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Application Stories:
Specifying metal inserts for molded plastics

Metal inserts are integral to the design and manufacturing of many molded plastic products. Yet, because inserts are often viewed as commodity items, it is not uncommon for product designers to write up basic specs for an insert without confirming that the insert selected is, in fact, the most suitable for an intended product and its application. With the number of inserts required often reaching the millions for a product line, failure to source the most ideal insert can significantly increase costs, scrap rates, and the risk of product failure, and also result in excessive manufacturing downtime.

Inserts and fasteners from Tri-Star Industries.



To solve this issue, many OEMs and plastic molders have discovered that teaming with insert manufacturers that offer engineering expertise throughout the design and manufacturing process can be worth its weight in gold. This not only creates an extension of the OEM's own technical team, but can assist in the discovery of non-standard insert designs that can overcome wear problems, improve performance or longevity, and provide major cost savings in the long run.

Solution teamwork
For Carlton Tanner, it is an all-too-common occurrence where he and an insert supplier team up to assist an OEM that is having issues with the inserts they selected. Tanner is the senior manager with Blackland Precision Hardware, a Texas-based distributor of electronic and commercial hardware, rivets, and inserts.

"This is particularly the case when the OEM gives us an insert order for a new product and the specs are sketchy, which happens fairly often," says Tanner.

Brass insert.



Since there are literally thousands of insert options available -- with each one designed to meet certain application requirements -- it's not always as simple as specifying OD, ID, length, and thread count.

Tanner emphasizes that even the most knowledgeable customers are wise to heed the advice of insert experts on their designs. He describes one such instance where an injection molder working for a major automaker had spec'd a threaded brass insert with diamond knurling for a new product they were set to produce.

"Fortunately, the OEM conducted an initial test with the insert where it became evident that the design they selected was somewhat problematic for their production process," Tanner explains. "Before they could place the million-plus part order, they needed a solution -- fast."

Fearing the issue could force production backups, cost run-ups, and missed deliveries for the OEM, Tanner took the issue to one of his primary insert suppliers, Tri-Star Industries. Tri-Star specializes in threaded inserts and compression limiters for plastics. It has an extensive catalog of available items as well as hundreds of custom inserts designed in-house since the 1990s.

"What we found was that the knurled surface of the inserts was chipping slightly, both in transit and as the inserts were bulk-loaded into the injection molding system (a standard operation)," explains Tanner. "As a result, the accumulation of these small chips was effectively gumming up the automated injection molding process, causing expensive unplanned downtime."

Knurls on insert.



Tri-Star's technical support team evaluated the specs from the injection molder and considered the end product's various requirements before developing drawings for Tanner to review with the molder. Those included a novel idea to modify the knurling, or the grooved pattern (often diamond-shaped) on the outside of the insert that helps keep it place.

Additionally, to prevent any minor chippings during shipment from entering the production, Tri-Star had another creative, yet cost-effective idea. They suggested adding a layer of mesh to the bottom of the insert shipping container and an adhesive coating to the bottom of the container. The mesh would function as a sieve, through which the chips are able fall during transit; these errant chips would then stick to the adhesive surface on the bottom of the container.

"These were inventive solutions and effective as well," says Tanner. "In fact, we never even had to use the adhesive-backed mesh solution because their initial idea to modify the knurls worked so well. As a result, our customer's manufacturing problem had been solved."

A unique insert design.



The stainless solution
In another instance, an OEM/plastics molder was sourcing inserts for use in commercial juice dispensing machine assemblies, the type often seen in cafeterias and restaurants.

"Since orange and grapefruit juice are highly acidic, the issue was that they would eventually cause a corrosion problem with a metal insert," explains Aaron Edelson, Tri-Star's sales engineer.

The beverage dispenser OEM had spec'd an insert that required an expensive, secondary plating operation that ensured a corrosion-resistant barrier was present.

"I typically review the specifications on each customer's orders and RFPs to make sure that the insert selected is not only suitable for their applications but offers the most cost-effective solution," says Edelson.

In this case, he recommended that the customer use a stainless steel insert instead. This would make it essentially impervious to corrosion, and thereby protect the juices from any impurities that would result from corrosion.

"Inserts composed of stainless steel are not available from the majority of insert manufacturers, so the OEM was amazed that it was even an option," he says.

Compression limiter.



Even though stainless steel is somewhat more expensive than common steel, "We saved the OEM both the added time and expense of a secondary operation," Edelson says. "With the large order they required, this added up to a huge savings."

He adds that the demand for stainless steel is significantly increasing in a number of industries from food and beverage, medical, and marine to integrated circuit manufacturers.

Ensuring the best solution
While it is important for an insert manufacturer's technical support team to review each design, they should also have the ability to read, write, and modify CAD files and isometric drawings. Some manufacturers, like Tri-Star for example, also include downloadable CAD information from their website for most standard catalog items. This allows engineers to visualize and validate their design concepts with the inserts they are considering for their application.

Tri-Star Industries provides both standard and custom inserts (like these special parts shown).



Edelson adds that, surprisingly often, an inquiry from him intending to confirm the specifications of an insert can lead a distributor or OEM/molder to a breakthrough in quality or service life.

"The teamwork among the engineering staff of the OEM customer and technical services/design staff of the insert manufacturer can create a type of synergy that results in superior product performance as well as cost and downtime savings," Edelson says. "It also demonstrates that these profitable outcomes offer conclusive proof that the customer doesn't need to try and go it alone."

Learn more and get the catalog at tristar-inserts.com.

Learn design guidelines and more at tristar-inserts.com/resources/.

Source: Tri-Star Industries

Published April 2020

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Specifying metal inserts for molded plastics]

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