Abstract:
A method for magnetically aligning non-particles within a polymer, involving adding a magnetic nano-particle filler to a plastic material, such as a molten thermoplastic. The magnetic property allows the filler or particles to be aligned through the use of magnetic fields during the molding process. In one embodiment, the nano-particles are synthesized to a specific size, and are made by applying suitable coatings to existing fillers.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an improvement in an injection molding process, which provides for greater predictability of shrinkage in an injection molded part, as well as improves the transfer efficiency of auto parts which make use of conductive paints. 
       BACKGROUND OF THE INVENTION 
       [0002]    Many different types of products are manufactured using an injection molding process where a thermoplastic or thermoset material is injected into a mold to form a desired part. Examples of parts made using an injection molded thermoplastic material include, but are not limited to, fascia, bumpers, door panels, rocker panels, and the like. The thermoplastic material is typically a polymer, and reinforcing fillers or particles are often used to reinforce the polymer matrix, improving physical properties (such as strength) and/or reducing cost. Various methods have been developed to improve the process of designing a molded part, such as using mold flow analysis software which predicts flow and shrink characteristics after the injected material is de-molded and the part is in a free state. Mold flow analysis involves modeling solid, liquid, and gas flow based on individual elements of the molten material being injected into the mold. This is known as finite element analysis. An example of an individual element is a solid particle within the polymer matrix of the polymer. The individual elements combine to comprise the entire part. 
         [0003]    One of the problems common with injection molded parts is that misaligned or randomly oriented reinforcing fillers or particles result in poor predictability of molded plastic part dimensions which adds cost to design and validation efforts. 
         [0004]    Furthermore, injection molded parts commonly experience shrinkage as the molten material cools in the mold and changes into a solid material. One problem resulting from the use of mineral fillers or particles in polymers (plastics) is the occurrence of non-uniform shrinkage in molded plastic parts. Along with other factors, particle aspect ratio (i.e., size) and orientation of the filler or particles in the polymer affect shrinkage in the molded part. In the instance of more complex-shaped parts, the prediction of shrinkage during the tool design phase requires finite element analysis. Using finite element analysis accounts for particle size and orientation within the polymer and provides for a prediction as to how a molded part may shrink to the required final dimensions. 
         [0005]    Because of the complexity of filler or particle alignment during tool-fill or injection of the molten material into the mold, simplifying assumptions with regard to shrinkage are often made to reduce design and development costs. Those assumptions can result in costly tooling adjustments later on in the design validation phase to make the part match to the print. If the assumptions are incorrect, the tool or die may need to be reshaped, increasing production costs. 
         [0006]    Accordingly, there exists a need for an improvement in orienting the fillers or particles during the molding process to provide better control over shrinkage characteristics, thereby reducing tooling adjustments of an injection mold. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention is directed to a method for magnetically aligning nano-particles within a polymer. The present invention involves adding a magnetic nano-particle filler to a plastic material, such as a molten thermoplastic. The magnetic property allows the filler or particles to be aligned through the use of magnetic fields during the molding process. In one embodiment, the nano-particles are synthesized to a specific size. This provides the advantage of the nano-particles being uniform in size. 
         [0008]    In another embodiment, the nano-particles are made by applying suitable coatings to existing fillers. The advantage of using existing mineral fillers would be lower initial cost, but with the disadvantage of size variation. In either case, a magnetic coating is applied to the particles. 
         [0009]    It is an object of the present invention to align filler particles (within the polymer) in a predictable manner to simplify the determination of shrink characteristics. This reduces design time and validation iterations of the mold to achieve a molded plastic part which meets customer expectations. Additionally, for parts which use conductive paints, the present invention improves the transfer efficiency of the conductive paint, reducing or eliminating the need for primer paints. 
         [0010]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0012]      FIG. 1  is a perspective view of a mold and a generator device used for an injection molding process, according to the present invention; 
           [0013]      FIG. 2  is a perspective view of an element of a molten material having a filler particle, used during an injection molding process, according to the present invention; 
           [0014]      FIG. 3  is an enlarged view of a section of a part made using an injection molding process prior to the filler particles being aligned, according to the present invention; and 
           [0015]      FIG. 4  is an enlarged view of a section of a part made using an injection molding process after the filler particles have been aligned, according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
         [0017]    Referring to the Figures generally, a mold used during an injection molding process according to the present invention is shown generally at  10 . Adjacent the mold  10  is a generator device  12  which is capable of creating a magnetic field around the mold  10 . In this embodiment, the generator device  12  is a coil of wires connected to a device for providing a current through the wires, and is a separate component relative to the mold  10 . However, it is within the scope of the invention that the generator device  12  may be integrated with the mold  10  such that the generator device  12  and mold  10  are a single component. In an alternate embodiment, the generator device  12  is a pair of permanent magnets located on each side of the mold; in yet another embodiment, the generator device  12  is an electromagnet located inside the mold. 
         [0018]    The generator device  12  is capable of generating a magnetic field, shown generally at  14 . The magnetic field  14  flows from a north pole, indicated at N, to a south pole, indicated at S. The generator device  12  produces the magnetic field  14  to properly align a plurality of particles, shown generally at  16 . 
         [0019]    Referring to  FIG. 2 , an element  18  of a component made using an injection molding process according to the present invention is shown. The element  18  represents an infinitesimally small part of the component used in a finite element analysis (FEA), but may represent a larger portion of the component, depending upon the type of FEA used. A larger amount of elements  18 , with the elements  18  being smaller, requires a larger amount of time to complete the FEA, but yields a more accurate prediction as to how much the component produced in the mold  12  undergoes shrinkage. A smaller amount of elements  18 , with each element  18  being larger in size, requires a shorter amount of time to complete the FEA, but yields a less accurate prediction as to the amount of shrinkage the component produced in the mold  12  may have upon completion. 
         [0020]    Each element  18  includes a length  20 , a width  22 , and a height  24 . Located within each element  18  is a magnetically interactive particle or filler  26 . The particle  26  has an oval or circular cross-section which includes a height  28  and a width  32 , with the height  28  being less than, equal to, or greater than the width  32 . The length of the particle  26  is substantially the same as the length  20  of the element  18 , and each particle  26  has a first end  34  and a second end  36 . In this embodiment, the element  18  is said to have an aspect ratio which is calculated in one of several ways, depending upon the height  28  and the width  32  of the particle  26 . If the height  28  and the width  32  of the particle are equal, the aspect ratio is the length  20  of the element  18  divided by the width  32  of the particle  26 . 
         [0021]    If the width  32  and the height  28  are not equal, then the aspect ratio is calculated by dividing the length  20  of the element  18  by the average cross-sectional dimension of the width  32  and height  28 . More specifically, the width  32  and the height  28  are added together and divided in half to give the average cross-sectional dimension, and the length  20  is divided by the average cross-sectional dimension. It is within the scope of the invention that various aspect ratios may be used, such as, but not limited to, between 1:1 and 20:1. In this embodiment, the particle  26  is made from wollastonite, a type of calcium inosilicate mineral, but it is within the scope of the invention that other types of materials may be used as the filler material. 
         [0022]    In one embodiment, the particles  26  are of different sizes, and each particle  26  has a magnetic coating  30 . During injection, the molten material, which is made up of the elements  18 , is injected into the mold  10 , and the particles  26  are in a random configuration, best seen in  FIG. 3 . While the molten material is still soft and has not hardened after cooling, the generator device  12  is activated to generate the magnetic field  14  which substantially aligns the particles  26 , best seen in  FIG. 4 . In one embodiment, the first end  34  aligns with the south pole S of the magnetic field  14 , and the second end  36  is aligned with the north pole N of the magnetic field  14 , best shown in  FIG. 4 . Once the particles  26  are aligned, as the molten material in the mold  10  begins to cool and shrink, the molten material shrinks more in the direction of the width  22  and less in the direction of the length  20 . The alignment of the particles  26  provides for better control of the shrinkage of the component after it is finished and removed from the mold  10 . The amount of shrinkage in each direction is controlled by the alignment of the particles  26 . This reduces the amount of adjustments that may need to be made to the mold  10  to compensate for shrinkage, thereby reducing the cost of producing the mold  10 . Furthermore, for parts which are painted with a conductive paint, the present invention improves the transfer efficiency of the conductive paint, reducing or eliminating the need for primer paints. 
         [0023]    In another embodiment, the particles  26  are synthesized to be of the same size, but still have the magnetic coating such that the particles  26  align when exposed to a magnetic field, as with the first embodiment. Furthermore, while the size of the particles  26  may be synthesized to be consistent relative to one another, the particles  26  may be synthesized such that all of the particles  26  are larger or smaller (but are still the same size relative to one another) to change the way the component shrinks in the mold  10  during cooling. 
         [0024]    In yet another embodiment, magnetic nanoparticles are used either alone or in conjunction with the above coated particles to provide shrinkage control. Useful particles are selected from the group of: iron oxide nanoparticles, nickel zinc ferrite nanoparticles, ferrous ferric oxide nanoparticles, ferrite nanoparticles having the formula MFeO 4 , wherein M is a divalent metal, preferably Ni or cobalt; magnetic nanowires including aligned magnetic nanowires; nanoparticles coated with any of these materials, and mixtures thereof. 
         [0025]    Typically, nanoparticles useful in the present invention are less than about one micrometer, and generally from about one to about 2500 nanometers, and preferably from about one to about 100 nanometers. In one embodiment, the particle size ranges from one nanometer to 20,000 nanometers. 
         [0026]    The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the essence of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.