Patent Application
20060125145
The present subject matter relates to plastics molding, and more specifically concerns accommodating a range of features in rotational molds and molded objects.
Rotational molding involves heating a plastic resin in a hollow mold while rotating the mold slowly in two axes, so as to melt and distribute the resin over the inside of the mold by gravity alone. Rotational molding can fashion large, sturdy objects beyond the abilities of spin molding or blow molding, which employ centrifugal force or pressure to distribute resin over the mold walls. More complex shapes are possible. Articles produced by rotational molding also possess uniform wall thicknesses and other desirable properties.
For these reasons, rotational molding is often employed in manufacturing large containers, such as storage and fuel tanks. As one example, fuel tanks for industrial and agricultural vehicles commonly have complex shapes to maximize capacity in oddly shaped available spaces. As manufacturers seek to reduce costs by converting from metal to plastic tanks, rotational molding offers sufficient durability for this purpose.
A typical rotational mold has a cast or fabricated aluminum, stainless steel, or similar body having a number of pieces that fit together along one or more parting lines. A framework, typically of metal tubing, seals the body pieces together and mounts the mold to a rotational molding machine. After the resin has cured, the body pieces are parted along one or more lines of draw perpendicular to the planes of the parting lines.
Some molded objects may require features to be molded outside or against a line of draw when demolding the finished object. Furthermore, some molded objects may have multiple versions that differ only in the position or orientation of a feature, so that a single reconfigurable mold would be less expensive than multiple complete molds for the different versions.
Such features have been molded with additional separate mold pieces that are mounted to one or more exterior surfaces of the mold before the mold is heated, and that are removed from the outside prior to parting the main body of the mold to remove the major portion of the cured object. Features of this type may include bosses, fill necks, handles, level-measuring structures, hinges, fillports for loading the mold cavity with resin, and many other shapes and structures.
Some molded objects, however, have features that may make them difficult or even impossible to mold with this process.
A need thus exists for configuring a feature body on a main body of a rotational mold to allow a greater range of feature structures and to allow fabrication of different versions of a rotational mold.
A feature body 220 defines a feature on the surface of the molded object. In this example, the feature is a “doghouse” for attaching an external fuel line to a fuel tank formed by main body 210. This feature may have many shapes and functions, such as bosses, fill necks, handles, level-measuring structures, hinges, and fillports for loading the mold cavity with resin. Features may be raised from the surface of main body 210, depressed into it, or both.
Feature body 220 comprises two feature-body pieces 221 and 222, separated by a feature-body parting line 223. Line 223 may deviate to a considerable extent, but it substantially defines a plane to which a feature-body draw line 224 is generally perpendicular. In addition to the portion that defines the desired feature shape, feature body 220 includes a flange 225 that extends along at least one surface 211 of main body 210. A periphery 226 of flange 225 sealingly engages an aperture 216 in main body 210. Although shown lying entirely within one of the main-body pieces 211, it may lie in any of the pieces, or may extend over more than one piece.
Feature parting line 223 extends across flange 225, so that the flange has separate portions associated respectively with feature-body pieces 221 and 222; stated alternatively, the pieces 221 and 222 have flange portions that meet along parting line 223.
Flange periphery 226—and thus also aperture 216—extend beyond any protrusion of feature body 210 that extends laterally in a direction perpendicular to main-body draw line 214, so that parting main body 210 therealong does not interfere with feature body 220. For example, aperture 216 extends beyond the horizontal projection of bosses 227A, 227B in the side of the feature body.
Feature body 220 is attached to main body 210 such that the main body can be parted before feature body 220 is parted and removed from the finished object.
Besides extending beyond lateral protrusions of the feature body, aperture 216 may permit mounting feature body 220 on main body 210 in multiple different positions or orientations. In the example of a fuel tank, a customer may desire several different orientations of bosses 227A, B, so that a fuel hose may lead off in different directions for different equipment models, or for multiple tanks on different sides of a single engine. In the example of
The present concept also allows a feature to be placed at different locations of a mold body.
After completion of operation 550, block 560 separates at least some of the main-body pieces along parting line 213, in the general direction of draw line 214. Parting may involve removing a frame or other operations. This releases a portion of the molded object from at least one of the main-body mold pieces such as 211-212, but leaves feature body 220 still attached to the molded object. After the object has been released from the main body at 561, operation 570 releases the object from the feature body by parting the pieces such as 221-222 along parting line 223. Pulling the pieces apart substantially in the direction of draw line 224 may accomplish this goal. Draw line 224 is usually not parallel with main-body draw line 214; in the examples illustrated herein, these draw lines are substantially perpendicular to each other. Because the planes of parting lines are perpendicular to their respective draw lines, corresponding relations hold with respect to the parting planes also. Detaching or disassembling part of a frame or subframe may be involved in this operation.
The examples in
As mentioned previously, a main body may include more than one feature body. In this event, operations 510-520 and 560-570 may be repeated, either sequentially or simultaneously, as indicated by arrows 521 and 571, for any additional feature bodies. Furthermore, a feature body may include a subfeature body, and so on. In this event, operations 510-520 and 560-570 may be iterated, substituting “subfeature body” for “feature body” and “feature body” for main body.” That is, nested features and subfeatures may permit the molding of more complex object shapes.
Different versions of a molded object may be manufactured by repositioning one or more feature (or subfeature bodies) as shown at 580, and then repeating operations 510-570 with the feature body in its new position, to produce a different molded object. A new position may involve a reorientation of one or more feature or subfeature bodies or moving it (or them) to different locations on the main or feature bodies.
The foregoing description and drawing figures illustrate certain aspects and embodiments sufficiently to enable those skilled in the art to practice the invention. Other embodiments may incorporate structural, process, and other changes. Examples merely typify possible variations, and are not limiting. Individual components, structures, and functions are optional unless explicitly required, and operational sequences may vary. Portions and features of some embodiments may be included in, substituted for, or added to those of others. The word “or” herein implies one or more of the listed items, in any combination, and singular forms include plural. The required Abstract is provided only as a search tool, and not for interpretation of the claims. The scope of the invention encompasses the full ambit of the claims and all available equivalents, and each claim represents a separate embodiment unto itself.
