Disposable Coffee Cups

 A coffee cup refers to a type of container from which hot beverages (including coffee) is consumed. Typical functions should include:

  • Easy portable
  • Providing good insulation to the beverage
  • Protecting the consumer from burns
  • Should avoid potential spills

Coffee cup sleeves – typically made of textured paperboard – are usually added for extra insulation against heat.[1] Disposable coffee cups are typically made of Styrofoam – which is made from foamed polystyrene.[2] The lids, on the other hand, is made from solid polystyrene (also a polymer).[3]

Material/ Mechanical Properties and how it influences the cup’s functionality:

Styrofoam/ foamed polystyrene (used as the coffee container):

Solids and liquids are generally good conductors of thermal energy, whereas gases are not – as common insulators use vacant space to increase insulation. Its low density (and high porosity level) also aids to good insulation.

Styrofoam is produced by polymerizing polystyrene as a gel with gas – gas “bubbles” are injected into the liquid styrene, and becomes “trapped” inside the foam. This process is the reason why Styrofoam has such extremely low heat conduction, providing good insulation to the beverage as well as protecting the consumer from burns. [4]

Not only is Styrofoam waterproof and lightweight, is it also very cost effective and economical feasible. Although technology exist to recycle Styrofoam cups (as it is not very biodegradable), it lacks in demand for recycled cups. [5]

The coned shape cup is used worldwide for improved handling, and easy portability. The solid circular surface (with the coffee cup sleeves) aids in preventing potential spills.

Polystyrene (used for the lid):

Polystyrene is a thermoplastic, which flows if it is heated above its glass transition temperature (which is roughly 100°C). It has a very low room density (1.05 g/cm³), and is very economically feasible to make. Polystyrene contains carbon and hydrogen, with the chemical formula (C8H8)n. It’s relatively strong, because of its strong intramolecular forces. [6]

The lids are designed in such a way, to be easily sealed onto the Styrofoam cup, with a small sipping hole (to consume the beverage). The lid reduces heat escaping the cup, as well as aiding to prevent spills. Therefore the sipping hole should be as small as possible. Lids will also set the consumer’s mind to ease, which will also improve faster portability.

Textured paperboard (for cup sleeves):

Paperboard is thicker than normal paper, which increases its tearing resistance. It is also a highly porous material, containing averagely 70% air (which, mentioned above, decreases heat conduction and therefore protects the consumer even more from burns).  [7]

Aside from that, is it very light in weight, and 100% recyclable. Paperboard provides a small amount of friction – but because it is textured, enough friction is established, which helps prevent spills. [7]

How Styrofoam cups are manufactured:

Styrofoam cups are produced by means of injection molding, where the polymer is heated to a melted state and forced to flow (under pressure) into a mold cavity. There it is cooled and solidifies. The foam is produced by forcing gas into the molten polymer in the injection unit. The injection unit consists of screw (which can be found inside the barrel), which turns (for mixing and heating the polymer) and moves forward to inject the molten polymer into the mold. A clamping unit is in place to hold the mold in proper alignment, and used to open and close the mold at appropriate times during the injection cycle. Usually ejector pins are used to push the part out of the cavity at the end of its cycle, whereafter sprues are cut off (extra material underneath the cup). [8]

The following is an illustration of how it works:

It makes it suitable to use, as cycling time is very fast (only a few seconds), and the process can produce multiple moldings per cycle, and is very economical for large production quantities. [8]

These coffee cups is off course not complete without its plastic lid, which can also be produced using injection molding or thermoforming – in which a flat thermoplastic sheet is heated and deformed into the desired shape. [8] The following image illustrates how:

Possible Design Changes That could be implemented:

  • Injection molding machines can be implemented which has more than one mold cavity, which will lead to more cups produced in shorter times. This will also help in times of high demands.
  • Although paper cups has higher heat conduction than Styrofoam, can it still be used which can be easier recycled than Styrofoam, and is more eco-friendly (which makes it an improvement towards the environment). However, according to a Canadian study, paper cups use more raw materials and energy than Styrofoam (which increases costs). [9]
  • A careful study could perhaps lead to a lower temperature to be used to heat the material to be injected into the mold cavity. This will result less energy to be used, and will reduce cost, although it will increase the time to produce high volumes of cups.

There are very little materials which can compete against Styrofoam (which insulates hot beverages to such a high standard). However, most of them is not currently economical feasible, or they lack in some properties as to assist in the coffee cup’s functions.

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References:

1. http://en.wikipedia.org/wiki/Coffee_cup_sleeve. Retrieved October 5, 2011

2. https://engineering.purdue.edu/MSE/AboutUs/GotMaterials/Manufacturing/sieckmann.html. Retrieved October 5, 2011

3. http://www.alibaba.com/product-gs/313177444/Disposable_Plastic_Coffee_cup_lid.html?s=p. Retrieved October 11, 2011

4. http://www.ehow.com/how-does_5314883_styrofoam-cup-insulation-works.html. Retrieved October 12, 2011

5. http://www.mnn.com/money/green-workplace/stories/recycle-styrofoam-cups-is-it-possible#. Retrieved October 12, 2011

6. http://en.wikipedia.org/wiki/Polystyrene. Retrieved October 12, 2011

7. http://www.paperonweb.com/paperpro.htm. Retrieved October 12, 2011

8. Groover, M.P. 2011. Principles of Modern Manufacturing. 4th Edition.

9. http://www.ecojoes.com/styrofoam-cups-vs-paper-cups/. Retrieved October 13, 2011