Mold clear detector

An apparatus for detecting the retention of non-ejected parts in the cavity of a plastic molding machine and inhibiting the operation of the molding machine in response thereto. The apparatus employs a set of photocells coupled via associated fiber optic cables to a comparator. The retention of a molded part subsequent to the completion of a molding cycle is indicated by the interruption of a light beam generated by at least one of the photocells. The optical signal is processed by the comparator and associated circuitry so as to activate a mold inhibiting relay.

TECHNICAL FIELD 
This invention relates to the the molding of plastic parts and more 
particularly to an apparatus for detecting the retention of parts in the 
cavity of a molding machine upon completion of the molding cycle and for 
inhibiting the molding operation in response thereto. 
BACKGROUND OF THE INVENTION 
One of the problems encountered in the fabrication of plastic parts, or 
parts fabricated from other similarly suitable materials, in a compression 
molding process is the periodic failure of parts to be successfully 
ejected from the mold cavity upon completion of the mold machine cycle. 
Clearly, retention of molded parts in the cavity will result in subsequent 
unsatisfactorally molded parts. 
In the past this problem has been addressed simply by having an operator 
monitor the molding process and turn the mold press off upon observation 
of a non-ejected part. However, because an operator could be expected on 
occasion to fail to observe non-ejected part and because, in any event, 
this approach introduced undesirable delay and attendant expense into the 
molding process, an improved solution was sought. 
Some limitations inherent in the operator-monitor approach can be 
circumvented by the installation of a miniature lamp in the mold machine. 
In a manner roughly analogous to the one described below, the lamps can be 
used to detect the retention of a molded part in the mold cavity and to 
appropriately modify the operation of the mold machine. However, initial 
installation of miniature lamps has proved costly and the expense of 
installation is exacerbated by the relatively short lamp life resulting 
from the hostile environment, that is, heat and vibration, presented by 
the molding process. 
DISCLOSURE OF THE INVENTION 
The above and other objects and advantages are achieved in one aspect of 
this invention by a mold clear detector that comprises a light generating 
element, for example, a photocell, in the mold cavity. The photocell 
develops an optical signal indicating the retention or ejection of a part 
from the mold cavity upon completion of a mold machine cycle. An optical 
transmission path in the form of a fiber optic cable couples the optical 
signal to a light-sensitive element. The light-sensitive element 
represents a part of a control device that inhibits the operation of the 
mold machine as a result of the non-ejection of a part from the mold 
cavity. The device also includes a comparator that has an input coupled to 
the light-sensitive element and that is, therefor, responsive to the 
optical signal developed by the photocell. A switching circuit driven by 
the comparator is adapted to inhibit the mold machine when necessary.

BEST MODE FOR CARRYING OUT THE INVENTION 
For a better understanding of the present invention, together with the 
objects, advantages and capabilities thereof, reference is made to the 
following disclosure and appended claims in conjunction with the above 
description of some of the aspects of the invention. 
Referring now to the drawing, the subject mold clear detector comprises a 
light-generating element in the form of a photocell 1. The photocell is 
suitably positioned in the mold cavity (not shown) so that a beam of light 
is normally focused on the cell but is broken when a molded part is 
retained, that is, not ejected from the mold cavity upon completion of the 
mold machine cycle. In this fashion the photocell can be viewed as 
generating an optical signal dependent on the ejection or retention of 
molded parts in the mold cavity. 
The optical signal is coupled from the photocell via an optical 
transmission path in the form of a fiber optic cable 2 to the input of 
control means 3. In a manner more fully disclosed below, control means 3 
operates to selectively inhibit the operation of the molding machine as 
the result of the retention of a molded part in the mold cavity. 
Control means 3 comprises a voltage divider 31 coupled between a first 
reference potential, +12 V, and a second reference potential, ground. The 
voltage divider includes a light-sensitive element in the form of a 
light-dependent resistor, VT1, coupled between the +12 V supply and an 
inverting (signal) input of a comparator Q1, and a fixed resistor R1 
coupled between the signal input of Q1 and ground. A variable resistor R2 
is coupled between the +12 V supply and ground and has a wiper coupled to 
the non-inverting (reference) input of comparator Q1. For this reason the 
voltage divider 31 and variable resistor R2 can be considered the signal 
and reference sources, respectively, for Q1. 
The output of Q1 is coupled to the input of a switching circuit 32 
including a logic element in the form of a NAND gate Q2, a transistor Q3, 
and a mold inhibiting device, relay means K1. In particular, the output of 
Q1 is coupled through a resistor R3 to an input of Q2 which is, in turn, 
coupled through a resistor R4 to ground. The output of Q2 is coupled 
through a resistor R5 to an input electrode, the base, of Q3. Q3 has an 
emitter connected to ground and an output electrode, collector, coupled to 
one end of the coil of R1. The other end of that relay coil is coupled to 
the +12 V supply. The normally open (NO) and normally closed (NC) contacts 
of K1 are adapted to be coupled to the mold machine (not shown). The mold 
machine, per se, is considered to be no part of this invention. 
During normal operation of the molding system, the mold machine 
successfully ejects mold parts upon completion of the machine cycle. Light 
impinging on the photocell will remain uninterrupted and will be 
transmitted by the fiber optic cable to the control means at VT1. Because, 
according to the commonly understood operation of light-dependent 
resistors, the effective resistive value of VT1 will be lower when VT1 is 
illumninated than when it is in a dark environment, the voltage at the 
inverting input of Q1 will be less than the voltage at its non-inverting 
input. Consequently the voltage at the output of Q1 will be high, a logic 
level ONE, and the voltage at the output of Q2 will be low, a logic level 
ZERO. (It is clear that in the configuration shown in the drawing Q2 is 
functionally equivalent to an inverter and its operation may be understood 
as such.) Transistor Q3 will be non-conducting and no current will flow 
through the coil K1, thereby allowing the molding machine to proceed to 
the next machine cycle. 
However should a molded part fail to be ejected at the end of a machine 
cycle, the beam of light impinging on the photocell will be interrupted. 
The interruption of light will cause the effective resistive value of VT1 
to increase and the voltage at the inverting input of Q1 to decrease 
concomitantly. The resulting logic level ZERO at the output of Q1 will be 
translated to a logic level ONE at the output of the NAND gate. Q3 will be 
rendered conductive, thereby "pulling in" K1 so that its contacts will be 
in a state opposite to that shown in the drawing. The contacts of K1 are 
wired to the molding machine so that its operation will be inhibited 
pending the removal of the part from the mold cavity. 
It should be noted that, although the subject invention has been at least 
implicitly described with reference and relation to compression molding of 
plastic parts, other types of molding techniques and materials and, for 
that matter, other machine operations are clearly within the contemplation 
of the invention as set forth above and claimed below. Furthermore it is 
an obvious expedient to extend the apparatus as in described above and 
shown in the drawing to a plurality of mold cavities. Accordingly, while 
there has been shown and described what at present is considered to be the 
preferred embodiment of a mold clear detector, it will be obvious to those 
skilled in the art that various modifications may be made therein without 
departing from the invention as defined by the appended claims. 
INDUSTRIAL APPLICABILITY 
This invention is useful in the automatic monitoring of various machine 
operations and, in particular, fabrication of molded parts.