Showthrough and Strikethrough

# Showthrough is a common print defect that is generally found in offset lithography.

 Paper opacity is responsible for this type of phenomenon while ink absorption and penetration through the paper results in the visibility of printing on the reverse side of a sheet of paper under normal lighting conditions termed as showthrough.

Reasons:- 

This printing problem occurs in thinner papers with less pulp to create opacity. 

Less fiber makes the paper more transparent. 

The main reason behind the showthrough problem is the combination of ink and paper. 

Tackiness of ink and excessive acidity of dampening 

 # Strikethrough is similar to the showthrough print problem where the main reason is the penetration of ink through the paper or the substrate. Generally, in porous or uncoated paper this type of problem arises because of the excessive absorbent nature of paper which allows the ink vehicle to penetrate. 

Moreover it can also occur when the refractive index of the ink vehicle is close to that of cellulose, making the printed area more transparent. 

The only remedy to avoid strikethrough is to use less absorbent substrate or porous paper, usage of quick set ink and maintain the printing pressure. The ink setting speed is very important criterion to avoid strikethrough as well as showthrough. 

Moreover, excessive printing impression pressure causes bleeding of the printed object through the paper.

The choice of resin and oil determines the wetting characteristics of ink vehicle. In newspaper printing an asphaltum complex solution in mineral oil with the correct addition (2 or 3%) is used which helps to avoid excessive strikethrough in printing.

Filler increases paper opacity, brightness, whiteness and smoothness and reduces ink strikethrough and showthrough by controlling ink absorption.

The most common fillers used for this purpose are calcium carbonate, titanium dioxide etc.

Paper sizing is one of the most common methods to control the penetration of ink through the paper. Rosin is common sizing agent that provides significant resistance to wetting paper by ink.

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