Patent Document

FIELD OF THE INVENTION 
   The invention relates to the field of septic tanks and more particularly to a process of forming septic tanks that allows unique structural and performance characteristics. 
   BACKGROUND OF THE INVENTION 
   There are various types of wastewater disposal systems in use such as municipal water treatment systems. In some situations, typically in rural areas where houses are spaced so far apart that a municipal or shared sewer system would be too expensive to install, people install their own treatment systems. These systems usually employ septic tanks. A septic tank system is simply a tank that is usually buried. The tank might hold 1,000 gallons (4,000 liters) of water. Wastewater flows into the tank at one end and exits the tank at the other end, typically into a leach field. Septic tanks often include an opening on the top surface which is meant to provide access to the tank in order to facilitate cleaning and maintenance of the tank. 
   Septic tanks are typically constructed from concrete, steel, fiberglass or plastic. Plastic septic tanks are typically constructed using rotational molding techniques. In contrast, the present invention uses blow molding, which is a plastic manufacturing process that manufactures hollow parts that can be irregular in shape. Examples include air ducts for cars, tool cases, toys, furniture and bottles/containers. 
   A variety of Federal, state and local regulations impose requirements for septic tank construction. Among other things, these requirements demand a watertight and structurally sound vessel. A monolithically manufactured poly tank using previous construction methods easily meets the watertight requirement. 
   The problems with existing poly tanks usually manifest in the structurally sound category. A typical problem associated with existing poly tanks is collapse of the tank or deformation of vessel walls resulting from the pressure earth of the fill material surrounding the tank. Because most poly tank are manufactured using a corrugated wall profile, which typically runs circumferentially around the tank, the existing poly tanks act like an accordion when subjected to the earth&#39;s pressures, a term know in the trade as “creep”. 
   The process of manufacturing rotationally molded tanks uses centrifugal force to spread poly resin against the mold walls which results in great variation and relatively little control in wall thickness distribution. Additionally, rotational molding is limited in its ability to manufacture parts that have significant sections with solid walls or structural stiffeners because the process cannot force poly resin into deep cavities within the mold. 
   The blow-molding process is a pressure process whereby poly resin is forced under pressure to seek the limits of a mold without respect to gravitational or centrifugal forces. The viscosity of the poly resin is such that this pressure can force the material into comparatively deep cavities integral to the wall structure. Hence, stiffening ribs and flanges can be integrally molded into the vessel walls to provide an increase in structural integrity. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a side elevation view of the septic tank according to the present invention. 
       FIG. 1B  is an end elevation view of the septic tank according to the present invention. 
       FIG. 1C  is a top view of the septic tank according to the present invention. 
       FIG. 1D  is a perspective view of the septic tank according to the present invention. 
       FIG. 2  is a top detail view of an anti-flotation wing of a septic tank according to the present invention. 
       FIG. 3  is a partial sectional view of ribs and flange portions of a septic tank according to the present invention. 
       FIG. 4A  is a partial top view of ribs and gaps in a septic tank according to the present invention. 
       FIG. 4B  is a partial sectional side view of ribs and gaps in a septic tank according to the present invention. 
       FIGS. 5A and 5B  are sectional end views of ribs at different locations on a septic tank according to the present invention. 
       FIG. 6A  is an exploded view showing components of a septic tank riser and cover system. 
       FIG. 6B  is a side view of a cover for a septic tank according to the present invention. 
       FIG. 6C  is a sectional side view of a cover for a septic tank according to the present invention. 
       FIG. 6D  is a perspective view of a cover for a septic tank according to the present invention. 
       FIG. 7  is a side view of a cover for a septic tank according to the present invention. 
       FIG. 8  is an exploded perspective view of a cover for a septic tank according to the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention includes a septic system including a riser system that is used to provide access to the septic tank wherein the septic tank and riser system are manufactured utilizing a blow-molding technology. The utilization of blow-molding technology allows for the easy incorporation of innovative and useful features over a typical septic tank system. 
   The general blow molding manufacturing process is well known. The manufacturing process for the various components of the septic tank system involves the same machine components as an injection molding process for the components of septic tank systems except for two variations. The first variation includes the gathering of the plastic material in an “accumulator head” which is located directly above a mold. The mold&#39;s head is filled with plastic and before the mold closes, a hollow plastic tube, referred to as the parison, is forced out of the head and held in place while the mold closes on it. The plastic in its molten state has a consistency similar to bubble gum. 
   The second variation in the blow molding process versus the injection molding process lies in the clamp, or “platen,” that holds the mold closed during the blow-molding process. Once the mold closes (and in some applications, as described below, before the mold completely closes), the soft parison is pierced with a needle prior to the introduction of air under pressure, typically about 100 psi air. In some cases, rather than using a piercing needle, there is a channel in the mold through which the air is introduced. With the introduction of pressurized air, the part is biased against the mold cavity and takes the shape of the mold. 
   Another advantage of the blow-molding process for the formation of the septic tank system is that it allows for the formation of irregular shaped parts that have a plurality of layers, because it uses a multi-layer co-extrusion process. This allows for the manufacture of a septic tank system wherein the material of each layer can be selected for it&#39;s specific performance qualities. This allows for the cost-effective construction of a watertight and corrosion resistant system. Material for each layer can be selected to provide different mechanical, chemical or aesthetic properties. The material for the exterior layer, for example, can be selected from a plastic material that confers rigid properties, such as a high molecular weight plastic. It can also include UV stabilizers and colorants for marketing purposes. Multi-layer blow molding techniques are more cost-effective than rotational molding, for example, because the use of such additives, which can be expensive, is limited to the layers in which they are necessary and most effective. Similarly, material for other layers can be selected based on its resistance to chemical activity, its food grade characteristics or for its relative cost or availability. One skilled in the art would recognize that there are numerous plastics materials that could be chosen for both the interior and exterior layers of the system. 
   Referring now to  FIG. 1 , there is shown a septic tank  10  that is formed using the blow molding technique. The tank  10  includes inlet and outlet ports  11 . The ports  11  are positioned at various locations about the tank  10  in order to provide for ease on installation. The tank  10  is formed using ribs  12  to provide reinforcement to the overall structure. The tank  10  includes anti-flotation winglets  14  along the outer periphery of the tank  10 . The winglets  14  help to stabilize the tank  10  when the tank  10  is buried in soil. The winglets  14  may be parallel to the soil surface or, more preferably, are inclined off of parallel in relation to the soil surface in order to provide greater stability to the tank  10 . The winglets  14  may include holes  15  ( FIG. 2 ) that can be used to provide an attachment point for securing tank  10  to an anchor (not shown) when a tank  10  is installed in conditions that require additional anti-flotation measures. During shipment of tanks  10 , holes  15  provide a means of connecting a number of tanks  10  together (bundling) and also provide a means of securing tanks  10  to the shipment vehicle. The ability to bundle the tanks allows for ease of loading at the manufacturing site, ease and safety of unloading at the customer&#39;s yard with the standard forklift equipment normally found at a distributors location, and allows for use of commercially standard open deck flat or step-deck trailers (does not require specialized basket or racked trailers). 
   Referring now to  FIG. 3 , the tank  10  also includes flange segments  13  around the periphery of the tank  10 , located between each adjacent pair of ribs  12 . The flange segments  13  are in the same plane as the anti-flotation winglets  14 . The flange segments  13  and winglets  14  form a continuous stiffening structure around the perimeter of the tank  10 , which prevents the present invention from suffering from “creep.” As described in more detail below, the flange segments  13  and winglets  14  are formed from compression molding techniques as part of the molding process of tank  10 . 
   Referring now to  FIG. 4 , the bottom and top surfaces of the septic tank  10  include paths defined on the interior of the tank  10  by the terminal portions of the ribs  12 . The liquid pass-through on the bottom surface of the tank  10  entails a path defined by gaps  18  formed between the interior terminal portions of the ribs  12 . The liquid pass-through allows for the liquids that are present in the tank  10  to have a free flow path unobstructed by the solids or sludge layers that may form in the septic tank during use, or by the ribs  12 . The tank  10  also includes a gas pass-through in the top surface of the tank  10 , which is shaped in the same fashion as the liquid pass-through and similarly allow gas to pass from one end of the tank  10  to the other end in an unobstructed fashion. 
   The gaps  18  are formed in the full height of the ribs  12 . This is possible in the present invention because the flange segments  13  and circumferential profile (described below) of the tank provide sufficient structural rigidity that it is not necessary to maintain any portion of ribs  12  through the gaps  18 . It is also possible because the gaps  18  in adjacent ribs  12  are offset from each other. 
   The liquid pass-through and gas pass-through are serpentine because the gaps  18  in each adjacent rib  12  are offset from each other. The gaps  18  are offset to provide additional structural strength to the tank  10 . A series of aligned gaps  18  would provide similar gas and liquid pass-through capability, but would create a line of structural weakness in tank  10 . By offsetting the gaps  18  to form serpentine paths for liquid pass-through and gas pass-through, liquid and gas flow is permitted without substantially weakening the structural integrity of the tank  10 . 
   Each rib  12  is approximately 2½ to 3 inches in height. Those skilled in the art will recognize that other dimension are within the scope of this disclosure. The relative height of each rib  12  varies throughout the tank. Specifically, ribs  12  near the ends of tank  10  approximate the ovo-rectangular profile of tank  10  when viewed from either end. Approaching the mid-portion of tank  10 , the ribs  12  become more circular, which enhances their structural strength. Thus tank  10  has a modified hourglass shape.  FIG. 5  depicts for comparison the profile of a rib  12   a  that is located near one end of a tank  10  ( FIG. 5A ) and the profile of a rib  12   b  that is located near the mid point of tank  10  ( FIG. 5B ). 
   The tank  10  includes access openings  16  to allow access for service and maintenance. Referring to  FIG. 6 , the access openings  16  in the preferred embodiment include threads, which interface with either a secure cover  30  or a riser system  40 . It is the blow molding process that allows for the manufacture of parts with much higher precision than in the prior art methods of septic tank system manufacturing. This higher level of precision allows for the manufacture of a threaded opening  16  in the tank  10 . The threaded openings  16  can then be closed and secured with a either a cover  30  or riser system  40 , both of which are manufactured with threaded connections. Covers  30  and riser systems  40  preferably are manufactured using blow molding techniques. The cover  30  is also manufactured to include a gasket  32  that is biased by the mechanism of the mutually engaged threads. The cover  30  also includes an additional advantage of the use of threaded connections in the riser system  40  is the easy retrofit of a riser system  40  into an existing concrete septic system  42 . Cover  30  is hollow, and includes one or more ports  34  that communicate to the interior of the cover  30 , which allows the installer to put insulation, ballast (such as sand or water), or other material into the cover  30 . Cover  30  preferably is formed with standoff pillars  36  to provide additional structural rigidity. 
   In some instances, it is preferable to supply a cover that fits into standard size openings such as a standard  24 ″ corrugated pipe. Typical corrugated pipe does not have a threaded connection. Referring to  FIG. 3 , there is shown a cover  50  that is manufactured with a very rigid flat top surface  52 , a gasket  54 , and a frustoconical lower section  56  that fits into the corrugated pipe  60  ( FIG. 4 ). Because corrugated pipe is typically not manufactured to exacting specifications, the size of the opening in the pipe often varies. In the present invention, the shape of the lower section  56  allows for a slip fit into the pipe  60 . 
   Cover  50  is manufactured using blow molding techniques, which allows for the easy incorporation of additional features into the cover  50 . The cover  50  includes security features  57  that provide the ability to secure the cover  50  to the pipe  60 . The cover  50  can also have ports  59  that communicate to the interior of the cover  50 , which allows the installer to put ballast, such as sand or water, into the cover  50 . Port  59  preferably is located on the bottom surface of cover  50 , but can be located anywhere that provides communication with the interior of cover  50 . The use of the security features  57  and ports  59  provide many benefits to the end user. Cover  50  preferably is formed with standoff pillars  58  to provide additional structural rigidity. 
   The process of manufacturing tanks  10  according to the present invention includes the steps of preparing a parison, placing the parison in a mold, pinching one end of the parison to form a “bottle”, and introducing gas into the bottle to cause it to conform to the shape of the mold. Preferably, the mold for a tank  10  according to the present invention comprises two pieces, each of which includes a mating face. When the parison is placed in the mold, the two pieces are separated to allow insertion of the parison. According to the present invention, the two pieces of the mold initially are only partially closed. That is, a gap sufficient to allow the parison to pass partially between the mating faces of the mold pieces is maintained initially between the mating faces of the pieces of the mold after one end of the parison has been pinched to form a bottle. At this point gas, preferably atmospheric air, is introduced into the bottle, causing it to expand. The natural expansion of the bottle causes it to force molten plastic material into the gap between the mating faces of the two pieces of the mold. At this point the pieces of the mold are closed together to compress the molten plastic between the mating faces of the mold pieces. This compression molds the portion of the bottle that is between the mating faces of the mold pieces. The flange portions  13  and winglets  14  according to the present invention are formed by this compression molding step. In this way, the present invention provides flange portions  13  and winglets  14  that have substantially more structural strength than can be achieved using rotational molding techniques. As gas is injected into the balloon, the portions of the balloon that are not compressed between the mold pieces thin slightly and consistently relative to the portion of the bottle in the area of the mold pieces mating faces. The wall thickness of tanks  10  thus varies in a consistent fashion so that the wall thickness in the area of the flange portions  13  is approximately 2–3 times the average wall thickness of the tank  10 . This provides additional structural rigidity to prevent installed tanks  10  from experiencing creep.

Technology Category: b