Plastic Part Design Aid

Plastic Part Design Aid

This software will guide a designer through the “best practice” methodology of designing a plastic part. In general, the steps are divided into four parts:

1) Specify the functionality of the part. What is the part supposed to do? Is the part hollow? Does it need to support a constant load? This information will help choose a manufacturing process. In this section, the software will ask a series of yes and no questions to define the part's functionality. Based on the responses, the software will recommend a process.

2) Verify the process. This section has explanations, movies, and example parts for each process. If the designer is unfamiliar with the process, he/she should learn about the process and verify that it is a good choice. Annual part volume should also be considered when choosing a process.

3) Define the part’s material based on the part requirements. The software will ask several questions about part requirements, including mechanical properties, optical properties, chemical resistance, etc. The software will search a database and recommend potential material families. All data in the material database is based on high average data of a material family, since material properties vary widely within each family.

4) Design the part to the process. There are several design sections for each process. Depending on the chosen process, the designer should learn about each design concept and use it when applicable.

IMPORTANT! This software will attempt to guide the designer down a reasonable path when choosing a process and a material. It is important to realize that the answers given by the software should be taken as RECOMMENDATIONS, and not absolutes.

This software will start with step 1 and proceed though the steps, aiding the designer in making logical choices.

In addition to these steps, the software also has some extras to help in other situations.

5) Guide to transferring existing business. If an existing job is transfer from one molder to another, there are certain steps to follow to ensure a smooth transition and minimize problems.

6) Price estimating guide. The software contains an Excel spreadsheet to help estimate the cost of producing a part. It also has instructions explaining how to fill it out.

7) Miscellaneous topics. Some other topics that do not belong anywhere else are also covered. These include optical properties, coloring, flammability, and resources.

Functionality

What is the part supposed to do? Is the part hollow? Does it need to support a constant load? These functionality questions will help the software choose a manufacturing process.

Manufacturing Process

After asking the functionality questions, the software will rank the manufacturing processes in order of feasibility. The software contains in-depth looks at all of the processes.

Injection Molding: Generally high annual volume to offset the expensive mold cost. Capable of complicated geometry. Injection molding is the most common process.

Blow Molding: Generally used to produce hollow parts, like bottles. Volume is generally high, but the mold cost is not nearly as great as for injection molding.

Thermoforming: High or low volume. Parts are formed from a heated plastic sheet, so geometry is limited.

Extrusion: High or low volume. Parts must have a constant cross section, like tubing or vinyl siding.

Rotational Molding: High or low volume. Parts are generally very large, with enclosed volumes (like storage tanks, gas tanks, garbage totes).

Casting: Low volume. Usually used as a prototyping method to make a few parts.

Compression / Transfer Molding: Generally high to medium volume. This process is used to produce parts made from thermoset material. Thermosets are plastics that cannot be remelted; they have excellent high temperature properties and will not burn. Compression and transfer molding is usually used when the parts have metal inserts (see the manufacturing section for more details). Thermosets are often used in electrical components.

Injection Molding (Thermosets): High volume. This process is similar to injection molding thermoplastics (the most common type of plastic).

Material

The software will ask several questions about part requirements, including mechanical properties, optical properties, chemical resistance, etc. The software will search a database and recommend potential material families. All data in the material database is based on high average data of a material family, since material properties vary widely within each family.

Part Design

In the design section of the software, the best practice design guidelines are presented for each process. The designer should carefully look over each topic relating to the appropriate process, and design the part to conform to these guides. Some of the most common guidelines are listed below.

· Use uniform wall thickness throughout the part to minimize filling problems, warpage, residual stresses, and cosmetic problems.

· Use generous radii at all corners to ease filling and reduce stress concentration.

· Use the least wall thickness compliant with the process, material, and design requirements. This will minimize material cost and cycle time to mold.

· Design parts to be removed from a mold. When a feature on a part cannot be pulled from the mold, it is called an undercut. Undercuts should be avoided whenever possible because they add cost to the mold and add time to the molding cycle. All walls perpendicular to the pull direction should be tapered (draft) to ease demolding.

· Use ribs or gussets to increase part stiffness instead of increasing wall thickness.

In addition to an explanation of these guidelines, the software will also show the designer how to properly design engineered part features like snap fits, living hinges, screw bosses, and ribs.