Attributes vs. Processes

Selecting the optimum process for producing a given part is an important part of a designer's responsibilities. The right choice can mean the difference between success and failure. Each process has unique capabilities and limitations. Rarely will any one process provide all of the most desirable attributes. Isolating the best process invariably becomes a compromise. An ideal process for any product has a combination of shortest delivery, lowest tooling and part costs, with maximum strength and design complexity. Unfortunately, this combination of properties does not exist.

Table 3 compares the attributes of the ten plastics processing techniques mentioned in this booklet. It is not possible to compare the processes factually as they are dissimilar and used to produce different kinds of products for widely varying applications. As a result, the numerical values assigned to each attribute-process combination are somewhat arbitrary.

Ratings given to each process are offered in good faith to assist designers who are unfamiliar with these processing options. No attempt has been made to favor one technique over another. However, there will be special cases where these ratings will be inaccurate. To minimize variations, the table is limited to relatively thick walled industrial parts. Large volume, thin walled, disposable parts and packaging applications are excluded.

The one-to-ten rating system is based on "10" as the best of any attribute; for example, the lowest cost or the highest stiffness. A "1" indicates the least desirable rating. An "NA" is used for non-applicable combinations such as sheet metal and molded-in stress or spray up and finished on both sides.

Pressure and vacuum forming are based on aluminum dies. Injection and compression molds are hardened steel. Structural foam and reaction injection molds are aluminum. Resin transfer, lay up and spray up are fiberglass dies. Sheet metal requires hardened steel forming dies.

Pressure and vacuum forming, injection, compression and lay up and spray up processes have approximately .125 inch thick walls. Structural foam, reaction injection, and resin transfer molding have .250 inch thick walls.

Thermo Pressure Forming is an exciting, relatively new and rapidly growing segment of the plastics industry. This unique process offers the designer one more option in the continual quest for the optimum process-material-cost combination. The number of choices available to the designer keeps increasing all the time and it is sometimes difficult to determine which process is best for a given product. The Attributes vs. Processes Chart (Table 3) may be helpful in making such decisions.

Thermo Pressure Forming of relatively thick-walled parts has to be considered to be an immature process. New tricks of the trade are being learned all of the time. The capabilities of the process expand every year.

The design guidelines presented in this booklet are offered as a record of the state-of-the-art as it is known today. These guidelines will change and new design details will be evolved as the industry continues to grow and more experience is gained. In the meantime, this booklet will be helpful to design engineers faced with the task of finalizing the design of a new plastic part that is going to be Thermo Pressure Formed.