The present invention relates to a molding method and to a molding apparatus for molding a resin product having a thick portion projecting outwardly on a rear surface thereof.
In the molding of a resin product having a thick portion projecting outwardly on a rear surface thereof, a sink mark (recess) may be formed on a surface of the thick portion of the resin product by the contraction of the resin. Due to this, the appearance of the resin product is deteriorated. In order to solve this problem, as disclosed in Japanese Unexamined Patent Application Publication No. 11-198165, a technique has been proposed in which pressure is applied to a melted resin filled in a cavity of a die, and the melted resin on which the pressure is maintained is solidified so that the contraction degree of the resin is compensated for, and the formation of a sink mark is thereby prevented. In this technique, while the pressure on the melted resin is maintained, a pressurized gas is simultaneously supplied from a rear surface side cavity surface of the die, which corresponds to the rear surface of the resin product, into the cavity, so that the melted resin is pressed on a surface side cavity surface of the die, which corresponds to the surface of the resin product.
However, in this technique, if the maintenance of the pressure on the melted resin is not appropriately performed, a recess will be easily formed on the surface of the resin product. In addition, since the supplying of the pressurized gas is simultaneously performed while the pressure on the resin is maintained, the pressure of the supplied gas must be set high, and it is therefore necessary to use an expensive molding apparatus. Since the maintenance process of the pressure on the resin is necessary, the production cycle is time-consuming, and productivity is thereby deteriorated.
The present invention was made in consideration of the above problems. Therefore, an object thereof is to provide a molding method and a molding apparatus which can prevent formation of recesses on surfaces of resin products, can reduce molding apparatus cost, and can improve productivity.
According to one aspect of the present invention, a molding method for a resin product having a thick portion projecting outwardly on a rear surface of the resin product was made in order to solve the above problems. The molding method includes: a first step for supplying a melted resin into a cavity of a die by an amount which is less than a volume of the cavity. The cavity has a surface side cavity surface corresponding to a surface of the resin product and a rear surface side cavity surface corresponding to the rear surface of the resin product. The molding method further includes: a second step for supplying a gas from the rear surface side cavity surface of the cavity into the cavity, so that the melted resin is separated from the rear surface side cavity surface of the cavity and closely contacts the surface side cavity surface of the cavity; and a third step for solidifying the melted resin maintained in the condition set in the second step.
According to another aspect of the present invention, a molding apparatus for a resin product having a thick portion projecting outwardly on a rear surface of the resin product was made in order to solve the above problems. The molding apparatus includes: a supplying control device for supplying a melted resin into a cavity of a die by an amount which is less than a volume of the cavity. The cavity has a surface side cavity surface corresponding to a surface of the resin product and a rear surface side cavity surface corresponding to the rear surface of the resin product. The molding apparatus further includes: a gas supplying device for supplying a gas from the rear surface side cavity surface of the cavity into the cavity, so that the melted resin is separated from the rear surface side cavity surface of the cavity and closely contacts the surface side cavity surface of the cavity; and a temperature control device for controlling a temperature of the surface side cavity surface so as to be higher than that of the rear surface side cavity surface.
In the above structure of the present invention, the gas is supplied from the rear surface side cavity surface of the cavity into the cavity, so that the rear surface side of the melted resin is separated from the rear surface side cavity surface of the cavity, and the surface side of the melted resin closely contacts the surface side cavity surface of the cavity. Therefore, solidification of the melted resin on the surface side starts, so that the surface side of the melted resin adheres to the surface side cavity surface of the cavity, and a sink mark (recess) is thereby formed on the rear surface side of the melted resin. As cooling progresses, solidification of the melted resin on the rear surface side starts. However, in this case, the surface side of the melted resin has been already solidified. As a result, no sink mark is formed on the surface side of the melted resin.
The present invention can use various structures. According to a preferred embodiment, a recess for forming the thick portion of the resin product is provided on the rear surface side cavity surface of the cavity, and the gas is supplied from a bottom surface of the recess and a neighborhood of the recess into the cavity. In this embodiment, since the gas can be concentrated at the thick portion and the neighborhood thereof, separation of the thick portion from the die can be performed efficiently.
According to the molding method or the molding apparatus of the present invention, no recess is formed on the surface of the product, the pressure of the gas supplied from the rear surface side cavity surface to the cavity is low, and apparatus cost can be reduced.
In addition, since maintenance process of pressure on resin is unnecessary, the cycle time is shortened and the productivity can be improved.
1 denotes a fixed die, 2 denotes a movable die, 3 denotes a cavity, 4 denotes a melted resin, 5 denotes a recess for forming a rib, 9 denotes a supplying control device, 12 denotes a temperature control device, 13 denotes a gas supplying device, 14 denotes a gas supplying passage, 14A and 14B denote opening portions, W denotes a product, R denotes a rib, and h denotes a sink mark.
An embodiment of the present invention will be described hereinafter with reference to the Figures.
Next, a molding method for molding a product, which uses the above molding apparatus, will be explained hereinafter.
The temperature of the die having the fixed die 1 and the movable die 2 is set at the following values by the temperature control device 12 in advance. Therefore, since the temperature of the die is higher than the typical value (40 degrees C. to 50 degrees C.), a melted resin can closely contact the cavity surface.
First, as shown in
After the supplying of the melted resin 4 is completed, cooling of the melted resin 4 is started by supplying cold water to the fixed die 1 and the movable die 2. The processing desirably changes over to the cooling immediately after the supplying of the melted resin 4 is completed. In consideration of variability in the action of the molding apparatus, in practice, time interval between the supplying and the cooling is about 0.2 second as shown in
Next, the gas supplying device 13 is activated, so that a gas is supplied from the cavity surface of the movable die 2 into the cavity 3 without a pressure being applied to the melted resin 4. As a result, the melted resin 4 is separated from the cavity surface of the movable die 2, as shown in
The amount of the melted resin 4, which is less than the volume of the cavity 3, is in the cavity 3. Therefore, when the gas is supplied from the movable die 2 into the cavity 3, the melted resin 4 is pressed on the cavity surface of the fixed die 1 by the pressure of the supplied gas. This action by the pressure of the supplied gas is referred to as “air assist” hereinafter. Since the temperature of the die is set to be higher than the typical value described above, force works on the melted resin 4 existing in the cavity 3, so that the melted resin 4 closely contacts the cavity surface of the fixed die 1 as shown in
Therefore, as shown in
On the other hand, since the rear surface side of the melted resin 4 is separated from the cavity surface of the movable die 2 by the supplying of the gas, and a heat insulating layer is formed between the melted resin 4 and the cavity surface of the movable die 2, the cooling rate of the rear surface side of the melted resin 4 is lower than that of the surface side of the melted resin 4. Therefore, formation of a solidified layer does not start on the rear surface side of the melted resin 4.
As the cooling progresses, a solidified layer 4b is formed on the rear surface side of the melted resin 4 as shown in
Since the supplied amount of the melted resin 4 is 3% to 20% less than the volume of the cavity 3, the cavity 3 has a space in which the melted resin 4 can move to the fixed die 1 even when the supplied pressure of the gas to the cavity 3 is low. That is, the air assist can be performed. When the unsupplied ratio of the melted resin 4 in the cavity 3 is less than 3%, the melted resin 4 is completely filled at a portion of the cavity 3, so that the cavity 3 does not have a space in which the melted resin 4 can move to the fixed die 1. On the other hand, when the unsupplied ratio of the melted resin 4 in the cavity 3 exceeds 20%, the absolute amount of the melted resin 4 is insufficient, so that it is difficult to maintain a predetermined shape of the product W.
It is necessary to set the temperature of the respective dies depending on the kind of resin as described hereinafter.
If the resin is a crystalline resin, the fixed die 1 has a temperature of from a crystallization temperature +50 degrees C. to the crystallization temperature −50 degrees C. If the resin is a noncrystalline resin, the fixed die 1 has a temperature of from a glass-transition temperature +50 degrees C. to the glass-transition temperature −50 degrees C.
If the resin is a crystalline resin, the movable die 2 has a temperature of from the movable die temperature −10 degrees C. to the movable die temperature −50 degrees C. If the resin is a noncrystalline resin, the movable die 2 has a temperature of from the movable die temperature −10 degrees C. to the movable die temperature −50 degrees C.
(1) In a case in which the temperature of the die is within the above predetermined range (see
(2) In a case in which the temperature of only the movable die 2 exceeds the above predetermined range (see
(3) In a case in which the temperature of only the fixed die 1 exceeds the above predetermined range (see
(4) In a case in which the temperature of only the movable die 2 does not reach the above predetermined range (see
(5) In a case in which the temperature of only the fixed die 1 does not reach the above predetermined range (see
The pressure of the supplied gas to the cavity 3 is desirably 0.1 to 0.6 MPa. When the pressure of the supplied gas is less than 0.1 MPa, the separation of the melted resin 4 from the movable die 2 is incompletely performed, so that the sink marks are easily formed on the rib R on the surface of the product W. On the other hand, when the pressure of the supplied gas exceeds 0.6 MPa, the gas intrudes to the cavity surface of the fixed die 1, so that the surface of the product W becomes corrugated. The delay time from the time at which the supplying of the melted resin 4 is started to the time at which the supplying of the gas is started is desirably from 0 to 5 seconds. The supplying time of the gas is desirably from 2 to 40 seconds.
In the embodiment, since maintenance process of pressure on the melted resin 4 is not performed, the pressure of the supplied gas can be low. In addition, since the air from the factory or the like can be used for the supplying of the gas, no special apparatus is necessary, so that the apparatus cost can be reduced.
In addition, as shown in
Number | Date | Country | Kind |
---|---|---|---|
2005-369144 | Dec 2005 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/JP2006/324364 | 12/6/2006 | WO | 00 | 6/20/2008 |