There are several factors to consider when selecting nip rolls or pinch rolls for any particular application:
Width: The first factor is roll width. It is always advisable to have the roller face at least 2” wider than the widest part that will go through the rollers. This will allow for lateral variation in placement of the substrates entering the rolls. It will also allow for cases where placement of the substrates occurs at a slight angle, which will cause the parts to travel sideways as they progress through the rollers, potentially missing a portion of the parts.
Roll Hardness: As the nip rollers press on a part, the surface of the roll flattens out against the part. This increases the surface area at the point of contact, and decreases the actual unit pressure (pounds per square inch) applied to the parts. On the other hand, if a roller is too hard, it can tend to bridge over minor variations in the substrate, leaving possible voids. The optimum roller will be soft enough to follow the surface contour of the parts but hard enough to squeeze out blisters and bubbles. For applications where the substrates are relatively flexible and high pressures are not required (such as label or print mounting) a soft face roller with hardness in the 40 – 50 Shore A durometer is ideal. If increased pressure is needed to ensure that parts come into full contact, the hardness of the roll surface should increase. For assembly work or bonding of pressure sensitive films a roller hardness of 60-65 Shore A will work well. The exception to this would be cases where the roller would need to conform to protrusions on the substrates or to shapes in the substrates. In these cases a soft faced roller with thicker rubber layer would be best.
Roller Movement: Since variations in part thickness are quite common, it is important to have some capability for one of the rollers to move sufficiently to follow these variations. This is almost always the upper roll. There should be sufficient travel in the moveable roller to follow the changes in shape while maintaining good nip pressure. The exception to this would be cases such as foam lamination where the substrate has sufficient “give” to absorb the thickness differences without the rolls changing position.
Initial Opening: Pinch rollers should be designed so that the initial opening between the rolls can be adjusted. The initial opening is usually set to be just slightly less than the thickness of the parts being laminated. This assures that the parts will receive sufficient laminating pressure but the rolls will not jump as the parts enter the nip point or slam together as the parts exit the rolls.
Both Rolls Driven?: In pinch rolls that are used for assembly of products that have a layer of wet adhesive on the surface, it is important that both rolls be driven at the same surface speed. This prevents putting any drag on one roller that could cause the substrates to slip relative to each other.
If the adhesive layer is very tacky or forms an immediate bond, it is possible to use a machine that has only one roll driven, since any shear load that is put on the adhesive layer would not be high enough to cause the parts to slip. The reason that some types of pinch rolls or laminators are built with non-driven upper rolls is primarily cost. It is more expensive to build a machine with both rollers driven.
On nip roll machines that are used as pulling machines, it is important that both rolls be driven. If not, all the pulling force is transmitted through only one surface of the substrate being pulled, and there is a greater possibility that slippage can occur.
Guarding: Because of the nature of nip rolls, the pinch point where the rolls come together must be guarded in some way to prevent injury. Several different types of guarding systems are used. On pinch rolls that exert relatively low pressures, a fixed guard may be sufficient to protect the operator. On pinch rolls with higher pressures better guarding systems are needed. Common methods of guarding can be fixed guards which keep the operator’s hands away from the pinch point, guard plates which can stop the rolls if the plate is touched, and light beams which trigger photocells if a foreign object (such as an operator’s hand) breaks the beam.
Speed: Since different types of work are laminated with nip rolls, the speed of the rolls should be adjustable. Some types of work may be sensitive and require lamination at low speeds, and other types of work can be put through the rolls at high speeds. To have the flexibility to handle all types of work, it is best to select a nip roller unit with variable speed drive. Hot laminating machines generally run at much lower speeds than cold laminating units, since time is needed for the heat from the rolls to melt the adhesive on the sheets being laminated. Variable speed is also important in this type of machine.
Roll Coverings: Most nip rollers are covered with either urethane or silicone rubber. The positive points of each type of roll covering are as follows:
· Urethane – is extremely durable and abrasion resistant. It is good for industrial applications where there will be many compression cycles. It is lower cost than silicone covering and offers greater durability. Urethane is resistant to most solvents so that rolls can be cleaned with a wide variety of solvents.
· Silicone – It’s more expensive than urethane, but has a tendency not to bond to other materials. If silicone rolls come into contact with adhesives or PSA films, the bonds to the silicone will be weak or will not form, making it easier to keep the rolls clean and to remove any adhesive residue. Silicone rolls also have a higher coefficient of friction than urethane rolls, and can generate higher pull forces. Silicone-faces rolls are used in hot laminators, since they will retain their properties for long periods at elevated temperatures.
Once all factors are taken into consideration, a nip roller machine can be selected that fits the particular application. In some cases, machines can be fitted with additional devices such as takeup spools for release paper or mandrels to hold rolls of substrate or adhesive films. Quite often this type of machine must be custom – built to fit the particular needs of a specific application. Whether your application can be done with a standard machine or requires a customized unit, please contact us to request more information or review your needs.