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Mechanical Engineering Technology

MEC 1012


MEC 1012 - Design Communication II
Assignment 2
Redesign for New Process, Injection Molding

This assignment is not due until the beginning of your TXL meeting following break (week 6)


Commonly, mechanical designers will attempt to preserve the functionality of an existing design while converting the design to a new manufacturing process. With this assignment you are asked to convert a design for sheet metal fabrication to a design that can be injection molded. The final package requested will include informal drawing views necessary to verify that design requirements have been met as well as the usual set of formal working drawings.

Injection Molding and Casting

In casting (generally metals) or injection molding (generally plastics), molten (liquid) material is forced into a hollow space between shaped pieces of steel (the mold). Here’s a typical mold layout.



Taper that allows part to release from mold


Primarily concave on steel / convex on part (the outside cosmetic surface)


Primarily convex on steel / concave on part (the inside where the knockouts show)

Parting Line

Boundary between moving parts of the mold like Cavity vs. Core


Where Cavity meets Core to ‘shut off’ the flow of plastic to form a hole through the part.


A surface on the part that cannot be reached with cavity or core
(like a hole in the side of a box)


Part of the mold made separate from cavity and core to from undercut features
(like holes) .Also called Side-Pull or Slide.


        Your assembly must preserve the basic functionality of the enclosure seen in the image above and defined by these working drawings: Case and Cover. Specifically, your design must:

a.      Rest on four ‘feet’ and stack with its duplicates or predecessors at an equal pitch (i.e. your design may have a different height, but the distance from a feature on one box to the same feature on the next box will equal the given design).

b.      Hold an equally large rectangular prism (i.e. the biggest block you could drop straight down into the given design is the block yours has to hold)

c.      Use 4 standard screw fasteners (screwed in from above) to hold the top on. Consider designing for the use of threaded inserts. There are good insert references at efunda.com and machinededign.com. Regardless of whether you’re using self-tapping screws (simplest solution) or inserts, you’ll want more than a wall thickness for threads, so modify your shell operation to include ‘unique face thickness’ at the tops of the screw bosses.

d.      Maintain a vertical exterior surface (1 inch diameter minimum) for an integrated dial and pointer. The ‘nose’ hole for the dial stays, but the ‘smiley’ face goes. Emboss ‘HI’ and ‘LO’ as in the original or emboss better graphics.

        Your parts must be designed for the injection molding process. Specifically, the minimum reasonable tolerance is +/- 1.0% but no tighter than +/- .05 mm or +/- .002 inches

Cavity Draft

1 degree min.

Core Draft

.5 degree min.

Shut-off and Parting Line Draft

5 degrees min.

Wall Thickness

.090 inches (2.3 mm)

Rib Thickness

Max 60% of Wall Thickness, Min .020 inches (.5 mm)

There are some very good guidelines for injection molding design at efunda.com (note menu items at left) and at protomold.com. Also, there are design guidelines for injection molding at engineersedge.com.

        Create and apply User Parameters for cavity draft, core draft, wall thickness, and rib thickness .

        Model your cover to be Adaptive to the size on the lower ‘case’ by projecting geometry from case to cover while editing the cover within an assembly of the two.

        Your documentation must adhere to the standards of completeness and quality outlined in Priorities 1 and 2 of the MEC 1011 Final Project

        Your tolerances must be consistent with your design guide’s stated process capabilities and minimum clearances (i.e. your parts must fit together and be ‘manufacturable’). This does not require geometric tolerances, but consider using them anyway.


        Produce a concept sketch of an assembly that meets the conditions above.

        Produce a complete set of working drawings based on your concept using Autodesk Inventor. Set up and use an Inventor project file in V:\MEC\1012\Spring_05\Assignment2\TXA\username. Use A-size portrait or B-size landscape sheets.

        Produce the models and drawings necessary to complement the working drawings to show that your design meets requirements. Include ‘check sections’ showing rectangular prism capacity, drafts, wall thicknesses, etc.

        Develop a design guide, using Word, to compile the rules of the design, ranging from aesthetics to manufacturing process plans and capabilities (capabilities include minimum reasonable tolerances and what geometric restrictions apply, like draft for casting). Your guide should also specify minimum clearances between mating parts. Consider table and outline structures for your design guide (see example). Save your design guide (.doc or .htm) in your V: drive folder.

        Exchange feedback with a peer. After making all necessary changes based on feedback, you must indicate who checked the drawings using the 'Checked by' iproperty.

        Turn in a paper package (stapled top left) consisting of:

e.      Design guide

f.        Concept sketch

g.      Set of working drawings (A-size portrait or B-Size landscape printed 1:1 with drawing scale of your choice. Fold B-size pages accordion style).


Keep it simple. Completeness and quality are more important than complexity.


Does your set of working drawings fully define an assembly of parts that are consistent with your complete design guide?

See Example of a Positional Tolerance Scheme

Typical stuff you’ve seen before shown in this color

Developed by Mary Waldo and Paul Johnson February 2005