Note: Descriptions are shown in the official language in which they were submitted.
2~
MAIN-rAINING PRESS PLATENS IN PARALLEL RELATIONSHrP
FIELD OF THE INVENl'ION
The invention relates generally to presses, and more particularly,
to maintaining press platens in parallel relationship during compression of a
5 workp;ece.
BACKGR(:)IJN~ OF ~IE INVENIION
Hydraulic presses are now used extensively for compression
rnolding of large and comparatively thin plastic products. Mold halves are fixedto upper and lower platens. The upper platen may typically be displaced under
10 very substantial driving forces to compress a molding charge. Very strict
control of platen parallelism is required to produce a satisfactory product. Theplanar platen surfaces that actually engage the molds may be required to deviatefrom a parallel relationship by no more than .001-.005 inches per foot. A basic
understanding of press parallelism control rnay be obtained by reference to U.S.l~ patent No. 4,076,780 which issued on February 28, 1978 to Ditto.
Many old hydraulic presses have a stationary lower platen and a
movable upper platen that is displaced by a large overhead hydraulic ram. The
guiding structures used to direct movement of the upper platen are generally
inadequate to maintain the strict platen parallelism required for contemporary
~0 compression molding. To adapt such presses for such strict parallelism control,
hydraulic leveling cylinders may typically be mounted at each corner on the
lower platen. These act against the upper platen and apply forces countervailingthe action to the central rarn. Alternatively, the leveling cylinders may be ~Ixed
to the upper platen and engageable with the lower platen. Each leveling cylindermay be associated with a platen spacing sensor that indicates the relative spacing
of the platens proximate to the leveling cylinder. Such sensors may comprise,
for example, a magnetic track that displaces with the upper platen and a
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complementaTy sensor head that is fixed to the stationary lower platen. In an
aIrangement described in U.S. patent No. 4,828,474 to Ballantyne, the platen
spacing sensors are actually built into the leveling cylinders. A microprocessorwill typically control the leveling cylinders in response to the spacing signals to
maintain a common spacing between the platens adjacent to each cylinder and,
assuming limited overall deformation of the platens, a parallel relationship. The
common spacing value is changed as the press proceeds through preliminary
compression and curing phases of operation.
A very significant problern in such prior presses appears to have
10 gone unnoticed. The inventor has initiated finite element analysis to assess
overall platen deformation in typical platens during c.ompression phases of
operation. Such analysis has revealed very significant local relative de-flection
of the platens in response to operation of the leveling cylinders themselves. The
amount of such deflection can approach the alignment tolerances that must be
1~ maintained in parallelism control and varies with the force exerted by each
cylinder. Since the spacing sensors identify such local relative deflection as part
of the spacing between the platens, the spacing signals produced by the sensors
do not reflect the actual alignment of the platens in the critical regions proxima~e
to the mold. Instead of maintaining parallelism in such critical regions, the
~0 control system may actually induce a significantly non-parallel relationship at
the mold itself. Such a problem may arise in more contemporary presses,
depending on their exact configuration. The present invention addresses this
problem.
BRI~F SUMMARY OP THE INVENT10~
2~ In one aspect, the invention provides a method of maintairling
platen parallelism while a molding charge is being compressed. The method is
applicable to a press comprising a pair of platens, a mold located substantiallycentrally between the platens, means for applying a driving force to one of the
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platens ~o displace it towards the other platen thereby to compress a molding
charge within the mold, and a plurality of leveling cylinders spaced from ~e
mold and acting between the platens to apply forces that vary with the operatingpressures of the cylinders to countervail the driving force. The method involves5 providing data identifying for each leveling cylinder the local relative deflection
of the platens expected in response to operation of the leveling cylinder at
various operating pressures (while the molding charge is being compressed).
The current operating pressure of each leveling cylinder is deterrnined. For
each leveling cylinder, the relative spacing of the platens proximate to the
10 leveling cylinder is sensed. The operating pressures of the leveling cylinders
ar~ controlled in response to the relative spacing of the platens as indicated for
each leveling cylinder and in response to the data identifying the expected local
relative platen deflection for each leveling cylinder at its current operating
pressure, to maintain a substantially parallel relationship between the platens
15 proximate to the mold. For purposes of this specification, "local relative
deflection of the platens" for a leveling cylinder should be understood as the
relative deflection of the platens away from one another proximate to the
leveling cylinder. Such deflec~ion affects readings of the sensors proximate to
the leveling cylinders, whether physically separate from or incorporated into the
20 leveling cylinders.
The data indicating expected relative deflection can be estimated
for a particular platen with a typical mold or equivalent workpiece. Such
estimates can be obtained by finite element analysis, but this is comparatively
costly. Ille data may be stored on disk or other non-volatile memory, or an
electronic means may be provided to generate the data. The data are preferably
obtained through a preliminary process (prior to actual production runs) which
involves positioning a workpiece centrally between the molds, and applying the
driving force compress the worlq)iece between the platens. While the
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workpiece is being compressed, each leveling cylinder is operated at various
operating pressures, the local relative deflection of the platens proximate to the
leveling cylinder is sensed, and the sensed local relative deflection at the various
operating pressures is recorded. This process may be used each time a mold is
5 changed, using the new mold, preferably but not essentially with a sacrificialmolding charge, as the worlcpiece, to condition press controls for subsequent
molding operations. This process can also be repeated after each shut down of
the press to recalibrate deflection data.
The invention also provides a press appropriately adapted to
10 implement the above method of platen parallelism control and apparatus for
retrofitting a pre-existing press for stricter platen parallelism con~rol. Various
aspects of the invention will be apparent from a description below of a preferred
embodiment and will be more specifically defined in the appended claims.
DESCRlPTION ~)~ THE DR~WINGS
The invention will be better understood with reference to
drawings in which:
fig. 1 is a fragment elevational view of a press embodying
control features of the invention;
fig. 2 diagrammatically illustrates local platen deflection
~0 proximate to two leveling cylinders of the press; and
fig. 3 diagrammatically illustrates a control system adapted to
accomlrlodate such local platen deflection; and,
flg. 4 diagrammatically illustrates stored data identifying
expected local relative platen deflection for a particular leveling cylinder as a
25 function of the operating pressure of the cylinder.
DE3~RlErrl0N OF PREFERRED EMBQDIMENT
Fig. 1 illustrates a press 10 adapted for compression molding.
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The press 10 includes upper and lower platens 12, 14 respectively carrying
upper and lower bolster plates 16, 18. Upper and lower halves 20, 22 of a
mold 24 are fixed in a conventional manner to the platens 12) 14. A molding
charge 26 has been diagramrnatically indicated in phantom outlin~ within the
5 lower mold half 22. A large central ram 28 suspended from a cross-head
permits substantial driving forces (typically up to 2000 tons) to be applied to the
upper platen 12 to displace it towards the lower platen 14 to compress the
molding charge 26.
Four leveling cylinders are fixed to the lower platen 14 in a
conventional rectangular spacing arrangement with one at each corner of the
lower platen 14. Only two forward leveling cylinders 32, 34 are illustrated,
and these are identified herein as the "left" and "right" l~veling cylinders,
respectively. The leveling cylinders are spaced outwardly from the mold ~4 and
the central regions of the platens 12, 14 to facilitate access to the mold 24. The
leveling cylinders act between the platens 12, 14 and apply upward forces to theupper movable platen to countenail the driving force of the central ram 28.
Each leveling cylinder has an internal spacing sensor, such as the spacing
sensor 36 associated with the left leveling cylinder 32, which detects relative
spacing of the platens 12, 14 proximate to the cylinder. Details of constructionof such spacing sensors will be apparent from U.S. patent No. 4,828,474 to
Ballantyne, whose teachings are incorporated herein by reference. Details of
the general construction and overall operation of such a press 10 will be
apparent to those skilled in the art.
The problem associated with the prior art will be described with
~5 reference to fig. 2. Fig. 2 shows the local relative deflection of the upper and
lower platens 12, 14 proximate to the two leveling cylinders, grossly
exaggerated. It should be understood that such deflection is expected to be in
the order of several thousands of an inch, which is nevertheless critical to
parallelism control. It is assumed that the relative spacing proximate to the
molds is actually at a value S wbich is substantially uniform about the mold 24
tallowing for acceptable tolerances). It is also assumed that the relative platen
spacing proximate to the left cylinder 32 has a value L that is significantly
greater than the value S (owing to a local relative deflection of the upper and
lower platens 12, 14) and that the relative platen spac;ng proxirnate to ~he right
leveling cylinder 34 is at a slightly smaller value R, still in excess of the value
S. In the control system of the present invention, such a spacing arrangement
would be maintained (as explained more fully below), which is desirable since
10 the platens 12, 14 are in fact substantially parallel in their critical central regions,
proximate to the mold 24.
In tlle prior art, however, the controls would identify that the
two corners of the platen are not spaced appropriately for a parallel relationship.
The controls would actuate the leveling cylinders to attempt to produce common
15 spacing at the corners. If, for example, the right leveling cylinder 34 were
treated as a master reference cylinder, as in one prior art fonn of leveling
control, then the operating pressure of the left cylinder 32 rnight be reduced
with the object of reducing the relative platen spacing L to the value R. If theaverage platen spacing as sensed proximate to the four leveling cylinders serves20 as a reference value, as in another prior art form o~ leveling control, then the
operating pressure of both cylinders might be adjusted, to equalize the spac;ng
values L and R at some intermediate value (assuming that the other pair of
cylind~rs are associated w;th comparable spacing values). In either case, the
substantially parallel relationship of the platens 12, 14 proximate to the mold 24
is disturbed.
Fig. 3 diagrammatically illustrates the control system that
regulates the leveling cylinders. Only components necessary for regula~on of
the left leveling cylinder 32 have been shown. The other leveling cylinders are
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regulated in a similar manner.
A controller 38 processes spacing and pressure signals and
regulates operation of the leveling cylind~r 32. The operating pressure of the
leveling cylinder 32 is ~mmediately controlled by a servovalve 40. A pressure
5 sensor 42 produces a signal indicating the current operating pressure of the
leveling cylinder 32. The spacing sensor 3~ contained within the leveling
cylinder 32 produces a signal indicating the relative platen spacing proximate to
the cylinder 3? (the value L indicated in fig. 2). Both sensor signals are
received by an adjustment block 44 of the controller 38.
The adjustment block 44 retrieves from a storage block 46 the
data identifying the expected local relative platen deflection for the leveling
cylinder 32 at its current operating pressure and for the particular operating
conditions of the press lO (primary the particular mold 24 and to a lesser extent
the driving force). This data has been graphically represented in ~lg. 4 as a
lS curve indicating the relationship expected between such deflection D (indicated
on a vertical axis) as a function of var~ous operating pressures P (indicated on a
horizontal axis). In this embodiment of the invention, the adjustment block 44
reduces the sensed spacer value L for the left cylinder 32 in accordance with the
retrieved data identifying the expected local deflection, essentially sub~acting20 the value of the expected deflection from the sensed spacing value S. The
resulting adjusted spacing value confolms more closely to the actual platen
spacing D proximate to the mold 24 (once again allowing for tolerable measure
of platen deflection in the order of .001-.002 inches per foot). In a similar
mansler, the adjusting block adjusts the sensed spacing value R associated with
25 the right leveling cylinder 34 to conform more closely to the actual spacing
bet veen the molds and adjust those of the other leveling cylinders.
A cylinder control block 48 then actuates the servovalve 40
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associaeed with the left cylinder 32 (and similar valves associated with the other
leveling cylinders) ~o maintain all adjusted spacing signals at a common value.
This causes the platen to remain substantially parallel in the vicini~ of the mold
24, essentially preserving the spacing D observed in ~ig. ~, for example. This
arrangement effectively ignores peripheral deformation of the platens 12, 14,
which is inconsequential to proper molding of the charge 26. It will be
understood of course that the controller 38 oth~rwise implements, in a
conventional manner, required velocity and pressure profiles appropriate for
initial compression and ultimat~ curing of the charge 26. Such additional
10 matters are well known, and will not be described.
Although the controller 38 is illustrated as distinct blocks and
might be constructed of elec~onic components, the prefel~ed implementat;on
involves a microprocessor. The adjustment and control ~unctions may be
implemented as appropriate algorithms, stored on disk or otherwise, and
1~ executed by the microprocessor. The storage block 46 may be a conventional
disk drive from which the expected relative deflection values for each leveling
cylinder at its various operating pressures may be retrieved, ~or appropriate
reduction of sensed spacing values. Alternatively, the stored data may be in theform of scaling factors (gain/attenuation) which are used to scale the sensed
~0 spacing value proximate to each leveling cylinder prior to regulation of the
operating pressure of the cylinder. In either case, the data may be stored
effectively as parameters of an equation specifying either the spacing reduc~ionor scaling factors, rather than discrete items of deflection and pressure data.
Rather than adjusting the sensed spacing value, the data indicating expected
'~ local relative platen deflection can be used to adjust reference signals (~orexample, a required spacing value for each leveling cylinder derived from a
master leveling cylinder or an average spacing value calculated for all levelingcylinders) to achieve the same result. It will be appreciated that platen
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parallelism proximate to the mold ~4 may be regulated in a variety o~ ways
relying on the current operating pressure of a leveling cylinder, sensed platen
spacing, and data indicating expected local platen deflection.
The deflec~on data required for each cylinder can be obtained in
:~ several ways. Finite element analysis or other estimating techniques can be
used to model the platen and its expected response to operation of the leveling
cylinders under various operating pressures. The same data may be used for all
leveling cylinders, if appropriate syrnmetry can b assumed. It should be noted
that the benefits of the invention can be obtained even if no exact compensationlO occurs.
A preferred method of generating the required data involves
deriving it from actual operation of the press 10 on an exemplary workpiece in
advance of actual production molding~ The workpiece is preferably the
particular mold 24 to be used and preferably containing a sacrificial molding
15 charge~ The workpiece may be positioned cen~ally between the upper and
lower platens 12, 14. The ram 28 is then used to apply the driving force to the
upper platen 12 to compress the workpiece~ While the workpiece is being
compressed, each leveling cylinder may be operated (separately or
simultaneously) at various operating pressures~ The sensors associated with the
2n leveling c.ylinders are used to sense the local relative deflection of the platens
12, 14 proximate to each leveling cylinder, essentially as changes from a
particular platen spacing value that serves as a datum~ These sets of values arerecorded in the storage block 46 as they are generated~ For each leveling
cylinder, the datum may be the platen spac;ng at any appropriate operating
pressllre, for example, a minimal pressure necessary to maintain contact with
the upper platen, a pressure midway in the cylinder's operating range or the
cylinder's maximum operating pressure. If the datum is the spacing for a non-
zero operating p~essure, the differential changes in spacing values recorded for
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a full range of operating pressures can be related to actual expected deflections
at the various operating pressures by simply adding a common increment to
each recorded differential spacing value. l hat increment is essentially the value
required to reduce to zero the sensed differential change in spacing at a zero or
5 near-zero operating pressure. It is preferred that this initial data-producingprocess be followed each time a new mold 24 is to be introduced between the
platens 12, 14, using the particular mold 24. This provides more exact
estimates of expected platen deflection for purposes of subsequent product
production.
A preliminary calibration process is normal in the operation of
such presses. Mold stops or a molding charge will be placed in the mold fixed
to the p~ess platens, and the rarn operated to compress the mold. The spacing
sensors are then effectively calibrated, establishing reference values for
subsequent platen parallelism control. This is done whenever the press is
15 shut-down and re-started. Such matters are well known to those skilled in the art and will not be described in detail.
The preliminary data-producing process associated with the
present invention is preferably fully automated and incorporated into the
preliminary calibration process otherwise reguired for operation of the press.
20 The control system is of course coupled to the leveling cylinders through thesenovalves to control their operation, to the pressure sensors to receive
operating pressures of the cylinders, to the spacing sensors to receive local
platen spacing proximate to each leveling cylinder, and to the storage means
both to read and write data. The controller 38 may additionally be coupled to
25 the ram 28 to actuate compression of the workpiece. In a data-recording mode
of operation, the controller 38 operates the rarn 28 to compress the mold 24,
steps each of the leveling cylinders through its various operating pressures by
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appropriate control of the servovalves, and automatically records platen
deflection in the storage block 46, the deflection values being sensed by the
spacing sensors as differential spacing changes from a preselected datum, in themanner described above.
Control systems for regulating leveling cylinders are now largely
microprocessor-based and operated by software algonthms. The necessary
programming of a microprocessor-based controller 38 to add the additional
control features o~ the present invention will be readily apparent to those skilled
in the art, as will the necessary programming to implement the data-recording
mode of operation as described above.
It will be appreciated that a particular embodiment of the
invention has been described and that modifications may be made therein
without departing from the spirit of the invention or necessarily departing ~romthe scope of the appended claims.
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