For unreinforced polybutylene terephthalate (PBT), moisture control is a kinetic constraint, not a pre-processing formality. Resin equilibrated at 23°C and 50% relative humidity absorbs approximately 0.08–0.10 wt% water, while melt-phase hydrolysis can measurably reduce molecular weight when residual moisture exceeds 0.02 wt% at melt temperatures above 240°C. Desiccant drying at 120°C for 4–6 h with supply air at a dew point of −30°C or lower is the standard precondition; residual moisture is verified by ISO 15512 or an equivalent Karl Fischer method. A vented co-rotating twin-screw extruder with L/D 40:1 can remove some volatiles, but a single-screw extruder without vacuum devolatilization cannot compensate for insufficient drying. Once hydrolysis initiates, carboxyl end group concentration rises autocatalytically, melt viscosity falls, and tensile elongation at break measured per ISO 527-2 deteriorates. The production failure mode is cumulative rather than immediate: screw recovery time drifts, nozzle drool increases, and parts show splay at gate locations. If opened containers remain exposed to ambient air above 60% RH, re-drying is required before molding; a sealed gaylord does not eliminate moisture uptake once the liner is opened. Buying neat PBT transfers drying discipline, hopper residence-time calculation, closed-loop dry-air conveying, and moisture verification to the molder.

What Is Actually Outsourced When the Compound Is Purchased?

When a molder buys compounded PBT pellets instead of neat resin, the visible transaction is a per-kilogram price difference. The actual transfer is a sequence of unit operations and quality controls. The compounder typically operates a co-rotating twin-screw extruder with L/D 40:1, side feeding for glass or mineral fillers, vacuum venting for volatiles, and melt filtration through 100–250 µm screen packs. The compounder selects the additive package: heat stabilizer, hydrolysis stabilizer, nucleating agent, mold release, colorants, and, where required, flame retardants. Dispersion quality is controlled by specific energy input, screw configuration, and residence time distribution. For a 30% glass-fiber-reinforced PBT, specific energy input on a 40:1 twin-screw line is typically 0.25–0.40 kWh/kg, depending on screw design and side-feeder position. Each lot is released against a certificate of analysis recording ash content per ISO 3451-1, melt volume-flow rate at 250°C/2.16 kg per ISO 1133-1:2022, tensile properties per ISO 527-2, and color coordinates per ISO 11664-4. The purchaser also outsources lot-to-lot variance control: filler content is commonly held within ±1 wt%, and color drift is controlled to ΔE <1.0 against a master standard. These are measurable outputs of a compounding line and its laboratory, not nominal resin attributes.

Precompounded 30% glass-fiber-reinforced PBT differs from neat resin dry-blended with chopped strand at the press in fiber wetting, length retention, and mechanical translation. In a precompounded grade, glass roving is fed downstream after the polymer melt seal, limiting fiber breakage and producing a mean fiber length after injection molding typically 200–400 µm, measured by microscopy per ISO 22314. When glass is mixed with neat resin in a single-screw reciprocating machine, fiber attrition during plasticating can reduce mean fiber length below 150 µm, and unwetted fiber bundles can appear as surface roughness or reduced notched Izod impact per ISO 180/A. The compounder also selects the silane coupling package on the glass; amino-silane sizes are common for PBT, and damaged or incorrect sizing cannot be repaired by the molder. Mechanical data reflect the difference: tensile stress at break for a 30% glass-filled compound is typically 130–140 MPa at 23°C per ISO 527-2, flexural modulus 9–10 GPa per ISO 178, and heat deflection temperature at 1.8 MPa approaches 205–215°C per ISO 75-1/2. Above 40 wt% glass loading, tensile strength gains plateau and notched Izod impact can decline sharply due to fiber packing and excessive melt viscosity. Barrel and screw wear also accelerates with glass; production lines therefore use bimetallic barrels and nitride-hardened screw elements to control dimensional wear over extended campaigns.

If the Molder Weighs Neat Resin and Masterbatch at the Throat

In-house blending of neat PBT with separate masterbatches places homogeneity, segregation control, and dosing accuracy on the molding floor. A gravimetric loss-in-weight feeder at the throat may hold feeding accuracy of ±0.25–0.5% at steady state, but that accuracy degrades during refill cycles, with low letdown ratios below 2 wt%, and with pellet-size mismatch between resin and masterbatch. Static charging on dried PBT can cause fines and masterbatch pellets to adhere to hopper walls, producing color streaks and composition drift. The injection unit then becomes the primary mixer; a general-purpose screw with L/D 20:1 provides limited distributive mixing compared with a barrier screw with a mixing section or a twin-screw compounding line. For flame-retardant packages, uneven dispersion can create localized regions that fail to meet UL 94 V-0 at the specified thickness. On a 1200 kN clamp force machine running a two-cavity connector tool, clamp force and injection pressure settings are not independent of viscosity: an uncompounded glass masterbatch can cause screw-recovery torque variation, requiring wider barrel-temperature bands and longer hold-pressure time to maintain gate seal. The cost of in-house weighing is therefore not limited to feeder hardware; it includes real-time homogeneity verification that is rarely feasible outside a compounding laboratory.

In flame-retardant PBT, the practical processing window is governed by additive chemistry rather than base-resin rheology alone. A halogenated PBT may use a brominated flame retardant with antimony trioxide synergist, while a non-halogen system may use an organophosphinate or melamine polyphosphate package. Shear heating can push local melt temperature beyond 270°C even when barrel set points are held at 240–260°C, and some non-halogen packages begin to degrade above 300°C. For certain phosphinate-based systems, the difference between complete melting and additive degradation is less than 10°C at the die, requiring closer barrel-temperature control than neat PBT. The compounder manages this by controlling screw temperature, specific energy, and residence time distribution; the molder who dry-blends a flame-retardant masterbatch inherits the same thermal sensitivity without the same screw-temperature instrumentation. Electrical and fire performance are thickness- and color-dependent: a V-0 rating at 1.6 mm or 0.8 mm per UL 94 does not automatically transfer to a differently pigmented lot. Comparative tracking index measured per IEC 60112 can fall below 250 V in some glass-filled flame-retardant systems, while selected unfilled grades may exceed 600 V. Glow-wire ignition temperature per IEC 60695-2-13 is commonly cited at 775°C for selected FR-PBT grades at 1.0 mm, but published data for this specific configuration is limited. The compound order also transfers regulatory documentation: compliance with RoHS Directive 2011/65/EU and REACH SVHC thresholds must be confirmed on the compounder’s certificate of compliance, not inferred from the base resin grade. Surface bloom of low-molecular-weight flame-retardant additives under damp-heat aging is a known failure mode; the compounder’s selection of higher-molecular-weight or polymeric synergists is part of what the purchased compound outsources.

Hydrolysis-resistant formulations and the cost of false equivalence

Two PBT compounds with the same nominal filler content and the same melt flow rate are not equivalent in humid service. Hydrolysis-resistant PBT for automotive connectors and sensor housings relies on chain-extension chemistry and carbodiimide or epoxy-based acid scavengers that cap carboxylic acid end groups and slow the autocatalytic degradation sequence. The test environment specified by IEC 60068-2-78 at 85°C/85% RH is the standard reference; after 1000 h, published datasheets for hydrolysis-stabilized grades typically report tensile strength retention above 75%, while unmodified PBT can fall below 50% under the same exposure. The molder buying neat PBT and adding only a color or glass masterbatch does not receive this stabilization unless it is explicitly included in the resin package. Amine-based additive packages should be avoided unless explicitly formulated for polyester, because nucleophilic amines can sever ester linkages and accelerate molecular weight loss at melt temperatures. PBT is generally not recommended for continuous exposure to water above 60°C unless a hydrolysis-resistant grade is selected, as documented in supplier chemical-resistance tables. The compounder outsources the compatibility of the stabilizer package with flame retardants, colorants, and coupling agents; some additive combinations can deactivate a carbodiimide stabilizer or produce plate-out on the mold. Batch-to-batch retention of the hydrolysis package is verified by tensile testing after damp-heat aging, not by melt flow alone. The purchaser of a compound acquires that aging data; the purchaser of neat resin assumes the burden of generating it.

When dimensional stability and color are specified on the same part drawing, the molder inherits nucleation and metering variables that a precolored compound would otherwise stabilize. A precolored compound is delivered with a spectrophotometric match typically held to ΔE <1.0 against the customer standard per ISO 11664-4; in-house color concentrate letdowns of 2–4 wt% shift the burden of metering, pellet-size compatibility, and regrind color shift onto the molder. Nucleation affects crystallization rate and post-mold shrinkage: a nucleated PBT grade processed at a mold temperature of 80–100°C produces a finer spherulitic structure and more uniform shrinkage than an unnucleated neat resin run at a lower mold temperature, with mold shrinkage measured per ISO 294-4. Regrind inclusion alters both average fiber length and color; in glass-filled grades, regrind levels above 20–30 wt% often reduce notched Izod impact per ISO 180/A and require either a compounder’s reformulation or acceptance of lower mechanical margins. The selection of a nucleating agent is not trivial: some talc-based nucleants can reduce notched Izod impact, while certain organic nucleants accelerate crystallization without shifting heat deflection temperature. Buying the compound outsources the experimental iterations required to balance nucleation, mold release, color, and impact at a specified mold temperature and regrind ratio.