Which injection molding resins are hygroscopic enough to cause splay?

Nylon, many PET and PBT polyesters, polycarbonate and most PC blends, and several TPU or TPE overmold grades commonly splay when processed wet. ABS is less hygroscopic than nylon, but wet ABS still streaks paint-ready or high-gloss housings. Name the family or grade on the RFQ and attach the supplier drying note rather than assuming the molder will classify the resin for you.

Can I skip drying on a short pilot run?

A short pilot does not remove moisture risk. Hygroscopic pellets can still splay on the first cosmetic shots if they sat open or arrived wet. Treat a pilot as a proof of the dry plan and the show-face rule, not as a reason to skip drying. If you must run a learning lot, write that splay on named show faces is still a reject, and state who dries that lot.

Is splay always caused by moisture?

No. Splay is a visual symptom. Moisture is one common cause on hygroscopic resins, but shear heating and contamination also produce silver or white streaks. Use the silver streaks defect page to separate those causes after T1. On the RFQ, still name dry-needed grades and show faces so moisture is controlled before you debate shear or dirt.

Does drying eliminate all splay risk on hygroscopic resins?

Proper drying reduces moisture-driven splay, but it does not guarantee a streak-free part. Shear heating, contamination, and regrind quality also cause silver streaks. If T1 shows splay after a documented dry plan, ask the molder to separate moisture from those other causes using the silver streaks defect page. On the RFQ, still name dry-needed grades and show faces so moisture is controlled before you debate shear or dirt.

Should I mark splay acceptance differently for textured vs polished faces?

Yes. A textured face can hide light splay that would be obvious on a polished show face. Still mark the textured face as a show face, and name the inspection lighting and viewing distance. A polished witness coupon in the same frame shows what the same splay would look like without grain. Grain can hide a dip; it does not hide every streak. See texture and sink marks on molded parts for that related limit.

Who owns drying responsibility when the resin is customer-supplied?

Ownership is an RFQ term, not a shop default. Write whether the molder dries incoming customer resin, or whether the customer ships pre-dried material and the molder only has to keep it closed. If packaging or mixed lots are unusual, say so in the same note. **Silent customer-supplied resin is how wet-bag disputes start.**

What should I upload if I only know the resin family, not the exact grade?

Upload the CAD, write the family, and mark the resin as hygroscopic or dry-needed until a grade is frozen. Attach any supplier literature you do have, even if it is a family brochure rather than a grade TDS. Ask reviewers to hold a drying-ownership note for the family, and update the RFQ when the commercial grade is chosen.

Hygroscopic Resins and Splay: A Buyer Checklist

What Are Hygroscopic Resins and Splay?

A hygroscopic resin absorbs moisture from ambient air. Splay is the silver or white streaking visible on the molded surface when that trapped moisture, or another volatile, flashes during the injection stroke. Buyers who search for hygroscopic resins and splay usually need a specification checklist they can put on an RFQ, not a dryer setup guide.

This page is that buyer checklist. It names the resin families that belong on a dry list, the RFQ sentences that assign drying ownership, and the show faces that cannot accept visible streaking. Use the resin drying glossary for the process definition. Use the silver streaks defect page when you need diagnosis after T1. Do not treat this article as a substitute for either.

Moisture in a hygroscopic pellet is a material property, not a shop secret. Once a bag is open, or a gaylord sits in a humid room, many engineering resins take on water fast enough to show on the next cosmetic shot. The molder still owns the process window. The buyer owns the requirement: name the grade, attach the supplier note, and say who dries.

Splay often looks like a silver comet, a milky fan, or a white haze that follows flow. It can start near the gate and then run along a thin rib or a high-shear feature. Not every streak is moisture. Shear heating and contamination produce similar marks. Your job on the RFQ is to say which faces cannot accept any of those marks, and to put drying ownership in writing before T1 photos are judged.

Key Facts

Nylon, many polyesters, polycarbonate, and several TPU or TPE grades require a defined drying plan before any cosmetic shot is attempted. Put that plan on the RFQ as a named requirement. Do not hope the hopper operator will classify the resin from memory.
Nylon has its own dedicated drying article. See why nylon must be dried for PA-specific handling. This checklist does not repeat that nylon page.
Clear PC splay has its own article. See clear polycarbonate silver streaks for optical and high-gloss PC. This checklist only flags PC as a dry-list family.
Splay is not always caused by moisture. Shear heating and contamination are separate root causes covered on the defect page. If T1 shows streaks after a documented dry plan, ask the molder to separate moisture from those other causes.
Responsibility for drying, molder or customer, must be stated on the RFQ when resin is customer-supplied. Silent RFQs create disputes over wet gaylords, open bags, and who pays for the next lot.
Dryer cycle times, moisture-target values, and barrel setpoints are not published here. Those process parameters belong to the molder and the supplier technical datasheet for the specific grade and equipment.

Write the key facts onto the RFQ in buyer language. "Hygroscopic family, molder dries, no visible splay on named faces" is a complete instruction. A copied press recipe is not a specification.

Which Resin Families Belong on the Dry List

Treat the list below as a buyer flag list, not a chemistry catalog. If the drawing names a grade in one of these families, assume a drying plan is required unless the supplier datasheet says otherwise and you attach that note.

Nylon (PA6, PA66, PA12) and glass-filled grades

Nylon (PA6, PA66, PA12) and many glass-filled nylon grades belong on the list first. Nylon is the family most buyers already associate with moisture. Glass-filled grades still pick up water. They can also make streaks easier to see on a paint-ready housing than on a dark, textured interior. Grain can hide a dip; it does not hide every streak. See texture and sink marks on molded parts. Do not copy a nylon drying recipe onto this RFQ. Use the nylon article linked above and attach the grade TDS.

PET, PBT, and polyester-based compounds

PET, PBT, and similar polyester-based compounds belong next. Polyesters used for housings, connectors, and structural clips are commonly hygroscopic. Wet polyester can splay on a cosmetic face and can also lose toughness on a clip or latch. Name the polyester family or the commercial grade. Do not assume the molder treats every polyester like nylon.

Polycarbonate and PC blends

Polycarbonate (PC) and most PC blends belong on the list, especially on clear or high-gloss faces. Clear and tinted PC shows moisture and shear marks more readily than an opaque, textured PC/ABS interior. If the show face is optical or high gloss, send reviewers to the clear PC article. On this RFQ, mark the face and name the blend.

TPU and TPE overmold grades

TPU and certain TPE overmold grades belong on the list when the processing window is moisture-sensitive. Soft overmold shots fail cosmetically when the elastomer arrives wet, or when the hard substrate sat open. Write the overmold grade and the substrate grade on the same RFQ. State that both need a dry plan if either family is on this list.

ABS on paint-ready or cosmetic faces

ABS is less hygroscopic than nylon, but wet ABS still produces visible splay on paint-ready or cosmetic housings. Buyers sometimes skip ABS because it feels less thirsty. That skip shows up as silver streaks on a bezel that will be painted or plated. If the ABS face is cosmetic, put it on the dry list and attach the TDS note.

Blends inherit the stricter parent

If the resin is a blend (PC/ABS, PA/ABS, PBT/PET), flag the more moisture-sensitive constituent. Do not assume the blend inherits the easier family. Write the full commercial grade when you have it.

Color and additive packages do not remove hygroscopic behavior. A black nylon, a UV-stabilized PBT, and a tinted PC still belong on the dry list. Filled grades (glass, mineral, carbon) still absorb moisture in the resin matrix. Fillers can make streaks more visible on a smooth face because they change how light breaks at the surface. Note the fill type so DFM can judge cosmetics. Do not turn that note into a dryer setting.

What Buyers Should Write, Not Tune

Buyers specify. Molders tune. The RFQ should name the risk and the owner. It should not invent a shop recipe.

Name the resin family or specific grade, and note that it is hygroscopic if you already know. "PC/ABS, hygroscopic, cosmetic A-side vs hidden face" is useful. "Engineering plastic, dry as needed" is not. If you only know the family, say so. Do not invent a grade code to look precise.

Name the risk and the owner, not a recipe

Attach the supplier's drying recommendation from the technical datasheet. Do not invent a shop recipe. A PDF page or a screenshot of the TDS drying section is enough. If the TDS is silent, write that it is silent and ask the molder to follow the supplier's published handling for that grade. Do not write dryer cycle times, moisture targets, or barrel setpoints on the drawing.

Three common drying-ownership patterns

State who is responsible for drying when the resin is customer-supplied. Three common patterns belong in one sentence. The molder dries incoming bags. The customer ships pre-dried resin in sealed containers and the molder keeps them closed until the hopper. Or the customer ships open gaylords and accepts that the molder must dry before a cosmetic shot. Pick one. If you skip this sentence, wet-material arguments start at T1. See who pays for startup scrap if the next lot is also a scrap argument.

Request T1 photos of the show face captured under the same lighting conditions your quality team will use during inspection. A phone photo in mixed warehouse light hides splay that a light booth will catch. Name the viewing distance and whether the face is inspected as molded, after paint, or after a hardcoat. If you already have a rejected lot from another shop, attach those photos too.

Write the acceptance rule in buyer language: no visible splay on the named show faces at the stated lighting. Do not write a process window. If a hidden face may show minor streaking, say that explicitly so the molder does not over-constrain the whole part.

When resin will change after T1, say so. A color chip, a UV package, or a move from a development lot to a production lot can arrive wetter or with a different additive. The dry-list flag stays. The TDS attachment should match the lot you intend to run.

Customer-supplied regrind or mixed bags raise the same ownership question. Write the blend rule on regrind blends buyer specification terms. If any reclaimed stream can enter the hopper, say whether that stream is allowed on show faces. Wet or dirty regrind is a known splay source. Keep blend rules on the quality plan. Keep drying ownership on the RFQ.

If the program uses two shots, write a dry note for each resin. A dry first shot does not protect a wet second shot.

RFQ Checklist

Use these five steps on every hygroscopic or splay-risk program. They fit in the notes field of an upload or on a one-page material callout.

Name the resin family or grade and indicate whether it is hygroscopic. Write the commercial grade when you have it. If you only have a family, write the family and mark it hygroscopic or dry-needed until the grade is frozen.
Attach the supplier's drying note from the TDS. A screenshot or PDF page is enough. Do not retype a recipe into the title block.
State who dries: the molder, or customer-supplied pre-dried material. If the customer pre-dries, say how the resin is packaged and how it is kept closed before the press. If the molder dries, say whether resin will arrive as sealed bags, open gaylords, or mixed lots.
Mark show faces that cannot accept any visible splay. Point to the CAD faces or the 2D zones. Separate paint-ready, high-gloss, clear, and hidden faces. If those faces share one tool, see mixing SPI finishes on one part. If a face will be painted, still mark it. Paint does not hide every streak.
Upload CAD through the instant quote platform for DFM review. Include the material note and the TDS excerpt in the same package so drying ownership is visible before tooling begins.

If you already ran a pilot elsewhere, add a free-text note: what the other shop saw, whether they dried, and which faces failed. Do not send a press recipe from that shop as a requirement. Send the photos and the acceptance rule.