The Basic Parts of a Glass Container

The three basic parts of a glass container are the finish, the body and the bottom (Figure 1).

These are formed by the three parts of the glass container molds in which they are made.   

FINISH--The finish is the part of the jar that holds the cap or closure, the glass surrounding the opening in the container. In the manufacturing process, it is made in the neck ring or the finish ring. It is so named because, in early hand glass manufacturing, it was the last part of the glass container to be made, hence the term "finish"

BODY----The body of the container is the portion that is made in the body-mold. It is, in most cases the largest part of the container and lies between the finish and the bottom.

BOTTOM--The bottom of the container is made in the bottom plate part of the glass-container mold.

THE FINISH

The finish of glass containers has several specific areas, as follows (Figure 1):

SEALING SURFACE--- The portion of the finish that makes contact with the sealing gasket or liner. The sealing surface is usually on the top of the finish, but may be a combination of the both top and side seal. 

Glass Thread or Lug---One of several horizontal, tapering and protruding ridges of glass around the periphery of the finish that permit specially-designed edges or lugs on the closure to slide between these protrusions and fasten the closure to securely with a partial turn. The number of lugs on the closure and their precise configuration is established by the closure manufacturer.

CONTINUOUS THREAD--A continuous spiral projecting glass ridge on the finish of a container intended to mesh with the thread of a screw-type closure.

TRANSFER BEAD--a continuous horizontal ridge of glass near the bottom of the finish used in transferring of container from one part of the manufacturing operation to another.

      Note: Not all glass containers have transfer beads. Some achieve the transfer in manufacturing through different means.

VERTICAL NECK RING SEAM--a mark on the glass finish resulting from the joint of matching the two parts of the neck ring, also referred to as the Mold Match or Parting Line.

NECK RING PARTING LINE---A horizontal mark on the glass surface at the bottom of the neck or finish ring resulting from the matching of the neck ring parts with the body-mold parts.

THE BODY

The characteristic parts of the body of a glass container are shown in Figure1:

SHOULDER--The portion of a glass container in which the maximum cross-section of the body area decreases to join the neck or finish area. The neck area is not shown in Figure 1 because most glass container for processed goods have very little neck. Actually, the neck would be any straight area between the shoulder and the bottom of the bead or, with beadless finishes, the neck ring parting line.

HEEL--The heel is the curved portion between the bottom and the beginning of the straight area of the side wall 

SIDE WALL-- The remainder of the body area between the shoulder and the heel.

MOLD SEAM--A vertical mark on the glass surface in the body area resulting from matching the two parts of the body-mold. The body-mold seam may or may not align with the vertical neck ring seam.

THE BOTTOM

BOTTOM PLATE PARTING LINE--A horizontal mark on the glass surface resulting from the matching of the body-mold parts with the bottom plate. 

BEARING SURFACE--The portion of the container on which it rests. The bearing surface may have a special configuration known as the stacking feature, which is designed to provide some interlocking of the bottom of the jar with the closure of another jar on which it might be stacked for display purpose.

DISCUSSION OF THE FINISH

     Many different finishes exist for closing glass containers. Figure 1 shows only three general types, which may be varied for use with specific closures.

Every type of closure for sealing glass containers has a specific glass finish with which the closure has been designed to function. Attempts to put a lug cap on a jar with a Press-on Twist-off (PT) finish would be futile. Several different types of lug closures are available, each of which has been designed to work best with a specific lug-style finish.

Fortunately, many glass finishes are standardized. For every finish standard designation, a specific set of dimensions, specifications and tolerances have been established by the Glass Packaging Institute, a trade association that works with glass manufacturers and closure manufacturers. Each finish standard drawing has a specific number and may be obtained directly from the Glass Packaging Institute or through the glass manufacturer.

LUBRICANTS OR GLASS SURFACE TREATMENTS

Surface treatments or lubricants on glass containers serve the beneficial purpose of easing the smooth flow of containers through conveying systems. They protect the outside surface of the container from abrasion during manufacture and distribution. Many different treatments are used. The use of or change in a surface treatment should be fully discussed with both the glass container and closure suppliers to prevent potential problems. For example, excess surface treatment may effect closure performance or label application.

DEFINITIONS OF TERMS FOR GLASS CLOSURES

     Among the terms commonly used for describing parts of metal vacuum closures are the following (Figure 2):

     PANEL-- The flat center area in the top of the cap.

     RADIUS or SHOULDER-- The rounded area at the outer edge of the panel connecting the panel and skirt.

     SKIRT--The flat, nearly vertical portion on the side of the cap. The skirt may be smooth, knurled or fluted and serves as the gripping surface.

     CURL- The round or rolled portion at the bottom of the skirt that adds rigidity to the cap and serves to protect the cut  edge of the metal.

     LUG--A horizontal inward protrusion from the curl that is seated under the thread on the glass finish and holds the cap in position.

     THREAD--The spiral groove on the skirt of a continuous thread closure that meshes with the tread on the glass finish.

     FACE-- The outside of the cap.

     REVERSE-- The inside of the cap.

     COATINGS and LITHOGRAPHY-- Coatings and inks that are used on the inner and outer surfaces of the cap to protect the metal from attack, adhere gasket materials and decorate the closure.

     GASKET--The actual sealing member of the cap that must make intimate contact with the glass finish at the proper point to form an effective seal. Gaskets may be made from plastisol compounds.

     PLASTISOLS--Suspensions of finely divided resin in a plasticizer, which are usually of two types: (1) flowed in--used in the standard lug or twist cap, and (2) molded-used in the PT cap. Plastisols are tailored to the product and process. For example, a closure intended for sealing a pasteurized product may not be suitable on a retorted product.

      SAFETY BUTTON or FLIP PANEL--A raised,  circular area in the center of the panel that is used only for vacuum packed products and serves two principal purposes:

     1.  Dud detection--In the packaging plant it aids in automatic on-line detection of low-vacuum or no-vacuum packages.

     2.  Consumer  indicator-It is an indicator to the consumer that the package is properly sealed when opened in the home. In addition to visual evidence of a disrupted seal, there is also an audible signal.

ROLE OF VACUUM IN OBTAINING GOOD SEALS

     Almost  all low-acid and acidified foods packed in glass containers are sealed with  vacuum-type closures. The following discussion deals exclusively  with this type of closure. The vacuum within the package and the resultant positive pressure on the outside of the cap play an important role in forming and maintaining a good seal. It is important to know how vacuum is formed, what may affect the vacuum level, and how, when, and where it should be measured.

VACUUM CAPPERS for GLASS CONTAINERS

     Two basic types of cappers apply caps while forming a vacuum in the container--the mechanical vacuum capper and the stream-flow capper. The mechanical vacuum capper is used primarily on dry products and applies the cap to the jar in an evacuated chamber. It is rarely used on low-acid processed foods.

     In stream-flow cappers, either straight line or rotary the container is subjected to a controlled stream atmosphere that displaces the headspace gases from the jar by a flushing action. The stream is trapped in the headspace as the cap is applied, then condenses to form a vacuum that helps hold the closure in place. As a aid to good sealing, the gasket in Plastisols-lined caps is softened by stream.

FACTORS AFFECTING VACUUM FORMATION

     Four primary factors affect vacuum formation:

     1.  Headspace is an important factor in efficient sealing, particularly in stream-flow cappers. For low-acid food products, sufficient headspace must be allowed to trap adequate stream in the container for forming a vacuum and to accommodate product expansion during retorting. The correct amount of headspace varies with product, processes and product design However, a rule-of-thumb indicator is that the headspace should be not less than 6 percent of the container volume when measured at the capping temperature. Inadequate headspace can result in displacement of deformation of the closure during retorting. This 6 percent headspace is not as critical with acidified products that are either hot-filled or pasteurized. There still has to be sufficient headspace, however, to allow for vacuum formation, a clean fill and some product expansion during pasteurization.

     2.  Product sealing temperature affects the final vacuum obtained due to the effect of product contraction upon cooling. Other factors being constant, the higher the product temperature at the time of sealing, the higher the final package vacuum. Product temperature may also affect the final vacuum by its interaction with the amount of air in the product. Usually, higher filling temperatures result in less air in the product.

     3.  Air in the product, as mentioned above, can have a direct effect on the final package vacuum and should be kept at a minimum for good sealing, product quality and product appearance. The more air that is trapped in the product, the lower the vacuum.

     4. Capper vacuum efficiency refers to the ability of the capper to produce vacuum in sealed containers. The most convenient, routine check on the vacuum efficiency of a stream-flow capper is the cold-water vacuum check. It is simple and quick; measurement are made with a vacuum gauge. The cold-water vacuum check shall be made prior to the start-up of actual filling operations or after extended break periods, at change-over from one container size to another , after a major jam, or whenever an unexplained significant change in vacuum level occurs in regular line samples. This check can serve the dual purpose of checking capper vacuum efficiency and cap application with the same jars.

METHOD of COLD-WATER VACUUM CHECK

     A series of jars is filled with cold tap water to the approximate headspace that will be maintained with the commercial product. These are then sealed in the capper after the capper has been allowed to warm up to operating  temperature and the normal stream setting attained. The jars are opened and re-run through the capper serves to deaerate the water and provide a truer vacuum reading. The vacuum obtained in the jars is the n measured by using a standard vacuum reading obtained  should be at least 22 inches, or as recommended by the closure supplier.

     The number of jars used to perform the cold-water vacuum check should be as follows: 

     1.  Straight-line capper: Four to six containers.

     2.  Rotary capper: One container for each capping head.

THE PRINCIPAL VACUUM CLOSURE TYPES

     Currently, two types of vacuum closures--lug or twist cap and PT cap--are widely used on low-acid food products. In addition, the Plastisols-lined continuous thread (PLCT) closure is used on acidified food products.

LUG OR TWIST CAP

     The lug or twist cap (Figure 3) has gained steadily in popularity to become the predominant vacuum-cap type. It is referred to as a convenience or utility closure, because it can be removed without a tool and forms a good reseal for storage.

     1.  Structural components--The lug cap consists of a steel shell and may have from three to six lugs, depending on its diameter; it normally contains a flowed-in Plastisols gasket.

     2.   Application and seal formation--The headspace of the glass container is swept by steam the same as the other closure styles. Lug caps are secured to the glass finish. It is desirable, in most instances, that the gasket be softened by heat in the capper to facilitate sealing. Both the lugs and vacuum, hold the cap in place on the glass finish, but vacuum is the most important.

          PT (Press-on Twist-off) Cap

     The Press-on Twist-off or PT cap is in widespread use for baby foods as well as other products (Figure 4).  It combines the simple application requirements of a press-on closure with the convenience of a lug cap:

      1.   Structural components-The cap consists of a steel shell that has no lugs. The gasket is molded Plastisols that covers a sealing area extending from the outer edge of the top panel to the curl of the cap, forming the primary top seal and a secondary long side seal. The standard baby food design contains a safety button or flip panel, as do most other PT caps.

      2.    Application and seal formation--Application requirements call for simply pressing the cap down on the glass finish after flowing steam over the headspace. The PT closure gasket must be properly heated prior to application. The glass threads form impressions in the skirt of the cap gasket that allow the cap to be cammed-off when twisted open. The PT closure is help in place on the finish primarily by vacuum with some assistance from the thread impressions formed in the gasket wall when the cap is headed then cooled  

PLCT   (Plastisols-lined Continuous Thread) Cap 

     The PLCT cap consists of  a metal shell with a threaded skirt that is knurled. It contains a flowed-in Plastisols gasket and is applied by screwing the closure onto the glass finish. The PLCT cap may be used in both steam and non-steam applications.

CLOSURE EVALUATION

     The two general types of closure inspections include: (1) visual, non-destructive, external observations or measurements made at frequent intervals, and (2) cap removal or destructive tests made at less frequent intervals, because the integrity of the seal is destroyed. Both of these tests and observations shall be made at the capper and after processing and cooling. The appropriate teats and observations for each type of closure being considered are listed in Table 1.

TESTS and OBSERVATIONS for CLOSURE APPLICATION and DEFECTS

     Cap Tilt--PT caps should be essentially level, not cocked or tilted, and seated well down on the finish. This is judged in relation to the transfer bead or shoulder on the glass container and should not exceed 3/32 of an inch.

     Cocked Cap (Figure 5)--The term cocked cap is used for both the lug cap and the PLCT cap. It is caused by a lug failing to seat under the glass thread. It occurs on a PLCT cap when the cap and glass threads fail to properly engage. As a result, the cap is cross threaded and becomes cocked.

     Crushed Lug (Figure 6)--A crushed lug on a lug cap may be visible on external examination. However, it may not be readily apparent, since it does not necessarily result in a tilted cap. It is caused by a lug being forced down over the glass thread by the capper's sealing mechanism.

     Stripped Cap (Figure 6)--A stripped cap is a lug cap that has been over-applied to the extent that the lugs have been "stripped" off the glass threads on the finish. On visual examination, the lugs appear to be pulled outward.

     Vacuum--In most cases, a vacuum will be formed in the package when it comes out of the capper, and the panel of the cap will show a concavity or dished-in appearance indicating the presence of a vacuum.

     On PT caps there must be at least five inches of vacuum out of the capper to avoid loose caps. However, a vacuum button, if present, may not be down at this point. After processing and cooling, the button must be down and return to the up position when the cap is removed.

     The exact amount of vacuum present is determined with a vacuum gauge and should read within the range for the product being run. This procedure is a destructive test that results in loss of the package integrity.

     Temperature- The product temperature should be within the normal range for that product being run and should be recorded in conjunction with vacuum.

     Headspace--In most cases, headspace should be not less than 6 percent of the container volume at the sealing temperature. Once the relationship of headspace volume for a  specific product is established for a given container, the headspace may be measured with a depth or headspace gauge rather than by volume.

     Gasket--After cap removal there should be a visible, even impression in the gasket 360 degrees around the circumference indicating tight contact with the glass finish.

     Cut-Thru--Cut-thru occurs when the top of the glass finish has pushed completely through the gasket on the metal. This problem results in a leaky seal and requires immediate corrective action.

     Removal Torque--Removal torque is the force required to remove a cap and can be measured on a standard torque meter. Removal torque should not be used as a measurement for the proper application of lug-style closures, but it may be valuable quality control tool for measuring removal torque trends. Removal torque, however, is used for the measurement of PLCT cap application. It is suggested that the optimum removal torque should be one-half the the diameter of the closure. This is not an absolute rule, and there is flexibility both above and below the optimum removal torque.

Pull-Up--Pull-up, also known as lug position, is a nondestructive method of measuring the engagement of the closure lugs on the threads of the glass finish. 

      This pull-up or lug position is defined as the distance between the leading edge of the cap lug and the vertical neck ring seam  on the glass finish. It is measured in 1/16 inch increments. To measure this position, first find the vertical neck ring seam on the glass finish. There are two vertical seams on the glass finish 180 degrees apart.