TechTips
5 Rules of Problem-Free Injection Molding
Design engineers don’t have to be fluid dynamics experts to injection mold plastic parts without a hitch. Pitfalls associated with flow
dynamics can still be averted through the use of simple designs and by following general guidelines. The following rules will help engineers
avoid problems when designing injection-molded plastic parts:
Rule 1: Keep Wall Thickness Consistent
Rule 4: Radius Corners Generously
Plastic part walls must be uniform in thickness. This is the most basic
design parameter, and strict adherence to it will eliminate many
manufacturing problems. Parts with uniform walls will not warp, will fill
properly and will fit together because variable shrinkage is minimized.
Wall thickness variations should not exceed 10% in high mold shrinkage plastics. In fact, even this slight disparity can introduce processing
and quality problems.
During injection molding, the molten plastic has to navigate turns or
corners. Rounded corners will ease plastic flow, so engineers should
generously radius the corners of all parts. In contrast, sharp inside
corners result in molded-in stress—particularly during the cooling
process when the top of the part tries to shrink and the material pulls
against the corners. If the inside and outside radii of a part are each
equal to half of the nominal wall thickness, a uniform wall around
the corner can be achieved. Both sides of the corner will display
equal amounts of shrinkage, and sink marks will be avoided entirely.
Moreover, the first rule of plastic design—uniform wall thickness—will
be obeyed. As the plastic goes around a well-proportioned corner,
it will not be subjected to area increases and abrupt changes in
direction. Cavity packing pressure stays consistent. This leads to a
strong, dimensionally stable corner that will resist post-mold warpage.
Rule 2: Provide for Proper Gate Location
If varying wall thickness cannot be avoided, then designers should
provide for proper gate location. If this is not supplied, then attaining
uniform pack of the molded part will be nearly impossible. The most
effective gate location is when the melt enters at the thickest part of
the cavity and then flows to the narrower areas.
Rule 3: Determine Optimal Wall Thickness
Theoretically, there is no maximum wall thickness for injectionmolded parts. But designers are more concerned with determining
the minimum wall thickness because thinner is almost always less
expensive. Two factors contribute to this: first, thinner parts require
less raw plastic material, and second, they cool faster. To determine the
most suitable wall thickness, engineers should first consider product
requirements. Generally, strength dictates the wall thickness. Engineers
can also rely on a finite analysis to select the optimal wall thickness.
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Rule 5: Select Suitable Draft Angles
From a cost and manufacturability viewpoint, the ideal draft angle is
the largest angle that will not lessen the customer’s satisfaction with
the product. The minimum allowable draft angle is harder to quantify.
Plastic material suppliers and molders are the authority on what is the
lowest acceptable draft. In most instances, 1° per side will be sufficient,
but between 2° and 5° per side would be preferable. If the design is not
compatible with 1°, then allow for 0.5° on each side. Even a small draft
angle, such as 0.25°, is preferable to none at all.