Material selection for multi-material overmolding and bonding applications requires a systematic view of which elastomer-substrate combinations produce reliable adhesion, which require process intervention, and which should be avoided. The compatibility ratings below reflect the practical adhesion outcomes achievable in production injection molding and adhesive bonding applications — not laboratory-optimized conditions. Ratings assume clean, properly dried substrates and appropriate processing parameters. Compatibility Rating Definitions A — Compatible without treatment: Cohesive failure achievable in standard overmolding conditions without adhesion promoters or surface preparation. Suitable for structural bonds. B — Compatible with process control: Adequate adhesion achievable when processing parameters (mold temperature, substrate drying, transfer time) are tightly controlled. Cohesive failure possible but process-sensitive. C — Requires adhesion promotion: Standard process produces adhesive failure. Silane primer, tie-layer materials, or plasma treatment required for structural adhesion. D — Not compatible without major intervention: Poor natural affinity. Specialized etching, corona treatment, or complete reformulation required. Not recommended for new designs. ABS (Acrylonitrile-Butadiene-Styrene) — Surface Energy: 38–42 mN/m Elastomer Rating Notes TPU (ether-based) A Strong natural affinity; cohesive failure without primers; widest process window TPU (ester-based) A Higher initial bond strength; avoid in humid service environments SEBS A Styrenic end-block compatibility; mold temp >60°C required SBS B Bonds well; UV/thermal degradation limits to protected, short-life applications COPE C Limited affinity for ABS; adhesion promoter required PEBA C Amide chemistry not matched to ABS; tie-layer required TPV C Inconsistent without coupling agent or surface treatment ### Polycarbonate (PC) — Surface Energy: 42–46 mN/m Elastomer Rating Notes --- --- --- TPU (ether-based) A Strong affinity; CSC risk requires PC-screened formulation; hydrolysis resistant TPU (ester-based) B Higher initial bond strength; CSC risk higher; avoid in humid service COPE A Ester-to-ester compatibility; requires mold temp >75°C; high-temperature capable SEBS C Inconsistent without adhesion promoter or tie-layer; UV stable TPV D Poor adhesion without plasma treatment or COPE tie-layer PEBA D Not matched to PC surface chemistry ### PA6 and PA66 (Nylon 6/6.6) — Surface Energy: 40–44 mN/m (dry-as-molded) Elastomer Rating Notes --- --- --- TPU (ether-based) A Strong urethane-amide interaction; moisture management critical TPU (ester-based) B Higher initial bond; degrades in humid service; use only for dry environments PEBA A Amide-to-amide compatibility; mold temp >80°C required SEBS C Requires silane primer for structural bonds TPV C Requires surface treatment or PEBA tie-layer COPE D Ester chemistry not matched to amide surface ### PA12 (Nylon 12) — Surface Energy: 35–38 mN/m (lower amide density) Elastomer Rating Notes --- --- --- TPU (ether-based) B/C Reduced amide density limits urethane-amide interaction; silane primer + mechanical interlocks needed PEBA B/C Better than SEBS; still weaker than on PA6; interlocks required SEBS D Poor natural affinity; not recommended without major adhesion promotion ### PET (Polyethylene Terephthalate) — Surface Energy: 40–44 mN/m Elastomer Rating Notes --- --- --- TPU A Urethane-to-ester interaction; similar to PC mechanism COPE A Ester-to-ester compatibility; strong natural affinity SEBS C Requires adhesion promotion PEBA C Amide chemistry limited affinity for PET ### Rigid PVC — Surface Energy: 38–42 mN/m Elastomer Rating…

Material selection for elastomeric overmolds and bonded assemblies fails most often not from lack of options but from applying the wrong selection criteria. Choosing by Shore hardness alone, or by cost per kilogram, or by whatever the previous similar product used — without evaluating substrate chemistry — produces designs that work in the sample room and delaminate in production. The compatibility-first selection process is systematic, and once the framework is understood, it applies to every elastomer-substrate combination encountered. Why Compatibility Must Come First The functional properties of an elastomeric component — how soft it feels, how long it lasts, how it responds to UV or temperature — only matter if the component stays bonded to the substrate it’s part of. An overmold that peels off in the first year of product life has failed regardless of its hardness or color accuracy. Compatibility — whether the elastomer can form a durable bond with the substrate — is the threshold requirement. Everything else is decided within the set of compatible options. Step 1: Identify the Substrate Chemistry Start with the structural substrate material. What plastic or material forms the rigid part that the elastomer will bond to? Common substrates and their surface chemistry class: - ABS: Polar; contains nitrile and styrenic groups - PC: Polar; contains carbonate ester groups - PA6, PA66, PA12: Polar; contains amide groups — also hygroscopic - PET, PBT: Polar; contains ester groups — also hygroscopic - Rigid PVC: Polar; contains C-Cl groups - PP: Non-polar; no functional groups for polar bonding - HDPE, LDPE: Non-polar; no functional groups for polar bonding - EPDM rubber: Hydrocarbon, moderately polar after surface prep - Silicone: Very low surface energy; requires specialized surface modification This classification immediately tells you whether a polar elastomer (TPU, SEBS, COPE, PEBA) will bond directly, or whether surface treatment and/or a different approach is needed. Step 2: Match Elastomer Chemistry to Substrate Chemistry For polar substrates, match the elastomer’s bonding mechanism to the substrate’s functional groups: ABS → TPU or SEBS. TPU bonds through urethane-nitrile interaction; SEBS bonds through styrenic affinity. Both are direct chemical matches. Either can be specified; the choice between them is a functional decision. PC → TPU or COPE. TPU bonds through urethane-carbonate interaction; COPE bonds through ester-to-carbonate interaction. Both are viable. COPE provides higher service temperature; TPU provides broader grade availability. Confirm CSC-safe grades for PC. PA → TPU or PEBA. TPU bonds through urethane-amide interaction; PEBA bonds through amide-to-amide chemistry. PEBA’s match is more direct; TPU is widely available across hardness grades. Both work with proper moisture management. PET/PBT → TPU or COPE. Both bond through ester chemistry. Aggressive pre-drying required for both substrates. PP → TPO. No polar elastomer (TPU, SEBS, COPE, PEBA) bonds reliably to PP without surface treatment. TPO provides polyolefin-to-PP cohesive failure bonds. This is the decision that most often goes wrong when PP compatibility is not analyzed: teams specify SEBS or TPU and discover poor adhesion late in development. HDPE/LDPE → Polyolefin-matrix TPE or adhesive…

Reliable multi-material bonding does not happen by accident. It follows from systematic material selection, controlled processing, and verified bond quality — disciplines that sound straightforward but are consistently undermined by time pressure, material substitutions, and process assumptions that carry over from single-material manufacturing. The practices below represent the engineering baseline for multi-material bonding across TPU and TPE systems, whether the joint is formed by overmolding or by a supplemental structural adhesive of the kind compared in UV glue vs epoxy for heavy-duty repairs. Practice 1: Select by Substrate Chemistry, Not by Material Category The most common multi-material bonding failure has nothing to do with processing — it is specifying the wrong elastomer for the substrate. Selecting “TPE” for a polypropylene housing without specifying “TPO” is selecting a category, not a compatible material. Standard TPE sub-classes (SEBS, COPE, PEBA) do not bond reliably to PP. Apply the substrate filter before any other selection criterion: - PA substrate → PEBA or TPU - ABS substrate → SEBS or TPU - PC substrate → COPE or TPU - PET/PBT substrate → COPE or TPU - PP substrate → TPO - HDPE → Specialty polyolefin TPE or adhesive bonding with CPO primer This filter eliminates incompatible candidates before Shore hardness, cost, or supplier discussions begin. Practice 2: Pre-Dry Hygroscopic Substrates Without Exception Moisture at the bond interface is the most consistent source of bond quality variation in multi-material overmolding. PA, PC, PET, and PBT absorb moisture from ambient air. At overmolding temperatures, this moisture converts to steam and creates voids in the bond layer. Non-negotiable pre-drying specifications: - PA6/PA66: 80°C, 4–6 hours minimum in dehumidifying dryer - PC: 120°C, 4–6 hours - PET: 160–180°C, 4+ hours - PBT: 120°C, 4+ hours Pre-drying must be followed immediately by overmolding or hermetically packaged storage. A PA substrate left on an open shelf for two hours after pre-drying in a humid environment may absorb enough moisture to degrade bond quality. Pre-dry the elastomer as well. TPU is hygroscopic and must be dried at 80–90°C for 3–4 hours before processing to maintain melt quality and bond strength. Practice 3: Specify Mold Temperature as a Critical Parameter Mold temperature is treated as an approximation in many injection molding operations — the setpoint is written in the setup sheet and rarely verified during production. For multi-material bonding on PA and PC substrates, mold temperature is a critical parameter that determines whether bonds are structural or marginal. TPU-PA bonds formed below 70°C mold temperature are substantially weaker than bonds formed above 80°C. PEBA-PA bonds follow the same relationship. TPU-PC bonds are less mold-temperature sensitive but still improve significantly above 60°C. Required actions: - Specify mold temperature with upper and lower limits (not just a target) - Verify mold temperature during first article with calibrated thermocouple at the bond zone, not just the mold surface setpoint - Include mold temperature in the process audit for bonded assemblies Practice 4: Design Mechanical Interlocks Into the Substrate From the Start Mechanical interlocks — through-holes,…

Durability in multi-material products depends on two independent factors: the mechanical properties of the elastomeric component itself, and the integrity of the bond between that component and its substrate through the product's service life. Specifying the most durable elastomer on the wrong substrate — or a compatible elastomer with inadequate mechanical properties for the application — produces the same outcome: premature field failure. The best material pairing is the combination that satisfies both the adhesion requirement and the mechanical performance requirement simultaneously. Durability Framework: Two Independent Axes Axis 1: Bond durability. The overmold adhesion must survive the loading, thermal cycling, and chemical exposure the product experiences. Bond durability depends on: initial bond quality (cohesive vs adhesive failure mode), resistance to environmental factors that degrade adhesion (moisture, UV, thermal cycling), and whether mechanical interlocks supplement chemical bonding. Axis 2: Intrinsic material durability. The elastomeric compound itself must maintain its mechanical properties through service life. Key properties: tensile strength, elongation at break, tear resistance, compression set, abrasion resistance, UV resistance, and chemical resistance to the product's operating environment. A material pairing that produces cohesive failure bonds on a matched substrate but has inadequate abrasion resistance fails on Axis 2. A material with excellent intrinsic properties but poor adhesion to the substrate fails on Axis 1. Both axes must be satisfied. TPU's Durability Profile TPU is the highest-durability elastomer in the standard overmolding palette by most mechanical measures: Abrasion resistance. TPU's abrasion resistance at comparable Shore hardness exceeds SEBS, COPE, and PEBA. Products with surfaces that experience continuous friction — tool handles, footwear soles, sports equipment contact points — benefit from TPU's resistance to wear. Tensile and tear strength. TPU provides higher tensile strength and tear resistance than SEBS or comparable-Shore COPE. Thin-wall overmolds that must sustain peel or tear forces without failure at the overmold edge benefit from TPU's structural properties. Flex fatigue. TPU maintains its properties through millions of flex cycles without crack initiation at ambient temperatures. Flexible zones on devices that experience continuous bending — wearable devices, handle flex sections, cable jacketing — benefit from TPU's fatigue resistance. Chemical resistance. Ether-based TPU provides the best hydrolysis resistance of the standard overmolding elastomers. For products exposed to water, moisture, and sweat, ether-based TPU is more durable than ester-based TPU, SEBS, or standard COPE in terms of maintaining mechanical properties over time. TPU's durability limitations: UV resistance requires additive packages for outdoor applications; ester-based TPU degrades in sustained moisture; high-temperature performance (above 100°C) is marginal without specialty grades. TPE Durability by Sub-Class SEBS: Lower intrinsic durability than TPU in mechanical measures (abrasion, tensile, tear). Acceptable for soft-touch and light-grip applications. UV-stable through saturated midblock chemistry — inherently more UV-resistant than unsaturated alternatives. Lower cost per kilogram than TPU at comparable Shore hardness. Appropriate for consumer product grip and soft-touch applications where mechanical durability requirements are moderate. COPE: Higher heat resistance than SEBS or standard TPU — usable to 120–140°C in some grades. For products operating in elevated temperature environments, COPE's temperature durability…

Nylon substrates demand a more deliberate elastomer selection process than ABS. The substrate hygroscopicity, significant adhesion differences between PA grades, and the narrow window of TPE sub-classes that actually bond to polyamide without intervention all make the choice more consequential than on more forgiving substrates. The right selection on nylon depends on the specific PA grade, the service environment, the production process’s ability to control moisture and temperature, and the mechanical demands placed on the bond. The Foundation: Adhesion Mechanism on Nylon TPU bonds to nylon through urethane-to-amide interaction. The urethane groups in TPU form hydrogen bonds with the amide groups in PA’s backbone, creating a polar chemical interface that on PA6 and PA66 is strong enough to produce cohesive failure under optimized overmolding conditions. This mechanism does not require sub-class matching — all TPU types bond to PA through the same urethane-amide chemistry. Within the TPE family, only PEBA (polyether block amide) bonds to PA through an equivalent mechanism — amide-to-amide compatibility. SEBS has affinity for ABS’s styrenic surface, not PA’s amide surface. TPV requires surface preparation. COPE matches ester substrates, not amide ones. The practical comparison for nylon applications is TPU versus PEBA. Where TPU Leads PA12 and difficult grades. TPU’s urethane-amide mechanism, while weaker on PA12 than on PA6, is better documented and more widely evaluated on difficult PA grades than PEBA-on-PA12. A broader range of PA12-screened TPU formulations with silane primer compatibility data is available from major TPU suppliers. Broad grade availability and supply chain. TPU for nylon applications is available from more suppliers, in more hardness and chemistry options, with shorter lead times and lower minimum order quantities than PEBA. For programs where supply chain flexibility matters, this is a practical advantage. Ether-based moisture resistance. Ether-based TPU’s hydrolysis resistance under sustained moisture exposure is a well-characterized and widely documented property. The ether-TPU product range is broad enough to cover virtually any Shore hardness and performance requirement while maintaining moisture resistance. Mechanical durability. TPU provides higher tensile strength and abrasion resistance than most PEBA formulations at equivalent Shore hardness — relevant for industrial PA applications where the overmold zone is subject to mechanical wear. Where PEBA Leads PA6 and PA66 adhesion chemistry. PEBA’s amide-to-amide mechanism is the most direct chemical matching available between a TPE and a PA substrate. On PA6 and PA66, PEBA can produce cohesive failure at mold temperatures slightly below what TPU requires, and the bond consistency under varying process conditions may be marginally better. Service temperature range. PEBA grades with service temperature ratings above 100°C are available, extending performance at elevated operating temperatures beyond what equivalent-hardness TPU typically delivers. For automotive and industrial nylon applications at high service temperatures, PEBA's high-temperature capability is meaningful, and the wider swing between ambient and peak service temperature also raises CTE-mismatch stress at the bond line that should be validated regardless of which elastomer is chosen. Flex fatigue performance. PEBA’s elastic recovery and fatigue resistance are strong, making it appropriate for applications involving repeated flex cycles…

A PC housing that looks perfectly molded and passes initial inspection can still develop hairline cracks weeks later, radiating outward from exactly where an elastomeric overmold or gasket contacts it. This is chemical stress cracking, and unlike a simple adhesion failure, it destroys the substrate itself — which means the diagnosis has to happen before production, not after a field return. Why PC Fails Differently Than Other Substrates Most substrate-compatibility problems show up as delamination: the elastomer lets go, the parts come apart, and the fix is a different chemistry or better surface prep. Polycarbonate under sustained mechanical stress and exposed to an incompatible compound doesn't just delaminate — its own molecular structure develops micro-fractures that can propagate over weeks or months, sometimes well after the part has shipped. This makes CSC risk assessment a different exercise than ordinary adhesion screening: a compound that bonds PC beautifully in a peel test can still be actively cracking it from the inside. Recognizing the Warning Signs Before Failure CSC on PC rarely announces itself immediately. The earliest indicator is often a faint whitening or crazing pattern at the edge of the elastomer-to-PC interface, visible under raking light or low magnification before any crack is large enough to see with the naked eye. Parts under sustained clamping load, snap-fit stress, or residual molding stress from the PC side are the most susceptible, since CSC requires both an aggressive chemical agent and a stressed substrate to develop — a compound that's perfectly benign on unstressed PC can still crack a part with residual molding stress at the same interface. Root Cause 1: Incompatible Additives in the Compound, Not the Base Polymer CSC risk on PC is not an inherent property of TPU or TPE as a material category — it's a function of the specific additive package in a given compound. Processing oils, certain plasticizers, and some flame-retardant packages are common CSC triggers regardless of whether the base elastomer is TPU, COPE, or SEBS. This is why two TPU compounds at the same hardness and from the same general chemistry family can produce completely different CSC outcomes on the same PC grade — the risk lives in the compound formulation, not the headline material name on a datasheet. Root Cause 2: Insufficient PC-Specific Grade Documentation TPU has the deepest bench of PC-specific, pre-screened grades among the common elastomer options, with urethane-to-carbonate ester chemistry that bonds consistently and supplier documentation that typically includes PC compatibility data directly. COPE can match TPU's adhesion performance through ester-to-ester chemistry and often provides better elevated-temperature capability, but fewer COPE grades carry the same depth of PC-specific CSC screening data, which pushes more of the qualification burden onto the molder. SEBS lacks a natural chemical affinity for PC's ester-dominated surface at all without adhesion promotion, and skipping that promotion step to save cost is a frequent, avoidable source of both adhesion failure and CSC risk together. Root Cause 3: Validation Under Sample Conditions Instead of Production Conditions A compound that…

A batch of overmolded parts fails peel testing on a Tuesday, passes on Thursday using the same material and the same mold — and the engineer who chases the wrong variable first can burn a week reformulating before discovering the actual cause was a process drift that had nothing to do with material chemistry at all. When Adhesion Testing Fails: Start With the Failure Mode, Not the Material Before investigating any process variable, determine the failure mode from the peel or lap-shear test itself. Cohesive failure — where the elastomer tears rather than separating cleanly from the substrate — means the bond exceeded the material's own strength and the joint is fundamentally sound even if the peel number looks low. Adhesive failure — a clean separation at the interface with the substrate surface left intact — means the bond itself is the weak point, and that's the signal to begin a process investigation rather than accepting the part as marginal. Skipping this distinction and reacting to a low peel number alone is the single most common reason troubleshooting starts in the wrong place. Root Cause 1: Substrate Moisture and Pre-Drying Gaps Polar substrates like nylon (PA6, PA66) are hygroscopic, absorbing ambient moisture that interferes with the chemical bonding mechanism during overmolding. A substrate that sat pre-dried on a shelf for an extended period, or was pre-dried to spec but exposed to humid ambient air during a delayed changeover, can produce a measurable adhesion drop compared to a freshly dried lot — often enough to shift a normally cohesive-failure combination into adhesive failure. Checking substrate moisture content directly, rather than assuming the original drying cycle still holds by the time of molding, is the fastest way to rule this cause in or out. Root Cause 2: Mold Temperature Below the Bonding Threshold Physical entanglement between the molten elastomer and the substrate surface requires both materials to be sufficiently mobile at the interface during the brief bonding window, which depends directly on mold temperature. A mold running even modestly below its qualified temperature — from a control system drift, a cooling channel partially blocked with scale, or a changeover that didn't allow full warm-up — can shift a process from reliable cohesive failure to inconsistent adhesive failure without any visible defect in the molded part itself. Logging actual mold-surface temperature at the cavity, not just the controller setpoint, catches this drift before it becomes a batch-wide quality escape. Root Cause 3: Contamination From Mold Release or Colorant Migration Internal mold-release agents compounded into the elastomer resin, and certain colorant or pigment carrier systems, can migrate to the bond interface during molding and measurably reduce adhesion — sometimes by half or more compared to the same base resin in its natural, uncolored form. This is a particularly easy cause to miss because the resin's own certificate of analysis for mechanical properties looks unaffected; the migration issue is specific to the bond interface, not the bulk material. Requesting adhesion validation data on the actual…

Overmolding TPU onto a rigid plastic substrate is not a single process — it is a family of processes whose viability depends on the substrate, the overmolding approach selected, and the production conditions maintained during manufacturing. The compatibility pattern follows predictable chemistry, but the process details that translate chemistry into production adhesion require discipline that goes beyond simply pairing compatible materials. The Compatibility Hierarchy TPU’s polar urethane mechanism creates a substrate compatibility hierarchy: Top tier — cohesive failure without primers: ABS, PC, PA6, PA66, PA12, PET, PBT, rigid PVC. On these polar engineering plastics, bond failure occurs within the TPU layer rather than at the substrate interface — the highest bond quality achievable, since adhesion exceeds the elastomer's own internal cohesive strength. Middle tier — adhesive failure with treatment: PP (flame or plasma activated), polycarbonate blends requiring CSC management. Adhesion is measurable and useful but does not reach cohesive failure; mechanical interlocks supplement chemical bonding. Lower tier — requires primer or alternate approach: HDPE, LDPE, PTFE, polyolefin-only substrates. Standard overmolding does not produce reliable adhesion; surface treatment alone is insufficient for structural bonds. Process Routes for TPU Overmolding Two-shot injection molding. The substrate (first shot) is injection molded, then transferred (by robot, rotary table, or indexed core) to a second cavity where TPU (second shot) is injected over it without fully cooling or demolding. Bond quality is high because the substrate is warm when TPU contacts it — retained heat enhances interdiffusion and reduces thermal shock stress. Two-shot molding requires both materials be processable in the same press — compatible injection pressure ranges, mold release requirements, and cycle time windows. Most polar engineering plastics and TPU are compatible in two-shot tools. Insert molding. The substrate is separately molded, cooled, demolded, and placed in the overmold cavity as an insert, then TPU is injected over the cold insert. Insert molding allows substrates to be sourced externally and provides more flexibility in geometry (including metal inserts and non-plastic substrates), but bond quality is generally lower than two-shot molding because the cold substrate cools the TPU melt faster, reducing interdiffusion at the interface. For insert molding of polar engineering plastics, substrate pre-heating (80–120°C depending on substrate and grade) before placing the insert improves bond quality by slowing the cooling rate at the interface. ABS: The Standard Case ABS is the most characterized substrate for TPU overmolding. Typical two-shot conditions produce consistent cohesive failure bonds: – Substrate mold temp: 40–60°C – TPU mold temp: 40–60°C (higher for better bond strength) – Substrate pre-drying: Recommended at 80°C for 4 hours for ABS blends – Mechanical interlocks: Recommended but not required for bond retention ABS/PC blends behave similarly to ABS. The PC fraction adds carbonate group compatibility with TPU’s urethane mechanism, maintaining or improving adhesion relative to ABS alone. PC: High Adhesion With CSC Management PC bonds strongly to TPU through urethane-carbonate interaction. Bond strength on PC often exceeds bond strength on ABS for the same TPU grade. The complication is chemical stress cracking: Certain TPU…

Polycarbonate appears in products where failure carries real consequences — automotive lamp lenses, aircraft interior components, and protective equipment all rely on PC’s combination of optical clarity, impact resistance, and thermal stability. When engineers add flexible seals, grip zones, or vibration-damping layers to these assemblies, thermoplastic polyurethane is frequently the first material evaluated. Understanding how TPU behaves on polycarbonate — and where the interaction can go wrong — determines whether that evaluation leads to a production-ready bond. Why TPU Bonds to Polycarbonate Polycarbonate is a polar engineering thermoplastic with surface energy typically in the 42–46 mN/m range — higher than ABS and substantially higher than polyolefins. The carbonate linkages in PC introduce ester groups to the surface, and these groups interact with TPU’s urethane chemistry through dipole-dipole interactions and hydrogen bonding at the interface. The result is a chemical affinity that supports strong adhesion without adhesion promoters in most injection overmolding applications. In well-executed two-shot overmolding on PC, TPU achieves cohesive failure — the elastomer tears before the bond line separates. This is the target result for structural overmolding and is reproducible on standard PC grades under controlled processing conditions. Chemical Stress Cracking: The Critical Risk The defining complication in TPU-on-PC bonding is chemical stress cracking (CSC). Polycarbonate is susceptible to crazing and cracking when exposed to certain chemicals while under mechanical stress — a phenomenon that does not require high chemical concentrations to trigger. Some TPU formulations contain residual solvents, plasticizers, or processing aids that can induce CSC at the PC interface, particularly in parts that carry mechanical load. To avoid this failure mode: – Specify TPU grades formulated with low solvent content and no aggressive plasticizers – Avoid TPU compounds with aromatic content that could interact with PC’s carbonate groups – Validate bond strength under sustained load — static stress combined with chemical exposure is the most damaging combination for PC – Test for crazing at the bond line after thermal cycling, not just immediately after processing Chemical stress cracking is most common in adhesive bonding applications where solvent-based adhesives or primers contact the PC substrate, but it can also occur in overmolding if the TPU compound’s additive package is not compatible. Selecting the Right TPU Grade for PC Grade selection for TPU on polycarbonate involves the same parameters as ABS applications, with additional attention to chemical compatibility: Base chemistry. Ether-based TPU is preferred for PC applications in humid or chemical environments — hydrolysis resistance protects bond integrity over time. Ester-based grades offer higher initial bond strength but degrade in moisture-exposed applications and may present a higher CSC risk on PC. Shore hardness. Softer grades (Shore 60A–85A) conform more readily to the PC surface during injection, increasing molecular contact area. Harder grades require tighter process control to achieve equivalent interfacial bonding. Additive compatibility. Request full formulation disclosure from the TPU supplier for any compound being evaluated on PC. Internal mold release agents, UV stabilizers, and flame retardant packages must be screened for CSC risk on polycarbonate. Flame retardancy.…

ABS, PC, and nylon dominate most discussions of TPU overmolding substrates — but engineering practice regularly encounters PET, polypropylene, and PVC in product assemblies where flexible zones are needed. Each of these three substrates presents a distinct compatibility profile with TPU, ranging from strong natural affinity (PET) to challenging non-polar surfaces (PP) to a nuanced situation driven by formulation variation (PVC). Understanding where TPU bonds well, where it requires intervention, and where alternatives may be more practical saves development time and prevents production failures on substrates outside the most common set. TPU on PET (Polyethylene Terephthalate) PET is a moderately polar polyester with surface energy in the 40–44 mN/m range, comparable to PA6 and ABS. Its ester linkages in the polymer backbone interact with TPU’s urethane groups through urethane-to-ester interaction — the same mechanism that drives TPU adhesion on PC. The result is a genuinely compatible pairing that produces cohesive failure in overmolding without adhesion promoters under controlled process conditions. PET applications where TPU overmolding appears include: – Packaging equipment components with flexible sealing zones – Electronic device substrates where PET’s optical and dielectric properties are required – Injection-molded PET structural components with integrated flexible grips Processing considerations for TPU on PET. PET must be dried aggressively before processing — at 150°C for four to six hours in a desiccant dryer — because it is more hygroscopic than ABS and less forgiving of residual moisture than PC. Undried PET at melt temperature undergoes hydrolytic chain scission, dramatically reducing molecular weight and producing severe viscosity reduction and part surface defects. TPU adhesion on improperly dried PET is unreliable because surface degradation changes the substrate’s bonding chemistry. PET processing temperatures (260–290°C) are higher than TPU’s preferred processing window (190–240°C), requiring careful management of substrate temperature at the moment of TPU contact in two-shot applications. TPU on Polypropylene (PP) Polypropylene is a non-polar, semi-crystalline thermoplastic with surface energy in the 29–31 mN/m range — below the threshold where TPU’s polar urethane mechanism can develop adequate adhesion. The mismatch between TPU’s polar chemistry and PP’s non-polar surface means that standard overmolding without surface preparation produces adhesive failure at low peel loads, regardless of process parameters. This does not mean TPU on PP is impossible — it means that surface activation is required: Plasma treatment. Atmospheric or vacuum plasma treatment oxidizes the PP surface, introducing polar functional groups (carbonyl, hydroxyl, peroxide) that increase surface energy to 60+ mN/m transiently. TPU overmolding must occur within 4–24 hours of plasma treatment before surface energy relaxes back toward baseline. Plasma treatment is effective but requires capital investment in plasma equipment and careful process timing. Flame treatment. Open-flame treatment oxidizes the PP surface through combustion products, producing similar surface energy improvement to plasma. More capital-accessible than plasma equipment; harder to control uniformly on complex three-dimensional geometries. Corona treatment. Used for film and sheet applications; less applicable to three-dimensional injection-molded PP substrates. Even with surface activation, TPU on PP typically produces adhesive failure rather than cohesive failure — structural overmolding…