1. Field of the Invention
The present invention relates generally to an injection apparatus; particularly an injection apparatus maintaining the nozzle and the injection gate at respective desired temperatures.
2. Background of the Invention
It has long been known that the temperature of a melt material is important to successful injection. This is particularly true when the melt material has a high melt temperature. For example, polyethylene terephthalate ("PET") is typically injected above 500.degree. F. A drop in the temperature of the melt material prior to reaching the injection cavity would lower the melt material temperature below that required for proper melt material flow causing less than ideal flow characteristics. These flow characteristics can cause deformed or defectively molded parts; particularly when injecting multilayer parts comprising very thin layers. Therefore, it is desirable to maintain the nozzle temperature at or above the temperature required to assure proper melt material flow as the melt material leaves the nozzle.
It is also known to maintain an injection cavity at a temperature relatively low compared to the temperature of the melt material to facilitate quick cooling of the melt material upon reaching the cavity. The colder the cavity temperature at the time the melt material is injected, the faster the melt material will solidify and allow removal of the solidified part from the cavity. Therefore, a relatively lower cavity temperature will decrease the overall cycle time for injection molding a part. Moreover, it is known that if the injection gate temperature exceeds the desired temperature of the melt material, `stringing` of the melt material will occur in the nozzle and gate area as the injected part is removed from the cavity after injection is complete. These `strings` either break off with the injected part and interfere with further processing of the part (e.g. blowmolding) or stay in the gate or cavity and cause a physical or aesthetic defect in subsequently injected parts.
For these reasons, it has been found desirable to prevent excessive heat transfer from the injection nozzle to the injection cavity. The melt material can thus be maintained at its appropriate temperature in both the nozzle and the cavity. Prior injection apparatuses were often designed to space a nozzle tip from an associated injection cavity during injection to leave a gap therebetween. It was thought that this gap would act as a thermal break between the nozzle and the cavity and allow the nozzle to operate at high temperatures while maintaining a relatively cool cavity. Unfortunately, the thermal break of this configuration could not be maintained at efficient cycle times. During the injection process, melt material would deviate from the injection path and flow into the gap between the nozzle and the cavity. The thermal break thus became a thermal bridge.
Other attempts to insulate an injection nozzle from a cavity have involved the use of nozzle inserts. For example, U.S. Pat. No. 4,279,588 issued to Gellert and entitled "Hot Tip Seal" disclosed a seal (12) located between the nozzle and the injection gate to limit heat transfer therebetween. The seal (12) of Gellert resided substantially within the nozzle and extended outward therefrom to contact the cavity. Similarly, U.S. Pat. No. 4,521,179 issued to Gellert and entitled "Injection Molding Core Ring Gate System" disclosed a nozzle seal (76). The seal (76) of Gellert also resided substantially within the nozzle and extended outward therefrom to contact the cavity.
It has been found that movement of the various parts within an injection apparatus will result from thermal expansion as portions of the apparatus are heated from ambient temperature to the temperature necessary to inject a melt material. Different injection apparatuses accommodate this thermal expansion in different ways. It has been found that the thermal expansion of some injection apparatuses results in movement of the nozzle both along the longitudinal axis thereof and perpendicular to that longitudinal axis. In other words, it has been found that the nozzles of some apparatuses will elongate and shift laterally as the apparatus is heated. Seals that attached to the nozzle, such as those of the Gellert patents discussed above, break or deform due to this lateral nozzle movement. Such seals are therefore inapplicable to apparatuses experiencing this lateral nozzle movement.
It has also been found that many seals cannot withstand the high temperatures and pressures associated with injection; especially when the high temperatures are maintained for long periods of time. Many prior inserts degraded after prolonged exposure to high temperatures resulting in rupture or deformation of the inserts which allowed melt material to leak into the area between the nozzle and the cavity causing in a thermal bridge.
It has also been known to supply a cooling means to a cavity to remove the heat transferred from the nozzle or melt material to the cavity. Cooling ducts circulating coolants such as glycol were typically employed. However, the distance between the part void and the injection gate has heretofore limited the proximity of the cooling ducts to the injection gate.