Why are Leveling Knives in Platen Diecutting Ineffective?

[Chris]

Why are Leveling Knives in Platen Diecutting Ineffective?
By : Kevin Carey

One of the hid­den chal­lenges fac­ing the diemaker and the diecut­ter, is the deflec­tion and dis­tor­tion of the diecut­ting press under the con­sid­er­able force required to con­vert a sub­strate into qual­ity prod­ucts with con­sis­tent per­for­mance attrib­utes. The degree of deflec­tion is related to the size of the steel rule die design/layout com­pared to the size of the press, see above right, and the loca­tion of the die, com­pared to the cen­ter of effort of the mech­a­nism. This is often char­ac­ter­ized as Cen­ter Deflec­tion and/or Off-Center Deflec­tion. See above left. Obvi­ously the pri­mary dif­fer­ence in loca­tion is based upon if the press is either a grip­per fed sub­strate sys­tem or a con­veyor fed sub­strate system.

To avoid the inevitable dam­age this would cause to the steel rule die, the press, and the diecut prod­ucts the diemaker coop­er­ates with the diecut­ter to mod­ify the tools to incor­po­rate bal­anc­ing devices. These include the use of Press Stops, see right, inte­grated into the periph­ery of the steel rule die, and designed to ensure the rec­i­p­ro­cat­ing platen can only close to a spe­cific gap, between the upper and lower surfaces.

A sim­i­lar tech­nique requires the inte­gra­tion of ‘Bear­ers’ into the periph­ery of the lay­out. These can be a fixed height, sim­i­lar to the press stops, or they can include the abil­ity to add or remove sev­eral adjust­ment ‘shims.’ See left. A com­mon alter­na­tive to using press stops and bear­ers is the use of the same height cut­ting knife, see right, to act as a press stop or a bearer, and limit the deflec­tion or dis­tor­tion of the platen mechanism.

The the­ory is, and it is impor­tant to state up front that the assump­tions made are fatally flawed and are incor­rect, that using the same stan­dard height cut­ting knife used in the die for the bearer knives, the inevitable com­pres­sive dam­age to the cut­ting knives in the steel rule die, would be matched by iden­ti­cal com­pres­sive dam­age in the bearer knives, so bal­ance across the entire platen would be main­tained. How­ever, this makes dan­ger­ous and incor­rect assump­tions.

The assump­tion is the lev­el­ing knives will bal­ance and sta­bi­lize the dis­tri­b­u­tion of pres­sure to pre­vent press deflec­tion and main­tain a per­fectly par­al­lel rela­tion­ship between the platen sur­faces and the tools. Unfor­tu­nately, when you exam­ine this the­ory in detail it makes lit­tle sense. In the dia­gram above you can see the stack of com­po­nents cen­tered on the steel rule die and the stack of com­po­nents cen­tered on the lev­el­ing knives … are not the same! What is miss­ing is the thick­ness of sub­strate from the lev­el­ing knives! There­fore, this stan­dard approach to using lev­el­ing knives has min­i­mal impact on press leveling.

Even when the sub­strate is inte­grated into the lev­el­ing knife pat­tern, see right, we still have a seri­ous imbal­ance. This is caused by the real­ity that the die is pen­e­trat­ing and burst­ing paper­board, which sig­nif­i­cantly increases the pres­sure or resis­tance gen­er­ated by the die com­pared to the pressure/resistance gen­er­ated by the lev­el­ing knives, which are not pinch­ing and pen­e­trat­ing a substrate.

The bot­tom line is inescapable. Using Lev­el­ing Knives in the stan­dard man­ner is largely inef­fec­tive in bal­anc­ing pres­sure across the cut­ting sur­face of the platen.

The solu­tion is to com­bine Full Height Creas­ing Rule with ‘Power Ejec­tion’ tech­niques. But more on that in another article…



Source : Die Cutting-Works