Compact cartridge hot runner nozzle

Details Compact cartridge hot runner nozzle Compact cartridge hot runner nozzle

2000-03-08

2002-05-28

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

C264S328150

Reexamination Certificate

active

06394784

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to injection molding and more particularly to an injection molding nozzle having an integral electrical heating element surrounded by layered dielectric insulation.
BACKGROUND OF THE INVENTION
Heaters for injection molding and hot runner applications are known in the prior art, as demonstrated amply by the following U.S. Pat. Nos. 2,991,423, 2,522,365, 2,769,201, 2,814,070, 2,875,312, 2,987,300, 3,062,940, 3,550,267, 3,849,630, 3,911,251, 4,032,046, 4,403,405, 4,386, 262, 4,557,685, 4,635,851, 4,644,140, 4,652,230, 4,771,164, 4,795,126, 4,837,925, 4,865,535, 4,945,630, and 4,981,431.
Heaters are of course also amply known in non-injection molding applications, as shown for example in U.S. Pat. Nos. 2,088,586, 2,378,530, 2,794,504, 4,438,322 and 4,621,251.
There are in general three types of heaters known for use in the hot runner nozzles. The first is so-called “integral heaters” which are embedded or cast in the nozzle body. Examples of such nozzles are disclosed in the following patents: U.S. Pat. No. 4,238,671, U.S. Pat. No. 4,386,262, U.S. Pat. No. 4,403,405 and EP 765728. The second is so-called “independent external heaters” which have their own support and that can be removed and replaced. Essentially, in such a design, shown in
FIG. 1
a
, the heating element H is external to the nozzle body N. Heating element H comprises a resistance wire W surrounded by electrical insulating material E and is encased in a steel casing C. Examples of such nozzles are disclosed in the following patents: U.S. Pat. No. 3,553,788, U.S. Pat. No. 3,677,682, U.S. Pat. No. 3,831,004, U.S. Pat. No. 3,912,907, U.S. Pat. No. 4,588,367, U.S. Pat. No. 5,360,333, U.S. Pat. No. 5,411,393, U.S. Pat. No. 5,820,900, EP 748678, EP 963829 and EP 444748. The third is so-called “attached external heaters” which are positioned spirally around the exterior of the nozzle or the nozzle tip but cannot be removed therefrom by reason of being brazed or embedded in the nozzle surface. Referring to
FIG. 1
b
, heating element H′ is embedded in a groove G′ in nozzle body N′. Examples of such nozzles are disclosed in the following patents: U.S. Pat. No. 4,557,685, U.S. Pat. No. 4,583,284, U.S. Pat. No. 4,652,230, U.S. Pat. No. 5,226,596, U.S. Pat. No. 5,235,737, U.S. Pat. No. 5,266,023, U.S. Pat. No. 5,282,735, U.S. Pat. No. 5,614,233, U.S. Pat. No. 5,704,113 and U.S. Pat. No. 5,871,786.
Electrical heaters have been also used in the design of the so-called hot runner probes. Unlike the hot runner nozzles, the hot runner probes do not comprise the melt channel. The probes are located inside the melt channel of the nozzle and thus create an annular flow. The melt is heated from the inside and this heating approach is not applicable to all materials and applications. Examples of such nozzles are disclosed in the following U.S. Pat. Nos. 3,800,027 3,970,821, 4,120,086, 4,373,132, 4,304,544, 4,376,244, 4,438,064, 4,492,556, 4,516,927, 4,641,423, 4,643,664, 4,704,516, 4,711,625, 4,740,674, 4,795,126, 4,894,197, 5,055,028, 5,225,211, 5,456,592, 5,527,177 and 5,504,304.
Injection molding nozzles having integral heaters typically have electrical heating elements, wound spirally around the nozzle, which offer an efficient response to the many critical process conditions required by modern injection molding operations. There has been a continuous effort in the prior art, however, to improve the temperature profile, the heating efficiency and durability of such nozzles and achieve an overall reduction in size. Most of these efforts have been aimed at improving the means of heating the nozzle.
For example, U.S. Pat. No. 5,051,086 to Gellert discloses a heater element brazed onto the nozzle housing and then embedded in multiple layers of plasma-sprayed stainless steel and alumina oxide. To avoid cracking of the ceramic layers caused by excessive thickness and the differing thermal properties of the ceramic and the stainless steel, Gellert employs alternating thin layers of stainless steel and alumina oxide. The heating element of Gellert is a nickel-chrome resistance wire (i.e. see W in
FIGS. 1
a
and
1
b
herein) extending centrally through a refractory powder electrical insulating material (i.e. see E in
FIGS. 1
a
and
1
b
), such as magnesium oxide, inside a steel casing (i.e. see C in
FIGS. 1
a
and
1
b
). The heating element is integrally cast in a nickel alloy by a first brazing step in a vacuum furnace, which causes the nickel alloy to flow by capillary action into the spaces around the heater element to metallurgically bond the steel casing of the element to the nozzle body. This bonding produces very efficient and uniform heat transfer from the element to the nozzle body.
Nozzles with this type of electrical heaters, however, are often too big to be used in small pitch gating due to the size of the insulated heater required. These heaters are also generally expensive to make because of complex machining required. Also, the manufacturing methods to make these nozzle heaters are complex and therefore production is time consuming.
U.S. Pat. No. 5,955,120 to Deissler which discloses a hot runner nozzle with high thermal insulation achieved by coating the electrical heater with layers of a thermally insulation materials (mica or ceramic) and high wear resistance material (titanium). Like Gellert, the heater element of Deissler has its own electrical insulation protection and thus can be placed in direct contact with the metallic nozzle body (see
FIG. 2
of Deissler). Also the heater element of Deissler is attached to the nozzle by casting (brazing) a metal such as brass. Deissler is thus similar to Gellert in that it discloses an insulated and brazed heater element. Again, as with Gellert, such a device requires many additional steps to braze and insulate the heater and is therefore time consuming. Also, as with Gellert, the use of an insulated element makes the size of the heated nozzle not well suited for small pitch applications.
In an attempt to reduce nozzle size, U.S. Pat. No. 5,973,296 to Juliano shows a thick film heater applied to the outside surface of an injection nozzle. The nozzle heater comprises a dielectric film layer and a resistive thick film layer applied directly to the exterior cylindrical surface of the nozzle by means of precision thick film printing. The thick film is applied directly to the nozzle body, which increases the nozzle's diameter by only a minimal amount. Flexibility of heat distribution is also obtained through the ability to apply the heater in various patterns and is, thus, less limited than spiral designs.
There are limitations to the thick film heater, however. Thermal expansion of the steel nozzle body during heating can cause unwanted cracking in the film layers due to the lower thermal expansion of the film material. This effect is particularly acute after a large number of injection cycles. The cracks could affect the resistive film heater because it is not a continuous and homogeneous material (as is a wire), but rather the fine dried powder of the conductive ink, as disclosed in Juliano '296.
Another heated nozzle design is disclosed in U.S. Pat. No. 4,120,086 to Crandell. In one embodiment, Crandell '086 discloses an electrically heated nozzle having an integral heater comprising a resistance wire heater disposed between two ceramic insulating layers. The Crandell '086 nozzle is made by wrapping a metal nozzle body with flexible strips of green (ie. unsintered) ceramic particles impregnated in heat dissipatable material, subsequently winding a resistance wire heating element around the wrapped green layer, wrapping a second layer of the flexible strips of green ceramic particles thereover, heat treating the assembly to bake out the heat dissipatable material and sinter the ceramic particles together, and then compacting the assembly to eliminate air voids in the assembly. In U.S. Pat. No. 4,304,544, also to Crandell, the inventor further describes the flexible green ceramic strip

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