CASE STUDY OF ATTRACTIVE QUALITY CREATION:

APPLICATION OF ROTOMOLDED POLYETHYLENE FOAM IN VERTICAL AGITATORS

Alongkorn K., Sirinun M.

Specialty Products Division

SCG Plastics Co., Ltd.

1 Siam Cement Road, Bangsue, Bangkok, 10800, Thailand

ABSTRACT

This paper presents a case study of Attractive Quality Creation (AQC) of SCG Plastics Co.,Ltd (thereafter refer to as SCG); a rotomolding resins supplier, through the development of rotomolding-grade polyethylene (PE) foam in a vertical agitator.

Due to the inherent nature of rotomolding process, the resulted parts tend to be hollow and of relatively medium-stiffness materials; as a result, rotomolded products are not suitable for demanding applications. Recognizing this shortcoming, SCG has made the initiatives to develop and introduce a rotomolding-grade PE foam to the Thai rotomolding industry. Some case studies of successful applications are discussed here. Initially, Customer ‘P’, one of its rotational molding customer, constructed the vertical agitator with stainless steel sheets, which is prone to corrosion, heavy in weight, and time-consuming to produce. Therefore, Customer ‘P’ would like to manufacture the part in PE by rotomolding process. However, PE by itself would not be stiff enough to withstand the hydrodynamic pressure experienced by the agitator when in service. To delight Customer ‘P’, SCG introduced the use of PE foam into their latest version of vertical agitator and created attractive qualities into their new product. To shorten the development time with the customer, SCG has deployed the latest technology in rotomolding computer software simulation and in real-time RFID to predict and measure molding conditions. SCG’s introduction of PE foam as reinforcement materials combined with new molding technologies has provided rotomolding customers with the possibility of many more demanding applications which has never been realized before with polyethylene. Specifically, Customer “P” is able to reduce the production costs and the fabrication time of the new vertical agitator, thus able to pass on the saving to its end customer resulting in its customer retention.

Keywords: Attractive Quality Creation (AQC), Polyethylene, Polyethylene Foam, Rotomolding, Rotational Molding, Computer Simulation Software, Real-time RFID

INTRODUCTION

Vertical agitator has been used in waster water treatment. It is normally fabricated from stainless steel sheets welded together, which is both time-consuming and labor-intensive. Because of its stainless steel construction, the agitator is also very heavy and requires high-torque motor to rotate the unit. Customer ‘P’ has developed their own design with steel as shown in Figure 1, however, his product was not well accepted due to very high power consumption from weight of steel and high corrosion from water pollutes.

Figure 1 - Steel vertical agitator and its brochure

Therefore, Customer ‘P’ would like to manufacture the part in polyethylene (PE) by rotomolding process as PE has higher chemical resistance and is not corroded. But due to the inherent nature of rotomolding process, the resulted parts tend to be hollow. PE by itself would not be stiff enough to withstand the hydrodynamic pressure experienced by the agitator when in service. Recognizing this shortcoming, SCG has made the initiatives to develop and introduce a rotomolding-grade PE foam to the industry.

CHALLENGES AND CRITICAL SUCCESS FACTOR

There are three major different options that are commonly used to produce this product, first by engineering plastic, second by PE rotomolding, and third by other metal. We try to evaluate the possibility of each technique as follow exhibit;

Table 1 –Evaluation of material and method to perform the task

Stiffness / Weight Reduction / Large size Molding / Water corrosion resistant / Investment / Total Score
Engineering plastic (ABS and Nylon) / 5 / 5 / 2 / 5 / 2 / 19
PE Rotomolding / 3 / 5 / 5 / 5 / 5 / 23
Other metal / 5 / 1 / 5 / 4 / 3 / 18

Note: weight 20% in each quality attribute

Score Rating 1 = the least, 5 = the most

Conclusion from evaluation in each quality attribute was shown that PE by rotomolding process should answer the question of customer ‘P’. So we decided to explore in details about implementing the scenario. The target of this task was to increase the stiffness of conventional PE to meet the requirement and increase productivity by reducing the total weight of new vertical agitator at 30%.

QUALITY AND TECHNOLOGY BREAKTHROUGH

SCG Plastics has been developing PE foam for rotomolding application but has limited success in developing the application. Parts of the reason were due to two reasons:

  1. Over 90% of rotomolding application in Thailand is water tanks, which do not need such a high stiffness.
  2. PE foam is difficult to process. It needs precise cooking temperature to activate the foaming and maintain bubble stability.

So the results of many experiments were not satisfied because we were able to mold PE as skin layer but PE foam inside was not developed well as shown in Figure 2.

Figure 2 - Failure of PE foaming after trial

To make PE foam more forgiving, several modifications were made:

1. To increase bubble stability, PE base resin with higher melt strength was selected.

2. To reduce runaway chemical reaction, less of exothermic chemical blowing agent was used.

Technology implementation

As a result of R&D investment strategy, in the second phase of PE foam application development, SCG Plastics has obtained an internal air temperature measurement device with radio telemetry capability called Rotolog®. This equipment turns out to be instrumental in helping the development team to successfully mold PE foam into the vertical agitator.

Rotolog® gives real-time measurements of the internal air temperature inside the mold, which rotates inside the enclosed oven as shown in Fig. 3. Having this piece of information helps the molders to decide exactly how long to heat the part without undercooking or overcooking it.

Figure 3 - Rotolog® equipment for measure internal air temperature

Figure 4 - Data of internal temperature vs. cooking time from Rotolog®

RESULT AND BENEFIT

The new version of vertical agitator that made of PE weights 52 kg, compared to the old one that made of stainless steel, which weighs 110 kg. It can be equipped with a motor ranging from 7 h.p. to 30 h.p., enable slow to fast agitation.

Figure 5 - Material usage comparison

There was a 60% reduction in the production costs by using PE foam in place of stainless steel. Due to this costs reduction, customer ‘P’ is able to pass on the saving to its customers by 40%, resulting in more units being sold.

Figure 6 - % Production cost saving

The field testing was done at a customer site, which was a tapioca-based enzyme production plant. The new PE vertical agitator was installed in a wastewater pond and run side-by-side the existing stainless steel ones for 24 hours a day, 7 days a week. A field visit by SCG Plastics team was made to observe the vertical agitator in operation and to interview the end-user. The plant manager had expressed satisfaction with the performance of new vertical agitator, which required less power consumption and was maintenance-free.

Figure 7 - Successful of PE foaming after the second phase development

Figure 8 - New vertical agitator from rotomolding process

CONCLUSION AND FUTURE PLAN

After we offered the latest technologies computer software simulation and in real-time RFID to predict and measure molding conditions, as a resultcustomer “P” is now earning more margins and acquires better product performances. After successful product testing for less than a year, customer ‘P’ has begun selling the rotomolded PE-foam vertical agitator as its standard product line. We are studying with the customer ‘P’ to apply more stiffness resins and to explore in new design of agitator which is targeted to consume less power than the existing model that would save operating costs of users about 20-30 %.