20 OCTOBER 2017
specifications takes a skilled team and advanced
manufacturing technologies. Generally speaking tight
tolerance parts for injection molding is + /- 0.002 inches
(Figure 2). Many factors are put into play such as materials,
part complexity, tooling, and the injection process.
Starting with a good part design will ensure tight
tolerance repeatability, improved manufacturability and
reduced costs of your plastic injection molded parts. Size,
geometry, and wall thickness requirements have an impact
on tolerance. Thicker walls produce different shrink rates
depending on the material, making repeatability difficult.
Before manufacturing, address and analyze your parts
making sure to receive both a mold flow analysis and Design
for Manufacturability review will help ensure a successful
injection molding process and reduce costly delays.
• Utilize low-shrinkage materials for parts with tight
RULES FOR RIBS
• Avoid tight tolerance areas around the
alignment of the mold halves (parting line) or
moving mold components such as sliders
• Design your parts to avoid tight tolerance in
areas prone to warpage or distortion
Often used for structure reinforcement, ribs
allow greater strength and stiffness in molded plastic parts
without the need to increase the wall thickness. Thicker
ribs will often cause sink and other cosmetic problems
on the opposite side
surface to which they
As a general rule,
design ribs that are
percent of the joining
wall thickness for
minimum risk for sink
marks. Glossy materials,
however, require a
thinner rib (40 percent
of wall thickness).
Keep in mind thin ribs
may be more difficult
to fill. Table 2 shows
the recommended rib thicknesses as a percentage of wall
Proper rib design involves five parameters:
• Thickness - Affects cooling rate and degree of shrinkage
which may cause warp.
• Location - Ribs added to uncritical areas can actually
reduce impact resistance.
• Quantity - It’s easier to add ribs than remove them so
they should be used sparingly and added as needed.
• Moldability - Thin ribs can be difficult to fill so to be sure,
obtain a manufacturability analysis.
BE THE BOSS OF YOUR BOSSES
Bosses are used for locating, mounting and assembly.
Following the guidelines for boss design will have an
impact on your final part. Wall thickness and height are the
Wall thickness - the wall thickness around a boss
design feature should be 60 percent of the nominal part
thickness, if that thickness is less than 1/8”. If the nominal
part thickness is greater than 1/8” the boss wall thickness
should be 40 percent of the nominal wall.
Height - the height of the boss will also have a role. As a
general rule, the height of the boss should be no more than
2-1/2 times the diameter of the hole in the boss.
An undercut is any indentation or protrusion that
prohibits an ejection of a part from a one-piece mold. Most
commonly categorized by either an internal undercut or
external undercut and requires an extra part to capture the
detail as part of the mold.
Undercuts (Figure 3) typically lead to increased mold
complexity and can lead to higher mold construction costs.
Usually, a simple re-design of your parts to eliminate or
minimize undercuts can lead to lower cost tooling and a
more efficient molding process.
When an undercut feature cannot be removed from
the part design, it will most likely require internal mold
mechanisms to help facilitate the ejection. Typically, the
mechanisms consist of side-action slides, jiggler pins, lifter
rails, collapsible cores and unscrewing mechanisms.
• When possible, design your part to eliminate undercuts
Table 2. Rib thickness should be the recommended
percentages of wall thickness shown here.
Figure 3. Proper mold design minimizes the use of undercuts, thus cutting costs.