Check the process, the mold, and the material.

* Successful troubleshooting requires a comprehensive analysis of the thermoforming process, mold design, and material. Thermoformers tend to focus on process modifications to solve problems. They may try longer cycle times, secondary rework operations, or equipment modifications. These olden have a negative impact on profits. When faced with a problem, we recommend a thorough and systematic analysis of all three elements of the thermoforming operation to pinpoint the exact causes of a problem and identify the most co-stand time-effective solutions. Below are three examples of how this approach worked in real-life situations.

When processing goes awry

In Case 1, a company produced thin-gauge horticultural products using reground HIPS and PETG on an in-line thermoformer. The company was experiencing more than 50% rejects. Parts were non-uniform and inconsistent, with thin spots and holes. Some parts were so distorted that they would not fit in trim fixtures.

After analysis, we concluded that the sheet used in making parts was not uniformly heated when entering the mold. The oven consisted of flat-panel heaters with a single control that set them all at the same temperature. When radiant heaters in an oven bank run at the same temperature they will always overheat the center of the sheet and under-heat the periphery. The chain rails, oven sidewalls, and openings where the sheet enters and exits can draw away heat from those areas of the sheet. Thus, it is essential that the oven heater banks be zoned in order to control the sheet temperature from side to side and end to end.

In this case, we brought in an infrared thermal-imaging camera to examine the temperature distribution across the sheet. It confirmed a significant temperature differential. Figure 1 shows a thermal image of the sheet before the problem was corrected. Temperatures at the edges (blue to red) were considerably lower than in the center of the sheet (yellow).

[FIGURE 1 OMITTED]

Individual control of heater panels was installed by means of PID temperature controls and thermocouples in each panel. Figure 2 shows the pattern of setpoints for the heater panels. As shown in Fig. 3, the zoned heating system produced a more uniform temperature distribution across the sheet. This is indicated by the yellow coloring throughout all but narrow edge bands on the sheet. The net result was to cut reject rates from 50% down to 10%.

[FIGURES 2-3 OMITTED]

Tweaking mold design

The company in Case 2 made automotive side panels and skirts out of PC/ABS. The problem was a complete inability to produce acceptable parts--a 100% reject rate. All the parts experienced severe bowing and twisting. The company instituted a labor-intensive secondary operation using 33 people to rework the parts so they would be acceptable for painting and shipping.

A manufacturing analysis revealed several problems: Mold temperature was not optimal, the mold vacuum system was inadequate, and sheet temperature was incorrect.

The first step was to determine proper mold temperature. The recommended temperature for ABS (185 F) was too cold for polycarbonate; the recommended 290 F for PC was too hot for ABS. We compromised at 240 to 245 F.

The second step was to determine proper sheet temperature. The recommended forming temperature for polycarbonate would burn the ABS. The recommended temperature for ABS was too cold to form PC. A temperature of approximately 340 F was determined to be the optimum temperature for this blend.

To produce the part without prematurely chilling the sheet, very fast forming was required. The mold design was revised to better distribute the vacuum by incorporating an open vacuum chamber on the back side of the mold. In addition, the capacity of the vacuum surge tank was increased to 10 times the volume of the parts being formed.

These changes cut the reject rate to less than 1%. The secondary hand operation was reduced from 33 people to three. The vast majority of parts now go directly from the forming station to the paint operation.

Is the material correct?

Case 3 involved producing plastic food containers. The application required a material with low odor and taste, heat resistance to withstand retorting, barrier properties to provide one-year or longer shelf life, and good formability. The material was a multilayer sheet of impact-modified PP with an EVOH inner barrier layer.

Upon start-up two major problems were immediately apparent: Parts had inconsistent wall-thickness distribution and they failed odor and taste testing.

In this operation, the sheet passed directly from the extruder to the forming station while supported by a conveyor belt. Inconsistent wall thickness resulted from poor melt strength. Excessive sag was evident in the unsupported length of sheet between the extruder die exit and the beginning of the conveyor belt to the forming station. Distortion of the sheet at the moment it left the die resulted in inconsistent formed parts.

We also noticed an unusual smell where the sheet exited the die. That led us to suspect that the sheet was being overheated and was charring--most likely the source of the odor and taste problem. Attempts to modify extrusion conditions to eliminate the charring and excessive sag were unsuccessful.

We then focused on the incoming raw material. To thermoform PP in the melt phase requires a high-molecular-weight, low-MFR material. The processor had requested a fractional-MFR grade from the resin supplier, but when we checked the MFR of the incoming pellets, we found that the MFR was twice what had been requested. The specified MFR was 0.5 g/10 min. The material as received was greater than 1.0 g/10 min. As shown in Fig. 4 propensity of a PP to sag increases with MFR. As a result, the entire lot of material was rejected and returned.

[FIGURE 4 OMITTED]

Further investigation revealed that the resin supplier had contracted a compounding shop to blend the impact modifier into the material. In an effort to increase throughput, the material was extruded at high temperatures. In doing so the material was degraded and burned.

A MFR specification of 0.6 g/10 or less was placed on all incoming material. The new specification forced the compounding house to lower the melt temperature and alter extrusion conditions to prevent material degradation. The result was a material that possessed adequate melt strength, which eliminated the wall distribution problem. Since the material was no longer overheated and charred, 100% of formed parts passed the odor and taste test.

Donald Hylton is an associate with the thermoforming consulting firm McConnell Co., based in Fort Worth, Texas. He is also associate director of the T.A. Cole Research Center for Science and Technology at Clark Atlanta University, Atlanta, Ga., where he does research on thermoforming. Hylton has more than 30 years' experience in thermoforming materials and process development with ExxonMobil Chemical Co. and General Electric. He can be reached at [phone] or e-mail [e-mail].