Injection mold assembly

Abstract: The present invention relates to a sliding member formed of a synthetic resin composed mainly of a polyethylene, said sliding member comprising a molded piece of a polyethylene having a viscosity-average molecular weight of 500,000 to 6,000,000 inclusive, having a low coefficient of friction and excelling in both its own wear resistance and its property of preventing the abrasion of an associated member.

Description: The present invention relates to an injection mold assembly for injection molding molded parts of a polyethylene having a molecular weight of 500,000 to 6,000,000. Heretofore, various materials have been used for guides for movable systems for magnetic tapes such as VTR and cassette tapes. For instance, known organic materials involve polyoxymethylene (POM for short) which are used alone or in combination with a fluororesin and even silicone. Some plastic parts contain carbon so as to release charges generated by frictional contact with the surface of a tape, since plastics tend to be electrified. Known inorganic materials include metals and ceramics, while known metallic guides are formed of stainless materials or brass plated on its surface with nickel or chromium. Such guides as mentioned above operate in two manners, according to one of which they rotate in association with the travelling of a tape. According to another, they are fixed relative to the travelling of a tape. In most cases, the fixed type of metallic guides are used for portions through which a tape travels under relatively increased tape tension with increased changes in the direction of its movement. Some problems with such conventional guides originate from the properties of the materials used and some arise from the processing techniques applied. Of noteworthy in connection with the materials used is that in view of the principles of friction or tribology, a material of larger momentum should basically be harder than its associated material of smaller momentum. In other words, a tape's material should fundamentally be harder than its associated guide. Under such conditions, the amount of abrasion is considered to be minimized. In view of tribology, however, the material used for the conventional guides is totally reversed in combination or not in ideal conditions at all. More specifically, the POM that is a polymeric material used for guides is much harder than PET that is a base film of magnetic tapes. In addition, stainless or chromium-plated brass materials used for fixed guides are harder than the POM. This implies that if guides become irregular even slightly on their surfaces, then magnetic tapes will be likely to be damaged. To avoid this problem, the surfaces of guides over which tapes travel, i.e., in contact with them, are now polished into mirror surfaces to increase the area of their surfaces in contact with the tapes, thereby applying local contact pressure to the tapes for the purpose of preventing the travelling resistance from increasing and so the surfaces of the tapes from being damaged. More illustratively, the guides are polished to such a surface roughness as expressed in terms of Rmax equal to or less than 0.1 µm. For the rotary guides, on the other hand, a roundness of 0.1 µm or below is required in addition to having such a surface roughness as mentioned. Thus, a problem with the conventional guides is that as a result of unavoidably making no account of the basic principles of tribology in view of some limitations placed on materials, it is required to increase their accuracy of processing to the highest level now achievable, leading to increases in product costs. In the case of the fixed type of guides in particular, it is still impossible to make them of plastics. Thus, metallic guides are still forcibly used even for cassette cases or decks, imposing considerable limitations on making them light, mass-producing them and cutting down their costs. As a result of various studies made by the present inventors so as to provide a solution to the above problems, it has now been found that a sliding material formed of a polyethylene resin having a relatively high molecular weight on the order of 500,000 to 6,000,000 can meet the basic principles of tribology, as already mentioned, and can be mass-produced at lower costs by making a high-molecular weight polyethylene injection-moldable, which has heretofore been considered difficult-to-mold. Accordingly, a method for producing a sliding member comprising a polyethylene having a viscosity-average molecular weight of 500,000 to 6,000,000 inclusive has been developed which is described in copending European patent no. 0 389 642. More specifically, the invention claimed in European patent no. 0 389 642 provides a sliding member of a synthetic resin containing a polyethylene as a main component, which is characterized by being formed of an injection-molded piece or part of a polyethylene having a molecular weight of 500,000 to 6,000,000 inclusive, being decreased in its coefficient of friction and excelling in both its own resistance to abrasion and its property of preventing the abrasion of an associated member. The problem underlying the present invention is to provide an injection mold which is suited for injection molding a polyethylene having a molecular weight of not less than 300,000, rendering it possible to mold sliding guides for magnetic tapes. The solution to the above problem is achieved by providing an injection mold assembly as defined in the claim. Figure 1 is a general view of a guide for magnetic tapes according to the present invention, Figure 2 is a sectional view showing part of a mold Figures 3-4 and 6-8 each are a sectional view of an injection-molding machine used in the present invention, Figure 5 is a sectional view illustrating a jar for making an ultrahigh-molecular polyethylene used in the present invention. In what follows, the sliding member which can be produced with the injection mold assembly of the present invention will be explained with reference to a guide for magnetic tapes as an example. Generally, the guide for guiding a magnetic tape is in such a form as illustrated in Fig. 1, e.g., in a cylindrical form of about 6 mm in outer diameter, about 4.85 mm in inner diameter and about 16 mm in length. This example is applicable to a 12,7 mm 1/2-inch magnetic tape. It is to be understood, however, that the outer and inner diameters and length of guides may vary depending upon the width of tapes and even the structure of cassette cases. It is also understood that the guide is not limited to such a cylindrical form and may be in any form best-suited for tapes to pass over it. Basically, however, it is suitable that the guide is in a columnar form which may be of any desired section. The material used for the guide produced with the injection mold assembly according to the present invention is a polyethylene having a molecular weight of 500,000 to 6,000,000 inclusive. A preferable molecular-weight range is between 600,000 and 2,000,000. As a result of the analysis of the results of tribological measurement of the surface of a polyester film that is a material of magnetic tapes and a polyethylene guide, it has been found that the polyethylene is worn down by the polyester in a molecular weight less than 500,000, whereas the polyester film is worn down in a molecular weight higher than 6,000,000. In other words, if the molecular weight is in the above-defined range, then it is possible to obtain a guide which successfully meets three requirements of (a) having a reduced coefficient of friction, (b) having increased abrasion resistance and (c) reducing the abrasion of an associated member (e.g., a polyester film) in a well-harmonized state. For more details regarding the material used for the guide it is referred to copending European patent no. 0 0389 642. Processing the polyethylene resin into a guide form is achieved by using the injection-molding technique provided by the invention of the present inventors, which will be described later. For molds for obtaining molded guides, use may be made of vacuum/compression molds to be described later. One typical basic structure of the mold used with the injection mold assembly of the present invention, which will be described in detail, is shown in Figure 2, wherein reference numeral 51 stands for a core pin, 52 a sleeve (for an ejector), 53 a cavity, 54 a cavity plate, 55 a seal ring, 56 a secondary sprue, 57 a film gate and 58 a molded piece or part. As an example, the molded piece is in the form of such a cylindrical guide as depicted in Figure 1. The cavity space, in which the composite polyethylene resin is to be filled, is vacuumized through a parting line of the mold. For vacuumization, a vacuum pump is used, which connects with a pipeline arranged externally of the mold. After filling, the resin is compressed by moving the sleeve 52 horizontally (on the drawing). Effectively, the sleeve 52 is actuated just after the resin is allowed to dwell under pressure following its filling in the mold cavity. Driving is effected by a hydraulic or pneumatic cylinder with the aid of an electric motor, etc. Reasonably, an actuation signal is outputted from the sequence of an injection-molding machine. The temperature of the mold is in a range of preferably 40-100°C, more preferably 60-90°C. The conditions for injection molding may basically follow the above molding conditions. In the case of the composite polyethylene, it is noted that a compression ratio at the screw of 1.1-1.6 gives satisfactory molded pieces and brings about stable plasticization. It is also noted that the lower the cylinder temperature of a feeder zone, which is to be preferably brought down by fan-induced forced cooling, the lower the molecular weight of the obtained molded pieces. Injection-Molding Machine Polyethylene resins increase generally in their melting viscosity with an increase in their molecular weight. Especially at a molecular weight exceeding 1,000,000, they are so likely to stagnate in the barrel of a general (in-line screw type of) injection-molding machine that they cannot be molded. When they are to be forcedly plasticized, the main molecular chain of polyethylene is cut out by high shear stress occurring between the barrel and the screw, resulting in the formation of polyethylene having its molecular weight reduced. Heretofore, the ultrahigh-molecular weight polyethylene has thus been molded by compression molding or extrusion molding. With such molding techniques, however, it is impossible to obtain molded pieces or parts of high accuracy at low costs. On the other hand, improvements in the rigidity of gears, etc. are considered to be achievable by making the ultrahigh-molecular weight polyethylene composite with an inorganic material such as carbon fibers. With compression molding, the inorganic material may be made composite with the ultrahigh-molecular weight polyethylene in powdery forms, since the former is fused onto the surface of each particle of the latter. With ordinary injection molding, however, the inorganic material is made composite with the ultrahigh-molecular weight polyethylene by way of the fusion of its respective particles, thus resulting in its molecular weight being reduced and its basic properties being lost. By using the injection mold assembly of the present invention, it is possible to efficiently obtain injection-molded pieces of high accuracy from a raw material comprising the ultrahigh-molecular with or without the inorganic material to be made composite with it by means of injection molding. The injection-molding machine used with the injection mold assembly of the present invention is an in-line screw type of molding machine in which a hopper is connected with an atmospheric gas supply pipe for melting an ultrahigh-molecular weight polyethylene feed in a non-oxidizing atmosphere, only the compression and nozzle zones of a cylinder with a built-in screw are provided with heaters and a heat-radiating zone is defined by a part of a feeder region. A more illustrative example of an injection-molding machine will now be explained with reference to the accompanying drawings. Figure 3 is a sectional view illustrating an injection-molding machine which may be used with the injection mold assembly of the present invention to make sliding members. Main parts of this injection-molding machine are a cylinder 2 with a built-in screw 1, a hopper 4 for inputting a feed stock 3 into the cylinder 2, a mold 6 having therein a molding cavity 5, in which the leading end of the cylinder 2 is located, a vacuum tank 7 for vacuumizing the cavity 5 and a rotary pump 8. The hopper 4 is connected with a pipe 9 for supplying to it an atmospheric gas such as nitrogen gas so as to melt the feed stock 3 fed into the cylinder 2 in a non-oxidizing atmosphere. At its lower portion, the hopper 4 includes a regulating plate 10 having therein a circular or rectangular slit 10a for regulating the input of the feed stock 3. Suitably, the screw 1 driven by a vane motor 11 gives rise to a compression ratio of 2.3 or below, esp., 1.3-2.0 with the length to diameter (L/D) ratio being in a range of preferably 10-25, more preferably 15-20. The pitch (P) of the screw is determined by: P = πDtan wherein  is preferably in a range of 10°-18°, esp., 11.5°-14.5°. At a compression ratio higher than 2.3, on the one hand, so higher a pressure than required is applied to the ultrahigh-molecular weight polyethylene powders that they remain stagnant and so lose flowability simultaneously with the initiation of crosslinking. A compression ratio lower than 1.3 is, on the other hand, insufficient to melt the surfaces of the ultrahigh-molecular weight polyethylene powders. At an L/D ratio exceeding 20, on the one hand, the ultrahigh-molecular weight polyethylene is likely to stagnate in the cylinder for a time longer than required. At an L/D ratio short of 15, on the other hand, the accuracy of heating control of the ultrahigh-molecular weight polyethylene is less than satisfactory. Determined by the flight angle , the screw pitch makes determination as to how much torque is applied to the ultrahigh-molecular weight polyethylene. At an angle  short of 10°, sufficient torque is obtained but the residence time is made longer than required with frictional energy applied to the ultrahigh-molecular weight polyethylene powders. An angle  exceeding 18° is unsuitable, since the torque applied to ultrahigh-molecular weight polyethylene is so insufficient that a burden is placed on the vane motor and no prescribed rpm is obtained, resulting in the polyethylene remaining stagnant in the cylinder over an extended period of time. Without recourse to any back-flow ring, the screw head has a flight extending to its extreme end, and is smaller in diameter than the inner diameter of the cylinder 2 by 0.1 to 0.2 mm. The cylinder 2 is divided into a rear section defining a feeder zone 2a and a front section defining a compression zone 2b with the leading end defining a nozzle zone 2c. Preferably, the length of the feeder zone 2a, over which a heat-radiating fin 12 is to be mounted, is 1/4 to 1/3 of the entire length of the cylinder 2. The ultrahigh-molecular weight polyethylene powders are delivered by the screw through the cylinder, and generate heat due to their friction, as the compression ratio increases. It is when the polyethylene reaches the compression zone of the screw that this state is achieved, and it is in this zone 2b that the powders are individually softened for the first time by external heating. In other words, in the zone 2a in front of the zone 2b, heating should be avoided as much as possible to radiate frictional heat, thereby preventing crosslinking. In the absence of the heat-radiating fin 12, heating control at the compression zone 2b is so impossible that the polyethylene powders can be prematurely crosslinked. For the instantaneous melting of the feed stock 3, the compression and nozzle zones 2b and 2c are provided with heaters 13. In what follows, the geometry of the screw head used with the present invention will be explained in detail. In order to obtain highly accurate injection-molded pieces of the ultrahigh-molecular weight polyethylene, it is essentially required to apply sufficient injection rates to the resin at the injection step and give sufficient pressures to the resin in the process of filling it in the mold cavity. From the flowability of the ultrahigh-molecular weight polyethylene, it has now been found that of particular importance for improving shapeability and dimensional accuracy is to maintain the pressure applied to the resin at the dwell step. As an outcome of various studies, it has been found that satisfactory results are obtained, if the means capable of maintaining sufficient pressures to the resin at the dwell step is designed as follows, with no provision of a back-flow ring to the screw, unlike conventional injection-molding machines. More specifically, the screw head portion following the screw portion of the screw body is constructed from a cylindrical section having a diameter equal to or smaller than the diameter of a groove in a metering region of the screw and a conical section contiguous to and extending from it. Part of the screw head portion of the injection-molding machine used with the injection mold assembly of the present invention is shown in the sectional view of Fig. 4, wherein reference numeral 31 stands for a screw, 32 a cylinder (a barrel) and 33 a cylinder nozzle. Reference numeral 34 indicates a screw head portion, which is constructed from a cylindrical section 35 extending from the screw portion and a conical section 36 contiguous to and extending from it. The cylinder nozzle 33 is threadedly fixed to an end portion of the cylinder 32 with its inner bore wall 37 following the shape of the screw head portion 34. When the screw 31 moves forward to such a position as illustrated, there is formed a gap or space between the cylindrical section 35 of the screw head portion 34 and the inner bore wall 37 of the cylinder nozzle 33, which defines a pressure-to-the-resin (hereinafter called the resin pressure) generating region 38. This pressure generating region 38 may be located between the cylindrical section 35 and the cylinder 32, not the cylinder nozzle 33. It is noted that reference numerals 39 and 40 stand for a nozzle tip and a lock nut, respectively. In the case of the conventional injection-molding machines, a conical section extends directly from the screw portion in the absence of an equivalent of the cylindrical section 35. Instead, a back-flow ring having a diameter substantially equal to the cylinder diameter is provided to the screw head portion, and is designed in such a way that it stops at the extreme end of the cylinder, even when it moves forward to a maximum. It is possible to prevent the back-flow of the resin at the time of dwell by meeting the condition of D > D' wherein D is the diameter of the screw 31 and D' is the diameter of the cylindrical section 35 of the screw head portion 34. However, the length L' of the cylindrical section 35 can be determined arbitrarily. Referring then to the resin pressure generating region 39, the diameter D' of the cylindrical section is preferably identical with or smaller than a groove diameter DM of the flight lying on the metering region of the screw 31. The length L' may meet 1/10D' ≤ L' ≤ 10D', preferably 1/2D' ≤ L' ≤ 3/2D'. The screw head portion 34 may be adapted to be separated from the screw 31. In this case, another screw head 34 having a length L' varied depending upon the nature of the resin used may be provided to regulate the generation of a dwell pressure arbitrarily. Reference will now be made to how to make molded pieces of the ultrahigh-molecular weight polyethylene with the use of such an injection-molding machine as mentioned above with the injection mold assembly of the present invention. The ultrahigh-molecular weight polyethylene in the form of fine powders having a particles size of about 30 to 100 µm is first supplied as the feed stock 3 into the hopper 4 shown in Figure 3. It is here noted that the input of the feed stock per time can be determined only by the regulation of the diameter 10a because, unlike ordinary polyethylene, the ultrahigh-molecular weight polyethylene is in powdery form, rather than in pellet form, so that the coefficient of friction among the polyethylene particles can be reduced. The regulation of the input of the feed stock to the feeder zone 2a means that the compression ratio of the screw 1 decrease relatively, with the result that when the ultrahigh-molecular weight polyethylene is plasticized, the compression ratio is so excessively increased that the stagnation of the feed stock in the cylinder 2 can be avoided. Then, the ultrahigh-molecular weight polyethylene fed into the cylinder 2 is delivered from the feeder zone 2a to the compression zone 2a, in which it is instantaneously heated. That polyethylene is in turn melted (on its surface alone) in the nozzle zone 2c. It is here noted that since the hopper 4 is filled with a nitrogen gas, the cylinder 2 is therein maintained in a non-oxidizing atmosphere to prevent the molecular weight of the polyethylene from being reduced by oxidation. The ultrahigh-molecular weight polyethylene has the property of starting crosslinking immediately upon melted. Thus, if the polyethylene is momentarily melted in the compression zone and, just thereafter, is cast through the nozzle zone into the cavity 5, as carried out in the present embodiment, then it is possible to cast the molten polyethylene in the cavity, while it remains flowable. Mold As means for molding the ultrahigh-molecular weight polyethylene into industrial small parts, etc., injection molding, compression molding and extrusion molding are considered available. However, the ultrahigh-molecular weight polyethylene shows no flowability at all, since its viscosity is not decreased even when heated due to its excessively high molecular weight. For that reason, it is impossible to fill the ultrahigh-molecular weight polyethylene to details of the cavity even with injection molding. Neither can the molded pieces be smoothened owing to their inferior accuracy. A problem with the plasticization of the ultrahigh-molecular weight polyethylene with the screw of an injection-molding machine is that it is oxidized and undergoes cutting of its main chain by heating under high shear load, resulting in a reduction in its molecular weight. In order to solve such problems, there have been filed a few patent applications, which have now been published under Japanese Patent Publication Nos. 57-30067 and 60-58010. However, to put the techniques set forth there to practical use has faced the following problems. For instance, it has been proposed to compress the resin injected into a cavity to a volume 1.5 to 3.0 times as large as the input of the resin injected. However, problems with this technique is that: (1) since there is a need for increasing the cavity volume, the mold is so complicated in structure that difficulty is encountered in molding small or sophisticated pieces and producing molded pieces in bulk; (2) due to a need for increasing a clearance between the sliding surfaces of the mold, there are formed burrs which have to be removed by post treatments; and (3) since only a pin-gate system can be utilized, some limitation is imposed on how to gate and what form molded pieces are to take, thus making the design and production of an optimum mold difficult. In order to solve such problems, the present inventors have already invented an injection-molding machine (Japanese Patent Kokai Publication No. 9723/60) in which, as illustrated in Figure 3, makes use of an ordinary mold with no provision of any compression mechanism. For conventional sliding guides for magnetic tapes, on the other hand, light metals such as brass are cut into guide forms which are chromium-plated or otherwise treated on their sliding surfaces for mirror finishing, thereby improving their heat resistance and wear resistance. Some sliding guides are formed of stainless metals. However, the above prior art techniques have the following problems. First, problems with such injection molding as in Japanese Patent Publication Nos. 30067/1982 and 58010/1985 already referred to are that: (1) how to gate is limited to a pin gate; (2) the mold is so complicated that difficulty is encountered in molding small or sophisticated pieces and producing molded pieces in bulk; and (3) residual stress due to compression force has a strain or otherwise adverse influence upon the products. A problem with such injection molding as disclosed in Japanese Patent Application No. 9723/1985 is that since the resin used is poor in flowability, there is a difference in the amount of contraction among the molded pieces, which varies in proportion with a distance from the gate. Especially in connection with precise molded pieces which are in cylindrical form and required to have roundness, there arises a problem that since the resin fed through the gate has a sharp pressure gradient in the cavity, it is impossible to maintain the dimensional accuracy of molded pieces at the end of a flow path. In view of conventional sliding guides for magnetic tapes, on the other hand, the following problems arise. (a) Since the conventional sliding guides for magnetic tapes are chromium-plated or formed of stainless on their surface, it is required to improve the surface roughness of the chromium-plated surfaces with a view to preventing the back sides of the magnetic tapes from being damaged or increasing frictional resistance. (b) Since metallic tape guides are basically harder than the base films of magnetic tapes, it is required to mirror-polish the metallic tape guides so as to clear them of irregularities. In view of tribology, the fundamental principle of material combinations are ideally that a tape of larger momentum must be harder than an associated tape guide. Thus, the combinations heretofore applied are contrary to this fundamental principle. To put it another way, it is essential that the material of tape guides be softer than the material of the base films of magnetic tapes. CH-A-407535 describes a device for keeping up the dwell pressure in an injection mold for thermoplastics, preferably for thick-walled molded pieces, consisting of a mold, at least one sprue and a spring biased riser, the mold being equipped with means for sealing such that the mold is separable from the injection nozzle before cooling, wherein the work pieces constituting the riser contain poorly heating conducting material. GB-A-2050928 describes an apparatus for molding ophthalmic lenses or other articles in which respective inserts having facing surfaces complementary to opposed surfaces of the finished article are mounted in first and second mold members relatively movable between an open position and a closed position for relative movement of the inserts toward and away from each other with the mold members closed to form a cavity. Further, the mold inserts are resiliently biased to move toward each other and are positively limited by stops or the like from separating beyond a predetermined extent. The mold according to the present invention, designed to solve the above problems, makes use of an injection mold fit for the flow characteristics of a polyethylene having a molecular weight not less than 300,000, rendering it possible to mold sliding guides for magnetic tapes, which have on their sliding surfaces a polyethylene whose molecular weight is not less than 500,000. The injection mold assembly for injection molding according to the present invention, particularly for injection molding molded parts of a polyethylene having a molecular weight of 500,000 to 6,000,000 inclusive, includes a compression piece 116 slidably located in a cavity and means 118 for energizing said compression piece 116 in which when said compression piece 116 is retracted by a resin-filling pressure, said means 118 generates a reaction compression force with respect to said resin pressure to compress the resin filled in the cavity with a pressure corresponding to the volume of the resin filled, and further includes a gate cutting piece 117 slidably located in said compression piece 116. Specifically, the injection mold according to the present invention is characterized by including a cavity in which a polyethylene having a molecular weight of 500,000 to 6,000,000 inclusive is filled through a gate suitable for the shape of pieces to be molded, a piece slidable in or out of said cavity, and a spring for generating a compression force counter to the resin pressure through said piece, whereby the leading end of the resin flowing through the cavity is compressed with a pressure corresponding to the volume of the resin filled. By the present invention, the polyethylene having a molecular weight not less than 500,000 can be injection-molded, thereby making it easy to transfer the mold surface to the pieces to be molded and reduce surface roughness; improving shapeability. A reduction in frictional resistance and the surface undulation of the molded pieces are also prevented, resulting improvements in the travelling characteristics of tapes. The reason is that the pressure distribution of resins differs generally at their flowing ends and at gates; it is higher at the latter than at the former. This tendency becomes particularly marked as the molecular weight increases. This causes the mold shrinkage factor to differ considerably at the gates and the flowing ends. As a result of intensive studies, the present inventors have found that the internal volume of the cavity is varied by the resin pressure and the spring for the die, whereby the resin pressure at the flowing end, which decreases as the resin is spaced away from the gate, can be rather increased by the displacement of the spring so that the resin pressure at the gate and the flowing end can both be made uniform. Thus, the present invention has been accomplished. More illustratively, the features of the mold according to the present invention are as follows. (1) The pressure applied to the resin by injection (measured by an in-mold sensor) is preferably in a range of, for instance 196 to 2942 bar (200 to 3000 kg/cm 2 ), especially 490 to 1961 bar (500 to 2000 kg/cm 2 ). Under such conditions, the compression pressure (applied directly to the resin) is preferably in a range of, for instance, 4.9 to 294 bar (5-300 kg/cm 2 ), especially 9.8 to 98 bar (10-100 kg/cm 2 ). (2) Selection of Gate Three types of gates, i.e., pin, side and film gates may all be applicable to injection molding. However, the pin and side gates are less likely to give strength and accuracy, since weldlines occur on the guide. It has been found that of the film gates, an annular type of film gate can be used to satisfy both strength and accuracy. The gate has a width of 0.1 to 2.0 mm, preferably 0.3 to 0.1 mm.The polyethylene having a molecular weight of 500,000 or higher is passed through the annular gate, thereby making it easy to fill it in the cavity for obtaining thin-wall molded pieces. The gate is also so concentric with the molded pieces that their short shot can be prevented with improvements in their roundness. Gate cutting is effected in the mold during molding, while a gate cutting piece (an inner core) is extended by a pressure cylinder for gate cutting. More effectively, gate cutting is effected just following the dwell step. In this manner: (1) In the cooling step just following the dwell step, the back-flow of the resin from the cavity to the sprue is prevented by cutting off the gate, so that stable molded pieces can be obtained. The above backflow phenomenon occurs when the pressure applied by the injection-molding machine to the resin filled in the cavity is released by the sprung pressure of the die according to the present invention. This also takes place by the elastic deformation of the molten resin. Since gate cutting timing is set just following the dwell step, the resin at the gate is kept at high temperatures. Thus, gate-free molded pieces can be obtained even by gate cutting making use of shearing, so that the products can sufficiently perform their own functions with no need of any post step. (3) The overall process can be cut short, since gate cutting can be carried out simultaneously with the molding step. It is understood that for the resinous guides the polyethylene having a molecular weight of 500,000 or higher may be used alone and, more effectively, it may be made composite with carbon or hydrocarbonaceous oligomers, polyethylene waxes, fluororesins and the like. Even in this case, no limitation is imposed on the present invention. In other words, although the melting viscosity of the resin is further increased by composite-making, yet it is possible to impart sufficient shapeability to high-viscosity materials, if the mold and injection-molding technique according to the present invention are used. Plastic materials are non-Newtonian fluids which decrease generally in their apparent viscosity at increased shear rates. This is also true of the polyethylene having a molecular weight of 500,000 or higher. In actual molding, this implies that viscosity decreases at increased shear rates so that thin-wall molding is feasible, thus giving rise to improvements in shapeability. As an example, it is possible to mold sliding guides for magnetic tapes by increasing shear rates and decreasing viscosity, because they are in thin-wall form. The problems then arising are that: (1) voids are generated, because the resin being molded entrains the air remaining in the mold; (2) short shots are generated, as if the volume of the cavity is decreased, because no air escapes from within the cavity; and (3) resin burning occurs due to the friction between the flowing end of the resin and the air, resulting in a lowering of its molecular weight. According to the above mold of the present invention, the cavity space is so filled with the insert piece at an initial stage of filling that the air prevailing in the runner and sprue portions is under reduced pressure. At the time of resin filling, therefore, the amount of air remaining in the cavity is so small that the above problems can be eliminated. Preparation of Feed Stock In what follows, reference will be made to how to composite the ultrahigh-molecular weight polyethylene inputted in the above injection-molding machine. Figure 5 is a longitudinal section of the jar used for compositing the ultrahigh-molecular weight polyethylene. A jar 18 includes a side wall filled with an oil 19 for heating, is provided at its bottom with an agitating blade 21 rotated by a motor 20 and is connected through its upper portion with a pipe 22 for feeding a non-oxidizing atmospheric gas such as nitrogen gas. The ultrahigh-molecular weight polyethylene and the material to be made composite with it are to be inputted in the above jar 18. That material is preferably an inorganic material such as CaCO 3 Al 2 (OH) 3 , Al 2 O 3 , Ca 3 (PO 4 ) 2 , glass fiber powders, carbon fibers powders, wood powders, paper powders, glass beads, metal particles and carbon, which are added in an amount of 30% by weight or less, particularly 3 to 30% by weight. At the same time, the aforesaid organic materials may be added. A mixture 23 of the ultrahigh-molecular weight polyethylene with the material to be made composite with it is inputted and mixed in the jar 18 fixed at a temperature of 80 to 120°C, preferably 85 to 100°C, while the speed of rotation the agitating blade 21 is rotating at 1,800 to 3,000 rpm. After the temperature of the mixture 23 reaches 110 to 120°C, the speed of rotation of the agitating blade blade is reduced to 90 rpm. The thus composited ultrahigh-molecular weight polyethylene is placed in a cooling jar in which it is cooled down to normal temperature at 60 rpm. That is to say, if the ultrahigh-molecular weight polyethylene particles are completely melt, then compositing cannot be achieved. By carrying out agitation in the above manner, however, the ultrahigh-molecular weight polyethylene particles absorb heat from the wall surface of the jar 18, and are then softened on their surfaces by frictional heat by agitation. Thereupon, the material to be composited is driven and deposited onto the softened particles' surfaces by acceleration by agitation. Thus, compositing is achieved. The thus composited raw material is inputted in the hopper 4 of the injection-molding machine shown in Figure 3, in which it is melted within such a short time as already mentioned. The melt is immediately injected into the cavity for molding. Illustrative Example of Injection Molding First of all, an ultrahigh-molecular weight polyethylene having a molecular weight of 3,000,000 (Hi-Zex million 240 M made by Mitsui Petrochemical Industries, Ltd.) was inputted in the hopper 4 of the injection-molding machine used with the injection mold assembly according to the present invention, and the slit 10a for regulating the input of the feed stock, located at the bottom of the hopper, was fully opened. The screw had then a diameter of 40 mm, a compression ratio of 1.4, a pitch of 30 mm, an L/D ratio of 20 and a speed of rotation of 180 rpm. The nozzle (an open nozzle) and compression zones 2c and 2b of the cylinder 2 were fixed at 220°C and 170°C, while the feeder zone 2a was not heated, but reached 70 to 100°C, as measured during molding. Further, the hopper 4 was fed with a nitrogen gas at a ratio of 1.0 liter/min. On the other hand, the mold 6 was fixed at 70°C, while the cavity 5 was vacuumized to 10 -3 torr. Molding was carried out with an annular gate (having a gate width of 0.8 mm) for the sliding guide for magnetic tapes so as to obtain molded pieces in which D = 6 mm, D = 4.85 mm and L = 16 mm, as shown in Figure 8. The molding conditions were an injection time of 0.3 sec., a dwell time of 3 sec., a cooling time of 20 sec., an interval of 3 sec. and an overall cycle of 26.5 sec. The pressure to be applied to the resin in the mold was fixed at 800 kg/cm 2 (as measured by a sensor), and the spring for producing a compression effect was built in the mold with a spring constant and deflection, both calculated and determined such that when the resin was filled in the cavity and the piece to be molded reached L = 16 mm, the spring produced a compression pressure of 15 kg/cm 2 . The above present invention, i.e. the characteristic features of the mold assembly, will now be explained in more detail with reference to the following embodiment. Figure 6 is a sectional view of a 12-cavity mold assembly for injection-molding 12 sliding guides for magnetic tapes according to the embodiment of the present invention. Referring to this figure, the mold assembly comprises a fixed bottom plate 103, a runner plate 104, a fixed retainer plate 105, a movable retainer plate 106, a back plate 107, a spacer plate 108, an ejector front plate 109, an ejector back plate 110 and a movable bottom plate 111 with a vacuum packing 119 interposed between the fixed and movable retainer plates 105 and 106 and O-rings (not shown) provided between the respective plates for complete sealing. While the mold is closed up, it is placed in a completely closed-up state and vacuumized through a hole 120 for vacuumization by means of a vacuum pump. Reference numeral 101 stands for a locating ring, 102 a sprue bush, and 112 a primary sprue formed in the sprue bush 102. A pressure sensor pin 126 is slidably fitted with respect to the runner plate 104 to transmit a load due to the resin pressure from a runner 113 to a sensor 127 therefor. The transmitted load is fed back to a machine control for closed-loop controlling. The cavity is constructed from a movable piece 115, a compression piece slidable by the resin pressure and a spring force and an inner core (a gate cutting piece) 117. When the cavity is unfilled, the compression piece 116 is in an advanced position with a gap of 1 mm located between it and the fixed retainer plate 105. The movable piece 115 is slidably fitted with the compression piece 116 in such a way that as the resin flows in the cavity and so the volume of the cavity increases, the compression piece 116 is retracted by the resin pressure. The compression piece 116 is connected and fixed with a spring 118 for generating a reaction force with respect to the resin pressure and determining the volume of the cavity by the resin pressure and spring force, thereby distributing the resin pressure uniformly throughout the piece to be molded. That is to say, the characteristic feature of the present invention is to vary the volume of the cavity by the amount of the resin filled, thereby generating a spring force corresponding to that volume or a force for making the cavity pressure distribution uniform. As illustrated in Figure 7, an annular gate 124 is located between the inner core (gate cutting piece) 117 and a secondary sprue 114. The inner core (gate cutting piece) 117 and the compression piece 116 are slidably fitted with each other, so that the inner core (gate cutting piece) 117 is ejected forward through the ejector front and back plates 109 and 110 for gate cutting by driving a pressure cylinder 123 for gate cutting, thereby cutting the annular gate 124. An illustrative example of the injection compression molding process will now be explained. The process is started from mold closing. In the course of mold closing, the vacuum pump is actuated to start vacuumizing the mold. At the time of completion of mold closing, the mold is vacuumized to about 10-3 torr. Afterwards, the weighed ultrahigh-molecular weight polyethylene is injected from the cylinder into the mold and filled in the cavity through the primary sprue 112, runner 113, secondary sprue 114 and annular gate 124. The resin is measured in terms of its pressure by the pressure sensor 127 and enters the dwell step after the preset pressure (785 bar; 800 kg/cm 2 ) is reached. Simultaneously with the completion of the dwell step, the pressure cylinder 123 for gate cutting is driven for gate cutting by the inner core (gate cutting piece) 117, Afterwards, the resin enters the cooling step and, in the meantime, the resin is weighed out in the cylinder for the next shot. As the cooling step is completed, the mold is so opened that the compression piece 116 is ejected forward by the spring 118 with the molded piece being pushed out of the mold. In this manner, a succession of molding steps are completed. As a result of measuring the dimensions of the thus obtained sliding guides for magnetic tapes, it has been found that they are in a range of 10 µm of the standard dimensions and have a roundness of 15 µm. According to the present method and apparatus in which, as explained above, a polyethylene having a molecular weight of 500,000 or higher is instantaneously melted and the melt is immediately injected into the cavity, it is possible to easily produce molded pieces of high precision such as sliding guides for tapes. By injection compression-molding a polyethylene having a molecular weight of 500,000 or higher, it is also possible to achieve improvements in shapeability such as easy transfer of the mold surface and a reduction in surface roughness. In addition, reductions in frictional resistance and the prevention of undulation of the molded pieces' surfaces are achieved, thus making it possible to improve the travelling characteristics of tapes. Because of having self-wettability, the obtained sliding guides for tapes dispenses with lubrication and so are applicable both in vacuo and in the water.

Claims: An injection mold assembly for injection molding molded parts of a polyethylene having a molecular weight of 500,000 to 6,000,000 inclusive, which includes a compression piece (116) slidably located in a cavity and means (118) for energizing said compression piece (116) and in which when said compression piece (116) is retracted by a resin-filling pressure, said means (118) generates a reaction compression force with respect to said resin pressure to compress the resin filled in the cavity with a pressure corresponding to the volume of the resin filled, and which further includes a gate cutting piece (117) slidably located in said compression piece (116).

Inventors: FUJII HIDEAKI, NISHIYAMA MASASHI, TAKAHASHI TATSUMI, FUJII, HIDEAKI, NISHIYAMA, MASASHI, TAKAHASHI, TATSUMI, FUJII, HIDEAKI, C/O DAI NIPPON INSATSU K. K., NISHIYAMA, MASASHI, C/O DAI NIPPON INSATSU K. K., TAKAHASHI, TATSUMI, C/O DAI NIPPON INSATSU K. K.
Applicants: DAINIPPON PRINTING CO LTD, DAI NIPPON INSATSU KABUSHIKI KAISHA
Application year: 1989
Main classification: B29C45
IPC codes: B29C45, G11B15