E: Application of STEM Principles

Overview

When designing the Cleaning pen, certain stem principles need to be observed so that the result is optimum. When designing the pen, things like strength, friction and weight need to be observed. Applying knowledge of STEM principles gives our design viability and scientific data to back our design.

Fluid Dynamics

To dispense liquid from the bottle to the lenses we must use a sprayer. Such technology, even as small as we need it, already exists, and we are not looking at developing a method of atomizing liquid.

Material

An ideal model of our device would be made out of polypropylene, due to its material qualities. The 3D printer that we will need to use to prototype will print with an ABS plastic. Polypropylene would be ideal because it offers advantages in chemical resistance, elasticity, toughness, and fatigue resistance.

This information found from Creative Mechanisms Blog talks about those ideal properties:

1. Chemical Resistance: Diluted bases and acids don’t react readily with polypropylene, which makes it a good choice for containers of such liquids, such as cleaning agents, first-aid products, and more.
2. Elasticity and Toughness: Polypropylene will act with elasticity over a certain range of deflection (like all materials), but it will also experience plastic deformation early on in the deformation process, so it is generally considered a "tough" material. Toughness is an engineering term which is defined as a material's ability to deform (plastically, not elastically) without breaking..
3. Fatigue Resistance: Polypropylene retains its shape after a lot of torsion, bending, and/or flexing. This property is especially valuable for making living hinges.
4. Insulation: polypropylene has a very high resistance to electricity and is very useful for electronic components.
5. Transmissivity: Although Polypropylene can be made transparent, it is normally produced to be naturally opaque in color. Polypropylene can be used for applications where some transfer of light is important or where it is of aesthetic value. If high transmissivity is desired then plastics like Acrylic or Polycarbonate are better choices.

Material Strength

Assuming an ideal prototype is made out of all polypropylene, our pen would be very tough and able to withstand much torsion, bending, or flexing, assuming the thickness is >1 mm. Our 3D model shows these stress tests:

Our pen will be able to handle normal rigors of everyday life, with the most stress likely coming from being in a pocket.

Information found Here

This stress analysis report shows that our design has a yield strength of almost 4400 psi, meaning that our device can withstand almost 4400 pounds of force before we see material failure.

Weight

Our bottle that we decided to begin our prototype with has a fluid capacity of 10mL. Completely full, the fluid would weigh 10g. Added to the weight of the plastic of the bottle we see a total weight of 14.5g.

Friction

When looking at our design we see the application of friction in two places. A friction fit is used to hold the caps of the pen in place similar to many ballpoint pens, and a friction fit is used to hold the brush into place with a more snug fit.

Here we see the coefficient of static friction for the ABS plastic that we are using to prototype is 0.46

Here we see the static coefficient of friction plastic to plastic for Polypropylene is 0.76

https://opentextbc.ca/physicstestbook2/chapter/friction/

Force necessary to overcome friction or, frictional force, is defined as the normal force multiplied by the coefficient of friction.

f_s ≤ μ_sN

f = 14.5g = .0145kg = .1421N

μ = 0.76

In the case of our cap, the force require to pull the cap off of the body should be greater than the weight of the pen itself, that way the pen does not fall apart if the user is holding it by the cap. Therefore if we set f = .1421 N and we know the coefficient of friction μ = 0.76, then we can calculate that N, or the normal force of the cap on the pen, should be greater than .186 N = 1.823 kg. measuring the normal force of a cap on a pen is tricky, but it can be done by comparison with other pens. If we measure the force that the cap can hold, and we know the coefficient of friction, we can calculate the normal force that the cap exerts and compare our calculation of 1.823 kg.