This invention relates to the moulding of plastics material articles,
more particularly but not exclusively in the context of making one off or small
quantities of articles, such as prototypes for testing designs later to be mass
produced. Some plastics materials, particularly some polymers including polyamide,
e.g. nylon, are not easily moulded with fine detail, except with complex and expensive
control equipment. This may be inappropriate for the kind of prototype manufacturing
apparatus mentioned. Accordingly, an object of the invention is to provide apparatus
for moulding which can produce a good result in the context but which is relatively
US-A-5 529 212 and US-A-3 773 300 disclose apparatuses for moulding
of plastics material articles according to the preamble of claim 1.
EP-A-1 166 989 discloses a device having two pressurized chambers
for vacuum casting plastic parts.
According to the invention there is provided apparatus for making
a moulded thermoplastics material article, the apparatus comprising a plurality
of reservoirs for receiving respective polymer thermoplastics intermediate components,
heating means for melting said components in said reservoirs and maintaining them
heated; mixing means for mixing the molten contents of the respective reservoirs;
piston and cylinder means for transferring the reservoir contents out of the reservoirs,
support means for supporting a rigid or soft rubber moulding; and dispensing means
for dispensing said plastics material into a rigid or soft rubber mould supported
by said support means; characterised in that said apparatus comprises:
said piston and cylinder means comprising, for each reservoir, a cylinder connected
to the respective reservoir via a first valve and to said dispensing means via a
second valve, and a piston connected to common drive means for said pump means to
supply respective controlled synchronised amounts of the contents of the reservoirs.
- a first compartment containing said reservoirs, said heating means, said mixing
means and pump means;
- a second compartment sealed from said first compartment and containing said
support means and said dispensing means; and
- a vacuum pump and control means for maintaining the ambient pressure in the
second compartment less than the ambient pressure in the first-compartment;
Preferably, the vacuum pump and control means are operable for maintaining
the ambient pressure in each of the two compartments less than atmospheric pressure.
Preferably, the mould is a silicon or polyurethane rubber mould.
Preferably, said dispensing means comprises mixing means, for example
a static mixing tube, operable for mixing said plastics material before dispensing
it to said mould.
Advantageously, the apparatus comprises apparatus wherein said reservoirs
and said cylinders are formed in respective metal blocks, said heating means being
coupled to said blocks for heating the blocks.
For a better understanding of the invention and to show how the same
may be carried into effect, reference will now be made, by way of example, to the
accompanying drawings, in which:-
- Figure 1 is a front view of equipment for casting prototype items in engineering
- Figure 2 is a diagram for explaining the construction and operation of a mechanism
forming part of the Figure 1 equipment and operable for mixing and dispensing plastics
- Figures 3 and 4 are respective sectional views of part of the Figure 2 mechanism
on lines III-III and IV-IV of Figure 5;
- Figure 5 is a plan view of part of the Figure 2 mechanism; and
- Figure 6 is a diagram of parts of the Figure 1 apparatus.
The Figure 1 equipment comprises a cabinet 1 divided into four compartments
2, 3, 4 and 5. The compartments 2 and 4 lie above respective ones of the compartments
3 and 5. The compartments 2, 3 and 4 have doors 6 (shown open) while compartment
5, which contains electro-mechanical control items and electronic control and computing
apparatus (not shown), has a front panel supporting an emergency cut-off button
7 and a touch screen control panel 8.
The compartment 5 contains electrical machinery including a vacuum
pump 9 while the compartment 2 contains a mechanism 10 for mixing and dispensing
plastics material intermediates. The compartments 2 and 3 are divided one from another
by a sturdy floor plate 11 strong enough for the two compartments to remain sealed
one from the other despite a substantial difference in vacuum within the two. The
plate 11 could be made of aluminium or other suitable material. A valve 100 with
a motor drive 101 is mounted in the plate 11 to permit the pressure in compartments
2 and 3 to be equalised or maintained equal when required.
The intermediates comprise thermoplastic materials with or without
fillers. The intermediates are chosen to give the desired mechanical properties
of the material of the cast component, i.e. the hardness or Durometer reading and
related properties such as tensile modulus, resilience, plasticity, compression
resistance and elasticity. For example, the intermediates might comprise commercially
available materials such as Caprolactum along with the appropriate activators and
catalysts. These components are intended to produce a polyamide plastics material
with particular, mechanical properties, namely a relatively rigid material. The
activator and catalyst might be replaced by an alternative catalyst and activator
in specific proportions to give different properties, for example a relatively tougher
plastics material. Also, fibrous material or filler, e.g. glass fibre, may be added
to give a fibre reinforced plastics material cast component.
As shown in the diagrammatic view of Figure 2, the mechanism 10 comprises
two closable reservoirs 20 and 21 each with a lid 22. A gas-tight bearing 23 is
provided in the lid 22 of each reservoir to receive a shaft 24 carrying, inside
the reservoir, a mixer paddle 25. Outside the respective reservoir, each shaft 24
is coupled to a respective electric motor 26. The lids of the reservoirs also have
openings 27 into which there can be mounted respective funnels 28 (one is shown
in the opening 27 of reservoir 21) or respective screwed plugs 29 (one is shown
in the opening 27 of reservoir 22).
In use of the mechanism 10 the funnels 28 are fitted to enable the
reservoirs to be filled with the required amounts of the chosen intermediates. The
funnels 28 are then removed and replaced with the screwed plugs 29 to seal the reservoirs.
The reservoirs are connected via respective gas taps 30 mounted in
the lids 22 and flexible tubing 31 to a supply 32 of inert gas such as nitrogen
or argon for forming a gas blanket over the contents of the reservoirs when the
mechanism is in use.
A heater, here illustrated as two electrical resistance heating elements
33 and 34, coupled to a source of electrical power 35 and located beneath respective
ones of the reservoirs 20 and 21, is provided for heating the reservoirs with a
view to melting the plastics material intermediates to be mixed.
The mechanism 10 also comprises two cylinders 36 and 37 containing
respective pistons 38 and 39. The pistons 38 and 39 are connected via respective
piston rods 40 and 41 to an electrically motorised drive mechanism 42.
The mechanism 42 could be replaced by two separate mechanisms one
for each cylinder. In either case, the cylinders 36 and 37 could be alongside one
another rather than coaxially opposed.
Each of the cylinders 36 and 37 is coupled via two conduits 43 and
44 and a valve 45 to a port 46 in the floor of the corresponding one of the two
reservoirs 20 and 21. Each cylinder is also coupled via conduit 47 and a valve 48
to one of two conduits 49, one for each cylinder. The conduits 49 terminate near
each other at an exit fitment 51.
Heater elements 33 and 34 are preferably arranged or additional elements
are provided, so as to maintain and regulate the temperature, not only of the reservoirs,
but also the valves 45 and 48, the conduits 43, 44, 47 and 49, and the cylinders
36 and 37 so as to ensure the molten plastics materials are kept at the right temperature.
In operation, the two reservoirs 20 and 21 are opened and, using the
funnels 28, the appropriate intermediates are inserted. With the closure plugs 29
fitted and the gas blankets applied, the intermediates are then raised to the appropriate
temperature, melting them if they are in the solid phase, and mixed by energising
the mixer paddle drive motors 26. The mixed molten components are then drawn via
valves 45 and conduits 43 and 44 into the cylinders 36 and 37 by appropriate movement
of pistons 38 and 39 and subsequently expelled by the pistons 38 and 39 through
valves 48 and conduits 47 and 49 out to the exit fitment 51.
The two valves 45 are coupled to respective drive units 54 while the
two valves 48 are coupled to respective drive units 58. The drive units 54 and 58
are operated in conjunction with the drive 42 coupled to the pistons 38 and 39 to
move the contents of the reservoirs 20 and 21 to the cylinders 36 and 37 and then
to the conduits 49 as appropriate. As a safety feature, the valves may be operable
so that, if the mixer tube is blocked say, the contents of the cylinders are transferred
back to the reservoirs 20 and 21.
The conduits 49 extend at specific angles to the vertical (60° for
example) and the exit ends to the conduits are at a specific distance one from another
so that the contents of the reservoirs exit from the conduits 49 as respective streams
which impinge together and become mixed. If required, a static mixing tube 52, i.e.
a tube incorporating angled baffles 53, can be coupled to the fitment 51.
The tube 52 leads down to a mould 60 positioned in the compartment
3 below the mechanism 10. If the mixing tube is not used, a plain plastics tube
(not shown) may be used to conduct the molten plastics material down to the mould
60 or the impinging streams exiting from the conduits 49 and 50 may be lead into
a funnel 61 (illustrated in dashed lines to show that it is alternative to the tube
52) which leads down to the mould 60. This also enables the connection of widely
differing mould sizes and geometries and gating points to the material outlet.
As shown in Figures 3, 4 and 5, the reservoirs 20 and 21 may be formed
as through bores in a solid metal block 62, the block 62 being clamped by bolts
(not shown) to the top of another solid metal block 63. The lids 22 and motors 26
are not shown in Figures 3 and 4. In Figure 5, a lid 22 is shown fitted over the
top of reservoir 21 while reservoir 20 is shown open. The motors 26 and paddles
25 are also not shown in this figure. The bottoms of the through bores are closed
by block 63 with respective gaskets 64 therebetween. Respective side-by-side horizontal
bores are provided in block 63 to form the cylinders 36 and 37.
In Figures 3 and 4, the cylinder 37 is hidden behind cylinder 36 and
underneath reservoir 21. It is generally the same as cylinder 36.
The pistons 38 and 39 may be elongate so as to extend back through
the cylinders 36 and 37 to where they are coupled to the motor drive 42 (not shown
in Figures 3 to 5). The motor drive 42 is operable to move the pistons by different
distances dependent upon the quantities of the intermediates in the two reservoirs
and to be mixed. This is done by providing a motor drive with respective motors
50 coupled via a suitable drive mechanism to the two pistons or by a single motor
with a differential drive mechanism. In each case, the drive mechanism can comprise
a ball-screw drive and the or each motor can comprise a stepper motor. The valves
45 and 48 are provided with the conduits 43, 44, 47 and 49 in a further block of
metal 70 bolted to the end face of block 63 so as to cover the forward ends of the
cylinders 36 and 37, a gasket 71 being provided between the block 70 and each cylinder.
The drive units 54 and 58 are mounted above the valves 45 and 48, for example, by
being fixed via a metal spacer 73 to block 62. The drive units 54 and 58 are coupled
to respective cylindrical valve members 74 of the valves 45 and 48 by valve rods
75, preferably with a degree of flexibility in the rods to take up any misalignment
of the valve members 74 in the valve seats 76. The heating elements 33 and 34 can
be embodied as stainless steel heating cartridges (not shown) located in bores in
the various blocks 62 and/or 63. The object of providing the reservoirs, cylinders,
conduits and valves in relatively massive, bolted together blocks of metal is to
achieve even temperature and less differential expansion of the parts of the mechanism.
Referring back to Figure 1, the mould 60 is located in compartment
3 supported on a hydraulic or manual or electric lifting platform 80. The mould
60 is a silicon rubber mould, for example the kind of mould which is made by embedding
a prototype of an article to be moulded, or an existing item to be copied in liquid
silicon mould making material in a mould box and allowing the material to set. The
mould is then cut, for example, by hand using a sharp knife, to divide the mould
in two parts so that the prototype or original item can be removed. The two parts
of the mould are then placed together and secured, for example, by wrapping adhesive
tape around them. A mould entry aperture and overflow apertures are also provided,
either by post-forming or by providing suitable plugs in the mould box.
The mould 60 could be of polyurethane rubber or other suitable 'soft'
material. Alternatively it could be a rigid mould, for example made of cast epoxy,
sprayed or cast metal, or made by machining.
To carry out a moulding operation, the mould 60 is installed and the
reservoirs 20 and 21 filled with fluid thermoplastics. For example, the aforementioned
Caprolactum and activator can be placed in one reservoir while the catalyst is put
in the other. Then with the reservoirs 20 and 21 and the doors 6 closed, the equipment
is set-up using the control touch panel 8 to carry out the operation automatically.
As noted, this involves heating the contents of reservoirs 20 and 21 whilst those
contents are covered by a gaseous (e.g. nitrogen) blanket, mixing the plastics intermediates
and dispensing them to the mould.
It may be desirable in some cases to maintain a vacuum within the
compartment 2 but this is not always essential. However, for the described embodiment
of the invention, a differential is maintained between the vacuum pressure of the
compartments 2 and 3 at least while the molten plastics material is dispensed from
the mechanism 10 to the mould 60. By way of preferred example, the chamber 2 could
be maintained at a pressure of -0.9 Bar (i.e. 90% vacuum) while the compartment
3 is kept at a pressure of -1.0 Bar (100% vacuum). However, these values of differential
vacuum can be changed depending upon choice having regard to factors such as the
particular plastics material used and, and mould cavity geometries appertaining
Figure 6 shows a diagrammatic view of the chambers 2 and 3 of the
casting equipment and indicating various controls connected thereto. The pressure
equalising valve 100 and motor drive 101 is shown as are the mechanism 10 and mould
60 in the top and bottom chambers 2 and 3 respectively. Also, a pneumatic supply
102, i.e. an inlet supplying air at about 70 kPa (10 pounds per square inch) leads
to the drives 54 and 58 (not shown in Figure 6) within the mechanism 10 for controlling
the valves 45 and 48. The motor drive 101 could also be pneumatically driven and
receive this supply 102 if desired. Item 103 is a simple analogue pressure gauge
connected to the chamber 2 and useful during setting up of the equipment for checking
the readings of pressure supplied by pressure transducer 105 to the electronic control
and computing apparatus within chamber 5 (not shown in Figure 6). Valve 104 with
a drive unit (not shown) controlled manually or by the control apparatus enables
the vacuum build up in chamber 2 to be released.
A further pressure transducer 106 is coupled to chamber 3 to provide
pressure readings to the control apparatus and this chamber is also coupled to the
vacuum pump 9. The vacuum built up within the chamber 3 can be released via valve
107. Valve 107 can be relatively larger than the valve 104 to permit a relatively
"fast leak" of the chamber 3 vacuum and a "slow leak" of the chamber 2 vacuum.
A vacuum line (not shown) to the chamber 2 can be provided if required,
e.g. to facilitate operation with different levels of vacuum both below ambient
as described earlier.
In operation, the plastics intermediates may be melted and mixed whilst
both chambers are at ambient pressure and the molten mixture transferred to the
mould whilst the upper chamber 2 is at ambient pressure and the lower chamber 3
is under a vacuum, say at circa 27 kPa (200 mm of mercury). When the mould is filled,
e.g. evidenced by the plastics material having just started to emerge from appropriate
apertures in the mould, filling ceases and the valve 100 opened to equalise the
pressure in the chambers 2 and 3, say so that the pressure levels at circa 14 kPa
(100 mm of mercury). Later the valves 104 and 107 are opened to admit ambient air
to the chambers, i.e. to raise the pressure in the chambers to ambient and hence
permit the doors of the chambers to be opened.
The apparatus shown can be used both in a differential pressure mode,
i.e. with different levels of vacuum in the compartments 2 and 3 at the appropriate
times, or in a non-differential vacuum casting mode with the pressure in the two
compartments equalised equal to or below ambient throughout the casting operation.