Punch system for perforating plastic sheets

A punch system for rigid and brittle plastic sheet materials such as polycarbonates comprising a punch body mounted in a carrier board and having an upwardly projecting head portion which defines the shape of the hole to be punched, a ring of high durometer urethane mounted on the punch head to serve as a stabilizer and stripper and a disk of high durometer urethane mounted on the under surface of the press platen directly opposite the punch to receive the punch head and the slug of punched material during the punching operation. The punch is used in combination with a steel rule die but can be used either with other punches or alone.

INTRODUCTION 
This invention relates to apparatus for punching holes in relatively thin 
sheets of rigid plastics such as polycarbonates. In particular, the 
invention contemplates the construction of a die which includes punch 
components either alone or in combination with steel rule elements for 
cutting specified shapes. 
BACKGROUND OF THE INVENTION 
Dies and punches for cutting and punching various material such as 
cardboard and plastic sheets are well known. Typical applications include 
mass production of display cards, instrument faces, semi-transparent film 
strips and backing materials. Typically a punch comprises a steel body 
having a work-engaging punch surface which is hollowed out to define a 
peripheral cutting edge and filled in the hollowed out portion with spongy 
material to eject the slug of punched material. These punches are 
relatively difficult and expensive to make, particularly in very small 
sizes. 
A problem arises in the punching of holes in relatively rigid plastic 
sheets such as polycarbonates. These materials have a tendency to crack or 
spall during the punching operation and the application of conventional 
techniques has not been satisfactory. 
SUMMARY OF THE INVENTION 
The present invention comprises an apparatus which is particularly adapted 
for punching holes in sheets of relatively rigid plastic materials such as 
polycarbonates, polyethylene, polyurethane and the like. The apparatus 
typically comprises a punch carrier such as a backing board, and a punch 
body which is mounted in or on the carrier and which has a head portion 
which defines the shape of the hole to be punched and which extends away 
from the carrier. The head portion has a leading surface which is not 
hollowed out but which is planar in character. The head portion of the 
punch is surrounded by a stabilizer/stripper body of durable resilient 
material such as hard urethane to both stabilize the work just prior to 
entry of the punch head and also to strip the work off of the punch head 
upon retraction of the punch operating press. The apparatus finally 
comprises a pad which is mounted on a reaction surface opposite the punch 
head for receiving the head portion during the punching operation. The pad 
is also preferably made of a durable resilient material such as hard 
urethane. 
The invention may be used for punching operations alone or in combination 
with steel rule cutting dies also mounted in the punch carrier board for 
cutting specific shapes and punching within said shapes at the same time.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENT 
Looking first to FIG. 1, a die 10 comprises a plywood board 12 which serves 
as a carrier for a length of conventional steel die rule 14 which defines 
a closed shape and a steel punch 16 which lies within the shape. The die 
10 is adapted to be operated by an upper press platen 18 which is actuated 
by mechanical, hydraulic or pneumatic means in the direction of the arrow 
to force a worksheet onto the upper sharpened edge of the die rule 14 and 
onto the head 20 of the punch 16 to cut out the shape defined by the rule 
14 and to punch a hole defined by the shape of the head 20. The actuating 
means then reciprocally withdraws the upper platen 18 to release the 
material in conventional fashion. 
The assembly shown in FIG. 1 comprises, in addition to the elements thus 
far described, a ring or annulus of high density relatively hard but 
resilient urethane which, as better shown in FIG. 2, rests on the 
shoulders 28 of the punch 16 to surround the head 20 and act as a 
stabilizer and stripper. In addition, a pad 24 of disk shaped urethane is 
bonded to the underside of the platen 18 in alignment with the ring 22 to 
resiliently receive the head 20 of the punch 16 during the punch operation 
as hereinafter is described in detail with reference to FIGS. 3 and 4. 
Looking now to FIG. 2, the die rule 14 and punch 16 are shown to be fully 
implemented in the carrier board 12 and also to rise to nearly the same 
height such that the upper portion of both the rule 14 and the punch 16 
extend above the upper surface of the board 12. The head 20 of punch 16 is 
smaller than the body 16 in at least one dimension thereby to define the 
shoulders 28 which receive the stabilizer/stripper ring 22. Gaps 26 in die 
rule 14 create uncut "bridges" in board 12 so that the routed-out channel 
for the rule 14 can be discontinuous; this is required to maintain the 
integrity of board 12. The punch 16 exhibits a flat, non-hollow 
workpiece-engaging surface 30, the shape of which defines the shape of the 
hole to be punched and which exhibits a shear or angulation the total 
vertical excursion of which is at least approximately equal to the 
thickness of the plastic sheet 32 which forms the workpiece in the 
assembly of FIGS. 2-4. Selecting typical dimensions for purposes of 
illustration only, the total height of the steel die rule 14 may be 0.937 
inches and the height from the bottom of the board 12 to the top of the 
ring 22 may be 0.907 inches. The thickness of the ring 22 is equal to the 
height of the head 20 and the thickness of the pad 24 is 0.060 inches. 
Ring 22 and pad 24 are preferably solid urethane with a durometer number 
of 90-100. 
The board 12 is made of wood, the rule 14 and the punch 16 are made of 
steel and the upper platen 18 is also made of steel. 
Looking now to FIGS. 3 and 4 a typical operation of the assembly will be 
described. As the platen 18 is driven vertically downwardly the plastic 
sheet 32 is forced onto the upper surface of the die rule 14 to cut the 
overall shape and also onto the head 20 of the punch 16. The ring 22 acts 
to stabilize the sheet material 32 as it meets the upper surface of the 
punch 16 and thereafter compresses as the sheet 32 is driven onto and 
through the punch head. The pad 24 compresses resiliently to receive the 
punch head 20 and also to receive the slug of plastic material which is 
removed from the sheet 32 as best shown in FIG. 3. 
As the upper platen 18 is withdrawn the stabilizer/stripper ring 22 
rebounds to full height and strips the material 32 off of the punch head. 
The upper pad 24 also reexpands to full thickness and a neat cut and punch 
operation is performed without cracking or spalling of the relatively 
brittle sheet 32 of plastic material. 
It is to be understood that the punch 16 together with the 
stabilizer/stripper ring 22 and the pad 24 may be used either alone or in 
combination with other similar punches or with the die rule 14 shown in 
FIGS. 1 and 2. The punch 16 may represent any shape according to the 
configuration of the head 20 according to the shape of the hole to be 
punched.