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.