Clamp pressure control method for injection molding process

The present invention relates to a method for injection molding thin resin substrates for optical disks such as CDs and DVDs, and a clamp pressure control method for same.

It is an object of the present invention to provide a clamping pressure control method for an injection-compression molding process, by which molding operation time is reduced without compromising the quality of molded articles, as well as the product quality of accurately molded articles is improved.

To achieve the above-described objects, the present invention provides a method for compression-molding an optical disk substrate, wherein molten resin is injected into a cavity between a stationary mold and a movable mold and molded into a desired shape by compressing the resin between the stationary mold and the movable mold.

When moving the movable mold to a predetermined position where a clamping force is generated between the stationary mold and the movable mold, positioning control of the movable mold is effected so as to move the movable mold at high speed. Upon detection of the movable mold having reached the predetermined position, the positioning control of the movable mold is switched to clamping force control, so that the clamping force generated between the stationary mold and the movable mold reaches a first target value, wherein the speed of the movable mold is regulated in accordance with a difference between a target clamp pressure value and the clamping force being applied. Upon detection of the clamping force having reached a second target value which is lower than the first target value, the clamping force control by speed feedback is switched back to the positioning control.

According to the clamping pressure control method of the invention, during the initial period of the injection-compression molding, high-speed position control is performed, which makes high-speed clamping control possible. Subsequently, clamping force control is performed by speed feedback based on a detected clamping pressure at a low speed, which enables fine regulation of clamp pressure in accordance with various pressure fluctuations in resin and thereby improves the molding accuracy. As a result, a reduction in operation time and stability in the product quality of an accurately molded article are both achieved.

While novel features of the invention are set forth in the preceding, the invention, both as to organization and content, can be further understood and appreciated, along with other objects and features thereof, from the following detailed description and examples when taken in conjunction with the attached drawings.

FIG. 1 is a schematic diagram illustrating the configuration of a mold clamping mechanism of an injection molding apparatus according to one embodiment of the present invention;

FIGS. 2A-2C are timing charts illustrating the clamping pressure control according to one embodiment of the invention;

FIG. 3 is a schematic block diagram given in explanation of the clamping force control by speed feedback according to the invention;

FIGS. 4A-4C are timing charts illustrating the clamping pressure control according to another embodiment of the invention; and

FIG. 5 is a schematic block diagram given in explanation of the clamping force control by speed feedback according to the invention.

FIG. 1 is a schematic diagram of an injection-compression molding apparatus according to one embodiment of the present invention. The apparatus is mainly composed of a mold 10 , a mold driving mechanism 7 for opening and closing the mold, and a control unit 8 . The mold driving mechanism 7 includes a crank mechanism 5 for converting the rotary motion transmitted from a servomotor 1 by way of a belt 4 into transverse reciprocating motion, and a toggle mechanism 6 for converting the transverse reciprocating motion into vertical motion. The control unit 8 controls the speed and torque of the servomotor 1 in accordance with analog signals fed from a distortion sensor 3 serving as a clamping force detector and disposed on a tie bar 2 . The mold 10 includes a lower mold 10 a mounted on a lower platen 9 free to slide along the tie bar 2 , and an upper mold 10 b secured to a stationary upper platen 11 and is provided with a nozzle, through which molten resin is injected.

The control unit 8 shown in FIG. 1 effects feedback control of the servomotor based on analog signals fed from the distortion sensor 3 and speed/position signals from the servomotor 1 . A clamping pressure control method according to one embodiment of the invention includes position control of the movable mold, and clamping force control which is effected by feed-back controlling the speed thereof based on detected clamping force.

FIGS. 2A-2C are timing charts respectively showing the changes in position of the lower mold 10 a , changes in the clamping pressure between the mold 10 , and changes in the speed of the lower mold 10 a during the injection-compression molding of resin. At T 1 , the lower mold is located at an injection start position P 1 where no clamping force is generated between the mold 10 . During the initial stage denoted at S 3 A in the drawing, the control unit 8 effects position control of the movable lower mold 10 a . That is, the lower mold 10 a is moved at high speed to a clamping position P 3 , where it applies a clamping force lower than a predetermined target value F 1 as shown in FIGS. 2A and 2B. Thus the lower mold 10 a is swiftly moved from the injection start position P 1 to the clamping position P 3 by the position control during the initial stage S 3 A from T 1 to T 2 . The clamping position P 3 may be predetermined such that a force slightly lower than a target clamping force F 1 is applied between the mold based on the correlation between the position of the lower mold and the pressure applied between the molds, and preset in the control program data.

Upon detection of the lower mold 10 a having reached the predetermined clamping position P 3 , the control unit 8 switches the control mode from position control to speed control which is based on the feedback of detected clamping force.

More specifically, from T 2 , the control unit 8 generates a speed command which is proportional to the difference between the detected clamp pressure that is being actually applied and a target pressure value. FIG. 3 is a schematic control block diagram illustrating the speed control method in accordance with the invention. In the drawing, F denotes a target clamp pressure whereas F′ denotes the detected pressure that is being applied. P represents a coefficient by which the difference between F and F′ is multiplied. V represents a speed command value input to the servo motor. The speed command V is obtained by adding an integral of the difference between F and F 1 multiplied by a predetermined coefficient I to the proportionally multiplied value of the difference between F and F′.

Thus the speed of the servomotor for driving the lower mold 10 a is controlled in accordance with the proportionally multiplied value of the difference between a target clamp pressure and the pressure that is actually being applied. While such control takes time, it enables slight pressure fluctuations to be responsively followed, whereby regulation of pressure to be applied is achieved with a high degree of precision.

Compression of the resin is performed such that the pressure applied by the mold is gradually decreased as shown in FIG. 2 B. That is, after a target clamp pressure F 1 has been reached and maintained for a while, the lower mold is lowered so as to decrease the pressure. Thus reduced pressure values F 2 and F 3 are reached stepwise during the period S 3 B precisely by the above-described speed feedback control.

Upon detecting the clamp pressure having reached a predetermined value F 3 , the control unit 8 switches the control mode from the clamping force control back to the position control. Thus from T 3 , the lower mold 10 a is swiftly moved to a predetermined position P 4 and then back to an initial position.

The switching of the control mode from the position control to the clamping force control may be effected automatically upon detection of the clamp pressure having reached the value F 1 ′ which is slightly lower than the target value F 1 .

As described above, according to the clamping pressure control method of the invention, after or during the injection of resin, the lower mold is swiftly moved to a predetermined position by the high speed position control, and thereafter, the control mode is switched to the clamping force control which enables fine regulation of pressure to be achieved in accordance with pressure fluctuations in the resin. After the compression of resin when such delicate control of clamping pressure is no more necessary, the control mode is switched back to the fast position control. In this way, the present invention achieves a considerable reduction in process time without compromising the quality of resultant molded articles by combining different control modes during the injection and compression of resin.

Next, another embodiment of the clamping pressure control method in accordance with the invention will be described with reference to FIGS. 4A-4C and FIG. 5 . The clamping pressure control method of this embodiment includes constant speed control of the movable lower mold 10 a and speed feedback control thereof which is based on a detected clamping force.

FIGS. 4A-4C are timing charts respectively showing the changes in position of the lower mold 10 a , changes in the clamping pressure between the molds 10 a and 10 b , and changes in the speed of the lower mold 10 a during the injection-compression molding of resin. At T 1 , the lower mold is located at the above-described injection start position P 1 . During the initial stage denoted at S 3 C in the drawing, the control unit 8 effects constant speed control of the movable lower mold 10 a . That is, the lower mold 10 a is moved at a predetermined, constant, high speed V 0 , until the pressure applied between the molds is detected to have reached a level −Δ 1 which is lower than a target value F 1 as shown in FIGS. 4B and 4C. The speed V 0 may be predetermined based on the correlation between the speed of the lower mold and the pressure applied between the molds, and preset in the control program data. Thus the lower mold 10 a is swiftly moved from the injection start position P 1 to a clamping position by the constant speed control during the initial stage S 3 C from T 1 to T 2 .

Upon detection of the clamping pressure having reached the predetermined level −ΔF 1 , the control unit 8 switches the control mode from the constant speed control to speed feedback control which is based on the detected clamping force.

At T 2 when the control mode is switched to the speed feedback control, the speed of the movable lower mold 10 a is lowered so that the target clamping pressure F 1 is precisely achieved. Here, the target clamp pressure F 1 must not be overshot to ensure uniform high quality of molded articles, and therefore the speed of the lower mold is instantly decreased upon the switching of the control modes at T 2 . Thereafter, during the period S 3 D, the above-described speed feedback control is effected.

The constant speed control is effected such that, the speed of the lower mold is maintained constant by a speed command which is proportional to the difference between a target speed and the actual speed. In order to prevent undesired overshoot of the lower mold when the control modes are changed at T 2 , a speed limit may be provided in the speed feedback control as shown in FIG. 5 . That is, in producing speed command control V, a limit may be given so that the speed of the lower mold will never exceed the predetermined value V 0 .

In this way, the lower mold 10 a is first moved at high, constant speed, whereby a reduction in process time is achieved. The control mode is switched to speed feedback control and the speed of the lower mold instantly decreased as soon as the clamp pressure level has reached the level −ΔF 1 which is lower than the target value F 1 . As a result, the target value F 1 is precisely achieved without the risk of overshooting.

While there has been described what are at present considered to be preferred embodiments of the present invention, it will be understood that various modifications may be made thereto, and it is intended that the appended claims cover all such modifications as fall within the true spirit and scope of the invention.