Non-metallic Fasteners: Selecting Materials for the Joint

Select a non-metallic fastener by the function of the complete joint, then qualify the specific material, thread geometry and assembly method. A requirement for electrical insulation, corrosion resistance or low weight does not establish the required mechanical performance. Plastic screws, glass-reinforced composite threaded rods and laminate-machined fasteners are not one interchangeable class. The purchasing specification must identify what carries the load and what evidence is required before the joint is released.

Define Why the Joint Needs a Non-Metallic Fastener

Write the primary function in one sentence: maintain a specified clamp load, locate a component, avoid an electrical path or reduce a thermal bridge. Then list the accompanying duties and the consequence of losing that function. This prevents a single attractive material property from becoming the entire selection decision.

Identify the working environment and every contacting substance, including process media, cleaning fluids and assembly products. State the temperature history and whether the fastener sees continuous loading, repeated assembly, vibration or load changes. A broad family-level statement about chemical resistance is not a compatibility assessment for every fastener material in that environment.

Separate Material Families Before Comparing Parts

RDS lists epoxy-glass fabric laminate, phenolic-cotton fabric laminate, wood laminate, engineering plastics and FRP pultruded rods among its non-metallic fastener material options. Ask which exact material and construction are proposed for the quoted component. FRP means fiber-reinforced plastic; the abbreviation does not identify its resin, reinforcement arrangement or grade.

Röchling’s composite fastener page likewise distinguishes material systems and applications. Use this as evidence that a grade-level decision is necessary. Do not copy a branded-grade temperature, strength comparison or electrical claim onto an RDS part. A common thread size also does not make two material systems equivalent.

Review the Load Path Through Threads and Bearing Faces

Follow the load from the head or nut through the bearing surface, engaged threads and connected parts. Identify whether tension, shear, bending or a combination is relevant. Ask the responsible designer to evaluate the joint and its failure modes; a tensile value for raw material does not establish the capacity of the finished threaded assembly.

Specify the mating nut or threaded insert, engagement length, bearing arrangement and any required washers. Review edge distances and support around the joint. For reinforced constructions, request information about the relationship between the manufacturing form and the finished thread. This is a question for the offered design, not a reason to assume that every machined or formed thread behaves the same way.

Establish an Assembly Method for the Actual Combination

Do not transfer steel-fastener torque settings to a non-metallic assembly without validation. Specify the intended tightening method and obtain the relevant recommendation for the offered fastener, mating component and surface condition. If lubrication or a locking product is proposed, include it in the material-compatibility and assembly review.

Decide whether the joint will be assembled once or routinely opened for maintenance. Define how thread damage, seating and any permitted reuse will be assessed. Where retained clamp function over time matters, request suitable evidence for that duty. An initial installation check alone cannot establish long-term performance through a different temperature or load history.

Evaluate Electrical Isolation at the Assembly Level

An insulating fastener is only one element of an electrical design. Review conductive paths through washers, sleeves, inserts, mating surfaces and contamination. If the purpose is isolation, specify the required assembly test and environmental conditions through the electrical design authority. Do not calculate an operating voltage from a generic material dielectric-strength value alone.

Keep electrical and mechanical acceptance connected. A geometry change that improves one function can affect the other. Record required clearances, inspection points and any restrictions on substitutions in the approved drawing package. This article concerns selection of the fastener itself; a flange-isolation kit using metal studs and insulating sleeves is a different engineered assembly.

Make the RFQ Comparable and the Sample Traceable

Include the part and assembly drawings, material designation, thread standard and size, tolerances, length, head or nut geometry, quantity and documentation requirements. Identify which features are mandatory and where the supplier may propose an alternative. Require each proposal to name its material and construction rather than quote only an equivalent plastic bolt.

For sample approval, separate dimensional inspection from functional joint evaluation. Retain the fastening procedure, sample identification and test conditions with the decision. If a thread or material changes later, reopen the affected review. This record gives procurement a controlled basis for repeat orders and gives engineering a clear reason to accept or reject a substitution.

Control a Proposed Replacement as a Joint Change

Consider an illustrative substitution request: a plastic bolt is offered in the same nominal size as an existing composite fastener. Begin by checking the approved material and joint function, then compare the proposed thread, mating nut and assembly method. A matching size may solve fit while leaving load retention or electrical behavior unanswered.

Place those unanswered items on the proposal review, with a responsible owner and the evidence needed to close them. Approve samples only for the stated evaluation purpose until the required joint evidence is available. If the final assembly uses a different nut, washer or lubricant from the tested combination, determine which checks must be repeated before treating it as the approved replacement.

Fastener Selection Questions

• Can a material datasheet replace a finished-fastener test? No. A datasheet can support screening; thread geometry and the complete joint still need appropriate evidence.
• Does non-metallic mean electrically insulating? Not for every possible material or assembled construction. Specify and verify the required electrical function.
• Can matching diameter establish interchangeability? No. Material, thread form, engagement, bearing surfaces and assembly method also matter. Approve the complete proposed combination.