Packaging technology

Published: 31-01-2012, 14:20

Plastic bottle design

Like many of today’s popular packaging techniques, blow molding of plastic bottles became popularized soon after WorldWar II (see Blow molding). The original applications took advantage of the flexibility of plastic material to create squeeze bottles for dispensing of deodorants or medicines. The availability of reasonably priced higher-density polyethylene for rigid containers in the 1950s led to the widespread use of plastic bottles for detergents (1).

As both molding and plastic material technology developed, conversion to plastic bottles expanded beyond household products into health and beauty aids, foods and beverages, and general goods. By the last decade of the twentieth century, most soft drink bottles were converted from glass to plastic, and oil and other automotive product containers were changed from metal to plastic. The 1990s and early twenty-first century brought technical breakthroughs in materials and manufacturing that allowed the economical production of high barrier, multilayer and high temperature resistant plastic bottles. As a result, foods and drugs requiring hot fill and good flavor, odor, oxygen, and vapor barriers (such as sauces and baby food) began to be packaged in plastic bottles and jars in increasing quantities.

The advantages of plastic over glass bottles are abundant. They are safer, lighter, easier to handle for consumer and manufacturer, and easier to manufacture, and they offer improved versatility in design. However, many of the earlier conversions suffered because of the marketer’s attempt to imitate the original glass bottle as closely as possible. It was feared that the consumer (especially in beauty aids) would perceive the change to plastic as strictly a cost-savings measure resulting in reduced quality. Today, the majority of plastic bottles are designed to take advantage of the material’s unique properties, and the consumer has come to expect all the inherent advantages in the items they purchase.

In the 1970s and 1980s, plastic bottle designers and manufacturers were faced with widely fluctuating plastic material and additive costs and availability. The latter 1980s and 1990s added the additional challenge of environmental concerns. The early twenty-first century has brought us dual concerns: high energy prices with increased costs and the potential for shortages, along with a renewed interest in environmental considerations. The package must not only minimize material usage to reduce costs and environmental impact, it must also take into account its eventual disposal.

BASIC STEPS IN THE PLASTIC BOTTLE DESIGN AND DEVELOPMENT

There are several basic steps for the proper design and development of plastic bottles. They can be summarized as follows:

  1. Define bottle requirements—product to be contained, use, distribution, aesthetics, and environmental issues. 
  2. Define manufacturing and filling requirements—types of molding available and filling and packaging systems. 
  3. Select materials. 
  4. Rough drawings. 
  5. Part drawing. 
  6. Model and/or rapid prototyping. 
  7. Mold drawing. 
  8. Unit cavity. 
  9. Unit cavity sampling and testing. 
  10. Finalize drawings. 
  11. Production mold. 
  12. Production mold sampling and testing. 
  13. Production startup. 

This is a basic list of steps for bottle production. Some items may be done in a different order, and some can be performed concurrently to save time. Many items will be performed more than once as the design is refined. In addition, mating fitments and secondary packaging are frequently being developed in the same timeframe and become part of the critical path.

DEFINING BOTTLE REQUIREMENTS

What Will the Bottle Hold?

Information about the product to be contained is of primary importance for bottle design, particularly in the selection of material and neck finish. Will the product be liquid, powder, or solid? What is its viscosity? Is it homogeneous, or does it tend to separate out? Will the product have to be shaken to be used? What plastic materials and additives is it compatible with? How sensitive is it to moisture or oxygen gain or loss? Does the product have components (such as those found in detergents) that could make the bottle prone to stress cracking? Will the product outgas into the headspace and cause a pressure buildup? Will the product absorb oxygen from the headspace and tend to collapse the bottle? Will the product be filled into the bottle hot or cold? How much product should the container hold (in weight or volume)? Is the product considered hazardous or in some way regulated?

WHAT ARE THE SPECIAL REQUIREMENTS FOR BOTTLE PERFORMANCE?

Product End-Use.

How will the product be used by the ultimate end-user for dispensing and storage? Examples of this are squirt bottles, roll-on bottles, trigger-spray bottles, drainback closure/measuring cup systems, bottles with integral funnels, and bottles that are stored on their caps. What are the storage conditions (temperature, humidity, pressure) of opened and unopened containers?

Secondary Packaging.

Will the primary package be sold in an intermediate secondary package? Examples of this are carton, blister, and tray.

Distribution Requirements.

Will the unitized product be sold in shipping case or partial-height tray? What type of inner partitions or shrink wrap can be used? Do intermediate customers require smaller unitized packs and if so, of what type? Will the bottle itself contribute to stacking strength? Does the final outer package have to fit on a standard GMA 40-in48-in. pallet (consider load optimization and stack heights)? Will cases be shipped in mixed loads at any time? Will shipment generally be by full truckload, partial truckload (LTL), or individual shipment by ground or air (such as United Parcel Service or Federal Express)? Does the product travel in a rack system or bulk package for any part of its distribution? Are there shelf size requirements for the retail market or for storage? Will the distribution system require the product to be shipped over high altitudes or in unusually hot temperatures or low and/or high humidity environments? Will international shipment lead to special conditions or requirements?

Aesthetic Requirements.

The plastic bottle is frequently required to act as a salesperson at point of purchase and after the sale. Common concerns include:

  1. Shelf facing size—maximization of width and/or height vs depth while maintaining package stability 
  2. Clarity, opacity, and color 
  3. Label area and label qualities 
  4. Requirement for recycled and/or recyclable material 

WHAT ARE THE SPECIAL REQUIREMENTS FOR BOTTLE MANUFACTURING AND FILLING?

Options for Bottle Manufacture

Ideally, the designer should be able to select the best manufacturing technique for the particular bottle. However, because of cost, available capital equipment, and other factors, it may be necessary to compromise if possible. The major choices are:

  1. Molding process—injection blow, extrusion blow, stretch blow (with variations) 
  2. New or modified molds (or parts of molds and mold bases) 

The various advantages and disadvantages of the different plastic bottle manufacturing methods and molding processes are summarized as follows:

1. Process: extrusion blow molding 

a. Description: A tube (parison) is extruded through an annular die. Two halves of a bottle mold are clamped over the parison, sealing the top and bottom except for a hole for air injection. Air is injected, expanding the parison to match the mold. Clamped material (excess material at the top and bottom commonly called necks and tails) is removed and generally put back into the system as regrind. 

b. Advantages 

  • i. Relatively inexpensive mold, mold modification and equipment costs for a basic system. 
  • ii. Multilayer bottles and extruded side stripes possible. 
  • iii. Larger size containers are economical. 
  • iv. In-mold labeling available. 
  • v. Good for handleware or other designs requiring a molded-in ‘‘hole.’’ 

c. Disadvantages

  • i. High built-in waste due to necks and tails. 
  • ii. Requires in-mold or postmold trimming of necks and tails. 
  • iii. Finish (neck area that typically receives a cap or fitment) dimensions and quality are not as consistent. 

d. Variations

  • i. Extrusion dies can be ovalized to improve material distribution in oval or rectangular bottles. 
  • ii. Extrusion dies can be programmed to open and close while extruding, providing a top to bottom variation in wall thickness. 
  • iii. For high-volume large-container production, continuous wheel machines can be used instead of molds that shuttle in and out. 
  • iv. Instead of direct blow into a bottle, a dome may be created that must be trimmed off the finish. A calibrated neck, on the other hand, uses the blow pin to help mold the finish. The top of the finish may be further improved by posttrim reaming. 
  • v. High blow ratio (finish to maximum width) or off-center neck bottles can sometimes be improved by blowing ‘‘outside’’ the neck; a wide parison is used and waste is trimmed off the sides of the finish on the parting line. 

2. Process: injection blow molding

a. Description: The neck finish and ‘‘cigar’’-shaped body are injection molded in the first phase. This parison mold preform is then moved to a blow station containing a mold shaped like the bottle body. Air is injected through the finish, and the completed bottle is removed from the pin. The injection molded performs can either be premade and stored for later transport to be reheated for the blow molding process, or can be molded and then blow molded all in one continuous process.

b. Advantages

  • i. Good quality and control of finish dimensions due to injection process. 
  • ii. Capability of control and design inside the finish area (as long as there are no undercuts). 
  • iii. Economical for large volumes of smaller containers. 
  • iv. Waste not built into molding process (injection mold is hot runner). 
  • v. Can use PET as material. 
  • vi. Finished container does not have a ‘‘pinchoff’’ area at the tail (the line where the extruded parison is actually cut off the container during the manufacturing process) that can provide a weak point for drop test or stress-crack failure. 

c. Disadvantages

  • i. Additional need for injection molds increases cost and lead times (although existing preform molds can sometimes be used to save costs and time). 
  • ii. In-mold labeling and handles (with ‘‘holes’’) not generally available. 
  • iii. Not good for larger containers or high blow ratios. 

3. Process: stretch blow molding

a. Description: This is a variation on injection blow molding wherein the hot parison mold, or preform, is stretched in length by a push rod placed in the bottom prior to blow molding. The preforms can be premade and stored for later transport to be reheated for the blow-molding process, or it can be molded and then blow molded all in one continuous process.

b. Advantages

  • i. Improved bottle impact and cold strength. 
  • ii. Improved transparency, surface gloss, stiffness, and gas barrier. 
  • iii. Good for pressure containers such as soda bottles. 
  • iv. Opportunity to save plastic material by optimizing strength through stretch process and good control. 

c. Disadvantages

  • i. Commonly used PET requires special handling and drying process. 
  • ii. Extra step required in process. 
  • iii. High capital equipment costs. 
  • iv. Additional need for injection molds increases cost and lead times (although existing preform molds can sometimes be used to save costs and time). 
  • v. In-mold labeling and handles (with ‘‘holes’’) not generally available. 

Bottle Filling and Packing

Bottle design needs to take into account the filling and packing operations for the bottle. Important criteria include:

  1. Current equipment and change parts. There are frequently size limitations (height, width, and depth on equipment). Likewise, change parts and changeover times can be minimized by determining common critical dimensions or easier adjustments (frequently height) with other bottles that might run frequently on the same line. 
  2. Forces and conditions imparted by the filling and packing operation on the bottle. Common conditions include downward compression due to insertion of items in the neck (such as plugs and balls) or on the finish (such as snap-on closures); bottle body torsion and neck distortion due to high-torque cap application; side-to-side compression due to labeling and starwheel pinch points; multidirectional compression and abrasion due to bulk handling such as in a bottle unscrambler; and bottom drop impact due to automatic drop packing into cases or trays (Figure 1). In addition, aesthetic damage can be done at various points to a labeled or unlabeled bottle due to abrasion at transfer points and handling throughout the line. In some cases, machinery can be selected or modified to reduce these conditions or insure that they occur at protected or strong points on the container. If this is cost prohibitive or not possible, the bottle needs to be designed with these requirements in mind (see Bottle Design and Specification). 

Common stresses on a bottle on a filling and packing line
Common stresses on a bottle on a filling and packing line. Figure 1.

BOTTLE DESIGN AND SPECIFICATION

Once the special requirements for the bottle have been determined, sketches and models of various bottle designs meeting these criteria can be produced. As the process continues, the finalists need to become more detailed (dimensions, angles, finish) and begin to incorporate good plastic bottle design principles. Computer modeling and rapid prototyping can be used as part of this process.

GENERAL DESIGN CONSIDERATIONS

  1. Sharp Edges and Changes in Direction Should Be Avoided. Surfaces should be blended and generous radii used as much as possible at bends and corners (2). Sharp edges can lead to thin and high-stress areas, resulting in stress cracking or failure on drop testing. 
  2. The bottle must have minimal or no undercuts. Thought must be given to how the bottle is to be removed from the mold. The inside of the finish must be stripped from the core rod or blow pin, and the front and back halves of the mold need to open up from the outside of the bottle. Any plastic that would hook onto the tooling or molds can cause the bottle to distort or even break. Very small and rounded undercuts, such as small retaining rings on the inside of the neck, can sometimes be successfully molded; trial and error may be necessary for a particular design and process to see how much is possible. 
  3. Wall Thickness Minimum and Distribution Should Be Considered. Thin spots can lead to stress cracking, failure on drop test, and even pinholing; thick sections can act as heatsinks after the bottle is molded, and the uneven cooling and shrinkage can lead to bottle distortion. In the case of product that will cool or outgas after the container is sealed, there is a potential for distortion due to negative or positive pressure. This can require special moldedin structures, expansion areas, or controlled wall thickness to minimize these effects. The preciseness of wall thickness guidelines should be determined by how critical it is for performance of the bottle. 
  4. Bottle Stability Is Critical. This is especially true for many of the tall, thin (depth) bottles frequently used in consumer products to maximize shelf impression. In injection blow molding, the height of the pushup (the center of the base of the bottle) can be adjusted. The center of the bottle base must be high enough to not ‘‘belly out’’ for a full or empty bottle, or the bottom of the bottle will be a round surface. Various aids to stability include molding bottom rings (radiused or with a flat land area) or feet (three or four). It is a good idea to put a slight depression in the vicinity of the parting line so that a slightly raised parting line will not contribute to instability. The best solution depends on the bottle’s general shape and center of gravity and on the controls available in the molding process. 
  5. Embossed or Debossed Decorations. These include logos, which can be a free or inexpensive way to add information to the bottle. However, care should be taken to avoid thin spots, undercuts, or sharp edges. 
  6. Information that Is Frequently Molded into the Bottom of the Bottle. This includes material identification symbol (recycling logo, legally required to be on the bottle in several states), mold and cavity number (for quality checks and troubleshooting), and a molder or company logo. Placement should be planned ahead of time, especially because this may affect bottle stability. These should generally not touch or cross the parting line. 
  7. The Bottle Sealing Area Should Be Closely Specified. For standard continuous thread closures, lined or unlined, the land seal area at the top of the container is critical. It needs to be flat, horizontal, and free of dips or nicks. If the flat surface is going to be angled, care should be taken to specify a continuous surface that will create a seal. A minimum land width (flat neck thickness at the top) can be specified. If a valve seal closure is to be used, the circumference inside the finish where it meets the valve is critical. 
  8. Anticipate Pressure Differentials. Special product, filling, and distribution circumstances can lead to pressure differentials between the inside and outside of the filled and sealed container over the course of product life. Examples of this include hot fill, product outgassing, product migration through walls causing suckback, and changes in external pressure and temperature during distribution, storage, and handling. Flexible panels or more flexible materials will not prevent dimensional change, but can prevent damage to the container and allow it to be aesthetically acceptable. Variations in wall thickness support ribs, and stiffer materials can strengthen containers against dimensional changes to some extent. Depending on the product, one- or two-way valves can be added to the closure or inner seal. 
  9. Labeling and Decoration. These are a major consideration in bottle design. A label panel must be flat or have a surface that curves in only one direction. Many label areas are recessed or provide top and bottom ‘‘bumpers’’ to protect the label against scuffing during normal handling and distribution. The maximum label area is determined by a combination of the tolerances of the label placement equipment, the label dimensions, and the usable label panel dimensions. The additive tolerances need to be subtracted from the specified label panel size. Major labeling or decoration methods include the following: (a) In-line labeling (filling line)—labels (generally pressure-sensitive or plain-paper and glue) are applied on the filling line; (b) postmold labeling— labels (generally pressure-sensitive or heat-transfer) are applied soon after the bottle is molded (further shrinkage in the bottle, particularly over the next 24 hours, must be taken into account); (c) in-mold labeling—labels (either plastic or paper) are applied inside the mold during the molding process (bottle versus label shrinkage must be designed into the bottle tooling, especially for paper labels); and (d) direct application to bottle—instead of applying a label, the container is decorated by printing or silk screening directly onto the bottle. 

HDPE or PP bottles usually require flame treatment prior to decoration or labeling. Round bottles may need a rotational guide in the bottom to help control them during either process. (See also Labels and labeling machinery.)

Industry Standards

As in any purchased item, specifications and tolerances should be set up between the customer and the supplier. However, the designer is greatly aided by industry standards. Currently, ASTM publishes D2911-94(2005) Standard Specification for Dimensions and Tolerances for Plastic Bottles with standard screw closure neck finish dimensions, threads, and tolerances, and tolerances for various ranges of bottle capacity (up to 5 gallons) and body dimensions. These standards are modified on an ongoing basis and should regularly be checked for updates. In addition to dimensional standards, there are procedures for stress crack resistance, crush strength, drop impact and additional tests (3). The existence of standard finishes greatly facilitates the interchangeability of various stock and custom closures that might be available. The standard tolerances also provide a good starting point for specifications. The designer might wish to tighten some of these tolerances as required, but should be prepared to work closely with any vendors ahead of time to ensure manufacturing capability.

Bottle Dimensions Table 1.

Bottle Dimensions

A typical dimensioned bottle is shown in Figure 2. Table 1 describes a typical container’s dimensions and their importance. The sample given is a standard continuous thread finish container, which uses a standard screw-on/screw-off closure that is common on many consumer bottles.

Typical dimensions of a plastic bottle. Bottle and finish nomenclature
Typical dimensions of a plastic bottle. Bottle and finish nomenclature. T, diameter of thread; E, diameter of root of thread; I, diameter of inside; S, distance of thread start to top of finish; and H, height of finish. Figure 2.

Materials and Colorants

Critical criteria for selection of bottle materials include the following:

  1. Cost and availability—should be calculated by the bottle, not just the pound. 
  2. Environmental—recyclability and availability of recycled material, toxicity or biodegradability. 
  3. Clarity or opacity. 
  4. Stiffness or squeezability. 
  5. Water or moisture vapor transmission rate (WVTR or MVTR). 
  6. Gas permeability. 
  7. Chemical resistance to type of chemicals required. 
  8. Temperature range. 
  9. Impact strength. 
  10. Environmental stress—crack resistance (ESCR) 

Materials generally used for bottles in the United States include high-density polyethylene (HDPE), polypropylene (PP), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), polyvinyl chloride (PVC), polyethylene terephthalate (PET), and polystyrene (PS). Of these, HDPE and PET make up the great majority of containers by number and weight. Other materials, including engineering plastics such as polycarbonate, are used for special applications. Individual material properties can be found in detail under their separate articles. Many materials can have their properties significantly modified by additives such as plasticizers, fillers, impact modifiers, antistatic agents, and UV (ultraviolet radiation) inhibitors. Copolymers and multilayer coextrusions can further enhance a material.

Colorants are added to provide color and/or opacity to the material. Pigments, an opacity agent (such as titanium dioxide), and a compatible base material are compounded and added to the regular plastic during the molding process. The colorant loading, wall thickness of the plastic, and color and opacity of the product all affect the final aesthetics of the filled bottle (see Colorants, plastic).

Different materials and to some extent different colorants all affect the shrinkage and final performance of a bottle. This must be taken into consideration for testing and qualification.

Computer Utilization in the Design Process

The computer is an integral part of the bottle design process, and new ways of using it are being added continuously. Uses include:

  • Specifications. Bottle specifications are computerized at both the customer and supplier level. 
  • Computer-Aided Design/Manufacturing (CAD/CAM). Bottle drawings are performed on CAD systems, with improved opportunity for analysis and exploration of options and variables. Mold makers frequently use a CAM system for production of the mold. Some forms of rapid prototyping can also be performed. 
  • Analysis and Data Recording. Test data can be both directly recorded and analyzed on the computer. Sophisticated engineering programs, such as finiteelement analysis, can be used to compute and visualize stresses at various points. 
  • Pallet Loads. Several programs, such as those produced by CAPE (4) or TOPS (5), allow quick calculations of finished pallet options based on bottle sizes. This allows for pallet load optimization at the initial design stage. 

In addition, there is various software to do everything from evaluating heat transfer in mold design to visualizing product placement on the supermarket shelf.

PROTOTYPING AND TESTING

Once the bottle is designed and unit-cavity samples have been produced in the proper materials, a testing program must be initiated. A testing protocol should be established that checks bottles against both their specifications and the requirement list that was generated.

SPECIALTY BOTTLE REQUIREMENTS

A number of specialty bottles have distinctive requirements. Some of these include:

  • Roll-On. A round ball is inserted in the finish; rolling the ball against a surface (clothes, parts of the body) dispenses a liquid. The ball is held in place between two rings in the finish. The most common method of sealing is to torque the cap so that it pushes the ball down tightly against the lower ring (Figure 3). 
  • Plug. A small orifice plug is placed into the finish so that product dispensing can be controlled (eye or nose drops, creams). The plug must be inserted without crushing the bottle body or neck finish and yet be difficult to remove. Controlled dimensions or retention rings are some of the solutions. 
  • Drain-Back Closures. These fitments provide pour spouts and matching caps for detergents and fabric softeners. The drain-back section is sometimes placed inside the finish similar to a plug, but frequently before filling. 
  • Child-Resistant Closure (CRC). There are a number of different types of these closures on the market, generally requiring special finishes. The popular ‘‘push and turn’’ instruction requires sufficient H dimension to allow the cap to be pushed down for thread engagement. The ‘‘line up the arrows’’ instruction requires a ring with an interruption under the arrow. Protocol testing includes both closures and containers (see Child-resistant packaging). 
  • Nonremovable Closure. Some dispensing closures are designed to prevent removal. One design has ratchets that can easily be overridden for tightening but prevent loosening (see Closures, bottle and jar). 
  • Tamper Band. Many products (drugs and foods) have tamper bands to show tamper evidence. The band must be able to anchor itself to something on the bottle—a ring is sometimes added to the outside under the finish area (see Bands, shrink; Tamperevident packaging). 

Typical special rollon finish
Typical special rollon finish. Figure 3.

ENVIRONMENTAL CONCERNS

Whether driven by legislation, consumer preference, or the desire to be a good corporate citizen, today’s packager has to take environmental concerns into account when producing a package. A new plastic bottle presents a special challenge and opportunity. The following is a brief look at plastic bottles and the ways of handling solid waste other than the landfill:

  1. Reduce. The preferred way of handling the problem— creating less waste in the first place—has another major advantage. The less plastic we use, the more money we save. This can be accomplished through optimizing structural design, eliminating oversizing, and taking advantage of the best materials for the job. 
  2. Reuse. Although returnable bottles may seem like an obvious answer, this is not necessarily the case: ‘‘to withstand the process of return and refill, a refillable container must be about twice as heavy as a one-way container of the same material.’’ This adds impact to energy usage and eventual disposal also (6). However, a number of consumer products, notably detergents and cleaning products, are sold in bottles that the consumer refills without having to undergo this process. The refill packages are designed with less plastic and frills, or are made of lighterweight or more easily crushable materials. 
  3. Recycle (Recyclable). Everything can be recycled by somebody, somehow, somewhere. However, unless a substantial percentage of the bottles can realistically be recycled in the near future, this doesn’t mean too much. Although the percentages of bottles that are recycled or could be from programs in place changes constantly, the bulk of recycling is with PET (mostly soda bottles, although some areas have added capabilities to accept other bottles) or HDPE (mostly homopolymer milk bottles, although some areas now accept other containers including copolymer detergent bottles). Plastic containers can be recycled into plastic lumber, piping, or even carpet liner as well as packaging containers. The recycling rate for plastic containers has been estimated at 24.3% for 2005, based on pounds of resin sold (7). To aid in potential recycling, as well as comply with the law in most states, a material identification logo, such as the SPI resin identification code, (8) should be added to all containers. 
  4. Recycle (Recycled). The relatively high cost of collecting, sorting, cleaning, and processing good-quality post-consumer HDPE generally makes it more expensive pound for pound than virgin resin. As the process becomes more efficient and if the cost of virgin continues to climb, this may change. Because of the unpredictability of the color and properties of much regrind, it is frequently used as a middle layer in a multilayer coextruded bottle or as a small percentage in a dark bottle holding nonaggressive product. Food and drug primary packages may not be permitted to use reground plastic directly, although repolymerization or special considerations can be investigated. 
  5. Incineration and Conversion to Energy. Plastics produce heat when burned, which, in turn, can be converted to other forms of energy. The presence of heavy metals (such as lead or cadmium used in some colorants) in a bottle will leave toxic waste residue. Several states have outlawed the use of these materials, and it is critical to avoid them. Older incinerators may also have a problem with chloride containing materials such as PVC (9).

ANDREA S. MANDEL

Andrea S. Mandel Associates,

Packaging Consulting Services,

West Windsor, New Jersey

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shimelis mekuria

3 июля 2012 01:25

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the given description on the book is good.but if possible at least one worked design is necessary. try this.
THANK YOU!.