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Plastic Parts by the Numbers

Designing an injection-moulded part and briefing its tool, from wall thickness to first-off approval

Design an injection-moulded part with numbers: wall, draft, ribs, bosses, snap fits, shrinkage, gating, clamp force, cycle time, tool cost and first-off approval.

10 chapters · About 100 minutes · Free to read · Intermediate product and tooling design

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KEVOS® LibraryMANUFACTURING1234Plastic Parts bythe NumbersKEVOS® ORIGINAL BOOK10 CHAPTERS

About this book

An injection-moulded part is decided twice: once by the designer who draws it and again by the toolmaker who cuts the steel to make it. Most of the expensive problems in moulding come from the gap between the two. A wall that is thicker in one place than another sinks and warps. A rib that is too thick shows through on the cosmetic face. A snap hook strained past its limit cracks on the first assembly. A tolerance copied from a machined part cannot be held in plastic at any price. Each of these is predictable with a little arithmetic and the material supplier’s data.

This book is for designers and engineers who model plastic parts in CAD and need them to mould well, assemble reliably and pass first-off inspection. It assumes familiarity with CAD and basic mechanics, but no previous moulding experience. Each chapter takes one design decision, explains the physics behind it, applies it to the running case with real numbers and ends with an exercise and its worked answer.

The running case

The book follows a fictional polycarbonate access cover for a small steel control box on a conveyor. The cover is 160 mm long, 100 mm wide and 20 mm deep, with a nominal wall of 2.5 mm. Two cantilever snap hooks hold one end; two screw bosses take thread-cutting screws at the other. A grid of ribs on the underside keeps the large flat top from flexing or warping. Annual demand is about 8,000 covers. The material is an unreinforced, easy-flow polycarbonate, and the book uses one published grade’s typical data sheet values as its reference properties.

All dimensions, quantities, process settings, prices and results are invented for teaching. This is an illustration. They are not KEVOS® projects, quotations or measured data. Money is in Australian dollars, and each example states whether GST is included.

Source limits and professional responsibility

The supplied CAD textbook informed the topic review. The method, case, calculations and figures are original. Design guideline values are taken from named material-supplier publications and attributed where used; they are general guidance from one supplier for its own materials, and other materials and suppliers may differ. Typical data sheet values are not specifications, and real parts should be designed with the supplier’s current data for the chosen grade and colour.

This book is educational. Parts that carry safety-critical loads, contain electrical equipment, contact food or medicine, or must meet flammability or other product standards have requirements beyond its scope. Qualified review, the moulder’s and toolmaker’s own expertise and testing of real parts govern real work.

Who this book is for

Mechanical and product designers, CAD users and manufacturing engineers who design moulded plastic parts and need to brief a toolmaker and approve the first parts with confidence.

What you will learn

  • Set wall thickness, draft, ribs and bosses from material data and explain each choice with a calculation.
  • Size a cantilever snap fit for strain, deflection force and assembly force.
  • Turn published shrinkage into mould dimensions and realistic part tolerances.
  • Estimate clamp force, cooling time, cycle time and part cost for single and multi-cavity tools.
  • Plan first-off trials, read common defects and approve a part against a written brief.

Inside this book

Chapters

  1. 01Define the part before you draw the mouldWrite a moulded-part brief covering function, loads, environment, appearance, assembly, volume and tolerances, then choose the process and material against it.
  2. 02Wall thickness sets nearly everythingChoose a nominal wall from stiffness, flow and cooling, estimate cooling time from the material's thermal data and remove thick sections before they sink.
  3. 03Draft, parting lines and undercutsChoose draft angles from texture and depth, calculate what draft does to dimensions and wall thickness, place the parting line and price each undercut before the tool is cut.
  4. 04Ribs, bosses and the sink they can causeSize ribs for stiffness without sink, bound the cover's deflection by hand, design screw bosses from supplier guidance and keep every feature steel safe.
  5. 05Snap fits that assemble and releaseSize a cantilever snap hook for permissible strain, deflection force and assembly force, check the worst-case undercut and design it to release for maintenance.
  6. 06Shrinkage, warpage and realistic tolerancesSize the mould from published shrinkage, set tolerances that moulding can hold, check the fit through temperature and keep critical dimensions steel safe.
  7. 07Gates, runners and how the part fillsChoose a gate position from flow length, weld lines, venting and appearance, compare cold and hot runners and use a mould flow analysis to check the choice.
  8. 08Clamp force, shot size and cycle timeEstimate the clamp force and shot a part needs, choose a machine size, build up a cycle time from fill, pack, cooling and ejection, and see where seconds can be saved.
  9. 09Tooling, cavities and part costBuild a part cost from material, machine time, set-up and tool amortisation, compare one and two cavities, find the break-even volume and state GST correctly.
  10. 10First-off trials, defects and approvalRun first-off trials with a plan, recognise and correct common moulding defects, prove a process window, inspect first articles and approve the part against its brief.

Apply This in Practice

Use these KEVOS® tools to try the method from this reading with your own numbers.

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 26 October 2026.

  • Ibrahim Zeid. Mastering SolidWorks: The Design Approach, second edition (2015). Pearson Education, publishing as Peachpit. Supplied source, reviewed for its subject map of injection moulding, mould design and part manufacture from a CAD model. No text, tutorials, figures, examples or problems are reproduced, and this book follows its own sequence and case.
  • Covestro LLC. Engineering Polymers: Part and Mold Design, Thermoplastics A Design Guide (undated edition). Reference for wall-thickness behaviour, rib thickness as a percentage of wall for polycarbonate, rib height and spacing, boss proportions and hole size for thread-cutting screws, draft, corner radii, blind-hole depth, shrinkage behaviour and the steel-safe practice. Guideline values are attributed where used; no figures or text are reproduced.
  • Bayer MaterialScience. Snap-Fit Joints for Plastics: A Design Guide (undated edition). Reference for the cantilever deflection, deflection force and mating force equations, the permissible short-term strain of 4% for unreinforced polycarbonate, the guidance to use about 60% of it for frequent separation, the secant modulus and friction data used in Chapter 5. Its calculation example is not reproduced.
  • Covestro. Makrolon 2405 product data (current product page). Typical properties of an unreinforced, easy-flow polycarbonate used as the reference grade: density, modulus, yield, moulding shrinkage, heat deflection temperature, expansion coefficient, thermal conductivity, processing temperatures and drying conditions. Typical values, not specifications.
  • Jeremy Williams, RJG Inc.. Injection Molding Cooling Time: A Breakdown (2017). Supports using the heat deflection temperature as the ejection temperature in cooling estimates, adding a margin to the calculated cooling time and the observation that cooling dominates the cycle. Its published equation uses a different constant from the one-term slab solution used in Chapter 2.
  • International Organization for Standardization. ISO 20457, Plastics moulded parts: Tolerances and acceptance conditions (2018). Named by number and scope only as the international standard for moulded-part tolerances. No values are quoted.

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