As is well known in the art, a typical multi-cavity hot runner injection molding system includes a heated manifold for conveying a pressurized melt stream from an inlet to a plurality of outlets. A heated nozzle communicates with each outlet to deliver the melt to a respective mold cavity through a mold gate. Manifolds have various configurations depending on the number and arrangement of the mold cavities.
Different heating arrangements are known for heating manifolds. A common arrangement is an electrical heating element that is received in a groove in a manifold outer surface, as described in U.S. Pat. No. 4,688,622 to Gellert, which issued Aug. 25, 1987. Other arrangements include cartridge heaters that are cast into the manifold as described in U.S. Pat. No. 4,439,915 to Gellert, which issued Apr. 3, 1984, and plate heaters with cast-in heaters that are secured along the surface of the manifold, as described in U.S. Pat. No. 5,007,821 to Schmidt, which issued Apr. 16, 1991. Manufacture and assembly of each of these heating arrangements requires machining of the manifold, the heater or both, which can be both costly and time consuming.
For certain large molded parts that require melt delivered from large heated manifolds, the melt stream is heated by either multiple smaller heater plates attached to the manifold or heater elements pressed within grooves machined into the manifold surface. Each of these solutions has its benefits and limitations.
Heater plates provide more consistent heat distribution than a heater element in contact with the manifold surface. Further, heater plates may include more than one heater element allowing for redundancy. However, heater plates are typically made by investment casting methods, which does not accommodate the manufacture of larger plates due to warpage and bending that occurs as the plates get longer. Therefore, multiple shorter plates, i.e., plates typically less than 170 mm, are utilized for larger manifold applications, which require more control zones to operate. Further, heater elements of current heater plates are cast within the heater plate and cannot be replaced once they fail, so that the entire heater plate must be replaced upon failure of the heater elements therein.
Alternatively, heater elements that are pressed-in machined grooves on the surface of a manifold may be removed for replacement, although machining such grooves is time consuming and expensive. In addition, redundancy is provided for by machining a corresponding groove in an opposing surface of the manifold and pressing a secondary heater element into the second groove, adding to the time and cost associated with this production method.
Accordingly, what is needed is a manifold heater arrangement that provides the improved heat distribution and redundancy of a heater plate and provides for replacement of failed heater elements and fewer control zones. In addition, a heater plate that may be efficiently constructed, particularly at longer sizes, is desired.