Injection Molding Guide: The Silicone Rubber Injection Molding Process

Liquid injection molding (LIM) is an industrial fabrication process that molds materials into a broad range of components and products. Unlike the standard reaction injection molding process, which relies on pressurized impingement mixing, liquid injection molding uses a mechanical mixing process that focuses mainly on liquid silicone rubber (LSR) and similar elastomeric materials.

Injection molding of High Consistency Rubber (HCR), the oldest form of producing silicone rubber parts, continues in widespread practice around the globe. Nonetheless, the liquid injection molding process has gained a reputation as the preferred process among many manufacturers of rubber parts, because of its superior end-of-product performance – durability, tensile strength, flexibility, and accuracy. In addition, the liquid injection molding process facilitates high levels of automation, and the ability for 24/7 production.

One of the primary keys to the flexibility of liquid injection molding has to do with the unique properties of liquid silicone materials. The material has superior heat and flame resistance characteristics, withstanding temperatures over 250°C, and as low as of -90°C. In addition, silicone material provides unparalleled formability qualities, which allows for transparency or coloring of the finished product.

Just about every industry imaginable has discovered the opportunities presented in the liquid injection molding process, including the following:

Injection Molding Industries

The thermal, chemical, and electrical resistance of LSR allows for a broad range of products, including components seals, o-rings, isolators, valves, cables, electronic components, and other parts. Liquid silicone rubber maintains its integrity under sterilization. The biocompatible features of LSR also make it safe for products that have contact with the skin.

How the Liquid Silicone Injection Molding Process Works

The materials commonly used in the LIM process are silicones and acrylics. The process requires the use of a spring-loaded pin nozzle to help prevent the machine hardware from becoming clogged with materials. The spring-loaded mechanism permits the injection pressure to be higher than the pressure of the extruder barrel, which keeps the channel unblocked.

Utilizing a pump the LIM method brings together an apportioned mixing and dispensing of LSR. One plunger holds the base forming plastic, which can be strengthened with additives and fibers. The other plunger contains the catalyst. Each will be pumped in a 1:1 ratio into a static mixer, which triggers the mixing reaction.

The liquid mixture is then injected into a sealed mold where it is heated at temperatures from 180 to 200 C, or 355 to 390 F. It starts out as a fluid and is then heated within the mold to initiate curing. Once it hardens, the molding machine ejects the nearly finished part.

Thermoplastics operate in reverse; the materials are heated in the injection barrel to their melting point and then cooled in the mold. Many manufacturers use computer-aided design (CAD) tools to make the LIM process more efficient. CAD allows them to run simulations to determine the most cost-efficient and effective processing regiment and conditions, evaluate results, and check integrated components.

In addition, thermal imaging technology can help pinpoint costly production mistakes, especially molding defects and design irregularities.

Liquid Injection Molding Machines

Multishot Injection Molding

Liquid silicon rubber material comes in a kit that consists of two components: “A” and “B”.

The key components of the conventional, two-part liquid injection molding process include the following:

  • Supply drums: The plungers, or liquid silicone supply containers, connect to the pumping system. Many two-container setups include a third container for pigment.
  • Metering units: The metering device pumps the two liquid materials in predetermined ratios, which ensures a concurrent release at a steady ratio.
  • Mixers: Once the liquid forming materials pass through the metering unit, a static mixer combines the materials. The blended material is pressurized and pushed into the mold.
  • Injectors: This device moves the LSR forming material into the pumping mechanism under pressurized force. The machine operator has the capability to adjust the pressure, as well as the injection rate. This parameter differs according to the project specifications.
  • Nozzles: The liquid compound flows into the mold through a nozzle that has an automatic shut-off valve. This valve prevents the mixture from leaking or the mold from overfilling.

In an ideal production environment, the basic liquid injection molding machine should be as lean and as compact as possible. Secondary devices should be configured to address the needs of a particular project.

Effective LIM Design

As with any manufacturing process, the LSR molding process starts with proper part design. Those familiar with designing parts for injection molding should find the design elements of the silicone rubber injection molding process to be alike. However, LSR has a very high shrink rate and the material has a tendency to flash very easily during molding. The designer can mitigate these issues by planning the correct tolerances and assimilating extra elements into the mold design to help reduce flash.

The liquid molding process offers greater freedom to part designers in comparison to conventional injection molding processes. LIM parts do not require the use of high heat and pressure to melt the material, which allows for a consistent flow of LSR into the mold. As a result, the designer has more flexibility and uniform part geometry.

Injection Molding Design Process

The Massachusetts Institute of Technology states that the design process determines up to 80% of a plastic injection molded component’s costs. It also has an impact on quality, reliability, functionality, serviceability, and manufacturability. The design also has a bearing on time to market, and it plays a crucial role as a driver of competitive advantage.

Designing plastic components involves a complicated task, which encompasses a variety of factors associated with the requirements of the part. It entails asking and answering a series of questions, including:

  • How the part will be used?
  • How does the part work with other components?
  • What are the weight, structure, impact, and load requirements?
  • Are there any environmental conditions that must be considered?
  • What are the cosmetic requirements?
  • Does the part have any unusual characteristics?

It’s important to consider the design parameters for your LIM program. Below are nine considerations.

1. Maximum Part Size

The following are approximate part sizes:

  • 5 inches-by-5 inches-by 2 inches (127-mm-by-127-mm-by-50mm)
  • No deeper than 2 inches (50 mm) from any parting line
  • Maximum projected mold area of 17.6 square inches (113.55 sq cm)
  • Maximum part volume of 4 cubic inches (65.55 cc)
  1. Wall and Rib Thicknesses

LSR has the capability to fill a thin wall section with fewer challenges. Parts can have a wall as thin as 0.010 in. (0.25 mm.), depending on the size of the wall and the location of contiguous thicker sections. The only limitation is whether the thin detail can be milled into the mold or ejected from the mold without damaging the part. Rib thickness should be approximately 0.5 to 1.0 times the thickness of the adjacent walls. The radius of inside fillets should be near the thickness of the wall because smaller or larger radii may cause porosity.

  1. Mass Reduction and Uniform Wall Thickness

Although liquid silicone rubber has superior ability to accommodate deviations in wall section, and it has almost no sink, the same rules for standard plastic part design apply.

  1. Parting Lines

Determining the location of the parting lines is one of the initial steps in the tooling process. To make the process easier, minimize parting lines to produce cleaner parts and save time.

  1. Undercuts

One of the key benefits of LSR molding has to do with the ability to create parts with undercuts. Most undercuts can be removed with minimal effort by the press operator or with  mechanical help. Then, each part gets evaluated on a case-by-case basis for viability.

  1. Part Ejection

LSR molding does not require ejector pins like the standard practice with thermoplastic molds. When designing the part, the entire part is retained on one half of the mold when it is opened at the end of the molding cycle. Under ideal conditions, the part features will rise above the parting line surface, which makes the part easier to demold.

  1. Draft

For the ease of the manufacturing process, LSR parts require draft similar to plastic injection molding. While 1 degree draft is commonly used, per inch (25.4mm.) of depth, on shallow components zero draft can be used occasionally. If mold construction allows, the inherent characteristics of LSR allows more flexibility with draft rules than it does with thermoplastics.

  1. Gating and Venting

The shear thinning characteristics of LSR parts require small gates compared to injection-molded parts. Although it’s not an absolute rule with liquid silicone rubber, a gate should feed the thickest cross-section of the component, like thermoplastics. Most LSR gates use some type of edge gate. Gates typically leaves a mark or blemish. Place gates on a surface that is not dimensionally or aesthetically important or provide a recess for gating.

  1. Expected Tolerances

Well-designed parts usually have a linear tolerance of 0.025 in/in (0.025 mm/mm). You will also need to consider flash allowance.

In addition to optimizing the design of a component, it is also crucial to select the right material for LIM programs.

Materials Selection for LIM Programs

LSR has been used in Europe for a number of years. Because of a lack of in-house expertise, many manufacturers in North American are only now beginning to understand and appreciate the performance advantages LSR has over other rubbers and thermoplastics.

One of the most critical factors to consider for a successful silicone molding program has to do with material selection. Standard or general purpose grade LSR does not have a high fill of silica, which makes it appropriate for applications that require basic physical characteristics. Advances in LSR material have led to significant improvements in the product beyond attributes, such as thermal stability, rubber-like qualities, and resistance to aging.

The addition of additives and other fillers give LSR the capacity to endure higher temperatures, oil, and other fluid environments. With the addition of phenyl units, LSR has greater capabilities in low-temperature settings. Adding phenyl fluid reduces the coefficient of friction, creating a part with a slippery surface as the fluid gradually bleeds out. Some varieties of LSR impart low friction chemically, which eliminates the need for fluid to bleed to the surface of the part.

The latest LSR technologies have a grade of self-adhesive material suitable for hard/soft overmolding applications or two-shot molding. This has eliminated the need to apply a separate primer (and another tool) to bond LSR to many commonly used thermoplastic. It also produces cooling cycles that closely adhere to the typical cooling cycle for the thermoplastic, and it permits in-mold bonding of the liquid silicone rubber to a thermoplastic.

Molders can select from many different types and grades of liquid silicone rubber, from tacky to soft touch, as well as an array of hardnesses. It is important for engineers to perform analyses to ensure that the material has chemical compatibility and wear resistance. It must also meet environmental and performance criteria as outlined in the program.

Prototyping

Before paying out for full production tooling, you may want to see an actual sample of the part in order to help ensure that the scale-up from prototype to production is a seamless transition. Prototypes can be crucial to the development process of liquid injection molded parts, because it can save you both time and money by making it possible to evaluate a part’s form, fit, and functionality.

The primary objective during the prototype stage involves the identification of any part performance or production process issues prior to the full-scale manufacturing phase.

The technologies for plastic injection molding prototyping range from traditional CNC machining to various full-range, additive manufacturing rapid prototyping technologies, including:

  • Traditional CNC machining
  • 3D printing

3D printing includes Stereolithography (SLA), which works well for evaluating part sizing, fit, and function, as well as providing a finished part for marketing campaigns. Fused Deposition Modeling (FDM) prototypes provide conceptual and engineering models, as well as functional testing capabilities.

LSR Inspection & Testing Protocols

To verify that the quality control standards for Silicon Rubber Liquid are met, and that the component material can carry out its purpose, the tensile properties of thermoset rubbers and thermoplastic elastomers must be tested (measured).

The testing standard is ASTM D412, which consists of two test methods: A and B. Method A, the most prevalent testing approach for silicone rubber liquid, tests a dumbbell-shaped sample on a universal testing machine, which has standard clamps. Method B employs a ring specimen that requires a special clamping mechanism.

There is also a variety of inspection and testing equipment used in proprietary internal manufacturing processes, including:

  • Elastomeric physical properties testing
  • Elastomeric rheometer
  • OGP Flash Inspection System
  • Video microspoe

A testing program should also include the sampling of parts, materials, and process to obtain qualitative data.

Pilot Production

When the tooling has been completed, you can run the new product design through the pilot production process phase so that you can look at samples of the part.

This is where you can:

Pilot Production Checklist

You can also work out other bugs and modifications to the mold, and refine processes if necessary. Once you have the tooling and processes refined, you can easily fine-tune materials and color without any problem. The attention given to the official pilot production stage will have a direct impact on the efficiency and effectiveness of the manufacturing process. Not only will it reduce manufacturing costs, but it can also assist in reducing the time-to-market and delivering the best product possible.

Full Scale Production

Most original manufacturers do not have the in-house staff necessary to successfully execute an LSR program. Consequently, many end up online in search of a vendor. Many companies have purchased machines and invested time and money learning about LSR injection molding technology in order to move into the silicone segment of the market. However, thermoplastic experience cannot substitute for LSR expertise in part design, mold design, and the manufacturing process because of the unique challenges of the liquid injection molding process.

For instance, silicone has stringent tooling standards, which can mean numerous challenges with gating, venting, and the increased risk of inadvertently damaging components. Methods also vary from supplier to supplier. Some vendors have  a manual process while others use flash-free tooling and an automated molding process. Each method has its advantages and disadvantages, but you will need to look at it in terms of the additional variables, including employees with the necessary knowledge and skill set and increased costs in an already capital intensive and complicated process.

Evaluation & Quality Control

The review process follows ISO 9001:2008 quality standards protocol to assure that all product output meets the criteria determined by the customers’ product specifications and delivery conditions, as well as in-house engineers and industry best practices.

To understand the processes and products, the evaluation and quality control program depend on a data-driven approach and continuous assessment based on proven, scientific concepts and best practices.

The quality control program should include:

  • Document and data control to ensure the most current, accurate information.
  • Inventory control and identification of materials.
  • Process control to get part production right the first time.
  • Inspection and testing of parts.
  • Non-conforming controls to avoid shipping inferior-quality parts.
  • Handling, warehousing, packaging, and preservation.

The idea is to fully document all stages of material procurement, product manufacturing, and distribution.

Contact SIMTEC Silicone Parts

At SIMTEC, we produce custom designed and manufactured, high-quality LSR parts and LSR 2-Shot components. We produce parts for the automotive and industrial markets, as well as medical and consumer applications. Contact us today for a free quote, or download our informational guide to LSR.


Liquid Silicone Injection Molding: A Versatile Solution for Demanding Conditions

Liquid Silicone Injection Molding

A Versatile Solution for Demanding Conditions

Liquid silicone rubber (LSR) can be molded via custom liquid silicone injection molding. Liquid silicone rubber is often used to make gaskets, seals and other products where difficult conditions are an issue.

The fast cure and high performance properties of liquid silicone rubber make it an ideal candidate for small molded rubber components. What makes liquid silicone rubber (LSR) a good solution, is its flexibility and elasticity at -70°F. It also retains its properties up to 4050°F.

Liquid Silicone Injection Molding

An advantage of liquid silicone injection molding process, is its reduction of labor, a major cost in molded components. Additionally, the elevated temperature and molding pressures of the liquid silicone injection molding enhances the cure of the process. Because we receive our liquid silicone rubber supply in airtight containers, humidity is not a concern. Therefore, generally speaking, the liquid silicone injection molding process is more consistent than the rubber compression molding.

Although liquid silicone injection molding is the most common term in the industry; it is also known as liquid silicone rubber molding, liquid silicone rubber injection molding, liquid silicone molding, silicone liquid molding, liquid silicone injection moulding, silicone injection molding, LIM molding, silicone rubber molding and silicone molding all used frequently and mostly interchangeably.

Silicone Prototyping: Chemical Compatibility of LSR

Silicone Prototyping

Chemical Compatibility of Liquid Silicone Rubber

The chemical composition of Liquid Silicone Rubber (LSR) makes it the perfect candidate for silicone prototyping. It is unique in comparison to many common elastomers. The inherent properties of  LSR make it an ideal choice for uses in specific chemical environments as well as for silicone prototyping.

Silicone Prototyping

The silicone-oxygen backbone of LSR has a higher bond strength than that of polyethylene or of a carbon-based material, and it is therefore mostly chemically inert. This inertness, as well as its natural hypoallergenic properties, make LSR a prime candidate for food, medical applications, and silicone prototyping.

In comparison with other rubber materials, LSR is exceptionally compatible with many diluted solutions of inorganic acids and bases (e.g., acetic acid, arsenic acid, boric acid, sulfuric acid, tartaric acid). Extending the variety of uses of LSR products, such as hosing and seals to the medical, food manufacturing, and automotive industries, LSR can be used as a propellant in food products, as filler for vehicle airbags or for silicone prototyping. The extensive list of LSR-compatible materials also includes ammonium hydroxide, ammonium phosphate, and alcohol bases, which are common ingredients of many household products.

In addition, LSR is highly suitable for use with water and ozone (which can be used in small quantities as a treatment for drinking water). This broadens the potential uses for LSR hoses, bellows, seals, and other components for municipal water systems or even agricultural irrigation systems. While this application may not seem exceptional, many other seal and hose materials expand over time, age, and crack under differential flow conditions or are unable to maintain mechanical integrity under varying internal and external temperatures. This same concept extends to the automotive industry, where LSR’s compatibility with many industry-standard oils and high-temperature air makes it an ideal candidate for gaskets, bellows, and electrical connectors among other applications.

Open Nozzle System or Valve Gate?

Choosing the right injection technology for LSR molding is key to a successful part, especially in the case of directly gated components (no sprue).  The two most popular systems involve open nozzle systems and valve gates.

For larger parts (weighing 200 grams or more), a valve gate system offers the advantage of a clean gate area. For open nozzle systems, the gate location is visible at roughly 0.7 mm in diameter and 0.5 mm in height.  Although on smaller parts, the gate size could be closer to 0.25 mm and 0.3 mm for open nozzle systems.

With high cavitation molds (16 cavities or more) the valve gates tend to have greater imbalance during the injection process due to the friction of the many moving parts. Maintenance on high cavitation molds that use the valve gate technology can be very time-consuming whereas open nozzle systems are seen as virtually maintenance free.  But, depending on the application, the open nozzle inserts may need to be reworked after roughly 1-2 million shots.

On any multi-cavity LSR mold, filling imbalances result in short shots or excessive flashing which can be controlled with adjustments to the hold-pressure times for instance. A significant difference between these two gating technologies occurs while the LSR is vulcanizing in the mold. For instance, on an open nozzle system allows the material to flow back into the cold runner during vulcanization when the LSR expands, acting as an equalizer.  Since the valve gate has to be closed before the LSR solidifies, any filling imbalances cannot be corrected after the needle is closed.

Molding paper-thin silicone membranes can be a challenge for any gating system and are often molded with sprues/ sub runners due to pin holes, wrinkles, and other surface defects. All LSR gating systems except UV curing systems have a thermal transition area between the hot cavity and the cold nozzle or valve gate body. Fig. 1 shows a valve gate system that has the potential of harboring vulcanized particles in the thermal transition area.  These then may dislodge on the next shot and become trapped in the thin areas of the part; creating pinholes. Alternatively, on open nozzle systems (see Fig. 2) the particles in the thermal transition area will remain attached to the parts themselves.  This prevents any vulcanized particles from entering the cavity and creating pinholes or other surface defects.

SIMTEC uses both systems in full-scale production runs and can assist with selecting the gating system that is right for your application.

 

© SIMTEC Silicone Parts, LLC

The information provided herein is to the best of our knowledge and it is believed accurate and reliable as of the date compiled. No representation, warranty or guarantee expressed or implied, is made as to the accuracy, reliability or completeness of the information provided herein. It is the user’s responsibility to determine the suitability and completeness of such information for the intended use. We do not accept liability for any loss or damage that may occur from the use of this information. Nothing herein shall be construed as a recommendation for uses which infringe valid patents or as extending a license under valid patents.

Two Shot Injection Molding: Efficient LSR Manufacturing

Two Shot Injection Molding

Efficient LSR Manufacturing

Modern LSR injection molding manufacturing reflects innovation and efficiency that are the results of decades of experience. Today, incredible cost-efficiency can be achieved without sacrificing time or quality, so long as one employs appropriate materials, selects a fitting manufacturing process and sets high standards.

two shot injection molding

Fig 1: Injection Molded LSR Over a PC Housing with Steel, Threaded Inserts

One advancement in manufacturing technology that has greatly improved the possibilities for high-efficiency and cost-savings is LSR two shot injection molding.

SIMTEC is a pioneer in the emerging field of Liquid Silicone Rubber (LSR) two shot injection molding and has a remarkable record of customer satisfaction, efficiency and insight which allows us to improve upon engineering and part design in ways that only a qualified team, such as SIMTEC’s can!

LSR two shot injection molding is a fantastic technology that bring several benefits. It not only combines LSR and a thermoplastic together, but is a flexible method which can be engineered to incorporate additional materials.

One example, seen in Figure 1, is a housing made out of a self-adhesive LSR and steel threaded insert within a polycarbonate (PC) housing. The self-adhesive LSR functions as: (1) a soft-touch illuminated button pad, (2) a gasket and, (3) protection for the PC housing to which it adheres. The housing was designed to provide a sturdy frame which also securely holds the threaded insert. Integrating these three functions into one component drives down not only the initial investment by only requiring one opposed to four tools, but also lowers inspection and other quality assessment costs.

Steel inserts, pins or terminals are incorporated within the initial manufacturing steps in insert molding. Material compatibility is determined before production. The finished housing, pictured below, does not require additional finishing or manual labor (such as flash or tail removal). Depending on tooling design, components are completed in seconds with SIMTEC’s LSR 2-Shot expertise!

At SIMTEC Silicone Parts, a leading company in manufacturing high precision parts and components, we are exclusively focused and specialized in the production of LSR and 2-Shot (LSR/Thermoplastic) components.

© SIMTEC Silicone Parts, LLC

The information provided herein is to the best of our knowledge and it is believed accurate and reliable as of the date compiled. No representation, warranty or guarantee expressed or implied, is made as to the accuracy, reliability or completeness of the information provided herein. It is the user’s responsibility to determine the suitability and completeness of such information for the intended use. We do not accept liability for any loss or damage that may occur from the use of this information. Nothing herein shall be construed as a recommendation for uses which infringe valid patents or as extending a license under valid patents.