# Materials | Custom Silicone Compound Selection > Choosing a silicone compound by structure, molding route and operating environment: hardness, rebound, temperature range, flame retardancy and clean grades. # Custom silicone material selection Silicone is not one single material: different applications need different compound systems. We select the compound from the product structure, the molding route and the operating environment, then adjust hardness, transparency, colour and feel to the project. ## The same word, very different performance Hardness, rebound, tear resistance, temperature range, flame retardancy and cleanliness all come from the compound formula, not from the molding machine. Ordering on the word "silicone" alone usually means the mismatch only appears at assembly or during reliability testing. We do not apply a generic formula. The product structure comes first, then the overmolding route (silicone onto metal, plastic or PCBA), and then the operating environment and reliability requirements decide the compound system, hardness and appearance parameters. The assessment looks at the load path, sealing needs, temperature cycling and safety requirements at the same time, so that formula choice is aligned with them from the start. That is where delamination, cracking, yellowing and missed performance targets are prevented — and it is the clearest difference from a molding-only subcontractor. Silicone plaques in different hardness and surface finish: hardness, transparency, colour and surface treatment are all project variables. Selectable properties ## Compound properties selected by application These properties can be selected individually or combined. Combinations usually involve trade-offs, so priorities are set during the assessment. Mechanics ### Mechanical - High tear resistance - High tensile strength - High rebound, fast or slow rebound - Cushioning and damping Environment ### Environmental resistance - High and low temperature (typically -60 °C to 250 °C; special grades up to 400 °C) - Ozone and ageing resistance - Acid and alkali resistance - Waterproof sealing Safety Grade ### Safety grades - Food-contact grade - Medical grade - Low odour - Low volatile release Function ### Functional safety - Flame retardant (such as UL94-V0) - Anti-static - Electrical insulation ### Hardness, appearance and feel Hardness, transparency, colour and feel are all adjustable, but they affect each other: raising hardness stiffens the feel, and fillers influence transparency. These are therefore confirmed together with structure and assembly. Colour and surface finish (matte or glossy) are normally checked against a physical sample at the sampling stage, so that volume production does not stall on different readings of the standard. Hardness Shore A 0 to 90, adjustable Transparency Transparent, translucent or solid colour Surface Matte or glossy Feel Anti-slip grip adjustable on request Property library ## When you need property-by-property comparison This page explains how to choose by application. The property library breaks the same ground down by compound system, mechanics, environmental resistance, function, safety and cleanliness, hardness and appearance, and substrate matching. System ### Compound systems Supply form, molding route, flash and consistency differences between solid silicone and liquid LSR, and the products each suits. Performance ### Mechanics and environment Tear, tensile, rebound and compression set; temperature range, ageing, ozone, chemicals and waterproof sealing. Function ### Function and safety grades Differences and test directions for flame retardancy, anti-static and insulation; food-contact, medical, low odour and low release requirements. Appearance ### Hardness and appearance Shore A 0 to 90, transparent through solid colour, matte or glossy finishes and feel, and what to confirm at sampling. Substrate ### Substrate matching What to watch for when overmolding onto metal, plastic, PCBA, batteries and electromagnetic parts. Verification ### Verifying a selection Four verification groups — mechanical and interface, weathering, electrical and cleanliness, dimensions and appearance — that confirm the choice. Selection matrix ## Working back from operating conditions to properties Once the operating conditions are listed clearly, the compound direction usually narrows quickly. These are the conditions we compare most often. | Operating condition | What to confirm | Property direction | | --- | --- | --- | | Load and assembly | Whether the part is stretched, compressed or repeatedly flexed, and how often it is assembled or removed | High tear resistance, high tensile strength, high or slow rebound, cushioning | | Sealing and water | Compression of the sealing face, contact media, and whether it stays in a damp environment | Waterproof sealing, matched hardness, stable rebound | | Temperature and outdoor use | Operating temperature range and long-term exposure to UV and ozone | High and low temperature resistance (typically -60 °C to 250 °C, special grades to 400 °C), ageing and ozone resistance | | Safety and certification | Human or food contact, and whether flame retardancy, anti-static or insulation is required | Food-contact and medical grades, flame retardant, anti-static, insulation, low odour | | Appearance and feel | Transparent or solid colour, matte or glossy, feel and appearance acceptance standard | Shore A 0 to 90, transparency, colour, anti-slip feel | Selection and defects ## Many defects are decided at the selection stage Some problems in overmolded parts come from the molding process; others are really a mismatch between compound and structure. Clearing that up during selection saves repeated rounds later. - Delamination Related to the bond interface and material compatibility. Substrate and silicone must be considered together, not the silicone alone. - Cracking Under repeated flexing or impact, insufficient tear strength and rebound show up first. The compound must be selected to the real load path. - Yellowing Products used outdoors for years need an ageing and ozone resistant direction, and colour stability is confirmed at sampling. - Missing performance targets State the reliability test requirements up front so that selection and sampling verification work to them, rather than discovering the gap before shipment. FAQ ## Questions about material selection What hardness range can silicone reach? Shore A 0 to 90. The value is confirmed against assembly feel, sealing compression, structural strength and the molding route, and can be checked with a physical sample at sampling. Are food-contact and medical grades available? Yes, selectable per project. State what the part contacts, the operating conditions and the certifications required; the compound direction is then set and verified at sampling. What flame-retardant level can be reached? Flame-retardant compounds can be selected, with common requirements such as UL94-V0. The exact level and the verification method are agreed during assessment and confirmed by sampling and test results. Can one part meet several requirements at once? Yes, but they must be balanced: high hardness against high rebound, or flame retardancy against high transparency, are trade-offs. List the priorities during assessment and confirm by sampling where necessary. How is a selection verified? Sampling covers trial assembly, appearance checks and functional verification, plus the tests agreed for the project. The confirmed compound batch, hardness and surface treatment then become the basis of volume production. What information is needed to select a compound? The part's use and operating environment, the load and assembly method, any certification or test requirements, and the appearance and feel targets. Drawings or samples help; without them we can still assess a direction from the description. ## Tell us the operating conditions and we will suggest a direction Describe the load path, the environment, the certifications and the appearance target. We start with a compound direction and hardness recommendation, then move to sampling. Request material advice