Designing in Plastic, Part 1 of 2

1 Apr, 2004 By: Adrian Scholes

Solid Edge delivers the tools to create stylized surfaces for plastic parts

Plastic parts, common in consumer products, present some interesting challenges for the designer. Not only do plastic parts need to be aesthetically pleasing, they must be functional and manufacturable as well. Solid Edge offers plastics designers flexible shape-creation capabilities as well as process-oriented tools to add typical engineering features. In this first of two columns about using Solid Edge to design plastic parts, I'll discuss some ideas for getting the form you need. Next month, I'll explore the many features, such as webs, mounting bosses, and vents, you can use to add function to your plastic parts.

Shape-Preserving Curves
Curves are the foundation of most shape modeling, so it's critically important that they represent the desired shape and be highly flexible for editing downstream. Solid Edge's Rapid Blue curves incorporate three types of points that meet these requirements. You can use standard constraint and dimension commands to edit and constrain these points in any combination (figure 1).

  • Edit Points lie along and remain connected to the curve.
  • Control Vertices modify more of the curve than Edit Points, so edits tend to result in smoother curves.
  • Silhouette Points identify the high and low points of the curve and can be used to attach other key elements that belong at that horizon point.

Figure 1. Using Solid Edge's flexible constraint system, you can define highly predictable curves that preserve your intended shape throughout any downstream changes.

Spend some time understanding and working with these different types of points. By incorporating different control options into your designs, you can easily capture and maintain your design intent with minimal edits. For example, when moving a single point on the curve, you can choose either Local Edit or Shape Edit from the ribbon bar. Using Local Edit, when you drag a vertex point on an unconstrained curve, no other vertex points move, so you affect only the shape of the curve around the edit point. When you use Shape Edit to move a point, the change affects the shape of the entire curve. Using Shape Edit, any shapes you create from the curves preserve their general characteristics through all but the most extreme edits.

You also can add or remove edit points along a curve. To add a point, click the Add button and then click the curve where you want to insert the point. When you add an edit point, the shape of the curve does not change. To remove a point, simply hold the Alt key and click the point you want to remove. When you remove edit points, the shape of the curve changes to reflect the remaining points. Click the right mouse button on a curve and use the Simplify Curve command to reduce the number of edit and vertex points.

BlueDots are a key element in Rapid Blue's flexibility. BlueDots abolish any order dependence among sketches so you don't need to worry about when to construct them (figure 2). Any edit to one sketch causes the appropriate edit to the others, regardless of the order in which you created the sketches.

To add a BlueDot, select the command and click a point on each of the two elements. You can choose key points or a point along the elements. If you previously disconnected the elements, they become connected and stay connected as you edit. You do not need to locate and edit the parent curve to drive the edit.

You can then edit the position of the BlueDot to change the shape of the elements and any associated surfaces:

  • Choose the Select Tool.
  • Click a BlueDot.
  • Click the Dynamic Edit button on the Select Tool ribbon bar.
  • Drag the BlueDot to a new position, either freely or using OrientXpres to restrict the movement to a particular axis or plane.

Figure 2. In a history-based system, you can't make Curve 1 dependent on Curve 5, because Curve 1 precedes it in history. BlueDots, however, allows sketches to exist anywhere in the feature tree, yet still be part of a peer-to-peer solution.

Convert to Curve
One of the interesting aspects of Solid Edge Rapid Blue technology is its inherent understanding that creative design requires exploring many alternatives, and that starting simple and adding further control is a desirable workflow. One feature that illustrates this is the Convert to Curve command. Convert to Curve lets you convert analytic elements, such as lines, circles, and arcs, to flexible curves with a single click, without destroying any downstream work you've already completed.

For example, you may start out using a simple arc to create a face on a part, but as the design evolves, you decide that further styling requires a more flexible curve. With traditional surfacing, you'd need to do a significant amount of rework after such a change, often manually repairing the history tree. In contrast, Convert to Curve lets you start with simple analytics and add style as needed during any part of the design process.

BlueSurf is a new surface-generation command that supports any combination of cross sections and guide curves. When you need more control, you can easily add curves or cross sections without harming downstream features (figure 3).

You can create a BlueSurf from cross sections only, or cross sections and guide curves, using sketches or part edges to define open or closed geometry. In the case of closed cross sections, the End Capping option specifies whether the ends of the feature are left open or closed. The Close Ends option creates a solid body.

To add more control to an existing BlueSurf, use the Insert Sketch Step on the ribbon bar and simply drag and drop to define the position of the new reference plane. The new sketch is guaranteed to match the surface, so no changes in shape result. BlueSurf is the first command that enables you to insert either a cross section or a guide curve and, once you create the new sketch, select and edit to refine the surface shape.

Figure 3. BlueSurf lets you start with minimal sections and guides to describe the shape, adding guides or sections only where further styling is needed.

You also can specify whether the new section or guide will be connected to the surface using Pierce Points or BlueDots. Base your choice on how you want to edit the feature later.

Pierce Points use connect relationships to tie the inserted sketch to any cross section or guide curve that it intersects, maintaining the parent-child history of the model. Modifying the original cross sections or guide curves updates the inserted sketch. Pierce Points, therefore, are most suitable for models that must conform to engineering data or dimension-driven criteria.

Connecting the inserted sketch using BlueDots gives you greater flexibility and is most suitable for models driven by aesthetic requirements. Moving a BlueDot changes not only the connected sketches but also the location of the reference planes, so you can manipulate the shape freely until it meets your needs.

New Level of Control
Solid Edge's Rapid Blue technology provides a new level of design control and flexibility for creating more stylized shapes. Have fun with these capabilities! Next month we'll look at how to add plastic-specific features to your models so that when you change the outside shape, the internal engineering features update correctly and retain the desired technical functions.

See you On the Edge next month.

AutoCAD Tips!

Lynn Allen

In her easy-to-follow, friendly style, long-time Cadalyst contributing editor Lynn Allen guides you through a new feature or time-saving trick in every episode of her popular AutoCAD Video Tips. Subscribe to the free Cadalyst Video Picks newsletter, and we'll notify you every time a new video tip is published. All exclusively from Cadalyst!

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