| Effects of Hot Melt Running Temperature on Book Quality |
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| A host of problems affecting book quality and bookbinding productivity arises when the hot melt temperature gets too hot or cold. The effects include excessive penetration, wrinkling, degradation, poor cover adhesion, poor page pulls, poor hot melt application, and trimming difficulties. |
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| Running the glue pot at the wrong temperature is extremely common even on newer binding equipment. Perhaps the thermostat is faulty or has failed. Perhaps someone turned up the hot melt temperature and neglected to turn it down later. |
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| Too Much Heat |
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| When hot melt gets too hot, the viscosity thins. The temperature might have risen in the glue pot, but the operators may not be aware that they are applying less glue. Standard EVAs require laying down a 20-mil film. A thin 10-mil film, resulting from the overheated hot melt, is not strong enough to permanently hold pages together. The book will eventually fail due to poor page anchorage or a weak spine. |
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| When glue thins, it also penetrates too far into the pages until it pulls ink off pages near the spine. An unsightly book results. |
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| When binding, an overheated adhesive can cause wrinkling of pages in the spine area. When the paper is still hot (from the overheated glue), it drives the moisture away from the spine area. When the paper cools, the moisture attempts to return to equilibrium. Since the hot melt has already solidified, locking the dry fibers in place, the only expansion that can take place in the fibers causes a wrinkle. |
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| Another problem, cover shift, can occur from excess temperature. Since the hot glue loses tackiness as it thins, the book block can no longer hold the cover securely. |
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| Thin, overheated hot melt also causes smearing at the trimmer. The trimmer knives smear the glue, creating a messy looking book and causing the paper to tack together. If the edges are to be sprayed or guilded, an unsightly spot will occur on the smear. The overheated glue may also build up and harden on the knife. The overheated glue will also cause coatings or laminates on the cover to become weak so that when the knife cuts the book block, chipping of the cover occurs. |
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| Trimmer jams bring on unnecessary maintenance, and lost production time to stop the line and clean the trimming knife. Over three shifts, such stoppages can delay production of hundreds of books. Too much heat also degrades a hot melt. |
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| Too Cool |
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| Another set of problems, particularly weak adhesion and weak page pulls, arises when the hot melt temperature is too low. |
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| A hot melt running "cold" pre-cures, losing adhesion quality. Cover adhesion becomes poor as the hot melt's surface cools, preventing penetration into the paper fibers. |
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| Weak page pulls also result from operating below recommended run-ning temperature. Viscosity becomes too high, making it difficult to drive the glue up and around the pages' fibers. It again becomes difficult to wet out the surface at the edge of the paper. |
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| In a typical glue pot, this high viscosity glue will flow poorly, starving the second application wheel. This will result in gaps or voids in the backbone and ultimately poor book quality. In high speed machines, the pot could overflow behind the first wheel due to poor flow characteristics. |
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| Running at low temperature can also cause stringing because of the "cold" hot melt's thicker viscosity. As the book runs over the wheel, a string of glue pulls out of the book block. The machine gets messy as strings of glue collect on the moving parts. |
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| Pay Attention to Temperature |
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| These temperature-related problems seriously affect plant produc- tivity and can impact book quality. Having accurate temperature gauges and keeping a close eye on temperature variation is fundamental to good bookbinding. |
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