Exterior trim parts, like virtually all automotive components, live or die by mandates for zero defects and cost reductions. Siegel-Robert production facilities in Tennessee, Missouri and Kentucky produce grills, door handles, body side moldings, nameplates and a variety of painted and electroplated decorative parts for OEMs in the automotive and other industries.
Many automotive grills start out as injection-molded plastic components, and additional components are attached mechanically or thermally to complete the finished part. A typical grill has three to five components, including an inner and outer piece, a center bar and a badge that identifies the vehicle brand and/or model. Grills are assembled on two-station dial machines from HA Industries (Sterling Heights, MI) that completely finish a grill in one setup.
These modular machines simultaneously hot-stake 25 or more plastic bosses on a grill fixtured on one side of the dial, and they drive clip fasteners on another grill fixtured on the other side of the dial. Staking the bosses is a particularly complex operation, because the bosses are oriented at different angles and located on different planes.
A continuous improvement program was the genesis for developing optimum staking and clip-driving processes that help achieve ever-more stringent quality assurance and cost-reduction goals. As process innovations were developed and proven on newer machines, older machines were refurbished to enhance their performance as well.
Better Stakes = Better Grills
If the bosses are not staked correctly, they can work loose from road vibration once the grill is on the vehicle, so the assembly standards are quite strict. Staking a chrome-plated plastic boss to form a rivet on a plastic part is a tricky proposition due to the hardness of the chrome coupled with the relatively low melting temperature of the plastic boss. The chrome acts like an insulator, and can keep the plastic boss from softening correctly.
Past practice had been to cover the bosses during the chroming process so that there was no chrome on the bosses to interfere with the staking process. This proved to be expensive because it added a step-chrome plating the headed bosses-to the assembly operation. Skipping this step offered cost savings but finding the ideal heating formula, in terms of temperature and duration, was the major challenge.
If the temperature isn't high enough, the chrome plating prevents the boss from being correctly upset. If the temperature is too high, the plastic inside the chrome-plated boss melts to virtually a liquid state, but the chrome is still intact and resists pressure applied by the staking tool to form a head on the boss. This often leads to a sudden breakthrough when the chrome is crushed, resulting in a "splat" that yields an irregularly shaped, unacceptable rivet head.
Refining the staking process was a matter of running a series of heating tests to identify the optimum staking temperature and cycle time. A new heater tube was also 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.
Heaters and Tooling
Longevity of the heater was also an issue addressed in the continuous improvement program. The heater tube sustains a fair amount of punishment as it cycles up and down on the parts, and heater tubes were being changed out far too often. Increasing the cooling air flow past the heater, and switching to more durable heater wire, resulted in longer heater life. Heater controls were also modified so that the heating parameters can be varied to accommodate chrome-plated, plain or painted plastic bosses.
Heater failure detection is critical to ensuring that all bosses 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.
Staking pressure is another critical parameter. Insufficient pressure fails to form heavily chrome-plated bosses correctly. Excessive pressure is likely to cause a "splat." Again, a series of tests, correlated with various heating levels, was key to developing a modified cylinder that provides consistently correct pressure.
The chrome plating presented another challenge. Because of its hardness, the chrome wears the staking tools much more quickly than would plain plastic bosses. A combination of specialty steels for the tools, plus unique coatings applied to the tool tips, was developed to prolong life and minimize the adverse effects of the chrome plating. Fixture nests were also improved to ensure that painted or chrome-plated surfaces were protected from scratching that could be caused by embedded chrome material.
Driving Clips for Quality
Driving the clips completes the grill assembly. The clip-driving station incorporates a variety of sensors to ensure that the correct clip is loaded in the driver, and that it is driven correctly into the correct location on the part. For certain parts, the clips are so similar that color coding is used to differentiate them. For these clips, fiber optic sensors verify that the correct clip is in the driver at the appropriate time in the assembly sequence.
Every clip is checked four times as it is loaded into the driver and driven. A sensor located near the driver verifies that the clip is fully inserted in the driver. Each driver head is made for one specific clip. If a wrong clip is fed to the driver it won't seat correctly. This in turn triggers a fault signal that prevents the machine from cycling. Therefore no value will be added to what would be a bad part, and no parts are made with a clip missing.
After the machine cycles, sensors verify that the clip is no longer on the driver head. Another set of sensors verifies that the correct clip is actually on the part, and in the correct location. Finally, the clip driving module physically tests the clip as it retracts to verify that the clip is securely fastened on the part.
A monitor within the operator interface keeps the operators at each station aware of any fault in terms of incorrect or incorrectly loaded parts, unacceptable rivet heads, faulty clip insertion, or other assembly errors. The location of the fault is also displayed.
The continuous improvement program has increased both uptime and throughput efficiency. Everything one can do to prevent a bad part from slipping through is cost-effective, simply because delivering a bad part to an automotive customer is terribly costly. Continuous improvements in manufacturing efficiency and product quality are mandatory in serving the automotive, appliance and other durable goods industries.
For more information on heat-staking and clip-driving, call 586-939-0550 or visit www.ha-industries.com.