Silicone Transfer Pads
Trans Tech transfer pads are available in a variety of shapes, sizes, and hardness. Constructed of silicone rubber combined with silicone oil, the function of the transfer pad is to transfer the image from the printing plate to the part. The shape, size, and hardness of the pad work in conjunction to effect the appearance of the printed image.
There are two basic shapes that all transfer pads imitate. The first and most common design is the cone-shaped pad. This shape consists of a pad body with a defined tip. The pad body radiates down and away from the tip at a specific angle. Most rectangular, square, and oblong pads utilize this feature. The second basic shape is commonly referred to as a “V-pad”. The V-pad resembles a rooftop with an apex running the length of the pad. The pad body radiates outward and downward from the apex.
Transfer pads vary dramatically in size. The smallest can weigh a few ounces, while the largest can weigh more than 50 pounds. Pad size is determined by a number of factors, including image area and machine dimensions.
Hardness, or durometer, is the third key variable in transfer pad design. Most manufacturers offer a variety of durometers to meet the diverse range of pad printing applications.
At Trans Tech, five levels of durometer are available. When measured on a 00 scale, durometer ranges from 30 shore to 70 shore with each level progressing at 10 point increments.
Let's Get Into the Details!
The transfer pad is able to pull the image out of the printing plate due to the ink tack that is created as the plate is doctored clean. As the pad compresses onto the plate a rolling effect occurs. This rolling action pushes air away from the image. Greater angles of descent enable less air entrapment, resulting in the elimination of pinholes. Conversely, shallow angles of descent prompt air entrapment and will result in the creation of pinholes. The same principle applies to the action of the pad compressing and then releasing the image onto the part.
For this reason, it is best not to position the image on the tip of the pad. The tip represents a shallow angle of descent. Whether you are working with a cone or v-shaped pad, the image should always be slightly offset from the tip or apex. Transfer pads with highly defined tips and apexes (sharp radius) will provide better print quality than pads with slightly defined tips and apexes (shallow radius) considered before making this decision.
Sizing a transfer pad to a specific application requires that a sufficient mass of silicone be present to avoid image distortion. As the pad compresses onto the printing plate and part, the print surface of the pad is absorbed into the pad body. The pad body acts as a support mechanism for the pad’s print surface. If the print surface is not adequately supported, it will deform, resulting in image distortion. It may seem that the simple solution is to oversize all transfer pads. However, a few factors need to be First, the transfer pad must fit into the machine. Specifically, it must clear all obstacles that are present (plate holder, doctor blade holder, ink cup, etc.) during the machine cycle. Secondly, when compressed, the pad must not stretch into the ink buildup that is present on the plate perimeter. Third, the machine must be capable of compressing the mass of silicone present. If these three criteria cannot be satisfied, then the pad is too large.
Transfer pad durometer, plays a key role in print quality. Generally, the harder the transfer pad, the better the print quality. A harder pad, when compressed, will maintain its shape much longer than a pad of the same design with a softer durometer. This retention of structure results in a better rolling effect and therefore is less likely to trap air.
Transfer Pad Maintenance
Transfer pad life ranges from 10,000 cycles to 50,000 cycles, depending on the application. Print surfaces that are flat and free of ridges or protrusions will promote longer life than surfaces that contain these factors. Through extended periods of operation and contact with solvents that are present in ink, the silicone oil within the transfer pad becomes depleted. As the pad dries over time, its ability to pick up and release an image is severely reduced. Proper maintenance of transfer pads can significantly enhance pad life.
One easy step includes the use of silicone oil (Long Life Oil; part # 141112). After each shift, remove the pad from the machine and rub a small dose of oil onto the pad’s surface. Then set the pad onto the shelf, positioned on its base. This will allow the newly applied oil to penetrate the pad body, thereby replenishing the oil that is lost in production.
Another recommendation applies to those facilities that utilize a multi-shift operation. It is best to alternate pads and not let them be used in consecutive shifts. The combination of applying silicone oil with pad rotation will greatly enhance pad life.
Now that we have reviewed the basic design features, we need to apply them to specific product situations. The first step in choosing the proper transfer pad begins with identifying the desired image size. Most transfer pad catalogs include image size capability with the individual pad illustrations. If the image size is 1” x 4”, then we need to choose a pad that has this capability. Remember that a pad with the capacity of 2” x 6” is the better choice than a pad with a capacity of 1” x 4”as it will probably be larger and be less likely to distort the image. However, be careful that the pad fits into the machine.
The next consideration will be the part’s surface. Specifically, you will need to know whether the surface is flat, curved, smooth or textured. On a flat and smooth surface, a standard durometer pad may be utilized. If the surface is flat but textured, a harder durometer pad is required. A pad with a harder durometer will do a better job of forcing itself into the bottom of the texture while retaining its basic shape.
A softer pad will deform by expanding at the sides and will bridge the texture peaks at the print surface. This will leave a void at the bottom of the texture. If a pad with a harder durometer is unavailable, a pad that utilizes a steep angle will perform the same function. Pads with softer durometer, the softer silicone will better conform to the part’s surface. The radius of the part will act to increase the pad’s angle of descent, reducing the potential for pinholes. A part that exhibits a smooth surface, but contains a radius, typically require s the use of a softer durometer. The softer silicone will better conform to the part’s surface. The radius of the part will act to increase the pad’s angle of descent, reducing the potential for pinholes.
Custom pads are available from most manufacturers and are designed for a specific application. A custom pad may be created with a void in the center to allow printing on a part like the skirt of a range knob. Other unique designs have been manufactured to allow printing within an electrical fuse block. Custom pads address the same issues discussed in this section. Contact your Consumables Specialist today to see if a Custom Pad is right for your new or current application.