Liquid Silicone Rubber (LSR) is a versatile Material used for a variety of purposes. Proper Testing of the LSR material and the molded parts are important elements in the production process and in assuring key functionality requirements.

When testing LSR materials, our primary objective is to analyze and verify the raw materials’ key properties. A wide range of LSR material testing methods are available to ensure quality and consistency.

Why LSR Parts Testing Matters

Material testing and part testing is an important step in the LSR injection molding process and serves many important purposes:

• Testing the raw material allows key properties to be observed and verified.

• Testing parts can detect potential issues in the injection molding process that unnoticed can lead to customers receiving non-confoming parts.

• Testing methods used to measure the presence of substances of high concern in LSR molded parts is critical for products that have strict regulations limiting these substances.

• Data and insight derived from testing can inspire new concepts for enhanced performance in mold design and in the LSR injection molding process.

LSR Materials and Parts Testing Methods

The following sections detail some of the various methods used for testing LSR materials and molded parts:

1. Infrared Spectroscopy (IR)

Infrared spectroscopy is a powerful tool used for early detection of fluctuations in materials for thermoplastics and LSRs. The IR testing method detects the wavelengths absorbed when the material is exposed to electromagnetic impulses. The test data collected also serves as a useful reference for noting fluctuations when comparing material lots and for identifying out-of-spec properties.

Figure 1: Typical IR spectra’s of LSR material

Some of the benefits of testing LSR materials with IR include:

• Material test results provide valuable input for making adjustments, within the defined process window, to the injection molding process

• Out-of-spec findings provide substantiation for rejecting material prior to processing.

• Provides insight into the chemistry behind the reaction by demonstrating the correlation between the chemical and mechanical data and a comprehensive view of all of the materials curing properties.

2. Rheology

Rheology is the analysis of a material’s crosslinking reaction. For LSR’s it is performed by measuring the shearing resistance after mixing the two LSR material components (A & B) at a defined temperature such as 110 degrees Celsius. With this data, the specs for the injection molds can be optimized resulting in enhanced process quality.

Figure 2: Curing graph from a fast curing LSR system

Rheology measurements are also used for analyzing thermoplastic materials by observing the behavior after melting the thermoplastic polymer. For analytical reasons, the measurement of the degradation of thermoplastics during the injection molding process is a useful tool for comparing the raw material with the finished component.

The following lists some of the benefits of using rheology for LSR materials testing:

• For liquid silicone rubber materials, observing crosslinking behavior provides a good overall view and the measurement of the material’s viscosity is the best method for providing this insight.

• Crosslinking behavior provides very important conclusions about the curing process in detail, especially for duration, speed, and other viscosity-related data.

• An understanding of how the material behaves allows for optimization of the injection molding parameters (holding pressure, cooling) and cycle time.

• Observing and comparing data from different batches during incoming inspection will provide valuable information about crosslinking behavior.

3. Mechanical Testing

Mechanical testing provides information about the material’s characterization. It has been the longtime standard used for incoming material inspections and quality releases, and has also been the standard method used for verification that the material properties are aligned with those found on the material certificates. The most common mechanical test methods used for polymers are tensile elongation, tear strength, and compression set.

For LSR polymers, the measurement of hardness (mostly Shore A) is also commonly performed.

Mechanical testing is also used for observing the influence of the injection molding process (sprue, injection parameters) on the mechanical properties of the silicone or thermoplastic material. Testing conducted prior to releasing production can help detect and prevent non-conforming parts from reaching the customer.

Benefits of mechanical testing include:

• Mechanical testing reduces the risk of changes in material lots influencing the processing.

• Performing mechanical testing allows for early detection of potential issues that affect the quality of the part.

• Mechanical test data provides inputs and an understanding of the effects of the molding process.

• This cause-and-effect knowledge is also valuable during molding simulations and new product designs, providing valuable insight about the part design and its suitability for the intended application.

4. Adhesion Testing

Adhesion testing is important for the confirmation of the bonding between silicone and thermoplastic materials in 2-shot molding operations. It is performed by “pull-off” tests measuring the maximum force on a standardized test specimen. For example, mechanical adhesion testing assesses the peeling properties and enables a proactive response to the material’s potential adhesion-related failures.

In general, a testing apparatus or fixture can be developed and utilized for testing specific application-related functionality and simulating the part’s life behavior, as closely as possible.

5. Additional Testing Methods

The following are testing methods used during inspection of injection molded parts that also provide useful findings:

• Polymer-filler test: This test reveals the effects of the interactions between the LSR polymer and various filler substances.

• Dynamic Scanning Calorimetry (DSC) testing: DSC testing analyzes the sample’s  behavior as it increases in heat. DSC testing is used for observing the melting and crystallization of thermoplastic materials as well as vulcanized and unvulcanized silicone elastomers.

• EN 12868: This testing is performed to analyze the release of nitrosamines from materials used in products such as baby pacifiers and bottle nipples according to EN 12868.

• RAMAN spectroscopy: This equipment is used to analyze, identify and observe the chemical structure and various chemical compounds that comprise the LSR product being tested.

• Thermogravimetric analysis (TGA): This test measures the stability of thermoplastics and silicone elastomers by analyzing how the specimen’s mass changes as it increases in temperature.

• EN 14350-2: Used to detect volatile organic substances according to EN 14350-2.  Typically used for children’s cups and other drinking products.

• The Karl Fischer method: Also called ISO 760, testing methodology is used to observe the reaction between an item’s water content and iodine, sulfur dioxide, pyridine, and methanol solutions.

Driving Innovation Through LSR Material Testing

Progressive and innovative manufacturers like the RICO GROUP have recognized the value and mutual benefit of partnering with outside research institutions and laboratories, academia, universities, and testing laboratories for joint-projects. Establishing working relationships with outside institutions creates an environment of rich collaboration for the purpose of research and testing. The experience and knowledge offered by manufacturers like RICO, combined with the well-equipped laboratories and willing participants found in many outside research facilities, contribute to the development of new manufacturing or processing methods. This type of mutually beneficial arrangement also expands the opportunities for improved testing and and quality results.

For RICO GROUP companies, the data and insight received from internal testing and cooperative R&D efforts is integrated into our innovative concepts for the high-performance molds we manufacture at our RICO tool shop, as well as in the state-of-the-art molding technology used by SIMTEC and the other molding sites within the Group. Exploring and adopting new technologies and the knowledge gained from testing leads to advances in processing and creates value to us as well as to our customers.

LSR products must meet customers’ product requirements to add value. Our internal and external testing methods help to ensure our LSR solutions maintain a high level of quality and customer satisfaction. With parts becoming smaller, and part geometries and performance requirements more challenging, we continue innovating. The applied knowledge gained by testing allows us to provide our customers with high-quality solutions to meet their needs.

LSR Testing Case Study: RAMAN Spectroscopy

The RICO GROUP material testing lab in Austria and the Johannes Kepler University Linz/Austria worked together on a project analyzing the bonding between LSR and a thermoplastic material for a 2-shot application.

Purpose: Testing was aimed at gaining a better understanding of the contact area and the extent to which the materials penetrate for bonding.

Test Method: RAMAN Spectroscopy

RAMAN scattering is the inelastic scattering of light on molecules or solids. RAMAN Spectroscopy can only be used in samples where the polarizability changes when it is irradiated by light. Energy is transferred from the stimulating medium to the sample (“Stokes”) or vice versa (“Antistokes”). The wavelength of the scattered light does not correspond to that of the irradiated. This is a non-contact method for testing material characterization.

Results: Using cryo-microtomy, samples were cut 90 degrees to the interface as required and then examined using RAMAN Spectroscopy.

Figure 2 displays the spectra from the thermoplastic and LSR areas. The RAMAN Imaging clearly shows distinguishable peaks. Using the defined peaks, the different phases (thermoplastic and LSR) can be made visible by RAMAN Imaging. The sample is now rasterized, and the spectra are picked up from the sample at certain intervals. By evaluating the intensity of the peaks marked above, one gets a visual depiction of the significance of the differences using mapping.

In Image 3, both materials, the thermoplastic and LSR, can be detected in the mixing area.

With mapping, it can be seen that the two layers (thermoplastic and LSR) are clearly distinguished and the glass fibers (filler in the thermoplastic) are also visible. The purpose of the study was to understand the degree of penetration, which is clearly evident using mapping.

With RAMAN Spectroscopy, as shown in image 4, mutual penetration can be observed (in the range of approximately 10 nm). It can be seen that the two phases penetrate, and therefore evidence that the bonding of the thermoplastic and LSR is beyond just at the surface.

LSR Material Testing From SIMTEC and the RICO GROUP

SIMTEC and sister companies in the RICO GROUP, have a wide range of testing capabilities used to ensure process and product quality. Material testing serves multiple purposes, typically the primary objective is for the analysis and verification of the materials’ key properties. This test data is used in optimizing processes and can also play a significant role in addressing issues that arise during the molding process. In some cases, functional testing is performed on the molded component at the request of customers to verify specific properties that are important to the part’s function.

The RICO GROUP of companies is driven by innovation and technology. We are constantly looking for ways to improve quality and productivity, remain agile to adapt and respond to new market requirements and opportunities, and to maintain our leadership position in the industry.

Contact SIMTEC today for more information.