Mold Temperature and Polyamide Crystallinity: How Cooling Controls Properties

Two identical polyamide pellets, one mold, the same machine — and two parts with different stiffness, different shrinkage and different behavior in the hot zone. The difference often comes down to a single, underrated parameter: mold temperature. For a semi-crystalline polyamide the mold is not just a "cold wall that freezes the melt," but a control element that decides how much crystalline phase has time to form and, therefore, what the properties of the finished part will be.

This article is about the physics of PA6 and PA66 crystallization within the injection molding cycle, and about how the process engineer controls it through mold temperature and the cooling profile. The numeric ranges here are given as typical for the class — always verify specific values against the TDS of the selected grade.

Why polyamide is a semi-crystalline, not an amorphous plastic

Polyamide belongs to the semi-crystalline thermoplastics. This means that in the solid state two phases coexist within it: ordered regions where the polymer chains are folded into dense lamellae (which assemble into spherulitic structures), and amorphous regions where the chains are entangled at random. It is precisely the hydrogen bonds between the amide groups (–CO–NH–) that make the crystalline phase of polyamide strong and heat-resistant — this is the physical basis of why nylon carries loads where an amorphous plastic is already flowing.

The key idea: the degree of crystallinity is not "hard-wired" into the material forever — it forms during cooling in the mold. PA6 and PA66 crystallize quickly, but not instantaneously. If the melt is cooled too abruptly (a cold mold), some of the chains "freeze" in the amorphous state without having time to pack into crystals. If crystallization is allowed to proceed more fully (a warmer mold, a controlled heat-removal rate), the part comes out denser, stiffer and more dimensionally stable — but with somewhat higher shrinkage.

What mold temperature actually changes in the finished part

Mold temperature affects several properties at once, and it is almost always a trade-off rather than "more = better."

A warmer mold (within the recommended TDS window) usually raises the degree of crystallinity, and with it the tensile modulus, heat deflection temperature under load (HDT), surface hardness, creep resistance and chemical resistance. The surface comes out glossier, and internal stresses are lower, which reduces the tendency to warp. A cold mold shortens the cycle and reduces primary shrinkage, but leaves more "under-crystallized" amorphous phase in the part. Such a part is prone to post-mold shrinkage: crystallization slowly continues outside the mold — during storage, and especially at the first heating in service — and the dimension "drifts" even after QC inspection.

For unfilled PA6 and PA66 grades the difference in properties driven by mold temperature is pronounced. For glass-filled grades (for example PA6 GF30, PA66 GF30) the picture is more complex: here glass-fiber orientation along the flow direction is superimposed on shrinkage and stiffness, so a warm mold improves weld-line fusion and surface, but does not eliminate the "along/across flow" shrinkage anisotropy.

The direction of the effect is typical for the PA6/PA66 class; the magnitude of the effect depends on the grade, filler and geometry. Take the absolute values and the mold-window limits from the TDS.

The practical temperature window: why "just colder" is not always faster

Process engineers are tempted to keep the mold cold for the sake of a shorter cycle. For polyamide this is often a false economy. First, a part with underdeveloped crystallinity gives post-mold shrinkage, and the batch "creeps" in dimensions between the day of molding and the day of shipment. Second, an overly cold mold worsens the fusion of flow fronts — the risk of visible weld lines and local strength loss increases. Third, the surface becomes dull or shows "silver streaking."

The typical mold-temperature window for unfilled PA6/PA66 lies roughly in the range from moderately warm to hot; for glass-filled grades and for parts where surface and dimensional stability matter, the mold is usually kept in the upper part of the recommended window. The exact values are determined solely by the TDS of the specific grade, because they depend on the melt viscosity, filler level, wall thickness and cycle requirements. The universal rule is different: choose the mold temperature to suit the part requirement, not the desired cycle time. If dimensional stability and stiffness are needed — a warmer, stably temperature-controlled mold; if the takt time is critical and the geometry is simple — a compromise is possible, but with post-mold shrinkage control.

It is worth separately distinguishing two independent levers: melt temperature (which controls viscosity and filling) and mold temperature (which controls crystallization and cooling). They are confused, yet they solve different tasks. And both work only on the condition that the pellet is properly dried — moisture in polyamide leads to melt hydrolysis and unstable properties regardless of how perfectly the mold is set.

The role of uniformity: not just "how many degrees," but "the same everywhere"

Even a correctly chosen average mold temperature does not help if the cooling is uneven. A temperature difference between the two mold halves, or between zones of a single part, means a different crystallization rate, different local shrinkage — and, as a result, warpage. That is why for critical polyamide parts what matters is not only the setpoint on the temperature controller, but also the uniformity of temperature control: balanced cooling channels, a stable coolant flow, control of both mold halves.

This is especially critical for glass-filled grades and thin-walled products, which already carry built-in anisotropy from fiber orientation. An uneven mold adds to it — and the part comes out of the mold already stressed. The practical takeaway for the designer: uniform wall thickness and well-thought-out placement of cooling channels often do more for stability than any "tweaking" of parameters on a finished mold.

How this relates to Material Wizard materials

In the Examid® line, the glass-filled polyamides — PA6 GF30 and PA66 GF30 — are grades for series molding, where it is precisely dimensional stability and batch repeatability that separate a successful project from endless fine-tuning. Technical material selection here is inseparable from process selection: we support the trials and help bring together the part requirement, the recommended mold window from the TDS and the real capabilities of the customer's machine. We do not publish the commercial details of formulations or specific processing settings — they are agreed for each project with a specialist.

Examid® PA6 GF30Dimensional stability · stiffness · batch repeatabilityRequest supply terms →

Examid® PA66 GF30Higher heat resistance · HDT · stability in the engine compartmentRequest supply terms →

On the role of glass-fiber reinforcement and the behavior of glass-filled polyamides — in the article Glass-Fiber Reinforcement; a comparison of the base PA6 and PA66 grades — in the article PA66 vs PA6; and the basic chemistry of amide bonds — in the article what is polyamide 6.

What to check before a series launch

Before signing off a part for series production, it is worth going through a checklist: is the pellet dried to the level specified in the TDS (moisture kills stability regardless of the mold); are both mold halves temperature-controlled with a stable coolant flow; is shrinkage measured not immediately but after conditioning and a short annealing (to "catch" post-mold shrinkage); is the geometry the same when measured along and across the flow (GF anisotropy); are dimensions stable between the first and last shots of the batch; and does the mold temperature not creep upward by drift over the course of a shift. These checks cost a few hours — and save weeks of reworking rejected parts.

Expert breakdown: 5 questions about mold temperature and crystallinity

Why does a polyamide part change dimensions even after quality control? The most frequent cause is post-mold shrinkage. If the mold was too cold, crystallization did not complete within the cycle and slowly continues outside the mold, especially at the first heating in service. The dimension "creeps" even after QC. The solution is a warmer, stably temperature-controlled mold within the TDS window and shrinkage measurement after conditioning rather than immediately after molding.

What matters more for properties — melt temperature or mold temperature? These are different levers. Melt temperature controls viscosity and filling, mold temperature controls the crystallization rate and, therefore, the final crystallinity. For the stiffness, HDT and dimensional stability of a semi-crystalline polyamide, mold temperature is the key one. Both parameters are set according to the grade's TDS, not "by eye."

Can a part's stiffness be increased without changing the material? Partly yes — a warmer mold (within the recommended window) raises the degree of crystallinity, and with it the modulus and heat resistance. But it is a trade-off: primary shrinkage increases and the cycle lengthens. For glass-filled grades the effect is less pronounced, because stiffness there is mostly set by the glass fiber.

Why does a cold mold produce weld lines and a dull surface? Under abrupt cooling the melt fronts fuse worse where they meet — the visibility of the weld line increases and strength drops locally. The surface layer freezes without having time to reproduce the mold's gloss, hence the dullness and the risk of "silver streaking." A warmer mold improves both fusion and surface.

How does uneven cooling lead to warpage? Different zones of the part crystallize at different rates and give different local shrinkage. This difference turns into internal stresses, which the part "releases" through warpage after removal from the mold. That is why for polyamide the uniformity of temperature control of both mold halves and balanced cooling channels are critical, not just the average value on the temperature controller.