Liquid Silicone Rubber Injection Molding vs High Consistency Rubber: Which Is Right for You?

LSR vs HCRIn the 1960s, the production of synthetic varieties of rubber surpassed natural rubber output. Today, synthetic rubbers comprise about two-thirds of the world’s rubber supply. Among the different categories of manufacturing materials, odorless and tasteless silicone rubber may be one of the most versatile engineering products available. Silicone rubber surpassed its image long ago as a material primarily used to make containers, waterproofing materials, elastic bands, and flexible tubing.

The unique characteristics of silicone rubber elastomers perform very differently from metals and plastics, especially the manner in which they deform and recover under load. Many components and products used in commercial and industrial applications require a high degree of elasticity. This ability of the material to regain its original shape and size after episodes of stretching, bending, twisting, and compressing, under an array of conditions makes it extremely attractive to manufacturers, regardless of the industry.

Silicone rubber has excellent resiliency. An attribute of inertness, resiliency is the measure of an item’s ability to snap back into place quickly. This characteristic of being non-reactive and safe for highly sensitive applications, appeals to not only the foodstuff and medical industries, but to the electrical, aerospace, automotive, machinery, paper, furniture, and construction sectors.

Nearly every product imaginable seems to require some degree of sealing, shock absorption, vibration damping, electrical resistance, thermal insulation, and other properties commonly associated with silicone rubbers. Common applications include the following:

  • Diaphragms
  • Food Preparation and Dispensing
  • Imaging
  • Infant and Baby Care
  • Personal Protection and Masks
  • Headlamps and Tail lamps
  • Data Entry Keypads
  • Safety Cables
  • Fuel Connectors
  • Engine Mounts
  • General Wire and Cable
  • Intercooler and Radiator Seals
  • Connector Seals and Gaskets
  • High Voltage Composite Insulators

Medical applications may require a special medical grade rubber.

Sealing and Non-Sealing Applications

The types of products made from elastomers fit under two general categories: sealing and non-sealing applications.

Sealing applications refer to precisely formed, molded, or machined shapes that seal fluids and gases, such as the cylinders and pistons used in the automobile industry or the keg seals and bottle lifters prevalent in the food sector.

Non-sealing applications include face masks, nasal units, ankle and wrist seals, transmissions, flexible hoses, and drive systems.

High Consistency Rubber Vs Liquid Silicone Rubber

Product manufacturers use a variety of forms, compounds, and fabricating methods. When choosing elastomers, the material options are typically narrowed down to high-consistency rubber (HCR) and Liquid Silicone Rubber (LSR).

For companies looking to manufacture a product, or enter a new market, the due diligence process for High Consistency Rubber vs Liquid Silicone Rubber can present a challenge. Designers, engineers, and managers will need to gather and evaluate data in order to make informed decisions about the type of silicone rubber they will use to produce their part. This decision will determine equipment and floor space requirements, as well as labor costs for the device manufacturing process.

Decision-makers will need to obtain as much data as possible about silicone rubber elastomers, their qualities, fabrication technologies, and benefits.

This guide will help your evaluation process as it relates to material options and fabrication methodologies.

What Is High Consistency Rubber?

Heat-cured rubber, high temp vulcanizing (HTV), or high consistency rubber contains polymers with a high molecular weight and long polymer chains. The properties of HCR epitomize those of rubbers, such as resiliency and recovery after elongation or compression.

HCR has excellent resistance to heat, cold, and other extreme weather conditions, and also has excellent electrical attributes. Fabricators routinely formulate HCR to unique specifications requirements by milling the base with additives, which modify the material’s physical properties. High consistency rubber has high viscosity and the consistency of putty.

High consistency rubber silicone that goes through the correct catalyzation process provides the raw materials that can be used to produce a broad range of applications that require finishing by molding and calendaring (sheets of varying thicknesses). HCR is the main material used for extruded tubing.

The HCR Fabrication Process

Manufacturers make solid silicone rubber in large batches, with the components mixed into a homogenous blend at high temperatures. The addition of peroxide catalyst facilitates the curing process. Fabricators allow the cross-linking of the molecules to begin but roll out the sheets of rubber materials for shipping or storage prior to the completion of the vulcanization process.

The actual process entails six steps:

Mill SofteningMill Softening and Catalyzation

This first step, required for both extrusion and molding applications, reverses any “crepe hardening” that results from the storage of the material at the supplier or fabricator. In addition, some processes use milling to add peroxide catalysts to a free radical elastomer, when needed, or to mix the two components of an addition cure elastomer prior to processing. This labor-intensive process requires the use of a two-roll mill, as the elastomer must cycle through the mill multiple times to blend the compound material into a homogeneous product. After the removal of sheet from the mold, it may require additional processing.

LSR HCR Preform PreparationPreparation of a Preform

The next step involves preparation of a preform, which requires a lot of labor, particularly in high-volume production environments that operate 24/7. The transfer molding process requires a die-cutter to cut a plug from the preform that fits in the transfer reservoir of the transfer press. The compression molding application entails a more complex process, because personnel must cut the preform into the same shape as the final component. For the extrusion and injection molding applications, the preform is prepared by cutting the elastomer sheet into strips and feeding them into the extruder.

LSR HCR ExtrusionExtrusion

The extrusion press for HCR involves the use of a single-screw extruder. The operator feeds preformed strips into to an extruder via a roller feed wheel. The elastomer material extrudes through a die and mandrel assembly, which shapes the desired profile by outfitting the extruder with a crosshead assembly. Another processing option, called the “support extrude,” requires the use of a crosshead assembly fitted onto the extruder, which passes the supporting geometry through the crosshead and extrudes a layer of silicone rubber over it.

LSR HCR MoldingMolding

Different high-consistency silicone applications require different molding equipment. Both transfer and injection molding fabrication requires personnel to load the elastomer into the equipment and to de-mold the finished components. In the compression molding process, workers must place preforms in each cavity of each mold. High-consistency rubber has slow cure cycles, which means the molds may have a large number of cavities.

LSR HCR VulcanizationVulcanization

Fabricators use hot-air vulcanizing ovens (HAVs) to achieve vulcanization of extruded products. This process increases the strength and durability of the material by cross-linking the molecules. Available in both vertical and horizontal models, the vertical oven has a variable speed drum at the top that pulls the extruded profile up through the oven. Some vulcanization processes use a steam autoclave oven or radiant heat.

LSR HCR Finishing InspectionFinishing

The extent of finishing depends on the application. For extrusion profiles, the finishing process involves a visual inspection and cutting the tubes into specified lengths. For extrusion components cured with peroxide, the profile must go through a post-cured process, which eliminates peroxide by-products. The injection molding process also requires trimming or de-flashing of molded parts. The worker uses a die-cutting tool to cut the parts from the large molded sheet. In some cases, the component may require post cure.

The high-consistency rubber fabrication wastes material and has a high labor cost. It also requires additional tools and equipment. However, the cost of the equipment is less expensive than the molds required in the manufacturing of LSR products.


What Is Liquid Silicone Rubber?
What Is Liquid Silicone Rubber?

The primary differentiator between liquid rubber and high consistency rubber is the “flow” or “liquid” nature of LSR materials. While HCR has a peroxide cure, LSR has addition/cure/platinum cure. LSR consists of silicone material that has a very low viscosity, which allows the material to be casted into rubber sheets or injection molded. Raw uncured material for LSR consists of Part A and Part B, liquid components that the fabricator mixes, which starts the curing process. The application of heat accelerates the curing process.

Unlike many thermoplastic elastomers such as TPEs and TPRs, LSR has performance characteristics and fast cure cycles, which makes it a popular material for small molded components. The material maintains its flexibility and elasticity down to temperatures of -60°C and retains these qualities up to 250°C. LSR often finds application in products that require a high-precision level, including the following:

  • Seals
  • Gaskets
  • Cushioning pads in portable communications
  • Data acquisition instruments
  • Electronic devices used in rugged environments
  • Infant products (bottle nipples)
  • Kitchen goods (spatulas)
  • Electrical connectors
  • Medical applications (hoses)

In addition, Liquid Silicone Rubber has become a common material used to over-mold onto other parts manufactured from a variety of plastics.

Meter-mixing

Utilizing hydraulic or pneumatic-driven reciprocating pumps, the meter-mixing delivers the two-part mixture at a 1:1 ratio, into a static mixer, within an accuracy of ±1%. The precision of the material allocation prevents off tolerance products and reduces material waste. The closed meter-mix system also keeps the product clean because the mixer seals the mix from dust and moisture. The system also allows for the controlled addition of pigments and other additives. 

Molding

The second stage of the liquid injection process requires the use of highly automated and modified plastic injection molding machines. Once operable, the machines require very little labor. The greatest capital expense incurred in the liquid silicone rubber injection molding process involves the cost of the mold design and fabrication.

Finishing

For most applications, LSR processing does not require finishing. Properly tooled molds cause minimal flash, which eliminates the need for trimming. The addition cure mechanism, used in liquid injection molding, makes a post-cure cycle unnecessary.

Some manufacturers add fillers during the mixing stage, which add additional qualities to the material. For example, reinforcing fillers strengthen the finished product. Fillers with particle sizes of 5 to 20 nanometers in diameter consist primarily of amorphous-fumed silicas and some precipitated silicas. Extending fillers provide a specific performance attribute, such as barium sulfate filler, which is used to create certain products for the medical industry.

Unlike other elastomers, silicone rubber requires few additives. Some fabricators add stabilizers, which optimize attributes, such as heat and media resistance. Silicone rubber has a transparent appearance, but fabricators can color the material as required. While some compounds deliver the highest value in terms of performance, manufacturers need to operate carefully when adding nonessential ingredients in order to avoid compromise of mechanical attributes and other properties.

The costs involved in the liquid injection molding process add up quickly due to the investment in tooling and equipment, especially during the design and development process. When comparing HCR vs LSR injection molding methodologies, note that although the liquid injection molding process involves fewer steps, all aspects of this fabrication method require expertise that goes beyond typical plastic manufacturing.

The key to a successful project involves identifying and collaborating with a supplier that has the required means and capabilities, a supplier that will guide you from the design and material selection process to full production.

Silicone Cure Mechanisms

Silicone rubber products have two major cure mechanics: free radical cure and addition cure.

Free Radical Cure

The addition of heat initiates the deposition of peroxide catalysts into two free radicals. These free radicals react with the either an alkyl or a vinyl species along the polymer backbone, which transfer the free radicals to the silicone polymer. The reaction of the free radical on the polymer chain, with an alkyl species on another polymer chain, terminates the cross-linking mechanism.

Addition Cure

Sufficient for both LSR and HCR, hydrosilylation, or addition cure, involves adding a silicon hydride to an unsaturated carbon-bond in the presence of a noble metal catalyst. Most hydrosilylation catalysts use platinum, but some processes employ rhodium and palladium. The silicone polymers in the elastomers must include vinyl or alkenyl functionality for the material to cure properly.

Materials that depend on the addition cure mechanism come in two-part kits. One part contains the catalyst species, and the other component contains a silicon hydridefuntional cross-linker, as well as an inhibitor. The inhibitor allows for working time after mixing the two ingredients.

Addition cure offers the advantage of producing a cure reaction that generates no by-products, which means that post curing is not necessary. Nonetheless, some applications require a post cure cycle to stabilize or enhance the properties of finished parts. The manufacturer must take great care to ensure that the material is not exposed to peroxide, sulfur, phosphorous, amines, tin complexes, peroxides, and peroxide by-products, which can inhibit the addition cure regiment.

Considerations for Material Selection

The elastomer material is available as a sheet that has a predetermined thickness or rolls of strip-formed material, divided into various thicknesses and widths, and ready for feeding into the injection molding machine and extrusions.

Decision makers must evaluate the efficacies of High Consistency Rubber vs Liquid Silicone Rubber materials and fabrication for their applications based on industry-wide criteria such as hardness, tear resistance, thermal resistance, and compression set.

Hardness

The Shore A Hardness Scale measures the hardness of materials, as well as the softness and flexibility of moldable rubbers. Rubbers can range from a hardness of very soft and flexible, to medium and somewhat flexible, to hard with almost no flexibility at all. This attribute is important to consider when it comes to removing the component from the mold.

LSR has a hardness range of 10 to 80 durometer Shore A. LSR with a 10 durometer has a soft, gum-like quality and feels softer than some sponge products or certain extra firm silicone foam. At the other end of the spectrum, LSR material with a durometer of 80 has a hardness that exceeds that of shoe heel. The hardness of most applications ranges from 20 to 60 durometer Shore A.

Tear Resistance

A tear resistance test provides a measure of how the silicone rubber responds to tearing. It is calculated by dividing the breaking force in pounds by the cross-section of the outstretched specimen in square inches. The typical tear strength of silicone rubber is around 9.8kN/m. For large molded products, reverse tapered forms, and complexly-shaped objects, use high-strength materials with a tear strength of between 29.4 kN/m and 49.0 kN/m. Manufacturers improve strength through polymer modification and the liberal use of fillers and cross-linkers.

Compression Set Resistance

This measurement determines the ability of silicone rubber material to recover from compression deformation. Silicone rubber has a consistent compression set over a broad range of temperatures, from -60C to +250 C.

Elongation

For products that require substantial stretching or seals to operate across large gaps, the fabricator will need to conduct elongation testing – a measurement of the strain at the point of rupture.

Depending on the application, manufacturers may perform other tests, such as modulus, thermal resistance, toxicity and flame resistance, permeability, abrasion resistance, electrical testing, and environmental testing.

Ultimately, the specifications and performance criteria for the finished application determines the minimal features required for the materials and fabrication process.

For example, a manufacturer of electronic parts may choose general purpose silicone rubber because it has good mechanical properties, a hardness in the 30 to 80 durometer range, tear strength of 8, and a compression set resistance between 12 to 26.

An O-rings and gaskets supplier to the automotive and aerospace industries may select a specialty material such as low compression set silicone rubber, which also meets other application requirements. However, the manufacturer of swimming caps or baby care items may choose a low hardness silicone rubber for its applications.

Choose Silicone Rubber for Your Project

Silicone rubber has been employed for many years in the manufacturing and commercial sectors, particularly for defense, electronics, and automotive. Advances in material technologies and the fabrication process have extended its use for food and water applications and for a variety of life-enhancing and critical-care applications.

The material choices and manufacturing processes for High Consistency Rubber vs Liquid Silicone Rubber depend on the fabricator’s preference and the requirements of the product or device. If you have additional questions or want to find out more about LSR and the injection molding process, contact an expert with multi-disciplinary experience who can help you find the right solution for your project.

Trust SIMTEC Silicone Parts

SIMTEC Silicone Parts Injection Molding

SIMTEC has produced millions of LSR parts since the inception of our company in 2002. We produce parts for clients in nearly every industry, and we look forward to helping you streamline your process by finding the right solution for your needs.


 

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.