3 Key points for Overmolding

Release time:2020-04-16

This design tip explores three important points of overmolding:

Bonding
Materials
Principles

What is Rapid Overmolding?

This process uses a mechanical or chemical bond (and oftentimes both) to permanently marry two parts together. This sidesteps assembly hassles, simplifies product design, and can improve the characteristics of many injection-molded parts.

In Ideal Group , it works by placing a previously molded part—the substrate—back into the press and injecting a second plastic or liquid silicone rubber (LSR) over, into, and around the original part. The two-shot process requires a pair of molds—one for the substrate, and one for the complete, overmolded product. It also needs a human to tend the machine, loading substrate parts and unloading completed products, a process known as “pick-and-place” overmolding.

What’s next? Before embarking on any overmolding design project, several design considerations should be explored first:

Bonding. A strong bond between the two materials is critical to overmolding.
Materials. Substrate and overmold materials should be physically, chemically, and thermally compatible.
Principles. The principles of moldability apply equally to overmolding, with a few additional considerations to keep in mind.

SUBSTRATE MATERIAL
OVERMOLD MATERIAL	ABS LUSTRAN	ABS/PC CYCOLO C2950-111	PC LEXAN 940-701	PBT VALOX 357-1001	PP PROFAX 6323	NYLON 66 ZYTEL 103 HSL NC010
TPU - Texin 983-000000	C	C	C	C	M	M
TPV - Santoprene 101-87	M	M	M	M	C	M
TPE - Santoprene 101-64	M	M	M	M	C	M
LSR - Elastosil 3003/30 A/B	-	-	M	M	-	M
TPC - Hytrel 3078	C	C	C	C	M	M
TPE-Versaflex OM 1060X-1	C	C	C	M	M	M
TPE-Versaflex OM 6240-1/td>	M	M	M	M	M	C
TPE-Versaflex OM 6258-1	M	M	M	M	M	C
TPE-Versaflex OM 1040X-1	C	C	C	M	M	M

M = mechanical bond (recommended)
C = chemical bond

There are many reasons to overmold. One of the most common is to improve a product’s grip while retaining its physical strength—the handle on a power tool, for example, or a non-slip grip for a surgical instrument. In this case, TPU over ABS is an excellent choice. Aesthetics and product branding are also readily achieved with overmolding—a sports franchise might use the team colors in two-piece overmolded mouth guards for its players, while a well-known tractor manufacturer could dress up its riding lawnmowers with green and yellow overmolded cowlings.

Following the Principles

Proper draft angles, uniform wall thickness, and smooth transition lines must be maintained in both parts.
The thickness of the overmold material should be less than or equal to that of the substrate below it.
The melting temperature of the overmolding material should be less than that of the substrate (as in our LSR example).
If chemical bonding isn’t practical, don’t despair. Mechanical interlocks are a great way to “hold it all together,” and should be used wherever possible.
Texturing of the substrate workpiece may help with adhesion. Texturing of the overmolded part may provide a better grip and more attractive surface.
The surface of the overmolded part should be even with or slightly below any adjacent substrate surfaces.

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