Working with Moulders

Injection moulding is a batch process, normally used in the production of automotive components, bottle caps and other packaging items. A very similar process to Blow moulding, whereby a screw will mix and convey polymer and masterbatch through a heat barrel. The molten polymer mixture is pushed forward and eventually injected to fill the void in the mould. Once cooled the mould will open and eject the moulded component.

This is the most effective way to make storage containers, bottles, plastic drums and tubs. These are usually produced in a batch process. The cycle time is defined as the time it takes to complete one cycle of the process: from injection of the molten polymer to the release of the final moulded product.

The blow moulding machine consists of a heated barrel with a single screw element inside. The feed polymer is fed through the heated barrel, being conveyed by the screw. Eventually the softened polymer is pushed through and out of a die hole to form a tube shape.

The softened polymer tube is pushed into 2 halves of a mould which closes tight around the molten polymer tube. Once squeezed and sealed, it is inflated with air to form the desired shape according to the mould. The cooling process will harden the resulting polymer mould which will be ejected.

Injection stretch blow moulding (ISBM) is best known for producing PET bottles commonly used for water, juice and a variety of other products. One of the major advantages of stretch blow moulding is the ability to stretch the preform in both the hoop direction and the axial direction. This biaxial stretching of material increases the tensile strength, barrier properties, drop impact, clarity, and top load in the container. With these increases it is usually possible to reduce the overall weight in a container by up to 15% compared to other methods.

Single-stage stretch blow moulding uses the extruder to inject the polymer into a preform mould where the plastic is rapidly cooled to form the preform. The preform is then reheated and placed in the bottle mould. The then softened polymer stretches to about twice its original length. Compressed air is then blown into the stretched parison to expand to the bottle’s mould. Once the bottle is cooled the mould is opened and the finished bottle is emptied from the mould cavity. This technique is most effective in specialty applications, such as wide-mouthed jars, where very high production rates are not a requirement.

Two-stage stretch blow moulding is the same as single-stage, except the preforms are already made. The single-stage process is usually done using one machine, where the two-stage process uses preforms that have already been made and cooled. This allows companies to either make or buy their own preforms. Because of the relatively high cost of moulding, this is the best technique for producing high volume items such as carbonated beverage bottles. In this process, the machinery injection moulds a preform, which is then transferred within the machine to another station where it is blown and then ejected from the machine. This type of machinery is generally called injection stretch blow moulding (ISBM) and usually requires large runs to justify the very large expense for the injection moulds to create the preform and then the blow moulds to finish the blowing of the container. This process is used for extremely high-volume runs of items such as drinks bottles.

There is a clear difference in definition between a sheet and film. To say ‘film’ to a polymer product, the thickness of the products must be below 0.5mm. A product between 0.5mm and 12.5mm, it is called a sheet. LDPE is generally used in film production.

In a similar way to other moulding processes, a screw will transport polymer through a heated barrel. The molten polymer will enter the die, and through the calendaring (cooling rollers) process in order to perfect the gauge and form the film or sheet. The calendaring process slowly cools the sheet until it is finished by either cutting to the desired size sheets or forming into roll stock.

Rotaional moulding provides a means of producing PE articles of a size and shape that cannot be manufactured economically in any other way. It is particularly suited for large, hollow, seamless parts such as industrial and agricultural tanks, kayaks, traffic bollards and marine floats – in fact anything that is hollow, enclosed or open-ended.

In this process a hollow mould is filled with a powdered polymer (typically LLDPE). The mould is slowly rotated around 2 perpendicular axis with heating, which encourages the polymer to melt and adhere to the inside of the mould. Mould rotation is constant and continuous throughout the heating and cooling phases to ensure an even wall thickness and to avoid deformation such as sagging, warpage and shrinkage.

3D printing is becoming increasingly popular. It’s ideal for low volume production runs, developing prototypes and intricate mouldings. Typically it uses CAD to create three-dimensional objects through a layering method. Usually using PA, ABS or PLA as the base resin, when heated the resins become pliable, they are deposited as layers according to the design and the process. This process extrudes the polymer, in filament form through a heated extruder to form layers and create the predetermined shape.

Because parts can be printed directly, it is possible to produce very detailed and intricate objects, often with functionality built in and negating the need for assembly.

This process is utilised to manufacture products for various industries. It’s widely used in construction applications such as pipes, window profiles, flooring and even glue sticks.

Extrusion begins in the same way as other plastic manufacturing processes. Raw plastic material, usually in the form of high-density polyethylene (HDPE), polyvinyl chloride (PVC), or other suitable polymers, is fed into an extruder. Within the extruder, the material undergoes a controlled melting process. A rotating screw conveys the material through a heated barrel. The molten plastic is then forced through a shaping die that imparts the desired cross-sectional profile to the material, resulting in a continuous tube or pipe. As the extrusion emerges from the die, it undergoes a cooling process to solidify and maintain its shape. It’s then cut to the desired length.