2005 Fall Archives » PostPress

by: Sheldon Cohen

Products that require two different vinyls to be welded to each other are at risk of distorting, usually in the form of wrinkles or bubbles. Understanding the causes of the distortion can be helpful in avoiding this problem. The three main factors that contribute to wrinkling are differences in plasticizers, differences in plasticizer levels and differences in rates of shrinkage.

I. Differences in plasticizers

What you should know
Although you might expect that “chemical” incompatibility be at the root of most wrinkling, it is, with today’s technology and vinyl manufacturing techniques, not the common cause for wrinkling. Chemical reactions usually manifest as random circular bubbles in the material.

The plasticizers commonly used in the calendaring of vinyl for heat sealing are liquids, usually with the viscosity of vegetable oils. They are added to powdered vinyl resins and are absorbed into the resin in a heating and mixing operation called compounding. The plasticizer molecules allow the resin molecules to “slide” against each other, which allows the vinyl film to be flexible.

There are many different types of plasticizers available that have been developed to provide specific end use properties. Most of the plasticizers that are used in vinyl calendering are from the phthalate group. The common ones are DOP (Di-n-octyl phthalate)*, DEHP, (Di (2-ethylhexyl) phthalate)*, andDINP, (Diisononyl phthalate). Note that DOP and DEHP are considered identical plasticizers.

Other plasticizers are from the adipate group. Adipates are usually used to impart better resistance to cold crack. A common one is DOA (Dioctyladipate).

Although the adipates are efficient plasticizers and perform very well in many applications, they have a different molecular structure than the phthalates, are more volatile, and have a greater tendency towards movement (migration).

What you can do
When welding two different vinyls together, check with your suppliers to be sure that the plasticizer systems that are used in each of their vinyls are compatible (i.e., from the same group). This is a routine caution and most suppliers will freely give such information.

Avoid using vinyl made from recycled scrap when welding two different materials. The formulations in the scrap can contain adipates and other incompatible ingredients.

II. Differences in plasticizer levels

What you should know
Differences in formulations are probably the most common cause for wrinkling. Wrinkling caused by differences in formulation can appear as a combination of irregular ripples and bubbles which are most seen near the corners of the welded product where the heat of the die has accelerated and exacerbated migration.

Every vinyl is formulated to perform in a specific end use application. The classic example is the end use differences in flexible opaque and clear vinyl. A producer of opaque vinyl for bookbinding would be primarily concerned with workability in the heat-sealing process, as well as some cold crack resistance, and would build his product accordingly. Since plasticizer acts to facilitate welding and helps to improve cold crack resistance, a typical opaque bookbinding formulation might contain 33-35 parts of plasticizer for every 100 parts of resin. Since the surface of the opaque product is usually embossed with a texture, the opaque producer is not concerned with the vinyl sticking together after sheeting.

The producer of double polished clear vinyl has different considerations. Since he will focus on transparency, his surfaces are as highly polished as glass. If he makes the vinyl too soft and flexible, it will stick together (block) when his customer sheets and stacks the vinyl. Therefore the clear producer will use very little plasticizer in his product (at the sacrifice of some welding and cold crack properties). Typical plasticizer volume in a “non-tack” clear vinyl formulation would be in the 24-25 parts of plasticizer per hundred of resin.

Here is a typical range of vinyl products from one manufacturer that shows their plasticizer content, going from soft to firm:

45 parts per hundred, 3-S
41 parts per hundred, 2-S
37 parts per hundred, 1-S (some medium bookbinding films)
33 parts per hundred, 1-H (most medium firm bookbinding films)
29 parts per hundred, 2-H (some clear non-tack overlay films)
25 parts per hundred, 3-H (most clear non-tack overlayfilms)

Even if the plasticizers in both materials are from the same chemical group, the plasticizer content of each material could be different. In a typical clear overlay binder, a material with 33 PHR plasticizer is placed in direct contact with a material with 25 PHR.

Because plasticizers are organic compounds, the materials, over time, will attempt to go into PHR balance. This means that plasticizer will attempt to leave the softer material and migrate into the firmer. When this happens, the firmer material (usually the clear) swells. Since the material is welded to the opaque and has nowhere to go, the displacement shows up as wrinkles in the clear. Another result is that as more plasticizer moves into the clear, the clear becomes softer than when initially produced.

Time is also a variable. Molecules of plasticized vinyl become more active as temperatures are elevated and will migrate faster. Another factor is how tightly the materials are pressed together. Heat, pressure, or any combination of both will accelerate migration.

What you can do
Specifying formulations that are closer together in PHR levels is one solution for very large custom runs but is impractical for everyday products. Be aware that decreasing the PHR in opaque will make the material more brittle and susceptible to cold cracking. Increasing the PHR in the clear vinyl will result in tackiness, which will make the clear vinyl difficult to handle for sheet fed work.

The best that the heat sealer can do when working with typical materials is to work to retard the transference of plasticizer:

Try to keep the thickness of the two materials as close as possible. A .006″ clear welded to a .014″ opaque will not absorb migrating plasticizers as well as a .008″ material and will show wrinkles more readily.
Wherever possible, slip sheet between the clear and opaque. This not only creates a barrier between the materials, the slip-sheet itself will absorb some of the plasticizer. Also, slip-sheets allow some air to circulate between the materials.
If the welded materials are stacked during or after manufacturing, keep the lifts as low as possible and never stack in lifts for long periods of time.
Ship products standing on end and mark the cartons: “This side up” and “Store away from heat.” Educate your customers accordingly.

III. Differences in shrinkage

What you should know
Both chemical and mechanical factors interplay to create shrinkage in vinyl. From a mechanical point of view, there must be some tension when winding up vinyl on rolls at the end of the calendar. A loosely wound roll would tend to telescope when moved. A tight wind helps to prevent the roll from flattening and reduces pressure marks during shipment and storage. Shrinkage caused by tension in wind up is called “snap back”. It usually occurs and is completed within a few hours after the vinyl is sheeted from the roll. A side effect of snap back is that as the vinyl shrinks in the calendaring direction, there is some slight increase in the width dimension. Attempts at introducing artificial shrinkage in low PHR vinyl by stretching at windup have not been successful.

A typical specification for mechanical shrinkage calls for not more than 1% change in dimension after 24 hours at room temperature.

After snap back is complete and as the vinyl ages, there will still be some residual shrinkage. Since flexible vinyl is a thermoplastic material, other stresses introduced in the manufacturing process will relax, causing the material to further condense over time. Another important consideration is the amount of plasticizer in the formulation. Softer vinyl tends to shrink more than firmer vinyl since their elasticity and elongation rates are higher. This brings us back to PHR of plasticizer differences in the formulations of two different vinyls that are being welded together. Normal bookbinding opaque vinyl with 33-35 PHR will tend to shrink more than a double polished clear vinyl with 25 PHR of plasticizer. Wrinkling occurs when two vinyls are welded together that have different rates of shrinkage. When one vinyl shrinks more than the other, the vinyl that shrinks less is displaced and this shows up as long undulating ridges of material. This wrinkling is exacerbated as the differences in the rates of shrinkage increase.

What you can do
Snap back can usually be handled in sheet fed work simply by giving the vinyl time to relax in low lifts after sheeting. Only a few hours is required to eliminate snap back problems. If roll feeding, dancer bars (accumulators) should be used to give the vinyl time to relax before reaching the welding die. Avoid introducing any tension in web feeding that could stretch the vinyl, especially when using softer materials.

With chemical shrinkage, since it is the nature of vinyl to shrink, heat sealers of two different materials should work with their suppliers to be sure that the vinyls that they are using have rates of shrinkage that are as close together as possible.

One highly respected bookbinder claims that he has minimized wrinkling by developing dies and heat-sealing techniques that weld the vinyl very tightly to the board and uses the board itself to staunch the vinyl from excessive shrinking.

As we have said before, when vinyl shrinks in the calendering direction, it can grow in the width. This sometimes causes a binder to look great coming off the heat sealer but “bag” a few days later. Vinyl sealed tight to the board will also minimize bagging.

Never cross grain materials when welding them together. Be sure that both pieces are taken from the roll in the same direction.

When packing heat-sealed products allow some space between the product and the top of the carton to absorb the weight of stacked cartons and not put pressure on the edges of the products.

Conclusion

Distortion (wrinkling), is an inherent problem when welding two dissimilar materials together. If the job is big enough to order specific vinyl formulations, the problem can probably be avoided at the outset if the heat sealer is willing to compromise some of the properties in each vinyl that makes them dissimilar. One can specify that the vinyl manufacturers use the same plasticizers and close the gap in the PHR proportions.

When this is not possible, the objective becomes to extend the amount of time that it takes for wrinkles to appear and to reduce the severity of the wrinkles by following some of the suggestions in this article.

The good news is that over the years, vinyl suppliers and their customers have made great strides in working together to create compatible products. The problems of incompatibility are much less frequent and much less severe today than they have been in the past.

Sheldon Cohen acts as Director of Marketing at Robeco, Inc. He can be contacted by calling (212) 986-6410.

by: Kevin Rickard, Rickard Bindery

In our industry, the mention of specialty folding often leads to discussions about intricate folding projects, such as the iron cross or some roll, gate, or accordion folds. But sometimes specialty folding simply refers to the ability to work with thick, thin or unusual paper stocks.

For specialty folding projects to be successful, bindery experts need to participate during the project design stage so they can help choose the right processes and equipment for each job. The best of these specialty folding experts can handle large volume projects and offer creative solutions to formidable challenges.

Specialty folding is, well, special, because an expert knowledge base is required to produce complicated folds, sometimes with glue, in high volumes with little or no error. Specialty folding experts know what equipment is suitable for which jobs. For example, buckle folders are less efficient with stock 12pts or thicker, and plow and knife folders should be used for such jobs. Knowing which resources to use prevents a lot of potential snags from becoming full-blown production problems. Some types of unusual miniature (at least one panel shorter than two inches) and oversized (large format) work qualify as specialty folding, too. Then there are the inline applications of EZ-release, permanent, or removable glue.

Machines and Paper Stress

Understanding specialty folding requires a grasp of science, or physics, to be exact. As sheets travel through the fold rollers and into fold plates, buckle folding machines apply different amounts of stress to the front and back ends of the sheets. Every fold has two sides: one heading into the plate, and the other being pushed from the back by fold rollers. The side of the fold that’s being pushed forms a “buckle” when the front end of the sheet hits the stop at the backend of the plate. As the sheet buckles, tremendous stress occurs in the paper on one side of the fold, but not the other. The side being pushed by the fold rollers gets a severe bend. If the stock is over 10 pts, fracturing of the paper surface or fibers can occur.

Folding speed stresses paper at a geometric rate of progression. For example, if the stress on a sheet is “5” when the speed is “5” (these numbers are only meant to be relative), doubling the speed to “10” means the stress jumps from “5” to “25” (instead of the linear “10”). In this case, doubling production speed translates to a five-fold increase in stress, perhaps enough to ruin a project. Therefore, many jobs that appear to be improperly setup are simply being run too fast.

Without proper planning, paper stress created during right angle folding can become a problem. The structure of paper is such that it is stronger under tension than under compression. When poorly designed folding sequences cause two panels to “fight,” the top one under tension always wins and the bottom one under compression always wrinkles. If a panel is trapped during right angle folding, wrinkling results.

In general, cracking usually occurs on the first fold. Why? As the buckle starts in the right angle section, the first fold bends around a very tight corner, greatly stressing the first fold. Unless preventative measures are undertaken, the result is poor quality.

Miniature Folds

Compliance demands led the pharmaceutical industry to find a way to include dosage instructions, risks, and even usage diagrams in all packaging – boxes, bottles, and other containers. That shift to miniature folding has caught fire in other consumer industries, such as personal electronics and cosmetics.

Now, companies seek to place even more information on larger sheets of paper. Folding those sheets down small enough to fit in the product package truly is specialty folding. When possible, get creative by using gate folds, onion skin stock, or maybe a miniature iron cross fold. The small size isn’t a limitation, but rather an opportunity for specialty folders to show off their creativity.

The applications for miniature folds in product placements continue to grow. For instance, companies that ship products to multiple countries must provide the same folded instructions to all markets, but in different languages. A specialty folder gives clients the option of including instructions in 10 or more languages on one sheet of paper, resulting in significant packaging and inventory savings.

Danger: Gatefolds

Running gatefolds multiple-up is fraught with danger. To avoid gatefold “pullout” (an unintended fold located approximately 1/8″ away from the intended fold), fold rollers should be loosely set. Although one-up work isn’t affected by running gatefold projects with loose fold rollers, multiple-up work certainly is. Since operators can’t get solid grips on gatefolds as they travel though the slitter shaft, slits are usually crooked, ragged, or both. Since two-up formats don’t yield much more product than one-up production, designing multiple-up gatefolds projects rarely makes sense.

Gatefolds may have short gaps (less than ¼” total gap), no gaps, and wide gaps (more than 2″ gap). Since most commercial gate folding plates don’t handle these types of projects well, make sure your bindery has the necessary specialty machinery for your job.

Tricky Folds

There are few universal rules about what’s possible in the world of tricky folds. Some projects are possible and some are not. This is true most commonly with folds with lots of panels, folds at unusual angles, diagonal folds, and unusual die cut shapes.

Naturally, some printing sales representatives try to avoid bidding on work involving tricky folds. But with a strong specialty folding partner, these types of projects become fertile sales areas. Printing sales representatives who figure out how to produce unusual projects are more valuable to clients than the ones who prematurely pass on tough requests. Here is a short list of some folds that are realistic and image enhancing:

Iron cross folding
Pop-up folding
Multi-directional die cut folding
Paper-doll style folding
“Staggered accordion” folding
“Swinger” folding
Narrow gap, no gap, and wide gap gate folding
No apparent side guide folding (ovals, circles, etc.)
Miniature folding with panels as short as 7/16″
“Layflat” miniature folding (with or without using glue)
Folding sheets as large as 80″ long

Expert specialty folding companies provide troubleshooting and design advice services to printers and their clients. Some of these companies have even invested in staff machinists and engineers that design and customize bindery equipment. When faced with a tough folding project, turn to a trade partner with expertise in folding. Specialty folding can be a profitable revenue stream with the right specialty folding company as a resource.

Kevin Rickard is vice president of Rickard Bindery and current president of the Binding Industries Association International. He can be reached at (800) 747-1389 or krickard@rickardbindery.com. Rickard Bindery serves printers coast to coast and specializes in discovering solutions to challenging bindery jobs.

Vinyl Compatibility: What You Should Know and What You Can Do