What is a ball point pen?

Ball_point_pen_componentsAt the heart of a ball-point pen, is a small ball which seals the chamber containing a viscous ink system. As the operator moves the pen across the surface the rotation of the ball transfer ink from the sealed chamber to the writing surface. The ball has two key functions which are to seal the ink chamber and to transfer the ink to the writing surface. The ink is fed by gravity to the upper surface of the ball. The image shows the main components for a particular pen. The centre item is the ink reservoir, the brass pen tip on the left is inserted in the inBall Point Pen tipk reservoir and the whole assembly is inserted in the pen holder on the right.

We have used a scanning electron microscope to look at the metal ball in the pen tip more closely as shown in the image to the right. At this scale or magnification the ball looks very smooth.

 

 

 

 

 

However, we can zoom in to a much higher magnification and see that the surface of the ball is far froPen surfacem smooth. This surface roughness will play a part in the transport of the ink from the reservoir to the paper. At this magnification, a human hair would completely fill the field of view.

 

 

 

 

 

 

The scanning electron microscope is a very versatile instrument and it can also be used in together with an x-ray spectrometer to mdistribution_elements_in_the_different_componentsap out the distribution of elements in the different components. The first image is the normal scanning electron microscope image, the remaining images are show the distribution of selected elements, such as Aluminium (Al) Cobalt (Co), Copper (Cu), Tungsten (W) and Zinc (Zn), in the pen tip. The brightness of the image indicates the amount of that element present. These images show very clearly that the lower part of the pen tip is formed from brass a metal containing copper and zinc. In contrast, the metal ball contains tungsten and is made from tungsten carbide.

Not all ball-point pens are made from the same materials and our studies have shown that for some brands the pens contain a ball made from steel or brass.

As with many household goods, there are international standards for ball point pens. The relevant ISO standards [1-3] define:

a. a pen as a 'writing instrument equipped with a feeding system which deposits a writing fluid on a surface';

b. a ball pen as a 'pen with a writing fluid feeding system based on a rotatable ball writing tip integrated either within the pen itself or within a refill;

c. a ball point pen as a 'ball pen which deposits a writing fluid with a dynamic viscosity greater than 1 000 mPa × s (1 000 cP) at 23 °C ± 2 °C.

ISO 12757-1:1998 [2] classifies the tips according to the ball diameter as detailed in Table 1.

Table 1

Tip Classification

Tip Code

Ball Diameter (mm)

Extra fine

EF

< 0.65

Fine

F

0.65 = 0.85

Medium

M

0.85 = 1.05

Broad

B

= 1.05

References

1. ISO 12756:1998 Drawing and writing instruments -- Ball point pens and roller ball pens -- Vocabulary

2. ISO 12757-1:1998 Ball point pens and refills -- Part 1: General use

3. ISO 12757-2:1998 Ball point pens and refills -- Part 2: Documentary use

This research involved Professor Geoff Mitchell (CfAM) and has been funded in part by BIC

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