Why does spot color ink produce color difference?

专色油墨（Spot Color Ink）相对于传统的CMYK四色油墨。它是指通过将一种或两种或多种墨水与某些辅助材料混合在一起而制备的新墨水。使它适应打印需求和颜色要求。专色的性能色域大大超过了RGB和CMYK的性能色域。使用专色油墨可以实现CMYK四色印刷油墨无法呈现的许多颜色。专色打印使用大面积的纯色块来打印颜色，色彩效果好，减少了由点套印和点变形引起的色差，颜色稳定，性能色域宽。因此，为了在香烟标签，酒标和包装产品中获得特殊的艺术效果，专色油墨被广泛用于印刷。专色印刷不仅可以解决套印和油墨平衡造成的四色印刷问题，而且还可以满足创新设计的质量要求，而且印刷过程中的油墨平衡也相对容易控制。

导致专色油墨打印中颜色差异的因素很多，但主要的因素有：材料的性能、油墨的性质、印刷条件、印刷品的表面整饰。这些因素都会影响专色印刷过程中的色彩再现。

一、承印材料(纸张)对油墨颜色的影响

1、纸张白度

影响纸张白度的因素有：①浆料的白度：重要的影响因素。②染料：选择染料，应使其在测量白度的主波长附近不影响其反射率，即不影响纸张白度。③填料：取决于其种类及等级。④涂料：涂料印刷纸的白度由原纸白度、涂料白度、涂布量来确定。白度不同及带有一定颜色的纸张(如有的纸张偏蓝后者偏黄)，对印刷墨层的颜色呈现有不同的影响。对同一种白板纸而言，白度不同，尤其是对具有较高明度值的颜色，影响特别明显，造成专色墨的配比差别很大。因此，在实际生产中应尽量选用白度相同的纸张印刷，以减小纸张白度对印刷颜色产生的影响。

2、纸张的平滑度和光泽度

从对印刷品质量的影响来看，纸张的光泽度与印刷品的光泽度之间有着十分直接的关系，无论是何种类型的油墨，印刷品光泽度均随纸张本身光泽度的提高而提高。纸张的光泽度与纸张的着墨效率有着直接的联系，光泽度高的纸张较光泽度低的纸张在纸面以相同墨膜厚度时能获得更高的印刷密度。因此，印刷品光泽与纸张本身光泽之间存在着密切的相关性。

3、纸张的油墨接受性能与吸收性能

油墨接受性能指纸张表面在印刷过程中在印刷机上压印瞬间接收转移油墨的能力。它与纸张的三个方面性能紧密相关：纸张表面接受印刷油墨润湿的能力；纸张表面吸收一定油墨组分的能力；纸张表面固定和保留均匀墨膜的能力。而纸张对油墨吸收性能发生在从油墨与纸张接触到完全固化在纸面的一个较长时间内，它与纸张毛细孔对油墨中的低粘度组分的吸收和油墨中某些组分向纸内渗透作用有关。印刷压力，印刷时间，油墨粘度，纸张毛细管半径都会影响纸张对油墨的吸收能力。同一种油墨在相同条件下印刷到吸收性不同的纸张上时，会有不同的印刷密度。纸张表面存在着由各向异性多相复杂的结构元素形成的孔隙结构，为了使纸面获得良好的均匀性和平滑度，一般要在纸张表面涂布不同厚度的涂料。涂料的性质和厚度决定了纸张表面对油墨的吸收能力。吸收能力不同，导致印刷墨层的颜色产生差异。

二、油墨的性质对颜色的影响

印刷品的最终效果和油墨的理化性能密切相关，油墨的组分决定了油墨的性质：

1、颜料颗粒的大小及其分散程度

虽然墨膜毛细管的作用是形成印刷品光泽的重要因素，但是，分散状态下的颜料颗粒的表观大小更重要，它直接决定墨膜毛细管的状态。

因而，颜料颗粒小，分散良好，有利于形成平滑的墨膜，对于提高印品光泽也是非常有利的。

2、连接料的粘度与颜料的含量

According to界面化学原理，毛细管渗透速度与液体粘度反相关，即渗透速度随粘度的增加而降低。油墨颜料颗粒间形成的毛细管网络结构是决定印刷品光泽的一个重要方面。在压印瞬间，油墨被整体地压入纸张较大的孔隙内；在压印后，连接料开始从油墨分离，渗入纸张较小的孔隙内。因此，油墨膜毛细管的大小决定着连接料的分离量。毛细管保留连接料的作用比印刷压力把连接料压入纸张孔隙的作用大得多。

3、干燥时间

墨膜在纸面迅速干燥可以减少连接料渗入纸张孔隙的量，从而提高墨膜的光泽度和平滑度。墨膜的光泽度高就意味着表面的镜面反射程度比较高，这样人眼在大部分情况下能看到表面反射的白光要比光泽度低的墨膜少得多，使得色彩的饱和度更高。所以光泽度高的印品看起来色彩更加鲜艳、饱满。专色油墨的印刷墨层大部分是半透明的或不透明的，在选择油墨配方时也要According to实际情况考虑到墨层的透光性对专色印刷的影响。

三、印刷过程对颜色的影响

1、墨层厚度及水墨平衡

印刷过程中对油墨色相的控制主要是通过控制墨层的厚度来实现。在墨层厚度变化0.1μm左右时就会引起△E=1.5～4.5NBS的色差，显然这一影响是很大的。胶印的墨色再现和水墨平衡密切相关，在同样的墨量情况下，水大水小会影响颜色的深浅。正确掌握水墨平衡，特别注意润湿液的PH值重要性无庸赘述。用水量大小的原则为不上脏的情况下，采用尽可能小的供水量。

2、印刷压力

印刷压力的大小对于油墨从印版上转移到纸张上的程度有很大的影响，当印刷压力不足时，油墨的转移不充分，不适宜印刷;当印刷压力过大时，由于印版上的油墨向图文以外的空白部分铺展，油墨转移率非但得不到提高，而且还带来其它弊病，同样不适宜印刷。只有在适当的印刷压力范围内，才能印出墨层厚实、图像清晰、阶调丰富和色彩再现性良好的印刷品。

3、印刷速度

印刷速度的增加，会使印刷面间的接触时间变短，网点发虚，油墨转移率降低，印刷质量下降。为了保证印刷面的充分接触，提高印刷质量，这就要求在一定印刷压力下，控制好印刷速度，以获得稳定的墨层厚度。

4、干退密度差

For the same paper, the sample just printed has a higher density, but after a period of time, the density value of the sample will decrease as the ink dries and the smoothness decreases. Due to the different density values ​​before and after the ink layer is dried, the color tone of the printed image is also different. For the printed matter just printed, the ink is still in a wet state, and there is a difference from the dry state. This is because the newly printed ink layer has a certain leveling property, so the surface reflection is mainly specular reflection, which looks bright in color and good in gloss. When the ink layer is in a dry state, the surface reflection is dominated by diffuse reflection, and the color is naturally duller than when it was just printed. Therefore, the ink color of the general printing proof is slightly darker than the standard proof, but how much is darker should be controlled digitally. When proofing, measure the density value of each color just printed. When printing, refer to these density values ​​to print. The ink color of the two is close to the same.

A densitometer with a polarizing filter can eliminate the light generated by the specular reflection on the surface of the ink layer. The measured wet color density is very close to the dry color density, and the measured density value is little affected by the dryness of the ink layer.

4. The influence of the surface finishing of the printed matter on the color

Surface finishing is to properly process the surface of the printed matter to improve the light resistance, heat resistance, water resistance, wear resistance, folding resistance, and chemical resistance of the printed matter; to increase the gloss and artistic sense of the printed matter; to protect the printed matter And beautify the role of printed matter, improve the value of printed matter. The surface finishing methods of packaging products mainly include glazing (printing varnish, coating varnish, matt oil, UV varnish, water-based varnish, etc.), waxing, and lamination (glossy film, matte film) , embossing, bronzing, etc. After the printed matter undergoes these surface treatments, there will be varying degrees of changes in hue and saturation, that is, differences in density. These changes can be divided into physical changes and chemical changes. Physical changes are mainly reflected in the increase of specular reflection and diffuse reflection capabilities on the surface of the product. Generally speaking, when the glossy film is applied, the gloss oil and UV varnish are applied, the color density increases; when the matte film is applied, and the matte oil is applied, the color density decreases. The chemical changes mainly come from the organic solvents contained in the coating glue, water-based varnish, and UV varnish, which will combine with the ink particles on the printed matter to change the color of the ink layer.

V. Conclusion

In short, in packaging and printing, there are many reasons for the color difference of spot colors. In actual production, it is necessary to analyze the specific problems caused by various reasons, try to control the deviation in the minimum range, and take corresponding countermeasures for the specific reasons. Provide packaging products that meet customer needs. Color can be better controlled as long as proper measures are taken during the color matching and printing process. It mainly regulates the deployment of spot colors, standardizes the printing process and manages data, while keeping the production environment relatively stable.