GB2051654A - A method of producing resin articles using a plaster mold - Google Patents

Abstract

A method of producing resin articles (A, B) using a plaster mold (6), comprises the steps of sensing the resin injection pressure in the plaster mold by a pressure sensing means (9) and controlling the injection conditions of a molding machine in accordance with a desired optimum pattern of correlation between the diameters of a sprue runner (4) and an air vent (5), which pattern is stored in the memory of a computer in advance. The injection conditions include the speed of injection of the resin which is controlled in order to change the internal pressure of the mold (6) to prevent damage to the article being moulded and to the plaster mold (6). <IMAGE>

Description

SPECIFICATION A method of producing resin articles using a plaster mold, and articles made thereby This invention relates to a method of producing in a plaster mold, resin articles such as dental prosthetic goods or industrial trial articles, and to products made by the method.

Heretofore, the resin charging step in the manufacturing process of bedded artificial teeth in the dental prosthetic goods field has typically been performed manually. However, because of this manual operation, the resulting product lacks dimensional stability due to non-uniform pressure polymerisation, thus making the manufacture of an article of accurate size impossible.

In place of such manual operation, there has been devised a method of resin molding using plaster molds in which a molding machine can be applied to a plaster mold. However, molding machines for plaster molds and for metal molds are basically different in their operating conditions. Moreover, the following problems are encountered with this kind of molding machine. In the molding machines in general, injection molding is a process wherein plastic material is fed by a screw into a heated cylinder and plasticised by external heating and by the inner heat of friction caused by the rotation of the screw. The resulting melted material is introduced and charged by mechanical energy into the metal mold, pressurised, and cooled to a molded article with the desired quality.Thus, the plastic material is changed, by external mechanical energy, from solid to liquid state, charged into the mold by external kinetic energy and again changed into a solid through absorption of the energy by the mold.

Thus, the process and the quality of the completed article are greatly influenced by external molding conditions.

In practice, molded products may have defects due to incorrect molding conditions because of short shots, flashing, flow marks, jetting, weld lines, gas burning, air bubbles, clouding, cold marks, strain, warping, cavities, noncompliance to standards, intershot standard errors and interrod standard errors.

On the other hand, there may be such molding conditions as cylinder heating temperatures for plasticising the material, screw RPM, the amount of the material to be stored in the injection cylinder for injection into the mold, metering strokes, screw back pressure, injection pressure for charging and pressurising into the mold, injection speed, injection time, cooling temperature for absorption of thermal energy, and cooling time. Thee conditions are critical and indispensable for producing molded articles. For instance, if the cylinder temperature is too high, the properties of the plastic material are lost through deterioration of the material. If the metering stroke is excessive, the resin will stay in the cylinder too long, resulting in deterioration of the material. If the injection energy is too high, the resin may ooze out from the parting line of the mold thus causing strain.If the injection energy is too low, the lustre property of the resin is lost, sometimes causing cold marks. Moreover, if the cooling time is short, the molded product may be unstable in shape and warped.

Because of these factors, skilled operators were required, that is, the molded products were checked and adjusted by visual inspection of the operators. The molding conditions are interrelated to one another and the molded product cannot be adjusted unless it is known how the conditions of the plastic material are changed under given conditions throughout the entire process, and what is happening in the mold.

As the molding conditions are not constant, but are changing at all times to a greater or lesser degree, the product quality for each shot cannot be maintained easily. In effect, the plastic material may be changed in its conditions through changes in hydraulic pressure, oil temperature, mold temperature and heating temperature. The molding machine must remain stable under the effect of these changes. Fluctuations or discrepancies in the product quality in the absence of these changes are attributable, among plasticising factors, to air discharge from the resin, material purity, mixing of regenerated articles, mixing of compound material and viscosity of regenerated articles. These factors have been confirmed experimentally to account for a majority of fluctuations occurring in the products and thee factors are extremely difficult to place under effective quality control.

The optimum injection molding should be so conducted that an optimum amount of evenly plasticised molten resin is charged into the mold at an injection energy suited to the mold, the injection energy is maintained accurately until sealing of the sprue and/or a gate and the product is cooled for a proper cooling time interval at a proper cooling temperature.

The problem is then how to control these factors. The status functions for the plastic material comprise three parameters such as resin pressure, resin capacity and resin temperature. It has been proposed to provide a sensor in the mold for sensing the resin temperature and feeding the sensed temperature back to heating means mounted at the entrance to the gate. However, such control is in practice a matter of great difficulty in view of a response in high injection speed of the resin which may be of the order of less than one second. The resin capacity can be made constant by maintaining the mold clamping state constant. An increase in the charged quantity until the cavity is filled with resin can be sensed indirectly through pressure increase in the resin. Thus, the basis for control is resin pressure.Fig. 8 of the appended drawings shows the relation between the mold inside pressure and time, such defects as flow marks, jetting, weld lines, gas burning, bubbles, cloud, cold marks, strain and warping occur during charging (time point A in the drawing); such defects as flashing, or strain occur immediately before completion of charging (time point B); such deects as short shot, flashing, warping, cavity nonconformance to standards and intershot standard errors occur during pressure holding (time point C) and such defects as warping or shot occurs during cooling (time point D). Thus, a majority of defects are caused during charging. Accordingly, the basic condition for molding of resin is the resin pressure.

Hence, it has been desired for some time to provide a method for the molding of resin under optimum conditions and by use of a plaster mold, wherein the correlation among the injection speed, inside pressure of the mold and the holding pressure on one hand and the sprue, diameters and lengths with air vent diameters and lengths on the other hand, are clarified and the corresponding technical measures are used for producing resin articles having the desired physical properties together with dimensional stability.

As the resin is pressurised and charged abruptly or at high speed from an injection opening into a mold, the air in the mold is compressed and the internal pressure of the mold becomes momentarily high; thus resistance within the mold becomes high. This momentary abrupt increase in the pressure in the mold lowers the speed of the resin about to be charged into the mold at a high speed of less than one second, thus inducing the cooling of the resin. However, in the injection mold, as the time in which the resin is cooled is inversely proportional to the distance traversed by the resin and the thickness of the resin product, the charging operation must be completed before the resin is cooled and hardened.Application of excessive pressure to the mold without regard to the above leads to an increased pressure remaining in the mold and reduced physical properties and stability of the resin. Moreover, molding may become impossible if the product has both thick and thin portions. It is therefore a primary object of the invention to provide a method and an apparatus in which the optimum correlation between the injection speed and pressure and the air vent diameter can be obtained.

It is a second object of the invention to provide a method and an apparatus for dealing with such a situation that, while abrupt pressure increase in the mold as encountered during injection molding can be avoided by air vent, the vent is stopped by hardened resin at the next moment and the resin so far charged into the mold starts to be hardened and be subjected to shrinkage.

Thus, the correlation between the air vent and sprue and the mold holding pressure must be clarified.

Moreover, if the molded product has considerable thickness, the dimensional stability cannot be maintained unless the holding pressure is applied immediately, because the resin once charged into the mold is subjected to cooling and shrinkage. The problem is then how to control this and how to cope with the situation that while decreased pressure due to shrinkage may be compensated by addition of resin, there is no way for charging of resin if the sprue is stopped completely through hardening before the molded article has assumed a completely stable condition. Thus, it is a third object of the invention to provide a method and an apparatus to overcome this problem through clarification of the correlation between the thickness and the sprue diameter and length.

It is a fourth object of the invention to provide a method and an apparatus for imparting physical properties and stability in case the molded product has both the thick and thin portions. It is of no avail to consider only the shrinkage caused during molding, and charging into the mold must be completed before hardening of the resin. To this end, injection and charging must occur in a very short time by using an elevated injection speed. However, if the air at the thick portion is compressed to dwell in the thin portion, the air resistance given by the thin portion will increase, delaying the resin flow during charging and resulting in cooling and reduced injection charging effect. Thus, a shorter article may be produced, or the physical properties and accuracy may not be obtained.On the other hand, increasing the injection speed and pressure recklessly leads to unstable quality and destruction of the plaster mold. In this case, the technical problem to be solved is that the correlation between the air vent and the diameter and length of sprue must be taken into consideration, which is capable of hardening the resin which enables the air vent to seal with respect to dimension and accuracy thereof to supplement and to stabilise the resin for contraction or shrinking so as to secure the discharge of air within the mold and properties thereof.

It is a fifth object of the present invention to provide a plaster mold cavity such that the shape of the wax pattern to be embedded in the plaster mold may be faithfully maintained.

These objects may be attained by the present invention wherein a wax pattern, a sprue, a gate and an air vent are mounted in the desired manner in a plaster mold, plaster is injected into the plaster mold, and the air bubbles in the plaster are then removed by vacuum defoaming, the mold is heated for removing the wax pattern, and the resin is injected from a molding machine after washing. At this time of injection, the air within the mold is removed by an air vent having a diameter decided by calculation having consideration to the correlation between the sprue, and gate diameters and lengths, and the pressure inside the mold while charging the resin. At the same time, the resin pressure is sensed, by a resin pressure sensor provided in the sprue, until the resin is filled to the terminal part of the cavity, cooled, and solidified.In addition, a setting pressure to be included within the maximum allowable range of the pressure in the mold, short of insufficient pressure or overpressure is determined. A controller is used to control the hydraulic system of the molding machine so that this setting pressure may be maintained until sealing of the mold gate.

In order that the invention may be more fully understood, embodiments in accordance therewith will now be described by way of example with reference to the accompanying drawings, in which:~ Figure 1 is a longitudinal section of a mold for uSe in a method according to the present invention showing the molding of a molded product having a uniform thickness; Figure 2 is a longitudinal section of a mold for use in a method according to the present invention showing the process of integral molding of a molded product having a thick portion A and a thin portion B; Figures 3 to 5 are perspective views of molded products; Figure 6 is a plan section showing the inside of the flasks provided with cooling and heating means;; Figure 7 is a longitudinal section of another mold for use in a method according to the present invention showing the molding of a dental prosthetic goods having a thin palatal surface; Figure 8 is a characteristic curve of the inner pressure of a mold and the time elapsed according to the prior art; Figure 9 is a characteristic curve of a setting pressure within a mold to be controlled; Figure 10 is an ideal characteristic curve of the inner pressure of a mold to be controlled according to the present invention; and Figure 11 is another characteristic curve of the inner pressure of a mold according to another embodiment of the method of the present invention.

Referring to the drawings, Fig. 1 shows a structure of a molding machine according to the present invention wherein a resin product (a) to be molded in a mold cavity 1, has approximately a constant thickness.

A wax plaster 6 is housed in known manner within a lower flask 2 and an upper flask 3 and there is provided a sprue runner 4 and an air vent 5 which extend in opposing directions respectively, along the joint surfaces of the upper and lower flasks 2, 3.

Plaster 6 is injected into the hermetically sealed flasks and air bubbles are removed from the plaster by vacuum defoaming. After the plaster has hardened, wax is removed therefrom by heating to provide a plaster pattern 7. In the vicinity of an injection opening 8 which leads to a sprue 4, there is provided a sensor pin 9 of an internal pressure sensing and controlling device (not shown) for sensing the internal pressure. An injection nozzle of an injection molding machine (briefly shown), having a very high speed injection capability (maximum injection speed 0.1 second) is applied to the opening 8 for pressure injection of ultra hard resin into the plaster mold 7 at an injection speed of 0.01 to 10 seconds and an injection pressure of 30 to 1200 kg/cm2.At this time, the air held in the mold 7 is compressed, resulting in a momentary increase in internal pressure and elevation in the resistance prevailing in the mold. This pressure is relieved smoothly by air vent 5 for preventing an abrupt increase in the internal pressure. After complete removal of the mold pressure, the air vent 5 is sealed hermetically. The numerical values showing an optimum correlation should be stored in a memory within a computer, with the thickness of the molded product and other conditions taken into account within the range of the diameter or perimetric length of 0.1 to 5 mm of the air vent 5 for the diameter or perimetric length of 0.5 to 20 mm of the sprue 4.

Granted that the abrupt increase in the pressure prevailing within the mold could be avoided by air removal through the air vent 5, the latter may be sealed at the next moment by hardened resin, and hardening shrinkage of injected resin is started after completion of injection. Therefore, a holding pressure must be applied immediately for a holding time of 0.05 to 60 seconds for compensating such shrinkage. At this time, the air vent 5, used for relieving the pressure within the mold, must be sealed completely.

In the embodiment of Fig. 1 in which the molded product has a uniform thickness, the molding operation does not encounter major difficulties. However, in the case of the molded product having a larger thickness, the higher the temperature to which the resin is heated, the more the shrinkage which the product undergoes when the latter is held and cooled in the mold. As a countermeasure to such shrinkage, the sensor pin 9 of the inner pressure detection and control device is provided for the sprue 4, and the destruction pressure of the plaster mold is memorised in advance in the computer so that a suitable amount of resin can be supplied into the mold for compensating resin shrinkage in accordance with the decrease in the internal pressure.It is to be noted that, if the sprue 4 has completely solidified before the injected resin has been molded completely and stably, the control function of the sensor pin 9 may be lost. With this consideration, the optimum correlation between the diameter and length of the sprue 4 and the thickness of the molded product is stored in the memory of a computer so that the optimum conditions may be instructed by the computer for the purpose of preventing the solidification of the sprue 4.

Fig. 2 shows an embodiment of the invention wherein the molded product has a nonuniform thickness and thus has a thick portion A and a thin portion B which must be molded simultaneously.

In this case, it is of no avail to take solely the shrinkage into consideration during molding, and hardening of the resin should not occur until the resin has been completely injected into the mold. Thus, an increased injection speed is required in comparison with that used in the embodiment of Fig. 1. First of all, if the air in the thick portion A is compressed and held in the thin portion B, the air pressure thereof becomes large and delays the resin flow during the injection moulding to cool the injected resin, thus resulting in a lowered injection effect and worsening physical properties of the molded product. An elevation in injection pressure and hence in injection speed may overcome the resistance in the mold. However, the plaster may be destroyed. The air vent 5 is essential for avoiding such increase in pressure in the mold.In addition, the air vent 5 should be of such a diameter and length as to be sealed by the injected resin itself after realisation of the above effect.

The diameters of sprue 4 and the air vent 5 are determined in accordance with the size of the products to be molded, but in order to compensate the discharge pressure and retain the suitable properties of the material by supplementing the resin for the shrinkage, a proper correlation between the sprue 4 and the air vent 5 is required, which enables the resin capable of sealing the air vent, to harden so as to prevent the secondary pressure from discharging.

The physical properties and dimensional accuracy may be realised with a product having both thick and thin portions by giving it numerical values indicative of such correlation stored in advance in a control device and having the optimum injection molding conditions instructed for the particular molding operation.

In injection molding not only of industrial products, but of dental prosthetic goods, where not many products are required and the injection molding is to be made in the plaster mold secured by the flasks, the internal pressure in the mold is sensed and adjusted by the sensor pin 9 for securing the physical properties and dimensional accuracy of the molded product. The sensor pin 9 is connected directly to the sprue 4. By such means, a product having a thin portion B with a thickness of 0.5 mm and a thick portion having a thickness of 20 mm, which otherwise had to be molded separately and connected to one another after molding, can be molded integrally and simultaneously, as shown in Fig. 4, through injection molding at ultra high speed and without the risk of destruction of the plaster mold. The pressure within the mold of the molding machine is controlled in the following manner.A pressure transducer or sensing means for sensing the resin pressure is mounted near a runner of an injection mold or a cavity gate near the sprue 4 as described before. The resin pressure is measured, with the aid of an oscillograph or other recorder (not shown), from the time that molten resin is injected from the nozzle of the molding machine into the mold until the resin has completely filled the cavity 1 and solidified through cooling. Thus, a desired setting pressure c is determined, said setting pressure being included within a tolerable range within the upper and lower limits a and b short of insufficient charging and overcharging, as indicated in Fig. 9. Then, the hydraulic system of the molding machine is controlled until sealing of the runner 4 is obtained so that the pressure in the mold may be included in the range of this setting pressure by a controller.

It may be remarked that, in the above construction, a variable hydraulic pressure supply device, such as a servo valve or a proportional magnetic valve, may be mounted on the injection means for providing a closed loop adapted for feedback of resin pressure within the mold so that the inner pressure in the mold may be controlled for each shot by the controller until sealing of the sprue 4 is performed in such a manner that the inner pressure in the mold is within the setting pressure as indicated in Fig. 10.

Alternatively, the variable hydraulic pressure supply device is applied in the above construction to the injection means of the molding machine and, a shown in Fig. 11, a desired programme control is formulated for the injection speed according to the actual mold configuration until the time of pressure increase of each section of setting pressure components, and the setting pressure is maintained until sealing of the air vent, with the programme prevailing so far being nullified as the setting pressure Ps is attained.

According to the present invention, in resin molding with the plaster mold 7, the resin is injected at high speed from a sprue of 0.5 to 20 mm in diameter or perimetric length for a pressure holding time of 0.05 to 60 seconds and by using injection molding conditions of 0.01 to 10 seconds of injection speed and 30 to 1200 kg/cm2 of injection pressure. The pressure in the mold having a tendency to increase abruptly is relieved by the air vent 4 with the diameter of perimetric length of 0.1 to 5 mm. After the mold 7 has been filled with resin, the air vent 5 is sealed by cooled resin, and an amount of resin is introduced so as to compensate for the shrinkage of the molded product. After filling, the sprue 4 is cooled and hardened. The sensor pin 9 of the control device connected direct to the sprue 4 senses the internal pressure for controlling this sequence of operations.Thus, the optimum correlation between the sprue 4 and the air vent 5 for securing the dimensional accuracy and desired properties of the molded product can be established at the optimum condition while controlling the internal pressure of the plaster mold, thus yielding the foregoing effect.

However, when the time has come for the sprue 4 to harden, the thick portion of the filled mold still undergoes cooling and shrinkage because of the fact that the thick portion is too thick, the desired properties and accuracy cannot be obtained. Thus, as shown in Fig. 6, a cooling pipe 10 is embedded in advance at the periphery of the thick portion, while a heating pipe 11 is embedded where the heating is required, so as to improve the overall physical properties and accuracy of the molded product.

Moreover, when manufacturing the plaster mold, mixing of plaster with water and stirring of the resulting mixture are carried out as known manner and when embedding, injection of the plaster is carried out in vacuo prior to the reaction of plasticisation thereof and at the same time vibration is applied thereto, which leads to an elaborate or smooth inner surface of the plaster pattern, thus improving the accuracy of the molded product.

Fig. 7 shows a cross-sectional view of the mold applied to a dental prosthetic goods, (such as artificial teeth) to which the above molding method according to the present invention is applied. In the figure, unless the thickness of the portion of the molded product corresponding to the upper palatal surface 12 of the mouth is less than 0.5 mm, it is normal in view of the human physiology that the physiological action of rejecting an alien or foreign substance entering in the mouth is always performed so as to defend the body by repelling the foreign substance out of the body. Accordingly, the portion must be very thin.In this respect, the artificial teeth heretofore obtained by the conventional method, however, were often broken at the thin portion because the latter cannot effectively withstand the chewing pressure; thus artificial teeth with a thin portion having sufficient strength cannot be obtained. Recently, resin with satisfactory properties in this area has been developed. However, such resin has to be heated to between 280 and 400 C or even higher, and exhibits poor fluidity during injection charging. Accordingly, in such resin, in order to compensate for the defects, the desired physical properties and dimensional accuracy cannot be attained unless the injection charging is completed at an injection speed of about 0.1 second.Moreover, products made according to conventional techniques have a property of absorbing water, and in turn have flexion so that the accuracy is poor. In the conventional injection molding process, it was not possible to manufacture artificial teeth with the physiologically ideal thin portions of less than 0.5 mm as shown in Fig. 7, in which the existing resins such as polycarbonate, polyallylate or polysulfonic resin were injected into the plaster mold and were molded. The injection molding of the ideal dental prosthetic goods according to the present invention has been made possible by an injection molding apparatus using a very high injection speed, which comprises the sensor pin 9 of means for sensing and controlling the inner pressure of the mold, and the cooling and heating means for the plaster mold operable in accordance with the correlation between the parameters of the sprue 4 and the air vent 5. Moreover, stable properties of the material within the plaster and the accuracy of the molded products can be realised by the present invention.

Claims (8)

1. A method of injection molding of a resin using a plaster mold, comprising the steps of providing an air vent in the plaster mold, sensing the injected resin pressure in the plaster mold by pressure sensing means, and controlling the injection conditions including the injection speed of the resin to be injected from a molding machine into the mold in accordance with the sensed pressure so as to change the inner pressure within the plaster mold in a predetermined optimum pattern stored in a memory of a computer below the destruction point of the product to be molded.

2. A method as claimed in Claim 1, wherein said injection speed is determined by the correlation between the diameter of a sprue and the diameter and the length of the air vent.

3. A method as claimed in Claim 1, wherein the optimum pattern for the injection mold is controlled in accordance with the following conditions; injection speed: 0.01 to 10 seconds mold internal pressure: 30 to 1200 kg/cm2 pressure holding: 0.05 to 60 seconds sprue diameter: 0.5 to 20 mmf air vent diameter: 0.1 to 5 mmf air vent length at least 30 mm.

4. A method as claimed in any of Claims 1 to 4, including the step of embedding a wax pattern in the plaster mold by vacuum defoaming.

5. A method as claimed in Claim 1 or Claim 2, further comprising the step of providing cooling pipes in the circumference of the thick portion or portions of the mold.

6 A method as claimed in Claim 1 or Claim 2, further comprising the step of providing heating pipes for a portion or portions to be heated so as to produce a heating effect on the mold.

7. A method of injection moulding a resin using a plaster mold, substantially as hereinbefore described with reference to the accompanying drawings.

8. A molded product manufactured by the method as set forth in any one of the preceding claims.