Liquid Silicone Rubber: The Ultimate Guide

Liquid Silicone Rubber The Ultimate Guide

Liquid silicone rubber (LSR) is a unique elastomer with unique physical and chemical properties. A reason this guide explores critical variables about this silicone grade.

So, before choosing LSR for your next application, read this guide.

What Is Liquid Silicone Rubber?

Liquid silicone rubber is a thermoset polymer known for low viscosity. Unlike other silicone rubbers, LSR is platinum cured.

In most cases, it exists as liquid silicone and platinum – hence the name 2-part liquid silicone rubber.

Benefits Of LSR

Liquid Silicone Rubber parts

LSR parts

  • Perfect for a Wide Range Of Temperatures Uses

At very high temperatures such as 250°C, the material maintains desirable physical, chemical, and thermal properties. It will remain stable even at extremely high temperatures.

On the other hand, at -100°C, LSR will neither crack nor become brittle. They will remain flexible.

  • Exhibit Excellent Resistance to Aging

LSR has excellent resistance to weathering or UV rays. As a result, they are perfect for most outdoor applications.

  • Ability to Retain Elasticity

LSR compression set ranges from 15% to 20%. Remember, this is not common with most materials.

It implies, when you subjected injection molded LSR part to compressive forces for a reasonable time, the possibility of deformation is virtual “zero”. Once you remove the load, the product retains its original shape.

Additionally, they can retain the desirable elastic properties for a long time.

  • Excellent Resistance to Chemicals and Most Solvents

LSR resists degradation by most chemicals. However, resistance may be limited to specific temperatures depending on the rubber formulation.

Ideally, they will remain stable on exposure to oxidation, UV rays, water, or ozone.

  • Exhibit the Best Non-toxic Properties

Hypoallergenic LSR grade neither reacts nor causes allergic reactions. Even for the most sensitive skin, these silicone products are usually safe.

Closely related to this is the biocompatibility. A reason LSR is a popular choice for most baby products.

  • It Offers Better Electrical Resistance

Silicone rubber has the best insulation properties. It can resist electrical stress and corona discharge. It is the main reason the material is perfect for most high-voltage applications.

Other benefits include:

  • It has an exceptionally smooth surface
  • Available in a range of colors
  • Odorless
  • Resistance to corrosion
  • Absorbs vibrations

Limitations of Liquid Silicone Rubber

Although LSR has many benefits, there are some limitations you should know. They include:

  1. LSR is a thermoset – it is not recyclable
  2. There are few grades of LSR to choose from
  3. Production cost is relatively high
  4. You should treat it to prevent the sticky feel, which may attract dust
  5. Materials such as sulfur may inhibit proper curing

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LSR Vs. High Consistency Rubber (HCR)

Figure 1 Medical grade liquid silicone rubber
Medical grade LSR parts

Well, HCR is a functional material you can use to make many products.

Let’s compare the two types of silicone elastomers:

LSR High Consistency Rubber (HCR)
  • Only additive curing with platinum is recommended
  • Contains silica as an inorganic filler
  • LSR is affordable compared to HCR
  • Lower compression strength than HCR
  • Superior elongation properties than HCR
  • Best for the compression molding process
  • Both platinum and peroxide work well during the curing process
  • Has silica as an inorganic filler
  • Although it is more expensive than the LSR, high-consistency rubber platinum catalyst is more expensive than those with peroxide catalyst
  • Higher compression strength than LSR
  • LSR has better elongation properties than HCR
  • HCR with peroxide is suitable for both compression molding and extrusion while platinum-cured HCR is suitable for the compression molding process

LSR Vs Silicone Rubber (Solid)

The primary material composition in the materials remains the same. However, the variation mainly comes in how the products exist under normal conditions and during processing.

Let’s compare the two:

LSR Solid Silicone
Known for its fluidity (liquid in nature) Exist in solid form
In most cases, it does not require a vulcanization agent during molding For successful molding, you should add a vulcanization agent
Final products are known for high transparency Final products have low transparency compared to LSR products

Chemical Composition Of LSR

Silicones Structure
Silicone structure

First, LSR is a type of silicone. The main elements in silicone are:

  • Silicon
  • Hydrogen
  • Oxygen, and
  • Carbon

In the entire structure, the main chemical chain is composed of silicon and oxygen. We refer to the chemical chain as siloxane.

However, during the curing process, manufacturers add platinum. A reason, LSR is also called platinum-cured plastic rubber.

This produces a high-performance rubber with high tear and tensile strength. All these desirable properties are due to reinforced polysiloxane chains with silica.

Furthermore, it is worth noting that siloxane bonds and platinum are responsible for the versatile properties of LSR.  Remember, depending on the specific application’s requirement, you can add fillers or other pigments.

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Liquid Silicone Rubber Properties

The resin is known for:

  • Highly biocompatible
  • Very durable
  • Highly chemical resistant
  • Works in a wide temperature range (between -60 °C and +250 °C)
  • Excellent insulation properties
  • High stable at high temperature
  • The compression set is generally low

LSR Manufacturing Process

How to get silicon from silica in LSR manufacturing
How to get silicon from silica

Silicones do not exist naturally. You must process the raw material – silicates.

Below is a summary of the entire process:

Step 1: Get raw material, which is mainly rocks or sand with silica

Step 2: Mix silica with carbon source then heat to get silicon and carbon monoxide

Step 3: Add methyl chloride to silicon then heat to produce Dimethyldichlorosilane

Step 4: Subject the compound to distillation then add water to produce disilanol and hydrochloric acid

Step 5: To produce polydimethylsiloxane, you should condense disilanol

Step 6: From here, you will subject polydimethylsiloxane to various chemical processes producing a range of rubber formulations such as:

  • Liquid silicone rubber (LSR)
  • Heat-cured rubber (HCR), just to mention a few

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Types of LSR

You may wish to have a conductive, self-lubricating, or self-bonding silicone grade.

Since silicone does not have such properties, liquid silicone manufacturers add special additives. It will depend on the specific application requirements.

LSR Type Properties
Medical grade LSR
  • FDA-approved and commonly used for cleanroom LSR injection molding
  • Must meet a high level of biocompatibility
  • Specifically formulated for highly sensitive applications
Conductive LSR
  • Contains carbon as an additive to improve the conductivity properties
  • Carbon also improves the mechanical properties
Self-lubricating LSR
  • Adding phenyl fluid by weight is about 1.5 to 3%
  • Generally, phenyl is incompatible and thus blooms to the cured LSR part making the surface slippery.
Self-bonding LSR
  • By adding promoters, the LSR becomes self-adhesive
Radio opaque LSR
  • For medical applications, adding barium sulfate allows for easy detection under X-ray
Fluorosilicone LSR
  • Improves resistance, especially in applications involving oils and fuels
Fast curing LSR
  • The number of functional groups will determine the curing rate of LSR
Flame retardant LSR
  • Although the normal LSR rubber can meet the UL 94 HB requirements, higher rating such as V1 or V0 requires other additives

Injection Molding Liquid Silicone Rubber

Liquid silicone injection molding process
Liquid silicone injection molding process

Injection molding LSR is a common practice due to:

  • Ability to achieve high-volume production
  • Parts are high quality, with consistent dimensions

LSR is known for its unique thermosetting properties. That is, you will subject LSR to intensive distributive mixing. The process should take place at a low temperature.

Step 1: Design tooling for injection molding LSR

Step 2: The LSR injection molding machine has two containers. One container will have liquid silicone while the second container will have platinum. Platinum is a catalyst in the injection molding process.

Step 3: The metering unit will release the liquid silicone and platinum in a ratio of 1:1. Of course, the machine will add any necessary pigments which will improve the quality of the final product.

Step 4: Injectors will pressurize liquid. As a result, the liquid will flow into metering pumps.

Step 5: A mixer will move to the heated mold cavity. At this point, vulcanization occurs thereby creating parts. At this point, the nozzle delivers the mixture into the mold.

Step 6: Once the parts cool, you will remove the mold clamps to access the product.

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Let’s look at another molding technique:

Clean Room Injection Molding Liquid Silicone Rubber

When it comes to clean room injection molding, the entire process takes place in a controlled environment. Maybe you can refer to it as a “sterile environment”.

As a result, the liquid silicone rubber parts are free from any form of contamination.

Normally, the clean room injection molding process eliminates possible contamination by;

  • Microbes
  • Dust
  • Tiny airborne particles

In liquid silicone rubber clean room injection molding, everything must comply with the ISO 14644 standards. Depending on the types of LSR products you want, the ISO 14644 Classes range between ISO Class 1 and ISO Class 9.

LSR clean room injection molding is suitable for making silicone parts in the laboratory, health, and medical industries.

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Liquid Silicone Rubber Overmolding

Unlike other silicone injection molding processes, here you will mold LSR in three different options:

  1. Over a substrate
  2. Under a substrate or,
  3. Through a substrate

By doing so, you will have complete parts with exceptional designs and aesthetic appeal.

However, LSR Overmolding is a challenging process for most product designers. Therefore, only companies with expertise can handle the process.

When it comes to liquid silicone rubber Overmolding, you can try any of the following techniques:

  • Multi-shot injection molding liquid silicone – In this technique, the first shot will form your substrate. Then your second shot of LSR will form the overmold.
  • Insert molding silicone – Once you have a specific substrate, you will start by placing it in the mold. Then you will cover the substrate either partially or fully with LSR.

In short, SLR offers a range of flexibility when it comes to designing and molding silicone parts.

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Uses of Liquid Silicone Rubber

Some common applications include:

  • Gasket and sealing parts in aerospace and automotive industries
  • Insulation systems in the electrical and electronics industry
  • Consumer products and hardware products
  • Keyboards and touchpads
  • Healthcare products such as hearing aids, respirators, wearable devices, masks, catheter tubing, etc.
  • Hand instruments
  • Valve parts and components, just to mention a few

Again, when it comes to choosing LSR for your applications, you can choose the general purpose or advanced option. Ideally, the variation comes in the formulation process to achieve high performance.

Conclusion

LSR plays an integral role in manufacturing a range of rubber parts. It is a high-performance thermoset rubber with unmatched physical and chemical properties.

Molding LSR is a popular technique for most fabricators.

At LegenDay, we offer a perfect solution for all your silicone rubber products – contact us now.

 

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