The short answer
Amorphous plastics (polycarbonate, ABS, acrylic, polystyrene, PSU, PEI, PVC) have randomly tangled chains. They soften gradually above their glass transition, shrink little and evenly, and can be clear. Semi-crystalline plastics (PP, PE, nylon, acetal, PBT, PET, PPS, PEEK, PTFE) contain ordered crystals. They melt sharply, shrink more and less evenly, and generally resist chemicals, fatigue and wear better.
Whether a resin is amorphous or semi-crystalline explains a lot of its behavior, from how it molds to how it fails. Once you know which group a resin is in, many data sheet numbers make more sense.
What the terms mean
In an amorphous plastic the long polymer chains are tangled at random, like cooked spaghetti, with no regular order. Below the glass transition temperature (Tg) the material is glassy and rigid. Above Tg it turns rubbery, then gradually more fluid as it heats.
In a semi-crystalline plastic, parts of the chains fold and pack into tight, ordered crystals as the melt cools. The rest stays amorphous between them. No commercial resin is fully crystalline; crystallinity ranges from roughly 30% to over 70% depending on the polymer and how it was cooled. The crystals melt at a fairly sharp melting point (Tm). See Tg vs. melting point for typical values.
Amorphous vs. semi-crystalline at a glance
| Trait | Amorphous | Semi-crystalline |
|---|---|---|
| Structure | Randomly tangled chains | Ordered crystals within amorphous regions |
| Thermal transitions | Glass transition (Tg) only | Tg and a melting point (Tm) |
| How it softens | Gradually, over a range above Tg | Sharply, at Tm |
| Clarity | Can be clear (PC, acrylic, polystyrene, SAN) | Usually translucent to opaque |
| Mold shrinkage, unfilled | About 0.2–0.8% | About 0.8–3% |
| Shrinkage direction | Close to even in all directions | Uneven, especially glass-filled; more warp |
| Chemical resistance | Fair; prone to stress cracking | Generally good to excellent |
| Glass fiber effect on HDT | Small (PC 124–132°C / 255–270°F to about 140–150°C / 284–302°F) | Large (PA66 70–90°C / 158–194°F to 240–255°C / 464–491°F) |
| Fatigue and wear | Generally lower | Generally better; used for gears and bearings |
| Mold temperature | Affects stress and surface finish | Also sets crystallinity and later shrinkage |
| Examples | PC, ABS, acrylic, polystyrene, PSU, PEI, PVC, modified PPE | PP, PE, nylon, acetal, PBT, PET, PPS, PEEK, PTFE |
General tendencies, with typical values from our material pages and processing chart. Individual grades vary, and fillers, modifiers and blends change the picture.
Clarity
Crystals and the boundaries between them scatter light, so semi-crystalline parts are usually translucent or opaque. Amorphous resins with no rubber or filler, such as polycarbonate, acrylic, polystyrene and SAN, can be water clear; PSU and PEI are transparent amber. ABS is amorphous but opaque because of its rubber particles. Exceptions on the other side include clarified PP and PET bottles. See the best plastic for clear parts.
Shrinkage and warpage
Chains pack more densely when they crystallize, so semi-crystalline resins shrink more: about 1.2–2.4% for acetal copolymer and 1.7–2.9% for injection HDPE, against about 0.5–0.75% for polycarbonate and 0.4–0.7% for ABS. Glass fibers cut total shrinkage but make it uneven, since the fibers line up with the flow and restrain shrinkage along it. Uneven shrinkage is the main cause of warp. See warped parts.
Chemical resistance and stress cracking
Tightly packed crystals are hard for solvents to get into, so semi-crystalline resins generally hold up better to fuels, oils and solvents. In amorphous resins, chemicals can penetrate the open structure. Combined with molded-in or applied stress, that leads to environmental stress cracking, a classic failure of polycarbonate and ABS around some cleaners and oils. It is a tendency, not a rule: nylon is attacked by strong acids, and PSU and PEI resist many chemicals. See chemical-resistant plastics and environmental stress cracking.
Heat resistance
An amorphous resin loses its stiffness at Tg, so its useful range ends somewhat below Tg. A semi-crystalline resin softens at Tg too, but the crystals keep it in one piece up to Tm. That is why glass fiber lifts HDT so much more in semi-crystalline resins: the fibers and crystals together carry load until the crystals melt. See HDT vs. continuous use temperature.
Fatigue and wear
Semi-crystalline resins such as acetal, nylon, PBT and PEEK generally handle repeated loading and sliding better than amorphous resins, which is why they are the usual choice for gears, bearings and clips, and why polypropylene is used for living hinges. Amorphous resins tend to have lower fatigue limits and can craze under cyclic stress.
Melt behavior and processing
Amorphous resins soften over a wide range, which gives a broad processing window and makes them easy to thermoform. Semi-crystalline resins need extra heat to melt the crystals, then become quite fluid; nylon's low melt viscosity is why it flashes and drools. On cooling they set up quickly once they crystallize, so gates freeze sooner. Starting temperatures for each resin are in processing parameters.
Mold temperature and crystallinity
For semi-crystalline resins, mold temperature sets how much crystallinity the part reaches. A cold mold freezes the chains before they crystallize fully. The part then keeps crystallizing later, especially when heated in service, and shrinks or distorts after it has passed inspection. Resins whose Tg is above room temperature are the most sensitive: glass-filled PET is molded at about 95–121°C (200–250°F), PPS at about 135–160°C (275–320°F), and PEEK at 170–200°C (338–392°F). For amorphous resins, mold temperature mainly affects molded-in stress and surface finish.
Drying
Whether a resin needs drying depends on its chemistry, not its structure. Polycarbonate, acrylic and ABS (amorphous) and nylon, PET and PBT (semi-crystalline) all need it; PP, PE and polystyrene generally do not. One structure-related point: amorphous PET flake or regrind must be crystallized before it is dried above its Tg, or the pellets soften and stick together. See the resin drying guide.
Lean amorphous when
- The part must be clear or very low warp
- Tight, even dimensions matter more than chemical resistance
- Heat stays well below the resin's Tg
- Examples: lenses, housings, medical and lab ware
Lean semi-crystalline when
- The part sees fuels, oils, solvents or cleaners
- It slides, wears or flexes repeatedly
- It must carry load when hot (glass-filled)
- Examples: gears, clips, under-hood parts, connectors
What to send us for a quote
- The part and what it must survive: chemicals, heat, load, wear
- Any need for clarity or tight flatness
- The grade you run now, if any
- Quantity and ship-to city
We aim to send your quote within one to three business days.
Common questions
Is nylon amorphous or semi-crystalline?
Standard nylon 6 and nylon 66 are semi-crystalline, with a sharp melting point. Transparent amorphous polyamides also exist, but they are a separate, specialty family.
Is ABS amorphous?
Yes. ABS, polycarbonate, acrylic, polystyrene, polysulfone, PEI, PVC and modified PPE are amorphous or close to it, so they soften gradually above their glass transition instead of melting sharply.
Why do semi-crystalline parts warp more?
They shrink more as the crystals form, typically about 0.8–3% unfilled versus about 0.2–0.8% for amorphous resins, and glass fibers make the shrinkage differ along and across the flow. Uneven shrinkage is what bends a part.
Which has better chemical resistance, amorphous or semi-crystalline plastic?
Semi-crystalline plastics generally resist solvents, fuels and oils better because the tightly packed crystals are hard for chemicals to penetrate. Amorphous plastics are more prone to environmental stress cracking, though there are exceptions on both sides.
Can a semi-crystalline plastic be clear?
Usually not in thick sections, because crystals scatter light. Exceptions include clarified polypropylene, thin oriented film, and PET that has been quenched or oriented so its crystals stay very small, as in bottles.
Does glass fiber help amorphous and semi-crystalline resins equally?
No. Glass raises stiffness in both, but it lifts heat deflection temperature far more in semi-crystalline resins. Nylon 66 goes from about 70–90°C (158–194°F) to 240–255°C (464–491°F), while polycarbonate gains only roughly 10–20°C (18–36°F).
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A polymer chemist reviews every request, then matches your grade or finds a qualified equivalent and prices it.
What to send for an accurate quote
- The grade name, from any supplier, or a data sheet
- Color (natural, black or a custom match)
- Fill type and percentage, if any
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