If you run a Heidelberg Speedmaster CD 74, you have almost certainly seen it: a faint, uniform film of ink creeping into the non-image areas of the sheet, usually starting at the lead edge or in the middle of the page. Press operators call it scumming \u2014 or tinting, or catching up \u2014 and on a CD 74 it is almost always a dampening problem, not an ink problem.<\/p>\n
Scumming looks different from other plate defects. A plate gone bad from exhausted fountain solution shows up as a slow build-up over hundreds of sheets. Scumming from the dampening unit tends to appear quickly, responds when you raise the water feed, and comes straight back the moment you lower it. When you see that pattern, the first place to look is the Alcolor dampening system \u2014 specifically the metering roller and the dampening form rollers.<\/p>\n
The CD 74 runs Heidelberg’s Alcolor continuous dampening system. A chrome metering roller runs against the water pan roller, and the film it carries is transferred to the dampening form rollers, which lay the fountain solution directly onto the plate. The whole unit only works when every roller in the chain is clean, round, and set at the right pressure.<\/p>\n
Scumming happens when the non-image areas of the plate stop receiving enough \u2014 or consistent \u2014 fountain solution. When those areas run dry, the desensitized coating can no longer repel ink, and the ink takes hold. That thin grey wash you see across the sheet is ink sitting where only water should be.<\/p>\n
Three dampening-roller failures are behind the vast majority of CD 74 scumming:<\/p>\n
Before swapping parts, work through this check in order:<\/p>\n
Once you have found the cause, the fix is usually quick.<\/p>\n
For a glazed form roller, you can sometimes recover it with a dedicated roller wash and a light re-conditioning, but if the rubber has hardened through, the only lasting fix is replacement. Reconditioning compounds can buy a few weeks; they cannot restore a roller whose surface is permanently gone.<\/p>\n
If the metering roller<\/a> is scored or dull, replace it \u2014 field resurfacing is rarely economical. While the roller is out, replace the small consumables that sit on the same shaft, because a worn O-ring<\/a> or a stripped bolt will throw the new roller’s setting off just as badly as the worn chrome did.<\/p>\n For pressure faults, re-strip the unit properly. Set the form rollers to the plate first, then work back through the dampening chain. A few minutes with the stripe gauge cures more scumming problems than any part replacement.<\/p>\n Then re-establish the chemistry: check the fountain solution pH and conductivity, confirm the alcohol (or substitute) dosing is correct, and make sure the water feed is steady. A dampening system can only do its job when the chemistry is right.<\/p>\n When you do replace rollers, keep these CD 74 dampening parts in stock so a scumming call does not turn into a full day of downtime:<\/p>\n A clean, correctly-pressured dampening unit should run a CD 74 for weeks without scumming, as long as the fountain solution is maintained. If the scumming comes back after a roller change, re-check the chemistry and the pressure before you blame the new parts.<\/p>\n When in doubt, replace the metering roller and the form rollers together. Running a half-worn dampening set is the fastest way to chase your own tail on scumming.<\/p>\n Disclaimer: RelyOffset parts are OEM-compatible aftermarket components. Brand names are trademarks of their respective owners.<\/em><\/p>\n","protected":false},"excerpt":{"rendered":" If you run a Heidelberg Speedmaster CD 74, you have almost certainly seen it: a faint, uniform film of ink creeping into the non-image areas of the sheet, usually starting at the lead edge or in the middle of the page. Press operators call it scumming \u2014 or tinting, or catching up \u2014 and on […]<\/p>\n","protected":false},"author":4,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_yoast_wpseo_metadesc":"Heidelberg CD 74 scumming is usually a dampening problem. Learn how worn metering and form rollers cause tinting \u2014 and how to fix it fast.","footnotes":""},"categories":[1241,1244],"tags":[],"class_list":["post-9815","post","type-post","status-publish","format-standard","hentry","category-heidelberg-parts-guide","category-troubleshooting"],"_links":{"self":[{"href":"https:\/\/relyoffset.com\/de\/wp-json\/wp\/v2\/posts\/9815","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/relyoffset.com\/de\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/relyoffset.com\/de\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/relyoffset.com\/de\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/relyoffset.com\/de\/wp-json\/wp\/v2\/comments?post=9815"}],"version-history":[{"count":1,"href":"https:\/\/relyoffset.com\/de\/wp-json\/wp\/v2\/posts\/9815\/revisions"}],"predecessor-version":[{"id":9871,"href":"https:\/\/relyoffset.com\/de\/wp-json\/wp\/v2\/posts\/9815\/revisions\/9871"}],"wp:attachment":[{"href":"https:\/\/relyoffset.com\/de\/wp-json\/wp\/v2\/media?parent=9815"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/relyoffset.com\/de\/wp-json\/wp\/v2\/categories?post=9815"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/relyoffset.com\/de\/wp-json\/wp\/v2\/tags?post=9815"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}Roller replacement on a CD 74 \u2014 what to keep on hand<\/h2>\n
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Clean vs. replace \u2014 a quick rule<\/h2>\n
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