How Tg defines the upper thermal limit of injection molded plastics — and why it matters for material selection, mold design, and part performance.
Glass transition temperature (Tg) is the temperature at which an amorphous thermoplastic transitions from a hard, glassy state to a soft, rubbery one. Below Tg, the polymer chains are locked in place and the material is stiff and dimensionally stable. Above Tg, the chains gain mobility, the material softens, and the part loses its structural integrity.
For engineers selecting injection molding materials, Tg is a critical number — it sets the practical upper thermal limit for amorphous thermoplastics and directly affects mold design, processing parameters, and part performance in elevated-temperature applications.
Tg vs. Melting Point — An Important Distinction
Glass transition temperature applies specifically to amorphous thermoplastics — materials that lack a defined crystalline structure. Examples include ABS, PC, PMMA (acrylic), PEI, and PS.
Semi-crystalline thermoplastics — PP, nylon (PA), POM/Delrin, PEEK, PET — have both a Tg and a crystalline melting point (Tm). For these materials, Tg marks the transition of the amorphous regions within the polymer, but the crystalline regions maintain their structure up to the much higher Tm. This is why semi-crystalline materials generally have better retention of mechanical properties at elevated temperatures than amorphous materials.
Key Distinction
Tg is not the same as melting point. Amorphous thermoplastics soften significantly when they exceed Tg, while semi-crystalline materials may retain useful mechanical properties above Tg because their crystalline regions remain intact until the much higher melting point.
| Material | Glass Transition Temperature / Thermal Behavior | Practical Design Consideration |
|---|---|---|
| ABS | Tg: 100–115°C | Practical service limit around 80°C with load. |
| Polycarbonate | Tg: 145–150°C | Service limit around 120°C with load. |
| PMMA / Acrylic | Tg: 100–105°C | Service limit around 80°C. |
| PEI / Ultem | Tg: 217°C | One of the highest Tg thermoplastics; suitable for sterilization environments. |
| PEEK | Tg: 143°C; Tm: 343°C | Semi-crystalline material with continuous use up to 240°C. |
| Nylon PA 6/6 | Tg: 50–80°C, depending on moisture; Tm: 255°C | Properties are maintained above Tg by crystallinity. |
| POM / Delrin | Tg: -60°C; Tm: 175°C | Highly crystalline material with mechanical properties maintained by crystallinity to near Tm. |
How Tg Affects Injection Molding Design
Material Selection
A part that must perform at 100°C continuously cannot be made from standard ABS with a Tg of approximately 105°C because there is no safety margin. Select materials with Tg or continuous use temperature ratings at least 20–30°C above the maximum service temperature to ensure adequate structural performance under sustained load.
Mold Temperature
Mold temperature directly affects the degree of molecular organization — and therefore the mechanical properties — of the molded part. For amorphous materials, mold temperature should approach but not exceed Tg: running PC molds at 80–100°C, below its Tg of approximately 145°C, relieves molding stress and improves dimensional stability.
For semi-crystalline materials, elevated mold temperatures promote crystallinity, which is essential to achieving the rated mechanical properties of the material.
Annealing
When injection molded parts must meet tight dimensional tolerances at elevated temperatures, annealing — heating the part above its service temperature but below Tg for amorphous materials, or near the crystallization temperature for semi-crystalline materials — relieves molding residual stress and allows the part to reach its stable dimensions before it goes into service.
Ejection Temperature
Parts must be cooled below a safe ejection temperature before the mold opens. For amorphous materials, ejection temperature is determined by Tg — the part must be cooled well below Tg before it can be ejected without deforming. This is why cooling time increases rapidly with wall thickness and why thick-walled PC or PEI parts have long cycle times.
Design Note
Glass transition temperature should be reviewed early in material selection. Tg affects service temperature limits, mold temperature strategy, cooling time, dimensional stability, and whether a molded part may require annealing before final use.
Frequently Asked Questions
Why does nylon have a low Tg but high service temperature?
Nylon is semi-crystalline. Its Tg is approximately 50–80°C and drops with absorbed moisture, but its mechanical properties are maintained well above Tg by the crystalline structure. The crystalline regions do not soften until the crystalline melting point, which is approximately 255°C for PA 6/6. This is why nylon brackets and gears function reliably at 120°C even though the amorphous regions have transitioned to a rubbery state.
What happens to a plastic part if it exceeds its Tg in service?
Above Tg, amorphous thermoplastics soften and lose stiffness — parts under load will creep, deform, or lose their shape. Dimensions shift and tolerances are lost. The transition is reversible: the part hardens again when cooled below Tg, but permanent deformation may remain if the part was loaded during the softening. For sustained elevated-temperature service, always use materials with Tg significantly above the maximum service temperature.
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