Mold Tool Design for Insert Molding

Cable assembly design is much like the build of a new home; it begins with a foundation design that is instrumental to the arrangement of other areas within, and aroun1d that house. That foundation is the beginning of the house’s overall strength, shape and eventual encapsulation of its future contents- furniture, photographic memories and people. Much like building a new house, the foundation, or mold tooling in this case, of a cable assembly will eventually encapsulate contents of value – contacts, PCBs, and other components. The manufacture of high-quality, custom medical cable assemblies typically begin with the foundation of proper mold tool design and fabrication.

Design Requirements for Mold Tooling

Whether designing an outer mold tool for aesthetics or tactile considerations, an inner mold to enhance mechanical integrity or facilitate encapsulation or unique strain reliefs, mold tool design is an important part of most new cable development projects. Engineering and design work is also required when the cable or connector project includes enclosures, handles, switches or custom nose pieces.

Design requirements can include:

• Encapsulation: An inner-mold for the sealing of critical components (such as PCBs, resistors, capacitors)
• Flexibility: For added flexibility, strain reliefs maintain the structural integrity, flexibility and overall robustness between the cable material and connector or grommet
• Gripping: Non-slip finger grips built into the connector over-mold for ease of use2
• Security: Latching features which lock connectors in place and prohibits disconnection during use
• Handling: Uniquely designed handles for maneuvering flexibility, often desirable for surgical applications.

Mold tooling may be designed for maximum versatility, meaning that the mold components may be designed to be interchangeable. Many customers have different configurations of a certain product; having interchangeable tooling allows for the accommodation of different cable diameters, number of lead wires, snaps, etc. Mold tooling is typically designed using a 3D modeling software such as SolidWorks or Pro/ENGINEER.

Mold Tool Components

The components that make up tooling for insert molding are common and typically include:
• Mold Cavity: The part of the tool which will produce the body of the molded component
• Loading Bars: Securely holds the connector housings, pins or other components in the proper position within the tool during the overmolding process
• Gripper Bars: Holds the cable exiting the strain relief in the proper position during overmolding. It is often desirable to be able to use cable with different overall diameter for the same connector. If the gripper bar tooling is separate from the strain relief tooling this is easily accomplished.
• Strain Relief: The portion of the tool which molds a solid or segmented bend relief joining and potentially sealing between the connector to the cable jacket material

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Mold Tool Design

While the intended use and functionality will drive overall mold tool design, the selection of cable material and connectors play a significant role. For both off-the-shelf connectors and custom connectors, the cable assembly will commonly require three tools: inner, outer and strain relief.
Many connector companies offer pre-manufactured slip-on “boots” as an alternative to custom over-molding a strain relief. Overall tooling cost may be reduced by using an off-the-shelf component, but the cost of a pre-manufactured boot is typically higher than a custom overmolded strain relief. Over the life of the program a custom overmolded strain relief will typically result in savings due to lower part cost.
If an off-the-shelf connector which meets all product requirements is not available, a custom connector or hybrid connector – a modified off-the-shelf connector – can be designed. Custom connectors typically require multiple tools to mold the connector housing, nosepiece/contact insulator, latch and other components.
In addition to considering product design, other important considerations when engineering mold tooling include what material will be molded, the shot size, wall thickness of the finished molded part and the material that the tool will be fabricated from.

Specifically for insert molding, the location and size of the gate(s) plays a significant role. The gate is the orifice through which the melted plastic is injected into the mold. Achieving uniform wall thickness, reducing shrinkage, eliminating mold voids and reducing gate vestige are all part of tool design. Engineers designing mold tooling will also take into account the material flow and composition. Typically, polyurethane requires a larger gate than thermoplastic resins such as Santoprene® or PVC.
Easily visible witness lines, also called parting lines, can occur where the two halves of the mold meet and are generally considered undesirable. Engineering mold tooling to reduce flash (excess material that escapes the mold) is another design consideration.

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Tooling for Ultrasonic Welding

Ultrasonic welding is an alternative to insert molding for connector bodies and housings. Typically, two hard, plastic shells, also known as clamshells, are injection molded. The shells are designed to properly fit and seal together through the ultrasonic welding process. One of the shells is typically designed with a spiked energy director which makes contact with the mating shell. The energy from the ultrasonic vibrations causes the material to melt at the point contact creating a strong and typically permanent joint.8

Tooling for ultrasonic welding includes the anvil or nest where the parts are held and the horn which augments and transfers the vibratory energy to the parts being joined. Anvils and horns are typically fabricated from aluminum alloys, titanium alloys or stainless steel. The design and fabrication of ultrasonic welding tools is often contracted to the manufacturer of the ultrasonic welding machine intended to be used.

Not Cut from the Same Mold

Mold tooling can be fabricated from a number of materials. Aluminum, stainless steel and hardened steel are the most common for production of medical cable assemblies. Choosing the most appropriate material to fabricate mold tooling from depends on production volume and life expectancy of program. Two of the most popular mold material styles include:

Aluminum:

• Commonly used for lower-volume production
• Often used for prototyping or to prove out tool and part design
• Lower service-life due to softer material- produces less parts than hardened steel
• Lower tool fabrication led time
• Lower cost than steel

Hardened Steel:

• Best for high-volume production
• Appropriate for proven designs, or designs which are frozen
• Excellent service life – typically up to one million parts
• Typically more costly to fabricate, but longer-lasting

The selection of the tool material is also driven by the resin which will be used. Thermoplastic resins (such as PVC, TPE/TPR, TPU) are room temperature materials that are heated up and injected into a cold mold. These resins can be molded using aluminum, stainless steel or hardened steel molds, and are typically chosen based on the product design, mechanical specifications and cost considerations. Most thermoplastic resins can be molded using aluminum or steel tools.

For Liquid Injection Molding (LIM) silicone applications, cool liquid silicone resin is injected into a hot mold where vulcanization takes place. Because of this, hardened steel is used for tools that run silicone resins. Liquid silicone flows easily and tends to be more prone to flash than thermoplastic resins. Therefore, silicone molds must be designed and fabricated with high precision so that there are literally no gaps for any material to escape from the mold as flash.

Single or Multi-Cavity Molds

Production volume over the lifetime of the product is a key factor when considering how many cavities should be designed into the tool. While typically more costly, mult9i-cavity molds allow two or more parts to be molded at once, increasing output and reducing the cost of each molded part. Designing and fabricating tools with a single cavity is less costly than for multi-cavity tools, but lower up-front costs may be offset by higher production unit costs.

Collaboration and Timeline

Customer communication and design input is essential to tool design and on-time completion. Early and regular collaboration between the customer’s engineering team and the cable manufacturer’s engineering team is important throughout the project, but is even more significant until the product design is frozen. Tooling fabrication is typically the longest lead item of a project, ranging from 8 – 14 weeks. While tooling is being fabricated, documentation is completed and components are ordered. Scheduling weekly design reviews, will help ensure that both the customer and cable manufacturing partner stay on-target and on-time while meeting established product requirements.

Tool Ownership

It is common for OEM customers to pay for and therefore own production tooling. When this is the case, the tooling is only used to produce parts for the tool owner. In some instances, the OEM may elect to share the cost and ownership of the tool in which case, use is not exclusive. Cable manufactures, such as Affinity, may own tooling for common connectors and components which may be used on a non-exclusive basis for little or no tooling cost.

Summary

The design of mold tooling is an essential part of most custom cable assembly projects, and should be the fundamental focus of the project from concept until design freeze. Understanding what the connector and cable assembly is expected to look like, how it will be used, and the desired life of the product is critical to tool design.

The Affinity engineering team has decades of experience designing mold tooling to meet the mechanical and aesthetic features for medical cable assemblies. Let us partner with you on your next new cable or connector project.

For additional information, contact your local Molex Sales Engineer or Account Manager or call Affinity at +1 949.477.9495 or email us at custcare2@molex.com.