PBT and PET Injection Molding: Glass Fibre, Drying and Warpage

Why PBT owns electrical connectors

Polybutylene terephthalate is a semi-crystalline polyester that does one job better than anything else in its price bracket: it holds thin walls, tight dimensions and a flame rating through a soldering process. Almost every connector body, bobbins, relay base, sensor housing and terminal block in a modern appliance is glass-filled PBT.

PET is the same chemistry with a different chain length and crystallisation behaviour. It is cheaper and tougher in some grades, but it crystallises slowly, which makes it harder to mould and mostly confines it to extrusion, blow moulding and bottles — though glass-filled PET is used for automotive and electrical parts where its higher melting point is worth the difficulty.

Grades

Table 1 — PBT and PET grade families. The flame-retardant and hydrolysis-resistant variants are the two that matter most in electrical work.

Drying: the same trap as PC, one step milder

PBT and PET are polyesters, and polyesters hydrolyse. Water at melt temperature cleaves the ester bond, reducing molecular weight. The limits are less severe than polycarbonate's but the mechanism is identical.

Table 2 — Drying limits. PET is twice as sensitive as PBT and needs a hotter dryer.

The practical point: PBT arrives in sealed foil bags with a moisture barrier, and it should stay there until it goes into the dryer. A bag left open overnight in a humid plant will pick up 0.1 – 0.2 %, which is four times the limit. If the plant has no desiccant dryer and relies on a hot-air oven, the resin will stabilise around 0.05 – 0.08 % on a humid day and the parts will streak.

Processing window

Table 3 — Processing windows. The PET mould temperature of 100 – 130 °C is what forces it to crystallise.

The PET mould temperature is not a preference. PET crystallises slowly, and below about 90 °C it cools into an amorphous state that is brittle, dimensionally unstable and will deform if it later sees heat — a phenomenon that shows up as parts warping in a reflow oven. A PET tool needs hot-oil mould temperature control, not water. That single requirement is why PBT dominates and PET is a specialist choice.

Warpage: the main PBT problem

Glass-filled PBT warps because the fibres align with the flow. Shrinkage in the flow direction is 0.2 – 0.4 %; across the flow it is 0.6 – 1.0 %. That difference is the warpage driver, and it gets worse as the flow length increases and the walls get thinner.

Table 4 — Warpage levers for glass-filled PBT, in order of effectiveness.

For a flat part, the most effective single change is usually the gate: a centre gate makes the flow radial, so the fibre orientation is symmetric about the centre and the part stays flat. An edge gate makes the flow unidirectional, the fibres align along the part, and it curls.

Electrical and flame-retardant grades

Table 5 — Electrical requirements. The wall-thickness caveat on V-0 is the one that catches designers.

That last caveat deserves detail: a V-0 rating is always quoted at a specific thickness. A grade rated V-0 at 1.5 mm is often only V-2 at 0.8 mm. If your design has a 0.6 mm wall in a flame-rated part, the certificate you were given does not apply to it. Always check the thickness column on the UL yellow card, not just the rating.

Design rules

Table 6 — PBT design rules. Glass-filled grades have low elongation, so snap fits need care.

Inserts and assembly

Most PBT parts are assembled with metal contacts, and the insert design determines whether the part survives.

Table 7 — Assembly methods. Self-tapping screws in 30 % GF PBT are a common field failure.

Defects and causes

Table 8 — PBT defects. The blister row is a moisture problem that appears a process step later.

Thin-wall filling: what PBT can and cannot do

PBT's reputation comes from its ability to fill thin sections at high speed, which is what connectors need. The limit is a combination of wall thickness, flow length and glass loading.

Table 9 — Flow length for PBT at 255 °C melt and 80 °C mould. Glass-filled grades are roughly 30 % shorter.

Below 0.6 mm the process becomes unforgiving: the melt front freezes before the cavity fills, and every variable — mould temperature uniformity, injection speed, gate size — has to be right. That is achievable in a connector tool built for it, and it is not achievable in a general-purpose tool being pushed past its design.

Hydrolysis: the slow failure

PBT and PET degrade in hot humid environments through hydrolysis of the ester bond. This is different from the moisture problem at the hopper — it happens in service, over months, and it is invisible until the part fails.

Table 10 — Hydrolysis resistance. The stabilised grade is roughly 3× the service life in hot humid conditions.

The applications that need it are obvious once you know: washing-machine components, dishwasher parts, automotive under-bonnet, anything in a bathroom, anything in a tropical climate. The applications that get caught out are the ones where nobody thought about humidity — a connector in an unheated warehouse in Southeast Asia, or a sensor housing on a pump.

PBT, PPS or PA for a connector

Table 11 — Connector material selection. PBT wins on cost plus stability; PPS wins on temperature; nylon wins on impact only.

The pin-position row is the deciding one for most connectors. A PA66 connector grows 0.3 – 0.6 % after moulding as it absorbs moisture, which on a 40 mm connector body is 0.12 – 0.24 mm of pin movement. That is more than the insertion tolerance of most terminals. PBT does not move, which is why it is the default.

Frequently asked questions