Method for reducing gate blush and mold design for the method

Abstract

A system and method for injection molding polymer articles are described. The system and method are designed to reduce gate blush. In one embodiment, an injection molding device injects a molten polymer composition into a mold cavity that is adjacent to an internal curved surface on the mold. The flow of the polymer material is parallel to a line tangent to the curved surface.

Description

[0001] Related Applications

[0002] This application claims priority to U.S. Provisional Application No. 62 / 818,975, filed on March 15, 2019, which is hereby incorporated by reference in its entirety. Background Art

[0003] In the relatively recent past, there has been an increasing desire to replace metal parts with parts made of polymeric materials, particularly high-performance thermoplastic polymers. This need has extended to almost every different application and field. For example, parts made of thermoplastic polymers are used in consumer electrical products, industrial processes, various transportation vehicles, and so on. For example, in the automotive field, there is an increasing need to replace metal parts with parts made of thermoplastic polymers. For example, parts made of thermoplastic polymers can be used in the exterior or interior of a vehicle.

[0004] For example, when used in automotive applications, parts made of thermoplastic polymers typically need to have an aesthetically pleasing appearance to further enhance the attractiveness of the vehicle. Therefore, in many applications, after molding, the parts are painted or otherwise decorated to match the colors of the environment in which they are used. Unfortunately, painting plastic parts is a significant expense in the production process of plastic parts. Having to paint the parts also increases the production time and significantly increases the capital consumption of the facilities for producing the parts.

[0005] In view of this, those skilled in the art have attempted to add colorants to thermoplastic polymers so that molded parts do not have to be painted. For example, in one embodiment, metallic pigments are added to a thermoplastic compound to provide molded parts with a metallic appearance. However, during the injection molding process of polymeric articles, various surface defects may occur during the production of the parts. For example, certain part geometries, mold layouts, gate locations, and molding conditions can result in various defects, including increased loss of gloss upon weathering and visual defects that affect the aesthetics of the part. A particularly recurring problem is known as "gate blush", which results in irregular dark patterns or dull or discolored areas near the gate location of the injection mold. Another recurring problem is that when the mold construction includes more than one gate, flow lines are formed in the finished part at the confluence of the two flow fronts. These flow lines are sometimes referred to in the art as "knit-lines".

[0006] In many applications, weld lines can be eliminated by a mold configuration having only a single gate or entry point for melting the thermoplastic material into the mold. On the other hand, attempts to eliminate gate blush are generally unsuccessful in many different molding configurations. For example, U.S. Patent Publication No. 2013 / 0123424 (which is incorporated herein by reference) discloses a method of reducing gate blush on a molded article by designing a runner of an injection mold that includes one or more overflow channels. The overflow channels temporarily divert the polymer stream during injection molding, which has been found to significantly reduce the occurrence of gate blush. However, there is still a need for other configurations or alternative configurations for reducing or eliminating gate blush. SUMMARY OF THE INVENTION

[0007] The present disclosure generally relates to a molding process and system for injection molded articles that reduces or substantially eliminates gate blush. More specifically, the present disclosure relates to an injection molding system in which gate blush is reduced by designing a gate for the flow of polymer material that connects to the mold cavity at a point tangent to a curved edge of the mold.

[0008] For example, in one embodiment, the present disclosure relates to a method for molding a polymer material. The method includes supplying molten polymer material through a gate leading to a mold cavity. The mold cavity includes an internal curved surface. The gate is positioned such that the molten polymer material stream exits the gate at a location tangent to the internal curved surface. The method further includes the steps of filling the mold cavity with the polymer material to form a molded article and removing the molded article from the mold.

[0009] For example, the internal curved surface of the mold cavity may follow an arc that extends greater than about 50°, such as greater than about 80°, such as greater than about 90°, such as greater than about 100° and generally less than about 300°. For example, the arc defined by the internal curved surface may generally follow a circle, such as a circle, an ellipse, or an oval. The exit of the gate may form an angle with an arc defined by two radii of the circle. The angle may be greater than about 30° and less than about 90°. For example, the arc may have a first end and an opposite second end. The angle may be formed on the circle, having a vertex at the center of the circle, and may extend between the first end on the arc and the point where the polymer material stream is tangent to the arc. For example, the angle may be from about 35° to about 65°, such as from about 40° to about 50°, such as from about 42° to about 48°.

[0010] In addition to introducing the polymer stream into the mold cavity along the internal curved surface, the polymer material stream may also fan out in a fan shape as it exits the gate. For example, the width of the fan-shaped stream may be at least five times the thickness of the polymer stream.

[0011] The polymeric material deposited into the mold cavity can include any suitable thermoplastic polymer. In one embodiment, for example, the polymeric material can include a polyoxymethylene polymer or a polyamide polymer. In one embodiment, the thermoplastic polymer can contain metallic pigments, such as aluminum pigments. In one embodiment, the metallic particles can have a flake shape. For example, the aspect ratio of the flake particles can be greater than about 4:1, such as greater than about 10:1 and generally less than about 40:1.

[0012] The present disclosure also relates to a system for injection molding a polymeric article from a thermoplastic material. The system includes a mold defining a mold cavity. The mold cavity includes an internal curved surface. The system also includes a compression zone for heating and compressing the thermoplastic material and delivering the molten thermoplastic material into at least one flow path leading to a gate. The gate is positioned to exit into the mold cavity. More specifically, the gate is positioned such that the molten polymeric material flows through the gate and exits the gate at a location tangential to the internal curved surface of the mold cavity. In one embodiment, the gate has a fan shape.

[0013] Other features and aspects of the present disclosure are discussed in more detail below. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In the remainder of the specification, including the reference drawings, the complete and enabling content of the present disclosure is set forth in more specific detail, in which:

[0015] Figure 1 is a cross-sectional perspective view of one embodiment of an injection molding configuration according to the present disclosure;

[0016] Figure 2 is a geometric diagram showing the relationship between a polymer stream and a curved surface on a mold according to the present disclosure;

[0017] Figure 3 is Figure 1 a cross-sectional view of the molded configuration shown;

[0018] Figure 4 is Figure 1 another cross-sectional view of the molded configuration shown; and

[0019] Figure 5 is a side view of a cross-sectional portion of an injection molding system according to the present disclosure.

[0020] In this specification and the drawings, repeated reference numerals are intended to denote the same or similar features or elements of the invention. DETAILED DESCRIPTION

[0021] Those of ordinary skill in the art will understand that this discussion is only a description of exemplary embodiments and is not intended to limit the broader aspects of the present disclosure.

[0022] Generally, the present disclosure relates to a system for producing molded articles while reducing or eliminating gate blush and a method for molding products while reducing or eliminating gate blush. Generally, the system of the present disclosure includes a mold that defines a cavity in fluid communication with an injection device. The injection device is configured to heat and inject molten thermoplastic material into the mold. There is a flow path between the mold and the injection device. The flow path terminates at a gate positioned adjacent to the cavity. According to the present disclosure, the molten polymeric material exiting the gate enters the cavity tangentially to a curved portion on the inner surface of the mold.

[0023] More specifically, the present disclosure relates to an injection mold gate that distributes a molten stream of polymer along a line tangential to a curved surface inside the mold. For example, the polymer stream can be tangent to the curved surface inside the mold at any point on the curved surface extending greater than 1° to generally less than about 89°. Even in cases where the polymeric material contains metallic pigments or other dispersed pigments, by having the gate distribute the polymeric material at a point generally tangent to the curved surface on the cavity, gate blush can be significantly reduced and in many cases eliminated.

[0024] Gate blush is a surface defect that can manifest as discoloration in the gate area of an injection molded article. The gate area is the portion of the exterior surface of the product that forms near the gate where the thermoplastic polymer is injected into the mold. For example, the gate area can be circular and have a diameter of less than about 5 mm 2 , such as less than about 4 mm 2 , such as less than about 3 mm 2 , such as less than about 2 mm 2 , such as less than about 1 mm 2 . The center of the gate area is concentric with the location of the gate. When the thermoplastic polymer is injection molded through the gate, gate blush can form and exhibit a series of curves located in the gate area.

[0025] Gate blush can be defined in different ways. In one embodiment, for example, gate blush includes a helical pattern of melt fracture that, in one embodiment, is visible to the human eye. In other embodiments, the helical pattern is visible when examined by a scanning electron microscope at a magnification of x100 and an angle of 45°. In one embodiment, at least some of the lengths of the melt fracture are at least 20 microns, such as at least 20%, such as at least 40%, such as at least 60%.

[0026] Gate blush can also be determined by a surface defect monitoring technique called BORG spectrophotometry, which is described in U.S. Patent No. 6,868,371 and is incorporated herein by reference. In this technique, a spatially resolved spectrophotometer is used to measure surface defects of molded plastic parts. The measurements of one or more of these sample plastic parts are then provided to a computerized device that appropriately filters the data and calculates the overall data shape, the average peak-to-valley offset, and a quality number indicative of the data slope. In this method, the sample molded part is mounted on a sample holder, which is optionally mounted on a motorized translation stage to scan features of interest on the sample surface. The computerized device automates the motion system and data collection and converts the raw data into color coordinates. This data is processed by considering instrument calibration data obtained beforehand and allows the user to optimize the signal-to-noise ratio by allowing parameter adjustment. The instrument then calculates the color index difference between the brightest and darkest points in the streak region.

[0027] Surface defects are typically detected by measuring a reduction in the color index (sometimes hereinafter referred to as “L”), the reduction of which physically corresponds to the defect (e.g., gate blush).

[0028] Using the above technique, the color index can be measured at surface defects in regions such as the gate of a molded article. The color index can also be measured away from the defect. The percentage difference in color shift can then be calculated from the two measurements.

[0029] According to the present disclosure, minimized surface defects result in a small difference in the color index measured in the gate region and the remainder of the cosmetic surface. For example, in one embodiment, the color index in the gate region can vary by no more than 20%, such as no more than 10%, such as no more than 5% relative to the color index of the remainder of the cosmetic surface.

[0030] A method using BORG spectrophotometry is described in U.S. Patent No. 6,825,266 and is incorporated herein by reference.

[0031] Referring Figure 5 , an embodiment of an injection molding system that can be manufactured according to the present disclosure is shown. As shown, the system includes an injection device 10 configured to melt or soften a polymer material and inject the molten polymer material into a mold cavity. The polymer composition can be fed into the injection device 10 in any suitable manner. In Figure 5 the illustrated embodiment, for example, the system includes a hopper 14 for receiving the polymer composition and supplying the composition to the injection device 10.

[0032] The injection device 10 includes a barrel 16 that contains a heater and a conveying device. The conveying device can include, for example, one or more feed screws 18. For example, in Figure 5 the illustrated embodiment, the barrel 16 includes a single feed screw 18. However, in other embodiments, the barrel 16 can contain a dual feed screw. The feed screw 18 is rotated by a motor 20. For example, the motor 20 can include an electric motor that is connected to the feed screw 18 by one or more belts or chains.

[0033] When the polymer composition is added to the hopper 14, the polymer composition is heated to a molten state within the barrel 16. The feed screw 18 conveys the molten polymer material from the barrel 16 into the flow path 22. The flow path 22 can communicate with the injection device 10 through an injection point. The polymer material is injected from the flow path 22 into the cavity of the mold 12 via the gate 24 to produce a molded article.

[0034] In the past, problems have been encountered at the location where the polymer material enters the cavity. Specifically, visual defects are formed at the location where the polymer material enters the cavity. These defects are called gate blush. In many cases, gate blush may appear as an irregular dark pattern or a reticulated defect, which greatly reduces the aesthetic appearance of the part and may cause the part to be rejected by the manufacturer.

[0035] Referring to Figures 1 to 4 , an embodiment of a mold configuration for reducing or eliminating gate blush according to the present disclosure is shown. Referring to Figure 1 , for example, the flow path 22 is shown, which is designed to receive a flow of molten polymer material from an injection molding system as Figure 5 shown. The flow path 22 is in fluid communication with the gate 24. The gate 24 directs the flow of molten polymer material into the cavity of the mold 12.

[0036] As Figure 1 shown, the mold 12 includes a curved portion 30 that defines an internal curved surface within the cavity. According to the present disclosure, the gate 24 is positioned such that the flow of molten polymer material exits the gate at a position tangential to the internal curved surface defined by the curved portion 30.

[0037] Referring to Figure 2 , a diagram is given that illustrates the relationship between the curved portion 30 on the mold 12 and the flow direction of the polymer material. As shown, the curved portion 30 includes an internal curved surface that generally follows the circumference of a circle 34. It should be understood that the internal curved surface of the mold does not have to have a perfect circular shape. Figure 2 For illustrative purposes only.

[0038] Line 32 is at Figure 2As shown, line 32 is tangent to the curved portion 30. According to the present disclosure, the gate 24 distributes the molten polymeric material in a direction parallel to line 32 on the interior of the mold 12. Generally, the molten polymeric material can flow into the mold cavity at any point on the curved portion 30 that is tangent to the curved portion. For example, as Figure 2 shown, the curved portion 30 defines an arc 36. The arc has a first end 38 and a second end 40. In Figure 2 the illustrated embodiment, the arc 36 defined by the curved portion 30 extends 90° along the circumference of the circle 34. Generally, the arc defined by the curved portion 30 can extend at least 50°, such as at least 60°, such as at least 70°, such as at least 80°, such as at least 90°, such as at least 100°, such as at least 110°, such as at least 120°, such as at least 130°, such as at least 140°, such as at least 150°, such as at least 160°, such as at least 170°, such as at least 180°, such as at least 190°, such as at least 200° and generally less than about 330°, such as less than about 300°, such as less than about 250°.

[0039] As Figure 2 shown, the point at which line 32 or the polymer stream is tangent to the arc 36 can be defined by an angle θ. The angle θ is defined by two radii of the circle 34. One side of the angle extends from the center of the circle 34 to the first end of the arc 36. The second side of the angle extends from the center of the circle 34 to the point at which line 32 is tangent to the arc 36. According to the present disclosure, the angle θ can be adjusted as needed to reduce or eliminate gate blush. Generally, the angle θ is greater than about 1° and less than about 89°. The angle θ for any particular mold configuration can depend on various factors, including the size of the mold cavity, the polymeric material flowing into the mold, the size of the arc 36 defined by the internal curved surface 30, the flow rate of the polymeric material, and various other factors. In one embodiment, the polymer material stream enters the mold cavity at a point tangent to the arc 36 such that the resulting angle θ satisfies the following conditions: the resulting angle θ is greater than about 10°, such as greater than about 20°, such as greater than about 30°, such as greater than about 40°. The angle θ is generally less than about 80°, such as less than about 70°, such as less than about 60°, such as less than about 50°. In one embodiment, the angle θ is from about 35° to about 65°, such as from about 40° to about 50°, such as from about 42° to about 48°. For example, in Figure 2 the illustrated embodiment, the angle θ is 45°.

[0040] See Figure 3 and Figure 4 , which shows a cross-sectional view of the mold configuration shown in Figure 1 . The mold 12 is in fluid communication with the gate 24. The mold 12 includes a curved portion 30 that defines an internal curved surface. The gate 24 directs the flow of the molten polymeric material such that the polymeric material enters the mold cavity of the mold 12 at a point tangent to the internal curved surface defined by the curved portion 30. AsFigure 4 As shown, a circle 34 can be drawn that most conforms to the inner curved surface of the curved portion 30. The polymer material stream enters the mold cavity at a point tangent to the inner curved surface. The point tangent to the inner curved surface forms an angle θ on the circle 34. This angle can be any angle from greater than 1° to less than 89°, such as greater than about 30° to less than about 60°.

[0041] In one embodiment, the mold 12 includes only a single gate 24. As Figure 1 shown, in one embodiment, the gate 24 can have a fan shape. For example, when the molten polymer material flows from the flow path 22 into the gate 24, the molten polymer material spreads out as it enters the mold cavity of the mold 12. For example, the fan shape of the gate 24 can have a width W that is much greater than the height H. For example, the width W can be at least 3 times the height H, such as at least 5 times, such as at least 7 times, such as at least 9 times. The width W is generally less than about 50 times the height H.

[0042] Still as Figure 1 shown, the gate 24 can generally have a height H that gradually decreases from the flow path 22 to the gate exit at the entrance of the mold 12. The gradually decreasing height is also shown in Figure 4 .

[0043] Generally, the systems and methods of the present disclosure are applicable to processing any suitable thermoplastic polymer in a molding process, particularly in an injection molding process. In one embodiment, the polymer composition used in the method of the present disclosure includes a thermoplastic polymer combined with a colorant for producing a molded article that is ready to use as soon as it exits the mold cavity. In particular, the presence of the colorant is to provide a molded article with aesthetic appeal without the need to paint or otherwise decorate the molded article.

[0044] Thermoplastic polymers that can be processed according to the present disclosure include polyacetal polymers, polyamide polymers, polyarylene sulfide polymers (such as polyphenylene sulfide polymers), polyolefin polymers (including polyethylene and polypropylene), polycarbonate polymers, polyester polymers (including PCT polymers), and the like.

[0045] Colorants combined with the thermoplastic polymer can include pigments, dyes, metal particles, etc. Pigment particles can include, for example, barium sulfate particles, titanium dioxide particles, calcium carbonate particles, etc. In another embodiment, the colorant can only contain mica flakes or a combination of mica flakes and dyes such as organic dyes.

[0046] In a particular embodiment, the polymer composition comprises a combination of polyoxymethylene or polyamide resin with a colorant, in particular metal flakes. The polymer resin may include homopolymers or copolymers and may include endcaps. Polyoxymethylene homopolymers can be obtained by polymerizing formaldehyde or trioxane, which can be cationically or anionicly initiated. The homopolymer may mainly contain formaldehyde units in the polymer chain. On the other hand, polyoxymethylene copolymers may contain oxyalkylene units and formaldehyde units. The oxyalkylene units may contain, for example, from about 2 to about 8 carbon units and may be straight-chain or branched-chain. In one embodiment, the homopolymer or copolymer may have hydroxyl end groups that are chemically stabilized against esterification or etherification degradation.

[0047] Together with the -CH2O- repeating unit, the polyoxymethylene copolymer may contain up to 50 mol%, for example 0.1 mol% to 20 mol%, in particular 0.5 mol% to 10 mol% of the repeating units of the following formula:

[0048]

[0049] wherein R 1 to R 4 are independently of one another a hydrogen atom, a C1-C4-alkyl or a haloalkyl group having 1 to 4 carbon atoms, and R 5 is --CH2--, --O--CH2--, or a methylene group substituted by C1-C4-alkyl- or C1-C4-haloalkyl, or the corresponding formaldehyde group, and n is from 0 to 3.

[0050] These groups can advantageously be introduced into the copolymer by ring-opening of cyclic ethers. Preferred cyclic ethers are those having the following formula:

[0051]

[0052] wherein R 1 to R 5 and n are as defined above.

[0053] Cyclic ethers that can be mentioned as examples are ethylene oxide, 1,2-epoxypropane, 1,2-epoxybutane, 1,3-epoxybutane, 1,3-dioxane, 1,3-dioxolane and 1,3-dioxepane, and comonomers that can be mentioned as examples are linear oligomeric formaldehyde or polyformaldehyde, such as polydioxolane or polydioxepane.

[0054] Formaldehyde terpolymers are also used, such as those prepared by reacting trioxane with one of the above cyclic ethers and a third monomer, which third monomer is preferably a bifunctional compound of the following formula:

[0055]

[0056] wherein Z is a chemical bond, -O-- or -ORO-- (R = C1-C8-alkylene or C2-C8-cycloalkylene).

[0057] Preferred monomers of this type are ethylene diglycide, diglycidyl ether and diether composed of glycidyl units and formaldehyde, dioxane or trioxane in a molar ratio of 2:1, and diether composed of 2 moles of glycidyl compound and 1 mole of aliphatic diol having 2 to 8 carbon atoms, such as diglycidyl ethers of ethylene glycol, 1,4-butanediol, 1,3-butanediol, 1,3-cyclobutanediol, 1,2-propanediol or 1,4-cyclohexanediol, to name just a few.

[0058] The polyacetal resin as defined herein may also include a blocked resin. For example, such a resin may have side hydroxyl groups. For example, such a polymer is described in U.S. Patent No. 5,043,398, which is incorporated herein by reference.

[0059] In one embodiment, the polyacetal polymer may contain hemiacetal end groups and / or formyl end groups. In particular, it is believed that the method of the present disclosure can advantageously significantly reduce the formaldehyde emission of the polyacetal polymer, even when the polymer contains hemiacetal end groups and possible formyl end groups. For example, in one embodiment, the polyacetal polymer may contain an amount greater than 1.0 mmol / kg, such as an amount greater than 1.5 mmol / kg, of hemiacetal end groups. In an alternative embodiment, the polyacetal polymer may contain an amount greater than 2 mmol / kg, such as an amount greater than 2.5 mmol / kg, of formyl end groups.

[0060] The method for forming the above polyoxymethylene polymer may vary according to the specific application. However, a method that results in a polyacetal resin with a relatively low formaldehyde content can be used. In this regard, in one embodiment, the polymer may be prepared by a solution hydrolysis process as described in U.S. Patent Application Publication No. 2007 / 0027300 and / or U.S. Patent Application No. 2008 / 0242800, both of which are incorporated herein by reference. For example, in one embodiment, a polyoxymethylene polymer containing aliphatic diol units or cycloaliphatic diol units may be degraded by solution hydrolysis using methanol and water with triolethylene.

[0061] The polyacetal resin or polyoxymethylene that can be used according to the present disclosure generally has a melting point greater than about 150 degrees Celsius. The molecular weight of the polymer can generally range from about 2,000 to about 1,000,000, such as from about 7,000 to about 150,000. The melt flow rate (MVR 190-2.16) of the polymer can be from about 0.3 g / 10 min to about 20 g / 10 min, particularly from about 2 g / 10 min to about 9 g / 10 min (ISO 1133).

[0062] Various different types of polyamide polymers can also be used to produce molded articles. For example, the polyamide polymer can be an aliphatic polyamide, such as nylon 6 or nylon 66.

[0063] Generally, the polymer composition can contain a thermoplastic polymer resin in an amount greater than about 60 wt%, such as in an amount greater than about 70 wt%, such as in an amount greater than about 80 wt%, such as in an amount greater than about 90 wt%. The polymer is generally present in an amount less than about 95 wt%.

[0064] As described above, the polymer resin can be combined with a metallic pigment, such as metal flakes. The metal flakes can have a flaky shape. The particles can be polished or otherwise have a high reflectivity. In one embodiment, the flaky particles can have an aspect ratio greater than about 4:1, such as greater than about 8:1, such as from about 10:1 to about 50:1.

[0065] The flaky particles can have a median diameter greater than about 12 microns, such as greater than about 14 microns. For example, the flaky particles can have a median diameter from about 15 microns to about 650 microns. In a specific embodiment, the flaky particles have a size from about 15 microns to about 30 microns.

[0066] The metallic pigment can be present in the polymer composition in an amount from about 0.01 wt% to about 20 wt%, such as from about 0.1 wt% to about 15 wt%. For example, the metallic pigment can be present in the polymer composition in an amount from about 0.25 wt% to about 10 wt%, such as from about 0.5 wt% to about 5 wt%.

[0067] The metallic pigment can contain any suitable metal, such as metals of Groups I-B, III-A, IV, VI-B, and VIII of the periodic table. Physical mixtures of these metallic pigments and alloys of these pigments can also be used. In a particular embodiment, the metallic pigment can include aluminum, bronze, brass, chromium, copper, gold, iron, molybdenum, nickel, tin, titanium, zinc, etc. In one embodiment, two metallic pigments having different average particle sizes can be combined.

[0068] In one embodiment, the metallic pigment can include an aluminum pigment containing the element aluminum. For example, the aluminum pigment can be very thin, with a thickness less than about 1 micron, and can have a median diameter as described above. For example, in a specific embodiment, the aluminum pigment can have a median diameter of about 12 microns to about 18 microns. The aluminum pigment can have a distinct flop, high brightness, and high reflectivity.

[0069] In one embodiment, the aluminum pigment can contain greater than about 80 wt% aluminum. The aluminum pigment can exist alone or in combination with other additives such as carriers. For example, the aluminum pigment can exist in combination with a thermoplastic polymer (such as polyolefin, purified medicinal white oil), or can exist together with a solvent (such as diisononyl phthalate).

[0070] In addition to the colorant and the thermoplastic polymer, the polymer composition can also contain various other components. For example, in one embodiment, an ultraviolet light stabilizer can be present. The ultraviolet light stabilizer can include benzophenone, benzotriazole, or benzoate. Specific examples of the ultraviolet light stabilizer include: 2,4-dihydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone, 2-(2'-hydroxy-3',5'-di-tert-butylphenyl)benzotriazole, 2-(2'-hydroxy-3'-tert-butyl-5'-methylphenyl)-5-chlorobenzotriazole, 2,4-dihydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone, 2-hydroxy-4-octoxybenzophenone, and 5,5'-methylenebis(2-hydroxy-4-methoxybenzophenone); 2-(2'-hydroxyphenyl)benzotriazole, for example, 2-(2'-hydroxy-5'-methylphenyl)benzotriazole, 2-(2'-hydroxy-5'-tert-octylphenyl)benzotriazole, 2-(2'-hydroxy-3',5'-di-tert-butylphenyl)benzotriazole, 2-(2'-hydroxy-3',5'-di-tert-butylphenyl)-5-chlorobenzotriazole, 2-(2'-hydroxy-3'-tert-butyl-5'-methylphenyl)-5-chlorobenzotriazole, 2-(2'-hydroxy-3',5'-dicumenylphenyl)benzotriazole, and 2,2'-methylenebis(4-tert-octyl-6-benzotriazolyl)phenol, phenyl salicylate, resorcinol monobenzoate, 2,4-di-tert-butylphenyl-3',5'-di-tert-butyl-4'-hydroxybenzoate, and cetyl 3,5-di-tert-butyl-4-hydroxybenzoate; substituted oxanilides, such as 2-ethyl-2'-ethoxyoxanilide and 2-ethoxy-4'-dodecyloxanilide; cyanoacrylates, such as ethyl-α-cyano-β,β-diphenylacrylate and methyl-2-cyano-3-methyl-3-(p-methoxyphenyl)acrylate, or mixtures thereof.

[0071] In one embodiment, the polymer composition may further comprise a formaldehyde scavenger, such as a nitrogen-containing compound. For example, the nitrogen scavenger may comprise a guanamine compound, such as benzoguanamine.

[0072] In one embodiment, the composition may comprise a nucleating agent. For example, the nucleating agent can increase crystallinity and may comprise a formaldehyde terpolymer. For example, in a particular embodiment, the nucleating agent can comprise a terpolymer of butanediol diglycidyl ether, ethylene oxide, and trioxane. The nucleating agent may be present in the composition in an amount greater than about 0.05 wt%, such as greater than about 0.1 wt%. The nucleating agent may also be present in an amount less than about 2 wt%, such as in an amount less than about 1 wt%.

[0073] Another additive that may be present in the composition is a sterically hindered phenol compound, which can be used as an antioxidant. Examples of commercially available such compounds are pentaerythritol tetra[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (Irganox 1010, BASF), triethylene glycol bis[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionate] (Irganox 245, BASF), 3,3'-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionylhydrazide] (Irganox MD 1024, BASF), hexamethylene glycol bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (Irganox 259, BASF), and 3,5-di-tert-butyl-4-benzyl alcohol (Lowinox BHT, Chemtura). Irganox 1010 is preferred, especially Irganox 245. The above compounds may be present in the composition in an amount less than about 2 wt%, such as from about 0.01 wt% to about 1 wt%.

[0074] In addition to the UV light stabilizer, the light stabilizers that may be present in the composition include sterically hindered amines. The hindered amine light stabilizers that can be used include N-methylated oligomeric compounds. For example, another example of a hindered amine light stabilizer includes the ADK STAB LA-63 light stabilizer available from Adeka Palmarole. The light stabilizer, when present, may be included in an amount greater than about 0.1 wt%, such as greater than about 0.5 wt%, but less than about 2 wt%, such as less than about 1 wt%.

[0075] Fillers that can be included in the composition include glass beads, wollastonite, loam, molybdenum disulfide or graphite, and inorganic or organic fibers such as glass fibers, carbon fibers or aramid fibers. For example, the length of the glass fibers can be greater than about 3 mm, such as 5 mm to about 50 mm. The composition can also include thermoplastic or thermosetting polymer additives, or elastomers such as polyethylene, polyurethane, polymethyl methacrylate, polybutadiene, polystyrene, or other graft copolymers as follows: the core of which is prepared by polymerizing 1,3-butadiene, isoprene, n-butyl acrylate, 2-ethylhexyl acrylate or a mixture thereof, and the shell of which is prepared by polymerizing styrene, acrylonitrile or (meth)acrylate.

[0076] In one embodiment, the composition can further include one or more lubricants. The lubricant can include a polymer wax composition. Lubricants that can be included in the composition include, for example, N,N'-ethylenebisstearamide. In one embodiment, a polyethylene glycol polymer (processing aid) can be present in the composition. For example, the polyethylene glycol can have a molecular weight of about 1000 to about 5000, such as about 3000 to about 4000. In one embodiment, for example, PEG-75 can be present. The lubricant can be present in the polymer composition in an amount of about 0.01% to about 5%. For example, the lubricant can be present in an amount greater than about 0.1 wt%, such as in an amount of about 0.1 wt% to about 1 wt%. The above polyethylene glycol polymer can also be present in an amount of up to about 5 wt%. For example, the polyethylene glycol polymer can be present in an amount of about 0.1 wt% to about 2 wt%, such as about 0.5 wt% to about 1 wt%.

[0077] In addition to the above components, the polymer composition can further include an acid scavenger. The acid scavenger can include, for example, an alkaline earth metal salt. For example, the acid scavenger can include a calcium salt such as calcium citrate. The acid scavenger can be present in an amount of about 0.01 wt% to about 1 wt%.

[0078] Any of the above additives can be added to the polymer composition alone or in combination with other additives. Generally, each additive is present in an amount less than about 5 wt%, such as less than about 2 wt%, such as less than about 1 wt%.

[0079] Almost an infinite variety of polymer articles can be molded according to the present disclosure. Such articles can include buttons, door handles, automotive panels, automotive interior parts such as trim, consumer appliance parts, etc., but are not limited thereto.

[0080] The present disclosure can be better understood with reference to the following examples.

[0081] Examples

[0082] The following examples are carried out to demonstrate some of the benefits and advantages of the present disclosure.

[0083] The polyoxymethylene polymer is mixed with aluminum pigment and fed through a single-screw extruder, and then injection molded into plates using different gate geometries. The first tested gate geometry is similar to the Figure 3 gate geometry shown, where the polymer stream enters the mold parallel to a line tangent to the curved surface on the mold. The second gate geometry is tested, where the gate distributes the polymer into the mold, where the polymer stream is not tangent to the curved surface on the mold. In particular, the gate is moved to a horizontal position (0°, as Figure 4 shown) relative to the inner surface of the mold. In the first tested gate geometry, the angle at which the gate enters the mold is 45°. The mold machine and injection molding parameters for both gate configurations are kept constant.

[0084] After molding, the plates are visually inspected. The plates manufactured according to the present disclosure have no visible gate blush. However, the comparative samples have a gate blush problem, where the gate blush covers more than half of the plate surface.

[0085] Without departing from the spirit and scope of the invention as more particularly set forth in the appended claims, those of ordinary skill in the art may make these and other improvements and variations to the invention. Additionally, it should be understood that aspects of the various embodiments may be wholly or partially interchanged. Further, those of ordinary skill in the art will understand that the foregoing description is merely exemplary and is not intended to limit the invention as further described in the appended claims.

Claims

1. A method for molding a polymeric material, comprising: feeding a molten polymeric material through a gate leading to a mold cavity, the mold cavity including an internal curved surface, and wherein the gate is positioned such that the molten polymeric material stream exits the gate at a location tangential to the internal curved surface; filling the mold cavity with the polymeric material to form a molded article; and removing the molded article from the mold; wherein the arc defined by the internal curved surface follows a circle and defines a first end and a second end, and wherein the outlet of the gate forms an angle with the arc defined by two radii of the circle extending from the center of the circle to the first end of the arc and from the center of the circle to the point where the polymeric material stream is tangent to the arc, the angle being greater than 30° and less than 90°; and wherein the angle formed between two radii of the circle extending from the center of the circle to the first end of the arc and from the center of the circle to the second end of the arc has an angle greater than 50° and less than 300°; the gate has a sector shape, the gate includes a width and a height, and wherein the width at the outlet of the gate is at least three times the height.

2. The method according to claim 1, wherein The angle defined by the outlet of the gate and the two radii of the circle extending from the center of the circle to the first end of the arc and from the center of the circle to the point where the polymeric material stream is tangent to the arc is from 35° to 65°.

3. The method according to claim 2, wherein, The angle defined by the outlet of the gate and the two radii of the circle extending from the center of the circle to the first end of the arc and from the center of the circle to the point where the polymeric material stream is tangent to the arc is from 40° to 50°.

4. The method according to claim 1, wherein The angle formed between two radii of the circle extending from the center of the circle to the first end of the arc and from the center of the circle to the second end of the arc has an angle greater than 70° and less than 300°.

5. The method according to claim 4, wherein The angle formed between two radii of the circle extending from the center of the circle to the first end of the arc and from the center of the circle to the second end of the arc has an angle greater than 80° and less than 300°.

6. The method according to claim 5, wherein, The angle formed between two radii of the circle extending from the center of the circle to the first end of the arc and from the center of the circle to the second end of the arc has an angle greater than 90° and less than 300°.

7. The method according to claim 6, wherein, The angle formed between two radii of the circle extending from the center of the circle to the first end of the arc and from the center of the circle to the second end of the arc has an angle greater than 100° and less than 300°.

8. The method according to claim 1, wherein When leaving the gate, the polymeric material stream unfolds in a sector shape.

9. The method according to claim 8, wherein, The width of the sector-shaped stream is at least five times the thickness of the polymeric stream.

10. The method according to claim 1, wherein, The polymeric material includes a polyoxymethylene polymer or a polyamide polymer.

11. The method according to claim 1, wherein, The polymeric material contains a metallic pigment.

12. The method according to claim 11, wherein, The metallic pigment includes aluminum particles.

13. The method according to claim 11 or 12, wherein, The metallic pigment includes metallic particles having a flaky shape, and the aspect ratio of the metallic particles is greater than 4 ∶ 1.

14. A system for injection molding a polymeric article from a thermoplastic material, comprising: a mold that defines a mold cavity, the mold cavity including an internal curved surface; a compression zone for heating and compressing the thermoplastic material; A flow path, the flow path being in fluid communication with the compression zone to receive a flow of molten polymeric material; and A gate, the gate being in fluid communication with the polymeric material flow path, the gate including an outlet leading to the mold cavity, the gate being positioned such that the flow of molten polymeric material exits the gate at a location tangential to the inner curved surface of the mold cavity, wherein the arc defined by the inner curved surface follows a circle and defines a first end and a second end, and wherein the outlet of the gate forms an angle with the arc defined by two radii extending from the center of the circle to the first end of the arc and from the center of the circle to the point where the polymeric material flow is tangent to the arc, the angle being greater than 30° and less than 90°; and wherein the angle formed between the two radii of the circle extending from the center of the circle to the first end of the arc and from the center of the circle to the second end of the arc has an angle greater than 50° and less than 300°; The gate has a sector shape, the gate including a width and a height, and wherein the width at the outlet of the gate is at least three times the height.

15. The system according to claim 14, wherein, The angle defined by the two radii of the circle extending from the center of the circle to the first end of the arc and from the center of the circle to the point where the polymeric material flow is tangent to the arc at the outlet of the gate is 35° to 65°.

16. The system according to claim 15, wherein, The angle defined by the two radii of the circle extending from the center of the circle to the first end of the arc and from the center of the circle to the point where the polymeric material flow is tangent to the arc at the outlet of the gate is 40° to 50°.

17. The system according to claim 14, wherein, The angle formed between the two radii of the circle extending from the center of the circle to the first end of the arc and from the center of the circle to the second end of the arc has an angle greater than 70° and less than 300°.

18. The system according to claim 17, wherein, The angle formed between the two radii of the circle extending from the center of the circle to the first end of the arc and from the center of the circle to the second end of the arc has an angle greater than 80° and less than 300°.

19. The system according to claim 18, wherein, The angle formed between the two radii of the circle extending from the center of the circle to the first end of the arc and from the center of the circle to the second end of the arc has an angle greater than 90° and less than 300°.

20. The system according to claim 19, wherein, The angle formed between the two radii of the circle extending from the center of the circle to the first end of the arc and from the center of the circle to the second end of the arc has an angle greater than 100° and less than 300°.

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