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Housings That Rebuild
Curves, continuity, surfaces, shells and split parts for a plastic housing model that survives change
Model a two-part plastic housing with hybrid surface and solid methods: clean curves, chosen continuity, shells that solve, robust references and a master model split into parts.
Start readingAbout this book
Most designers can build a block with fillets. Fewer can build a curved housing that still rebuilds after the industrial designer raises the crown by 2 mm, the electronics team lengthens the board by 10 mm and the moulder asks for a thicker wall. This book is for the second job. It treats an advanced part model as a structure of decisions: which curves control the shape, how faces meet, which features are allowed to depend on which, and how one master shape becomes several parts.
You should already be comfortable with sketches, constraints, extrudes, revolves, fillets and simple assemblies in a parametric CAD system. Command names follow SOLIDWORKS where an example helps, because that is where the supplied material came from, but the methods apply to any history-based modeller with surface tools. Read the chapters in order the first time. Each one builds a piece of the same model and ends with an exercise and a worked answer.
The running case
This is an illustration. The book follows a fictional handheld moisture-meter housing moulded in PC/ABS. It is 160 mm long, 72 mm wide and 34 mm tall, with a 2.0 mm nominal wall, a crowned top that rises 6 mm across its width, plan-view corners of R10, a 50 mm by 30 mm display window and a narrowed sensor nose. The housing splits into a top cover and a base at a parting height 14 mm above the underside.
Every dimension, mass, failure and test result in the case is invented for teaching. None of it describes a KEVOS® project, client or product. Calculations were checked by running them, and figures carry the same numbers as the text.
Source limits and professional responsibility
The Library already introduces curves, surfaces, face editing and design intent in its edited CAD collection. This book goes deeper on one connected case and on the failure modes that the overview chapters only mention. The supplied slide deck informed the topic review only; nothing from it is reproduced.
The book teaches modelling method. It does not certify a part for moulding, impact, ingress protection, electrical safety or any regulated use. Draft, wall, boss and joint dimensions in the case are starting points for discussion with a moulder and resin supplier, not design values. A real product needs qualified review, supplier confirmation and its own testing.
Who this book is for
Mechanical and product designers who can already build solid parts in a parametric CAD system and now need curved housings that shell, split and survive design changes.
What you will learn
- Capture a housing's design intent in a small set of controlling sketches, planes and parameters.
- Build curves with few controls and judge G0, G1 and G2 joins with curvature and zebra evidence.
- Choose between extrude, sweep, loft, boundary and fill features, and avoid degenerate patches.
- Diagnose and fix shell, knit and rebuild failures by testing rather than guessing.
- Split a master model into separate parts with lips, bosses and draft that update together.
Inside this book
Chapters
- 01Decide what the model must keepTurn a housing brief into controlling sketches, planes and parameters so later changes move the right faces and nothing else.
- 02Build the curves firstReplace chains of arcs with a few controlled splines, read curvature combs and fix imported outlines before any surface depends on them.
- 03Choose the continuity at each joinDecide where a housing needs position, tangent or curvature continuity, check it with combs and zebra stripes, and count the cost in setback and radius.
- 04Pick the feature that fits the patchChoose between extrude, sweep, loft, boundary and fill surfaces for each region of a housing, and avoid the three-sided patches that later refuse to offset or shell.
- 05Trim, knit and make it solidJoin overbuilt surfaces into a closed skin, turn it into a solid, and use delete face, replace face and cut with surface to move between surface and solid work.
- 06Make the shell solvePredict shell failures from radius and topology, isolate them with offset tests and diagnostic cuts, and fix them without weakening the design.
- 07Test the model by changing itFind fragile references before a colleague does: flex-test a housing with planned changes, resolve ambiguous edges and choose references that survive topology changes.
- 08Split the master into partsDivide one master housing into a cover and a base that update together, add lips, bosses and draft to each part, and check the pair before release.
Apply This in Practice
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References and source notes
The manuscript and worked examples are original KEVOS® writing. References inform particular concepts; their authors do not endorse this book. Web references were consulted on 25 October 2026.
- Mark Biasotti. Advanced Modeling Techniques (presentation slides) (c. 2003 (PDF dated April 2003)). Supplied conference-style slide deck reviewed for its topic list: design intent, curve continuity, loft and fill choice, surface utilities, shelling problems and split-part methods. No slides, wording, images, exercises or sequence are reproduced; the case, calculations and figures in this book are original.
- Dassault Systèmes. SOLIDWORKS Help (current online help). Checked for the names and general behaviour of commands mentioned as examples, such as Fill Surface, Knit Surface, Thicken, Replace Face, Split and multibody parts. Other parametric CAD systems offer equivalent tools under different names.
- Les Piegl and Wayne Tiller. The NURBS Book, 2nd edition (1997). Springer. Background for B-spline and NURBS curves, control points, degree and knot behaviour described in Chapters 2 and 3.
- Gerald Farin. Curves and Surfaces for CAGD: A Practical Guide, 5th edition (2002). Morgan Kaufmann. Background for parametric and geometric continuity and curvature as used in Chapter 3. The clothoid-pair blend calculation is this book's own numerical example.
- Robert A. Malloy. Plastic Part Design for Injection Molding: An Introduction, 2nd edition (2010). Hanser. Background for draft, nominal wall, bosses and lip-and-groove joints in Chapter 8. Specific dimensions in the case are teaching choices to be confirmed with the moulder and resin supplier.
- International Organization for Standardization; American Society of Mechanical Engineers. ISO 10303 (STEP) and ASME Y14.41, Digital Product Definition Data Practices (current editions). Named by number and scope only, as the exchange format and model-based definition practice that a released model may need to satisfy. No clauses are quoted.