Abstract:
Gas-assisted injection molding is used to form molded parts having relatively thick cross-sections, such as a support section of a chair back and seat portion of a chair. Gas is injected into a mold cavity using the same nozzle and screw injector that injects molten resin into the mold cavity. The injected gas creates a void in the molten resin that allows a reduced amount of resin to be used to fill a mold cavity without sacrificing the integrity of the molded part.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
       [0001]    The present invention relates generally to a method of forming relatively thick parts using gas-assisted injection molding. 
         [0002]    Injection molding is a process whereby a heated resin or plastic is injected into a mold, typically by either a reciprocating screw or a ram injector. Once in the mold, the heated resin is allowed to cool and thus solidify. Gas-Assisted Injection Molding (GAIM) is an injection molding process whereby gas is injected into to the mold as the heated resin cools. One of the benefits of GAIM is it that allows a reduced amount of resin to be used. More particularly, part of the volume defined by the mold, commonly referred to as the mold cavity, is partially filled with gas that is trapped as the resin cools and solidifies. Smaller and thinner plastic parts are commonly formed using GAIM but it is generally difficult to form a thicker plastic part, such as a part having a thickness in excess of one inch, with a GAIM process. 
         [0003]    Generally, during the injection molding process, the heated resin begins to cool as the resin comes into contact with the cooler interior surfaces of the mold. For thinner parts, it is generally possible to fill the mold with the heated resin before the resin cools and solidifies. However, for thicker parts, the heated resin begins to cool before the mold cavity is filled. The resin however does not cool evenly throughout the mold cavity, which can result in the cooled resin presenting an uneven or contoured surface or base for the heated resin as it is continues to be injected into the mold. This can result in cosmetic defects being formed in the part. In addition, the uneven surface presents issues with injecting the gas properly. 
         [0004]    It is therefore an object of the invention to provide a GAIM process capable of molding parts having a thickness in excess of one inch. 
         [0005]    It is another object of the invention to provide a part, such as molded chair back and seat for a chair, having a thickness greater than one inch and formed using a GAIM process. 
         [0006]    Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The drawings illustrate the best mode presently contemplated of carrying out the invention. 
           [0008]    In the drawings: 
           [0009]      FIG. 1  is a schematic view of a gas-assisted injection molding machine; 
           [0010]      FIG. 2  is a schematic view of an injection screw for use with the machine of  FIG. 1  and having an elongated hole formed in the screw that allows gas to be passed through the screw; 
           [0011]      FIG. 3  is an isometric view of a mold usable with the machine of  FIG. 1  for making a part with a thick section, such as a seat and chair back portion of a chair; 
           [0012]      FIG. 4  is an isometric view of the mold of  FIG. 3  with the mold sections separated from one another showing the mold cavity defined by the mold sections; 
           [0013]      FIG. 5  is a section view of the mold of  FIG. 3  taken along line  5 - 5  of  FIG. 3 ; 
           [0014]      FIG. 6  is a section view of the mold of  FIG. 3  taken along line  6 - 6  of  FIG. 3 ; 
           [0015]      FIG. 7  is an isometric view of a thick section part, such as a chair back and seat portion of a chair, that may be formed using the gas-assisted injection molding machine of  FIG. 1  and the mold of  FIG. 3 ; 
           [0016]      FIG. 8  is a section view of the chair back and seat portion of  FIG. 7  taken along line  8 - 8  of  FIG. 7  showing a thick support section formed using gas-assisted injection molding according to one embodiment of the present invention; and 
           [0017]      FIG. 9  is a flow chart setting forth the steps of an exemplary gas-assisted injection molding process for making the chair back and seat portion of a chair shown in  FIGS. 7 and 8 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    A gas-assisted injection molding machine is shown in  FIG. 1 . The machine  10  includes a reservoir or hopper  12  from which resin or other plastic material is fed to an injection barrel  14 . Generally, the resin is fed into the barrel  14  under the force of gravity, but is contemplated that the resin could be pumped into the barrel  14 . The barrel  14  extends through a heater assembly  16  that heats the resin. The resin is injected through a nozzle  18  by a screw injector  20 , shown in  FIG. 2 , which is reciprocated within the barrel  14  by a hydraulic gear and motor assembly  22 , to which hydraulic fluid is fed by a hydraulic pump  24 . 
         [0019]    The mold  26  includes a pair of mold sections  28 ,  30 . Mold section  28  is held by a stationary platen  32  whereas mold section  30  is held by a movable platen  34 . The platens  32 ,  34  are connected to a pair of tie bars  36 ,  38 . The movable mold section  30  is moved into contact with and away from the stationary mold section  28  by a clamping unit  40  connected to the movable platen  34 . The clamping unit  40  is also connected to the tie bars  36 ,  34 . An ejector  42  ejects the formed part once the molding process is complete. 
         [0020]    The machine  10  further includes a gas supply  44  that supplies gas, under pressure, to the barrel  14  and through the screw injector  20 , which has a threaded body  44  and an elongated hole  46  formed along the body  44 . The elongated hole  46  allows gas to be presented to the nozzle  18  and ultimately the mold  28  from the gas supply  44  though the screw injector  20  during a gas-assisted injection molding process. 
         [0021]    The machine  10  can be used with various types of molds to form different types of injection molded products. One exemplary type of product is the chair back and seat sections of a chair. A mold  48  for molding such chair back and seat sections is shown in  FIGS. 3-6 ; although, it is contemplated that the invention may be used to mold other types of parts, including those having thick sections as well as thin sections. In the orientation shown in the figures, the mold  48  has an upper mold section  50  and a lower mold section  52 . The mold sections  50 ,  52  collectively define a mold cavity  54 , shown in  FIGS. 5 and 6 . In particular, an undersurface  56  of the mold section  50  and the upper surface  58  of mold section  52  cooperate to define the mold cavity  54  when the mold sections  50 ,  52  are clamped together. The mold cavity  54  generally defines a seat section  60  and a chair back section  62 , as shown in  FIG. 5 . The seat section  60  and the chair back section  62  intersect at a recessed section  64 . 
         [0022]    Referring briefly to  FIG. 7 , the mold  48  is used to form the chair back and seat portions of a chair shell  66 , designated by reference numerals  68  and  70 , respectively. The recessed section  64  is used to mold a support portion  72  defined at the backside of the chair  66 . The support portion  72  is integrally formed with the chair back and seat portions  68  and  70 , respectively; yet, is thicker than the back and seat portions. 
         [0023]    Referring back to  FIGS. 3 ,  5  and  6 , the mold  48  includes a port  74  along which the nozzle  18  of the barrel  14  is aligned. The screw injector  20  may be inserted into the port  74  to force molten resin into the mold cavity. In one representative embodiment, the mold  48  includes a sprue  76  that extends from the recessed section  64  of the mold cavity  54  and provides a passage  78  from port  74  to the mold cavity  54 . During the injection molding process, molten resin is forced by the screw injector  20  into the mold cavity through the sprue  76 . Before gas is injected into the mold cavity, the screw injector  20  is translated into abutment with the sprue  76  and with the elongated hole  46  aligned with passage  78 . It is recognized however that gas could be injected with the screw injector  20  spaced from the sprue  76 . 
         [0024]    Referring again to  FIG. 7 , which shows a molded chair back portion  68  and a seat portion  70  of a chair  66 , a molded sprue  80  is formed extending from the backside of support section  72  during the molding process. The molded sprue  80  is formed as a result of molten resin filling the mold sprue  76 , cooling and then solidifying. After the molded chair  66  is ejected from the mold  48  the molded sprue  80  is removed, such as by a cutting device (not shown) and the hole formed in the support portion  72  is plugged using a suitable plug. For instance, a piece of hardened plastic could be sonic welded to the support portion  72  to plug the hole. 
         [0025]    As shown in  FIG. 8 , the support portion  72  generally includes a thickened body  82  extending from a curved surface  84  that seamlessly joins the seat portion  70  and the chair back portion  68  of the chair  66 . As will be explained in greater detail below, after heated resin is injected into the mold  48 , gas is injected into the mold  48 . The gas forces the heated resin outwardly against the mold surfaces  56  and  58  that define the mold cavity  54 . The heated resin cools and solidifies while the gas is being injected resulting in a void  86  being formed in the thickened body  82 . More particularly, the gas forces the heated resin against the inside surfaces of the mold thereby forming walls  88 ,  90 , and  92 , seamlessly connected to one another that collectively define void  86 . As shown in  FIG. 8 , the molded sprue  80  extends from wall  92  that is generally parallel with curved surface  84 . As noted above, when the molded sprue  80  is removed, e.g., cut, a hole  94  is formed in the wall  92   
         [0026]    It will be appreciated that the thickness of the support portion  72  is considerably thicker than the chair back and seat portions  68  and  70 , respectively. In one embodiment, the thickness of the support portion  72  is approximately two inches as defined from wall  92  to curved wall  84 . 
         [0027]    The steps of an exemplary GAIM process  94  are set forth in  FIG. 9 . In setting forth the steps of the process  94 , reference will also be made to the components of the machine  10  shown in  FIGS. 1-2 . After the machine  10  has been properly set up and mold  48  has been loaded into the machine  10 , the process  94  begins with the filling of the barrel  14  at block  96 . The amount of resin loaded into the barrel  94  is a function of the size of the part to be molded. For example, for a chair back and seat, with a thickened support, as described with respect to  FIGS. 7-8 , the barrel is loaded with approximately 12.0 cubic inches of resin from hopper  12 . The amount of resin loaded into the barrel  14  is commonly referred to as the “shot size”. 
         [0028]    Once the barrel  14  is loaded, the resin is heated by heaters  16  at block  98 . In one embodiment, the heaters  16  include five heating elements that are heated approximately to 425, 425, 415, 410, and 400 degrees Fahrenheit, respectively. After the resin is heated, the heated resin is injected into the mold cavity  54  at block  100 . In one embodiment, the resin is injected under a pressure of approximately 800 psi at approximately 1.5 inches per second for a fill time of approximately 12.0 seconds. 
         [0029]    After the resin has been loaded into the mold cavity  54 , a cooling timer is started at block  102 . In one embodiment, the cooling timer is set to approximately 75 seconds to correspond to a resin cooling time of approximately 75 seconds. Gas is then injected into the mold cavity at block  104  following a predefined delay period. In one embodiment, the gas injection is commenced after an approximate 12 second delay and is injected for approximately 40 seconds at a pressure of approximately 750 psi. As described above, the gas forces the molten resin outwardly against the interior mold surfaces  56 ,  58  to form a void  86 . 
         [0030]    After the gas is injected, the resin is allowed to cool, which in one embodiment, corresponds to approximately 11 seconds after cessation of gas injection. After the molded resin has cooled and thus solidified for the predefined cooling period, the molded resin is ejected from the mold  48  at block  108 . The molding process results in a molded sprue  80  being formed which is removed at block  110  resulting in the hole  94  being formed that is filled by sonic welding a plug thereto at block  112 . 
         [0031]    It can thus be appreciated that the sprue  80  is formed when the gas is injected into the mold cavity after the molten resin has been injected into the mold cavity  54 . Thus, the sprue  80  has an open end facing the gas injection nozzle, and a passage that extends from the open end completely through sprue  80  and into communication with the void  86  that is formed in support portion  72 . 
         [0032]    The present invention thus contemplates a gas assist injection molding process which has been found to be suitable for molding significantly thicker parts than has been possible in the prior art. The system and process of the present invention allows gas assist injection molding of a thick part in a central location on the thick part, in which the void area to be occupied by the gas extends outwardly in all directions from the injection location. The sequence of steps carried out in accordance with the present invention provides a part that does not warp or shrink, which has typically been the case when attempting to injection mold thick parts using a gas assist process. 
         [0033]    It is understood that the values set forth above are merely representative and that other values may be used for other types of molded parts. Additionally, the values set forth above may fall within a range of suitable values for gas-assisted injection molding of a given part. 
         [0034]    Various alternatives and embodiments are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter regarded as the invention.