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Guides by topic
Injection molding defects
Splay and silver streaksThe usual causes of splay, silver streaks and bubbles, and how to tell which one you have.Short shots and parts that won’t fillHow to tell whether incomplete fill comes from the process or the material.Sink marksWhy dimples form over ribs and bosses, and the order to check pack, cooling and wall thickness.Voids and bubblesHow to tell a vacuum void from a gas or moisture bubble, and what fixes each one.Warped partsWhy molded parts warp, and the material changes that reduce it.Weld linesWhy weld lines show and where they weaken a part, especially in glass-filled grades.Flow lines and flow marksWavy lines, gate blush and hesitation marks, and how to tell them apart.Burn marksHow to tell trapped air at the end of fill from material degraded in the barrel.FlashWhy material escapes at the parting line, and what to check before blaming the tool.JettingWhy a snake-like stream forms at the gate, and how gate, speed and temperature affect it.Black specks and contaminationWhere black specks and streaks come from, and how to stop them.Color that varies lot to lotWhy color shifts between lots, and how to keep it consistent.Yellowed or discolored partsParts that come off the press yellow or brown from too much heat history.DelaminationWhy a molded surface peels in thin layers, and how to find the contamination behind it.Stringing and nozzle droolWhy strings pull from the sprue and the nozzle drools between shots.Parts sticking in the moldOverpacking, draft, cooling and shrinkage, in the order to check them.Parts out of toleranceWhy dimensions drift shot to shot or lot to lot, and where to look first.
Extrusion and film
Melt fracture and sharkskinWhy an extruded surface turns rough or matte, and what lowers the stress at the die.Die lines and die build-upHow to tell a damaged die lip from build-up that breaks off, and what causes the build-up.Extruder surging and gauge variationWhy output pulses or thickness wanders, read from how fast the swing repeats.Gels and fisheyes in filmHow to tell unmelts, crosslinked gels and contamination apart in blown and cast film.Blown film bubble instabilityBreathing, snaking and a hunting frost line, and what steadies the bubble.Blown film that tears or puncturesWhy blown film tears or punctures, and the resin changes that make it tougher.Film blockingWhy film layers stick together, and what antiblock and slip cost you.Heat seal failuresWeak, leaking or burned-through seals in film and bags, and what to check first.
Material handling
Part performance
Brittle or cracking partsThe common reasons molded parts turn brittle or crack, and the material fix for each.Parts wearing outWhy gears, bushings and slides wear out, and which base resins and fillers fix it.Fading, yellowing or chalking outdoorsWhy parts fade, yellow and chalk in sunlight, and the materials that hold up outdoors.Cracking near chemicals (stress cracking)What environmental stress cracking is, and which resins are vulnerable to which chemicals.Nylon parts swelling or changing sizeWhy nylon parts grow and soften as they absorb moisture, and what to use instead.Parts softening under heatWhy parts sag or creep when hot, how to read heat ratings, and the next resin up.Static buildup and ESD-safe resinsWhy parts build up static, and how to choose between dissipative and conductive grades.Replacing metal with plasticWhen replacing metal makes sense, which resins are used, and what to watch.Making a part lighterHow to cut part weight through resin choice and lighter reinforcements.
Compliance and requirements
Getting a UL 94 flame ratingWhat the UL 94 ratings mean, why thickness and color matter, and how to choose an FR resin.FDA food-contact resinsWhat FDA food-contact compliance means for resin, and what to ask your supplier for.Adding recycled content (PCR or PIR)How to meet a recycled-content requirement with PCR or PIR resin.
Supply and price
Polycarbonate alternativesWhat can replace PC, and what to check before you switch.Chemical-resistant plasticsWhich resins hold up to acids, bases and solvents.Glass-filled plastics explainedWhat glass fiber changes, and how to pick the right level.High-temperature plasticsWhich resins handle heat, and which heat number to trust.Best plastics for outdoor useWhich resins hold up in sunlight, and how outdoor grades are protected.PEEK alternativesLower-cost high-performance resins, and when they can replace PEEK.PA66 shortage alternativesWhat to use when nylon 66 is short or on allocation, and how to switch safely.Resin on allocation or force majeureA checklist for the first 48 hours and the options that keep your line running.Your grade was discontinuedWhat to do when a grade is discontinued or goes on allocation, from buying time to qualifying a replacement.Price went up or lead times slippedHow to check a price increase, compare delivered cost and protect your supply.
Compare materials
HDPE vs. UHMWTwo polyethylenes with very different wear, toughness and processing.PVDF vs. PTFETwo fluoropolymers compared, and when to use each.Nylon 12 vs. nylon 6Low moisture and chemical resistance vs. strength and cost.PP vs. ABSPolypropylene and ABS compared, and when one can replace the other.PPA vs. nylon 66When glass-filled PA66 runs out of heat or moisture resistance.PC vs. PC/ABSPolycarbonate and its ABS blend compared.ABS vs. HIPSStrength and finish vs. cost, and when HIPS can replace ABS.Masterbatch vs. precolored resinCost and flexibility vs. color consistency.TPE vs. TPUSoftness, wear and oil resistance: which elastomer suits your part.Glass-filled vs. mineral-filledStrength and heat vs. flatness and cost, and when to blend them.HDPE vs. LDPEDensity, stiffness and which polyethylene suits your part or film.Acetal homopolymer vs. copolymerPOM-H vs. POM-C: strength, stability and which suits your part.Glass-filled PP vs. nylonA common cost-down: when coupled glass-filled PP can replace nylon.Glass-filled nylon vs. PBTMoisture, heat and chemicals: which glass-filled resin suits your part.Nylon 6 vs. nylon 66PA6 and PA66 compared on heat, strength, moisture, processing, cost and swapping one for the other.ABS vs. polycarbonate vs. nylonABS, polycarbonate and nylon compared on impact, heat, clarity and cost.GPPS vs. HIPSCrystal and high-impact polystyrene compared on clarity, toughness and use.Acetal (POM) vs. nylonAcetal and nylon compared for gears, bushings and precision parts.Polypropylene vs. HDPEPP and HDPE compared on stiffness, heat, cold impact and chemical resistance.LDPE vs. LLDPELDPE and LLDPE compared for film: processing, clarity and toughness.PBT vs. PETPBT and PET compared as engineering resins: molding, stiffness, heat and moisture.Polycarbonate vs. acrylic (PMMA)Polycarbonate and acrylic compared on impact, clarity, scratch and UV resistance.ABS vs. ASAABS and ASA compared on outdoor UV resistance, color stability and cost.PEEK vs. PEIPEEK and PEI compared on service temperature, chemical resistance and wear.Carbon-filled vs. glass-filled nylonCarbon-filled and glass-filled nylon compared on stiffness, weight and static control.
Guides & reference
Engineering vs. commodity plasticsThe three tiers of plastic resin, and how to pick one.How to read a resin data sheetWhat each property means and how to compare two grades fairly.What is a COA?What a certificate of analysis shows, and how to check one.Resin glossaryPlain-English definitions of MFI, HDT, GF30, PCR, COA, UL 94 and more.Order sizes: gaylord, LTL, truckload, railcarHow much resin is in a gaylord, a truckload and a railcar, and how each ships.Prime, wide-spec, off-grade and regrindWhat prime, wide-spec, off-grade and regrind mean, and when each makes sense.Melt flow index explainedWhat melt flow index means, how it is tested, and typical ranges by process.Melt temp, mold temp and shrinkage chartMelt temperature, mold temperature and shrinkage for 35 resin families.Resin drying chartDrying temperature, time and moisture targets for 31 resin families.How to choose a resinA practical way to choose a resin from mechanical, thermal and chemical needs.Why set up a second supplierWhy single-sourcing resin is a risk, and how to qualify a second source.
Tools
Defect finder for injection molding
Find your defect in the left column. A dot marks a common cause. Then open the guide for the checks in order.Find your defect. The tags under it are the common causes. Tap the defect for the checks in order.
| Defect | Wet resin | Melt too hot | Melt or mold too cold | Pack and hold too low | Poor venting | Fill too fast | Contamination | Resin or lot change |
|---|---|---|---|---|---|---|---|---|
| Splay and silver streaks | ||||||||
| Voids and bubbles | ||||||||
| Short shots | ||||||||
| Sink marks | ||||||||
| Weld lines | ||||||||
| Burn marks | ||||||||
| Flash | ||||||||
| Jetting | ||||||||
| Black specks | ||||||||
| Color varies | ||||||||
| Warped parts | ||||||||
| Flow lines and flow marks | ||||||||
| Delamination | ||||||||
| Stringing and nozzle drool | ||||||||
| Parts sticking in the mold | ||||||||
| Yellowed or discolored parts | ||||||||
| Parts out of tolerance |
- Splay and silver streaksWet resinMelt too hotContaminationResin or lot change
- Voids and bubblesWet resinMelt too hotMelt or mold too coldPack and hold too low
- Short shotsMelt or mold too coldPack and hold too lowPoor ventingResin or lot change
- Sink marksMelt too hotPack and hold too lowResin or lot change
- Weld linesMelt or mold too coldPack and hold too lowPoor venting
- Burn marksMelt too hotPoor ventingFill too fast
- FlashWet resinMelt too hotFill too fastResin or lot change
- JettingMelt or mold too coldFill too fast
- Black specksMelt too hotContaminationResin or lot change
- Color variesMelt too hotContaminationResin or lot change
- Warped partsMelt or mold too coldPack and hold too lowResin or lot change
- Flow lines and flow marksMelt or mold too coldPack and hold too low
- DelaminationWet resinMelt or mold too coldContamination
- Stringing and nozzle droolWet resinMelt too hotResin or lot change
- Parts sticking in the moldMelt too hotResin or lot change
- Yellowed or discolored partsMelt too hotContaminationResin or lot change
- Parts out of toleranceMelt or mold too coldPack and hold too lowResin or lot change
General guidance. Tooling and part design also cause most of these, and each guide covers them.
Resin selector
Answer two questions to see which material families fit. Then send us the part and a polymer chemist will confirm the grade.
Order-size calculator
Enter your part weight and monthly volume to see how much resin you use.
Resin per month
Gaylords per month
Resin per year
Estimate only. Assumes about 1,000 lb per gaylord; the actual fill weight depends on the resin.
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