Neck Finish Specifications

The effectiveness of a plastic bottle as a container depends on the creation of a seal between the bottle and its closure to keep the contents from escaping, while permitting easy opening and resealing of the closure. To achieve a proper seal requires compatibility between the mating bottle and closure threads and other points of engagement; that is, compatibility requires standardization. When plastic blow molding began, standard neck finish specifications developed by the glass industry were used, but subsequently the plastics industry adopted specifications developed by The Plastic Bottle Institute, a division of The Society of the Plastics Industry, Inc. See Fig. 1. for bottle-finish terminology; the three most commonly used finish specifications: SP-400, SP-410, and SP-415. Other standard finish specifications are available from The Plastic Bottle Institute. Having a copy of their technical bulletin PB1-2 on hand will be very helpful.

Fig. 1.

Although standard specifications are widely used, container manufacturers produce many nonstandard neck finishes to mate with specialized closures and dispensing fitments. Such nonstandard finishes have proliferated during recent years to meet packaging requirements for

• cost reduction by using snap on and linerless closures;
• improved assembly of fitments which lock in place to dispense products by drop, stream, or spray;
• consumer convenience by dispensing through the closure; and
• child and pilfer resistance in accordance with government safety regulations.

Specifications for such nonstandard finishes tend to be proprietary to each container manufacturer and are not generally available for publication.

It should be noted that the ability of certain plastic blow molding processes (such as injection-blow) to mold neck-finish configurations and close tolerances (not possible in glass), and the unique properties of plastics used in containers and mating fitments, have provided package designers with great freedom and latitude to develop new package forms.

CONTAINER TERMINOLOGY

Many of the terms referring to various sections of blown plastic containers have been adopted from glass terminology. The following are definitions of the terminology used in Fig. 1

Neck: The section of the container above the shoulder, where the cross-sectional area is smaller than the body of the container.

Base of Neck: The point where the neck meets the shoulder of the container. The base of neck (B.O.N.) often is a reference point for measuring container fill level.

Finish: Shaping of the neck section with a thread form or other configuration suitable for attaching a closure.

Bead: An enlarged diameter of the neck finish used for various purposes: aesthetics；’ stabilization of the closure, and as a location point for secondary machining of the neck.

Shoulder: The section of the container where the body decreases in size to meet the neck. The shoulder surface generally is a radius shape.

Base: The bottom surface, on which the container stands upright.

Push-up: A section depressed upward within the base surface. The depression assures that the container will stand upright without rocking on projections caused by parison pinch lines, gate marks, engraving, coding, or other interruptions in the base surface.

venting for plastic mold making

Air in the empty cavities must be displaced by the melt as it flows into the mold during injection. Unless removed quickly, trapped air may actually compress enough to ignite and leave a characteristic burn mark. The faster the fill rate and the less viscous the melt, the more difficult it may be to completely vent, and bubbles or incomplete fills may occur. For high-viscosity melts, vent grooves milled into the parting plane of the mold should not be more than about 6-mm wide by 0.05-mm deep. For lower viscosities, the vent groove may have to be less than 0.05 mm deep where it meets the cavity and deepened, further on. Vents should be at the end of flow paths and appropriately distributed around the part. Large runners should be separately vented. Other methods for venting include the use of clearances machined into ejector pins; porous plugs next to the cavities, which can exhaust air into the water coolant flowing through the cores, the coolant being arranged to be under a slight vacuum at these points; and vacuum applied to the mold cavities before injection (rare at present).

plastic mold maker cooling principle

For minimum cycle time and consistent part quality, the mold designer must provide efficient and uniform cooling in the cavity and core plates. For some plastics, as little as a 5 °C variation from point to point in a cavity can significantly affect part quality. It is not enough simply to run the mold as cold as possible—often a high mold temperature and slower injection produces better parts. It is up to 山e mold designer to estimate the cooling load, size and select the external heat exchanger, and lay out cooling circuits for the cavity and core plates. The main steps in this task may be outlined as follows.

• Calculate shot weight, including runners and gates.
• Calculate heat to be removed to cool the shot from incoming melt to demolding temperature.
• Estimate overall flow and coolant velocity needed to remove this amount of heat during the proposed cooling time.
• Select diameter and distribution of coolant channels in the cavity plate and shape and type of cooling for the cores .
• Check to make sure that coolant channels and thermocouples will fit between cavity inserts, and that they will not weaken the mold unduly.
• 

Design of the Molded Part

The following factors can significantly influence the design of the molded part:

• Flow properties of the melt,
• Solidification behaviour,
• Pressure transfer in the cooling-down melt,
• Molecular and fiber orientation,
• Shrinkage and its dependence on process parameters, on the gate position and on the direction of measuring,
• Warpage, which increases remarkably through fiber reinforcement.
• When designing, it has to be especially taken into account:
• Demoldability，demolding draft angle,
• Permissibility of weld lines,
•  Permissibility of marks by gates, ejectors, sliders and splits,
•  Surface structure,
•  Required tolerances.

The area around the gate has normally far more molecular orientations than other areas. Additionally, there is often an overloading due to a too high melt compression or too long holding pressure. This forms an area very liable to fracture. The practical experience coincides with the fact that two thirds of fracture lines pass through the gate. The gate position is often indicated by stress crack lines

Important to consider the following fundamental rules：

• No gates and weld lines in high-stressed areas and edges!
• Avoid wall thickness differences. The ideal injection molded part has equal wall thickness.
• If mass concentrations cannot be avoided, they have to be positioned as close as possible to gate. Figure 1.9 shows a negative example.
• Minimize wall thicknesses and only as big as absolutely necessary!
• Optimize wall thicknesses, position and number of the gates to result in a uniform and inclusion- free flow front progress. Therefore, it is essential to perform a rheo-logical analysis with finite elements. Several experienced simulation programs for example Cadmould or Moldflow can be used for this application.
•  Avoid sharp inside edges.
• Use simplifications： Often a few minor design changes are enough to process without the use of sliders or jaws. This decreases the mold costs drastically.

Conventional structure of an injection mold.

a: Locating ring of the fixed half; b: basic plate of the fixed half; c: guide pin; d: guide bushing; e: locating bushing; f: spacer fo「the ejector box; g: basic plate of the ejector half; h: insulation plate (in case of higher mold temperatures); i: screw locking device, j: main screw k: frame plate of the fixed half containing the inserts; I: frame plate of the ejector half; m: support plate; n: support bar; o, p: plates of the ejector traverse.The mechanical and thermal design can also be done using different CAE programs. The integration of cooling channels allows a detection of “hot spots” in the mold. These “hot spots” are determining the cooling time and have to be eliminated

The tolerances for plastic molded parts are standardized in DIN 16901. However, in practice, narrower tolerances are demanded  and achieved.

PET preform mould for plastics materials (PET) have been made in some manner for centuries. Some of these PET preform moulds were fine works of art, as, for example, the moulds used by craftsmen in glass. The coming of the modern plastics moulding materials brought about great advancement in this old art, and transformed it into a science. Mass production PET preform moulding machinery has been developed, and new PET preform mould steels and alloys have been introduced to withstand severe service. Accuracy is a requirement in modern manufacturing, and PET preform moulded plastics are produced by steadily mounting standards of precision, which have necessitated new machine tool applications and methods.

A PET preform mould may be defined as a form for shaping a plastic material ”PET” into a finished product-here is PET preform. PET preform moulds are made of plain carbon steel or of alloy steels, and are hardened to provide compressive strength and hard surfaces to take and maintain a high polish under severe wearing conditions. PET preform moulding materials require heat and usually pressure to achieve the plasticity necessary for them to flow into the shape of the mould cavity. Pressure is required to force the material into the cavity and to hold it to shape until it is set, and to give the casting or finished product the required strength. A PET preform mould must be polished to give the casting a good finish，and to allow it to be ejected easily.

A mould for the general run of PET parts is divided into two halves which meet at the parting line. These halves are mounted on backing plates which are drilled to allow passage of steam or cooling water, and which carry the guide pins which aligning the halves of the PET preform mould. The mould halves and backing plates assembled constitute the PET preform mould proper.

The fundamentals of mould design are discussed and applied to representative type PET preform moulds. The important compression PET preform mould types are classified for study; transfer and jet moulding are described; injection PET preform moulds are presented both as units and broken down into elements of design and construction. Methods of moulding screw threads are discussed; methods of PET preform mould sinking and applications of mould base standards are shown. mould building methods and equipment, moulded parts finishing, product design considerations and estimating methods are included as background information. A summary of practical points in PET preform mould design and construction, shrinkage charts and a nomenclature section provide a basic fund of data required by the serious learner.

Since many factors enter into the design of plastics PET preform moulds, and into the design of products to be PET preform moulded, it is well for product designers, tool designers, and tool makers to have a common understanding of plastics PET preform moulds and PET preform moulding in order to cooperate to the fullest extent in making possible PET preform moulded products of high standards of quality and economy.

Requirements for PET preform mould Designing

To design plastics PET preform moulds, a plastics engineer should have an intimate knowledge of a proper design procedure which is based upon a knowledge of the characteristics of materials; of the technique of PET preform mould building; of the economics of each production schedule; of the tooling cost both to purchaser and to the PET preform mould manufacturers; of PET preform moulding equipment operation; of the special mould steels and alloys; and of the moulding and finishing facilities of his own plant.

The design of plastics PET preform moulds includes, besides the design of the mould proper, the provision for mounting the mould in a press; the provision of means to eject the finished PET preform moulded casting; and a provision for temperature control. There are also finishing tools to be designed, such as drilling jigs, buffing attachments, holding fixtures, cooling fixtures, gages, and other devices for obtaining accurate and economical production.

Two General Types of Plastic moulding in General Use

There are two general types of moulding in general use compression and injection. A compression Mould is one which is open when the material is introduced, and which forms the material by heat and by the pressure of closing. An injection Mould is one which is closed before the material is introduced from an external heating, or plasticizing cylinder. Compression Moulds are usually operated in hydraulic presses; while injection Moulding presses have been developed for either mechanical or hydraulic operation.

The Mould designer does not always have complete information on the product his Mould must produce. Usually a sketch or drawing is supplied, and sometimes a model. The model is useful since a Moulding or finishing feature may appear in three dimensions which would escape notice on a drawing. A model, while desirable, is not absolutely necessary, and most Moulds are built without their use. As the die or the die casting designer, the Mould designer can very frequently find details which may be altered slightly to allow more convenient and economical Moulding. The Mould designer can render a real service by making such suggestions for approval before completing his design.

Injection Molding

In contrast with compression molding, this method is the most widely used for the forming of thermoplastic materials. With some modifications for a quicker adjustment of the temperature, some thermosetting materials, especially formulated alkyds, may also be injection molded.

In injection molding, the granular plastic molding compound is fed intermittently from the hopper in predetermined amounts into a heated cylinder, where it is softened by the heat and pushed forward by a plunger or ram through a nozzle opening into the cavity of a mold, much the same as toothpaste is squeezed by applying pressure to the tube. After filling the mold, the hot plastics material is set by chilling to a solid, the mold is opened and the molded part discharged. In the meanwhile, the plunger has been pulled to the back of the chamber ready to receive a fresh charge of molding compound. Accurate temperature control is maintained within the cylinder by use of thermostats, and by controlling the flow of electricity to the band heaters placed around the cylinder walls. The cylinder heating is divided into three zones: (1) charging section, (2) melt section and (3) discharge section. In the melt section a “torpedo” or hollow oblong sphere is placed to force the molden plastic against the sides of the heated cylinder, thus assuring complete melting and mixing of the material.

A schematic diagram of injection molding is shown in Figure 1. In the upper drawing, the press is closed having just received a charge of molding powder from the hopper, and the plunger is pushing it forward, causing the molten plastic in the forward part of the cylinder to be forced through the nozzle into the mold cavities. In the lower drawing, the press has been opened, following the injection and chilling of the molten plastic in the mold. The mold has been parted and the molded plastics pieces are about to be ejected from the mold. Simultaneously the plunger has been withdrawn to the rear of the cylinder, thus allowing another charge of molding powder to enter.

The injection mold is composed of two parts which are held in tight contact by a locking device which prevents them from being forced apart by the pressure of the molten plastic when it enters the mold. Before injection, the mold is heated to a temperature just below the softening point of the plastic to allow a smooth flow of the molten plastics material to all parts of the mold through the sprue or opening, and the runners to the mold cavities. The air and any vapors present are pushed out of the mold cavities ahead of the molten plastic by means of vent holes on the opposite side of the mold. After filling the mold cavities with the molten plastics, cold water is admitted to the embedded coils of the mold, thus lowering the mold temperature and absorbing the heat from the soft plastics mass，causing it to harden. As soon as the material is solidified, the mold locking device is released, permitting the mold to open. The molded articles are removed from the mold by ejector pins.

Most injection molding machines are automatically controlled by electronic timing mechanisms. The molding cycle for most thermoplastic materials averages from 10 to 30 seconds. By the time a part has been ejected from the mold, a fresh charge of material has been admitted to the cylinder and the previous charge has melted, ready for injecting into the mold. The rapid succession of injection of the molten plastic and ejection of the solid molded article has permitted this method to become the leader among mass production methods for producing plastics articles. Consequently injection molding has made it possible to produce a large variety of molded plastics at a relatively low cost.

Certain modifications of injection molding have been introduced to facilitate the molding of special types of plastics resins. The problem with the injection molding of thermosetting materials is that under heat these plastics will first soften and then harden to an infusible mass if the heating is continued beyond the gel stage. Thus it is essential to restrict the time and the extent of heating of the thermosetting maerial while in the injection cylinder or nozzle. In contrast, the heating of the injected material in the mold must be continued until the material cures or sets to a solid, before discharging. Consequently the time cycle for thermosetting injection molding is 10 to 20 times longer than for the thermoplastic materials. For this reason, jet molding or transfer molding, which are modifications of injection molding, are recommended for those thermosetting plastics capable of being injection molded. These methods are discussed later in this chapter.

A new modification of injection molding, known as “extrusion injection”, substitutes an extruder screw operated intermittently, in place of the plunger of the usual injection molding machine. This not only permits easier regulation of the amount of plastic to be injected during each cycle, but also provides a better mixing of the plastics material to assure a greater homogeneity of the melt, and in addition allows for mixing of colorants with the molten plastic while being molded, rather than requiring that a colored plastics molding compound be employed. This method will be more fully described in a later chapter under “Future Developments.