Posted by Paula Hynes | 05 / 12 / 15 0 Comments

There are four key factors that go into making a high-quality plastic injection molded part: part design, tool building, material selection and manufacturing.

In this article, we will start at the beginning of the process, with the part design.

The process of plastic injection molding is designed to produce precision parts at a low cost.  The part design must be developed to maximize the efficiency inherent in high-volume molding.  With the right design, parts can be made consistently and with quality.  Without a good design, costly processing mistakes can be made.

Around the industry, most molding professionals agree that there are key design elements that must be held to a high standard for the injection process to work properly. They include wall thickness (t), rib design, boss design, corner transitions and weld lines and gate placement, and properly placed vents.

Wall thickness

Maintaining a uniform wall thickness in your part design will resolve most of the defects that can occur during manufacturing.  Plastic, when melted, will flow into areas of least resistance.  If you have a part with both thick and thin-walled sections, depending on the gate placement, the melt will flow into the thick walled areas first.  The thin areas may not fill and pack properly.  In addition, thicker areas will tend to cool more slowly and may be subject to voids or sinking defects.  If your part design does not have a consistent wall thickness, there are some things you can do to avoid improper fill.  You can design the cavity, so the thicker areas transition gradually into the thinner areas.   The gate placement can also help ensure the cavity fills properly.   However, it is still best to have a uniform wall.  Uniform wall thickness helps the mold fill and cool properly. Thicker areas in the part design can be cored or hollowed out to maintain uniformity.  These modifications will improve part quality and appearance.

poor part design drawing example of a part with sharp angles and edges                               proper part design example: part with rounded corners

Rib design

Ribs are used to help reinforce the overall strength of a part.  Like flying buttresses, they support the walls and other dimensional components of a design.  Depending on the material used, they should be between 50% to 70% of the wall thickness.   Greater width could cause sinking to occur.  To avoid this problem, a designer will often core out some material to reduce the shrinking. In addition, ribs cannot be too tall or too thin.  The height recommendations are generally no more than 3 x the wall thickness.  The corners should include radii and the height should include a draft (.5 to 1.5 degrees). The draft angle allows the part to be ejected from the mold.

Boss design

A boss is included in a part design to accommodate part assembly through screws or pins.  They should conform to the same thickness as ribs.  Thicker bosses will create sink defects as the part cools.  As an alternative to a thick wall on a boss, the designer can use ribs to support the boss cylinder.  And as with ribs, the boss also needs to have some draft to aid in ejection.

 3D boss example

Corner transitions

As we mentioned earlier, plastic melt flows into the area of least resistance.  Sharp corners or angle changes impede this flow.  These abrupt geometries can cause the cavity to not fill or pack properly, creating a part with defects.  Smooth transitions are all important in the injection molding process.  So corners should have a curve versus an angle. The radius should be consistent on the inside and outside of the wall creating a uniform thickness.   By using this design, the material will be able to flow throughout the cavity.

Weld lines

All plastic injection molded parts have weld lines.  They key is to design the tool so that they do not compromise the part integrity.  The part and tool can be built with varying geometries, wall thickness and strategic gating to place the weld in a position that does not interfere with the part performance. 

Welds are formed when two material flows come together.  This juncture often has cosmetic and structural problems.  In fact, weld line considerations are one of the top issues facing injection molders.  Computer aided flow simulators such as Solidworks Flow Simulation can help designers determine where the weld line will occur.  This flow analysis gives the designer information to adjust the part design and tool gating to minimize the impact.


The gates are the key to ensuring your mold fills properly.  They direct the plastic flow from the runners for distribution throughout the part.  Choosing the type of gate and its placement and will greatly impact the part quality and integrity.  The wall thickness and the geometry dictate the size and placement of the gate.  And, their location should minimize the flow length to avoid flow marks.  Gates that flow from thick to thin walls will fill and pack better than the reverse.  In some
instances, additional gates may be needed to avoid cooling problems and shrinkage.

Vent placement

Vents will allow the gas (air) in the mold cavity to escape without causing super-heating or burning of the resin.  They will also assist in minimizing weld lines if put in the proper locations.

Improved part design

Reputable plastic injection molding companies help their clients improve part design to advance the overall functionality and appearance of the  part.  At The Rodon Group, we have worked with a variety of industries providing innovative solutions to their plastic part designs.  We know how to create the best quality product at a competitive price.  If you would like to learn more about our capabilities, please watch our new video.

Go inside The Rodon Group factory  Watch how we've remained a leader  in U.S. plastic injection molding since 1956





Topics: Plastic Injection Molding, Manufacturing, Plastic Injection Molds and Presses

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