Abstrict A method of constructing a desiccant pot by trapping a spun-bonded
low density membrane between two separate parts constituting either
the pot lid or the pot body, these two separate parts being made
of high density plastic, and then fusing the membrane and the separate
parts together, in which one of the separate parts has upon its
touching surface an energy directing protrusion, arranged such that
when the three components are assembled so that the two separate
body and lid parts are pressed together with the membrane between,
the energy directing protrusion extends from one touching surface
to the other to space them slightly apart.
Claims The invention claimed is:
1. A method of constructing a desiccant pot by trapping a low density
membrane of spun bonded plastics between opposed touching surfaces
of two separate parts constituting the pot lid and the pot body,
these two separate parts being made of high density plastic, and
then fusing the membrane and the separate parts together by ultrasonic
welding,wherein one of the separate parts is provided with a fine
protuberance in the form of one or more ridges or points extending
from its touching surface towards the touching surface of the other
separate part to space the touching surfaces slightly apart, and
pressing the two parts together with the membrane trapped between
the touching surfaces so that the membrane is compressed at the
point of contact with the protuberance thereby increasing its density
at the point such that, at the time of ultrasonic welding, the protuberance
acts as an energy directing protrusion, and the ultrasonic vibrations
pass from the protuberance on the one part through the membrane
at the point where it is compressed to increase its density to the
other part.
2. A method according to claim 1 in which, the energy directing
protrusion forms a continuous ridge around the periphery said part.
3. A method according to claim 1 in which, the energy directing
protrusion comprises a series of discrete protrusions arranged around
the periphery of said one part.
4. A method according to claim 1 in which the energy directing
protrusion projects from the touching surface a distance of between
0.5 and 0.7 mm.
5. A method according to claim 1 in which the point or edge of
the energy directing protrusion is as sharp as possible to minimise
the cross-sectional area of the tip of the protrusion.
6. A method according to claim 5 in which the body is a tubular
component and the energy directing protrusion is a sharp knife like
edge at the end of the tubular body component.
7. A method according to claim 6 in which the energy directing
protrusion is formed by moulding a chamfer to the outside edge of
the tube wall.
8. A method according to claim 7 in which the angle of chamfer
is of the order of 60.degree..
9. A method according to claim 1 in which the energy directing
protrusion is arranged such that it bears against a peripheral area
of the membrane.
10. A method according to claim 1 in which the separate parts
of the pot body and pot lid are of a high density polyethylene.
11. A method according to claim 1 in which the pot body has first
and second ends and each end is provided with a pot lid and a membrane
trapped between the pot lid and the adjacent end of the body.
Description CROSS-REFERENCE TO RELATED APPLICATION(S)
This application is the national stage of PCT Application No. PCT/GB03/01730
filed Apr. 22 2003 for "Method of Constructing a Desiccant
Container" by Desmond Charles Drummond and Andrew Robertson
Drummond, which in turn claims priority from British Application
No. 0209523.0 filed Apr. 26 2002 and British Application No. 0218637.7
filed Aug. 12 2002.
BACKGROUND OF THE INVENTION
This invention relates to desiccant stoppers for a wide range of
containers, and concerns in particular plastic stoppers, comprising
a breathable plastic membrane to absorb any free moisture (or odours
and the like) in the container.
Desiccant stoppers are used to control the moisture or odour vapour
levels of air, within a sealed container, such as a bottle, jar,
bag or box, and to control the closed atmosphere to the benefit
of sensitive products such as pharmaceuticals packaged within. They
are produced in a number of sizes and types relevant to the size
and nature of the container and the content to be protected. They
must be non-toxic, resistant to water, strong, sterile, and able
to provide a microbial barrier. They must also be dust-free, and
dust-proof.
The present invention relates to the type of desiccant stopper
described in WO 02/038465 which comprise a suitably-sized capsule,
rather like a small pot or jar, as the desiccant holder, and after
this has been filled with the chosen desiccant it is capped with
either a porous-type material wad (such as a thin disc of cardboard)
crimped into place, or capped with a moulded plastic lid with cast-in
perforations.
Dependent upon their end use, desiccant stoppers can be filled
with a wide variety of desiccant-material content. In the event
that they are required to control moisture, suitable absorbent materials
are silica-gel, or molecular sieve, while for the control of odours,
granulated carbon is used. Some devices can be used to control the
level of a specific gas within a sealed container where the gas
might have an ill effect upon the contents. One such gas is oxygen;
oxygenation can often lead to serious content deterioration. In
some instances, a mixture of several different materials, and types
of material, will be formulated and used, and there are a number
of proprietary brands of admixtures on the market.
Regardless of their content, these stoppers are generally referred
to as "desiccant stoppers"; their use is extensive within
Pharmaceutical, Veterinary, Foodstuffs, and Electronics goods packaging.
WO 02/038465 discloses a method for the production of such a stopper,
in which method a fibrous fabric sheet is held in place between
the two relevant parts--the stopper body and the lid--and these
two are then fused together and to the sheet so that on cooling
and solidifying they form a solid, integral plastic supporting frame
around the sheet fused thereinto. The term "fused" as
used herein means that the materials of the body and lid and the
materials of the sheet have become one integral body, as though
the entire stopper had been cast in a single piece. Thus, the body
and lid materials are not merely attached to either side of the
fibrous sheet, and do not even merely extend integrally through
the sheet, but are instead actually integral with--have become one
with--the sheet.
The fibrous fabric sheet is that spun-bonded breathable plastic
(Low Density Polyethylene, or LDPE) material known as TYVEK HBD
1059 and manufactured by Dupont
Spun-bonded materials made from LDPE or the like--referred to hereinafter
for convenience simply as "spun-bonded plastics"--are
immensely strong, and will not tear. They will bond with other plastic
parts of similar plastic specification providing temperature profiles
are strictly observed (being extremely thin, typically as little
as 0.15 mm thick, they are easily performance-impaired, even destroyed,
by the application of too much heat).
Spun-bonded plastics, and particularly the TYVEK types of material,
are most suitable to act as a permeable membrane for desiccant stoppers
due to the superb transfer through them of moisture and odour vapours.
The microporosity of the material controls dust emission, and the
high tear strength and puncture resistance is perfect for ensuring
the integrity of the final finished article. Another example of
such a spun-bonded plastics is that material available under the
name TEIJIN, and manufactured by Unisel (now amalgamated with Dupont).
The method might be said to be characterised by the manner in which
the membrane and body/lid parts are disposed to enable the required
sonic fusing to take place. A high density pot/lid plastic can most
reliably be fused with a low density membrane plastic provided that
the pot/lid make physical contact with each other, to transmit the
vibrations and make them melt, with the membrane retained in a recess
between the two such that it, too, melts--to become fused integrally
with the pot/lid--but only at its very outermost edge.
BRIEF SUMMARY OF THE INVENTION
According to the invention, there is provided a method of constructing
a desiccant pot by trapping a spun-bonded low density membrane between
two separate parts constituting either the pot lid or the pot body,
these two separate parts being made of high density plastic, and
then fusing the membrane and the separate parts together, in which
one of the separate parts has upon its touching surface an energy
directing protrusion, arranged such that when the three components
are assembled so that the two separate body and lid parts are pressed
together with the membrane between, the energy directing protrusion
extends from one touching surface to the other to space them slightly
apart.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A & 1B show in plan and in section a desiccant stopper.
FIGS. 2A & 2B show in section both final and exploded part
views of the stopper.
FIG. 3 is an enlarged part section of the desiccant stopper with
the membrane omitted for clarity.
FIG. 4 is an enlarged part section of the desiccant stopper illustrating
the membrane in place.
DETAILED DESCRIPTION
Thus in this method of ultrasonic welding, when joining two or
more High Density plastic parts, (end cap(s) and body) and a Low
Density plastic spun-bonded material membrane, the high density
plastic components are profiled, so that at least one of those components
has an energy director ridge(s) or point(s) designed into the component.
An energy director profile is a section of the moulding which is
situated at the intended point of an interface, when that interface
will marry with another separate interfacing component. It is a
fine protuberance, either a ridge(s) or point(s) standing higher
than the underlying land area from which it protrudes.
Thus the energy directing protrusion may form a continuous ridge
around the periphery of one of the parts or could comprise a series
of discrete protrusions arranged around the periphery of the part.
Preferably the distance the protrusion projects from the touching
surface is between 0.5 to 0.7 mm.
When the parts to be welded are placed together and are touching,
the height of the energy directing ridge(s) or point(s) separates
the two flat interface surface areas, which can only then initially
interface across the/those highest points.
The effect of the energy directing protrusion is to do exactly
as the term implies, to direct high intensity friction to an exact
position, designed as a sharp edge(s), or a sharp point(s). Preferably
the point or edge is designed to be as sharp as possible--with the
cross-sectional area of the tip of the protrusion being minimised.
Because that sharp edge(s) or sharp point(s) is the first touching
point of interface with the other component to which it is to be
joined, and whilst it is subjected to a holding pressure, the force
applied to hold the components together is concentrated at that
point(s), in consequence, all of the energy released at the moment
of ultrasonic vibration is also directed to that point(s).
The vibratory action applied, causes intense frictional heat at
those first interfacing points. This spreads deeply into secondary
interface areas now brought together by the collapse and dispersal
of the energy director profile(s). The areas of contact become molten
and fuse together. The result is a strong weld.
The stopper is pot-like--that is, it is in the shape of a small
container (perhaps 0.6 in [15 mm] across, and 0.8 in [20 mm] deep)
for holding in use the desiccant (or other) material contained by
the stopper. The stopper can be of any convenient cross-section,
but a tubular section is generally most suitable, fitting into most
containers of pills or the like.
In the case of this application, the energy director is a sharp
knife like edge, at the end(s) of the tubular body component.
By moulding a chamfer to the outside edge of the tube wall, a knife
like edge around the entire periphery of the end(s) of the capsule
body is formed to produce a suitable energy director. In this case
it is preferred that the angle of chamfer is of the order of 60.degree..
Preferably the energy directing protrusion is arranged such that
it bears against a peripheral area of the membrane.
The stopper ends up as a one-piece (integral) object, but for manufacturing
purposes it is formed from at least three pieces, namely a body
portion, a lid (or cap) portion that fits into the body portion,
and a membrane portion that is placed between and fuses to the body
and lid portions. The lid (or cap) portion is conveniently a simple
one-piece object that fits into the body portion, the membrane portion
being between and fused to the body and lid portions.
Of course, although the body portion may be formed with its other,
non-lid end, already closed, it is also possible to have the body
portion as an open-ended central ring portion, with a cap/lid at
each end.
The flat bottom surface of the body portion, and the flat top surface
of the (or each) lid/cap portion, provide the two end faces of the
stopper; one or both of these is made from the fibrous fabric sheet
of plastics material fused sealingly around its periphery to the
main/side parts--the wall portions--of the body or lid appropriately.
For the stopper a typical preferred material is a High density
Polyethylene (HDPE), and examples of this are that material known
as LADENE (an HDPE manufactured by SABIC Marketing Ltd. Saudi Arabia
and distributed in the UK by ALBI Ltd., of Knutsford. Cheshire),
and the material sold by BP Chemicals as "BP High Density Polyethylene"
(and distributed in the UK by Distropol Ltd., of Chertsey. Surrey).
Each of these grades of HDPE are FDA approved, and are also suitable
for near field and far field ultrasonic welding.
Alternatively high melt polypropylenes can be used.
For the membrane the preferred material is, as intimated above,
that known as Perfecseal HBD 1059B TYVEK, manufactured by Dupont.
This is a low density polyethylene. Dupont produce a range of materials
under the Trademark TYVEK, each of which have specific end uses.
Many of the products from this range are suitable for the purposes
which are here described. Another suitable material is that available
under the name TEIJIN, and manufactured by Unisel. The principal
purposes of these types of materials are as breathable fabric membranes
used to construct bags or sachets, or to cover plastic or foil tray-like
containers, to which they are fastened using conventional heat sealing
techniques. For best results a large area of contact is required
between the two materials which are to be joined.
At the time of assembly of the components, and when they are correctly
positioned, each to each other, the positions are as described:
a) The Tyvek membrane is placed into the end cap, where it is retained
being an interference fit. b) The capsule body is then placed into
the open end of the end cap(s) and pressed into place. c) The assembled
components are then as a sandwich with the Low Density plastic spun-bonded
membrane trapped firmly between the two High Density plastic components,
(Being the end cap(s) and the body of the capsule.) The energy director
profile is in direct contact with the spun-bonded membrane material,
and is held in position under pressure.
At the time the ultrasonic welding process is applied, the parts
are forced together by the actuator, which applies a holding pressure
before the ultrasonic vibratory welding period begins.
The sharp knife-like edge of the High Density tubular body of the
capsule, is forced against the Low Density spun-bonded membrane,
situated and supported in the High Density end cap(s). The Low Density
spun-bonded membrane, typically only 0.15 mm thick, compresses at
the area of the point of contact with the High Density energy director
profile, closing the porous nature of the membrane material, substantially
increasing its density at that point. When the ultrasonic vibrations
are applied, the energy director leading edges are the first points
of interface to be agitated, and are subsequently the first areas
of interface to melt and fuse together.
The friction induced melt, progresses through the extremely thin,
(lower temperature affected) spun-bonded material, to the high density
plastic (higher melt temperature) component on the other side of
the membrane, spreads and fuses, becoming an integrated mass, which
when cooled forms a strong weld, fusing the end cap(s), membrane
and capsule body, into a solid single part.
This method relies upon the technique of sonic--that is to say,
"ultrasonic"--welding of thermoplastic parts to fuse the
body and lid parts together, embedding the membrane therewithin.
This technique is now described in more detail.
The principle of ultrasonic assembly involves the use of high-frequency
mechanical vibrations transmitted through thermoplastic parts to
generate a frictional heat build-up at an interface. The effect
of the vibrations causes intense friction between separate but touching
parts, causing the materials to heat and melt and weld/fuse together.
This vibrational movement is effected by a vibrating component
called a "sonotrode", which is applied at right angles
to the surface of a part to be welded. The latter starts to vibrate
throughout due to a series of stationary waves, with a maximum amplitude
in the area of contact of the two parts to be joined. Ultrasonic
plastic welding techniques, have been successfully used for more
than 25 years by engineering companies within the plastics industry.
After cooling, which is rapid, a solid homogeneous weld results
between the parts of the assembly. The membrane is thus integral
with the solid plastic of the body and lid parts, and so is inseparable
therefrom. It thus provides an impermeable barrier to the passage
of microparticulate matter, preventing escape of the chemical ingredients
of the capsule, and so making the stopper unit both dust-free and
dust-proof. The membrane is presented as a window to the stopper,
allowing unimpeded passage of moisture or odour vapours, in either
direction, through the microporous material.
One important factor relevant to the use of sonic welding for this
purpose is the compatibility of the body/lid material and the membrane
material. The preferred choice for the spun-bonded plastics membrane
material is TYVEK, which is a Low Density Polyethylene (LDPE) product.
It might then be thought that a low density material, and specifically
a low density polyethylene, should also be used for the body/lid
material. However, it transpires that low density materials such
as this are in fact rather difficult to weld sonically. The reason
seems to be that they are too "soft", and vibrations generated
in one part tend to get damped out too easily, and are not transmitted
through to the other part, so that the two do not move against each
other in a manner sufficient to produce fraction and heat--and without
that heat, of course, they do not melt, and so do not weld/fuse
together.
Another crucial factor is temperature. Spun-bonded plastics such
as TYVEK are extremely thin, and are prone to the effect of high
temperatures which causes them to shrivel. Exposure to temperature
in excess of 125.degree. C. renders TYVEK useless, as the matrix
which provides the permeability melts, collapses, and congeals into
an impermeable structure. The melt point at which HDPE becomes molten,
is approximately 150.degree. C.; this temperature is higher than
the destruction point temperature of TYVEK, so it might be thought
that fusing a low density membrane to a high density stopper body/lid
would be impossible. However, while merely fusing a sheet of an
LDPE material such as TYVEK to a body of an HDPE material is indeed
fraught with problems, these seem to be banished in the case where
the low density sheet is trapped around its periphery between two
high density surfaces; surprisingly, placing the low density membrane
between two high density parts in the form of a sandwich does in
fact produce a reliable fusion of the components. Accordingly, the
use of the energy directing protrusion reduces this problem of high
temperature upon the former, as the melt temperature of the high
density parts is contained local to the fusion point of the two,
leaving the greater expanse of the low density membrane completely
unaffected.
The success of this technique depends entirely on the ability of
the materials to propagate vibrations without damping them; excellent
results can be obtained with suitable thermoplastic rigid materials
with a high modulus of elasticity. The method permits the welding
of objects of very complex design with a sonotrode which is very
simple in form.
As referred to hereinbefore, the stiffness of the polymer to be
welded will influence its ability to transmit the ultrasonic energy
to the joint interface. Generally the stiffer a material the better
its transmission capability. It is usually not possible to weld
materials of different types by ultrasonics, due to the differences
in fusion temperature. If the macromolecular structure is not the
same for both materials, it will prevent interpenetration.
During the period of time that the ultrasonic vibration welding
method is applied, a degree of pressure must be exerted to hold
the components together. Each high density component must be in
contact with the other in order that the ultrasonic vibrations can
be transmitted through the component nearest the sonotrode. Moreover,
each component should be designed to provide a sufficient surface
area to be in firm contact with an equal surface area on the part
to which it is to be welded. The fusion takes place at the interface
of the two surfaces. Each component is held together by applied
pressure at the point(s) at which they become welded.
As the ultrasonic vibration is applied, and with the simultaneous
application of pressure holding the combination together, the ultrasonic
vibration passes through one of the high density parts and on into
the second, causing friction effects at their interface. The friction
produces heat sufficient to melt the plastic--both of the body/lid
(the two contacting high density components) and the membrane (the
low density component), allowing the plastics to flow, and by the
pressure being exerted upon the two parts forcing them together,
fuses all three into an inseparable bond. Thus, the membrane is
securely fused to the container, being embedded at all of its outer
edges, and at any interface with the capsule body and/or the retaining
end cap(s). The edges of the membrane and some of the surface membrane
material close to the edges is penetrated by the flowing plastic,
over moulded and through moulded by the molten plastic whilst liquid
and under pressure. The molten plastic of the body end cap(s) fuses
to the spun filaments of the fibrous spun-bonded plastics material
which is the membrane, and as it cools solidifies and forms a solid
plastic frame around and through the membrane. The spun-bonded plastics
material itself is also part welded in the operation, as some degree
of sonic vibration is effective between the low density membrane
and the high density capsule body; this further enhances the fusion
bond between the body and lid high density components.
The following points should be borne in mind when using sonic welding.
1. The cap/lid is to be welded to the body, and while this could
be a butt weld it is preferred to chamfer each abutting face in
a matching manner, to form a larger weld surface. Specifically,
the edge of the side wall of the cap(s) is moulded to a form recommended
as a correct interface profile for ultrasonic welding. 2. The edge
of the side wall of the body is correspondingly moulded to a form
recommended as a correct interface profile for ultrasonic welding,
but also incorporates a section which, when the two plastic components
(body & cap) are placed together with the TYVEK type material
also in place, acts as a snap fit to temporarily secure the components
together, with the underside of the cap in close proximity with
the uppermost side of the top edge of the inserted profiled wall
of the plastic body. 3. When a membrane window is required at both
ends of the stopper, the process described is repeated at the opposite
end of the container, which is moulded to suit. 4. It is normally
most convenient to assemble the stopper one end at a time, in an
upright position, with the end cap placed on top at the time of
assembly and ultrasonic welding. 5. Once correctly positioned, with
the membrane held therebetween, the body/lid mouldings are ultrasonically
welded together to form an integrally-joined capsule. The or each
porous membrane is encapsulated within the previously separate components,
held in place by the weld between the body and the relevant cap
or end.
In this way the fabric sheet--the spun-bonded plastics TYVEK-type
material--is fused around its periphery to the material forming
the stopper body/lid combination. The membrane is thus presented
as a window to the stopper container's body or lid portion, and
thus in use allows unimpeded ingress by moisture or odour vapours.
Moreover, reinforced as it is by the plastic frame in which it is
totally suspended, the membrane acts as a structural form securing
the contents of the stopper from loss or damage.
As can be inferred from what has been said above, there is a choice
of spun-bonded plastics material membrane at one or more positions
on the desiccant stopper. Typically the position of a single membrane
could be at the end of a stopper, whilst a stopper with two membranes
could have them situated one at either end (the purpose of two membranes
would be to allow a faster ingress of vapours).
And as also noted above, in the case of a stopper with one membrane
only, it will be seen that there is a requirement for two parts.
One is the body portion--the receptacle into which the desiccant
is placed, while the other is the lid portion. Either may carry
the membrane as its end wall, but usually it is more convenient
to use the lid for this.
When two membranes are required within a single desiccant stopper
the unit can be constructed in a variety of ways. One preferred
way is to manufacture the unit in three or more parts, comprising
two separate cap/lid-like end parts and one (or more) central body
part open at both ends. The end parts--each identical in manufacture--carry
the membrane as a tight fit, and each is welded separately to the
central body part to form the complete container (into which the
fill content is placed before attachment of the second "lid"
end to complete the structure).
One further advantage of the described method of assembly is the
ability to pre-prepare the components ready for use. The spun-bonded
plastics membrane discs can be pre-cut and inserted into the cap
ends where they are held fast being an interference fit, before
being fitted to the container body. Similarly the body of the capsule
can be pre-assembled to one cap end with the spun-bonded plastics
membrane already inserted, and sonic welded together, as a pre-form
produced for the filling machinery.
Where the several parts of the stopper--the body and one or more
end cap/lid--are manufactured separately (and then joined together)
it is of course possible to give them different colours. This may
be used, if wished, for identification purposes--to indicate, perhaps,
either what is inside the stopper (what desiccant is used) or what
the stopper is to be employed with (what materials or articles it
can be utilised to keep dry, say).
The area of membrane left exposed as a window of permeable material
to the container, is not affected in any detrimental way, and its
efficacy is not impaired. The design allows ingress or egress of
gaseous vapours through the membrane, to or from the encapsulated
chemical materials contained within the capsule, but prevents leakage
of the contents, due to the high integrity and reliability of the
seal, caused by the fusion of each of the components to become one
integral part.
Ultrasonic Welding
The Process
The process of ultrasonic welding as here applied utilises the
ability of thermoplastic resin to form a welded joint at the interface
of two parts when, by generating heat at the interface by applying
directed mechanical vibratory energy, the plastic melts, and becomes
fluid. Once the resin is molten at the joint, it flows at the point(s)
of contact, and if a force is applied during the time of the heat
generation the parts are pressed and fused together to form an inseparable
homogeneous link. This technique is fast, efficient, non-contaminating,
and does not require consumables.
Ultrasonic assembly systems are available in several levels of
frequency, and can be controlled in terms of time, energy, energy
compensation, distance and force applied. Suitable frequencies for
a system capable of the type of assembly required in this invention
are around 20 kHz, rated at between 1100 and 3300 Watts.
The Equipment
Branson Applied Technologies Group, of Hayes, Middlesex manufacture
a wide range of ultrasonic vibratory welding machines. Ultrasonic
Engineering Ltd, of Singapore Road, London manufacture a specialist
range of ultrasonic vibratory welding machines.
A typical ultrasonic vibratory welding system consists of four
essential components. 1) The power supply--electronically converts
the 50 Hz mains supply to the required 20 kHz operating frequency.
2) The converter--also driven from the power supply--consists of
a number of polarised piezoelectric discs which have a natural resonance
to 20 kHz. When subjected to the ultrasonic frequency output from
the power supply, the discs increase, and then decrease their physical
size, in the same time frequency as the power supply, in other words
at a speed correspondent to 20 kHz. In this way vibrations are caused
by the electrical signal being converted into mechanical energy,
and they are passed directly to the horn/(sonotrode), which is then
brought into contact with the work piece for a determined time and
set pressure, to produce intense friction and a subsequent weld
in a very efficient and controlled way. 3) Acoustic tooling--used
to couple, and boost energy from the converter to the parts to be
welded. The tooling is manufactured from either titanium or aluminium
alloy and is tuned to resonate at the same frequency as the converter.
The amplitude, or vibration movements, from the converter alone
is often insufficient to generate a weld, so acoustic tooling, or
horns/(sonotrodes) as they are called, are installed to amplify
the amplitude to a corrected value. In order to obtain the correct
resonant frequency, and a magnification of amplitude to the correct
level, to provide and transfer uniform movement to the working face
of the horn/(sonotrode), great care is necessary to determine the
correct design of shape and mass of the tooling, and the material
from which it is manufactured.
Acoustic tooling is made from two parts: a booster horn which modifies
the amplitude from the converter to the correct value for the specific
application; and the work horn/(sonotrode) designed for the job.
The work horn/(sonotrode) is configured to suit the geometry of
the parts to be welded.
The ideal situation is to provide a flat face for the work horn/(sonotrode)
to channel energy into the parts.
Operating the Method
Vibratory energy under pressure is passed into the work piece by
the acoustic tool or horn which is brought into contact with one
of the parts to be welded. Vibratory energy is passed through the
work pieces resulting in localised frictional heating at the joint
interface surfaces, causing the plastic to melt. On termination
of the energy input, the plastic solidifies forming a high strength
molecular bond. The weld is achieved in a time of typically less
than one second, the heat generated is local to the joint area.
4) The actuator--a simple, pneumatically controlled device which
brings the acoustic tooling into contact with the work piece. It
must be of rigid construction, able to consistently present the
welding horn/(sonotrode) accurately onto the work piece, and maintain
a set pressure for the duration of the weld time.
Usually it is mounted on an compressed air driven cylinder, which
has controlled movement up and down, or through other single axis
directions.
Efficiency
Two factors which establish a thermoplastic resin's ability to
weld, are how well it transmits energy from the welding horn to
the joint interface, and how good is its coefficient of friction
and melt temperature.
For ultrasonic welding purposes, resins are divided into two major
groups, Amorphous or Crystalline. These groupings refer to the molecular
structure of the plastic. In the solid state, amorphous plastics
have no ordered structure. This enhances the materials ability to
transmit energy.
Amorphous materials do not have a precise melt point. With heat
they become soft before they then become a viscous liquid. This
progressive melt to molten state aids the welding process by allowing
at the point of interface, fusion of the liquid plastic. The process
is easy to control.
Far and Near Field Welding
Clearly distance to the joint line from the horn land area is a
critical consideration. To assist with defining a resin's ability
to transmit ultrasonics the terms "near field" and "far
field" have been adopted for defining distance of energy transmission.
These terms are generally applicable to 20 kHz operation.
Near field welding is defined as the distance between horn/(sonotrode)
and joint interface if less than 6 mm. Reasonable energy transmission
in rigid amorphous materials can be obtained over this distance.
Far field welding is defined as the distance between horn/(sonotrode)
and joint interface when greater than 6 mm.
Other Factors
Two other factors of consideration are component rigidity and the
energy route to the joint or interface.
The actuator will apply a pressure of between 20 and 80 Newtons
to the parts to be welded at the time of the welding process. The
assembly must be designed to withstand such pressure during the
operation. The application of a set measure of pressure is important
to the mechanical action of the Horn/(sonotrode) at the time that
it is in contact with the plastic component(s), and it must be progressive,
as the plastic material when molten partially compresses whilst
under pressure from the actuator.
Good component design is crucial for successful welding. In order
for the process to work, energy has to be transmitted through the
components one to the other to promote a good weld, the strength
of which is determined by the depth of penetration. A weld depth
of 0.5 1.0 mm is considered sufficiently strong for the application
described.
To ensure good repeatability of weld, the settings of the equipment
should be observed and committed to record. Ultrasonic equipment
can be integrated into automated systems. Any information relevant
to weld parameters, such as time, amplitude, frequency, energy,
or distances, if available digitally from the welder equipment,
can be extracted, recorded, and stored in a database.
A desiccant stopper, in accordance with the invention, will now
be described, though by way of illustration only, with reference
to the accompanying diagrammatic Drawings (the Drawings are based
on cylindrical and circular designs, but other shapes are also suitable
for moulding).
FIGS. 1A & B are almost self-explanatory. They show a desiccant
stopper (generally 11) of the invention. The stopper is tubular,
and of circular section, and thus is like a small pot.
The version of stopper shown here has a tubular main body portion
(12) and two annular lid portions (13), one at each end. Each lid
13 fits on to the body using mating recesses (14a in the lid, 14b
in the body, as shown). The top (as viewed) surface of the lid 13
is a "window" (15) made of a porous fibrous fabric sheet
material membrane; this is shown in FIG. 1B with diagrammatic perforations,
to indicate that it is a porous sheet, but in fact it has no visible
perforations at all. All around its edge (16) the sheet is trapped
between, and integrally fused to, the lid and the body.
In the embodiment of FIG. 1 the membrane 15 is located between
the body 12 and the lid 13. This area is shown only generally in
FIGS. 1 and 2 with the specific form shown in detail in FIGS. 3
and 4.
FIG. 2 illustrates generally the steps in manufacture and depicts
the snap fitting 17/18 between the mating body 12 and lid 13.
Both the pot lid 13 and a pot body 12 are of high density plastic.
A spun-bonded low density membrane 15 is trapped between the lid
13 and body 12. After trapping the membrane 15 in position the membrane
15 and the separate parts 12 13 are fused together
One of the separate parts 12 13 (in this case the pot body 12)
has upon its touching surface 20 an energy directing protrusion
21 arranged such that when the three components are assembled so
that the two separate body and lid parts are pressed together with
the membrane between, the energy directing protrusion 21 extends
from one touching surface 20 to the other 22 to space them slightly
apart.
The protrusion 21 comprises a circular ridge with a sharp edge
formed by a 60.degree. chamfer 24 on the outside edge of the wall
of the pot body 12 around the periphery. The pot ridge 21 extends
between 0.5 and 0.7 mm from the touching surface 20.
At the time of assembly of the components, and when they are correctly
positioned, each to each other, the positions are as described:
a) The Tyvek membrane 15 is placed into the end cap 13 where it
is retained being an interference fit. b) The capsule body 12 is
then placed into the open end of the end cap(s) 13 and pressed into
place. c) The assembled components are then as a sandwich with the
Low Density plastic spun-bonded membrane 15 trapped firmly between
the two High Density plastic components, (Being the end cap(s) and
the body of the capsule.) The energy director profile 21 is in direct
contact with the spun-bonded membrane material 15 and is held in
position under pressure.
At the time the ultrasonic welding process is applied, the parts
are forced together by the actuator, which applies a holding pressure
before the ultrasonic vibratory welding period begins.
The sharp knife-like edge 21 of the High Density tubular body 12
of the capsule, is forced against the Low Density spun-bonded membrane
15 situated and supported in the High Density end cap(s) 13. The
Low Density spun-bonded membrane 15 typically only 0.15 mm thick,
compresses at the area of the point of contact with the High Density
energy director profile 21 closing the porous nature of the membrane
material, substantially increasing its density at that point. When
the ultrasonic vibrations are applied, the energy director leading
edges are the first points of interface to be agitated, and are
subsequently the first areas of interface to melt and fuse together.
The friction induced melt, progresses through the extremely thin,
(lower temperature affected) spun-bonded material, to the high density
plastic (higher melt temperature) component on the other side of
the membrane, spreads and fuses, becoming an integrated mass, which
when cooled forms a strong weld, fusing the end cap(s), membrane
and capsule body, into a solid single part.
The assembly process of the desiccant pot is divided into six stages.
For the purpose of explanation, each stage is described as a single
action, but in production, the action will be constantly repeated.
It is implied that at the completion of each single action, at all
stages, the process is repeated by command of an electronic instruction.
All stages are simultaneously active, synchronised to repeat at
the same time.
Stage 1) Plastic Injection Moulding of the Plastic Component Parts
The body of the capsule, and the end cap(s) are produced using modern
injection moulding techniques, from High Density Polyethylene with
a melt index of 30. Multi cavity moulds are employed to produce
high quality mouldings of each of the components required, from
moulds incorporating hot runner systems. This method has the effect
of producing cosmetically pleasing mouldings, to satisfactory dimensional
tolerances, with very little scrap or reject rate, due to the absence
of sprue and runner. Use of a high melt material assists in the
moulding cycle time, particularly in the manufacture of thin wall,
or fine detail items. The mouldings can be produced to colour of
choice. It is known that High Density Polyethylene materials which
have a high melt index figure are also likely to be stiffer after
moulding than lower melt index materials made from the same material.
The effect of the stiffness factor in the finished components assists
the ultrasonic welding processes that follow. High Density plastic
materials respond well to ultrasonic welding processes, but weld
performance is enhanced when the stiffness of the plastic material
contributes to an improved flow path for the sonic vibrations to
the component(s) interface, which is the required point of fusion/weld.
Both that material known as LADENE and the material sold by BP Chemicals
as "BP High Density Polyethylene" are satisfactory.
Stage 2) Preparing the Spun-Bonded Breathable LDPE Membrane The
spun-bonded breathable LDPE membrane was made from TYVEK HBD 1059.
Tyvek is extremely strong, and is exceedingly difficult to cut repeatedly,
without the need to constantly re-sharpen the punch tooling. A better
and totally consistent method, is to use Laser cutting techniques
which do not rely on sharp edge forms to cut. Using a laser--typically
a Carbon Dioxide Laser--to cut material is in general well known,
and needs no detailed comment here. It is, however, well suited
to adaptation as a device for marking and cutting plastic materials,
and is particularly efficient and economic with thin film-like materials.
A suitable laser cutter is available from Alltec UK Ltd., of Maltby,
South Yorkshire, while Laserex/Hi-Tech UK Ltd., of Rotherham, South
Yorkshire, manufacture a range of ancillary equipment essential
to Laser operation. Tyvek type material can be procured in reel
form, and can be slit to any width required. Tyvek is very inclined
to static interference during handling, and discs pre-cut and stored
together, are prone to collect in difficult to separate masses.
Precautions are recommended to be taken to prevent the problems
that this state causes. The manufacturers recommend a number of
solutions to the problem in their literature. One preferred method
of control of the Tyvek discs whilst being cut to size from the
reel web, and to avoid the problems of static interference (the
discs stick together), is to hold the Tyvek material prior to the
time of the Laser cut with a vacuum-operated suction cup, and then
to Laser-cut around the suction pad, to the disc size required,
the Laser cut to be completed whilst the Tyvek is held firmly in
place by the suction pad. The advantage of this method of handling
is that first of all, the Tyvek disc is under control as a single
item, and is not allowed contact with other discs, so avoiding the
static which is caused when discs in multiples are in contact with
each other. Secondly, the cut disc, firmly held, can be positioned
very precisely into the presented end cap of the capsule, at the
following stage of the operation. Thus: this method of operation
is conducted by passing the Tyvek web of material from the primary
reel, over the suction cup section of a pick-and-place pneumatically-operated
dextrous robot arm, which upon command will move up and under the
Tyvek web, and will fasten itself by suction onto the underside
of the Tyvek material prior to the Laser cut. The Laser cutting
equipment is then situated over the Tyvek web, and cuts in a downward
motion around the vacuum suction cup, which at completion is left
holding the separated disc. The robot can now remove the disc from
the path of the overhead reel by pulling it downwards, and because
of its dextrous capability can then place it into the end cap, at
an adjacent point, before returning to repeat the process. The method
can be reversed. After the previous Laser cut, the Primary reel
of Tyvek indexes forward on command, to present a new area of uncut
web to the suction cup section of the pick and place robot arm,
and the expended perforated section of the reel of Tyvek is wound
onto a Secondary reel, as scrap. The process then repeats. It will
be recognised that a bank of suction pads can be arranged to accommodate
a number of discs to be cut either singly by the computerised Laser
beam moving from one location to the next, or by a number of Laser
beams working simultaneously.
Stage 3A Emplacing the Membrane in the Lid The end caps are presented
to an adjacent point of access for the robot arm, in an open, end-up
position, delivered to that point via a vibratory bowl feeder and
collation device, which sorts the components into a continuous stream
of right-way-up components ready to receive the Tyvek disc. The
disc is pressed into the end cap, and is retained by an interference
fit between the outer edge of the disc and the inner dimension of
the outer wall of the end cap which is raised around its circumference
for that purpose. The end cap together with the inserted Tyvek membrane
is now conveyed to the next stage. A second stream of end caps,
together with inserted Tyvek membrane discs is conveyed to Stage
5 to cap the other end of the capsule body, and so form the finished
product. The caps in this second stream of end caps are inverted
on route to enable presentation to the capsule by the robot arm
in the correct attitude. The Tyvek membrane is retained in place,
being an interference fit.
Stage 3) First Assembly of Plastic Components The end cap is now
part assembled with the inserted Tyvek membrane disc, and is in
the correct attitude of open end/right way up. Tubular bodies, identical
at each end, are delivered in a continuous stream from a vibratory
bowl feeder and collation device, and they are presented to an adjacent
point of access for the dextrous robot arm, to pick and place. Each
tubular body is picked up by the robot arm, and is placed precisely
into the open side of the end cap, where it is pressed into place,
held in that position by the snap fit design, trapping the Tyvek
membrane therein firmly between the (bottom) end cap and the tubular
body. The part-assembled capsule, with the bottom Tyvek membrane
held firmly in place by the snap-on bottom end cap, is now conveyed
to the next stage.
Stage 4) Filling the Capsule At this stage, capsules are correctly
positioned for the filling process, being in an upright position
with the open end at the top. The empty capsules are passed beneath
a volumetric portion filling head, and the correct amount and type
of content is discharged from the filling head into the capsule.
The filled part-assembled capsule is now conveyed to the next stage.
Stage 5) Second Assembly of Plastic Components The "other
end" caps complete with inserted Tyvek membrane disc are presented
to an adjacent point of access for the dextrous robot arm, in an
inverted ready to pick position, delivered to that point by conveyor
from the stage 2 operation. The Tyvek is retained by an interference
fit. The inverted cap end is picked up by the robot arm, and is
placed precisely onto the open top of the filled capsule body, where
it is pressed into place, held in that position by the snap fit
design, trapping the Tyvek membrane therein firmly between the cap
and the tubular body. The completely assembled capsule is now prepared
for ultrasonic welding.
Stage 6) Ultrasonic Welding the Desiccant Stopper Together The
completely assembled capsule is presented to a position directly
beneath the Actuator. The Actuator is set for distance of downward
travel, and also set for the degree of force to be applied during
the ultrasonic vibration period. It is also set for the length of
time of contact with the work-piece. The Converter is set for the
degree of required Amplitude and the length of time of energising
the Horn/(sonotrode). The welding process is commenced with an electronic
command from the controller; the duration of the entire operation
is less than two seconds. It will be understood that more than one
capsule can be welded at a time. Dependent upon the type of ultrasonic
vibratory equipment employed, and the physical size of the components,
more than one weld can be executed on the same work-piece at the
same time by tuning the Converter and the Acoustic Tooling to accommodate
both near field and far field distances of energy transmission.
Success is conditional on component design, size, and the suitability
of the materials being used. |