MEC 1012 - Design Communication II
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)
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
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:
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).
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)
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
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.
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
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
1 degree min.
.5 degree min.
Shut-off and Parting
5 degrees min.
.090 inches (2.3 mm)
Max 60% of Wall
Thickness, Min .020 inches (.5 mm)
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.
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
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
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.
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:
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
by Mary Waldo and Paul Johnson February 2005