Industry Information
Rubber Molding Defects Explained: A Glossary for Seal Buyers
Short shots, flash, flow marks, vulcanized adhesive - what each molding defect term means, which process condition produces it, and the detection boundaries that matter when you specify inspection.
Rubber Molding Defects: The Vocabulary Problem in Seal Inspection
When buyers and suppliers talk about seal quality, the vocabulary of rubber molding defects - terms like "short shot" or "flow mark" - gets used as if everyone means the same thing. But the same word can describe different appearances on different parts, and each defect has a specific process condition behind it. This glossary gives the cause-side: what each defect is, and which molding condition produces it. What each defect does on the sealing line - the leakage path - is covered in Why Seals Leak: the surface defects vision inspection catches (/posts/seal-leakage-defects-vision-inspection), so this page does not repeat it. The defect families below are drawn from the inspection lists in our machine brochure - the molded-part list, the extrusion list and the tensile-ring list - and a family is tagged where it is line-specific. The causes are general molding engineering context.
Short Shots
A short shot is an incomplete part: the mold cavity did not fill fully, leaving a section thin, missing, or empty. On seals it appears as an incomplete cross-section or a missing lip segment. Cause: trapped air in the cavity, insufficient injection pressure or material, or a blocked gate. It is on our machines' molded-seal inspection lists - and a missing segment cannot be fixed downstream.
Pits, Voids and Bubbles
Pits are small depressions on the surface; voids are empty cavities inside the cross-section; bubbles sit on the extrusion line's list. Cause: trapped air or volatiles during curing, or insufficient packing pressure while the material sets. A void near the surface may show as a pit; a fully internal one is where camera-based checking meets its boundary - what cameras see is the surface.
Flash and Burrs
Flash is a thin fin of rubber that squeezed out at the mold parting line; burrs are excess at edges and gates. Cause: mold wear or misalignment opening the parting line, or excessive injection pressure - with trimming and handling feeding burrs as often as molding does. For what a fin at the parting line does to the sealing contact, see the leak-path article above.
Flow Marks
Flow marks (flow lines, known in some shops as knit lines) are surface seams where two flow fronts met and did not merge fully. Cause: injection speed too low, uneven mold temperature, or gate position. They matter in inspection because they are low-contrast - one of the harder surface families to image, which is exactly why marked flow-mark samples belong in your sample test package.
Mold Residue
Mold residue (mold fouling) is buildup from release agents, cure byproducts, or rubber debris left on the mold surface and transferred to the part. Cause: incomplete mold cleaning between runs. It earns its place on the inspection list because it appears and disappears with cleaning schedules - a part that was fine yesterday can carry residue today, with no change to the process itself.
Cracks and Cuts
Cracks are breaks in the surface; cuts are incisions from handling or trimming. Cause: demolding stress, sharp mold edges, or trimming and assembly errors as much as molding itself. One detection boundary worth knowing: a crack that stays closed while the part is relaxed may not separate optically until the ring is under pressure - which is why our brochure's extrusion machine checks 8-35 mm rings under applied pressure. For molded rings, whether a closed crack separates on your compound is a sample-test item.
Impurities and Contamination
Impurities are foreign particles in or on the rubber: release-agent residue, mold debris, or environmental contamination. Cause: dirty molds, poor handling, or airborne particles during curing. The detection boundary: a particle changes the surface appearance where it sits, but very small or subsurface particles are a sample-test confirmation item.
Dents and Deformation
Dents are local depressions from impact or compression; deformation is broader shape change from handling, poor support during curing, or post-mold pressure. Cause: handling and storage practice as much as the molding step itself - these families appear even when no press setting has changed. They matter for purchasing because the gland design assumes the shape the mold produced; the sealing consequence is the leak-path article's territory.
Discoloration and Material Mixing
Discoloration is a surface color change; material mixing means two compounds in one part. Cause: wrong compound loaded into the press, incomplete purging between batches, or mixed material at the machine. Our brochure lists material mixing as its own check item, visible where the mixing changes the surface appearance. The boundary: same-color mixing can be invisible to a camera - that is a material-control problem upstream of any inspection machine.
Vulcanized Adhesive (Tensile Ring Lines)
On the tensile ring lines, our brochure lists vulcanized adhesive - residue of the vulcanizing step - among the checks. Cause: the curing and joining steps themselves, which is why it appears on lines where parts are vulcanized or bonded rather than only pressed. Like other residue defects, it is visible where it changes the surface appearance.
Two Terms the Industry Uses That the Lists Don't Name
Two families that O-ring buyers meet daily sit outside the brochure's inspection lists. Backrind and spew - mold parting-line backflow and excess cured rubber forced out during cure - are appearance defects where they show at the parting line, but how your machine configuration handles them is a sample-test question. Out-of-round (ovality) is a geometry deviation, not a surface appearance - it shows between measured diameters rather than as a visible mark, so catching it depends on the dimensional checks confirmed at the sample test.
Frequently Asked Questions
What causes short shots in rubber molding?
Trapped air, insufficient injection pressure or material, or a blocked gate - the cavity simply did not fill. On seals it shows as an incomplete cross-section or a missing lip segment.
Which process condition should I fix first?
Your own defect history ranks that - the glossary lists possible causes, not your process's actual ones. Sort your recent NG log by defect family - the method in how to define defects for OK/NG inspection (/posts/defining-defects-ai-visual-inspection) maps directly to it - and the dominant cause usually shows.
Can inspection replace fixing the molding problem?
No - inspection screens parts; it does not change the mold, the material, or the press settings. Plants use the machine's NG statistics to see which family is growing, and the cause-side above to point at the likely process condition.
Does the machine catch every defect in these families?
Whether each family is caught on your parts, at your sizes and materials, is what the sample test demonstrates - with the images and OK/NG decision log returned to you. Same-color mixing and subsurface voids are typical boundaries to confirm on your parts.
Next Step
Sort your NG log by the families above, mark which process conditions plausibly feed them, and send your seals with the marked defect list. The sample test (/posts/vision-inspection-sample-test) returns the images and OK/NG decision log showing exactly which defects the machine catches on your parts. Contact the team (/contact-us) - Monica (+86 138 5726 5705) or Arya (+86 173 2430 9735), or email monica@aivisioncheck.com.
