Consider process attributes for successful thermoforming

Introduction

How do you select the right manufacturing process for your part? You can start by comparing the attributes of several processes and ranking how well they benefit you. Pressure forming offers a superior balance of features, material usage, and cost.

This article originally appeared in Profile’s Design Guide. Plastics industry legend Glenn Beall wrote our first edition in 1985. Hidden away in the company archives for years, the concepts presented here can offer valuable insight for creating successful parts today.

How do you select the right process for your part?

Selecting the best process for making a part is an important for a designer. The right choice can make the difference between success and failure. Each process has unique capabilities and limitations, and rarely will any one process provide all the most desirable attributes. Invariably, it becomes a compromise. 

An ideal process for any product offers a combination of the lowest tooling and part costs, the shortest delivery time, with maximum strength and design complexity. Unfortunately, this combination of properties does not exist. 

Compare process attributes

There are many different attributes that can be compared between the different processes available for manufacturing plastic parts. However, the processes are dissimilar and used for different kinds of products in very different applications, so they can’t be compared directly. 

Use a rating system for each process, which can assist designers when selecting between the options. No attempt has been made here to favor one technique over another. However, there will be cases when these ratings are 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, identifying 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. 

Where does pressure forming fit?

Pressure forming offers the designer a great option in the ongoing quest for the optimum process-material-cost combination. The number of choices available to the designer increases all the time. Sometimes, it’s difficult to determine which process is best for a given product. 

Pressure and vacuum forming use aluminum dies for production runs. Injection and compression molds are hardened steel. Structural foam and reaction injection molds are aluminum. Resin transfer, lay up fiberglass and spray up fiberglass use fiberglass dies. 

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

The Attributes vs. Processes Chart (Table 3) may be helpful in making such decisions.

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

Conclusion: Using these guidelines to finalize your design

The recommendations presented in the Design Guide are offered as a record of the state-of-the-art as it is known today. These guidelines change and design details evolve as the industry continues to grow and gain experience. In the meantime, the Design Guide will help design engineers faced with the task of finalizing the design of a new plastic part that is going to be pressure formed.