The present invention is directed towards a molded plastic tank having an improved drain opening with a cast-in seat to provide a leak-proof seal with a drain fitting.
Plastic tanks of all sizes are used in various industries for storage and containment of liquids. Such tanks typically are manufactured using a rotational molding process with medium or high density polyethylene. Tanks often range in size from 20-10,000 gallons, although other size tanks may also be molded. A drain opening is normally provided near the bottom of the tank for drainage of liquid via a fitting mounted in the drain opening. A hose and/or valve may be connected to the fitting for drainage of liquid from the tank, as needed.
These conventional molded plastic tanks have walls normally thicker at the bottom of the tank to handle the increased stresses from the liquid. The inner and outer wall surfaces generally are not parallel, due to the limited control of wall thickness during the molding process. For example, the wall thickness may vary 10-20% over a relatively short distance of a few inches. When a conventional bulk head style or compression fitting is installed in the drain opening, the varying wall thickness and non-parallel nature of the inside and outside wall surfaces makes it difficult to provide a good seal, such that leakage is not uncommon. Ideally, there should be no more than 0.50 inch variation on the wall surfaces for proper sealing. The molded wall around the drain opening is normally outside this tolerance. Therefore, the inside wall surface must be manually ground or shaved so as to be substantially parallel with the outside wall surface. This grinding process is slow and tedious, requiring multiple measurements with calipers until a precise surface is provided, which may take 1.5 hours or more per drain opening. Thicker sealing gaskets are sometimes used to overcome defects and variations in the wall thickness, but are less than ideal, with leakage of liquid from the tank still a risk.
Accordingly, a primary objective of the present invention is the provision of an improved molded plastic tank having a cast-in fitting seat to provide a leak-proof seal for a drain fitting.
Another objective of the present invention is a method of manufacturing a plastic tank with a cast-in fitting seat.
Another objective of the present invention is the provision of a molded tank having a drain opening with parallel inner and outer wall surfaces surrounding the opening for installation of a leak-proof fitting.
A further objective of the present invention is the provision of a molding process for plastic tanks using a mandrel for forming a stepped seat around a drain opening in the tank wall.
Yet another objective of the present invention is the provision of a plastic tank molding process which minimizes the manufacturing time for the tank.
Still another objective of the present invention is the provision of an improved molded plastic tank which is economical to manufacture, and safe and durable in use.
These and other objectives will become apparent from the following description of the invention.
The molded plastic tank of the present invention has a cast-in seat formed during the molding process and extending around the drain opening for receiving a leak-proof fitting. The inner and outer walls surrounding the drain opening are substantially parallel. A stepped seat is formed on the inner surface of the wall around the drain opening during the molding process through the use of a mandrel. The mandrel has a non-stick surface for easy removal upon completion of the molding process. When the mold process is complete, the tank is ready for installation of a drain fitting without further processing or delay.
The tank 10 may take various sizes and shapes without departing from the scope of the present invention.
The improved tank 10 of the present invention includes a cast-in seat 40 surrounding the drain opening 19, as shown in
The seat 40 is formed during the rotational molding process using a mandrel 50. The mandrel 50 is shown in
The invention has been shown and described above with the preferred embodiments, and it is understood that many modifications, substitutions, and additions may be made which are within the intended spirit and scope of the invention. From the foregoing, it can be seen that the present invention accomplishes at least all of its stated objectives, and overcomes the problems in the prior art.