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The InfraStake Process - Seeing the Light
by Scott Beute, Extol, Inc.
The practice of forming a molded stud or boss in order to
retain another component is nothing new. Long ago, engineers recognized the
benefits of eliminating glue, screws, and other fasteners, and as a result, they
developed several different methods of heat staking that have been around for
quite some time.
backdrop, a nonconventional staking process called InfraStakeģ, a relative
newcomer in the business, is continuing to gain popularity. As the name
suggests, the energy source is infrared light, and the heat transfer mode is
radiation (the safe type). The InfraStake module clamps the parts together,
heats the boss evenly with precisely directed infrared light, and forms the
plastic with a non-heated punch driven by a low-force pneumatic cylinder.
Several established methods for staking plastic parts exist.
These methods typically can be used for any type of thermoplastic. Some of the
more common materials that are staked include polyethylene, polypropylene, ABS,
polycarbonate, acetal, and polyamide. The process also works with plastics that
contain fillers such as glass or talc. The traditional methods include hot air
staking, thermal staking (heated punch), and ultrasonic staking.
Although these processes all melt and form plastic, they each
have some weaknesses when it comes to minimizing cycle time and producing parts
with acceptable joint strength. Some of the weaknesses are poor part clamping,
stress-induced show surface marking opposite the stud (ultrasonics), high
operating costs, danger to machine operators, plastic stringing or sticking to
the punches, and process variation. The struggle to overcome these weaknesses
ultimately led to the development of the InfraStake process.
The InfraStake process can be broken down into four basic
steps: Clamping, Heating, Forming, and Retracting.
Industry experts have recently discovered that plastic parts
are not always known for their dimensional stability. One of the first
challenges when staking plastic parts together is making certain that they are
properly assembled. Because actual parts donít always match the CAD models, an
assembly process that can compensate for some part variation (warp) can be a
The InfraStake process clamps the parts together at each
stake point, ensuring that they are firmly pressed together throughout the
entire cycle. This completely eliminates the need for secondary clamps in most
The InfraStake process works with materials of all different
colors and transparencies. Dark, opaque materials absorb infrared energy more
quickly than light, translucent materials and typically allow faster cycle
times. The energy source used is a 12-volt, 100-watt technical-grade halogen
lamp, not unlike those used in many automotive fog lamps. Once the lamp is
energized, a reflector directs the infrared energy from the filament into a
column around the punch. This column of energy travels downward until it is
focused on the full perimeter of the boss by the concentrator.
Itís Safe. The
InfraStake module transfers the infrared energy from the lamp to the boss
through radiation. Because the working area is completely encapsulated, the
operator cannot access the area where the infrared energy is being focused once
the cycle has begun. Even when the operator can contact the outside of the
InfraStake module during the staking process, the surface temperature remains
safe to the touch. This is possible because the reflective surfaces are plated
with gold, which is very efficient at reflecting infrared energy. This improves
both the safety and the efficiency of the process by keeping the energy where it
is needed, and not anywhere else. Without the risk of burning the machine
operator, a simple press mechanism often can be used on machines that would
otherwise require heavy guarding, a shuttle, or a turntable for operator
Itís Energy Efficient. The ability to focus the infrared
energy precisely where it is needed also minimizes the power consumption. Unlike
a typical hot-air system, which uses a 400-watt heater for each stake point, an
InfraStake module uses only a 100-watt lamp. Since the price of electricity
hasnít been going down recently, this has a real impact on the operating cost of
the equipment. Additionally, because the system does not have any internal
components that have to Ďwarm upí before use, no preheating is necessary.
Finally, the system uses only a small amount of cooling air, which minimizes the
amount of compressed air required.
Itís a Controlled Process. The InfraStake process heats
the boss uniformly until the plastic reaches a semi-molten state. The infrared
energy is focused only on the portion of the boss that extends through the
mating part, so there is very little chance of affecting the show surface
opposite the boss. The standard round profile works well for many different boss
shapes, but the reflective surface geometry also can be tailored to non-circular
bosses, like blade or tab studs, if necessary.
The process parameters for the heating portion of the
InfraStake cycle are quite simple. The lamp is either on or off, and the amount
of infrared energy transferred to the boss is determined by the amount of time
that the lamp is on. The other parameter that affects the heat cycle is the
cooling airflow rate. A small amount of cooling air flows through the InfraStake
module during the staking cycle for several reasons. It stabilizes the
temperature of the lamp and lamp holder and maintains positive pressure in the
area surrounding the polished reflective surfaces. Positive pressure keeps the
surfaces clean by keeping out dust, airborne contaminants, and potential
out-gassing contaminants. The cooling air also helps regulate the surface
temperature of the boss to prevent overheating. Finally, the airflow keeps the
punch relatively cool so it can effectively remove the heat from the plastic
once it makes contact with the boss. The InfraStake controllers error-proof the
process by monitoring the current draw of each module during the heat cycle and
alerting the operator if there is a problem.
The InfraStake body has two integral tracks that accept
proximity sensors triggered by the position of the punch coupler magnet. These
sensors can be used to verify that a boss is present before the cycle and to
verify that the punch has extended completely at the end of each cycle.
process produces consistent radial heating of the boss. As a result, a
low-force, pneumatic cylinder driving the punch is sufficient to form the stud,
minimizing stress and potential part damage. This is especially advantageous for
PC board assemblies and other delicate applications.
The punch cavity is designed to match the volume of plastic
in the unstaked boss. This ensures that each stake is fully formed. The punch is
gold-plated to reflect the infrared energy, and its location (outside of the
focal point of the reflective surfaces) and the cooling airflow help it maintain
a reasonable temperature so it can draw heat out of the molten plastic quickly
and prevent sticking. When staking glass-filled materials, which can be
abrasive, the punch detail often is machined from A2 tool steel, which is
hardened to resist the abrasion.
Several different geometries can be used for the punch
cavity. For round bosses, the dome style and the rosette style are quite common,
and the strength is actually very comparable between the two. In fact, the
diameter of the clearance hole in the mating part often affects joint strength
more than the punch geometry. The dome punch design minimizes the distance that
the plastic must be displaced, and the lack of a center pilot reduces its
sensitivity to bosses that are slightly off-center. As a result, it often can
provide a larger process window than a rosette design. Rectangular shapes also
can be used if necessary when staking features such as tabs.
The final step in the InfraStake cycle is to retract the
punch. Now it is important that the plastic has resolidified to the point where
it can maintain its shape and structure. If it does not, the stake joint may
loosen or the plastic may stick to the punch. The InfraStake concentrator
functions as a stripper plate, holding the part down and preventing it from
lifting as the punch retracts. The InfraStake module is only removed from the
part after the punch has retracted from the staked boss.
process can be used to stake bosses of many different sizes. The IS125 model,
which is the most popular size, is 31mm (1.25 inches) in diameter at the
concentrator. Due to recent design improvements, it can now stake bosses up to
8mm (0.313 inch) in diameter and up to 12mm (0.5 inch) tall. There also are two
other standard modules, the IS170 and the IS230, which can stake larger bosses,
but those applications are far less common. In cases where the bosses are too
close together for standard modules to fit, custom heads also can be designed.
The InfraStake technology offers a flexible approach to
joining plastic parts. Whether the application requires a complete machine or a
package of integration components, the InfraStake process can provide unique
advantages over conventional staking methods through safety, efficiency, and
Extol, Inc., Zeeland, Mich., is a manufacturer of innovative plastics joining
equipment and custom, automated assembly machinery. Extol can provide complete,
turn-key production InfraStake machines ranging in size from small standard
presses to large, custom machines. The company also offers a complete line of
InfraStake components and controllers specifically engineered for integration.
To contact Scott Beute, manager - technology and education, call (616) 748-5029
or e-mail Scott.Beute@Extolinc.com. To contact Bill Reed, national sales
manager, call (616) 748-9955 or e-mail Bill.Reed@Extolinc.com. For more
information on Extol, Inc., visit www.Extolinc.com.