The compression and transfer molding process, using plastic and high consistency rubber (HCR) as raw materials, was once the dominant manufacturing process in medical, aerospace, electrical, construction and industrial applications. In recent years, however, Liquid Silicone Rubber (LSR) 2-Shot injection molding has been the manufacturing process of choice across a range of industries for the fabrication of molded parts. The LSR 2-Shot molding process enables manufacturers to produce parts in a wide range of sizes and technical configurations, one of the many advantages 2-Shot injection molding has over compression and extrusion molding.
LSR 2-Shot injection molding is part of an overall growth story for the silicone elastomers market — the global market value of silicone elastomers was estimated to be $6.5 billion in 2013 and is projected to grow at a CAGR of about 9.28% between 2014 and 2019.
SIMTEC Silicone Parts is a producer of custom designed and manufactured LSR parts and LSR 2-Shot (LSR onto a substrate) components. LSR’s unique characteristics make it a good fit for a variety of industries and applications, and at SIMTEC we have produced billions of parts since our founding in 2002. While many injection molding companies started with plastic as the raw material and then expanded into LSR, SIMTEC was founded as a custom LSR parts manufacturer. We use our vast knowledge of LSR and the LSR two shot injection molding process to produce the highest quality parts for customers in a wide range of industries, including healthcare, automotive, industrial and electronic consumer products.
This article discusses the benefits of LSR as a material and the many advantages LSR two shot injection molding has over other common manufacturing methods.
LIQUID SILICONE RUBBER 2-SHOT INJECTION MOLDING: TWO IN ONE SAYS IT ALL
LSR 2-Shot injection molding is also called multi-component injection molding, over-molding, and twin shot molding. LSR 2-shot molding consists of molding LSR directly onto a substrate (typically a thermoplastic) right after the substrate itself has been molded, using the same mold and injection molding machine. It is even possible to over-mold LSR onto LSR, with two separate durometers and colors. The benefit for manufacturers is the ability to integrate several functions, parts, and materials into one component — permanently fused.
Twin shot silicone rubber molding also allows manufacturers to take advantage of the best features of both materials and reduce costs because it reduces or eliminates post-molding assembly. The two-step process requires the use of only a single machine cycle because the mold incorporates both the thermoplastic and LSR injection. By consolidating multiple cycles to one, 2-shot molding lowers production costs and requires fewer operators, while producing more items per run.
HERE’S WHY LSR IS A BETTER RAW MATERIAL THAN THERMOPLASTICS
Liquid silicone rubber is a versatile material that is used in countless applications in a variety of market sectors. One common use for LSR is in sealing applications due to its ability to maintain or return to its original shape despite repeated compression, its water repellant properties, and its thermal resistance. Some of the most significant advantages LSR has over thermoplastics include the following:
- Environmental friendliness: LSR does not generate by-products nor does it break down at high temperatures, ensuring lasting products that are not simply single-use disposables. Silicone also has material advantages in purity, chemical inertness and the ability to survive multiple sterilization processes, making it the popular choice in the food and beverage, medical device, and baby care products industries.
- Flexible design: Although LSR has some general design guidelines that are similar to thermoplastics, it’s a much more pliable material and provides designers with significantly more flexibility. It flows easily into the mold, lending itself towards micro-designability and thin wall geometry while also avoiding typical flow concerns common to thermoplastics.
- Sink: Thermoplastic materials are susceptible to surface depressions, or sink, limiting the thickness of features designers can incorporate into the parts they create. Sink is less of an issue for LSR, therefore designers can utilize thicker features with less concern of deformation.
- Undercuts: LSR allows designers to create products with complex geometries and undercuts that would be impossible to create using more rigid thermoplastic materials. Although fabricators can mold thermoplastic components with undercuts, the process is complicated and requires pick-out inserts or side-actions to produce the often basic feature. It is important to note that various grades of LSR have elongation values reaching up to 800%, further facilitating undercut designs.
- Low chance of contamination: The LSR two shot fabrication process takes place in a closed system (as opposed to insert-overmolding), which reduces the possibility of contamination significantly. Not only does the operator not have to touch the material, but it’s also not exposed to the environment during the molding operation.
- Biocompatibility: Liquid silicone rubber is highly compatible with human tissue and bodily fluids compared to other elastomers, so naturally it is the material of choice in medical applications. It is resistant to mold and bacteria growth, does not stain or degrade other materials and is tasteless and odorless. LSR formulations comply with FDA regulations, and the material can be sterilized using a number of methods, including steam autoclaving, ethylene oxide (EtO) gas treatment, and gamma and electron beam (e-beam) irradiation.
- Durability: LSR parts continue to perform well even in extreme temperature environments, which makes them well suited for automotive applications — particularly under the hood and in close proximity to car engines. LSR parts are also fire retardant and will not melt, unlike injection-molded thermoplastic components.
- Transparency and pigmentation: Liquid Silicone Rubber is naturally transparent, and therefore there are limitless pigment options available. This transparency is also capitalized upon in optical-grade LSR, which serves as a functional offset to other glass alternatives like PC and PMMA while still providing the same benefits of silicone rubber (thermal stability, flexibility, micro-designability.)
LSR is often more expensive than other materials such as thermoplastics and HCR, but the value added from liquid silicone rubber far exceeds the initial investment — especially for fabricators of high-volume parts. LSR enables extended production runs with minimal labor and material waste. In fact, it’s possible to completely automate the process in which one operator can monitor and maintain multiple liquid injection molding machines simultaneously. This is a significant cost-saving benefit for manufacturers looking to increase process efficiency in the face of rising labor costs.
HOW DOES IT WORK?
The most important aspect of two shot injection molding is the design of the tool. The tool must be designed to allow both stages of the part to be molded in the same tool to achieve maximum production efficiency. The design must consider two separate injection stages that can be rotated 180 degrees or oriented to allow a robot to transfer the components. In stage one, the substrate parts are molded and then rotated/transferred to the second stage to be over-molded.
During the second phase, another substrate part is being produced in the first position, resulting in a highly efficient, continuous cycle.
The two shot process can also be accomplished with an indexing system (a roundtable rotating around a horizontal axis) with a primary and secondary station. While the first injection is accomplished at station number one, the second injection is accomplished at station number two. Each injection station is run by an independent injection unit. This allows injection speeds and pressures to be controlled for each material being utilized.
BENEFITS OF LIQUID SILICONE RUBBER FOR 2-SHOT INJECTION MOLDING
There are a number of benefits of LSR 2-Shot injection molding for manufacturers, including but not limited to:
- Production of complex components from two separate polymers during a single machine cycle. This allows for the creation of parts that utilize characteristics of two distinct materials and/or colors. The design can embrace a chemical bonding between the two materials when applicable or incorporate mechanical interlocks when needed. The bottom line for designers is the freedom to easily and accurately create complex components with a high level of repeatability.
- Creation of innovative parts that are high quality and can be produced quickly. As described earlier, two shot injection molding allows for the creation of parts consisting of various material combinations, resulting in products that are visually pleasing and that feature the most desirable properties of both materials. One example is a handle (thermoplastic) requiring a soft grip (LSR) — the two separate parts could be molded by one machine in a single tool and in a single process.
- Cost reduction benefits are inherent in a process that permits complex components to be molded within a single tool. This ability eliminates the need to manually join separate materials together or apply color to components, while also reducing labor and production costs.
- Better quality because the entire production process occurs within a single cavity in the molding tool, so each component produced is theoretically identical. Advanced machining technology currently used in the LSR 2-Shot injection molding process allows for incredibly tight tolerances, ensuring consistent quality across all parts.
- Reduced cycle times because LSR 2-Shot molding consolidates component production into a single cycle that allows only a short break as the tool repositions. Complete parts can be produced in very short periods of time as compared to the alternative insert-overmolding.
- Improved aesthetics because 2-Shot products are more visually appealing to the consumer with the use of multiple materials and colors.
- Improved ergonomics since the LSR 2-Shot injection molding process allows for the application of soft-touch surfaces onto hard substrates. The resulting items can be equipped with ergonomically designed handles or other features, a particular benefit for tools, medical devices, and other hand-held items.
LSR 2-SHOT INJECTION MOLDING IS A POPULAR CHOICE ACROSS MANY INDUSTRIES AND APPLICATIONS
Some of the industries LSR two shot injection molding is particularly useful in include:
- Medical /Healthcare: LSR 2-Shot injection molding has made great strides in the medical industry in recent years because of the efficiency of the process and the excellent physical, chemical and heat resistance properties of LSR. Other advantages of LSR 2-Shot molding include biocompatibility, easy sterilization, transparency, and softness — the latter making LSR an obvious choice for medical devices that come into contact with the body. LSR products also meet strict FDA standards.
- Automotive: LSR is extremely durable with its high tensile strength, thermal stability, and fatigue and tear-resistance, all of which make this material an excellent choice for under the hood applications in vehicles. Many OEMs and top tier automotive suppliers use LSR injection molding to produce a number of standalone parts, including O-rings, buttons, and gaskets. Another benefit of LSR is that it can also be molded directly onto plastic or metal parts in an overmold or 2-Shot operation.
- Consumer products: LSR is a versatile material that is used to manufacture a range of consumer products, ranging from kitchen bakeware and plastic showerheads to electronic products and bottle dispensers.
- Specialty applications: These include a number of types and grades of LSR used to produce a wide-range of products, including valves and gauges, membranes and seal rings, irrigation and filtration parts, pressure exits, check and one-way valves, and lighted panel buttons.
In a time of limited resources and cost-cutting, budgets for new product design and development may also be limited. In some companies, the resources and experience necessary to successfully complete an LSR molding project are equally limited. SIMTEC is a leading provider of LSR 2-Shot injection molding services, and we can help you complete those projects successfully, economically and on time. Contact us today for more information about how SIMTEC can help you with your LSR 2-Shot injection molding needs.