Irrigation Valve Bodies Crazing Where Fertilisers Contact

A moulded valve body develops fine crazes and weeps after weeks in contact with a concentrated fertiliser solution, away from any loaded joint.

What the failure looks like on the line

Agricultural mouldings live outdoors, see fertiliser and wash-down chemicals, and are judged a season later rather than on the day they are moulded.

Fine surface crazes appear on the valve body near the threaded ports after service, followed by a slow seep under low pressure. The crazes follow no weld line and the part was not mechanically overloaded.

Environmental stress cracking occurs when an active chemical penetrates the amorphous regions and accelerates craze growth under a residual tensile stress left by moulding. A surface-active fertiliser salt or surfactant lowers the energy needed to open a craze, so the part fails at stresses well below its short-term strength. Residual stress from over-packing or a cold tool supplies the driving tension without any external load.

Buyers and suppliers usually agree on the drawing and disagree on the conditions under which the drawing was checked. On a buyer's specification this distinction matters: a part can measure inside tolerance and still fail in use, because the drawing records geometry while the complaint records behaviour. Holding pressure is usually the first value both sides compare, and it is also the one most often quoted without the conditions attached to it.

The process window that decides it

Four variables move this result more than the rest. The ranges below are illustrative engineering guidance for a first discussion; confirm them on your own tool, material grade and part geometry before they become contract terms.

Table 1. Starting window for a joint process trial. Values are engineering guidance, not a measured survey.

Holding pressure is typically held at 50-80 MPa. Over-packing freezes high residual tensile stress near the ports Record it together with the measurement point and the moment in the cycle; a single number without a location is not a process setting.

Mould temperature is typically held at 60-90 C. A cold tool locks in orientation and residual stress When this drifts, the symptom usually appears at one cavity, one sheet position or one lane before it appears across the whole batch.

Resin moisture content is typically held at < 0.2 % by mass. Hydrolysis shortens chains and lowers craze resistance A change here is cheap during a trial and expensive after tooling release, so treat it as a controlled variable with a documented re-approval.

Annealing condition is typically held at 2-4 h at 80-100 C. Skipping stress relief leaves the craze driving stress in place Put it on the change-control list, otherwise an unannounced material or setting substitution will be mistaken for ordinary process noise.

Reading the response curve

The left panel shows craze-free valve bodies (%) as service exposure (weeks) increases: the response falls from 98 to 16 on the illustrative 0–100 scale. The dashed line at 60 marks a proposed acceptance limit, not a universal standard. Move that line only after the test method and the sample plan have been agreed, otherwise two suppliers can both be compliant and still deliver different parts.

Read the shape rather than a single point. A curve that changes quickly near the limit is more sensitive to ordinary process noise than one that approaches it gradually, and that difference should be reflected in how tightly the setting is controlled and how often it is checked.

Where the causes actually sit

The right panel of Figure 1 splits audited failures of this type into five groups. The percentages are illustrative engineering guidance for prioritising an investigation, not a measured survey and not a prediction for your line.

Table 2. Cause split and the corresponding first check. Shares are indicative and sum to 100.

High moulded-in residual stress accounts for roughly 34% of the illustrative split. Inspect the crazed area under polarised light for frozen-in stress patterns. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Surface-active fertiliser chemistry accounts for roughly 26% of the illustrative split. Confirm the chemical identity and concentration of the contacting fertiliser solution. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Over-packing at the ports accounts for roughly 18% of the illustrative split. Compare holding pressure and mould temperature records for the affected lot. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Checks before sign-off

Run the same sequence on a retained good sample and on the disputed batch so the comparison means something.

1. Inspect the crazed area under polarised light for frozen-in stress patterns.
2. Confirm the chemical identity and concentration of the contacting fertiliser solution.
3. Compare holding pressure and mould temperature records for the affected lot.
4. Section a port and check for over-packing or a dense, stressed skin.
5. Request and verify the resin chemical-resistance documentation coverage from the supplier.

Keep a retained reference sample beside every result. When a batch is disputed, the comparison that settles it is almost always between the retained part and the current one under the same method, not between either part and a number on a drawing. Record the instrument, the operator, the conditioning time and the position on the part or sheet for each reading.

Writing it into the specification and the evidence pack

A defined sample of valve bodies shall be exposed to the declared service solution under agreed stress and duration; any part developing crazes or seepage within that window is rejectable, with the acceptance condition set from an agreed limit sample rather than an absolute rating.

Write the requirement as a behaviour at a stated condition, not as a brand of material or a machine setting. State the test method, the conditioning, the sample plan and the rule for deciding a pass. Where a document is needed, ask the supplier for documentation that covers the actual construction and check its scope against your own market before approval; a document for the base material alone does not describe the finished part you are buying.

At first-article approval, freeze the drawing revision, the material and colour identification, the process window, the tool and cavity identification, and the inspection method. Any later change to grade, site, tool, or window should trigger a new first article rather than a paper waiver. Keep retained samples from the approved lot and from each production lot so that a future comparison is possible.

Table 3. Minimum evidence pack for a first-article decision and a later batch dispute.

None of the figures above should be read as a measured survey. They are engineering guidance for framing a joint trial; confirm them on your own tool, grade and geometry before they become contract terms.