to and controlled by the control means to disconnect the pump when a predetermined injection pressure is reached, and to reconnect the pump if the pressure drops below a lower level of pressure within said tolerance range.

1 0. Apparatus as claimed in claim 7, 8 or 9, further including interlock means sensing relative positioning of the injection vessel and alignment thereof with the mould structure and the piston-cylinder unit, and preventing operation of the piston-cylinder unit unless the piston-cylinder, the injection vessel, and the mold structure are in properly aligned position to prevent application of the high injection pressures by the piston-cylinder unit, and hence possible damage to apparatus or injury unless said injection vessel, mould structure, and piston-cylinder unit are in proper operating position.

11. Apparatus as claimed in claim 7, 8 9 or 10, wherein the means for applying closing pressure to the mould structure is connected to and controlled by the control means, the control means controlling the closing pressure application means to apply the closing pressure prior to application of fluid powered to the piston-cylinder unit and to release the closing pressure only after injection pressure from the fluid powered piston-cylinder unit has been released.

1 2. Apparatus as claimed in any of claims 7 to 11,'wherein the control means comprises separable control elements which individually control timing of processing steps; timing of application of injection pressures in addition to controlling the closing pressure and said injection pressure.

1 3. Apparatus as claimed in any of claims 7 to 12, further including an automatic drive coupled to the injection vessel and operable to place the injection vessel, selectively, in aligned injection position with the piston-cylinder unit, or out-of the aligned position, to permit access to the vessel for filling thereof.

14. Apparatus as claimed in claim 1 2, wherein the control means is interconnected with the means for applying the closing pressure, and control the closing pressure application means in accordance with a predetermined command programme.

15. Apparatus as claimed in any of claims 7 to 14, wherein the fluid powered pistoncylinder unit and/or the means for applying closing pressure comprises hydraulic pressure operated means.

1 6. Apparatus as claimed in any of claims 7 to 15, wherein the fluid powered pistoncylinder unit and/or the means for applying closing pressure comprises hydro-pneumatic pressure operated means.

1 7. Apparatus as claimed in any of claims 7 to 16, wherein the control means comprises individually replaceable, separable circuit carrying sections.

1 8. Apparatus as claimed in any of claims 7 to 17, including an operating drive connected to the injection vessel and moving the injection vessel in a horizontal plane in and out of alignment with the separable mould structure and the fluid powered piston-cylinder unit, operation of the drive being controlled by the control means.

19. Apparatus as claimed in claim 18, wherein the injection vessel and the drive therefor are permanently and securely connected together.

20. Apparatus as claimed in any of claims 7 to 19, and further including a stirring and mixing apparatus connected to and controlled by the control means for selective engagement in the injection vessel prior to its closure, for intimately mixing and stirring the liquid and solid components, and withdraw therefrom, under command of the control means; and wherein the control means, only after withdrawal of the mixing and stirring means, permits movement of the injection vessel into alignment with the separable mould structure and the piston-cylinder unit.

21. A method of manufacturing a dental prosthesis substantially as hereinbefore described with reference to the accompanying drawings.

22. Apparatus for manufacturing dental prostheses, substantially as hereinbefore described with reference to the accompanying drawings.