Liquid silicone injection molding

PHOTOS COURTESY OF SUMITOMO DEMAG

More medical applications demand users skilled in liquid silicone rubber (LSR) molding, according to Reports & Data, which predicts a 7.3% compound annual growth rate (CAGR) to a $3.8 billion global industry by 2026. Nigel Flowers, managing director of Sumitomo (SHI) Demag UK explores the recent popularity of LSR and how molders can adapt to this lucrative market.

Today’s Medical Developments (TMD): What makes LSR so different to other molding materials?

Nigel Flowers (NF): LSR is fairly unique in that it remains flexible and elastic down to -50°C, yet retains its properties up to 200°C. This is why LSR is starting to be used more extensively in encapsulated electronic components, cables etc., where insulation is required for user safety. Additionally, LSR stands up well to ultraviolet (UV) and chemical exposure.

TMD: How can LSR be used in medical applications?

NF: For many years LSR has been used in the baby care market, for pacifiers and bottles. Now, LSR is being used to create implantable devices, wearables to track blood pressure or heart rates, as well as smaller medical devices with more intricate designs and micro parts.

One key characteristic of LSR is its biocompatibility and the fact it is bacteria resistant, hydrophobic, odorless, and can withstand repeatable sterilization at high heat. All of which lend the material well to healthcare applications.

TMD: Can you overmold with LSR?

NF: Overmolding works extremely well for several devices that require special aesthetics, for example a comfortable soft-touch grip on tool handles or disposable razors. The ability to add adhesion to the material assists with this and helps reduce the number of production steps for a molder.

Critically, LSR can be molded onto both bondable and non-bondable materials, including metal, glass, and ceramics. This assists with manufacturing efficiency, helping reduce the number of individual components and consequently reduce assembly costs.

TMD: How does LSR processing vary from other materials?

NF: The low viscosity of the material requires high precision processing. Compared to polymer granules, LSR is almost honey-like in consistency. It behaves in the opposite way to conventional polymers during the molding cycle. To keep LSR in its semi-liquid state before it arrives in the mold, the injection barrel is water cooled. Once it arrives in the mold, the material is heated and cured.

TMD: Is special equipment required?

NF: To mix and process the material, you need equipment that can deliver the stability required to handle an optical grade silicone material. Although there are examples where traditional molding machines have been adapted with an LSR injection unit, molders may find that part quality or efficiency is compromised.

Sumitomo Demag recently launched a ready-to-use precision IntElect LSR injection molding package. The all-electric IntElect 130 is equipped with a special screw, a non-return valve, vacuum pump, and toggle technology. The cell was developed with numerous partners, including Nexus.

Nexus designed the four-cavity LSR matrix light mold and supplied the mixing and dosing unit, which stabilizes dosing and ensures a precise mixing ratio, irrespective of dosing volumes. The unit also performs automatic venting to detect and purge air from the material before it is fed into the machine and mold.

To optimize LSR component quality, Nexus offers a cold-runner control technology that enables direct injection into each individual cavity via a needle valve gate, balancing each mold cavity filling.

TMD: How is the molding machine different from the standard IntElect?

NF: Special screws, measuring between 14mm and 45mm in diameter, with a sealing system at the shaft, are adapted to the material and machine size. Currently, the LSR package is available on IntElect machines ranging from 50 metric tons to 180 metric tons.

Other special features include a spring-loaded non-return valve to avoid uncontrolled backflow of material, a shut-off system designed for LSR, and a pressure controlled vacuum sequence to extract air and prevent flash during mold filling.

Sumitomo Demag’s Centre Press Platen ensures high rigidity and uniform distribution of clamp force for more balanced pressure distribution. A robust linear guidance system controls the mold opening and closing sequence with a built-in staged clamp force, further reducing flash risk.

TMD: Do manufacturers need a cleanroom?

NF: Typically, a standard all-electric molding machine such as the IntElect is an enclosed cell and holds little risk to cleanliness. However, parts being produced for the medical and life sciences sector are governed by stricter manufacturing hygiene guidelines. These instances may require investment in a certified cleanroom, limiting exposure to airborne particulates, contaminants, and pollutants.

Sumitomo Demag