Challenges with Screen Printing Polyethylene and Polypropylene
How do you determine what type of hot stamping die is the best choice for different plastic applications? The two basic choices are silicone rubber and metal dies. However, the subject is much more complicated than metal versus silicone rubber. Let us take a closer look into both and explore the potential applications of each.
Several factors must be analyzed before deciding on a metal die for hot stamping a plastic part or product. The main difference that a metal die will provide is an actual debossing of the image versus laying the image on the surface of the part. This may be the look you want for either aesthetic or functional purposes. Aesthetically, the debossing of the image can provide a slight 3-dimensional look. Functionally, by basically melting the image into the plastic, the hot stamping foil may last longer than an image that is applied with a rubber die. In addition, if you are hot stamping a word or phrase, debossing the image into the plastic will still be readable even if the hot stamping foil eventually wears off.
For the most part, metal dies are limited to flat plastic parts or sheets. If there is much of a contour or unevenness to the part, silicone rubber is the best choice. If the part has an extremely uneven or dimpled surface, you may want to consider pad printing the image (see “Spotlight on Pad Printing,” page 7, Plastics Decorating Jan./Feb. 2000).
There are several different types of metal dies to choose from. Here is a summary of the most common metal dies.
Steel dies are machine engraved and are practically indestructible. Steel may be the best choice for extremely long runs or plastic parts that are hot stamped on a continuous basis. The downside to steel is the high expense of manufacturing the die and the relatively low heat conductivity of steel versus other die metals. However, it may pay for itself with reduced downtime over very long runs. In addition, steel can be machined to fit contoured shapes for certain applications.
Brass dies can be machine engraved or photo-etched. Machine engraved brass dies can be used for similar applications as steel, although brass is not nearly as durable. However, the cost savings versus steel are considerable, and brass is much stronger than magnesium and somewhat stronger than copper. Brass also holds its heat very well, which becomes important in high-speed hot stamping applications. A photo-etched brass die is a good choice if there is a detailed graphic or a lot of copy in the image. However, copper and magnesium etch much easier and deeper than brass, making them a better choice in most circumstances where a photo etched die is being used.
Photo etched copper dies are an excellent choice for detailed graphics or a large amount of copy. Copper etches reasonably fast and is available in different etching reliefs. It also provides a durable die choice at a price range less than brass or steel. As you would imagine, copper is also an excellent heat conductor and recuperates heat quickly for high-speed applications were the motion of the hot stamping press can cool the die and foil.
Magnesium is the most common choice for hot stamping plastics with a metal die. It can be photo-etched extremely fast and is available in a variety of etching reliefs for certain applications. It is the least expensive type of die and turnaround time is usually only 1 or 2 days. The draw back to magnesium is the softness of the metal. Steel, brass, and copper are all much stronger metal choices. Magnesium is the best choice for short to medium sized runs for small plastic parts or advertising specialty applications.
Silicone rubber has become the most popular choice for many hot stamping and heat transfer applications. The most obvious reason silicone rubber is such a popular choice is its ability to conform to the surface irregularities that are present in most molded plastic parts. Because of this, silicone rubber requires less pressure and produces a more consistent quality stamp, resulting in fewer rejects and faster set-up times.
Silicone rubber is formulated in several different hardnesses (durometer), depending on the application. 80 durometer is the most common and is used on the majority of plastic stamping applications. However, there are many situations were a different durometer is recommended. Below are guidelines to follow:
Hard Durometer Rubber – (85, 90, & 95 durometer) – recommended for very fine copy or graphics.
Soft Durometer Rubber – (50, 60, & 70 durometer) – recommended for bold copy or graphics, applying heat transfers, or tipping raised graphics with a flat or contoured pad.
High Temperature Rubber – Withstands high temperature applications (up to 600° F) without permanent deterioration of the die.
High Conductivity Rubber – Transfers and recovers very quickly. The best choice for very high-speed applications.
Dual Durometer Rubber – Has a hard durometer rubber on the face to hold fine copy without distortion and a softer durometer on the backing to allow for part variation (commonly the face is 95 durometer and the backing is 60 durometer).
Zoned Durometer die – Includes a hard durometer rubber in areas for fine copy and a softer durometer in areas with bold copy or graphics.
It is important to remember to be cautious of the heat stability of a silicone rubber die for two reasons. The silicone rubber is bound to a metal backing for structural stability and thermal conductivity reasons. Exceeding too high of a temperature on the face of the silicone rubber will shorten the life of the die itself and cause premature break-down of the rubber-to-metal bond.
Silicone rubber dies for hot stamping and heat transfer applications are available in a variety of configurations, including sheet stock, molded flat dies, molded contour dies, and rollers.
Sheet stock is silicone rubber bonded to a metal backing, usually aluminum and sometimes steel. The silicone rubber is molded in different thicknesses ranging from 1/32″ to 1/4″. Sheet stock is used for tipping graphics and beads that have been molded into the part as well as for applying heat transfers to flat plastic surfaces.
Molded flat silicone rubber dies are made from photo engraved molds (usually magnesium) with the image molded in the silicone rubber surface. The height of the raised image on the silicone rubber is called the relief, with 1/32″ or 1/16″ being the most common. Molded flat dies are used for stamping all types of plastic parts with reasonably flat surfaces.
Molded contoured dies are manufactured the same as flat dies except the mold is machine engraved to match the contour of the part being stamped. It is very important that the foundations are machined to exactly match the contour of the part to allow a uniform thickness of rubber on the molded die. Contour silicone rubber dies are used to hot stamp rounded plastic containers or other parts with a contoured shape.
Silicone rubber rollers are used to apply hot stamping foil in a continuous stream for extruded parts and large flat or slightly contoured surfaces. Picture frames and the banding of bottle caps are good examples. For large coverage areas of foil, rollers are used to eliminate air entrapment where vertical hot stamping machines are impractical because of the tonnage required to apply the large foiled area.
Although choosing the correct type of hot stamping die is an important part of the hot stamping process, there are many other factors to consider. The tooling, or nesting fixture that holds the part in place, the machine size and drive type, the type of foil or heat transfer being used, the type of plastic, and the shape of the part all have a great deal of impact on the final results. It is always best to consult your die manufacturer and provide all the details of your hot stamping job to assure that you are using the right tools for the right application.
Plastics Decorating would like to thank Ted Schwerdtle of Schwerdtle Incorporated (800-535-0004) and Keith Hillestad of United Silicone Inc. (716-681-8222) for their assistance with this article.