Injection-molding device and injection-molding method using volume expansion mechanism

Abstract

Provided is an injection-molding device capable of remarkably reducing residual air in a cavity and producing a wax mold that has higher fidelity to the shape of the cavity.　An injection-molding device (10) has: an injection part (24) capable of abutting a rubber mold (3); a wax tank (21) for pressurizing and storing wax; a vacuum tank (22) connected to a vacuum pump; a wax valve (31) for opening and closing a passage (21L) which connects the injection part (24) and the wax tank (21); and a first suction valve (32) for opening and closing a passage (22L) which connects the injection part (24) and the vacuum tank (22). The injection-molding device (10) further has: a volume expansion chamber (23); a volume expansion mechanism (40) for expanding the volume of the volume expansion chamber (23); and a second suction valve (33) for opening and closing a passage (23L) which connects the injection part (24) and the volume expansion chamber (23).

Description

Injection molding apparatus and injection molding method using a volume expansion mechanism

[0001] This application relates to an injection molding apparatus and injection molding method for wax used in lost-wax casting.

[0002] Jewelry, accessories, and decorative items, primarily made from precious metals, are small, complex, and have precise shapes. The demand for mass production of such products has led to the development of the lost-wax precision casting method in this industry. Of course, it can also be applied to the manufacturing of other precision industrial parts.

[0003] Lost-wax precision casting in the jewelry manufacturing industry involves the following steps: The first step is to create a prototype. The second step is to create a rubber mold with a cavity the same shape as the prototype. The third step is to inject molten wax into the rubber mold and remove it to produce a wax mold with the same shape as the prototype. Generally, the third step is repeated to produce a large number of wax molds. The fourth step is to attach the numerous wax molds produced in the third step to a wax rod in a tree-like manner, melting the ends of the sprue of the wax molds around it, then to place this in a cylindrical heat-resistant container and pour in plaster to create a plaster mold. The fifth step is a firing process using an electric furnace or gas furnace or other means to melt and remove the wax inside the plaster mold at a low temperature, completely burn off the wax adhering to the inside of the cavity at a medium temperature, and then raise the temperature further to give the plaster mold sufficient strength to withstand the impact when precious metal is poured in, after which it is lowered to a temperature suitable for pouring in the precious metal and left to await. The sixth step is the casting process, in which precious metal is poured into a plaster mold that has many cavities just like the original model. The seventh step is the finishing process, in which, once the precious metal has hardened, the plaster mold is rapidly cooled with water to break the plaster into pieces, the tree-like precious metal is extracted, the excess is cut off, and the precious metal is polished to the same shape as the original model.

[0004] Referring to FIG. 1, a method for manufacturing a rubber mold in the second step will be described. In the second step, a mold frame (not shown) sized according to the size of the rubber mold 3 to be created, a prototype 1, and a part 2 (runner 2a and nozzle 2b) are used. The bottom of the mold frame is sealed with a plate, a material before vulcanization of silicone rubber is placed in the lower half, and on top of it, the prototype 1 with the part 2 adhered thereto is arranged. Then, a material before vulcanization is placed in the upper half, the upper part of the frame is sealed with a plate, and it is vulcanized by raising the temperature while pressing. Instead of vulcanizable rubber, two-component curing type silicone rubber may be used.

[0005] When the silicone rubber has hardened and has the original elasticity of rubber, the rubber mold 3 is taken out from the above-mentioned frame to form a notch 3C, the prototype 1 and the part 2 adhered thereto are taken out, and separated into an upper rubber mold 3U and a lower rubber mold 3L. Thereby, a rubber mold 3 having cavities 3H and injection ports 3P with the same shape as the prototype 1 and the part 2 inside is obtained. By providing such a notch 3C throughout the separation surface, when the upper rubber mold 3U and the lower rubber mold 3L are combined, they can be accurately aligned, and the internal cavity shape can also be accurately reproduced.

[0006] Such a notch 3C also has the effect of facilitating vacuuming and sealing of the pressurized wax when injecting wax into the rubber mold 3 to create a wax mold. When taking out the wax mold from the rubber mold 3, after separating the upper rubber mold 3U from the mating surface (notch) 3C, if the lower rubber mold 3L is deformed during the removal operation, even a wax mold with a complex shape can be easily taken out without being damaged. In some cases, the inside of the rubber mold may be divided into several parts or cores may be inserted.

[0007] Since the prototypes have various sizes, the sizes and thicknesses of the rubber molds also vary accordingly. Also, the hardness of the rubber is often changed according to the durability of the rubber mold and the difficulty of taking out the wax mold from inside the rubber mold.

[0008] FIG. 2 shows a conceptual diagram of a conventional injection molding apparatus 10' used in the third step. The injection molding apparatus 10' has a main body apparatus 20 and a clamp unit 50.

[0009] The main unit 20 includes a tank 20T having a wax tank 21 for pressurized storage of wax and a vacuum tank 22 connected to a vacuum pump, an injection unit 24 that can contact the rubber mold 3, and a switching device 30 for selectively connecting the injection unit 24 to the wax tank 21 and the vacuum tank 22. The wax in the wax tank 21 is heated by a heater 20H.

[0010] The injection unit 24 can be selectively connected to the wax tank 21 and the vacuum tank 22 via a switching device 30. The switching device 30 has the function of a valve (wax valve 31) that opens and closes the passage 21L connecting the injection unit 24 and the wax tank 21, and a valve (first suction valve 32) that opens and closes the passage connecting the injection unit 24 and the vacuum tank 22.

[0011] The clamp unit 50 has the function of clamping the rubber mold 3 and pressing the injection port 3P of the rubber mold 3 against the injection unit 24.

[0012] By pressing the rubber mold 3, which is clamped by the clamp unit 50, against the injection unit 24 and opening the first suction valve 32, air can be sucked from the cavity 3H through the injection unit 24 (suction step). Subsequently, by closing the first suction valve 32 and opening the wax valve 31, wax can be injected from the injection unit 24 into the cavity 3H (injection step).

[0013] Figures 3(a) and 3(b) show an exemplary structure of the switching device 30. The switching device 30 has a wax valve 31 and a first suction valve 32. The wax valve 31 opens and closes the passage 21L by the movement of the wax valve stem 31a, and the first suction valve 32 opens and closes the passage 22L by the movement of the first suction valve stem 32a.

[0014] Typically, the vacuum pump used for suction can achieve a fairly high vacuum, around 10 Pa in absolute pressure. A perfect vacuum is theoretically zero Pa, or -101.325 kPa in gauge pressure. However, due to the passage of several pipe fittings, the involvement of numerous seals within the device, and the fact that air from cavity 3H is drawn into the vacuum chamber at intervals of several tens of seconds during the operation of the injection molding device 10', the vacuum pressure in the vacuum chamber 22 fluctuates to around -100 kPa, and can even drop to around -95 kPa if not properly managed. Therefore, it is impossible to completely remove the air from cavity 3H in the above suction step, and several percent of air, equivalent to atmospheric pressure, will inevitably remain as residual air within the cavity.

[0015] Compared to injection molding equipment for resins, which injects molten resin into a mold at high pressure, injection molding equipment for jewelry making, which injects wax into a rubber mold, uses significantly lower injection pressure, making it impossible to completely eliminate any residual air. Furthermore, in injection molding using molds, a constant amount of resin can be injected using screws or plungers, allowing for the inclusion of an air vent mechanism in the mold. In contrast, in injection molding using rubber molds, the wax is injected under constant pressure conditions, making it impossible to include an air vent mechanism. (If an air vent mechanism is included, wax will enter the mechanism and cause clogging.)

[0016] This causes various problems in the third step of wax mold creation. The rubber mold shown in Figure 1 is the simplest design, but if the wax is injected immediately after the cavity is evacuated, the wax will pass through the injection port 3P and branch out in the cavity 3H, and then rejoin at the point furthest from the injection port 3P. As a result, the aforementioned few percent of air will be collected at this confluence point and condensed to a pressure equal to the injection pressure.

[0017] Condensed air can cause bubbles or wrinkles, leading to a mismatch between the cavity 3H and the shape of the wax mold, resulting in a decrease in the quality of the wax mold. This is especially true for ring-shaped objects like those shown in Figure 1, such as rings, where intricate designs are often found at the confluence point. For example, if there is a protrusion at the confluence point, the protrusion may become rounded, or the wax may not fill to the tip, making the above problems more pronounced.

[0018] Currently, one way to address this problem is to apply baby powder or a similar substance to the rubber mold 3. This creates a very small gap in the cutout 3C of the rubber mold, which is expected to allow any remaining air in the cavity to be expelled during wax injection.

[0019] Increasing the amount of powder applied improves the efficiency of removing residual air, but the powder adheres to the surface of the wax mold, making the surface rough. Also, leakage from the gaps in the notches 3C formed by the powder increases, reducing the vacuum level of the rubber mold 3. Conversely, reducing the amount of powder applied prevents sufficient removal of residual air.

[0020] As described above, it is currently impossible to completely remove the air from cavity 3H, and therefore impossible to manufacture a wax mold that perfectly matches the shape of cavity 3H. In particular, the problem of residual air becomes even more pronounced as the design becomes more complex. For this reason, adjustments such as shifting the position of clamping force, changing the injection pressure, and changing the wax temperature are being made, but the problem is not fully solved, and making such adjustments every time the design is changed is excessively laborious.

[0021] One method for solving the above problem is disclosed in Patent Document 2. In Patent Document 2, residual air in the cavity 3H that is condensed during the wax injection process is sucked in through the wax blocking section. However, the method of Patent Document 2 cannot be applied to the many existing rubber molds. Furthermore, in the method of Patent Document 2, the achievable vacuum level of the cavity 3H is limited to the vacuum level of the vacuum chamber 22 (see

[0012] of this specification).

[0022] International Publication No. 2017 / 068670, Patent No. 6756958

[0023] As described above, after vacuuming, a few percent of the cavity volume remains as residual air in cavity 3H, converted to atmospheric pressure.

[0024] Therefore, if, after the air in the cavity 3H is removed by the vacuum pump and vacuum chamber 22, the air in the cavity 3H can be further removed by another means that does not rely on the vacuum pump and vacuum chamber 22, a high vacuum can be achieved and the residual air can be significantly reduced.

[0025] If this can be achieved, most of the aforementioned problems will be resolved, and it will be possible to manufacture rubber molds with more complex designs. Furthermore, it will also promote the evolution of decorative design. The objective of the present invention is to dramatically reduce residual air in the cavity. Another objective of the present invention is to enable the manufacture of wax molds that are more faithful to the shape of the cavity. Yet another objective of the present invention is to enable the manufacture of wax molds with more precise or fine shapes.

[0026] The present invention is disclosed below. <Aspect 1> An injection molding apparatus comprising: an injection unit capable of contacting a rubber mold; a wax tank for pressurized storage of wax; a vacuum tank connected to a vacuum pump; a wax valve for opening and closing a passage connecting the injection unit and the wax tank; and a first suction valve for opening and closing a passage connecting the injection unit and the vacuum tank, further comprising: a volume expansion chamber; a volume expansion mechanism for expanding the volume of the volume expansion chamber; and a second suction valve for opening and closing a passage connecting the injection unit and the volume expansion chamber. <Aspect 2> An injection molding method using the injection molding apparatus of Aspect 1, comprising: a preparation step of closing the wax valve and the first and second suction valves to bring the rubber mold into contact with the injection section; a first suction step of opening the first and second suction valves following the preparation step; a second suction step of closing the first suction valve and expanding the volume of the volume expansion chamber following the first suction step; and a wax injection step of closing the second suction valve and opening the wax valve following the second suction step.

[0027] The method for manufacturing the rubber mold 3 is shown. A conventional injection molding apparatus 10' is shown. (a) One form of the conventional switching device 30 is shown. (b) Another form of the conventional switching device 30 is shown. The injection molding apparatus 10 of one embodiment of the present invention is shown. The integrated assembly 70 of one embodiment of the present invention is shown. The peripheral part of the switching device 30 of the integrated assembly 70 is shown. The operation of one embodiment of the present invention is shown.

[0028] Figure 4 shows a conceptual diagram of the injection molding apparatus 10 of the present invention. The injection molding apparatus 10 of the present invention has a main body 20 and a clamp unit 50.

[0029] The main body device 20 of the present invention has a wax tank 21 and a vacuum tank 22 similar to those of the conventional device 10', as well as a volume expansion chamber 23 and a volume expansion mechanism 40 for expanding the volume of the volume expansion chamber 23. Furthermore, the switching device 30 of the present invention has the same functions as the wax valve 31 and first suction valve 32 similar to those of the conventional device 10', as well as the function of a valve (second suction valve 33) that opens and closes the passage 23L connecting the injection unit 24 and the volume expansion chamber 23.

[0030] The clamp unit 50 is the same as that of a conventional injection molding apparatus 10.

[0031] The operation of the injection molding apparatus 10 will now be explained. First, in the preparation step, with all valves 31, 32, and 33 closed, the rubber mold 3 is brought into contact with the injection section 24.

[0032] In the subsequent first suction step, the wax valve 31 is closed and the first suction valve 32 and the second suction valve 33 are opened. As a result, the air in the cavity 3H of the rubber mold 3 and the air in the volume expansion chamber 23 are drawn into the vacuum chamber 22. As a result, the vacuum level of the cavity 3H can reach a level similar to that of the suction step of the conventional apparatus 10' (see

[0011] ). As the drawn air flows into the vacuum chamber 22, the vacuum level of the vacuum chamber 22 may decrease slightly from its initial value.

[0033] In the subsequent second suction step, the volume of the volume expansion chamber 23 is increased after the first suction valve 32 is closed.

[0034] Let V1 be the volume of the rubber-type cavity 3H, V2 be the sum of the volume of the passage 23L and the volume of the volume expansion chamber 23 before expansion, and V3 be the volume expansion of the volume expansion chamber 23 in the second suction step. Then, in the second suction step, V1 + V2 changes to V1 + V2 + V3.

[0035] Therefore, by increasing the volume ratio of V3 to V1 + V2, the residual air in the cavity 3H mentioned in

[0012] can be significantly reduced. For example, if the volume ratio of V3 to V1 + V2 is increased tenfold, the residual air in the cavity 3H will be (V1 + V2) / (V1 + V2 + V3) = 1 / 11 = 0.091. If the volume ratio is increased twentyfold, the residual air will be 1 / 21 = 0.048. Typically, the volume V1 in the case of a ring is about 1.5 cc, and it is easy in design to set V2 to about 1.5 cc and V3 to 30 cc or more, so a volume ratio of 10 times or more can be easily achieved.

[0036] Thus, compared to conventional methods, the present invention makes it possible to dramatically reduce the residual air in the cavity 3H and dramatically increase the vacuum level. Furthermore, by injecting wax into the cavity 3H with a dramatically increased vacuum level, it is possible to manufacture a wax mold that more faithfully reproduces the shape of the cavity 3H.

[0037] The integrated assembly 70 will be explained using Figures 5 and 6.

[0038] As shown in Figure 5, the integrated assembly 70 includes a switching device 30 and a volume expansion mechanism 40. Preferably, the integrated assembly 70 houses the switching device 30 and the volume expansion mechanism 40 within a single housing.

[0039] As shown in Figure 6, the switching device 30 has a wax valve 31, a first suction valve 32, and a second suction valve 33.

[0040] The wax valve 31 is for opening and closing the passage 21L that connects the injection unit 24 and the wax tank 21. The wax valve 31 has a wax valve stem 31a and a wax valve seat 31b. The wax valve 31 is closed and opened by the wax valve stem 31a seating against and separating from the wax valve seat 31b. The wax valve stem 31a is integrated with the piston 61a of the wax valve cylinder 61 shown in Figure 5, and can be moved to the left in the drawing (in the direction of closing the wax valve 31) by the biasing force of the return spring 61b, and to the right in the drawing (in the direction of opening the wax valve 31) by the pressurization of the port 61c.

[0041] The first suction valve 32 opens and closes a passage 22L connecting the injection section 24 and the vacuum chamber 22. The first suction valve 32 has a first suction valve rod 32a and a first suction valve seat 32b. The first suction valve 32 can be closed and opened by the tip of the first suction valve rod 32a seating on and separating from the first suction valve seat 32b. The first suction valve rod 32a is integral with the piston 62a of the first suction valve cylinder 62, and can be moved downward in the drawing (in the direction of closing the first suction valve 32) by the biasing force of the return spring 62b, and can be moved upward in the drawing (in the direction of opening the first suction valve 32) by the pressurization of the port 62c.

[0042] The second suction valve 33 opens and closes a passage 23L connecting the injection section 24 and the volume expansion chamber 23 shown in FIG. 5. The second suction valve 33 has a second suction valve rod 33a and a second suction valve seat 33b. The second suction valve 33 can be closed and opened by the tip of the second suction valve rod 33a seating on and separating from the second suction valve seat 33b. The second suction valve rod 33a is integral with the piston 63a of the second suction valve cylinder 63, and can be moved upward in the drawing (in the direction of closing the second suction valve 33) by the biasing force of the return spring 63b, and can be moved downward in the drawing (in the direction of opening the second suction valve 33) by the pressurization of the port 63c.

[0043] By selectively opening and closing the wax valve 31, the first suction valve 32 and the second suction valve 33, the passages 21L, 22L, 23L connecting the injection section 24 to the wax tank 21, the vacuum chamber 22 and the volume expansion chamber 23 can be selectively opened and closed.

[0044] Further, the injection section 24 has a passage 24L extending from the opening 24a of the injection section 24 towards the switching device 30. The passage 24L is preferably linear. The passage 24L branches into three branches leading to the wax valve 31, the first suction valve 32 and the second suction valve 33 respectively via a cross-shaped intersection X. The branch leading to the wax valve 31 is arranged linearly with respect to the passage 24L, the branch leading to the first suction valve 32 is arranged in a direction perpendicular to the passage 24L, and the branch leading to the second suction valve 33 is arranged in a direction perpendicular to the passage 24L.

[0045] As shown in FIG. 5, the volume expansion mechanism 40 includes a volume expansion cylinder 41 and a drive cylinder 42.

[0046] The volume expansion cylinder 41 has cylinder chambers 41L and 41R on both sides of a volume expansion piston 41a. The left cylinder chamber 41L is a volume expansion chamber 23, which is connected to a passage 23L. The port 41b of the right cylinder chamber 41R is always in an exhaust state (or open to atmospheric pressure).

[0047] The drive cylinder 42 has cylinder chambers 42L and 42R on both sides of a drive piston 42a. Each of the cylinder chambers 42L and 42R has ports 42b and 42c.

[0048] The volume expansion piston 41a and the drive piston 42a are connected by a connecting rod 43 so as to move integrally.

[0049] The cylinder chamber 41L and the cylinder chamber 41R are sealed by a sliding packing on the outer periphery of the volume expansion piston 41a, the cylinder chambers 42L and 42R are sealed by a sliding packing on the outer periphery of the drive piston 42a, and the cylinder chamber 41R and the cylinder chamber 42L are sealed by a sliding packing on the outer periphery of the connecting rod 43, respectively.

[0050] In the volume expansion mechanism 40, by pressurizing the port 42b and exhausting the port 42c, the volume expansion piston 41a and the drive piston 42a can be moved to the left in the drawing, and by exhausting the port 42b and pressurizing the port 42c, the volume expansion piston 41a and the drive piston 42a can be moved to the right in the drawing. When the volume expansion piston 41a moves to the left, the volume of the volume expansion chamber 23 decreases, and when the volume expansion piston 41a moves to the right, the volume of the volume expansion chamber 23 increases.

[0051] FIG. 7 shows the operation of the injection molding apparatus 10 of the above embodiment when the integrated assembly 70 is used. The operation of the injection molding apparatus 10 includes the following steps (1) to (4). (1) Preparation step (2) First suction step (3) Second suction step (4) Injection step The above preparation step, first suction step, second suction step, and wax injection step may be carried out in the order of (1) to (4) above.

[0052] Figure 7(a) shows the preparation step. In the preparation step, the wax valve 31, the first suction valve 32, and the second suction valve 33 are all closed, and the rubber mold 3 clamped to the clamp unit 50 is pressed against the injection unit 24. At this time, it is preferable to move the volume expansion piston 41a to the left to minimize the volume expansion chamber 23.

[0053] Figure 7(b) shows the subsequent first suction step. In the first suction step, the first suction valve 32 and the second suction valve 33 are opened. The first suction step draws the air from the cavity 3H and volume expansion chamber 23 of the rubber mold 3 into the vacuum chamber 32. This allows the vacuum level of the cavity 3H to be raised to the same level as that of a conventional injection molding apparatus 10'.

[0054] Figure 7(c) shows the subsequent second suction step. In the second suction step, the first suction valve 32 is closed, and then the volume of the volume expansion chamber 23 is expanded (Figure 7(c)). By closing the first suction valve 32, the cavity 3H is connected only to the volume expansion chamber 23 via the passage 23L, and is isolated from the wax bath 21 and the vacuum bath 22. In this state, by expanding the volume of the volume expansion chamber 23, the remaining air in the cavity 3H is drawn into the volume expansion chamber 23. This makes it possible to significantly increase the vacuum level of the cavity 3H compared to the conventional injection molding apparatus 10'.

[0055] Figure 7(d) shows the subsequent wax injection step. In the wax injection step, the wax valve 31 is opened after the second suction valve 33 is closed (Figure 7(d)). This causes the wax from the wax tank 21 to be injected into the cavity 3H.

[0056] By performing the above preparation steps, first suction step, second suction step, and wax injection step, one cycle of the wax injection process is completed. Multiple wax molds can be created by repeatedly performing multiple cycles of the wax injection process.

[0057] The injection molding apparatus 10 may have a control device for controlling the operation of each valve 31, 32, 33 and the volume expansion mechanism 40. The control device may include a computer. The control device may perform each of the above steps automatically.

[0058] In the injection molding apparatus 10 described above, the residual air in the cavity 3H can be significantly reduced by performing a second suction step in addition to the first suction step. As a result, the manufacturing yield of wax molds is improved, and / or it becomes possible to manufacture wax molds that are faithful to the cavity shape, and / or it becomes possible to manufacture wax molds with fine and / or complex shapes that are difficult to manufacture with conventional technology.

[0059] The injection molding apparatus and injection molding method of the present invention can be used in lost-wax casting for the manufacture of small items such as jewelry and accessories, as well as industrial products.

[0060] 1... Prototype 2... Parts 3... Rubber mold 3C... Cutout 3U... Upper rubber mold 3L... Lower rubber mold 3H... Cavity 3P... Injection port 10, 10'... Injection molding machine 20T... Tank 20H... Heater 20... Main unit 21... Wax tank 22... Vacuum chamber 23... Volume expansion chamber 24... Injection section 24a... Opening 21L, 22L, 23L, 24L... Passage 30... Switching device 31... Wax valve 32... First suction valve 33... Second suction valve 31a, 32a, 33a... Valve stem 31b, 32b, 33b... Valve seat 40... Volume expansion mechanism 41... Volume expansion cylinder 42... Drive cylinder 43... Connecting rod 41a, 42a... Piston 41b, 42b, 42c... Port 41L, 41R, 42L, 42R, 43L, 43R... Cylinder chamber 50... Clamp unit 51... Table 52... Clamp force generator 53... Clamp plate 54... Pressing mechanism 61... Wax valve cylinder 62... First suction valve cylinder 63... Second suction valve cylinder 61a, 62a, 63a... Piston 61b, 62b, 63b... Return spring 61c, 62c, 63c... Port 70... Integrated assembly

Claims

1. An injection molding apparatus comprising: an injection unit capable of contacting a rubber mold; a wax tank for pressurized storage of wax; a vacuum tank connected to a vacuum pump; a wax valve for opening and closing a passage connecting the injection unit and the wax tank; and a first suction valve for opening and closing a passage connecting the injection unit and the vacuum tank, further comprising: a volume expansion chamber; a volume expansion mechanism for expanding the volume of the volume expansion chamber; and a second suction valve for opening and closing a passage connecting the injection unit and the volume expansion chamber.

2. An injection molding method using the injection molding apparatus of claim 1, comprising: a preparation step of closing the wax valve and the first and second suction valves to bring the rubber mold into contact with the injection unit; a first suction step of opening the first and second suction valves following the preparation step; a second suction step of closing the first suction valve and then expanding the volume of the volume expansion chamber following the first suction step; and a wax injection step of closing the second suction valve and opening the wax valve following the second suction step.

Images