HCR Parts That Stay Soft: Reading Under-Cure Before Post-Cure Is Blamed

Low hardness and high compression set after demoulding often come from an incomplete primary cure that a post-cure oven cannot fully repair.

What the failure looks like on the line

High-consistency rubber is mixed, milled and cured in more steps than LSR, and each step adds a place where batch history can be lost.

Freshly demoulded HCR parts read below the specified Shore A band and feel soft; a post-cure cycle improves them only partly.

Peroxide decomposition and crosslink density depend on the time-temperature integral at the thickest section. If the core never reaches its activation window, the network stays under-crosslinked; post-cure can drive off volatiles but cannot rebuild the missing crosslinks.

The useful question is not whether the part conforms, but which condition was never written into the specification. 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. Mould temperature 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.

Mould temperature is typically held at 160-190 C. Below the window peroxide decomposition is incomplete Record it together with the measurement point and the moment in the cycle; a single number without a location is not a process setting.

Cure time per section is typically held at 30-90 s/mm. Short dwell leaves the core under-cured in thick sections When this drifts, the symptom usually appears at one cavity, one sheet position or one lane before it appears across the whole batch.

Post-cure temperature is typically held at 180-220 C. Too low a hold fails to remove decomposition volatiles 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.

Post-cure duration is typically held at 2-8 h. Short holds leave residual by-products in the bulk 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 crosslink completeness (%) as mould dwell within cure window increases: the response rises from 35 to 96 on the illustrative 0–100 scale. The dashed line at 90 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.

Mould temperature below window accounts for roughly 30% of the illustrative split. Measure hardness at the thickest section, not only at the surface. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Short cure dwell at section accounts for roughly 26% of the illustrative split. Compare a rheometer curve from the lot against the agreed cure window. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Peroxide dosing drift accounts for roughly 18% of the illustrative split. Section a scrap part to look for a soft, under-cured core. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Checks before sign-off

Work through these in order; measuring a later variable before an earlier one hides the cause rather than finding it.

1. Measure hardness at the thickest section, not only at the surface.
2. Compare a rheometer curve from the lot against the agreed cure window.
3. Section a scrap part to look for a soft, under-cured core.
4. Verify oven air circulation and record the actual part temperature.
5. Re-check hardness after conditioning before accepting a post-cure fix.

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

Require a documented cure window and a hardness reading at the thickest section after conditioning, with post-cure treated as a finishing step rather than a cure substitute.

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.