Minimum vestige nozzle assembly for plastics injection molding

A hot tip nozzle assembly for injection molding plastic components comprises a nozzle housing defining a hot runner passage and a nozzle tip section which is received in the hot runner passage. The nozzle tip section defines a hot runner tip passage in flow communication with the hot runner passage. The nozzle tip section slides freely within the hot runner passage during injection molding. A stop is provided on the nozzle tip section to limit downward movement relative to the cavity gate and automatically position the nozzle tip relative to the injection gate of the mold. In addition, axial thermal expansion of the nozzle housing is not imparted to the nozzle tip section. The nozzle tip section also has a restricted length that can axially thermally expand. Accordingly, the position of the nozzle tip is only minutely affected by even larger temperature changes in the nozzle assembly. This controlled thermal expansion together with the sliding movement of the nozzle tip section relative to the nozzle housing maintains the nozzle tip at a substantially constant position during the injection molding operation and minimizes vestige formation in the molded components.

BACKGROUND 
The invention relates to plastic injection molding apparatuses for molding 
plastic components and, more particularly, to a hot tip nozzle assembly 
which minimizes and controls the formation of vestiges in molded plastic 
components. 
Pressure injection molding nozzle assemblies for molding plastic components 
comprise a nozzle housing and a nozzle tip, commonly referred to as a "hot 
tip." In the known assemblies, the nozzle tip is fixedly secured to the 
nozzle housing, typically by mating threads formed in the nozzle housing 
and on the nozzle tip, to prevent relative movement. The nozzle assembly 
is fixtured in the mold plates and the nozzle tip is positioned relative 
to the injection gate. 
In operation, a plastic resin melt is supplied to the nozzle assembly from 
a manifold and directed into the mold cavity through the hot runner 
passage defined by the nozzle housing and the nozzle tip. Temperature 
changes and heating effects occur during this process that have important 
effects on the injection molding nozzle assembly. Such temperature changes 
are required for molding various plastics. The temperature changes cause 
thermal expansion of the hot runner system, affecting the dimensional 
stability of the nozzle assembly and, consequently, the quality of the 
formed plastic components in the known nozzle assemblies. 
Thermal effects can result in the plastic components having unacceptable 
vestiges. A vestige is a visible surface flaw such as a bump, stringing or 
raised material that may form on the component surface during the 
injection molding process. Vestiges may occur due to improper nozzle tip 
positioning relative to the injection gate during the injection molding 
process. The plastic resin melt is injected at a high temperature into the 
injection gate of the cooled mold core. Following injection and after the 
cooling cycle, the component is moved away from the nozzle tip. A residual 
amount of the plastic resin melt can be sheared away with the component, 
forming a vestige. 
Vestiges can vary in size and shape in molded plastic components, generally 
depending on the nozzle tip position relative to the injection gate. Some 
vestiges may be acceptable in certain applications where the surface 
condition is not critical. Vestiges are unacceptable, however, in plastic 
components that require a near flawless outer surface, such as various 
medical components and components requiring a highly cosmetic appearance. 
When poor vestiges occur in such components, the components must be 
scraped at a financial loss to the manufacturer. 
Attempts have been made to reduce the problem of vestige formation. These 
attempts have typically included calculating the expected thermal 
expansion of the nozzle assembly at a specific temperature and 
constructing an apparatus, that when operated at that temperature, expands 
by such an amount that the nozzle tip is placed in the ideal position 
relative to the injection gate and to the component. The ideal position is 
substantially flush with the component. This approach requires that the 
nozzle assembly components be machined to precise size tolerances because 
component dimensions are critical to achieving the ideal nozzle tip 
placement. Accordingly, manufacture of the known nozzle assemblies is 
highly demanding. 
This known approach to nozzle tip positioning has proved to be somewhat 
unsatisfactory due to the influence of variables during the actual 
injection molding process. Factors such as the actual thermocouple sensor 
position, the plastic resin melt composition, and the composition of the 
nozzle assembly components, can significantly affect the actual position 
of the nozzle tip during operation, as the nozzle assembly expands and 
contracts due to wide temperature fluctuations. These and other factors 
can cause the position of the nozzle tip to move from the ideal position, 
resulting in the formation of poor vestiges on the components. 
The known nozzle assemblies are often unable to compensate for such 
variable factors and achieve a substantially constant nozzle tip position 
during the injection molding process. If the temperature inside of the 
nozzle assembly is not accurately sensed, excessive heat can be applied to 
the nozzle assembly, causing it to be overheated. This overheating 
produces greater expansion of the nozzle assembly than calculated to place 
the tip at the ideal position and, consequently, results in the nozzle tip 
being positioned too close to the injection gate. 
The plastic resin melt composition also affects the nozzle tip position. If 
the nozzle assembly is used to inject a plastic resin material having a 
processing temperature different from that of the material used to 
calculate the ideal tip placement, the corresponding temperature change in 
the nozzle assembly can result in the nozzle tip being displaced from the 
ideal position due to the different amount of thermal expansion. 
The change in length due to thermal expansion of a given component is equal 
to the product of the temperature change, the coefficient of thermal 
expansion of the component material, and the portion of the total length 
of the component that can expand when heated. For example, increasing the 
temperature of a three inch long steel component by 100.degree. F. due to 
overheating produces a corresponding linear thermal expansion of about 
0.002 inch. This amount of expansion is typical in the known nozzle 
assemblies due to overheating as a result of inadequate temperature 
control. Raising the temperature of the same component from ambient 
temperature to 500.degree. F. causes a linear expansion of about 0.008 
inch. Temperature changes of this magnitude are typical in plastic 
injection molding processes between the cold, ambient temperature, 
condition and the operating temperature. The resultant expansion of the 
nozzle assembly is sufficient to change the position of the fixedly 
attached nozzle tip from the ideal position and, consequently, cause poor 
vestige formation in the molded components. 
Thus, there is a need for a hot tip nozzle assembly for use in injection 
molding of plastics that (i) controls the thermal expansion of the nozzle 
assembly so as to maintain a substantially constant nozzle tip position 
during processing, such that vestige formation on the manufactured parts 
is minimized; (ii) achieves a substantially constant nozzle tip position 
for various plastic resin materials having different processing 
temperatures; (iii) comprises an automatic positioning nozzle tip that 
achieves precise location during operation; (iv) has less areas requiring 
precise machining tolerances and is easier to manufacture than the known 
nozzle assemblies; and (v) does not require precise locating of the nozzle 
tip during assembly of components. 
SUMMARY 
The present invention is directed to a hot tip nozzle assembly for use in 
injection molding processes for plastic components that satisfies the 
above needs. The nozzle assembly (i) limits thermal expansion so as to 
maintain a substantially constant tip position, such that vestige 
formation is minimized; (ii) achieves a substantially constant tip 
position for various plastic resin materials having a range of processing 
temperatures; (iii) comprises an automatic positioning nozzle tip that 
achieves precise locating during operation; (iv) has less areas requiring 
precise manufacturing tolerances and is easier to manufacture than the 
known nozzle assemblies; and (v) does not require a precise locating of 
the nozzle tip during assembly of components. In addition, the nozzle 
assembly comprises a versatile nozzle housing compatible with various 
nozzle tip configurations. 
The present invention comprises a nozzle assembly for injecting a plastic 
resin melt into a mold cavity through a gate. The nozzle assembly 
comprises a nozzle housing and a nozzle tip section. The nozzle housing 
defines an axial hot runner passage therethrough for flowing a plastic 
resin melt. The nozzle tip section is received in the hot runner passage 
and is axially movable relative to the nozzle housing. The nozzle tip 
section defines an axial hot runner tip passage in flow communication with 
the hot runner passage for flowing the plastic resin melt. 
The nozzle housing is typically formed of steel. A case hardened inner 
surface is preferably formed in the nozzle housing to provide high 
lubricity so as to enhance sliding movement between the nozzle housing and 
the nozzle tip section. The inner surface is typically case hardened by a 
nitriding process. 
The nozzle tip section comprises a nozzle tip retainer which comprises the 
nozzle tip and defines the hot runner tip passage. The hot runner tip 
passage can be straight, or it can optionally be spiral shaped. 
The nozzle tip retainer is preferably formed of a material having a higher 
coefficient of thermal expansion than the material forming the nozzle 
housing. Typically, the nozzle tip retainer is formed of a material such 
as beryllium-copper. This difference in the coefficients of thermal 
expansion of the two materials provides a plastic resin melt seal between 
the nozzle housing and the nozzle tip section at the operating 
temperature. This seal also allows the nozzle tip section to slide within 
the axial hot runner passage. The nozzle tip retainer is preferably also 
nickel plated to reduce wear and erosion and also to provide a slippery 
outer surface. 
The nozzle tip section comprises stop means for limiting axial movement 
thereof relative to the gate. The stop means ensures that the nozzle tip 
is properly positioned relative to the gate during the injection molding 
process to minimize vestige formation. 
The stop means also controls linear thermal expansion by restricting the 
length of the nozzle tip section that can thermally expand. Accordingly, 
even as the temperature of the nozzle assembly is increased, the nozzle 
tip section only minimally thermally expands such that the position of the 
nozzle tip relative to the gate remains substantially constant.

DESCRIPTION 
As illustrated in the drawings, the present invention is directed to a hot 
tip nozzle assembly 50 for injection molding plastic components. The 
nozzle assembly 50 produces improved quality components, having minimal 
vestiges, as compared to the known nozzle assemblies. 
A typical known nozzle assembly 10 is illustrated in FIG. 1. As shown, the 
nozzle assembly 10 is fixed in a retainer plate 12, a cavity retainer 
plate 14 and a mold cavity plate 16, such that the nozzle assembly 10 is 
vertically oriented. 
The nozzle assembly 10 comprises a nozzle housing 18 which defines an axial 
hot runner passage 20. A heating unit 22 surrounds a portion of the nozzle 
housing 18. 
A nozzle tip section 24 is attached at the lower end of the nozzle housing 
18. The nozzle tip section 24 is commonly referred to as a "hot tip." The 
nozzle tip section 24 includes external threads which engage internal 
threads formed in the nozzle housing 18 as depicted at 26, to fixedly 
attach the nozzle tip section 24 to the nozzle housing 18. The nozzle tip 
section 24 defines a hot runner tip passage 28 in flow communication with 
the hot runner passage 20 formed in the nozzle housing 18, and a pair of 
nozzle tip openings 30 through which a plastic resin melt is injected 
through an injection gate 32 and which is disposed on a mold core 34. 
A plastic resin melt is supplied to the nozzle assembly 10 through a melt 
passage 36 of a supply manifold 38. The plastic resin melt is flowed 
through the hot runner passage 20 and hot runner tip passage 28, and 
directed through the nozzle tip openings 30, into the gate well 43 and 
through the injection gate 32. The plastic resin melt flowed through the 
nozzle assembly 10 is heated by the heating unit 22 controlled by a 
thermocouple 23. Upon cooling of the plastic resin, a molded component 40 
is formed. 
The known nozzle assembly 10 relies on axial thermal expansion for nozzle 
tip placement and consequently is unable to consistently produce molded 
plastic components having minimal vestiges. During operation, the nozzle 
housing 18 axially thermally expands due to changes in the operating 
temperature. The nozzle tip section 24, being fixedly attached to the 
nozzle housing 18, must also move axially along with the nozzle housing 18 
as it expands. As a result, the position of the nozzle tip 42 is changed. 
The nozzle tip 42 position is not controlled. As the length of the nozzle 
housing 18 and the tip section 24 increase, and the temperature change 
increases, the amount of thermal expansion increases. This increased 
expansion causes an increased change in the position of the nozzle tip 42 
relative to the injection gate 32. If the nozzle tip 42 moves downwardly 
too great a distance due to this expansion, the nozzle tip 42 can protrude 
into the component, resulting in unsuitable vestige problems. Vestige 
problems can also occur if the thermal expansion is less than calculated 
and the nozzle tip 42 is consequently positioned too far from the 
injection gate 32. 
The nozzle assembly 50 according to the present invention overcomes the 
problem of thermal expansion and produces components of high quality with 
minimal vestiges. This is achieved by controlling thermal expansion and 
restricting the nozzle tip movement during the injection molding process. 
The nozzle tip section is also automatic positioning. Thus, the nozzle 
assembly can be used to process a variety of plastic resin materials 
having different processing temperatures without ever having to use a 
different nozzle assembly. This feature of the nozzle assembly 50 
liberalizes the range of machining tolerances that are required as 
compared to the known nozzle assemblies and, consequently, makes the 
nozzle assembly 50 easier to manufacture. 
In addition, the nozzle assembly 50 has increased versatility in that it 
comprises a nozzle housing which is compatible with a variety of nozzle 
tip section configurations. 
Referring to FIG. 2, the nozzle assembly 50 is shown fixtured in a manifold 
plate 52, a cavity retainer plate 54 and a cavity plate 56. The nozzle 
assembly 50 comprises a nozzle housing 58 and a nozzle tip section 60 
which is freely slidably received within the nozzle housing 58, as 
described in greater detail hereinbelow. The nozzle tip section 60 
comprises a nozzle tip 90. 
The nozzle housing 58 includes an upper portion 62 having an upper surface 
64 substantially flush with a plastic resin melt manifold 66. 
A locating tensioner collar 68 is fitted on the nozzle housing 58 and 
supported by the manifold plate 52. A timing dowel 72 extends through a 
bore (not shown) formed in the locating tensioner collar 68. The locating 
tensioner collar 68 holds the nozzle housing 58 against the manifold 66 
during the cold mold start up procedure, minimizing the possibility of a 
plastic resin leak between the nozzle housing 58 and the manifold 66 if 
the plastic resin is prematurely injected prior to the mold reaching the 
operating temperature. 
The nozzle housing 58 comprises a reduced outer diameter portion 74 which 
may be flush at one end with the cavity retainer plate 54. A longitudinal 
groove (not shown) is formed in the outer surface of the portion 74. A 
thermocouple 76 is positioned in the groove to monitor the temperature of 
the nozzle housing 58. 
A band heater 78 is disposed on the portion 74 and surrounds the 
thermocouple 76. The band heater 78 applies a controlled amount of heat to 
the nozzle housing 58, based on the temperature sensed by the thermocouple 
76. A retainer ring 80 is fitted in a circumferential groove 82 formed in 
the nozzle housing 58 to prevent axial movement of the band heater 78. 
The nozzle housing 58 defines an axial hot runner passage 84 having an 
upper portion 86 and an expanded lower portion 88. The cross-sectional 
area of the lower portion 88 is typically from about 2-3 times greater 
than that of the upper portion 86. The nozzle tip section 60 is received 
in the expanded lower portion 88. The thermocouple 76 is positioned in 
close proximity of the lower portion 88 and the nozzle tip section 60. The 
nozzle tip section 60 is slidably received within the lower portion 88 so 
that the nozzle tip section 60 is able to slide within the lower portion 
88 during the injection molding process. Consequently, any thermal 
expansion of the nozzle housing 58 is not imparted to the nozzle tip 
section 60. This non-fixed construction enables the nozzle tip 90 to 
remain in the ideal position during the injection molding operation as 
described below, and, as a result, minimizes the formation of undesirable 
vestiges in the molded components 130. The nozzle housing 58 is preferably 
case hardened by a process such as nitriding to provide an inner surface 
having high hardness and high lubricity to enhance sliding movement. 
Referring to FIG. 3, the nozzle tip section 60 comprises a nozzle tip 
retainer 92 which defines a hot runner tip passage 94 therethrough in flow 
communication with the hot runner passage 84 in the nozzle housing 58. The 
nozzle housing 58 is typically formed of steel which has a low coefficient 
of thermal expansion. The nozzle tip retainer 92 is formed of a material, 
such as beryllium-copper, having a higher coefficient of thermal expansion 
than the steel nozzle housing 58. There is a running clearance typically 
of about 0.0006 in. to about 0.0008 in. between the outer surface of the 
nozzle tip retainer 92 and the inner surface of the nozzle housing 58, as 
depicted at 96 in FIG. 2, in the cold condition of the nozzle assembly 50. 
This clearance is selected so that at the operating temperature of the 
nozzle assembly 50, the nozzle tip retainer 92 expands radially outwardly 
a greater distance than does the nozzle housing 58, thereby forming a seal 
between the nozzle tip retainer 92 and the nozzle housing 58. This seal 
reduces leakage of the plastic resin melt between the nozzle tip retainer 
92 and the nozzle housing 58, and also increases the heat transfer area 
between the nozzle tip retainer 92 and the nozzle housing 58 as compared 
to the known nozzle assembly 10, in which the nozzle tip section 24 is 
threaded to the nozzle housing 18. 
The nozzle tip retainer 92 is preferably nickel plated to reduce wear and 
erosion, and also to provide a slippery outer surface which allows the 
nozzle tip section 60 to form the seal with the nozzle housing 58, yet 
also allows the nozzle tip section 60 to slide freely axially relative to 
the nozzle housing 58 due to hydraulic pressure within the hot runner 
system. 
A spiral flow tip member 98 including the nozzle tip 90 is disposed within 
the nozzle tip retainer 92. The spiral flow tip member 98 includes a 
shoulder 102 which abuts a shoulder 104 formed in the nozzle tip retainer 
92. The spiral flow tip member 98 is preferably formed of a material 
having high abrasion resistance to plastics, such as high speed steels. 
The spiral flow tip member 98 is typically brazed to the nozzle tip 
retainer 92. The plastic resin melt is sheared as it flows axially over 
the spiral flow tip member 98. The spiral flow tip member 98 provides a 
washing action to the plastic resin melt which, when the color of the 
plastic resin melt fed into the nozzle assembly 50 from the manifold 66 is 
changed, enhances removal of the original colored plastic resin melt from 
the hot runner system so that components of the new color are produced 
more quickly. The plastic melt resin emerges from the nozzle tip section 
60 at the nozzle tip 90. 
FIG. 4 illustrates an alternative embodiment of the nozzle tip section 106 
which can be used in the injection nozzle assembly 50. The nozzle tip 
section 106 defines a straight passage 108 through which the plastic resin 
melt is flowed. The nozzle tip section 106 has a unitary construction and 
comprises a tip portion 110 having a pair of holes 112 extending 
therethrough at an angle relative to the axis of the passage 108. The 
plastic resin melt emerges from the nozzle assembly through the holes 112. 
Referring to FIG. 5, a nozzle tip locator support 114 is mounted to the 
nozzle tip retainer 92. The nozzle tip locator support 114 comprises an 
upper portion 116 having an upper face 118 which abuts the nozzle tip 
retainer 92 and a stop face 120, and a lower portion 122. The outer 
diameter of the lower portion 122 is slightly less than the width of an 
air gap 124 in the cavity plate 56 to enable the lower portion 122 to 
slide within the air gap 124 during the injection molding process. The 
gate well 140 is predominantly an air gap when cold. The nozzle tip 
locator support 114 is typically comprised of titanium. 
The nozzle tip locator support 114 minimizes the size of the plastic bubble 
125 around the nozzle tip 90. If this plastic bubble is too large, 
degradation of sensitive material due to prolonged residence time and 
overheating can occur. There can also be a poor color change of the 
plastic resins if the plastic bubble is too large. 
FIG. 5 illustrates the preferred injection position of the nozzle tip 
section 60 during the injection molding process. As shown, the positive 
stop face 120 contacts a face 126 of the mold cavity plate 56 in a cavity 
138, preventing further downward movement of the nozzle tip section 60 
relative to the injection gate 128 and the component 130 supported on a 
mold core 132. A cooling element 134 is provided in the mold core 132 to 
rapidly cool the injected plastic resin melt. The distance Z.sub.1 between 
the stop face 120 and the injection gate 128 is fixed. This distance is 
selected such that, in the depicted fully downward position of the nozzle 
tip section 60, the nozzle tip 90 is positioned at the preferred position 
relative to the injection gate 128. 
The nozzle tip locator support 114 limits the length of the nozzle tip 
section 60 that can axially thermally expand in the downward direction so 
that the nozzle tip 90 is automatically positioned at the preferred 
position substantially flush with the injection gate 128. Particularly, 
the expansion length is approximately Z.sub.2, which is the distance from 
the surface 118 of the upper portion 116 to the nozzle tip 90. This length 
is preferably only about 0.3 inch. A temperature increase of 500.degree. 
F., for example, produces a thermal expansion of the length Z.sub.2 of 
only about 0.0009 inch for beryllium-copper, resulting in a minute change 
in position of the nozzle tip 90 relative to the injection gate 128. Such 
a minute change in the nozzle tip 90 position is much less than the change 
that occurs in the nozzle tip 42 position as a result of the same 
temperature change in the nozzle assembly 10. Thus, nozzle assembly 50 
overcomes the problem of improper nozzle tip 90 positioning. 
During operation of the nozzle assembly 50, a plastic resin melt is 
supplied from the manifold 66 and flowed through the upper portion 86 of 
the hot runner passage 84 and into the expanded lower portion 88. The 
plastic resin melt is sheared as it passes over the spiral flow tip 98 
before exiting the nozzle assembly 50. The temperature of the plastic 
resin melt within the nozzle assembly 50 is controlled by the heating unit 
78. The plastic resin melt is injected into the mold through gate 128 and 
cools to form the plastic component 130. 
Referring to FIGS. 2 and 5, the force of the hydraulic pressure in the 
nozzle assembly 50 during the injection molding process acts on the nozzle 
tip section 60, causing it to slide relative to the nozzle housing 58. 
Downward force acts on an upper face 105 due to the larger surface area at 
this location. Downward movement of the nozzle tip section 60 is limited 
by the stop face 120 of the nozzle tip locator support 114. When the stop 
face 120 contacts the face 126 of the cavity plate 56 at the full extent 
of the nozzle tip section 60 movement, the nozzle tip 100 is in the 
preferred position relative to the injection gate 128 and the component 
130. 
The preferred position of the nozzle tip 100 is maintained by the force of 
the hydraulic pressure exerted on the upper face 105 of the nozzle tip 
retainer 92 by the plastic resin melt injection pressure. The 
cross-sectional area at the upper face 105 is greater than the 
cross-sectional area of the flow passage at the plastic bubble 125, 
resulting in a downward acting force on the nozzle tip section 60. Thus, 
the nozzle tip section 60 slides forward during normal operation of the 
nozzle assembly 50. 
Because the nozzle tip 90 position is automatically controlled, the nozzle 
assembly 50 eliminates the need to precisely manually position the nozzle 
tip 90 as in the known nozzle assembly 10. This automatic positioning 
provides several important advantages. Many machining tolerances for the 
nozzle housing 58 and the nozzle tip section 60 are less precise. 
Accordingly, the nozzle housing assembly 50 is easier to manufacture. 
In addition, the nozzle assembly 50 can be used to mold various plastic 
melt resins, having a range of processing temperatures, without having to 
replace the nozzle tip section 60 or manually adjust the position of the 
nozzle tip 90 relative to the injection gate 128. The nozzle tip 90 is 
automatically seated relative to the injection gate 128 by the hydraulic 
pressure of the plastic resin melt and the contact between the stop face 
120 and the face 126 of the cavity plate 56. The short length of the 
nozzle tip locator support 114 significantly limits the length of the 
nozzle assembly 50 that can thermally expand. This controlled expansion 
ensures that the nozzle tip 90 remains substantially stationary in the 
preferred position despite even large temperature changes. 
Although the present invention has been described in considerable detail 
with reference to certain preferred versions thereof, other versions are 
possible. Therefore, the scope of the appended claims should not be 
limited to the description of the preferred versions contained herein.