Landmark Plastic Corporation of Akron, OH specializes in the manufacture of plastic horticultural plant packaging and growing systems. One popular product is a six cavity transport tray thermoformed from recycled polypropylene. Continuous improvement efforts identified an unacceptably high reject rate upon start up caused by tear-outs in the raised corners of the part. This offered an opportunity to review process and tooling. The critical areas of the part are shown in Figure 1.

Figure 1. Critical Part Area

Landmark was using a plug assist made from HYTAC®-W. HYTAC®-W is an entry level epoxy based syntactic foam that works well for many applications. Following a review of application requirements and current results, CMT suggested Landmark run trials with HYTAC®-B1X. HYTAC®-B1X combines the toughness of an engineering thermoplastic with the low thermal conductivity of a syntactic foam. Compared to HYTAC-®W, HYTAC®-B1X has greater friction at the plug-sheet interface which allows it to carry more material into the mold and improve material distribution. Evaluation trials run with HYTAC®-B1X achieved dramatic improvements. With HYTAC®-B1X, the line started up faster and the amount of scrap was reduced to near zero percent. Parts made with HYTAC®-B1X and W plugs were measured for thickness to determine differences in material distribution. In each of the critical corner areas, six measurements were made as shown in Figure 2. As illustrated in Figure 3, the average thickness at the top of the critical corner area increased by 10% when using the HYTAC®B1X plug.

In addition, the minimum thickness values in the critical top corner area were 41% greater with the HYTAC®-B1X plug, as depicted in Figure 4. As a result of the trials, Landmark found the choice of plug material had a direct correlation with material distribution, startup time and scrap rates. Selection of a plug assist material with properties optimized for the sheet and application requirements resulted in significant material and operational savings.

Figure 2. Corner Measurements on Critical Tear-out Area

 

Figure 3. Average Corner Thickness

 

Figure 4. Minimum Corner Thickness

A large thermoforming company recently worked with CMT specialists to implement a new material specifically designed to meet the challenges of heavy gauge forming. The company had already switched from heated aluminum to HYTAC syntactic foam plugs several years ago, realizing a 12% savings in plastic usage due to more consistent and repeatable forming. The finished part is now made from 0.150″ (3.81mm) HIPS on both rotary and single-stage machines.

Thanks to a new continuous improvement initiative, the company wanted to increase plug durability and reduce down-time. An initial fact-finding discussion revealed that the mounting locations for the large plug had changed from prior years and was likely contributing to a stress mismatch between the plug and the base plate. Rather than stop at a change of mounting location, the company decided to replace their older plugs with a new HYTAC LPX formulation and monitor the process.

HYTAC LPX, a tough formulation with friction enhancers, creates a solid syntactic outer shell over a lightweight composite core. Over a period of several months, the company installed three HYTAC LPX plugs.

According to the plant manager, operators have seen dramatic improvements in performance: “Our plastic moves differently on the LPX which gives us the ability to pull material from the back of the part which is the top surface [of the plug]. This has been a problem with previous plug materials. In addition, the time frame to achieve a good part from a cold start is much quicker and we have been able to see some cycle time reduction.” The company has also reduced downtime thanks to proper plug mounting techniques and other best practices.

Ultra-low thermal conductivity of HYTAC syntactic foam allows for optimal forming while reducing stress in the sheet. Thanks to new formulations, HYTAC LPX is quickly becoming the syntactic foam of choice for the heavy gauge industry. LPX may be cast to near net shape and/or machined using conventional equipment.

It is one of our favorite thermoforming questions: “Which is more important – the plug material or the plug geometry?” It’s really a trick question, though, because the answer is usually, “It depends!” CMT Materials was recently approached by a customer with a stubborn plug mark on a cup. Together, we performed a test with 2 types of HYTAC® plug assist where multiple variables were involved.

The cups in question had a plug mark at 20mm from the bottom of the cup. After experimenting with the plug geometry and the plug material, there was a noticeable improvement in the appearance of the cup. It was not clear, however, if the improvement was a result of the different type of plug material or because of the different shape of the plug. In order to get a better understanding, we ran a controlled trial.

General details

• Machine: tilt-mold, cut-in-place
• Part: Ø70,5mm (2.77″)
• Material: White PP 1,05mm (0.041″)
• Machine oven temperature: 408°C / (766°F) (top and bottom)
• Sheet temperature: ± 150°C (302°F)

The test was run with 4 different types of plugs in the same row of the tool.

1. HYTAC XTL: Small Radius (R 4.0) Angle 4.0° (Cavity 5)
2. HYTAC FLX: Small Radius (R 4.0) Angle 4.0° (Cavity 3)
3. HYTAC XTL: Large Radius (R 8.0) Angle 5.0° (Cavity 2)
4. HYTAC FLX: Large Radius (R 8.0) Angle 5.0° (Cavity 4)

The machine was cycled for approximately 3 minutes with cups measured from the same shot.

1. HYTAC XTL: Small Radius (R 4.0) Angle 4.0° (Cavity 5)
   There is a plug mark at 20mm from the bottom of the cup. Enough material in the bottom; bottom thickness is good.
2. HYTAC FLX: Small Radius (R 4.0) Angle 4.0° (Cavity 3)
   There is a plug mark at 20mm from the bottom of the cup. Less material in the bottom; bottom thickness is acceptable.
3. HYTAC XTL: Large Radius (R 8.0) Angle 5.0° (Cavity 2)
   There is no line at the bottom of the product. Enough material in the bottom; bottom thickness is good.
4. HYTAC FLX: Large Radius (R 8.0) Angle 5.0° (Cavity 4)
   There is no line at the bottom of the product. Less material in the bottom; bottom thickness is acceptable.

Conclusion & Discussion
The plug with the small radius (R 4.0) created a plug mark at 20mm from the bottom of the product. HYTAC XTL carried more material to the bottom compared to HYTAC FLX. Plug No. 3, HYTAC XTL with Radius = R 8.0, gave the best result in this test. What the test revealed is that small changes can have a large impact on the quality of the final part. With increasing demands from global brand owners, converters must be able to meet quality standards from the first part to the last. These trials revealed that a deeper understanding of how plastic interacts with the both the plug material and the plug geometry can be the difference between a successful part and a rejected part.

The following comments came from a thermoformer who participated in a recent workshop where CMT was invited to speak. We think it provides excellent insight into the daily challenges faced by operators and how a small change can have an outsized impact.

“I took what was learned in your class concerning material flow over the plug assist and applied that to the design of our mold. I started with 1/8” offset from the cavity, applied blends in strategic areas based upon my new understanding, and had the tool maker polish the HYTAC® FLX plugs to a 1500 grit. These three things together resulted in a wall thickness that was more than double what we had seen in the part’s troubled areas.

I’ve only been involved with this project for a relatively short time (8 months or so), however, I know that over the years (several years, in fact) at least two engineers and a host of operators and tooling people have all tried their hand at carving plug assists, adding on to the mold length, and various other forms of ‘black art’ in the attempt to thicken up those troublesome thin walls. In short, the training we received from you proved that there is more of a science to thermoforming than most people recognize.” HYTAC® FLX is a top choice for optical clarity with PET and provides excellent polymer yields as a result of better material distribution. FLX has almost 3x the flexural toughness as standard thermoset epoxy syntactics. It is easy to machine and polish, offering a superb finish.

What does your plug do for you?