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Two-Shot Liquid Silicone and PC—A Union of Medical Materials

Fri, 10/21/2011 - 11:11am
Eric Bishop
The Challenge: A previously identified material selected to be overmolded onto polycarbonate would not properly bond without pre-treatment.

The Solution: Utilizing a liquid injection molding system product, a replacement material that did not require pre-treatment and offered the ability to perform a higher production run than originally scheduled resolved the bonding challenge.

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 Production efficiency in the KE2090-40 product allowed for the increased initial run beyond the original production schedule slated for the previous self-bonding material.

Although the following case study involves the union of self-bonding liquid silicone rubber and polycarbonate (PC) in an industrial application; the material combination can and has been used successfully in numerous medical products. PC is commonly specified in a wide variety of medical components. Shin-Etsu Silicones’ KE-2090 series of Select-Hesive LIMS are USP Class VI and ISO 10993 compliant and bond aggressively to PC. KE-2090 has been utilized in many medical applications, including respirator and CPAP masks, needle-free IV valves, breast-pump straps, introducers, and instrument grips.

Overmolding Challenge
Shin-Etsu Silicones of America recently provided a primer-less solution to resolve production issues in overmold bonding for a client who was producing a next-generation industrial thermo-controller for a leading manufacturer. Custom injection molding manufacturer Amerimold Tech Inc. contacted Shin-Etsu and identified the key production issue originating from an existing material’s inability to bond (without pre-treatment) to the front cover of the product’s overmolded polycarbonate chassis, cover, and pushbutton keypad.

Whereas multi-material molding is designed to take economical advantage of two or more materials with uniquely different properties by incorporating them into a single molded component, the injection molded substrate bonding issue, if not resolved, would result in major tooling, scrap, time, and ultimately, cost constraints to complete the final part.

Implementing Shin-Etsu’s KE2090-40 Select-Hesive LIMS (liquid injection molding system) product, which is engineered to provide primer-less adhesion to a variety of thermoplastic substrates, all parties were able to streamline an effective bonding solution based on the ensuing parameters to overcome bonding barriers.

Pre-Treatment Production Challenge

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 Next-generation industrial thermo-controller’s front cover overmolded without pre-treatment to its polycarbonate chassis, cover, and pushbutton keypad using Shin-Etsu’s KE2090-40 Select-Hesive LIMS.

Amerimold’s client had an existing loyalty to a self-bonding material that was already specified in the production of the thermo-controller component. Unfortunately, the self-bonding material supplied was not sufficiently able to overmold to the polycarbonate housing without a pre-treatment. This would have required additional equipment and an additional cost for the material to function correctly. The client also pursued overseas material solutions that, after testing, resulted in too much waste in rejected parts and a 60% scrap rate as the self-bonding silicone did not adhere properly to the keypad.

A final alternative solution was tested that would have involved the purchase of additional variable frequency light pumping (VFLP) equipment to enhance and engage the surface substrate bonds—at an estimated expense of $17,000. The VFLP equipment process was tested as the substrate was shot and treated with light, but still failed.

Primerless Solution
Having already invested upwards of $70,000 on tooling for the two-cavity LSR (liquid silicone rubber) mold, Amerimold was faced with the expensive option of adding even more equipment to resolve the self-bonding production dilemma. Recognizing the need to streamline an efficient solution swiftly to avoid incurring more costs on equipment, pre-treatment, and waste, Amerimold President Michael Schon contacted Shin-Etsu to overnight a five-gallon sample pail of its KE2090-40 LIMS product to test run with the LSR mold.

Approximately three to four tests were conducted on the two-stage molding system utilizing KE2090-40 without pre-treating the substrate. Conducting an initial physical inspection of the bond to see if it pealed back, Schon noted that the KE2090-40 parts bonding passed the test. According to Schon, “Incredibly, the sample run was successful on the first shot, creating a perfect bond with no UV light treatment required and, more importantly, no rejects.”

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 Side-view of thermo-controller housing; achieved good self-bond with no UV light pre-treatment required, and no waste/rejects.

Additional tests were expedited by the client and they passed immediately, resulting in a go-ahead on an initial 25,000-piece production run. Of note, the production efficiency in the KE2090-40 product allowed for the increased initial run beyond the original 10,000-piece production schedule slated for the previous self-bonding material.

Ke2090-40 Series Properties
The KE2090 Select-Hesive LIMS Series boasts fast cure and outstanding bonding to various engineering thermoplastics that can withstand high heat, humidity, and thermal cycling. With regard to the Amerimold case, the key is that the substrate does not need to be modified or pre-treated to use the KE2090 self-adhesive materials.

The KE2090 products provide primer-less adhesion, which is optimal for multi-component silicone-to-plastic molding, and delivers the following performance benefits and physical properties:

  • Short curing time yields a chemical bond, producing a firmly integrated composite part
  • No primer/pre-treatment of the substrate is required
  • Outstanding bonding strength under extreme conditions
    • High temperature (120°C)
    • High humidity (85°C/85% RH)
    • Thermal cycling (-40 to 120°C)
  • Bonds to PC, PBT, PPO PEEK, and PPSU engineering thermoplastics
  • Compliant with USP Class VI and ISO 10993-1 regulatory standards for biocompatibility
  • Hardness ranges from 10 to 70 Shore-A

Conclusion
Overmolding of rigid plastic substrates with liquid silicone rubber can provide a soft feel, improve grip, weather seal, dampen vibration, and protect against impact. A wide range of colors, textures, and durometers are available as they are increasingly being adopted in applications ranging from automotive, consumer products, electrical/electronic, industrial, and healthcare.

With the continued growth of two-shot molding, the Select-Hesive LIMS products save time and the cost of pre-treatment, which is often required in an overmolding system. This series provides designers and molders an efficient means to manufacture a bi-material component in a fully automated manner by eliminating the need for hazardous priming, pre-treatments, and costly secondary assembly operations.

Eric Bishop is the North America marketing manager at Shin-Etsu Silicones of America. He can be reached at 513-232-8917 or ebishop@shinetsusilicones.com.

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