Electrostatic discharge (ESD) has been problematic for centuries. According to the ESD Association, “As early as the 1400s, European and Caribbean military forts were using static control procedures and devices trying to prevent inadvertent electrostatic discharge ignition of gunpowder stores. By the 1860s, paper mills throughout the U.S. employed basic grounding.” Various motion and activity can create static charge.
Preventing static damage is possible by selecting the proper ESD protective totes and packaging. ESD protective materials are categorized as conductive, static dissipative, or antistatic, and are measured by surface resistivity units of ohms per square.
Conductive surface resistivity
When conductive totes are grounded they bleed off their charge to the surface upon which they make contact, preventing electrostatic discharges. When conductive totes are enclosed with covers they also provide a Faraday cage —an electrically continuous conductive enclosure that provides electrostatic protection.
Conductive plastic material is usually made by compounding carbon particle material into plastic resin.
This process permanently changes surface resistivity, essentially changing an electrical insulator to electrical conductor.
Static dissipative surface resistivity
Static dissipative material should make direct contact with the electrostatic discharge-sensitive components. Many advocate for static dissipative material because the rate of discharge is slower than conductive material. Static dissipative surface resistivity can also be achieved by compounding carbon particles material into plastic resin.
Antistatic surface resistivity
Antistatic material resists high amounts of charge accumulation thereby preventing triboelectric charging when two dissimilar objects are separated. Antistatic materials do not need to be grounded. Some antistatic additives are dependent on the relative humidity of the environment to attract moisture to the material’s surface. However, the antistatic property may not be permanent. The additive may migrate to the surface and evaporate in the air.
Corrugated paper and paperboard
ESD protective corrugated paper and paperboard provide lower cost and protection. Typically, this is achieved by coating the surface of the corrugated paper or paperboard with carbon black. Some manufacturers also laminate the surface with foils and other materials to achieve ESD protection. However, paper packaging often emits small fibers, dust, and corrosive sulfur contaminates which can damage electronic components. Standard sizes are readily available from many manufacturers and industrial supply companies.
Molded plastic
ESD protective injection and thermo formed molded plastic totes are available in conductive, static dissipative, and antistatic material. These totes have a level of durability and can be reused many times. However, if the optimal molded tote design and size is not standard from a supplier, customizing molded totes can be cost prohibitive. For example, tooling costs can be in the five to six figure range for injection molded totes. Many times manufacturers require minimum order quantities that may exceed the need for the project.
Corrugated plastic
Profile extruded plastic corrugated is also available in ESD protective conductive and antistatic material in thicknesses ranging from 2 to 6mm. Conductive corrugated material is recommended for long term use since antistatic material will not give permanent static protection.
Corrugated plastic costs more than corrugated paper but it is more durable. Plastic corrugated containers are available in stock sizes from some manufacturers and industrial supply companies. Plastic corrugated is fully customizable for a wide variety of in-plant uses such as material handling totes with partitions, storage bins, and shipping containers. Prototypes and small production runs readily available.
Determine the use
The checklist below contains some design considerations and questions to be asked. As always, it is useful to obtain input from all parties involved in using the ESD protective material.
• Will the tote be used as an in-process, shipping, or storage container?
• What is the size of shelves, carts, conveyers, etc. that must store or move the tote?
• What is the part size or variation of sizes and weights that will go into the tote?
• Is there a certain part orientation in the tote that is ideal?
• Do the containers need to be stacked? If so, how high?
• What is the minimum part lot size, if any, per container?
• What is the maximum load that can be manually handled?
• Would using the tote vertically or horizontally be an advantage to save space?
• What is the best placement of hand holes for manual lifting to reduce awkward bending or reaching?
• Are hand holes enclosed for Faraday cage protection?
• Will the components be bagged? How will they be separated? By dividers or slotted guide systems?
• What is the component or part sensitivity to shock or vibration? Is there additional need for static protective foam inserts and dunnage?
• How is accompanying paperwork handled? Are document holders needed?
• Does the container need to be identified or tracked? Will it need to be labeled repeatedly?
• Does the tote have loose parts and can they be minimized?
Time frame and quantity of totes needed
To get an idea on how long an order will take, ask for a conservative lead time estimate from suppliers. For custom totes, consider lead time to complete tooling. Also consider country of origin to allow for any extra shipping time.
When trying to determine quantity needed for returnable “closed loop” containers, consider how many containers needed in your facility, in transit, and for outside vendors or customers. Some time allowance may be required for damaged and lost containers.
Sample prototype and testing
Ask for prototypes and samples from the supplier to confirm that the tote will fit the components and process.
Also confirm that the totes will fit on shelves, carts, conveyors, etc. as planned. A common mistake made when designing or selecting the container is that one may think in terms of inside dimensions and forget the effect that the outside dimensions may have on the material handling equipment.
We recommend getting a small quantity of containers for a realistic ship test. Confirm that the container will hold up in shipping and the contents will be undamaged. Use a surface resistivity meter to check if the surface resistivity is within required specifications.
Total cost
For custom orders, consider tooling and other onetime non-recurring costs. Also, consider what type of tooling is required and how long the manufacturer will store the tooling. For returnable shipping containers, plan for replacement costs. It is common that some will be damaged in shipping or lost by outside vendors or customers. Some companies plan for a loss of approximately three percent of the quantity of the returnable shipping containers in use per year.
Also consider return freight costs for returnable containers. This cost is frequently overlooked but can be reduced by using collapsible designs if feasible.
End of life
Many manufacturers have “take-back” programs that will pay a set amount per pound of container weight. Some companies specialize in buying used totes to refurbish and resell.
The last alternative is to have the totes recycled, disassembled or collapsed to make the regrinding and recycling process easier. Most ESD protective plastic is made from polypropylene, typically marked as a number 5 resin recycling code.
There is much to consider when packaging ESD-sensitive items. One faces options involving surface resistivity and material choice, along with the possibility for customization. Considering purpose, frequency of use, contents, variation of size, and the effect of outside dimensions are all essential in terms of packaging such components. This makes possibilities broad and finding the perfect fit a realistic goal.
References
1. Dangelmayer, G. Theodore. ESD Program Management: A Realistic Approach to Continuous, Measurable Improvement in Static Control. New York: Van Nostrand Reinhold, 1990.
Mills Industries is a third generation family owned and operated company. www.millsind.com
This article appeared in the April 2014 issue of Controlled Environments.
