Abstract:
A method of forming a component of a vehicle heating, ventilation, and air-conditioning system from polymeric material includes providing a molding system including at least one mold cavity defining the component. A base resin is introduced to the mold system via an inlet. A chemical foaming agent is blended with the base resin to form a composition. The composition is then further heated and blended under pressure, wherein the chemical foaming agent decomposes within the composition. The composition is introduced to the mold cavity, wherein a reduced minimized pressure of the mold cavity facilitates initiation of a nucleation of the composition, wherein the composition expands to fill the mold cavity.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This patent application claims priority to U.S. Provisional Patent Application No. 62/233,733, filed on Sep. 28, 2015, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method for producing a heating, ventilation, and air conditioning (HVAC) box for a vehicle, and more particularly, to producing a HVAC box for a vehicle using chemical foaming. 
       BACKGROUND 
       [0003]    There is a continuing effort in the automotive industry to reduce vehicle weight in order to improve vehicle efficiency. Particularly, a trend exists to minimize weight of polymeric components of heating, ventilation, and air-conditioning (HVAC) systems through changes that reduce part thickness and densities. 
         [0004]    One current solution for minimizing the weight of polymeric HVAC components is known as physical foaming. Physical foaming involves entraining a compressed gas, such as Nitrogen, into a molten flow of polymeric material to form a homogenous mixture within the barrel of a molding system. The homogenous mixture is then introduced to a molding chamber and pressure is reduced, thereby allowing the homogenous mixture to nucleate, wherein the compressed gas within the mixture expands to form a suspension of bubbles within the polymeric material. 
         [0005]    However, physical foaming processes involve high capital investment, as specialty molding equipment is required to inject the gas into the polymeric material, and to maintain the molten polymeric material in a highly compressed state prior to introduction into the molding chamber. Once the polymeric material is cooled in the mold, inherent stresses may form within the molded component, leading to deformation and failure over the life cycle of the component. 
         [0006]    Another method for forming foamed polymeric HVAC components involves the blending of hollow glass bubbles into a base resin. The hollow glass bubbles serve to displace the base resin, thereby forming hollow cavities within the material to reduce overall density of the material. 
         [0007]    Unlike physical foaming, hollow glass bubble foaming does not require auxiliary equipment to inject a compressed gas. Thus, conventional molding systems may be utilized. However, the addition of hollow glass spheres to the polymeric material increases overall material costs. Additionally, hollow glass sphere-containing resins are not offered by many suppliers, making sourcing of suitable materials more difficult and costly. 
         [0008]    Yet another known method for producing lighter weight HVAC components involves the blending of alternative filler materials and/or reinforcing agents, or to use less filler materials and/or reinforcing agents in the injection molding resins. For example, one common type of base resin used in injection molding is a polypropylene containing approximately 20% talc as a filler material. However, talc has a higher density than polypropylene, thereby increasing the overall weight of the material. Thus, it may be advantageous to reduce the concentration of talc within the base resin in an effort to minimize overall weight. Alternatively, at least a portion of the talc may be substituted with filler materials having a lower density. 
         [0009]    However, the reduction of the concentration of talc may be undesirable for multiple reasons. Initially, the physical properties of the base resin may be negatively affected by removing or substituting the talc. Additionally, base resin blends having less than 20% talc are not commonly manufactured by suppliers, and costs to obtain these alternative base resins may be prohibitively high. 
         [0010]    In addition to the aforementioned shortcomings in the art, part fit-and-finish and dimensional control is difficult to achieve due to increasingly complex part geometries combined with the desire for reduced wall thicknesses. For example, thinner wall sections make it progressively harder to inject molten material into a mold and achieve even pack pressure. There is also a desire in the art to minimize residual stresses created during cooling and re-crystallization of the thermoplastic, and to prevent the anisotropy of fillers and reinforcing agents. 
         [0011]    Accordingly, there exists a need in the art for an improved means of forming polymeric components of a HVAC system, wherein the process utilizes conventional injection molding equipment, minimizes raw material costs, and minimizes inherent stresses. 
       SUMMARY OF THE INVENTION 
       [0012]    In concordance with the instant disclosure, an improved process for forming polymeric components of a HVAC system, wherein the process utilizes conventional injection molding equipment, minimizes raw material costs, and minimizes inherent stress is surprisingly discovered. 
         [0013]    In one embodiment, the foaming means involves the introduction of endothermic chemical foaming agent to an injection molding resin prior to molding. The introduction of chemical foaming agent results in molded articles having a reduced weight, reduced cycle times, reduced pressure and energy consumption, and improved dimensional control, thermal insulation, and noise and vibrational damping compared to those of the prior art. 
         [0014]    A method of forming a component of a vehicle HVAC system from a polymeric material includes providing a molding system including at least one mold cavity, including a die configured to form a component of a vehicle HVAC system. A composition including a base resin and a chemical foaming agent is then provided to the mold cavity, wherein a pressure drop within the mold cavity is configured to initiate a nucleation of the foaming agent within the base resin. Nucleation of the chemical foaming agent forms a plurality of gas bubbles, creating a cellular structure within the composition and causing the composition to expand to fill the mold cavity. 
         [0015]    A system for forming a component of a vehicle HVAC system from a polymeric material includes a mold and an injector. The mold includes a mold cavity having a definition corresponding to a profile of an HVAC component. The injector is in fluid communication with the mold cavity. The system further comprises a composition including a base resin and a chemical foaming agent. The injector is configured to heat the composition to a first temperature configured to initiate a decomposition of the chemical foaming agent, and a pressure of the mold cavity is configured to initiate a nucleation of the chemical foaming agent in the composition. 
         [0016]    A component for a vehicle HVAC system includes at least one thin-walled section formed of a polymeric material. The polymeric material is formed of a composition including a base resin and a chemical foaming agent, wherein the thin-walled section of the component has a cellular core and a solidly formed surface layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The FIGURE is a schematic cross-sectional elevational view of an injection molding system for forming HVAC components according to an embodiment of the instant disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The following detailed description and appended drawings describe and illustrate various embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner. In respect of the methods disclosed, the steps presented are exemplary in nature, and thus, the order of the steps is not necessary or critical. 
         [0019]    As shown in the FIGURE, a molding system  2  for carrying out an embodiment of the disclosure is shown. The molding system  2  includes an injector  4  and a mold  6  in fluid communication with each other, wherein the injector  4  is configured to provide a flow of a composition  8  to the mold  6 . 
         [0020]    The injector  4  includes a barrel  10 , a feed system  12 , and a head  14 . The barrel  10  of the injector  4  includes at least one inlet  16  in fluid communication with the feed system  12 , and an outlet  18  in communication with the head  14 . The barrel  10  further includes a screw  20  rotatably disposed therein and configured to convey the composition  8  from the feed system  12  to the head  14 . 
         [0021]    The feed system  12  of the injector  4  is configured to provide the composition  8  to an interior of the barrel  10  through the inlet  16 . In the illustrated embodiment, the feed system  12  includes a plurality of hoppers  22 ,  24  for containing a supply of various ingredients  26 ,  28  of the composition  8 . As shown, the feed system  12  includes a first hopper  22  and a second hopper  24 , wherein the first hopper  22  contains a volume of a first ingredient  26  and the second hopper  24  contains a volume of a second ingredient  28 . As shown, the first hopper  22  and the second hopper  24  converge in a single mixing chamber  30  configured to blend the first ingredient  26  and the second ingredient  28  in a predetermined proportion to form the composition  8 . As discussed further below, the first ingredient  26  of the composition  8  may be a base resin, and the second ingredient  28  of the composition  8  may be a foaming agent. In alternate embodiments, the feed system  12  may include additional hoppers containing additional ingredients, such as nucleating agents and coloring agents, for example. Alternatively, the feed system  12  may include a single hopper, wherein the composition is mixed prior to provision to the feed system  12 . 
         [0022]    The head  14  of the injector  4  is disposed adjacent the outlet  18  of the barrel  10 , and includes a nozzle  32  configured to convey the composition  8  from the barrel  10  to the mold  6 . The head  14  may further include a shut-off valve  34  disposed therein, and configured to control a flow of the composition  8  into the mold  6 . In one embodiment, the shut-off valve  34  may be a gate-valve system, wherein a plunger  36  is slidingly disposed within the nozzle  32  to selectively control a flow of the composition  8  into the mold  6 . Other types of shut-off valves will be appreciated by those skilled in the art. 
         [0023]    The mold  6  of the molding system  2  is configured to form the composition  8  into one of a plurality of components  38  for a vehicle HVAC system. The mold  6  includes a mold cavity  40  defined by a pair of dies  42 , wherein each of the dies  42  is coupled to a respective platen  44 . As shown, a first one of the platens  44  may be stationary, while a second one of the platens  44  may be moveable between an open position and a closed position to selectively enclose the mold cavity  40 . 
         [0024]    In the illustrated embodiment, a profile of the mold cavity  40  corresponds to a profile of a portion of a housing for a HVAC system. Particularly, the mold cavity includes a series of thin-walled legs corresponding to at least a first sidewall of the housing and a second sidewall of the housing. However, in alternate embodiments, the mold cavity  40  may define flow-control doors, vent panels and grills, actuating hardware, conduits, and other components commonly utilized in the assembly of vehicle HVAC systems. 
         [0025]    The feed system  12 , the barrel  10 , and the dies  42  may each include at least one temperature control unit  46  for maintaining the composition  8  at a predetermined temperature. For example, heating temperature control units  46  may be included in at least one of the hoppers  22 ,  24  and/or the mixing chamber  30 , wherein a temperature of the ingredients  26 ,  28  of the composition  8  is elevated above a melting temperature of the base resin to facilitate blending of the ingredients  26 ,  28 . As shown, the heating temperature control units  46  of the injector  4  are heater bands at least partially circumscribing the barrel  10  of the injector  4 . However, in alternate embodiments, the temperature control units  46  of the injector  4  may include both heating and cooling capabilities. 
         [0026]    Additionally, at least one of the dies  42  of the mold  6  may include both heating temperature control units  46  and cooling temperature control units  46 , wherein the heating temperature control units  46  are used to control decomposition of a chemical foaming agent  28 , as described below, and the cooling temperature control units  46  are used solidify the base resin  26  and to further cool the HVAC component  38  after nucleation is complete, thereby expediting removal of the molded HVAC component  38  from the mold cavity  40 . In the illustrated embodiment, the temperature control units  46  of the mold  6  comprise a plurality of conduits formed integrally with the dies  42  of the mold  6 , wherein a heat transfer fluid is provided from an external source (not shown) to control a temperature of the mold cavity  40 . In one embodiment, a single circuit of conduits is formed in the mold  6 , wherein a single heat transfer fluid is used for heating and cooling of the dies  42 . In alternate embodiments, a first circuit of conduits may be used for a cooling heat transfer fluid and a second circuit of conduits may be used for a heating heat transfer fluid. 
         [0027]    The base resin  26  may be a pelletized or a fluid form of an organic thermoplastic such as polyethylene; ethylene-vinylacetate copolymer; ethylene-ethyleneacrylate; ionomeric polyethylene; polypropylene; polybutene; polymethylpentene; polystyrene; impact-resistant polystyrene; styrene-acrylonitrile copolymer; acrylic-butadienestyrene copolymer; acrylonitrile styrene acrylate; polyvinylcarbazole; polyoxymethylene; polyester; polyamide; polyvinyl chloride; polytrifluoroethylene; polytetrafluoroethylene-perfluoropropylene; polyvinylidene fluoride; ethylene-tetrafluoroethylene copolymer; polymethylmethacrylate; chlorinated polyether; phenoxy resin; polyphenylene oxide; polysulphone; polyethersulphone; polyphenylenesulphide; polyurethane elastomer; cellulose acetate; cellulose propionate; cellulose-acetobutyrate, or a combination thereof. Other thermoplastics or elastomers will be appreciated by those of ordinary skill in the art. 
         [0028]    A passive nucleating agent may also be blended with the base resin  26  to provide a starting point from which gas bubbles begin to grow during formation of foam cells. In one embodiment, the passive nucleating agent is a solid material blended with the base resin. For example, the base resin  26  may include about 20% talc blended therewith. In alternate embodiments an active nucleating agent, such as the chemical foaming agent  28 , may actively serve as the nucleating agent, thereby minimizing or eliminating the need for solid nucleating agents. Using the chemical foaming agent  28  has been discovered to be more efficient, and capable of providing a smaller and more uniform cellular structure than the use of solid nucleating agents. 
         [0029]    The chemical foaming agent  28 , also referred to as a blowing agent, is blended with the base resin  26 . The chemical foaming agent  28  may be provided as an additive to the base resin  26  in powder form, wherein the chemical foaming agent  28  is contained within the second hopper  24 , and blended with the base resin  26  in the mixing chamber  30  of the feed system  12  immediately prior to introduction into the inlet  16 . The chemical foaming agent  28  may be mixed with the base resin  26  using a passive mixing means such as a gravity feed, or an active mixing means such as a screw, for example. Alternately, the base resin  26  may be provided as a master batch in a granular form, wherein the chemical foaming agent  28  is pre-blended with the base resin  26  in a desired proportion. In yet another embodiment, an operator may blend the base resin  26  and the chemical foaming agent  28  prior to provision of the composition  8  to the first hopper  22 . 
         [0030]    The chemical foaming agent  28  is configured to produce a cellular structure within the composition  8  by decomposing within the base resin  26  at a predetermined processing temperature and pressure. The decomposition of the chemical foaming agent  28  brings about the development of a blowing gas within the composition  8 . In one example, the decomposition of the chemical foaming agent  28  may bring about the development of a CO 2  gas. The decomposition of the chemical foaming agent  28 , and subsequent formation of gas bubbles within the composition  8  is often referred to as nucleation. 
         [0031]    The chemical foaming agent  28  may be an endothermic chemical foaming agent. The endothermic chemical foaming agent requires an input of energy to initiate and maintain decomposition. Examples of the endothermic chemical foaming agent include sodium bicarbonate and citric acid. In a particular embodiment, the endothermic chemical foaming agent is based on monoesters and diesters of citric acid. Particularly, it has been surprisingly discovered that a chemical foaming agent  28  formed of a monoester or diester of citric acid having up to 8 carbon atoms performs particularly well in the formation of thin-walled HVAC components. Those of ordinary skill in the art will appreciate that other endothermic chemical foaming agents may also be utilized. 
         [0032]    Alternately, the chemical foaming agent  28  may be an exothermic chemical foaming agent. In contrast to the endothermic blowing agent, the exothermic chemical foaming agent requires an input of energy to initiate decomposition, but releases energy once decomposition has started. In exothermic reactions, decomposition continues spontaneously until all of the chemical foaming agent  28  is consumed. Examples of the exothermic chemical foaming agent include hydrazines and azo or diazo compounds. 
         [0033]    The use of the endothermic chemical foaming agent in the manufacture of HVAC components provides several advantages over the use of a physical blowing agent and the exothermic chemical foaming agent. By requiring a continuous input of energy to maintain the decomposition process, the reaction rate can be controlled and reaction products can be retained in solution until nucleation can be initiated via the reduced pressures and temperatures present in the mold cavity  40 , thereby allowing a density and a volume of the composition  8  to be precisely controlled. Nucleation using the endothermic chemical foaming agent  28  also has the advantageous effect of consuming energy from the mold  6  during nucleation, which allows a temperature of the mold cavity  40  to be minimized. The minimized temperature of the mold cavity  40  is advantageous, as it allows the HVAC component  38  formed within the mold cavity  40  to be removed from the mold cavity  40  more quickly, thereby minimizing process times. The minimized temperature of the mold cavity  40  also provides the benefit of allowing outer surfaces of the HVAC component  38  to be rapidly cooled upon introduction to the mold cavity  40 , thereby minimizing surface nucleation to allow formation of a smooth outer “skin” on the part. A smooth outer skin is particularly beneficial in HVAC components  38 , as it eases manufacturing and assembly of individual HVAC components  38  by maximizing dimensional control, providing better aesthetic appearance, providing greater physical property retention, and maximizing aerodynamic performance of individual components  38  by minimizing surface drag. 
         [0034]    Within the feed system  12 , the composition  8  is maintained at a first temperature range. The first temperature range is below a melting point of the base resin  26  and a decomposition temperature of the chemical foaming agent  28 , wherein the base resin  26  remains in a solid form. The composition  8  is then conveyed from the feed system  12  and into the barrel  10  under the action of gravity. Within the barrel  10 , energy is input into the composition  8  to transition the base resin  26  from a solid form to a molten form, and to initiate decomposition of the chemical foaming agent  28  within the composition  8 . Energy may be input to the composition  8  by at least one of the temperature control units  46 . Energy may also be input to the composition  8  by the screw  20  in the form of shear and pressure forces. Particularly, a temperature of the composition  8  within the barrel may be maintained at a temperature between 150° C. and 300° C. Optimal temperature ranges will depend on a type of base resin  26  and chemical foaming agent  28  included in the composition, wherein a selected temperature will be sufficient to initiate decomposition of the chemical foaming agent  28  at a desired rate, while maintaining the base resin  26  in a suitable physical state. 
         [0035]    As the chemical foaming agent  28  decomposes, the composition  8  is maintained under pressure within the barrel  10  by the screw  20 . Accordingly, the blowing gas formed by the decomposed chemical foaming agent  28  within the composition  8  is maintained under pressure and remains entrained within the composition  8 , thereby minimizing nucleation. 
         [0036]    The composition  8  is then introduced into the mold cavity  40  through the nozzle  32  of the injector  4 . A predetermined amount of the composition  8  is fed into the mold cavity  40  based on several factors including: final part volume and wall thicknesses, chemical foaming agent type, and chemical foaming agent concentration. The predetermined amount of the composition  8  may be an amount sufficient to partially fill the mold cavity  40 , thereby allowing space in the mold cavity  40  for expansion of the composition  8 . Introduction of the composition  8  into the mold cavity  40  may be metered in several ways. For example, a speed of the screw  20  may be controlled to effect a volumetric flow rate of the composition  8  into the mold cavity  40 . Alternately, the shut-off valve  34  may be relied upon to selectively control the volumetric flow rate of the composition  8  into the mold cavity  40 . 
         [0037]    Upon introduction of the composition  8  into the mold cavity  40 , the reduced pressures within the mold cavity  40  allow the blowing gas to begin nucleation, wherein a suspension of gas bubbles is allowed to form and grow within the composition  8 , thereby forming a cellular structure within the composition  8 . Nucleation is controlled by a combination of a temperature of the mold cavity  40 , a pressure of the mold cavity  40 , and a thickness of a wall of the part, among other factors. The temperature of the mold cavity  40  may be maintained at an elevated state sufficient to sustain decomposition of the chemical foaming agent  28  within the composition  8 , as desired. 
         [0038]    The use of chemical foaming agents in the manufacture of HVAC components offers several benefits over the prior art. For example, the use of chemical foaming agents provides a foam material having superior noise and vibrational damping and thermal insulation compared to HVAC components formed according to the prior art. By using the disclosed method of forming HVAC components, a weight of the component and energy consumption during formation of the component are minimized while a solid surface layer and cellular core are maintained. 
         [0039]    The use of chemical foaming may also provide manufacturing benefits, such as allowing the foaming process to be implemented without the need for specialized injection molding equipment or increased raw material costs. Additionally, when an endothermic chemical foaming agent is used, process times are minimized by maintaining a relatively cool mold cavity  40  compared to physical foaming and exothermic chemical foaming. HVAC components formed using the disclosed method also exhibit improved dimensional accuracy through reduced differential shrinkage, increased speed of manufacture, and an ability to fill a mold cavity  40  quicker and with reduced resistance to material flow compared to physical foaming and hollow glass bubble foaming. Reduced shrink and therefore better contact with the mold surface will further add efficiency to the cooling. 
         [0040]    The use of the disclosed method minimizes molding cycle times by minimizing the temperature of the mold through use of an endothermic chemical foaming agent. Additionally, the disclosed method minimizes energy consumption of the mold system  2 , as a viscosity of the composition  8  may be minimized by the inclusion of the chemical foaming agent  28 . Furthermore, the disclosed method may provide lower press clamp tonnage, improve dimensional control, and increase a flexural modulus of the material with minimal loss of strength or smooth surface appearance. The use of the disclosed method also provides improved HVAC component performance such as improved noise and vibration damping, and improved thermal insulation over the prior art, for example. 
         [0041]    From the foregoing description, one ordinarily skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications to the invention to adapt it to various usages and conditions.