Rotational molding oven

Details Rotational molding oven Rotational molding oven

2000-04-29

2002-01-15

Nguyen, Nam (Department: 1722)

Plastic article or earthenware shaping or treating: apparatus

Female mold including tamping means or means utilizing mold...

Including means imparting diverse motion

C264S310000, C425S435000

Reexamination Certificate

active

06338623

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to ovens and, more specifically, to a rotational molding oven able to evenly heat and cool a mold filled with resin materials while rotating the mold to manufacture primarily hollow or partial shell objects wherein the resin is evenly distributed throughout the object.
2. Description of the Prior Art
Numerous methods and apparatuses for forming an object have been disclosed in the prior art. On such example is rotational molding. Rotational molding is a method of manufacture for primarily hollow or partial shell shaped plastic objects. This process utilizes a shell mold having a cavity bounded on five sides by the mold. The sixth side of the cavity is formed by a cover attached by clamps or bolts to one of the adjacent sides. When the cover to the mold is open, a powdered (possibly colored) plastic resin is placed into the cavity of the mold. The cover is then sealed to restrict access to the cavity and the mold is placed in a heated environment in which it is rotated about two axes. The heat causes the resin to melt against the heated inside surface of the mold. The melted resin flows within the cavity to form a viscous membrane conforming to the mold's inner surface. The mold (and the plastic inside) is then cooled while rotation continues causing the resin to harden in the shape of the cavity filled thereby.
When the hardened resin is cool enough to handle (normally below 150° F.) the rotation is stopped and the mold is opened. The hardened resin forming a desired part is removed. The part is then trimmed and cut to the form the desired final contour(s) for the part. These parts are generally of uniform wall thickness, colored throughout, and unstressed, i.e. the parts will not deform if subject to cyclical heat or heating/cooling.
The quality of the part produced is dependent on both the heating cycle and the cooling cycle. The heating cycle is divided into four stages. The initial stage is the warm up of the oven interior and mold support structure to a process temperature for the mold shell. This stage is an unproductive time and may contribute to uneven heating patterns which for the most part are undesirable. Generally, the shorter the warm up cycle, the better and more efficient the process. Excessive heat infusion may cause momentary overheating and/or mold distortion. Therefore, time and temperature control is important. Equally important is the uniformity and rate of convective heat transfer, the higher the gas velocity about the mold, the greater the heat transfer and the more uniform the heating of the mold.
The second stage of the heating cycle is attaining the resin melting temperature within the oven. At this stage, the resin is still predominantly in powder form and heat transfer must be maintained at maximum level. The mold wall temperature must be kept below the rapid oxidation temperature or else discoloration (the first indication of burning) or oxidation will occur. High gas velocities are important during this stage to ensure that no portion of the mold is cooler or hotter than the bulk of the mold and that the initial melt of the resin is uniform.
The third stage starts after the resin begins to melt. At the initiation of the third stage, the majority of the resin is in contact with the mold wall and heat transfer has begun to slow down. The mold wall temperature begins to rise, approaching the oven gas temperature. If left unchecked, the resin in contact with the mold wall may start to discolor. At this stage the oven temperature must be reduced. In some systems this reduction is progressive over the heating cycle. Alternatively, in many ovens in use today, the second stage is ignored and the temperature is held at the third stage temperature limits throughout the process. This prevents burning but also slows the process cycle thereby sacrificing speed of production in order to obtain satisfactory quality for the produced part.
The forth stage is the cool down of the mold. In some ovens the cool down is left to nature, e.g. convective air cooling is applied and the mold cools slowly. Heavier and shielded parts of the mold cool slower than lighter unshielded parts. The large exposed sections of the mold cool quickly. Factoring the irregular cooling during this stage of the overall heating cycle into the manufacturing cycle of a part is an inexact art form. Modern ovens use water spray cooling to quickly cool or de-superheat the mold and mold support structure. Preferably, the spray is in the form of a mist. A spray mist provides an enhanced cooling effect which is more uniform and regulated than convective air heating. Excessive cooling such as water deluge will cause uneven shrinkage of the mold and molded part and may damage some molds. When the plastic part inside of the mold is below the melting/viscous point of the resin, the heating stages are complete.
The cooling cycle occurs over three stages. The initial stage is a continuation of the last stage of the heating cycle. The mold and structural support attachments must be cooled to a point where heat flows out of the mold. Since the resin is a poor conductor of heat, the inner surface of the molded part cools much slower than the surface in contact with the mold. As the cooling continues, the part begins to shrink. Shrinkage will cause some portions of the molded part to detach from the mold wall and these areas will now cool slower than the balance of the molded part. This may cause some degree of distortion. In this stage, excessive rate of cooling causes the part to warp.
The second stage of cooling begins when all of the part has cooled and is released from the mold wall. At this point, the temperature of the part is completely below the viscous temperature. The heat transfer rate is at its slowest at this point due to a lack of part-to-mold contact. The part can now be cooled quickly with little fear of increased deformation. An increased use of water spray is the generally accepted procedure for further cooling the part.
The last stage of cooling occurs after the part has cooled to a point where it could be safely removed from the mold. The part may still be soft and additional cooling may help post-molding operations, otherwise the part is complete and is held waiting for operator attention. This stage is therefore non-critical to the overall process. In interconnected, multiple mold systems, this stage is often required so that other molds can be processed.
Rotation of the mold can include either complete revolutions about two axes or complete revolutions about a single axis with partial revolutions about a second axis. The latter type of rotation is called “rock and roll” as the partial revolutions are similar to a cradle being rocked. In both cases, the two axes of rotation are mutually perpendicular and horizontal rotation about the vertical axis is not required.
The method of heating the mold may be either direct or indirect. Direct heating by an open flame or radiant panels is not considered here. Heating by an open flame is a very old technique characterized by uneven heating, a potential for flame impingement and low energy efficiency. The cost of equipment is very low. Direct heating using radiant panels is still under development and presents limitations for molds having complex shapes and/or curvatures.
Indirect heating is performed in an oven and is currently the preferred method used by most of the industry. Using a direct flame inside the oven is a special case and is subject to the same quality limitations for open direct flame methods mentioned hereinbefore. The indirectly heated oven is discussed hereinbelow.
Another type of system is based on bi-axial rotation. Bi-axial rotational systems, i.e. ovens with mold handling mechanisms having two axes of complete rotation, are the most popular commercially made ovens. These ovens provide the most universal rotational patterns and are well suited for a large portion, but not all, of the marketplace. Commercial systems of this type

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