Quality & And Metrology for Injection Molded Liquid Silicone Rubber Parts

Whether you are a manufacturer of LSR molded components or a manufacturer that uses (or is considering) LSR molded parts in your product, this useful how-to guide provides an understanding of quality and metrology as it relates to LSR injection molded parts, common defects to look out for, and best practices for minimizing defectives parts in product shipped to customers.

Quality and Metrology for Liquid Silicone Rubber (LSR) Molded Components

Every manufacturer knows making a perfect product all of the time is impossible.  The laws of physics and the statistical variation of natural processes prevent this from happening.  In Liquid Silicone Rubber (LSR) injection molding there are a number of defects that can occur that can be difficult to predict, however this does not mean the product is doomed for failure. The objective is to supply your customers with product within a specification window. It should also be remembered that an imperfection doesn’t mean the part is defective.  The quality expectation your customer has for incoming products must be consistent with the definition of quality your manufacturing facility has for outgoing quality. In order to accomplish this for LSR products we must employ the tools of quality engineering, and understand the specialized metrology that apply to LSR parts.

WHAT IS QUALITY?

Quality engineering may best be described as a scientific business approach that utilizes a large set of problem-solving tools. 

Quality Tools

There have been hundreds of different tools developed over the years, and some of the simplest tools are also some of the most effective. These quality tools are required for a successful LSR project. 

Essential quality tools include:

  • Blueprints
  • Control Plans
  • Procedures
  • Visual Standards
  • FMEA’s
  • Measurement Equipment
  • Control Charts
  • Technical Problem Solving Methods

The application of these tools must start early in the product life cycle (as early as the quoting phase) and should continue to be applied throughout the cycle.  In fact the product life cycle can be thought of as a series of sequential quality tools, as illustrated in the figure below.  These quality activities make up the essential deliverables of a project.

Figure 1. Product lifecycle and high-level quality deliverables 

Quality Characteristics

Quality can also be defined through the functional features and performance characteristics of the product, using the Form, Fit, and Function analysis.  It is essential to have a good understanding of your customer’s part’s function and expectations.  Features such as critical dimensions, durometer, and surface finish help to understand the real interpretation of the tolerance window and level of acceptable imperfection.

Another key aspect of quality is the characterization of defects. By defining and characterizing defects, we can plan for those worst-case scenarios and make adjustments to the manufacturing process accordingly.  Good characterization and planning mean you’ll have improved Design for Manufacture (DFM) and good Process Failure Mode and Effect Analysis (PFMEA).

QUALITY BEST PRACTICES

To apply the principles of quality to LSR projects, focus is placed on four essential activities:

1)  Embed Quality Tools into the Project Timeline

2)  Visual Inspections

3)  Dimensional Measurements

4)  Material & Functional Tests

1) Quality Tools & Project Timelines

Embedding quality tools into the project timeline starts with developing a mutual understanding of the blueprint with the customer and establishing the proper critical dimensions and features. 

  • Ensure there is at least one critical parameter that is indicative of the process. The objective is control of the process and the part. For example, the primary concern for silicone injection molding is achieving a good fill of the mold. The basic overall dimensions (such as ID, OD, part thickness) are important indicators of the overall health of the process.
  • During the print review process, it is important to incorporate the customer-specific requirements that may cover some items that are not on the print and may include basic definitions of part imperfections. 
    • Be aware these definitions may be based on plastics terminology and may need some interpreting to apply to silicone and important to clarify this terminology with the customer. (For example a gate location may be referred to as a sprue) 
  • Apply the key learnings from the print and codify them into a control plan. Include also pertinent measurement procedures with the control plan.

2) Visual Inspection. Once control plans are in place and the quality tools are being applied, the second essential activity is to ensure successful visual inspection.  To correctly perform visual inspections, it is a good practice to create Visual Standards. The visual standards should properly define the various defect modes.  

The table below is a list of potential defects that can occur based on possible problems at each stage of the injection molding process.

Some of the more common and troublesome defect modes that occur through the different stages of the injection molding include:

Injection Tails/Wells: Sometimes referred to as a high gate, high sprue, gate void, or inverted tail. 

  • Injection tails/wells are excess material (tail) or missing material (well) located at the injection site of the part.  
  • Typically a cylindrical remnant or void that occurs due to the curing kinetics and ejection mechanics of the part. 
  • Injection tails/wells are typically present to some degree on the part and are dictated by the gate geometry, injection pressures and speeds, and can vary due to fluctuations in material properties.  

The images below illustrate each of the conditions.

Buildup: Sometimes referred to as mold fouling and is actually the root cause of the imperfection.

  • Buildup refers to the injected material that has hardened and stuck to the mold
  • This mold buildup causes the parts from subsequent shots to have a void, notch, or pitting appearing on the part.  
  • Buildup is often associated with a particular material and the residue from its by-products visible after the curing process. 

The following depict examples of imperfections in parts as a result of buildup

Short/Last Fill: Perhaps one of the most common injection molding problems, short shot in a part is the result of an inadequately filled the mold.

  • Sometimes this means that the part is grossly “short” of its final form.
  • Other times there is only a small area with void-like imperfection at the area of last fill.
  • Short shots can happen for number of reasons, including low injection volumes, venting issues that lead to trapped air, and vacuum issues in the system.

The images below illustrate the short shot and last fill void conditions.

3)  Dimensional measurements: To complement visual inspection controls, dimensional measurements must also be utilized. 

  • LSR’s flexibility and compression set characteristics are benefits of LSR and desirable attributes for a parts performance. However these same characteristics are what make measuring silicone parts challenging. 
  • It is recommended to have an earnest conversation with your customer about acceptable tolerances and a reminder to the customer’s design team that silicones differ from plastics and metals.  Tolerances and imperfections in silicone rubber take on a different character — often more forgiving.  For example, flash on an LSR part will not play as large of a role mechanically as it might with thermoplastics.

Hard Gages

When using hard gages, measurements will require special considerations related to the contact force used by quality technicians. Custom designed fixtures and training are recommended to ensure accurate consistent dimensional and functional measurements.

When force is being applied to the component for hard gage measurements, we have to be aware of the force and also the indicator geometry so that we are applying the correct pressure on the part. ASTM D3767 provides the guidance for this and defines a pressure that can be used for materials that are either above or below 35 on the IRHD of Shore A scale.

  • Softer or lower durometer parts (below 35 shore A) require a smaller specified pressure of 10 +/- 2 kPA . 
  • Greater than 35 require a pressure of 22 +/- 5 kPA.

Optical CMM

Because of the challenges when using hard gauges in measuring LSR parts, the bulk of the dimensional measurements use an optical CMM.  To successfully and repeatedly measure silicone rubber parts, customized fixtures are often required.  These fixtures will need to establish a proper datum plane that allows the part to maintain “flatness” as well as integrity of the features.  

Regardless of how good the fixture is, it will also be important to train quality team members to understand what a flat part should look like and how it can be manipulated. It is to be expected that false readings may occur from time to time, and these will need to be recognized and corrected.  

Shown below are images of two different custom fixtures that have been developed at SIMTEC Silicone Parts. The image at right depicts a fixture using mirrors allowing the operator to capture the orthogonal view in a single program without manipulating the part.

4) Material and functional tests: Implemented to meet control plan needs.  Shore hardness testing, tensile testing, and pressure/leak testing are perhaps the three most common tests.  As stated with the other methods, proper equipment, fixtures and training are necessities to ensure parts are measured properly.

In conclusion, remember to employ your basic quality engineering tools early and often in the product life cycle.  Quality engineering is a set of problem solving tools that will allow you to apply a scientific approach to your manufacturing.  Remember, a good quality management system is founded on good data and its application.

ABOUT SIMTEC SILICONE PARTS

SIMTEC Silicone Parts is a pioneer in LSR injection molding and a preferred supplier to the global leaders in the medical, life sciences, automotive, consumer and building technology industries. With decades of experience, SIMTEC has amassed a deep understanding of LSRs and the LIM manufacturing process, and we apply this knowledge to every part we manufacture – delivering consistently high quality, high volume precision LSR injection molded components to our customers.  For more information about our quality system or to discuss an upcoming project involving LSR molded components, contact SIMTEC today.