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Cutting costs, with no loss, of making shelf-stable packaging.

Plastic packaging for shelf-stable foods previously sold from refrigerator cases represents a rapidly growing segment of the food-packaging market. However, because no single resin matches all requirements for container performance and economics, multilayer structures have been developed to take advantage of the benefits that individual resins can bring to the finished product. A benefit obtainable from polypropylene (PP), for example, is resistance to distortion in microwave ovens; polyvinylidene chloride (PVDC) can provide a barrier to oxygen transmission. While polystyrene (PS) might be used for its good thermoforming properties, ethylene vinyl acetate (EVA) can be used as a tie layer for dissimilar resins.

The use of multilayer structures for shelf-stable food packaging began in the early 1980s. Sales of these containers are expected to reach 2 to 3 billion units/yr by the year 2000, thus providing a sizable market for the resin suppliers: the projected resin requirements for sales of this magnitude are as high as 60 million lbs/ yr.

Multilayer containers can be made by a variety of processes, including coextrusion blowmolding, solvent-based coating of monolayer thermoformed containers, and pressure thermoforming of coextruded or laminated sheet. Each processing option has its own parameters that affect when and where it will be used. However, a common thread throughout is the need to supply an economical container that meets all performance requirements. How to improve the economics of the process while maintaining container performance is the subject of this article.

Although we focus on a coextrusion and thermoforming process, the principles described in this article may be applied to any container-forming operation. Manufacturing Economics-Controlling waste. The production of multilayer containers generates waste in many places (as illustrated by the sheet coextrusion and thermoforming processes). Manufacturing coextruded sheet generates about 0.5% waste sheet during start-up and shutdown. It is significant to note that the length of time between shutdowns strongly affects the quantity of start-up and shutdown waste. Processing material for "getting on spec" can easily produce an additional 2% waste. This amount is influenced by the product mix; shorter runs produce more waste. Process disruptions, either from equipment breakdowns or difficulties in resin processing, can add another 1 % to the waste. Making a prime product also generates waste, primarily in the form of edge trim. This waste, plus such miscellaneous items as samples for quality control, can contribute another 4% to 8% to the total waste figure for coextrusion. By the time the coextruded sheet is made, waste can constitute 10% or more of the processed material.

In a thermoforming operation, the major contributor to total process waste is web scrap, or skeleton, which can constitute 15% to 50% of the coextruded sheet, depending on container shape, web width, and type of forming. Start-up and shutdown operations for thermoforming equipment can produce an additional 1 % waste; off-spec containers, another 0.5%.

If all of these contributions to process waste are combined, total waste from production of multilayer containers could be as much as 63% of the coextruded sheet. Regardless of source, all material that does not leave the shipping dock as prime product is waste. If the manufacturer disposes of the waste, he sacrifices material, time, and money, and increases the waste material's impact on the environment. However, manufacturers have other options: they can recycle much of the waste by reusing material, and thus reduce the overall waste stream. They can also adapt their processes, through more efficient operation or better design, to minimize waste. Either approach should be a top priority for reducing costs in any multilayer-container manufacturing facility.

Downgaging. As manufacturers continue to strive for the most performance for the least cost, they must constantly reevaluate the benefits that each layer brings to a given container. For example, the manufacturer can consider using a thinner layer of barrier polymer that has lower permeability to oxygen. If the material has better barrier properties at higher cost, the question arises as to how economical it will be. Often, simple calculations involving resin price, specific gravity, and permeability can help answer this question. In some cases, however, a change in barrier material requires still more changes in tie resins or in the positioning of the barrier layer within the structure. To effectively evaluate these and other similar situations, one must consider all relevant factors.

Structural layers can also play a role in downgaging. Although it is possible to change to a new structural polymer, technological innovations in resin manufacturing and in sheet and container processing often allow thinning of existing layers. Any consequent advantages to the processor should be calculated and balanced against the cost of implementing the proposed technology.

Increased production rate. Assuming that all output can be sold, an increase in production throughput can lower the cost per unit by spreading the fixed costs over more containers. However, rarely do all pieces of equipment, such as extruders, have matched capabilities. Often, one extruder will limit an overall increase in rate; if its extrusion rate can be improved, new questions arise as to the potential limits of other extruders. One must consider the capabilities of the equipment when calculating cost reductions from increased production rates.

Mass balance. Determining the effect of cost-reduction options on manufacturing cost is a primary concern of manufacturing engineers. A mass balance around the manufacturing processes can show the effects of waste minimization and recycling, and can be used to optimize an existing process with respect to selecting resins.

One should include waste, scrap, and recycle streams in the mass-balance equations. Waste is all of the processed material that is not product; it is divided into scrap, which is thrown out, and recycle," which is reused. "Recycle" is expressed as "recycled material" in Figs. 1-3. By convention, recycle ratio equals pounds of recycled material divided by pounds of material extruded. Similarly, scrap ratio equals pounds of scrap divided by pounds of material extruded. One can easily add other operating costs, such as utilities and labor, to the material costs in approximating the true manufacturing cost.

The basic steps to develop the mass-balance equations are: balance all flows around the processes; use balance equations to calculate pounds of each raw material and additive; and multiply pounds by resin cost to determine product cost.

The overall mass balance sets the total pounds of feed equal to the total pounds of product plus the total pounds of scrap. The mass balance around the coextrusion process sets the total pounds of feed plus total pounds of recycle" equal to the total pounds of coextruded sheet. The mass balance around the forming process sets the total pounds of coextruded sheet equal to the "recycle" plus scrap plus product.

In a similar manner, individual component balances for each component of the container should be made. For example, if xb denotes the weight fraction of the barrier resin, the individual balance for the barrier resin can be stated as:

xb x total feed = (xb x total

product) + xb x total scrap).

It is important to remember that at steady state, the composition of each stream is the same. Therefore, xb is the same in the feed, product, scrap, and all other streams.

After the overall and individual balances have been made, they may be simplified and combined with the definitions of scrap and recycle ratios. One may use container-structure definitions, such as volume percent of barrier layer and tie layer, and process designators such as scrap and recycle ratios, to calculate the pounds of each component required to make a container. By multiplying these individual parts by their respective costs, one can derive an estimate of raw material costs for a container. Addition of other costs, such as depreciation, labor, utilities, and maintenance, will yield an estimate of cost for manufacturing a multilayer container.


The Table describes a hypothetical multilayer container whose oxygen ingress of 1.0 cc/yr is based on the permeability, provided by a SARAN (Dow Chemical Co.) barrier polymer, of 0.10 cc x mil/100 in' x atm x day. Figure 1 shows the calculated manufacturing cost, including operating costs, for the multilayer container described in the Table. A cost of $500/hr was estimated for equipment depreciation, labor, utilities, and other operating expenses-container cost is $46/1000 units (see point A in Fig. 1) if none of the 50% waste that is generated is recycled.

Economics for the container improve as more waste is recycled; this is apparent by following the "50% waste" line in Fig. 1 to the right, toward higher recycle ratios. At "50% recycled material," the cost to manufacture this container drops to $38/1000 containers (see point B). The line labeled "40% waste" shows the effect of improving process efficiency from 50% waste to 40%: equivalent manufacturing costs are obtained at the points of "40% waste with 6% recycled material" (point C) and "50% waste with 50% recycled material" (point B).

Figure 2 shows the container costs and effect of recycling for a container that uses ethylene vinyl alcohol (EVOH) polymer for the barrier layer. The thickness of EVOH necessary for allowing 1.0 cc/yr oxygen ingress was calculated using a permeability for EVOH of 0.179 cc x mil/100 in [.sup.2] x atm x day. The permeability value was estimated from published data for a 44-mole % ethylene polymer at 23 degrees C and 85% RH. However, simply calculating that a recycle rate has favorable economics does not mean that the technology is available. Today, 45% is the maximum percentage of recycled material found in a commercial container.

Figure 3 shows the overall effect of waste reduction on economics. Just reducing the generation of waste from 60% to 50% is as economical as recycling all of the 60% waste.

Sensitivity Analysis

Sensitivity analysis shows the effect that various factors have on container costs. Some items to consider include barrier polymer performance; changing the thickness of one or more layers; changing fixed or variable costs; changing extrusion rate; and changing color loading. The effects of each factor are easily calculated, and are shown in Fig. 4. In the examples shown, waste reduction is again the best way to reduce cost; recycling is second. This type of analysis is useful in evaluating the effects of other, smaller variables such as tie-resin price.

Sensitivity analysis can be used to focus the reduction of manufacturing costs on high-impact factors. It is also helpful in determining expectations of how competing processes or products will compare with each other.

Other Costs

The manufacturing costs calculated with the techniques described above constitute only a portion of the costs associated with multilayer containers. Costs of research, sales, administration, advertising, and design are just a few of the costs that are often incurred in addition to those discussed here.

In any event, mass-balance calculations nd sensitivity analysis yield valuable information for reducing the costs of manufacturing multilayer containers. By minimizing and recycling waste, manufacturers can improve product economics while maintaining product performance.

(Tables and other figures omitted)
COPYRIGHT 1990 Society of Plastics Engineers, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Author:Powers, J..; Jenkins, S.R.
Publication:Plastics Engineering
Date:Mar 1, 1990
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