Title: Module for a modular conveyor belt having a microcellular structure
Abstract: A module for a modular conveyor belt, is described. The module is of a micocellular polymeric foam produced by an injection molding process. The foam polymeric material exhibits excellent mechanical properties required for modular conveyor belts capable of withstanding long service lives. Also, the reduced weight in comparison to conventionally molded modules helps decrease wear on the conveyor belt support structures including the drive sprockets.
Patent Number: 6,942,913 Issued on 09/13/2005 to Cediel, et al.
| Inventors:
|
Cediel; Luis (Wollerau, CH);
Fandella; Sergio (Mogliano Veneto-TV, IT)
|
| Assignee:
|
Habasit AG (Reinach, CH)
|
| Appl. No.:
|
961708 |
| Filed:
|
September 24, 2001 |
| Current U.S. Class: |
428/54; 198/957; 428/52; 428/53; 428/304.4; 428/308.4; 428/314.4 |
| Intern'l Class: |
B32B 007/08; B32B 003/08; B32B 003/10; B32B 003/26 |
| Field of Search: |
428/54,52,53,304.4,314.4,308.4
198/957
|
References Cited [Referenced By]
U.S. Patent Documents
| 3882191 | May., 1975 | Balatoni et al.
| |
| 3929026 | Dec., 1975 | Hofmann.
| |
| 4043958 | Aug., 1977 | Whelan.
| |
| 5253749 | Oct., 1993 | Ensch.
| |
| 5372248 | Dec., 1994 | Horton.
| |
| 5377819 | Jan., 1995 | Horton et al.
| |
| 5706934 | Jan., 1998 | Palmaer et al.
| |
| 6017586 | Jan., 2000 | Payn et al.
| |
| 6169122 | Jan., 2001 | Blizard et al.
| |
| 6231942 | May., 2001 | Blizard et al.
| |
| Foreign Patent Documents |
| 0 068 475 | Jan., 1983 | EP.
| |
Primary Examiner: Pyon; Harold
Assistant Examiner: Bruenjes; Chris
Attorney, Agent or Firm: Hodgson Russ LLP
Claims
1. A modular conveyor belt, which comprises:
a) a plurality of modules, each module comprised of a first plurality of link
ends, a second plurality of link ends and an intermediate section integrally formed
with and joining the first and second plurality of link ends, wherein the link
ends of each of the modules are releasably engaged between link ends of an adjacent
module except for individual link ends of modules disposed at the extreme sides
of the belt and wherein at least some of the modules comprise a polymeric material
including a plurality of cells having an average cell size of less than 100 microns
and at least some of the modules include a nucleating agent; and
b) a pivot rod pivotally connecting the modules at engaged link ends.
2. The conveyor belt of claim 1 wherein the average cell size is less than about
60 microns.
3. The conveyor belt of claim 1 wherein the average cell size is less than about
20 microns.
4. The conveyor belt of claim 1 having a void volume of greater than 10%.
5. The conveyor belt of claim 1 having a void volume of from about 10% to about 50%.
6. The conveyor belt of claim 1 wherein the polymeric material is selected from
the group consisting of polyethylene terephthalate, polylactic acid, nylon 6, nylon
6/6, polyethylene, polypropylene, syndiotactic polystyrene, polyacetal, and mixtures thereof.
7. The conveyor belt of claim 1 wherein the polymeric material comprises at least
about 80% of one of the group selected from polypropylene, medium density polyethylene,
and high-density polyethylene, by weight, of the polymeric material.
8. The conveyor belt of claim 1 wherein the polymeric material comprises from
about 2.5% to about 7% of a nucleating agent, by weight, of the polymeric material.
9. The conveyor belt of claim 1 wherein the nucleating agent comprises an inorganic solid.
10. The conveyor belt of claim 1 wherein the nucleating agent is selected from
the group consisting of talc, calcium carbonate, titanium oxide, barium sulfate,
zinc sulfide, cellulosic fibers, and mixtures thereof.
11. The conveyor belt of claim 1 wherein the cells are characterized as having
been formed by introducing a blowing agent into the polymeric material.
12. The conveyor belt of claim 11 wherein the blowing agent is either a chemical
blowing agent or a physical blowing agent.
13. The conveyor belt of claim 12 wherein the physical blowing agent is selected
from the group consisting of dichlorotetrafluoroethane, iso-pentane, n-pentane,
trichlorofluoromethane, dichlorodifluoromethane, carbon dioxide, nitrogen, and
mixtures thereof.
14. The conveyor belt of claim 12 wherein the chemical blowing agent is selected
from the group consisting of azodicarbonamide, azodiisobutyronitrile, p,p-oxibis,
trihydrazinotriazine, barium-azodicarbonate, p-toluenesulfonyl semicarbazide, azobisisobutyronitrile,
diazoaminobenzene, N, N′-dimethyl-N,N′-dinitrosoterephthalamide,
N,N′-dinitrosopentamethylenetetramine, benzenesulfonyl hydrazide, toluene-(4)-sulfonyl
hydrazide, benzene-1,3-disulfonyl hydrazide, diphenylsulfon-3,3′-disulfonyl
hydrazide, 4,4′-oxybis(benzenesulfonyl hydrazide), and mixtures thereof.
15. The conveyor belt of claim 12 wherein the polymeric material is essentially
free of residual physical blowing agent and chemical blowing agent and reaction-by-products
of the physical blowing agent and the chemical blowing agent.
16. The conveyor belt of claim 1 wherein the cells are primarily closed cells.
17. The conveyor belt of claim 1 wherein the polymeric material includes an additive
material selected from the group consisting of an antimicrobial material, an electrical
conductivity material, a flame retardant, a pigment, and mixtures thereof.
18. The conveyor belt of claim 17 wherein the additive material is present in
the polymeric material in an amount of about 0.15% to about 10.5%, by weight.
19. The conveyor belt of claim 1 wherein the modules are selected from the group
consisting of flat top modules, radius modules, flush grid modules, raised rib
modules, and flight modules.
20. A modular conveyor belt, which comprises:
a) a plurality of modules, each module comprised of a first plurality of link
ends, a second plurality of link ends and an intermediate section integrally formed
with and joining the first and second plurality of link ends, wherein the link
ends of each of the modules are releasably engaged between link ends of an adjacent
module except for individual link ends of modules disposed at the extreme sides
of the belt;
b) a polymeric material comprising at least some of the modules including a nucleating
agent in an amount between about 2.5% and about 7%, by weight, of the polymeric
material; and
c) a pivot rod pivotally connecting the modules at engaged link ends to thereby
form the modular conveyor belt.
21. The modular conveyor belt of claim 20 wherein the nucleating agent is selected
from the group consisting of talc, calcium carbonate, titanium oxide, barium sulfate,
zinc sulfide, cellulosic fibers, and mixtures thereof.
22. A method for manufacturing a modular conveyor belt, comprising the steps of:
a) molding a plurality of modules, each comprising a first plurality of link
ends, a second plurality of link ends and an intermediate section integrally formed
with and joining the first and second plurality of link ends, and including molding
at least some of the modules of a polymeric material comprising a plurality of
cells of an average cell size of less than about 100 microns and including associating
a nucleating agent with the polymeric material to thereby provide the cells; and
b) releasably engaging the link ends of each module between the link ends of
an adjacent module except for individual link ends of modules disposed at the extreme
sides of the belt by a pivot rod pivotally connecting the modules at engaged link
ends to thereby form the modular conveyor belt.
23. The method of claim 22 including introducing a blowing agent into the polymeric
material to provide the cells.
24. A method for manufacturing a modular conveyor belt, comprising the step of:
a) molding a plurality of modules, each comprising a first plurality of link
ends, a second plurality of link ends and an intermediate section integrally formed
with and joining the first and second plurality of link ends;
b) molding at least some of the modules from a polymeric material comprising
a nucleating agent in an amount from about 2.5% to about 7%, by weight, of the
polymeric material; and
c) releasably engaging the link ends of each module between the link ends of
an adjacent module except for individual link ends of modules disposed at the extreme
sides of the belt by a pivot rod pivotally connecting the modules at engaged link
ends to thereby form the modular conveyor belt.
25. The method of claim 24 including selecting the nucleating agent from the
group consisting of talc, calcium carbonate, titanium oxide, barium sulfate, zinc
sulfide, cellulosic fibers, and mixtures thereof.
26. The method of claim 24 including introducing a blowing agent into the polymeric
material including the nucleating agent.
27. The method of claim 26 including providing the blowing agent as either a
physical blowing agent or a chemical blowing agent.
28. The method of claim 27 including selecting the physical blowing agent from
the group consisting of FREON R114, iso-pentane, n-pentane, trichlorofluoromethane,
dichlorodifluoromethane, nitrogen, carbon dioxide, and mixtures thereof.
29. The method of claim 27 including selecting the chemical blowing agent from
the group consisting of azodicarbonamide, azodiisobutyronitrile, p,p-oxibis, trihydrazinotriazine,
barium-azodicarbonate, p-toluenesulfonyl semicarbazide, azobisisobutyronitrile,
diazoaminobenzene, N,N′-dimethyl-N,N′-dinitrosoterephthalamide, N,N′-dinitrosopentamethylenetetramine,
benzenesulfonyl hydrazide, toluene-(4)-sulfonyl hydrazide, benzene-1,3-disulfonyl
hydrazide, diphenylsulfon-3,3′-disulfonyl hydrazide, 4,4′-oxybis(benzenesulfonyl
hydrazide), and mixtures thereof.
30. The method of claim 26 including providing the blowing agent at about 0.1%
to about 7%, by weight, of the polymeric material including the nucleating agent.
31. The method of claim 24 including subjecting the polymeric material including
the nucleating agent to a pressure drop of greater than 1,000 psi as it is being
injected into a mold for the module.
32. The method of claim 24 including subjecting the polymeric material including
the nucleating agent to a pressure drop of greater than 3,000 psi as it is being
injected into a mold for the module.
33. The method of claim 24 including subjecting the polymeric material including
the nucleating agent to a pressure drop rate of about 0.5 GPa/s to about 10.0 GPa/s
as it is being injected into mold for the module.
34. The method of claim 24 including providing the module as one selected from
the group consisting of flat top modules, radius modules, flush grid modules, raised
rib modules, and flight modules.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to conveyor belts and, more particularly,
to modular conveyor belts formed of rows of plastic belt modules pivotally interlinked
by transverse pivot rods. Preferably, the modules are of polymeric materials of
microcellular foams including additives. Methods for the production of such modules
are also described.
2. Prior Art
Because they do not corrode, are light weight, and are easy to clean, plastic
modular conveyor belts are gaining increased usage in a wide range of industrial
applications. Modular conveyor belts are built from molded plastic modular links,
or modules, arranged side-by-side in rows of a selected width. A series of spaced
apart link ends extending from each side of the modules include aligned openings
that receive a pivot rod. The link ends along one end of a row of modules are interconnected
with the link ends of an adjacent row. The pivot rod journaled in the aligned openings
of the side-by-side and end-to-end connected modules form a hinge between adjacent
rows. Rows of belt modules are connected together to form an endless conveyor belt
capable of articulating about drive sprockets positioned at opposite ends of the
conveying surface.
Modules for modular conveyor belts are typically formed of polyolefinic materials,
for example, polypropylene or polyethylene. A modular conveyor belt system typically
comprises a support frame upon which the conveyor belt rests as drive sprockets
spaced at intervals along the belt length provide a motive force. A preferred material
for modules is high density polyethylene having a density in excess of 0.94 g/cm
3.
When the weight of the conveyor belt becomes too great, the drive sprockets and
other support frame components, and even the belt modules themselves, experience
excessive wear and must be replaced. Therefore, belt and support structure longevity
can be an important factor in a customer deciding to continue using one manufacture
over another.
In that light, the modules of the present invention are of polymeric foams having
a plurality of voids, called cells, in the polymeric matrix. By replacing solid
plastic with voids, such microcellular polymeric foams use less raw material than
solid plastics for a given volume. The modules of the present invention comprising
microcellular polymeric foams instead of solid plastics are less expensive in terms
of material costs and are of a comparatively reduced weight. This latter attribute
translates to a conveyor belt having a weight reduction of about 30% in comparison
to a similarly sized belt of a solid polymeric material. A belt of reduced weight
leads to prolonged module wear and prolonged conveyor belt support structure service.
SUMMARY OF THE INVENTION
The present invention provides a modular conveyor belt of modules comprised of
a polymeric foam and, particularly, a microcellular polymeric foam, including a
nucleating agent and a blowing agent. The microcellular foam is produced by an
injection molding process and the resulting modules exhibit excellent mechanical
properties required for modular conveyor belts capable of extended duty.
One embodiment of the present invention provides a method for forming a module
for a modular conveyor belt, the module comprising a microcellular polymeric material.
The method includes conveying a polymeric mixture through a polymer processing
apparatus. The polymeric mixture comprises a semi-crystalline polymer and a nucleating
agent present in an amount of about 2.5% to about 7%, by weight, of the polymeric
material. At these concentrations, the nucleating agent effectively functions as
a filler replacing solid polymeric material in a non-negligible amount.
In another embodiment of the present invention, the manufacturing process further
includes introducing a blowing agent into the polymeric material as it moves through
the polymer processing apparatus. The blowing agent is preferably present in an
amount of about 0.1% to about 7%, by weight, of the polymeric material. The resulting
module of a semi-crystalline polymer provided with the nucleating agent and blowing
agent incorporated therein is of a microcellular structure having an average cell
size of about 60 microns. This results in a considerable cost savings attributed
to the microcellular structure replacing solid polymeric material without detracting
from the mechanical strength of the module. In still a further embodiment, the
present invention includes the step of introducing a pressure drop rate of less
than 1.0 GPa/s as the mixture of the nucleating agent/blowing agent and polymeric
material moves through the polymer processing apparatus.
In that respect, certain advantages of the present invention include producing
modules for a modular conveyor belt with the modules being formed of a microcellular
polymeric foam with low blowing agent percentages and/or low pressure drop rates
due to the presence of the nucleating agent. Using low blowing agent percentages
results in cost savings associated with the blowing agent and also may improve
the surface quality of the resulting modules. Employing low pressure drop rates
as opposed to high pressure drop rates generally permits greater freedom in module
design and, in some cases, allows for the production of modules having thicker
cross-sectional dimensions.
In many cases the microcellular foams have uniform and fine cell structures despite
the presence of the nucleating agent and the blowing agent. The interconnectivity
between cells is generally minimal, and the foams can be produced over a range
of densities. In particular, relatively high density foams can be produced having
properties comparable to solid, unfoamed plastics.
These and other aspects of the present invention will become increasingly more
apparent to those skilled in the art by reference to the following description
and the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic, partly in cross-section, showing an extrusion system
for producing conveyor belt modules of a microcellular polymeric material according
to the present invention.
FIG. 2 is a schematic, partly in cross-section, showing a multihole blowing
agent feed orifice arrangement and extrusion screw.
FIG. 3 is a cross-sectioned view of a mold for producing a conveyor belt module
of a microcellular polymeric foam according to the present invention.
FIG. 4 is a side elevational view of an exemplary flat top module according
to the present invention.
FIG. 5 is a plan view of the flat top module shown in FIG. 4.
FIG. 6 is a perspective of a radius module of a microcellular polymeric foam
according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The various embodiments and aspects of the present invention will be better understood
from the following definitions. As used herein, "nucleation" defines a process
by which a homogeneous, single-phase solution of polymeric material, in which is
dissolved molecules of a species that is a gas under ambient conditions, undergoes
formations of clusters of molecules of the species that define "nucleation sites",
from which cells will grow. That is, "nucleation" means a change from a homogeneous,
single-phase solution to a multi-phase mixture in which, throughout the polymeric
material, sites of aggregation of at least several molecules of blowing agent are
formed. Thus "nucleation sites" do not define locations, within a polymer, at which
nucleating agent particles reside. "Nucleated" refers to a state of a fluid polymeric
material that had contained a single-phase, homogeneous solution including a dissolved
species that is a gas under ambient conditions, but, following a nucleating event
(typically thermodynamic instability) contains nucleation sites. "Non-nucleated"
refers to a state defined by a homogeneous, single-phase solution of polymeric
material and dissolved species that is a gas under ambient conditions, absent nucleation
sites. A "non-nucleated" material can include a nucleating agent such as talc.
A "nucleating agent" is a dispersed agent, such as talc or other filler particles,
added to a polymer and able to promote formation of nucleation sites from a single-phase,
homogenous solution. A "filler" is a dispersed particle added to replace solid
polymeric material.
The term "blowing agent" describes two basic types of materials—those which
are "physical blowing agents" and those which are "chemical blowing agents". Physical
blowing agents are liquids with boiling points below the softening points of the
polymeric resins used. Chemical blowing agents are substances that decompose over
a narrow temperature range to produce gases.
A suitable physical blowing agent for use in constructing conveyor belt modules
according to the present invention must have the following properties: a relatively
high solubility in the resin without drastic change of the resin's viscosity or
glass transition point, low diffusion tendencies, and fast vaporization during
expansion to achieve low residual gas concentration in the polymeric cell wall.
The main criteria for a suitable chemical blowing agent is that the temperature
of decomposition lie within the processing temperature range of the polymeric resin.
In addition, the rate of decomposition to gaseous products must not be too slow.
It is also advantageous that the following conditions be fulfilled: the products
of decomposition must not discolor the polymeric resin, the products of decomposition
must not be corrosive, and the products of decomposition themselves should act
as nucleating agents.
One embodiment of the present invention provides a module for a modular conveyor
belt, the module being of a semi-crystalline microcellular foam having a nucleating
agent incorporated therein. The foam includes from about 2.5% to about 7%, by weight,
of the nucleating agent. A more preferred range of the nucleating agent is about
3% to about 7%, and still more preferably, about 5% to about 7%, by weight. Suitable
nucleating agents include a variety of inorganic solids such as talc, calcium carbonate
(CaCO
3), titanium oxide (TiO
2), barium sulfate (BaSO
4),
zinc sulfide (ZnS), and mixtures thereof. Organic solids such as the cellulosic
fibers may also function as nucleating agents. In some cases, the nucleating agents
may also enhance electrical conductivity, enhance crystallinity, function as a
pigment, and serve as a flame retardant.
Typically, the nucleating agents are particles, though in some cases they
may be fibrous or have other forms. The nucleating particles can have a variety
of shapes such as spherical, cylindrical, or planar. Generally, the particles have
a size in the range of about 0.01 microns to about 10 microns, and more typically
about 0.1 microns to about 1.0 microns. In some embodiments, the particles may
be surface treated with a surfactant to enhance dispersibility within the polymeric
melt and to prevent particle agglomeration.
The present modules of a microcellular foam are composed, at least in part, of
any semi-crystalline polymeric resin including, but not limited to, the following
materials: polyethylene terephthalate (PET), polylactic acid, nylon 6, nylon 6/6,
polyethylene, polypropylene, syndiotactic polystyrene, polyacetal, and mixtures
thereof. In certain cases, the semi-crystalline polymer may be blended with non-semicrystalline
polymers. In preferred cases, the semi-crystalline resin is a polyolefinic material
such as high-density polyethylene having a density of greater than about 0.94 g/cm
3.
In preferred cases, the weight percentage of high-density polyethylene is greater
than 80% of the polymeric material, and more preferably greater than 90%. In a
particularly preferred case the polymeric material consists essentially of high
density polyethylene, that is, there is no other polymeric resin components other
than high-density polyethylene.
Optionally, the foam polymeric mixture may include other additives in
addition to the nucleating agent. These include additives such as plasticizers
(e.g., low-molecular weight organic compounds), lubricants, flow enhancers, anti-oxidants,
and mixtures thereof.
According to another embodiment of the present invention, a physical or
a chemical blowing agent is introduced into the molten polymeric resin. Suitable
physical blowing agents include fluoro-chlorinated hydrocarbons, aliphatic hydrocarbons,
nitrogen and carbon dioxide. There are many fluoro-chlorinated hydrocarbons sold
under the various trade name: FRIGEN, KALTRON, FREON, and FLUGENE. For foamed PS
and PVC, trichlorofluoromethane (CCl
3F) called R11, or dichlorodifluoromethane
(CCl
2F
2) called R12, or a 50:50 mixture of them is used.
For foaming of polyolefins, FREON R114 is preferred. Iso-pentane and n-pentane
are also suitable blowing agents, even though they are flammable and form explosive
mixtures with air.
Azodicarbonamide (ADC) is the most preferred chemical blowing agent.
The best gas yield of about 220 cm
3/g is achieved at a temperature of
about 210° C. This blowing agent decomposes into solid and nitrogen. The temperature
of decomposition, however, is too high for several of the temperature sensitive
thermoplastic resins. The decomposition temperature can be reduced by the addition
of initiators (metal compounds such as zinc oxide and zinc stearate). Other suitable
chemical blowing agents and their properties are listed below in Table 1.
| TABLE 1 |
| |
| |
Short |
Processing |
|
|
|
| |
nomen- |
temp. |
Gas yield |
Concen- |
Used |
| Blowing Agent |
clature |
range ° C. |
cm3/g |
tration % |
with |
| |
| Azodicarbonamide |
ADC |
165 to 215 |
220 (210° C.) |
0.1 to 4.0 |
PP, PS, |
| (Azobisformamide) |
|
|
|
|
ABS, PE |
| |
|
|
|
|
(hard), |
| |
|
|
|
|
PVC |
| Azodiisobutyronitrile |
AZDN |
110 to 125 |
130 (110° C.) |
0.5 to 6.0 |
PVC |
| p,p-oxibis (benzol- |
OBSH |
150 to 200 |
160 (160° C.) |
0.5 to 2.0 |
PE |
| sulphonhydrazine) |
|
|
|
|
(soft), |
| |
|
|
|
|
EVA |
| Trihydrazinotriazine |
THT |
250 to 300 |
220 (270° C.) |
0.1 to 1.0 |
PA, AC |
| Barium-azodicarbonate |
BADC |
250 to 300 |
200 (270° C.) |
0.1 to 1.0 |
PVC, PA, |
| |
|
|
|
|
PC, ABS |
| p-toluenesulfonyl |
TSSC |
180 to 210 |
200 (200° C. |
0.5 to 2.0 |
PE |
| semicarbazide |
|
|
|
|
(hard), |
| |
|
|
|
|
PP, PS, |
| |
|
|
|
|
PVC) |
| |
Other chemical blowing agents useful with the present invention include azobisisobutyronitrile,
diazoaminobenzene, N,N′-dimethyl-N,N′-dinitrosoterephthalamide, N,N′-dinitrosopentamethylenetetramine,
benzenesulfonyl hydrazide, toluene-(4)-sulfonyl hydrazide, benzene-1,3-disulfonyl
hydrazide, diphenylsulfon-3,3′-disulfonyl hydrazide, and 4,4′-oxybis(benzenesulfonyl hydrazide).
Even though in some cases the amount of nucleating agent and/or blowing agent
is greater than about 2.5 weight percent, the polymeric foams have a relatively
uniform and fine cell structure. Conveyor belt modules of a foam polymeric material
according to the present invention have an average cell size of less than about
60 microns, preferably less than about 50 microns, more preferably less than about
20 microns, and more preferably still less than about 5 microns. The microcellular
material preferably has a maximum cell size of about 100 microns.
Preferably, the cell structure of the microcellular foam material comprising
the modules of the present invention is a closed cell structure. A substantially
closed cell structure has limited interconnection between adjacent cells and, generally,
is meant to define a polymeric material that, at a thickness of about 200 microns,
contains no connected cell pathway through the material. It is believed that the
closed cell structure advantageously contributes to enhancing the mechanical properties
of the foam due to the absence of long interconnected pathways which could act
as sites for premature failure of the conveyor belt modules.
Microcellular polymeric foams comprising the modules of the present
invention can be produced over a wide range of densities. In a particularly preferred
embodiment, the microcellular foam has a void volume from about 10% to about 50%.
Foams within this preferred void volume range exhibit excellent mechanical properties
such as tensile strength and tensile modulus while still having a significant density
reduction from that of solid plastic.
Another attribute of the present invention is that the microcellular polymeric
foams provide the conveyor belt modules with a desirable surface quality because
they are produced with low blowing agent percentages, thus limiting the amount
of gas that diffuses through the foam surface. As is known in the field of microcellular
foam processing, gas diffusion through the module surface generally leads to surface
roughness and imperfections.
Referring now to the drawings, FIG. 1 shows an injection molding system
10 for the production of modules
12 and
14 (FIGS. 3 to
6)
of a microcellular polymeric foam
16. The microcellular polymeric foam
16
includes a nucleating agent in a range of about 2.5% to about 7%, by weight, and/or
a chemical blowing agent in a range of about 0.1% to about 7%, by weight. The injection
system
10 comprises an injection molding apparatus
18 for injecting
the microcellular polymeric foam
16 into a mold
20. The injection
molding apparatus
18 comprises a screw
22 that rotates within a barrel
24 to convey, in a downstream direction (arrow
26), a polymeric material
in a processing space
28 between the screw and the barrel. The polymeric
material is injected into the mold
20 through an injection conduit
30
fluidly connected to the processing space
28 and fixed to a metering section
32 at a downstream end of the injection barrel
24.
Typically, the polymeric material is gravity fed into polymer processing
space
28 through orifice
34 from a standard hopper
36. The
polymeric material is preferably in a pelletized form. Though the polymeric material
can include a variety of semi-crystalline materials or blends thereof, preferably
the polymeric material includes a polyolefin such as polypropylene and medium or
high-density polyethylene.
In some cases, the nucleating agent or the chemical blowing agent, or both, are
added in a concentrate pellet form, for example 40% by weight, blended with the
semi-crystalline polymer pellets as a master batch. The concentrated pellets are
blended with suitable amounts of semi-crystalline pellets to produce a polymeric
material having between about 2.5% and about 7%, by weight, of the nucleating agent,
and/or about 0.1% to about 7%, by weight, of the chemical blowing agent. In that
manner, the percentage of, for example talc, in the polymeric material composition
is adjusted by controlling the ratio of the nucleating agent concentration to that
of the pure polymer pellets. In other embodiments, the nucleating agent and/or
chemical blowing agent in particulate form is added directly to the polymeric material.
Any other techniques well known in the art may also be employed for incorporating
the nucleating agent and/or the chemical blowing agent into the polymer composition
in controllable amounts.
Injection screw
22 is operably connected at its upstream end to
a drive motor
38 which rotates the screw. Although not shown in detail,
injection screw
22 includes feed, transition, gas injection, mixing, and
metering sections, as described further below.
Temperature control units
40 are optionally positioned along injection
barrel
24. Control units
40 can be electrical heaters, can include
passageways for temperature control fluid, and the like and are used to heat the
stream of pelletized or fluid polymeric material within the injection barrel. This
helps to facilitate melting or effecting cooling the polymeric stream to control
viscosity, skin formation and, in some cases, blowing agent solubility. The temperature
control units
40 operate differently at different locations along the barrel.
That is, they may heat at one or more locations and cool at one or more different
locations. Any number of temperature control units can be provided.
From hopper
36 pellets are received into the feed section of screw
22
and conveyed in the downstream direction
26 in polymer processing space
28 as the screw rotates. Heat from injection barrel
24 and the shear
forces arising from the rotating screw act to soften the pellets within the transition
section. Typically, by the end of the first mixing section the softened pellets
have been gelated, that is welded together to form a uniform fluid stream substantially
free of air pockets.
The physical blowing agent is introduced into the polymer stream through a port
42 in fluid communication with a source
44 thereof. The port is positioned
to introduce the physical blowing agent at any of a number of locations along the
injection barrel
24. Preferably, as discussed further below, the port
42
introduces the physical blowing agent at the gas injection section of the screw,
where the screw includes multiple fights.
A pressure and metering device
46 is provided between the physical blowing
agent source
44 and port
42. Blowing agents that are in the supercritical
fluid state in the extruder are especially preferred, in particular supercritical
carbon dioxide and supercritical nitrogen.
Metering device
46 is used to meter the amount of the physical blowing
agent introduced into the polymeric stream within the injection barrel
24.
In a preferred embodiment, metering device
46 measures the mass flow rate
of the physical blowing agent. The physical blowing agent is generally less than
about 15% by weight of the polymeric stream and blowing agent. The presence of
the previously introduced nucleating agent is believed to enhance the driving force
for nucleation thus enabling the production of a microcellular foam at low blowing
agent percentages, for example about 0.1% to about 2.5% blowing agent by weight
of the polymeric stream and blowing agent. This attribute of the nucleating agent
applies for both chemical and physical blowing agents.
FIG. 2 is an enlarged view showing a preferred embodiment of two physical blowing
agent ports on opposite top and bottom sides of the injection barrel
24.
In this preferred embodiment, port
42 is located in the gas injection section
of the screw at a region upstream from mixing section
48 of screw
22
(including highly-broken flights) by no more than about four full flights, and
preferably no more than about one full flight. Positioned as such, the injected
physical blowing agent is rapidly and evenly mixed into the fluid polymeric stream
to promote production of a single-phase solution of the foamed material precursor
and the physical blowing agent.
Physical blowing agent port
42 is a multi-hole port including a plurality
of orifices
50 connecting the blowing agent
44 source with the injection
barrel
24. Preferably, a plurality of ports
42 are provided about
the injection barrel
24 at various positions radially and longitudinally
aligned with each other. For example, a plurality of ports
42 are placed
at the 12 o'clock, 3 o'clock, 6 o'clock and 9 o'clock positions about the injection
barrel, each including multiple orifices
50. Where each orifice
50
is considered a physical blowing agent orifice, the present invention includes
an injection molding apparatus having at least about 10, preferably at least about
100, more preferably at least about 500, and more preferably still at least about
700 blowing agent orifices in fluid flow communication between the barrel
24
and the source
44 of the blowing agent.
Also, the orifice or orifices are adjacent full, unbroken flights
52.
As the screw rotates, each unbroken flight
52 periodically passes or "wipes"
each orifice
50. This wiping increases rapid mixing of the physical blowing
agent and the fluid foamed polymeric precursor by essentially rapidly opening and
then closing each orifice periodically blocked by the flight
52 in alignment
therewith. The result is a distribution of relatively finely-divided, isolated
regions of physical blowing agent in the fluid polymeric material immediately upon
injection and prior to any mixing. In this arrangement, at a standard screw revolution
speed of about 30 rpm, each orifice
50 is passed by a flight
52 at
a rate of at least about 0.5 passes per second to about two passes per second.
In preferred embodiments, orifices
50 are positioned at a distance of from
about 15 to about 30 barrel diameters from the beginning of the screw at its upstream
end, adjacent to drive motor
38.
Referring again to FIG. 1, the mixing section
48 of screw
22,
following the gas injection port
42, is constructed to mix the physical
blowing agent and polymer stream to promote formation of a single phase solution
of blowing agent and polymer, including the nucleating agent, if used. The mixing
section
48 includes unbroken flights
54 which break up the stream
to encourage mixing. Downstream from the mixing section, the metering section
32
builds pressure in the polymer-blowing agent stream prior to injection conduit
30 connected to mold
20.
As shown in FIG. 3, mold
20 includes mating first and second mold halves
56 and
58 forming a cavity, for example of the shape of the exemplary
flat top module
12, connected to injection conduit
30 through which
the polymer stream flows from the polymer processing space
28. In a broader
sense, however, mold
20 can have any variety of module configurations including
flat top modules, flush grid modules, raised rib modules and radius modules, as
is well known in the art. Mold
20 further has rods
20A and
20B
to provide the module with openings in its link ends. This will be described in
detail hereinafter.
The metering section
32 may also perform the function of nucleating the
polymer and blowing agent, whether it be of the chemical or the physical type,
into a single-phase solution. The pressure in the single phase solution drops as
the polymeric mixture flows through the metering section
32. This pressure
drop causes the solubility of the blowing agent in the polymer to decrease, which
is the driving force for the cell nucleation process. Typically, the metering section
32 is designed to provide a pressure drop suitable for cell nucleation in
accordance to microcellular foam requirements. Under processing conditions, the
pressure drop across the metering section
32 is generally greater than 1,000
psi, preferably greater than 2,000 psi, and more preferably greater than 3,000 psi.
In some embodiments, the metering section
32 is also configured, as known
in the art, to provide a pressure drop rate (dP/dt) as the single-phase solution
flows there through. Pressure drop rates also effect the cell nucleation process.
Typically, a sufficient pressure drop rate must be induced to achieve appropriate
nucleation conditions for microcellular polymeric materials. The presence of the
nucleating agent at an amount of from about 2.5% to about 7%, by weight, is believed
to lower the pressure drop rate required. In certain cases, it is desirable to
use a process that employs low pressure drop rates. Generally, lower pressure drop
rates allow for more freedom in mold construction and the resulting conveyor belt
module dimensions. In certain embodiments, the pressure drop rate in the solution
is less than 1.0 GPa/s, in some embodiments less than 0.10 GPa/s, and, in other
embodiments less than 0.05 GPa/s. In still other embodiments higher pressure drop
rates are utilized, for example greater than about 10.0 GPa/s.
As a result of elevated temperatures, the microcellular polymeric foam is typically
soft enough so that the nucleated cells grow. As the foam cools in the mold and
becomes more solid, however, cell growth is restricted. In certain embodiments,
it is advantageous to provide external cooling means such as cooling air or water
to speed the cooling rate of the foam.
Now that the injection molding system
10 has been described in detail,
the exemplary flat top module
12 and the exemplary radius module
14
of the present invention will be described. More particularly, the modules
12
and
14 are manufactured by the injection molding system
10 shown
in FIGS. 1 and 2 and are of a polymeric material comprising a microcellular foam.
The flat top module
12 includes a generally rectangular plate-like body
60 having a first plurality of link ends
62 and a second plurality
of link ends
64 extending in opposite directions therefrom. A transverse
rib
66 extends across the width of the underside of the body
60 to
form opposed channels
68 and
70 terminating at respective edges
72
and
74 from which the respective link ends
62 and
64 project.
The rib
66 and the inside of the link ends
62,
64 are adapted
to mate with corresponding sprocket teeth of a sprocket wheel (not shown) to impart
a driving force to a conveyor belt formed by the interconnected modules
12.
The under structure of the module
12 formed by the transverse rib
66
serves to strengthen the module and to prevent any significant binding of the module
10 about its longitudinal or transverse axes.
The link ends
62 and
64 circumscribe corresponding aligned cylindrically
shaped openings
76. The openings
76 receive pivot pins or rods (not
shown) adapted to pivotally connect a plurality of the modules
12 in an
end to end configuration while laterally aligning adjacent modules to form a modular
conveyor belt (not shown). Preferably, the modules
12 are of link end configuration
to be end-to-end reversible. In other words, either end of a module can mate with
either end of any other link module.
FIG. 6 shows another embodiment of a module, in this case a radius module
14,
comprised of a microcellular polymeric foam. The module
14 is referred to
as a radius module because, as will be described in detail below, it is adapted
for construction of conveyor belts that are capable of traveling around a radius
turn. The module
14 further has an intermediate section
78 supporting
a plurality of first link ends
80 and a plurality of second link ends
82.
The first link ends
80 are disposed in the direction of belt travel and
the plurality of second link ends
82 extend opposite the first link ends
80. The intermediate section
78 is comprised of an upper, transverse
stiffening web
84 forming into a lower corrugated portion
86 (only
partially shown in the drawing) having a sinusoidal shape. Along with the transverse
web
84 of the intermediate section
78, the ridges (not shown) of
the sinusoidal shape extending toward the right of FIG. 6 support the first link
ends
80 while the ridges (not shown) of the sinusoidal shape extending toward
the left in the drawing support the second link ends
82.
Module
14 further includes generally cylindrically-shaped pivot rod
openings
88 in link ends
80. Similarly, oblong slots
90 are
disposed through the link ends
82 transverse to the direction of belt travel.
With a plurality of modules
14 forming a conveyor belt, a pivot rod (not
shown) passes through the openings
88 in the first link ends
80 and
through the slots
90 in the second link ends
82. The pivot rod preferably
cannot move in the direction of belt travel inside the openings
88. However,
due to the oblong shape of slots
90 the pivot rod pivots inside of them.
This enables a conveyor belt constructed of a plurality of the modules
14
to travel around a radius turn by collapsing on one side while the other side fans
out due to the pivoting of the pivot rod in the oblong slots
90. For a more
detailed description of a radius module, reference is made to U.S. application
Ser. No. 09/579,090, filed May 25, 2000, which is assigned to the assignee of the
present invention, and incorporated herein by reference.
It is further contemplated by the scope of the present invention that the microcellular
polymeric foam material comprising the exemplary flat top module
12 and
the exemplary radius module
14 may include additives such as those that
enhance electrical conductivity (carbon black and graphite particle fillers), flame
retardants and pigments. These additives are preferably provided in the polymeric
material in a concentration of about 0.15% to about 10.5%, by weight.
While the present invention has been described with respect to an exemplary
flat top module
12 and a radius module
14, that is by way of example
only. Those skilled in the modular belt arts will readily recognize that the present
injection molding process can be used to manufacture a variety of modules including
flush grid modules, raised rib modules and flight modules as well as various accessories
for modular conveyor belts such as sprockets, pivot rods, side guards, finger boards,
and the like. In short, the injection molding process of the present invention
can be used to manufacture any component for a modular conveyor belt where it is
desired to have a microcellular polymeric foam comprising the component.
It is intended that the foregoing description be only illustrative of the present
invention and that the present invention be only limited by the hereinafter appended claims.
*
