why light weight important for plastic design

Light weighting

It must be understood that the cost of any product consists essentially of two major factors:

  1. The cost of the plastic itself, and
  2. the sum of all other costs, such as the mold cost attributed to the product, the machine hour cost,power, handling, overhead, etc.

In mass production of many plastic items, the first factor—the cost of the plastic一is often as high as 75% of the total cost of a product. For some products, such as disposable items, this percentage may be even higher. There is,therefore, a compelling incentive for the manufacturer to reduce the mass (weight) of any product, and more so when it comes to disposable items. As long as the product will meet the necessary demands, such as utility and safety, it should be acceptable in its lightest weight design.

Example 1 A typical example is the history of the disposable seven-ounce drinking cup, made from readily available and low-cost clear, crystal polystyrene (PS). From an original model of such a cup at 15 g per unit, the side walls’ the rim, and the bottom were gradually reduced, while adding thin ribs not only for strength but to make it moldable at all with such a narrow thickness. This resulted in cups of about 9 to 11 g, which was better than before but still not good enough.

Further reduction of the mass of the cup was not possible, not only due to the limited capabilities of the injection unit (molding machine), which must provide extremely high pressure and very fast injection speed to fill the very thin walls before the plastic freezes, but also because the plastic used became so heavily flow oriented that the product became too brittle for the intended use. Instead of the readily available plastic, a better grade had to be developed, but, even then, a cup weighing 9 g was at the lowest practical limit.

However, when the designer resorted to another way of manufacturing-in this case, injection blow molding-these obstacles disappeared. During the injection molding step, the rim and the portion to be blown has an easily moldable thickness. The rest of the cup shape, which will be blown in the second step,is much thicker. This facilitates the first step by filling the thin rim through the heavier body walls. (Before, the heavy rim was filled through a very thin side wall, a condition which should generally be avoided in molding any plastic product.) After being transferred to the blow station, the heavy side walls and the bottom are blown to the final shape, resulting in extremely thin cross sections. While blowing, the plastic is biaxially oriented and thereby loses the brittleness formerly experienced. Even though it is now much thinner, the plastic is not brittle.

This example is given to highlight the necessary cooperation between the product and the mold designer, the materials supplier, the mold maker, the molding machine builder, and the molder to design a product which is more than a simple run-of-the-mill article. Typically, such cooperation applies whenever the quantities required make these efforts (high development costs) economically viable.

The result of the changes in the above-cited example was an excellent cup weighing only 6 g, or about 40% less than (he original. Considering that one machine can produce 1000 cups per hour, the redesign saved about 40,000 g, or 40 kg/hour. At a plastic cost of 1.00/kg, this is a substantial savings, considering that the machine runs at least 6,000 hours/year. The annual savings in material] alone was $240,000/machine-more than enough to pay for the additional costs associated with designing and building special molds, machines, and suitable handling equipment.

Another example in the same product area shows that a certain product, such as a container, also can be produced using an entirely different method:

Example 2 Consider a margarine container molded from polypropylene (PP), which is easily injection molded. The product designer must consider several facts. First, the designer must.be aware of the limits set by the injection molding machine in the area of injection capability. The thinner the product walls, the higher the injection pressures and speeds required to fill the product with a suitable resin that will remain strong enough the purpose. Easier flowing materials will improve the molding capacity, but there is a danger of losing required ‘product strength. Higher resin cost is also a factor.

A similar container could, however, be produced using an entirely different process-thermoforming-in which an extruded thin, wide strip of PP is reheated before entering the forming station and shaped in a multicavity forming mold into the desired shape. After forming, the strip advances to a trimming station; from there the products are conveyed, for stacking, etc. The unused portion of the strip (the web) is then, out up and can be recycled.

This method is, in many cases, as well as injection molding, and is cost competitive. The designer must, however, be aware that there are certain disadvantages connected with this method of container production which concern the actual product:

  1. Injection molding permits better dimensional accuracy, which may be of important advantage for proper sealing (fit) of the matching lid.
  2. Ribs to stiffen the rim are not possible with thermoforming, and any stacking features are usually less accurate than with injection molding.
  3. For a long time, it was not possible or economical to thermoform crystal clear containers from plastics acceptable for the food industry; however, this drawback may change with new developments in resins.

If none of these points are important for the final shape of a new design, the designer has the choice to design either for injection molding or for thermoforming. The final decision should come from the molder, who will decide on economic grounds (availability of injection molding machines, investment in new machines or in a thermoforming machine line) which method to use. The actual product design drawing used will be slightly different for injection molding than for thermoforming, even though the main dimensions will be the same for either method.

Light weighting has other advantages which are also very important:

  • The thinner the wall thickness, provided it is more or less uniform throughout the product, the faster can be the molding speed, and the higher the productivity.
  • There are additional savings in the cost of transporting the raw material, the finished goods, the cost of recycling, land fill costs,etc. All these costs represent savings in energy (electricity or oil) and result directly in a decrease in pollution.
  • The efficiency of an automobile or aircraft increases when their mass is reduced. The lighter the structure, the more efficient it is, and the greater are the possible payloads.
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