The Production Part Approval Process (PPAP) approach has its roots in the automotive marketplace.  This general approach has been adopted by many companies in the industrial and consumer goods markets.  Meanwhile many medical device manufacturers are more apt to use the installation qualification, operational qualification, performa­nce qualification (IQ-OQ-PQ) methodologies.  These two methodologies overlap significantly in their approach, but each methodology has different strengths. As a manufacturer of parts to both industries, at SIMTEC we believe each has its advantages depending on the application.

A temptation would be to combine the multiple systems into one comprehensive risk management methodology; however, an additional key input is the consideration of value added (i.e. cost).  Doing more than is necessary can significantly add to the timeline and cost of development of a product, so a blanket approach will have negative timeline and cost elements. The preferred approach is a targeted combination based on experience from multiple sources in the supply chain.

Regardless of which approach a company uses, in the evaluation of risk, there is no substitute for experience.  When available, the most common input for risk analysis is similar products or applications.  The effectiveness of this methodology can be markedly improved by involving the whole supply chain. The best approach to gathering experience is to get input from all parties in the supply chain (e.g. material suppliers, contractors, manufacturers, packagers, consumers, etc.) Each will have unique insight as to what variation is in their process and what can go wrong. Unfortunately, this integrated approach for the development of a risk management strategy is too often skipped.

A more typical scenario of risk analysis for companies that use the PPAP methodology is when the part designer identifies “critical” dimensions and properties on a print. The part manufacturer has a standard Process Failure Mode Effect Analysis (PFMEA) used for the manufacture of similar products. One lot of raw material will be used in the manufacture of pilot/PPAP parts. In some cases, the disconnect may be even more severe if the manufacturer of the part does not know how the part is used or understand the interactions between the part and other parts in an assembly. This can all lead to an inadequate risk analysis strategy. This can all be done using boiler plate PPAP templates that meet the requirements of the standard.

The IQ-OQ-PQ methodology is likely to be more comprehensive by design and one of the reasons for this is the protocol requirement. This requires the initiator to develop a specific plan for the validation of the product for all three stages individually. The installation qualification (IQ) verifies that equipment was installed correctly and functions as intended. The operation qualification (OQ) verifies that the product meets the requirements at defined process limits.  The performance qualification (PQ) verifies that the product is capable of meeting the requirements over time.  The protocol typically requires review and approval by all involved parties. This customization can help the process be more effective.

The OQ stage is an important step, and it is a fundamental difference between the two methodologies in regards to part performance. This step generally requires parts to be produced at “high” and “low” process limits and the evaluation of the product at those limits. This can uncover unpredicted product variation and evaluate the significance of this variation.  On the other hand, this step can add significant cost and time to the product realization process, and should therefore be avoided for non-critical parts.

The PPAP equivalent of the OQ step is the identification of “critical dimensions” on the print. This likely means that the part designer has some intimate knowledge, or has made some assumptions, of what is thought to be critical to the part and/or what must be controlled in production. However, the major difference between these two approaches is that these assumptions are not tested at the process limits. So, the OQ process can be an important enhancement to risk analysis in evaluating a critical part.

Whereas the OQ process is not present in the PPAP methodology, the Gage Repeatability & Reproducibility (R&R) process within a PPAP package is a strength and is not covered in the IQ-OQ-PQ process. A Gage R&R is a verification of the measurement system. It quantifies the variation caused by the measurement system and personnel in order to determine if the measurement system is capable of detecting a change in the product.  The gage R&R will tell how sensitive a measurement system is to changes and will determine if improvements are needed.  Control limits can be set that consider the variation in the measurement system.

Neither of these two methodologies have a specific requirement to consider different lots of material, although this is more common by practice in the medical device and pharma industries. This can be an important step when material variation is expected to have an impact on the end product. In validations where material variation is evaluated, it is typical to use 3 lots of raw material. Ideally, although difficult to get in many cases, raw materials that cover a specification range would be used.

In the end, no matter which industry a customer is in, a customized validation strategy with input from the customer, the manufacturer, and raw material supplier, will result in a better understanding of risk and ensure that proper controls are put in place to minimize that risk.

All of the parts we produce are critical parts with tight tolerances used in demanding applications. In addition, LSR 2-shot parts always require tight tolerances and process controls to ensure specifications are met. SIMTEC is well positioned to work with customers in order to customize a validation strategy that utilizes appropriate methods for your critical application. With input from our highly qualified team members with significant design, tooling, engineering, production, and quality experience, we can help develop an effective validation strategy to ensure success, not only for the launch of the product, but also for the life of the program.

© 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.

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.

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 2-Shot molding. SIMTEC is a pioneer in the emerging field of Liquid Silicone Rubber (LSR) 2-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 2-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.

Fig. 1: Multi-Station Rotational Mold

In addition, over-molding allows the LSR to be injected directly to critical function zones on the part. The reduction in material used for the part increases the cost effectiveness of the part and decreases cycle time. The process also eliminates the additional assembly that may have been required.

Following the trend of this emerging technology and assuring its position as leader in the two shot LSR injection molding industry, SIMTEC Silicone Parts added two additional 250 ton machines in the first half of 2011.

Another multi-station injection mold is a rotating mold. Find out more about rotating mold presses!

Additional information can be found in our “Tech Info” section.

[1] Schmachtenberg, E. and Johannaber, F., 2007, “Montagespritzgießen – Verfahrensprinzipien und Definition,” Technical Conference Montagespritzgießen, Institute of Polymer Technology, Erlangen, Germany, p. 1-18

[2] Schmachtenberg, E., Schuck, M., and Kuehnert, I., 2007, “Forming and Assembly in One Process,” Kunststoffe International, p. 8-13

[3] Kazmer, D.O., 2007, Injection Mold Design Engineering, Hanser Publishers, Munich

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.

One method of multi-component injection molding employs a rotating mold and multiple injection units. Once the insert is molded, a hydraulic or electric servo drive rotates the core and the part by 180 degrees (or 120 degrees for a three-shot part), allowing alternating polymers to be injected.

Fig. 1: Multi-Station Rotational Mold

When compared to other methods, such as insert overmolding, this is the fastest and most common method because two or more parts can be molded in every cycle. However, this is also presents a challenge, as the manufacturer has to fine-tune the process precisely in order to reach the perfect degree of cure in the LSR and still have a short cooling time in the thermoplastic component. Another advantage of this process is the possibility of more complex part geometries than that with the core-back technology, while the general design of the mold remains simpler.

As the 2-Shot molding allows the insertion of the LSR exactly where its function is needed, the production is very cost-effective. Following the trend of this emerging technology and assuring its position as leader in the 2-Shot LSR injection molding industry, SIMTEC Silicone Parts added two additional 250 ton machines in the first half of 2011.

Another multi-station injection mold is a transfer mold. Find out more about transfer mold presses!

Additional information can be found in our “Tech Info” section.

[1] Schmachtenberg, E., and Johannaber, F.,2007, “Verfahrensprinzipien und Definition,” Technical Conference Montagespritzgießen, Institute of Polymer Technology, Erlangen, Germany, p. 1-18.

[2] Schmachtenberg, E., Schuck, M., and Kuehnert, I.(2007), “Forming and Assembly in One Process” Kunststoffe International, p. 8-13.

[3] Kazmer, D.O.,2007, Injection Mold Design Engineering, Hanser Publishers, Munich.

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.

The relationship between viscocity and temperature over time coorelates to the gel point and pot life of liquid silicone rubber (LSR), as is charted above.

In injection molding the curing is delayed while it flows through the cooled barrel.  The viscosity decreases in the barrel due to shearing of the material and viscous dissipation. As soon as the material is injected, the viscosity increases because of the exothermic reaction and increased mobility of the resin in the heated mold. The drop in viscosity due to temperature increase is overshadowed by the increase of viscosity due to cross-linking. The curing process becomes the driving factor until the gel-solid transition is reached. After the gel-solid state is reached, viscosity stays on a constant high level.

[1] Osswald, T. A.; Baur, E.; Brinkmann, S.; Oberbach, K.; Schmachtenberg, E.: International Plastics Handbook; 4^th edition, Hanser Gardner Publications, Cincinnati, 2006

[2] Lopez, L.: Modeling the Vulcanization of Liquid Silicone Rubber; Master Thesis, University of Wisconsin-Madison, 2004

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.

Abrasion Resistance: the ability of a material to resist mechanical wear; an attribute of Liquid Silicone Rubber (LSR).

Absorption: the uptake of a substance.

Accelerator: a chemical that speeds the vulcanization of an elastomer, so that it takes place in a shorter period of time or at a lower temperature.

Adhesion: the action or process of binding to a surface or object; for LSR, the tendency to stick to a contact surface, resulting from chemical or physical interlocking.

After Cure: the uncontrolled continuation of vulcanization after the desired cure is effected and after removal of the heat source; note: not the same as Post Cure.

Aging: the change of the characteristics of LSR or other materials after exposure to light or heat.

Ambient Temperature: the surrounding temperature relative to the given point of application.

American Society of Testing Materials (ASTM): an international standards organization which evaluates and publishes standards for materials and processes.

Assembly Injection Molding: see Two Shot Injection Molding.

Autoclave: a strong, heated container used for chemical reactions and other processes involving high pressures and temperatures.

Backrind: the distortion at the parting line, usually in the form of a ragged indentation.

Bake-Out: the process whereby a vacuum system is heated for a given time at a predetermined temperature to de-gas components such as gauges, fittings, valves, seals, etc.

Batch: a quantity or consignment of goods produced at one time.

Blister: a raised spot or bubble on a material’s surface, or a separation between layers, usually forming a void or air-filled pocket in the vulcanized article.

Bloom: a dusty or milky-looking deposit caused by migration of a liquid or solid to the surface; may occur after curing when excess additives are blended within the rubber; also known as bleeding.

Break-Out: the amount of frictional force required to start a body in motion over a surface.

Bundesinstitut für Gesundheitlichen Verbraucherschutz und Veterinärmedizin (BgVV): German Federal Institute for Consumer Health Protection and Veterinary Medicine; in 2002, the BgVV dissolved into two entities: the Bundesinstitut für Risikobewertung (BfR) and the Bundesamt für Verbraucherschutz und Lebensmittelsicherheit (BVL).

Bundesinstitut für Risikobewertung (BfR): German Federal Institute for Risk Assessment; the scientific agency responsible for assessing the safety of food, feed, substances, and products.

Bundesamt für Verbraucherschutz und Lebensmittelsicherheit (BVL): German Federal Office of Consumer Protection and Food Safety; like the FDA in the US, the BVL makes recommendations applicable for medical and food applications.

Catalyst: the chemical that triggers or accelerates the cure of LSR; does not usually become a component of the end product.

Cavity: the hollowed-out portion of a set of molds that will be used in shaping a container as it is formed on a bottle machine.

Chemical Bonding: an attraction between atoms or molecules, resulting in the formation of a chemical compound.

Coefficient of Thermal Expansion: describes the expansion of an object with changes in temperature and may be linear or volumetric; the coefficient of linear thermal expansion is the change in length per unit of length for a one degree rise in temperature; the coefficient of volumetric thermal expansion is the change in volume divided by the product of the original volume and the change in temperature.

Compound: the mixture of polymers and other ingredients to produce a usable rubber material.

Compression Modulus: the ratio of compression stress (force) to the resulting compression strain (noted as a percentage of the original specimen thickness).

Compression Molding: a thermoset molding technique in which the uncured LSR compound is put in a heated, open mold cavity and the mold is closed under pressure (often in a hydraulic press); the material flows to completely fill the cavity and pressure is maintained until curing is complete.

Compression Set: the amount by which a rubber specimen fails to return to its original shape after the release of a compression load; the numerical value is the percentage of the amount compressed that does not return.

Co-Polymer: a polymer consisting of two different monomers chemically combined.

Cracking: the sharp breaks or fissures in a rubber surface caused by excessive strain and/or exposure to detrimental environmental conditions, such as ozone, weather, or ultraviolet (UV) light; also known as crazing.

Creep: the increasing deformation of a rubber material under a constant compressive load; also known as cold flow.

Cross-link: bonds that link one polymer chain to another.

Cure: the heat- or mixing-induced process whereby long chains become cross-linked by a vulcanizing agent to form three-dimensional elastic structures. This reaction transforms soft, weak, non-cross-linked materials into strong elastic products; also known as Vulcanization.

Cure Date: the date when the rubber product was molded (e.g., 1Q01 means the 1st quarter of the year 2001).

Cure Moderator: a substance that delays or controls the rate of the cure.

Daylight Opening: the distance between two edges of the open mold.

Deflashing: the various processes used to remove the waste edge from a molded part.

Dielectric Strength: the maximum electric field that a material can withstand without breaking down.

Durometer: an instrument for measuring the hardness of rubber by its resistance to surface penetration of an indenter point; also the numerical scale indicating the hardness of rubber (e.g., Shore A Durometer).

Elasticity: the property of an article which tends to return to its original shape after deformation or the extent to which it tends to do so.

Elastomer: any synthetic or natural material with resilience or memory sufficient to return to its original shape after major or minor distortion.

Elongation: generally, “Ultimate Elongation,” or the percent of increase in original length of a specimen at time of break.

Endothermic: a reaction that requires heat; absorbing heat.

Exothermic: a reaction that releases heat such as vulcanization or cure; releasing heat.

Fiber Reinforced: describes a structural matrix of materials used in polymers to gain structural support.

Filler: the chemically inert, finely divided material added to the elastomer to aid in the processing and improvement of physical properties.

Flash: the excess rubber remaining around a rubber part after molding due to space between mating mold surfaces.

Food and Drug Administration (FDA): the US body which regulates medical and food applications.

Gate: the point of rubber entry into a mold cavity.

Goniometer: an instrument used for the precise measurement of angles.

Hardness: resistance to a distorting force, measured by the relative resistance of the material to an indented point of a standard hardness-testing instrument.

Hardness, Shore A: rubber durometer hardness as measured on a Shore “A” gauge; higher numbers indicate harder material (e.g., 30 shore is soft, 90 shore is hard).

Heat Resistance: the capacity of a rubber compound to undergo exposure to a specified level of elevated temperature while retaining a large majority of its original properties.

High Cure Rubber (HCR): rubber that is processed on a rubber mill having much higher viscosity than liquid silicone rubber (LSR); belongs to the HTV family.

High Temperature Vulcanization (HTV): high-temperature-curing rubbers.

Hydrolysis: the chemical breakdown of a compound due to its reaction with water.

Hydrophilic: having a tendency to mix with or be wetted by water.

Hydrophobic: having a tendency to repel or fail to mix with water.

Hysteresis: the phenomenon in which the value of a physical property lags behind the changes in the effect causing it.

Injection Molding: a process in which preheated rubber is injected under pressure from the heating chamber through a series of runners and sprues and into a closed, heated mold cavity, where it is then vulcanized; injection molding is ideal for high-volume, high-precision production of molded rubber parts.

In-Mold: refers to the processing of two materials within the same mold to form a component with parts of multiple materials; see Two Shot Injection Molding.

Innovation: (from Merriam Webster’s Collegiate Dictionary) the introduction of something new; a new idea, method, or device.

Liquid Injection Molding (LIM™): refers to the processing of liquid silicone rubber (LSR).

Liquid Silicone Rubber (LSR): the silicone rubber utilized in the LIM™ process; the liquid form of HTV.

Low Temperature Flexibility: the ability of an elastomeric product to resist cracking or breaking when flexed or bent at low temperatures.

Material Safety Data Sheet (MSDS): a form containing data regarding the various properties of a particular substance.

Maximum Cure: the point at which an elastomer has reached its highest degree of cure.

Mechanical Bond: a form of adhesion in which material physically locks into the crevices of the surface of another material.

Melt Flow Index: a measure of the ease of flow for the melt of a thermoplastic polymer.

Millable Gum Rubber: rubber that is processed on a rubber mill having a much higher viscosity than liquid silicone rubber (LSR); as also known as high consistency rubber.

Mismatch: the misalignment of the halves coming together in a mold.

Modulus: the tensile stress at a specified elongation (usually 100% elongation for elastomers).

Modulus of Elasticity: the ratio of the stress (force per unit area) to the strain (increase in length per initial length) as measured on a rubber specimen; also known as Young’s Modulus (E).

Mold: to shape or process a material into a usable form (verb); the metal tools, usually steel or aluminum, machined and assembled so as to create openable cavities for the purpose of shaping and vulcanizing rubber (noun).

Mold Cavity: a hollow space within the mold in which uncured rubber is shaped and vulcanized; also known simply as a Cavity.

Mold Finish: an uninterrupted surface produced by the intimate contact of rubber with the surface of the mold at vulcanization.

Mold Shrinkage: the dimensional loss in a molded rubber product that occurs during cooling, after it has been removed from the mold.

Monomers: the unit of a chemical compound that is polymerized into a polymer.

Mooney Viscometer: a shearing disk device used to gauge the viscosity of a rubber sample under heat and pressure. Named for developer Melvin Mooney, this was once the standard tool for determining processing characteristics, but has now largely been replaced by the Rheometer.

Moving Die Rheometer (MDR): an instrument used to measure the cure degree of liquid silicone rubber (LSR) and rubber by a percentage of maximum torque, utilizing heated die.

Multi-Component Injection Molding: see Two Shot Injection Molding.

Nitrile Butadiene Rubber (NBR): a material with a wide temperature resistance that is resistant to oil, fuel, and other chemicals.

Occlusion: the mechanical process by which vapors, gases, liquids, or solids are entrapped within the folds of a given substance during working or solidification.

Oil Resistant: the ability of a vulcanized rubber to resist the swelling and deterioration effects of various types of oils.

Oil Swell: the change in volume of a rubber article due to absorption of oil or other fluids.

Oscillating Disk Rheometer (ODR): an instrument used to measure the cure degree of liquid silicone rubber (LSR) and rubber by a percentage of maximum torque, utilizing an oscillating rotor.

Outgassing: a vacuum phenomenon wherein a substance spontaneously releases volatile constituents in the form of vapors or gases. In rubber compounds, this may include water vapor, plasticizers, air, inhibitors, etc.

Over Cure: when a longer than optimum vulcanization causes some properties to be thermally degraded. There are two types of over cure: (a) the material continues to harden, the modulus rises, and both tensile strength and elongation fall; and (b) the rubber begins to break down, the material softens, and the modulus, tensile strength, and elongation all decrease.

Overflow: a spot for excess material during molding.

Over-Molding: see Two Shot Injection Molding.

Parting Line: the place at which two (or more) parts of the mold meet.

Permanent Set: the deformation remaining after a specimen has been stressed in tension for a definite period and released for a definite period.

Permeability: the rate at which a liquid or gas under pressure passes through a solid material by diffusion and solution. In rubber terminology, it is the rate of gas flow expressed in atmospheric cubic centimeters per second through an elastomeric material one centimeter square and one centimeter thick (atm cc/cm squared / cm/sec).

Pigment: the material, usually a powder or paste, used to color rubber.

Polymer: a material formed by the joining of many (poly) individual units (mer); i.e., of one or more monomers.

Post Cure: the controlled continuation of vulcanization (usually in an oven) to complete the curing process; provides stabilization of parts and drives off decomposition products resulting from the vulcanization process.

Pot Life: the period of time after mixing a two component system wherein the material retains sufficiently low viscosity for processing.

Purging: the flushing or removing of material.

Resilient: being capable of returning to original size and shape after deformation.

Reversion: the condition in an elastomer caused by thermal or chemical attack whereby chemical bonds are broken with a resulting loss in physical properties.

Rheometer: a cure meter which determines the state of cure for a given time and temperature by measuring the viscosity of the vulcanizing rubber; typically either an Oscillating Disk Rheometer (ODR) or a Moving Die Rheometer (MDR).

Room Temperature Vulcanization (RTV): generally silicone rubber, which after mixing will cure when exposed to air at low temperatures; comes as a soft paste or a viscous liquid.

Rubber, Natural: raw or crude rubber obtained from vegetable sources.

Rubber, Synthetic: manufactured elastomers.

Runner: the distribution system within a mold; connects the Sprue to the Gates.

Styrene Butadiene Rubber (SBR): material with very good resilience and tensile strength, but poor ozone and oil resistance.

Screw: the rotating component of an injection molding machine which delivers the material into the mold.

Scorching: the premature curing or setting up of raw compounding during processing.

Shore A Durometer: an instrument used to gauge soft to medium hard rubber based on its resistance to a frustum (truncated) cone indenter point; most accurate for materials below 90 Shore A.

Shore D Durometer: an instrument used to gauge hard rubber based on its resistance to a sharp 30° angle indenter point; most accurate for materials at or above 90 Shore A.

Shrinkage: the decreased volume of material; usually caused by extraction of soluble constituents by fluids followed by air-drying.

Silicone Rubber: an elastomer that retains beneficial properties through an extra wide temperature range.

Specific Gravity: the ratio of the weight of a given substance to the weight of an equal volume of water at a specified temperature.

Sprue: the initial entry point of rubber into a mold. In a simple mold, it is the same as a gate.

Sprue Marks: the marks (usually elevated) left on the surface of a rubber part after removal of the sprue or cured compound from the gate through which the compound is injected or transferred.

Static Mixer: a device without moving parts used to mix two components together.

Swell: the increased volume of material; often caused by contact with a liquid.

TC 2: the time at which 2% of the maximum torque is reached in a MDR test; indicates the start of cure.

TC 90: the time at which 90% of the maximum torque is reached in a MDR test; indicates that demolding is possible due to sufficient degree of cure.

Tear Resistance: the resistance to growth of a cut or nick when tension is applied to the cut specimen; commonly expressed in pounds per inch thickness.

Tensile Strength: the force per unit area [lb/in2 or kN/m2] required to cause the rupture of a specimen of a rubber material.

Thermal Expansion: the expansion caused by an increase in temperature; may be linear or volumetric.

Thermoplastic: describes a polymer that becomes plastic when heated and hardens when cooled, and is able to repeat the process.

Thermoplastic Elastomer (TPE): a copolymer or physical mix of polymers, usually of a plastic and a rubber.

Thermoset: a polymer that becomes permanently hard and rigid when cured.

Transfer Molding: a process in which an initially molded component is moved automatically or manually to a new mold for injection.

Two Shot Injection Molding: a special injection molding process during which one material is injected into a mold to form the initial part or section of a component. Once formed, a second material is injected into the mold with a second injection unit to form the remainder of the component. This process requires that the two materials be compatible (chemically similar) or no bonding will occur; also known as multi-component injection molding, over-molding, in-mold, or assembly injection molding.

Under Cure: the degree of incomplete vulcanization resulting in undeveloped physical properties and tackiness.

Undercut: an area in a mold where there is a overhanging relief.

Vent: an opening in a mold that allows air, gas, or liquid to pass out of the cavity.

Viscosity: the measure of the resistance of a fluid which is being deformed by either shear stress or tensile stress; may be thought of as a liquid’s internal friction.

Volume Change: the increase (swell) or decrease (shrinkage) in the volume of a specimen; noted as a percentage of original volume.

Volume Shrinkage: the volumetric decrease of an elastomeric material when in contact with a fluid; also known simply as Shrinkage.

Volume Swell: the volumetric increase of an elastomeric material when in contact with a fluid; also known simply as Swell.

Vulcanization: the heat- or mixing-induced process whereby long chains of the rubber molecules become cross-linked by a vulcanizing agent to form three-dimensional elastic structures; transforms soft, weak, non-cross-linked materials into strong elastic products; also known as Cure.

Warpage: the dimensional distortion of a material or object after molding.

Weld Lines: the point at which two different flow fronts meet in the molding process.

Young’s Modulus (E): a measure of material stiffness; defined as the ratio of the stress to the strain (increase in length relative to initial length) as measured on a rubber specimen; also known as Modulus of Elasticity; not the same as tensile modulus.

Both curves have corresponding glass transition temperatures, ,T-g1. and ,T-g gel., for the glass transition temperature of the fully cured resin and at its gel point, respectively. The glass transition temperature of the uncured resin, ,T-g0., and an S-shaped curve labeled “vitrification line,” are also depicted. The vitrification line represents the boundary at which the glass transition temperature becomes the processing temperature. Hence, to the left of the vitrification curve, the curing process is controlled by a very slow diffusion process. The TTT-diagram shows an arbitrary process temperature. The material being processed reaches the gel point at ,t=t-gel. and the vitrification line at ,t=t-g.. At this point, the material has reached a degree of cure of ,c-1. and the glass transition temperature of the resin is equal to the processing temperature. The material continues to cure very slowly (in other words it is diffusion controlled) until it reaches a degree of cure just below ,c-1..

The various regions in the diagram delineate the material properties of thermosets. The region labeled “viscous liquid” is the one where the resin is found from the beginning of processing until the gel point has been reached. The flow and deformation that occurs during processing or shaping must occur within this region. The region labeled “char” must be avoided during processing, since at high processing temperatures the polymer will eventually undergo thermal degradation.

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.

As with other properties, the specific volume is greatly affected by the temperature and pressure.  Temperature and pressure are both important for processing and is described in the post “Expansion or Shrinkage”.

Pictured above: Hollow LSR parts with air pockets that float and LSR parts without air pockets that sink.

[1] ASTM D 792: Density and Specific Gravity (Relative Density) of Plastics by Displacement; ASTM International, 2008

[2] ISO 1183-1: Plastics – Methods for determining the density of non-cellular plastics – Part 1: Immersion method, liquid pyknometer method and titration method; International Organization for Standards, 2004

[3] Osswald, T. A.; Baur, E.; Brinkmann, S.; Oberbach, K.; Schmachtenberg, E.: International Plastics Handbook; 4^th edition, Hanser Gardner Publications, Cincinnati, 2006

[4] Campo, E. A.: Industrial Polymers

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.

Method A is the most common testing approach. This method uses a dumbbell shaped specimen (see illustration below) and can be performed on a universal testing machine with standard clamps (tensile testing machine). Method B uses a ring specimen which requires special clamping equipment. Test data is recorded at 100% and 300% modulus values for tension set and set after break, peak and break load as well as elongation. Due to the extensibility, extensometers are also used to accurately measure the elongation or stretch of a sample.

[1] ASTM D 412: Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers – Tension; ASTM International, 2008

[2] Campo, E.A.: Industrial Polymers; Hanser Gardner Publications, Cincinnati, 2007

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 LSR/Thermoplastic (Two-Shot) 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.