How to design PET preform mould

    How to design PET preform mould

    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 bottle mould, 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 preform bottle mould, 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 bottle mould 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.


    Single Stage Blow Molding Machine


    While PET bottle development was proceeding in the US, a large manufacturer of injection molding machines in Japan, was leading a project to develop a machine to make biaxially oriented PP (polypropylene) containers. They recognized that the prototype machine could be used to produce the new PET bottles and, in December 1975, the One-stage ASB-150 injection stretch blow moulding machine for making the new biaxial oriented PET bottles was unveiled. All one-stage injection stretch blow moulding machines derived from this original Stretch Blow design are referred to as classic one-stage machines, as the concept has long since been extended into other PET developments. The classic one-stage machines design is extremely versatile in that the same basic machine design can be used to make a wide variety of bottles and jars in all shapes and sizes.


    Two Stage Blow Molding Machine


    In the early developments, performs were made by continuously extruding a PET tube. To make these prefroms, a perform manufacturing machine that took a continuously extruded PET tube, heated and closed one end, and then heated the other and formed a thread finish by blow moulding. This process had a faster output rate, at 12000 preforms per hour, than the early injection moulding routes of 8 and 16 cavity moulds. Being extruded, the performs could be multilayered with barrier materials. The system was overtaken by injection moulded performs as the cavitation increased to 32 and beyond. The quality of the injection moulded (IM) neck, adding for example vent slots, made the IM finish preferable. Moreover, IM technology is available from more than one company, giving customers greater technical and commercial choice. Two-stage technology machines with six blow moulds operating at around 4000 bottles per hour. Subsequent mould and cooling development increased the output to 6000 bottles per hour.


    Two stage PET processing includes:


    Making performs by Injection Molding


    Blowing bottles by Stretch Blow Molding


    Because it is more flexible than one step processing, it is widely accepted in Plastic packaging market. You may choose to make preforms only or blow bottles only if you want to do one of them specially.


    Integrated Two Stage Blow Molding Machine


    In Integrated Two Stage approach the performs were made by more conventional injection moulding routes (with the number of cavities optimized to match the required output) and then, while still hot, were carried to a separate blowing machine with the optimized number of blow moulds to suit the required output. This was the first ‘integrated ‘approach to PET bottle making. Equipment developers took the ‘single-stage’ approach. Here the equipment had the same number of injection cavities as PET blowing mould. This was a more compact approach and proved ideal for small batch output with excellent glossy surfaces.

    Blow molding is a type of plastic forming process for creating hollow plastic products made from thermoplastic materials. The process involves heating and inflating a plastic tube known as a parison or preform. The parison is placed between two dies that contain the desired shape of the product. Air is then supplied to expand the tube causing the walls to become thinner and conform to the shape of the mold. Once the blowing process is complete, the product is then cooled, ejected, trimmed, and prepared for the secondary processes.Bottling and packaging are the main applications of blow molding which comprise about 49% of the global blow molding market share. This is followed by building and construction, consumer products, and transportation industries.

    The global market share of blow molding was estimated at around $78 billion in 2019 and is expected to grow annually by 2.8% from 2020 to 2027. Typical raw materials used are polyethylene (PE), polyethylene terephthalate (PET), and polypropylene (PP).Plastic blow molding originated from the ancient process of glass blowing. Both glass and plastic in their molten state can be formed easily by introducing small amounts of air inside the molten material. The first injection molding process for glass, known as free-blowing, was created around the 1st century BC. Glass mold-blowing was later developed as an alternative process that came about around the 1st century AD.The first blow molding material used other than glass was natural rubber. The process was patented by Samuel Armstrong in the 1850s. The next major advancement came around the 1930s with the development of the first blow molding machine by Plax Corporation. The process uses cellulose acetate as the raw material. Come 1939, low-density polyethylene (LDPE) was introduced by Imperial Chemical Industries from England. LDPE is more suitable for blow molding which then paved the way for further process development. By the 1950s, the commercialization of blow molding was completed, and the industry grew exponentially.

    This chapter discusses the general blow molding process. Blow molding is a fairly straightforward process that includes melting, homogenizing, extruding, molding (blowing), cooling, and ejection. Different manufacturing plants can employ additional processes such as additional cooling or heating cycles and additives and colorants compounding. These additional processes depend on the design and intended application of the final product.


    Plastic Resin Feeding or Charging: The first step in the blow molding process is plastic feeding. This is done by conveying the plastic pellets into the extruder hopper. Vacuum pumps draw pellets from big bags or bulk containers and transfer them into the raw material silos or hoppers. A rotary feeder at the bottom of the silo controls the rate of feeding into the plastifier or extruder. Compressed air is then used to convey the stored pellets to the extruder hopper. In other systems, plastic pellets from big bags or bins can be vacuum conveyed directly to the extruder hopper without the need for a separate conveying air system.


    Plasticizing or Melting: As the plastic resin enters and goes through the extrusion machine, it is melted by continuous kneading and heating. Electric heating elements or heating bands are wrapped around the extruder barrel to provide heat for melting the polymer. The extruder screw has different sections that serve a specific purpose. These are feeding, compressing, and metering. The extruder screw is designed to provide sufficient shearing and compression to homogenize and extrude the plastic.


    Parison Extrusion or Preform Injection: This process is the preparation of the parison or preform to be inflated. This is done by extruding the plastic through free extrusion or injection into a preform mold. The different processes in producing the preform are discussed in detail.


    Sealing or Clamping: Sealing or clamping involves a split die that captures the preform. The ends of the parison (extrusion blow molding) are sealed except for one hole, typically the container opening, where the compressed air will be injected.


    Inflation or Blow Molding: This step is where the plastic takes its form. Compressed air is introduced inside the preform. This inflates the preform until it is molded according to the profile of the die.


    Cooling and Ejecting: The next step is the cooling process. Typically, as the plastic touches the die, it cools at a predefined rate which stabilizes the dimensions of the product. After cooling, the mold opens and ejects the product.


    Trimming: Flashing is generally evident in extrusion blow molding. Most blow molding machines have auto deflashing features as the dies clamp the preform. But in some instances, flash is present at the top and bottom parts of the product, especially at the opening where compressed air is injected. This excess material is trimmed by a rotating knife. To minimize wastage, some systems collect the excess materials, grind them, and feed them back to the extrusion machine.


    Leak Test: This is the typical quality control method used in manufacturing bottles or packaging materials. In this step, either vacuum or compression is generated inside the container. The machine will then check if air enters or escapes the container by monitoring the pressure. If a leak is sensed from the container, it is rejected and fed back into the system.