Plastic article or earthenware shaping or treating: apparatus – Female mold and charger to supply fluent stock under... – With means to heat or cool
Reexamination Certificate
2000-09-14
2003-08-12
Heitbrink, Tim (Department: 1722)
Plastic article or earthenware shaping or treating: apparatus
Female mold and charger to supply fluent stock under...
With means to heat or cool
C264S328900, C264S328150
Reexamination Certificate
active
06604933
ABSTRACT:
TECHNICAL FIELD
This invention relates in general to thermoplastic injection molding nozzle tips used on injection molding machines. In particular, the present invention is directed to a thermoplastic injection molding nozzle tip having predetermined temperature manipulation characteristics to effect clean cut-off of molten material.
BACKGROUND ART
An injection nozzle tip on an injection molding machine is in contact with a metallic mold. The mold is cooled to a relatively low temperature for the formation of a desired object from the molten material sent through the injection nozzle. As a result, the injection nozzle tip is generally cooled by the mold even though the injection nozzle tip is independently heated.
A number of problems arise from this arrangement. In particular the end of the injection nozzle tip which is in contact with the mold is cooled by the heat sink action of the mold so that this part of the injection nozzle tips tends to be lowered to a temperature much less than that of the molten material handled by the nozzle tip. Compensation for this situation is provided by a heating coil at the opposite end of the injection nozzle tip. As a result, much of the nozzle tip becomes overheated to compensate for the heat sink effects of the mold. This leads to a very uneven distribution of heat throughout the injection nozzle.
Because of massive temperature differences between the two ends of the injection nozzle, control of the temperature of the nozzle tip and the material passing therethrough becomes extremely unreliable. For example, if the nozzle tip front end is set to an appropriate temperature, thereby compensating for the heat sink operation of the mold, the other end of the injection nozzle tip must be set to such a high temperature that the molten material being handled is often burned or otherwise degraded.
Also, when overheating occurs an uncontrolled flow of resin is pushed through the injection nozzle tip, leading to undesired phenomenons such as “stringing” or “drooling”. If, on the other hand, the nozzle tip is insufficiently heated so that the portion of the injection nozzle tip against the external mold is relatively cool, clogging of the nozzle will occur, along with the formation of large plugs.
Even minor clogging of the nozzle tip greatly increases the amount of pressure necessary to push the molten material into the mold. Insufficient pressure will cause uneven or incomplete filling of the mold by the molten material, thereby degrading the resulting product.
A number of developments have occurred in the evolution of injection molding nozzle tips, resulting in the present conventional designs. The three most common nozzle tip designs are: (a) general purpose nozzles used for both crystalizing and amorphous materials; (b) ABS reverse taper nozzles which are dedicated for use with amorphous materials; and, (c) nylon taper nozzle tips which are directed to controlling drool of free-flowing molten materials such a nylon. All three of these conventional designs are illustrated in the materials included with the Information Disclosure Statement accompanying this application. All of these designs, however improved over the years, have limitations as to their specific uses.
A general purpose nozzle tip is relatively effective for the basic conveyance of both amorphous and semi-crystalized resins. However, there is some difficulty with controlling strings, which result from materials being drawn out from the nozzle tip after the mold has been filled and the flow of resin cut off. This occurs because the mold acts as a heat sink which pulls heat from the nozzle tip at a moderately consistent rate, and due to the polymer behavior during cooling.
The tip opening area of the nozzle tip gives up the most heat due to the physical contact between the nozzle tip and the mold through the process of conduction. As a result, the plastic forms within the nozzle tip due to the action of this heat sink action of the mold. Very often, the resin cools to an unstable glass transition state which is very difficult to control if the mold sprue (at the opening of the mold) must break cleanly and be free of strings. Because the resin at the sprue opening of the mold must freeze for removal from the mold, there is a tendency of the resin to freeze at the interface between the mold and the nozzle tip. Such freezing also occurs immediately within the nozzle tip. However, beyond the sprue opening of the mold, the plastic must stay molten in order to completely fill the mold, and to be at the proper process temperature for the next molding cycle.
Strings occur when molten plastic is drawn out from the nozzle tip by an attachment to the frozen plastic at the sprue opening. The existence of strings is caused when there is not a clean freeze point or &Dgr;t transition at which the molten resin is cleanly separated from the frozen resin. The strings can contaminate both the mold and the injection nozzle tip itself. In particular, strings can be drawn into the open molds, degrading the product. Further, strings can contaminate the overall processing area causing a variety of different problems. Residue of strings, like frozen slugs or drool remaining within the injection nozzle tip can alter the thermal characteristics as well as the flow characteristics of that nozzle tip, thereby altering the overall performance of the nozzle tip and degrading the resulting product formed within that mold.
Various techniques are used to compensate for string formation. In one example a much smaller injection nozzle tip orifice (interfacing with the sprue opening of the mold) is used than would be recommended for a particular size of mold sprue orifice in order to reduce the contaminating effects of strings. Unfortunately, this technique causes high pressure loss. As a result, there may not be sufficient pressure to properly fill and pack the mold with the resin. Consequently, the product quality and uniformity will be substantially degraded. Further, the reduction in orifice size of the nozzle tip leads to additional undesirable freeze-off in the nozzle tip. The result is a frozen residue within the nozzle tip that compromises the nozzle tip's performance for the next injection cycle. Also, the required pressure (to fill the mold properly) is also increased.
One expedient to control the occurrences of freeze-off and strings has been use of a cardboard buffer between the injection nozzle tip and the mold. This technique has been used as a way of moving or otherwise controlling the &Dgr;t point to allow a proper break between the molten resin and the frozen resin. Unfortunately, this is usually a temporary expedient, and an extremely inefficient way to use standard injection nozzle tips. Also, the cardboard rapidly degrades, causing variations between “shots” of resin injection. Other expedients, such as the use of ceramic buffers between the nozzle tip and the mold sprue opening have also proved ineffective and quite inefficient.
Accordingly, even using all of the ingenuity available in modem molding processes, the conventional general purpose nozzle tip is a device in which resin flow is difficult to control, and the overall molding process using such nozzle tips ultimately becomes very inefficient.
The ABS tip was developed specifically to control stringing, an inherent property of ABS (amorphous) resins. Unfortunately, conventional models of such nozzle tips provide only marginal improvement, and can provide only an inconsistent &Dgr;t or break point for the mold-nozzle tip interface. This performance results in long chunks of frozen material being pulled inconsistently from the nozzle tip, or strings. Since ABS tips are designed for amorphous resins, they do not work particularly well with crystalline resins due to the enormous strings that are inherent with this type of nozzle tip design. Attempts have been made to control the strings through the use of lowering the temperature. However, this technique also causes increased plastic pressure drops which effect the finished molded product.
Heitbrink Tim
Lev Robert G.
LandOfFree
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