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Designing overmolded parts? Time to consider the tool

May 31, 2001

5 Min Read
Designing overmolded parts? Time to consider the tool

Ask any forecaster familiar with the consumer market if the trend toward soft-touch TPE overmolded products is growing, and the answer is a definite "yes." If you ask designers to name the most important factor in designing parts for these products, however, few will point to preliminary mold design. 

Yet that is exactly the point that Mark Mitkowski, application engineer at GLS Corp., wanted to drive home when he jointly authored a technical paper on overmold design. "When designing a TPE-overmolded part," he says, "shutoffs, gates, and runner designs play a critical role in creating a successful part. They are as essential as a properly designed substrate [also called the first shot or rigid engineering thermoplastic]. Sending a part design to a tool designer without specifying these areas first is a bit like playing Russian roulette." 

At the heart of this advice lie sound scientific principles. TPEs flow well, unlike rubber, as long as they are injected quickly for shear thinning. Runners and gates, therefore, must be sized to produce this effect. Shutoffs have to ensure that the high-viscosity TPE doesn't flash. And because overmolded parts have an aesthetic goal, gate vestiges need to be minimized. 

For these reasons, part and product designers need to stretch beyond their area of expertise by adding mold design specifications that ensure the part will be produced correctly. Mitkowski and colleagues at GLS recommend the following dos and don'ts for overmolded part design. 

•••DO test runner balance for multicavity tools. In naturally balanced runner systems, molten material is delivered to each cavity at the same time. Test runner balance by performing a short-shot study, both by software means before tooling is cut, and in reality when the tool is complete. The amount of material entering all cavities should have equal weight if the system is properly balanced. Also, keep the length from the center of the sprue to each gate constant for flow balance. Family molds with different cavity geometries and part sizes should be avoided for high-tolerance parts. 

Unbalanced runners can cause inconsistent part weights and dimensional variability. Cavities closest to the sprue may be overpacked, resulting in flash. Another potential problem, parts with molded-in stress, can generate warpage at elevated temperatures. 

•••DON'T assume all runner configurations are created equal. For TPE molding, full-round runner cross sections are optimum (Figure 1). They provide maximum shear heating and the least resistance to flow. They also have less mold contact than other configurations to minimize cooling requirements, which can maximize pressure and increase the material's ability to fill long thin-wall sections. 

Shutoffs, gates, and runner designs are critical to a successful overmolded part.

While full-round runners are best, a modified trapezoid with 5° to 10° tapers can be used to reduce tool cost. This shape closely simulates the performance of a full-round runner, but is cheaper to machine with a tapered ball cutter. 

•••DO pay attention to runner diameters. For multicavity tools, diameter of the runner that feeds the cavity should be larger than the wall thickness at the gate. This practice minimizes pressure drop and maximizes velocity in the runner. Guidelines for calculating runner cross-sectional areas are shown in Figure 2. 

•••DON'T overlook sprue puller designs. Sprue puller designs vary with material hardness. For harder-durometer TPEs (40 to 70 Shore A), use an undercut sprue puller combined with sleeve ejectors. Materials of less than 40 Shore A require more aggressive designs, such as pine tree pullers with sleeved ejectors. 

•••DO include venting on all tools. Overmolded parts need vented tools for good finished part quality. Vents evacuate the air in the sprue, runner, and cavity. Without them, parts can have poor surface appearance, discoloration, short-shot issues, and weak weldlines. As the sprue, runner, and cavity size increase, more vents are required. Complex geometry parts should incorporate vents at the last place to fill in areas where weldlines are present. 

To find the best areas for venting, identify potential air traps with flow simulation software. After tool construction, short shots can be used to find additional venting locations. 

The total length of venting on the parting line should equal 25 to 30 percent of the cavity perimeter at the parting line. Typical vent size for TPEs should be less than or equal to .001 inch. Vent through ejector pins if possible, because the vents are self wiping and don't require much maintenance. 

•••DON'T take gate design for granted. Gate types and their location, relative to the part, can affect a variety of factors, including part packing, gate removal or vestige, cosmetic appearance, and warpage. In addition, part filling and venting rely on well-designed and well-located gates. 

To prevent jetting of the elastomer and ensure surface quality, place the gate in an area where the flow of TPE impinges, or is interrupted, against the substrate. To find the best combination of design and location, analyze various candidates using flow simulation software. 

•••DO consider texturing and thickness for part aesthetics and function. Weldlines and other surface imperfections in the TPE, such as gate vestiges, can be masked by texturing the mold. It is also a good practice to texture the mold surface for part grips, which can be tailored for soft-touch, wet-grip, or a unique feel. To ensure that the overmolded TPE will meet softness targets, thickness should be specified at .040 to .060 inch for the elastomer. 

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