Published 1/28/2022

The Role of Model-Based Definition in Precise Mold Design

Model-based definition replaces technical drawings to communicate precise product definitions between designers and manufacturers.

John Wright McCullough, General Manager Marketing, Kubotek3D

Traditionally drawings have been an indispensable and unambiguous definition of the finished product—a vital part of the service agreement between the customer and those responsible for tooling and inspection. Unfortunately, for many shops, 3D models are still only a supplement to the official authority of the drawing. However, a growing number of companies involved in product design, mold design, machining and inspection are finding ways to effectively communicate across these functions using model-based definition (MBD)—without any drawings.

A primary benefit of MBD is faster turnaround time. This is important, as a project begins with the customer submitting their part design to the molder as a 3D CAD model—including nominal sizes and locations of the geometry—that are accurate to five or six decimal places. This model serves as the basic, unambiguous definition of size and form with no required dimensions.

Using MBD, the customer can code tolerances, surface finishes and special features onto the faces of the model using color attributes. The model file can also include a table defining each color, as this often varies from shop to shop. These face colors on the product model are then carried directly into the mold design and CAM software to automate cycle parameters, eliminating the need for workers to re-enter critical information from the drawing manually.

Turnaround speed can be increased further by limiting this MBD strategy to classes of parts that support the fastest NC machining and process capabilities. For example, smaller parts without any extra-tight design tolerances and moldable in standard aluminum mold bases.

Drawing Challenges

The digital age has changed two fundamental strengths of drawings into weaknesses:

1. Drawings are read by humans.

Part characteristics are designed for humans to read on a physical sheet of paper, which lacks the benefits of modern information technology data structures that drive automation. These structures include efficient databases that can be quickly scanned for valuable traits from numerous perspectives.

Digital CAD drawings demonstrate that storage on paper is not required, yet they still hold onto the powerful but dangerous concept of scale that originated from the physical constraints of paper size. Global adjustment factors like scale present the opportunity for global errors in product definition.

When engineers started to use computers to improve product definition and communication processes, the trend toward definition based on full-scale 3D models began.

2. Drawings are inherently a secondary representation of essential data.

Think back to your high school technical drawing textbook. The part drawing views are derived from the 3D part using the rules of orthographic projection. Unfortunately, this translation step increases the potential for errors in creation and reading.

Another risk level is created when critical annotations and dimensions are linked to the derived drawing views. Since most design changes are done on the 3D model, the CAD software must execute complex rules to automatically update drawing views and annotations to reflect any model changes.

MBD and STEP

STEP (Standard for the Exchange of Product Data) is a standard 3D CAD file format controlled by the International Organization for Standardization. It is a popular neutral 3D CAD file format that is interoperable among various CAD programs. Over the last five years, CAD vendors have been implementing a new STEP application protocol (AP) that builds on top of the popular STEP AP 203 and 214 formats. Basically, the color-based MBD process used in moldmaking today is supported by the AP 214 and AP 203 edition two formats, and the new STEP AP 242 adds the ability to connect tolerance information directly to the model.

Shown here is a STEP AP242-based position tolerance definition attached to a set of faces in a part model. This machine-readable definition of thread requirements can be read directly by process planning and inspection software. Photo Credit: Kubotek3D

AP 242 for managed model-based 3D engineering defines critical manufacturing annotations, including geometric dimensioning and tolerancing (GD&T) and their relationship to the faces of the 3D model. These annotations and the nominal size and position of the precise geometric model provide an unambiguous part definition. This form of MBD provides process efficiency by eliminating the need to translate the design into detailed drawings and better support automation of downstream activities such as NC machining and inspection.

STEP AP 242 adds “machine-readable representation of manufacturing and assembly information, such as assembly tolerances, surface finish and manufacturing process information.”

The extended definitions in STEP AP 242 can improve the MBD process by providing a model that includes standard tolerance, surface finish and unique feature definitions that can be used instead of face color attributes. All of this means that the part information the molder needs to automate CAD, CAM and CMM processes can be in one MBD file directly from the customer. This eliminates potential errors in mapping non-standard face colors from the customer model to their specific processes.

The ultimate goal of STEP AP 242 for MBD is to share specific product, manufacturing and inspection definitions across all software in the supply chain using a standard, machine-readable 3D format, eliminating reliance on technical part drawings.

STEP Review

The core geometric definitions of the STEP format, like common boundary representation (aka B-Rep) solids, were driven by the aerospace industry and first published as AP 203 in 1995. The next widely adopted version of STEP named AP 214 came out of the automotive industry and added the support for face colors critical to the MBD process described above. AP 214 also added layers, annotations and kinematic structures. The color, layer and annotation extensions of AP 214 were later added to AP 203 edition two. Finally, AP 242 includes all of 203 and 214 and is intended to supersede both and unify further development of the standards.

The Bottom Line

Humans make mistakes. People reading critical information from drawings and re-entering it into software takes time and introduces the risk for errors at several points during the manufacturing process. The bottom line is that technical drawings are a significant source of miscommunication and add complexity to the definition process.

This change may be a slow one for many because of the widespread training needed in geometric tolerancing and significant process and cultural changes across extended teams. However, if your shop reacts early to this transition, you’ll have a competitive advantage.