JP2013226684A - Method for manufacturing fresnel lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing an Fresnel lens with good appearance.SOLUTION: A position of an arcuate annular belt forming face nearest to the gate 29, out of an arcuate annular belt forming faces each having both ends that come into contact with two sides adjacent to an apex farthest from a gate 29 on a surface formation face of a mirror surface piece, is defined as a reference position 235a. Until molten resin reaches the reference position 235a, a speed of injecting the molten resin R from the gate 29 to a cavity 28 is set to be a first speed (first process). After the molten resin reaches the reference position 235a, the speed of injecting the molten resin R from the gate 29 to the cavity 28 is set to be a second speed slower than the first speed (second process).

Description

  The present invention relates to a Fresnel lens manufacturing method for manufacturing a Fresnel lens by gas-assisted injection molding with gas injection into a mold.

  In recent years, as the accuracy and functions required for resin injection-molded products have increased, various injection molding methods have been developed to meet such demands, and gas-assisted injection molding methods have attracted attention. In this gas assist molding method, a method has been proposed in which a gas is injected into a mold during resin injection molding, and the viscosity of the resin is lowered by causing the gas to act on the resin (see Patent Document 1).

  In this method, first, the mold cavity is filled with a gas such as carbon dioxide, and then the cavity is filled with resin. As a result, the gas is absorbed at the flow front of the fluid resin, or enters the interface between the mold and the resin and dissolves in the resin surface layer. The gas dissolved in the resin acts as a plasticizer, lowering the melt viscosity of the resin, and in particular, selectively lowering the solidification temperature of the resin surface. As a result, the solidification temperature is lowered only for the thin resin surface layer, and the solidification temperature is set to the mold surface temperature or lower, thereby suppressing the solidification of the resin during the resin filling process. By such a technique, the mold surface transferability of the molded product is improved.

  By the way, the Fresnel lens to be molded by gas assist molding has an annular structure in which lens surfaces are cut out and arranged concentrically on a flat surface, and can be regarded as an assembly of prisms. Generally, when power is given to the lens (decreasing the focal length), the curvature increases and the lens becomes thicker. However, by using a Fresnel lens, it is possible to have a condensing function while being flat, so it is compact. And high integration. This Fresnel lens is applied to, for example, a rear projection screen, a condenser lens of an overhead projector, a focusing screen in a camera finder, and the like.

  A typical Fresnel lens has a spherical lens surface, and the height of the annular zone gradually increases from the central annular zone to the outer annular zone, and the annular pitch is constant at a few tens of μm from the inner circumference to the outer circumference. It is.

JP 11-245257 A

  However, when a Fresnel lens with a polygonal outer shape is manufactured by gas-assisted injection molding, it is difficult to form all the annular zones uniformly, and a streaky appearance defect along the arc-shaped annular zone occurs. There was a place to do.

  The annular zone in the inner peripheral portion that is accommodated in the outer shape is a circular shape of 360 °, whereas the annular zone is divided along the outer shape in the outer peripheral portion. In the mirror face piece of the mold for injection molding, a circumferential groove without an open portion is formed between two adjacent annular zone forming surfaces at a location corresponding to a circular annular zone. In a portion corresponding to the arc-shaped annular zone, an arc groove is broken at the end.

  In the gas assist molding, the gas filled in the cavity of the injected resin is melted, but there is no gas escape portion in the circumferential groove, and the melting of the gas is promoted. On the other hand, in the arc groove, as the resin flows in, the gas is discharged so as to be pushed out from the groove cut at the end of the arc, so that the melting of the gas is hindered. This difference in formability causes a shape difference between the inner and outer ring zones. The appearance defect of the Fresnel lens is that the temperature of the molten resin decreases and the viscosity increases as it goes to the outer ring zone, so the corners of the outer shape, specifically, the two sides adjacent to the vertex farthest from the gate Appears in an arcuate zone where both ends touch.

  Accordingly, an object of the present invention is to provide a method of manufacturing a Fresnel lens that can manufacture a Fresnel lens having a good appearance.

  The present invention provides a plurality of circular annular zone forming surfaces arranged concentrically on a polygonal surface forming surface and a plurality of arc-shaped annular zone forming surfaces outside the plurality of circular annular zone forming surfaces. And a mold body surrounding the mirror piece to form a cavity, injecting molten resin into the cavity through a gate communicating with the side wall surface of the cavity, and injecting gas into the cavity In the method of manufacturing a Fresnel lens in which the shape of the surface forming surface is transferred to the resin surface by gas-assisted injection molding to manufacture a Fresnel lens having a plurality of annular zones, the surface forming surface is farthest from the gate. Of the arc-shaped ring zone forming surface whose both ends are in contact with two sides adjacent to the apex, the position of the arc-shaped ring zone forming surface closest to the gate is used as a reference position, and the molten resin is A first step of setting a speed at which the molten resin is injected from the gate into the cavity to a first speed until reaching the position; and after the molten resin has reached the reference position, And a second step of setting a speed at which the molten resin is injected to a second speed slower than the first speed.

  According to the present invention, the injected molten resin is positioned at the position of the arc-shaped ring zone forming surface closest to the gate among the arc-shaped ring zone forming surfaces whose both ends are in contact with the two sides adjacent to the vertex farthest from the gate. After reaching, the injection speed is reduced from the first speed to the second speed. Thus, the gas is sufficiently dissolved in the molten resin before the molten resin reaches the arc groove. Therefore, the viscosity of the molten resin can be effectively reduced, and a Fresnel lens having a good appearance can be manufactured.

It is explanatory drawing which shows schematic structure of the principal part of the injection molding apparatus which concerns on embodiment of this invention, (a) is a longitudinal cross-sectional view of the principal part of an injection molding apparatus, (b) is crossing of the principal part of an injection molding apparatus. FIG. 4C is a diagram showing a state in which molten resin is injected into the cavity. It is explanatory drawing of a mirror surface piece, (a) is a top view of a mirror surface piece, (b) is AA sectional drawing of a mirror surface piece. It is explanatory drawing of a Fresnel lens, (a) is a top view of a Fresnel lens, (b) is BB sectional drawing of a Fresnel lens. It is a schematic diagram of the surface formation surface of a mirror surface piece. It is explanatory drawing which shows the manufacturing process of a Fresnel lens, (a) is a figure which shows the state before injection | pouring of molten resin, (b) is a figure which shows the state which has injected the molten resin at 1st speed, (c) ) Is a diagram showing a state in which molten resin is injected at a second speed. It is a schematic diagram of the surface formation surface of the specular piece when the molten resin is filled in the cavity, (a) is a plan view of the surface formation surface, (b) is a cross-sectional view of the surface formation surface. It is a top view which shows other embodiment of a mirror surface piece, (a) shows the case where the surface formation surface of a mirror surface piece is a rectangle, (b) has shown the case where the surface formation surface of a mirror surface piece is a pentagon. It is a figure which shows the focusing screen used as the shaping | molding object in Example 1, (a) is a top view of a focusing screen, (b) is a front view of a focusing screen. 2 is a schematic diagram of a mold movable side cavity used in Example 1. FIG. It is explanatory drawing which shows the manufacturing process of the focusing screen in Example 1. FIG. (A) is a figure which shows the state before injection | pouring of molten resin, (b) is a figure which shows the state which has injected the molten resin at the 1st speed, (c) has injected molten resin at the 2nd speed | rate. (D) is a figure which shows the state which has injected the molten resin at the 2nd speed | rate. It is a figure which shows the focusing screen used as the shaping | molding object in Example 2, (a) is a top view of a focusing screen, (b) is a front view of a focusing screen. 6 is a schematic diagram of a mold movable side cavity used in Example 2. FIG. It is explanatory drawing which shows the manufacturing process of the focusing screen in Example 2. FIG. (A) is a figure which shows the state before injection | pouring of molten resin, (b) is a figure which shows the state which has injected the molten resin at the 1st speed, (c) has injected molten resin at the 2nd speed | rate. (D) is a figure which shows the state which has injected the molten resin at the 2nd speed | rate.

  Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings. FIG. 1 is an explanatory diagram showing a schematic configuration of a main part of an injection molding apparatus according to an embodiment of the present invention. Fig.1 (a) is a longitudinal cross-sectional view of the principal part of an injection molding apparatus, FIG.1 (b) is a cross-sectional view of the principal part of an injection molding apparatus. FIG.1 (c) is a figure which shows the state which has injected the molten resin to the cavity.

  The injection molding apparatus 100 of this embodiment manufactures a Fresnel lens by gas assist injection molding. The Fresnel lens has an annular structure in which a lens surface is cut out and arranged concentrically on a flat surface, and has a lens power despite being flat. Generally, the annular zone of the Fresnel lens to be subjected to plastic molding is formed at an equal pitch of several tens to several hundreds of μm.

  As shown in FIG. 1A, the injection molding apparatus 100 includes a mold 200 and a plasticizing unit 300. The plasticizing unit 300 includes a screw 302 that heats and kneads a resin raw material that becomes a molten resin and plasticizes the resin 302, and a cylinder 301 that houses the screw 302. The mold 200 is connected to the gas supply device 12 through a pipe 11 for gas assist molding.

  The mold 200 includes a fixed die set 21 and a movable die set 22 that serve as a mold body, and a mirror piece 23. A fixed side mounting plate 24 is attached to the fixed side die set 21. A movable attachment plate 25 is attached to the movable die set 22.

  The movable die set 22 is formed with a substantially rectangular parallelepiped recess 221, and the mirror piece 23 is accommodated in the recess 221. The movable die set 22 is formed with a through-hole penetrating the recess 221, and an ejector rod 27 extending from an ejector plane 26 fixed to the movable-side mounting plate 25 is inserted into the through-hole. The mirror surface (surface) 23a of the mirror surface piece 23 is a surface forming surface formed in an inverse shape for transferring the surface shape of the Fresnel lens. A cavity 28 is formed by being surrounded by the mirror surface 23 a of the mirror surface piece 23, the side wall surface 22 a of the concave portion 221 of the movable side die set 22, and the flat surface 21 a of the fixed side die set 21.

  Further, as shown in FIG. 1B, the movable side die set 22 and the fixed side die set 21 are connected to the gate 29 and the gate 29 communicating with the side wall surface 28a of the cavity 28 (the side wall surface 22a of the recess 221). The runner 30 is formed. A sprue 31 (FIG. 1A) connected to the runner 30 is formed on the fixed die set 21. A cold slug well 32 is provided between the runner 30 and the sprue 31.

  As the screw 302 advances in the cylinder 301 of the plasticizing unit 300, the molten resin passes from the cylinder 301 to the inside of the cavity 28 through the sprue 31, the runner 30 and the gate 29 as shown in FIG. R is injected.

  Carbon dioxide gas is injected into the cavity 28 through the gas pipe 11. In order to hold the injected gas in the cavity 28 of the mold 200, a sealing material is incorporated in the mold material dividing portion, but the illustration is omitted.

  2A and 2B are explanatory diagrams of the specular piece, FIG. 2A is a plan view of the specular piece, and FIG. 2B is an AA cross-sectional view of the specular piece. 3 is an explanatory diagram of the Fresnel lens, FIG. 3A is a plan view of the Fresnel lens, and FIG. 3B is a BB cross-sectional view of the Fresnel lens.

  As shown in FIG. 2A, the surface forming surface 23a of the specular piece 23 is formed in a polygonal shape in a plan view, and in this embodiment, a square shape. The surface forming surface 23a is formed in an inverse shape that forms the surface shape of the Fresnel lens 500 of FIG. 3, and has a plurality of circular shapes arranged concentrically around the center point of the surface forming surface 23a. The annular zone forming surfaces 231 to 235 are formed. In the present embodiment, the surface forming surface 23a is not a circle but a square, and therefore, a plurality of arc-shaped annular zone forming surfaces 236A to 236D and 237A are formed outside the circular annular zone forming surfaces 231 to 235. ~ 237D are formed.

  As shown in FIG. 2 (b), two adjacent zone forming surfaces, for example, zone forming surfaces 231 and 232, end 231 a of one zone forming surface 231 and end of the other zone forming surface 232. The part 232b is connected by a connecting surface 241 serving as a wall surface. The end portion 231a of one annular zone forming surface 231 is a peak portion, and the end portion 232b of the other annular zone forming surface 232 is a valley portion. Therefore, the end part which becomes a trough part of circular ring zone formation surfaces 232 to 235 forms a circumferential groove, and the end part which becomes a trough part of arc shaped ring zone formation surfaces 236A to 236D and 237A to 237D The arc groove is formed.

  In the present embodiment, since the surface forming surface 23a is square, four arc-shaped annular zone forming surfaces are formed on the circumference of the same radius outside the circular annular zone forming surface 235 on the outermost periphery. Is formed. In FIG. 2A, the circular first zone forming surface 231 to the fifth annular zone forming the first annular zone surface 501 to the fifth annular zone surface 505 of the Fresnel lens 500 shown in FIG. A surface 235 is formed. The circular first annular zone forming surface 231 to the fourth annular zone forming surface 234 are accommodated in the outer shape of the mirror piece 23 (that is, the surface forming surface), and the number of divisions is zero. The circular fifth annular zone forming surface 235 has a defect formed by four sides of the surface forming surface 23a, but is not divided. On the outer side of the circular fifth annular zone forming surface 235, four arc-shaped sixth annular zones forming the sixth annular zone surfaces 506A to 506D shown in FIG. Surfaces 236A to 236D are formed. Further, on the outside of the arc-shaped sixth annular zone forming surfaces 236A to 236D, four arc-shaped seventh wheels forming the seventh annular zone surfaces 507A to 507D having the number of divisions shown in FIG. Band forming surfaces 237A to 237D are formed.

  With the above configuration, when manufacturing the Fresnel lens 500, gas (carbon dioxide in the present embodiment) is injected into the cavity 28. Then, by advancing the screw 302 in the cylinder 301 of the plasticizing unit 300, the molten resin R is injected into the cavity 28 through the gate 29 that communicates with the side wall surface 28a of the cavity 28 as shown in FIG. Is done. The molten resin R proceeds in a fan shape in the cavity 28 starting from the gate 29. Since carbon dioxide gas is supplied to the cavity 28, the carbon dioxide gas is dissolved on the surface of the molten resin R injected into the cavity 28, and the viscosity of the molten resin R decreases.

  By this gas assist injection molding method, the shape of the surface forming surface 23a of the mirror piece 23 is transferred to the surface of the resin, and as shown in FIG. 3A, a plurality of annular zone surfaces 501 to 507 (507A to 507D) are formed. The Fresnel lens 500 is produced. In the Fresnel lens 500, a gate cut mark 510 remains as shown in FIG. Here, as shown in FIG. 3B, the pitch P of the annular zone is defined as a horizontal distance from a valley portion to a valley portion of the adjacent annular zone.

  By the way, the circumferential groove for transferring the first annular zone surface 501 to the fifth annular zone surface 505 by the mirror piece 23 is closed over the entire circumference, and the gas escapes even if the molten resin flows into the circumferential groove. And the gas acts on the molten resin sufficiently. On the other hand, the arc grooves for transferring the sixth annular surfaces 506A to 506D and the seventh annular surfaces 507A to 507D are opened at the end of the mirror piece 23, and when the resin flows vigorously here. The gas is pushed out from the end portion without allowing the gas to act on the resin.

  Therefore, microscopically, the first annular surface 501 to the fifth annular surface 505 are different from the sixth annular surface 506 (506A to 506D) to the seventh annular surface 507 (507A to 507D). Will be formed. Accordingly, when molten resin is injected from the first annular zone forming surface 231 to the seventh annular zone forming surfaces 237A to 237D at a high rate of fire, there is a minute shape difference at the boundary between the arcuate annular zone surface and the arcuate annular zone surface. Occurs, resulting in poor appearance. On the other hand, in order to increase the amount of gas that acts on the molten resin, when the molten resin is injected from the first annular zone forming surface 231 to the seventh annular zone forming surfaces 237A to 237D at a low injection speed, the resin skin layer increases. As a result, sink marks and shorts occur.

  Therefore, in this embodiment, the molten resin is filled into the cavity 28 at a high injection speed until halfway, and is switched to a low injection speed in the latter half of the filling. Hereinafter, a method for manufacturing the Fresnel lens 500 will be specifically described.

  FIG. 4 is a schematic diagram of the surface forming surface 23 a of the mirror piece 23. In the present embodiment, among the annular zone forming surfaces having the largest number of divisions and the longest arc length, the annular zone forming surfaces located farthest from the gate 29 are the sixth annular zone forming surfaces 236A to 236D. Of these, the sixth ring zone forming surfaces 236A and 236B on the opposite gate side are formed. That is, among the arc-shaped annular zone forming surfaces 236A and 237A whose both ends are in contact with the two sides 23d and 23e adjacent to the vertex 23b farthest from the gate 29 on the surface forming surface 23a, the arc-shaped annular zone closest to the gate 29 is formed. The surface 236A is set as a reference position. In this embodiment, since the gate 29 is located at the center of the side 23g, the vertex 23c is also the farthest vertex from the gate 29 on the surface forming surface 23a. Therefore, the arc-shaped ring zone forming surface 236B closest to the gate 29 may be used as the reference position among the arc-shaped ring zone forming surfaces 236B and 237B whose both ends are in contact with the two sides 23e and 23f adjacent to the vertex 23c.

  The reference position may be any position on the annular zone forming surface 236A (236B), but in this embodiment, the boundary between the fifth annular zone forming surface 235 and the sixth annular zone forming surface 236A (236B) is used as the reference position. Yes. More specifically, the reference position is set to an end portion (arc portion) 235a that is a peak portion of the fifth annular zone forming surface 235. In addition, when the surface forming surface 23a is viewed from the direction perpendicular to the surface, the arc portion 235a is substantially at the same position as the end portion that becomes the valley portion of the sixth annular zone forming surface 236.

  FIG. 5 is an explanatory view showing the manufacturing process of the Fresnel lens. 5A is a diagram showing a state before injection of molten resin, FIG. 5B is a diagram showing a state in which molten resin is injected at a first speed, and FIG. It is a figure which shows the state which is injecting resin at the 2nd speed | rate. In FIG. 5, the movement of the screw 302 and the progress of filling the resin R reaching the cavity 28 from the cold slag well 32 through the runner 30 are shown in an interlocking manner. In FIG. 5, the illustration of the sprue is omitted.

  As shown in FIG. 5B, the screw 302 is moved at a high speed in the direction of the arrow until the molten resin reaches the arc portion 235a which is the reference position from the state before injection shown in FIG. That is, the speed at which the molten resin R is injected from the gate 29 into the cavity 28 is set to a first high-speed (first step). Thereby, the molten resin R is injected from the gate 29 into the cavity 28 through the cold slag well 32 and the runner 30 at the first speed.

  The first speed may be constant or may change stepwise. Until the molten resin R reaches the gate 29, the injection speed may be set to a third speed that is faster than the first speed. Cycle time can be shortened by filling sprues and runners 30 that are not directly related to the molded product at high speed and switching the shooting speed to the first speed appropriate for manufacturing the molded product after the gate 29 is passed. And productivity can be improved.

  After the molten resin reaches the arc portion 235a which is the reference position, the screw 302 is moved at a low speed in the direction of the arrow as shown in FIG. 5C until the resin filling of the cavity 28 is completed. That is, the speed at which the molten resin R is injected from the gate 29 into the cavity 28 is set to a second speed that is slower than the first speed (second step). Thereby, the molten resin R is injected into the cavity 28 from the gate 29 through the cold slag well 32 and the runner 30 at a second speed lower than the first speed.

  That is, in the injection molding method according to the present embodiment, the resin is supplied to the cavity 28 at a first high velocity at least until the resin reaches the arc portion 235a. The second speed is switched to complete the filling.

  Here, when injected at the first speed, the molten resin R may pass through the arc portion 235a. That is, not only when the second speed is set when the molten resin R reaches the arc portion 235a serving as the reference position, the injection speed is changed after the molten resin R passes through the arc portion 235a serving as the reference position. A speed of 2 may be set. It is more preferable that the injection speed is set to the second speed before the molten resin R reaches the grooves of the sixth annular zone forming surfaces 236A and 236B.

  As described above, according to the present embodiment, after the injected molten resin R reaches the arc portion 235a which is the reference position, the injection speed is reduced from the first speed to the second speed. Thus, the gas is sufficiently dissolved in the molten resin R before the molten resin R reaches the arc grooves of the sixth annular zone forming surfaces 236A and 236B. Therefore, the viscosity of the molten resin R can be effectively reduced.

  In addition, on the sixth annular zone forming surfaces 236C and 236D and the seventh annular zone forming surfaces 237C and 237D on the gate side, since the molten resin R has a sufficiently high temperature and a sufficiently low viscosity, it reaches the arc groove well. it can. Then, on the sixth annular zone forming surfaces 236A and 236B and the seventh annular zone forming surfaces 237A and 237B on the opposite gate side, the viscosity is lowered by the action of carbon dioxide gas on the molten resin R that has reached these, The resin R can reach the arc groove well.

  As described above, since the molten resin R is sufficiently distributed in the arc groove, the difference in moldability that occurs with the reference position as the boundary line is minimized, and a Fresnel lens having a good appearance that suppresses the occurrence of sink marks and shorts is manufactured. be able to.

  FIG. 6 is a schematic view of the surface forming surface 23a of the mirror piece 23 when the molten resin R is filled in the cavity 28. FIG. 6 (a) is a plan view of the surface forming surface 23a, and FIG. 6 (b). These are sectional views of the surface forming surface 23a. As shown in FIG. 6A, when the molten resin R reaches the arc portion 235a, the sixth annular zone forming surfaces 236C and 236D and the seventh annular zone forming surfaces 237C and 237D on the gate side are completely filled. Looks like. However, as shown in FIG. 6B, only the outer frame shape is completed, and the molten resin may not be completely contained in the groove for transferring the annular zone in the mold. Even after this state, the resin filling the sixth annular zone forming surfaces 236A to 236D and the seventh annular zone forming surfaces 237A to 237D, regardless of the gate side or the non-gate side, by lowering the shooting speed to the second velocity. It is possible to make the gas act more effectively.

  In the present embodiment, the second speed is constant when the injection speed is changed to the second speed after reaching the arc portion 235a, but the present invention is not limited to this. The second speed may be set so that the molten resin R gradually decelerates toward the position of the vertex 23b (23c). By switching the injection speed to a lower speed just before the final filling portion, a molded article having a more preferable appearance can be obtained.

  In the present embodiment, the case where the surface forming surface (mirror surface) 23a of the mirror piece 23 is square has been described. However, the present invention is not limited to this, and the present invention can be applied to any polygonal shape. . For example, FIG. 7 is a plan view showing another embodiment of the specular piece, FIG. 7A shows a case where the surface forming surface of the specular piece is rectangular, and FIG. 7B shows the surface of the specular piece. The case where a formation surface is a pentagon is shown.

  As shown in FIG. 7A, the surface forming surface 123a of the mirror piece 123 may be rectangular. If the center of the rectangular Fresnel lens and the center of the Fresnel annular zone coincide with each other, the inner annular zone from the annular inscribed circle of the long side of the rectangle is a 360 ° circular shape. The outer ring zone is divided into two parts to form an arc shape with a central angle of less than 180 °. Further, the outer ring zone is divided into four, and the number of the same radius ring zones is four.

  In this case, the arc-shaped ring zone forming surface 123h closest to the gate 29 among the arc-shaped ring zone forming surfaces whose both ends are in contact with the two sides 123d and 123e adjacent to the vertex 123b farthest from the gate 29 in the surface forming surface 123a. Is the reference position. Since the gate 29 is located at the center of the side 123g, the vertex 123c is also the farthest vertex from the gate 29 in the surface forming surface 123a. Therefore, the arc-shaped zone forming surface 123i closest to the gate 29 may be used as the reference position among the arc-shaped zone forming surfaces whose both ends are in contact with the two sides 123e and 123f adjacent to the vertex 123c. As described above, a non-defective product having a good appearance can be obtained even in a Fresnel lens having a rectangular outer shape.

  Moreover, as shown in FIG.7 (b), the case where the surface formation surface of 123 A of specular pieces may be a pentagon may be sufficient. In the pentagonal Fresnel lens, the annular zone is divided into five arcs from a circular shape with the inscribed circle as a boundary. In this case, the arc-shaped zone forming surface 123m closest to the gate 29 among the arc-shaped zone forming surfaces whose ends are in contact with the two sides 123k and 123l adjacent to the vertex 123j farthest from the gate 29 is set as the reference position. That's fine.

[Example 1]
Next, gas-assisted injection molding in Example 1 will be described with reference to FIGS. 8, 9 and 10 below. FIG. 8 is a diagram illustrating a focusing screen to be molded in Example 1, FIG. 8A is a plan view of the focusing screen, and FIG. 8B is a front view of the focusing screen. FIG. 9 is a schematic diagram of the mold movable side cavity used in the first embodiment. FIG. 10 is an explanatory diagram illustrating a manufacturing process of the focusing screen according to the first embodiment. In FIG. 10, the injection speed control is expressed by interlocking the filling of the resin in the cavity with the movement of the screw in the cylinder.

  The resin material used in the gas assist injection molding of Example 1 is an acrylic resin, and the gas is carbon dioxide. The carbon dioxide taken from the liquefied carbon dioxide cylinder by the gas supply device is heated and pressurized to 50 ° C. and 8 MPa, and the signal of the position of the screw 302 of the plasticizing unit is used as a trigger to link the gas to the mold. An injection was made. The molding machine and the gas supply device were connected by an SUS pipe having an inner diameter of 1.6 mm and a length of 5 m.

  The object to be molded is a focusing screen which is one of camera parts. The thickness C of the focusing screen in FIG. 8B is 1.5 mm, and the outer shape in FIG. 8A is a substantially rectangular shape with a longitudinal dimension A of 40 mm and a lateral direction length B of 28 mm. The pitch of the Fresnel zone is 35 μm, and the height of the zone at the outermost periphery is about 27 μm.

  As shown in FIG. 9, the mold has four cavities 28 corresponding to the size of the molded product, and the gate 29 is a fan gate having a thickness of 1 mm. FIG. 9 illustrates a movable cavity including a runner 30 and a cold slug well 32. Since the lens outer shape is rectangular, the reference position for switching the shooting speed is the arc portion 36 on the side opposite to the gate of the short side inscribed circle (the arc portions on the inner side of the annular zone forming surfaces 123h and 123i in FIG. 7A) or Outside the arc portion 36.

  In order to hold the gas injected from the gas supply device, a sealing material was arranged on the mold divided parts such as the ejector pins and the spool bushes in addition to the outer periphery of the cavity 28 (not shown).

  In order to confirm the position where the injection speed is set and changed from the first speed to the second speed, a short shot product was obtained by intentionally reducing the weighing. As a result, when the plasticizing unit 300 having a screw diameter of 25 mm was injected for 31 mm, it was confirmed that the resin reached the position of the arc portion 36 from the nozzle.

  In the above apparatus configuration, gas assisted injection molding was performed by changing the firing speed setting, and a comparative study was performed. Table 1 shows the molding conditions.

As a common molding condition, the injection start position D0 shown in FIG. From the injection start position D0 to the reference position D1 for switching the injection speed shown in FIG. 10B, the injection speed is a constant first speed of 10 mm / s (injection rate conversion 1.2 cm ³ / s per cab). ).

  Under the molding conditions A and E, the injection speed was a constant second speed from the reference position D1 to the injection end position (cushion position) D3 shown in FIG.

  In the molding conditions B, C, D, and F, the second speed is gradually reduced. From the reference position D1 to the intermediate position D2 shown in FIG. 10C, the second speed is a constant speed lower than the first speed, and from the intermediate position D2 to the injection end position D3 shown in FIG. 10D. Until then, it was set to a lower constant speed.

For example, under the molding condition B, the screw position is filled at 10 mm / s (injection rate conversion 1.2 cm ³ / s per mold) from 41 mm, which is the injection start position D0, to 9 mm, which is the reference position D1, and the injection speed Resin has reached the switching area (FIGS. 10A and 10B). From 9 mm as the reference position D1 to 6.3 mm as the intermediate position D2, the resin was filled at 2 mm / s (injection rate conversion per cab: 0.25 cm ³ / s) (FIGS. 10B and 10C). )). From the intermediate position D2 of 6.3 mm to the injection end position D3, filling was performed at 1.5 mm / s (injection rate conversion 0.18 cm ³ / s per cab) to complete the filling (FIG. 10 (c ), (D)).

As Comparative Example A, the resin was filled from the injection start position D0 to the injection end position D3 at a first speed of 1.2 cm ³ / s. In Comparative Example A, the firing speed was too fast, resulting in poor appearance.

  Under the molding condition E, there was no noticeable appearance defect and it was improved over Comparative Example A. Further, under the molding condition F, although slight sink marks were confirmed, there was no poor appearance and it was improved over Comparative Example A.

Under the molding conditions A to D, the appearance was better than the other molding conditions E and F. That is, in the molding conditions A to D, the second speed is switched to the range of 0.12 cm ³ / s or more and 0.37 cm ³ / s or less in terms of the injection rate per cab, and the second speed is within the range. It was confirmed that a non-defective product could be obtained by setting the value inside.

[Example 2]
Next, gas assist injection molding in Example 2 will be described with reference to FIGS. 11, 12, and 13 below. 11A and 11B are diagrams illustrating a focusing screen that is a molding target in Example 2. FIG. 11A is a plan view of the focusing screen, and FIG. 11B is a front view of the focusing screen. FIG. 12 is a schematic diagram of a mold movable side cavity used in the second embodiment. FIG. 13 is an explanatory diagram illustrating a manufacturing process of the focusing screen in the second embodiment. In FIG. 13, the shooting speed control is expressed by interlocking the filling of the resin in the cavity and the movement of the screw in the cylinder.

  The resin material used in the gas-assisted injection molding of Example 2 is an acrylic resin, and the gas is carbon dioxide. Carbon dioxide taken from the liquefied carbon dioxide cylinder by the gas supply device is heated and pressurized to 50 ° C. and 8.4 MPa, and the signal at the position of the screw 302 of the plasticizing unit is used as a trigger to move to the mold. Gas injection was performed. The molding machine and the gas supply device were connected by an SUS pipe having an inner diameter of 1.6 mm and a length of 5 m.

  The object to be molded is a focusing screen which is one of camera parts. In FIG. 11 (b), the focus plate has a thickness C of 1.5 mm, and in FIG. 11 (a), the outer shape is a substantially rectangular shape having a longitudinal dimension A of 38 mm and a lateral direction length B of 27 mm. The pitch of the Fresnel zone is 30 μm, and the height of the zone at the outermost periphery is about 17 μm.

  As shown in FIG. 12, the mold has two cavities 28 corresponding to the size of the molded product, and the gate 29 is a fan gate having a thickness of 1 mm. FIG. 12 shows a movable cavity including the runner 30 and the cold slug well 32. Since the lens outer shape is rectangular, the reference position for switching the shooting speed is the arc portion 36 on the side opposite to the gate of the short side inscribed circle (the arc portions on the inner side of the annular zone forming surfaces 123h and 123i in FIG. 7A) or Outside the arc portion 36.

  In order to hold the gas injected from the gas supply device, a sealing material was arranged on the mold divided parts such as the ejector pins and the spool bushes in addition to the outer periphery of the cavity 28 (not shown).

  In order to confirm the position where the injection speed is set and changed from the first speed to the second speed, a short shot product was obtained by intentionally reducing the weighing. As a result, it was confirmed that when the plasticizing unit 300 having a screw diameter of 25 mm was injected for 15.5 mm, the resin reached the position of the arc portion 36 from the nozzle.

  In the above apparatus configuration, gas assisted injection molding was performed by changing the firing speed setting, and a comparative study was performed. Table 2 shows the molding conditions.

As a common molding condition, the injection start position D0 shown in FIG. 13 (a) was set to 25.5 mm. From the injection starting position D0, to the reference position D1 to switch the morphism speed shown in FIG. 13 (b), the injection speed, constant first velocity 4 mm / s (1 per cavity injection rate conversion 1.0 cm ³ / s ).

  Under the molding conditions G and K, the injection speed was a constant second speed from the reference position D1 to the injection end position (cushion position) D3 shown in FIG.

  Under the molding conditions H, I, J, and L, the second speed is gradually reduced. From the reference position D1 to the intermediate position D2 shown in FIG. 13C, the second speed is a constant speed lower than the first speed, and from the intermediate position D2 to the injection end position D3 shown in FIG. 13D. Until then, it was set to a lower constant speed.

For example, under the molding condition H, the screw position is filled at 4 mm / s (injection rate conversion 1.0 cm ³ / s per mold) from 25.5 mm which is the injection start position D0 to 7.5 mm which is the reference position D1. Then, the resin reached the firing speed switching region (FIGS. 13A and 13B). From the reference position D1 of 7.5 mm to the intermediate position D2 of 4.5 mm, the resin was filled at 1.5 mm / s (injection rate conversion 0.37 cm ³ / s per mold) (FIG. 13 (b ), (C)). From the intermediate position D2 of 4.5 mm to the injection end position D3, filling was performed at 0.5 mm / s (injection rate conversion per mold 0.12 cm ³ / s) to complete the filling (FIG. 13 (c ), (D)).

As Comparative Example B, the resin was filled from the injection start position D0 to the injection end position D3 at a first speed of 1 cm ³ / s. In Comparative Example B, since the shooting speed was too fast, appearance failure occurred.

  Under the molding condition K, there was no noticeable appearance defect and it was improved over Comparative Example B. Further, under molding condition L, although slight sink marks were confirmed, there was no poor appearance and it was improved over Comparative Example B.

Under the molding conditions G to J, the appearance was better than the other molding conditions K and L. That is, in the molding conditions G to J, the second speed is switched to a range of 0.12 cm ³ / s or more and 0.37 cm ³ / s or less in terms of the injection rate per cab, and the second speed is within the range. It was confirmed that a non-defective product could be obtained by setting the value inside.

  The present invention is not limited to the embodiments and examples described above, and many modifications can be made by those having ordinary knowledge in the art within the technical idea of the present invention.

  In the embodiments and examples described above, the case where carbon dioxide is used as the gas has been described. However, any gas may be used as long as it dissolves in the molten resin and reduces the viscosity of the molten resin. For example, a hydrocarbon-based gas may be used as the gas.

21 ... Fixed side die set (mold body), 22 ... Movable side die set (mold body), 23 ... Mirror surface piece, 23a ... Surface forming surface, 23b ... Vertex, 23d, 23e ... Two sides, 28 ... Cavity, DESCRIPTION OF SYMBOLS 29 ... Gate, 200 ... Mold, 231-235 ... Circular ring zone formation surface, 236A-236D, 237A-237D ... Arc-shaped ring zone formation surface, 500 ... Fresnel lens

Claims (4)

A plurality of circular annular zone forming surfaces arranged concentrically on a polygonal surface forming surface and a plurality of arc-shaped annular zone forming surfaces outside the plurality of circular annular zone forming surfaces are formed. Gas-assisted injection in which a cavity is formed by the mirror piece and a mold body surrounding the mirror piece, and molten resin is injected into the cavity through a gate communicating with the side wall surface of the cavity, and gas is injected into the cavity In the manufacturing method of the Fresnel lens, the shape of the surface forming surface is transferred to the surface of the resin by molding, and a Fresnel lens having a plurality of annular surfaces is manufactured.
Among the arc-shaped zone forming surfaces whose both ends are in contact with the two sides adjacent to the vertex farthest from the gate on the surface-forming surface, the position of the arc-shaped zone forming surface closest to the gate is used as a reference position for melting. A first step of setting a speed at which the molten resin is injected from the gate into the cavity to a first speed until the resin reaches the reference position;
After the molten resin reaches the reference position, a second step of setting a speed at which the molten resin is injected from the gate into the cavity to a second speed that is slower than the first speed;
A method for manufacturing a Fresnel lens, comprising:   The method for producing a Fresnel lens according to claim 1, wherein the surface forming surface is rectangular.   3. The method for manufacturing a Fresnel lens according to claim 1, wherein the second speed is set so that the molten resin gradually decelerates toward the position of the apex. 4. 4. The method for manufacturing a Fresnel lens according to claim 1, wherein the second speed is in a range of 0.12 cm ³ / s to 0.37 cm ³ / s. 5.

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