Automotive Grille Assembly Yields Process Efficiencies

Exterior trim parts, like virtually all automotive components, live and die by zero defect and cost-reduction mandates. Component suppliers have looked beyond molding process economies to examine every aspect of producing the part. In many cases, the needed process controls and efficiencies have come from straightforward, semi-automated operations.

As a full-service supplier of injection-molded plastic components, St. Louis-based Siegel-Robert Automotive & Diversified Products focuses heavily on the "value added" part of the equation. Production facilities in Tennessee, Missouri, and Kentucky produce grilles, door handles, body side moldings, nameplates, and a variety of painted and electroplated decorative parts for OEMs in the automotive and other industries.

One good example of their attention to detail can be found within the evolution of "assembly assists" used to produce automotive grilles. Process economies for both clip driving and hot air staking have paid off, via 2-station dial assembly machines that completely assemble grilles in one setup. The system represents a commitment to continuous improvement to the assembly process, and a number of older machines have been refurbished with the problem-solving innovations discussed here.

Better Stakes Make Better Grilles

Staking a chrome-plated boss (or tab) to form a rivet on a plastic part is a tricky proposition due to the hardness of the chrome, contrasted with the melt temperature of the plastic boss. In the past, Siegel-Robert had covered the bosses during the chroming process so there was no chrome on the bosses to interfere with the staking process. Since it added a step to assembly operations, this proved to be expensive.

"We wanted the cost savings of skipping this step and then dealing with the problem of getting a properly formed rivet despite the ''full metal jacket'' on the boss," said Rod Schuh, plant manager at Siegel-Robert''s SR of Tennessee facility. "Zero defects is the name of the game. If the tabs aren''t staked correctly, they can work loose from road vibration once the grille is on the vehicle. So we''re held accountable for some very strict standards."

The staking problem arises because the chrome acts like an insulator, keeping the plastic boss from melting. Finding the ideal heat formula, both in terms of temperature and duration, was a major challenge: if it''s not hot enough, the combination of plastic and chrome prevents the boss from being properly "upset." If it''s too hot, the plastic inside the chrome-plated boss melts to virtually a liquid state, though the chrome is still intact. When the staking tool then begins to reform the boss, the chrome provides unwanted resistance to the staking pressure, causing a sudden breakthrough when it''s crushed. The result can be a "splat" that yields an irregular-shaped, unacceptable rivet.

Siegel-Robert worked with HA Industries (Sterling Heights, MI) to refine the staking process. HA is a specialist in designing and building machines and systems that optimize the assembly of functional and decorative plastic components, such as automotive interior and exterior trim. Hot air/cold stake systems, as well as fastener driving machines, are their mainstay.

HA tackled the challenge by working with Siegel-Robert engineers on a series of heat tests to find the optimum staking temperature and cycle time. A new heater tube was designed and built for the application. Once proven out, the heater became the standard for the process, with features such as individual heater element monitoring to ensure continuous process control.

Heater longevity was also an issue. "The heater tube has to withstand a fair amount of punishment as it cycles up and down on the parts," Rod explains. "We were changing out heaters far too often, so we worked with HA to improve the air flow and the quality of the heater wire. Now we get much better heater life."

Also researched and refined was the pressure used during the staking process. If there''s not enough pressure, the more heavily chrome-plated bosses could not be properly reformed. A series of tests, cross-referenced with heating parameters developed for this application, resulted in the creation of a modified cylinder that provided consistently correct pressure.

HA has, to date, built numerous 2-station dial machines for Siegel-Robert for automotive grille assembly purposes, as well as many other single-station fixtures and machines for other assembly applications. Designed from modular components, HA Industries'' machines are custom applied to simultaneously stake 25 or more bosses on these grilles at a number of angles and planes. On the dials, staking is performed in one station, and the fastener-driving operation is completed in the other. The optimized staking process balances the heat requirements with the goal of minimizing cycle time for the highest throughput per machine.

Keith Morie, program manager at Siegel-Robert, explains: "Six or 7 years ago the heating controls were very inflexible. Now HA can vary the heating parameters based on whether or not the tabs are chrome-plated or painted. Our customer can also design the grilles with stakes at strange angles and in places that, in the past, were too difficult to get at. The multiple-piece grilles are really a more complex assembly than you might think. This is because of the high number of tabs, clips, and mating components in each assembly . . . and every part of it must meet our customers'' quality standards."

Tooling Issues

A second challenge was dealing with wear on the staking tools. The chrome plating tended to eat away at the stake tips, causing them to wear excessively. HA looked at a number of specialty steels and tip coatings to determine the best combination to prolong wear and minimize the effect of the chrome. They then modified the tools to prevent accelerated wear.

Fixture nests were also improved to ensure that painted or chrome-plated surfaces were protected from scratching caused by embedded chrome material.

The second station on the dials drives the clips for completion of the grille assembly. The station incorporates comprehensive sensing technology to ensure that the proper clip is put into the driver and that it actually gets driven into location on the part. For certain parts, the clips are so similar that color coding is used to differentiate them, along with fiber optic sensors to verify that the correct clip is in the driver. During assembly, clips are checked four times, at the following stages:

• Clip is on the driver: A sensor located near the driver verifies that the clip is fully inserted in the driver. The wrong clip won''t seat correctly in the driver head and the fault will be detected; the controls will then prevent the machine from cycling. The result: no value will be added to what would be a bad part, and no parts are made with a clip missing.


• Clip has left the driver: After the machine cycles, sensors verify that the clip is no longer on the driver head. In many assembly applications, this is where the sensing and verification ends, but since the clips may have fallen out of a driver, or fallen off the part, or may not have been fully seated on the part, a final check is needed.


• Clip is in place in the part: Another set of sensors verifies that the right clip is actually in the part, and in the location, as it should be. On some machines, where the clips are very similar, the color of the clip is sensed for verification. The fiber optic sensors are designed so that fluctuations in plant lighting don''t affect their performance.


• Clip will stay on the part: HA and Siegel-Robert devised a way for the clip-driving module to physically test the clip, as it retracts, to ensure that the clip is securely fastened on the part.

"This attention to the clips has really evolved into a new level of assembly inspection for this type of product," said Keith Morie. "Anything and everything you can do to prevent a bad part from slipping through is cost effective, simply because a bad part delivered to an automotive customer is going to cost you more."

On the 2-station dial machines, a monitor within the operator interface keeps both operators (one at each station) aware of any fault in terms of incorrect or incorrectly loaded parts, clip insertion, or other assembly parameters. The location of the fault is also displayed. A typical grille has 3-5 components, including an inner and outer piece, a center bar, and a badge.

Heater Failure Detection also plays an important role in assuring that all parts are fully staked. If a heater malfunctions or expires, an alarm sounds and the machine won''t cycle. The monitor indicates which heater must be replaced.

Problem-Solving Partnership

"HA engineers and our people work together well as a team," said Rod. "HA is very thorough, and they listen well. By continuing to look at how we can improve the process, we keep problems from happening. The best relationship is one where you share responsibility to, first, make it work, then make it even better." Prior to designing and building the first of the dials, HA spent time in each of the Siegel-Robert plants to get suggestions regarding machine reliability, durability, and maintenance.

Keith Morie sums it up: "Our program of continuous improvement has given us uptime and throughput efficiency that is right where it needs to be to meet our customer''s production requirements. In this environment, where we have to continuously look for manufacturing efficiencies and improve our product quality, it''s the only way to operate."