In injection moulding, the plastic granules are fed via a feed hopper into the externally heated injection moulding cylinder of the injection moulding machine. In larger systems, the granules can also be conveyed via a suction line directly from the material container to the hopper above the screw conveyor.
During the dosing phase, a continuous supply of material to the screw conveyor must be ensured. The plastic granules, which are still cold, enter the space between the flights of the rotating screw.
The rotational movement conveys the granules towards the tip of the screw. At the same time, they are heated by the heat supplied to the barrel from the outside, as well as by the heat generated by the friction and shearing of the material. As the temperature rises, the plastic granules are plasticised and gradually transition into a flowable state.
As it rotates, the screw moves against what is known as static pressure. This resistance helps to homogenise the plastic mass and ensures that a uniform melt accumulates in front of the screw tip.
As soon as the quantity of material required for the next injection cycle has been metered, the so-called melt cushion is formed in front of the screw tip. By this point, the plastic material must be fully plastified and homogenised so that it can subsequently be injected evenly into the mould.
The unit comprising the injection cylinder, screw and feed hopper – known as the injection unit – moves forwards and the injection nozzle makes contact with the mould. The injection mould is secured on the machine side between a rigid clamping plate and a movable mould half, known as the ejector side.
The hot plastic material required to fill the cavity is then injected into the closed mould under high pressure. The molten plastic first flows through the runner system and is directed via the gate points into the designated mould cavities of the injection mould.
The geometry of the component has a direct influence on the flow of material. Varying wall thicknesses, long flow paths, narrow sections or poorly positioned gate locations can lead to varying filling rates and cooling conditions.
Such differences may become apparent later on, for example, in the form of warping, seam lines, air pockets or visible surface changes.
During the filling phase, the injection pressure ensures that the molten plastic completely fills the cavity. Hold pressure is then applied to compensate for the volumetric shrinkage that occurs as the molten plastic cools. This prevents the component from contracting too much as it solidifies and cools, which could otherwise lead to sink marks, for example.
The clamping force keeps the mould halves securely closed during the injection and holding pressure stages, preventing plastic from escaping at the parting line and forming flash.
The cavities of an injection mould are temperature-controlled to ensure that the plastic part cools with as little stress as possible. During the subsequent cooling phase, the holding pressure is maintained to compensate for the volumetric shrinkage of the still-hot plastic material and to stabilise the shape of the moulded part.
Once the cross-section of the injection point has cooled completely, the holding pressure can be released. The injection nozzle of the injection moulding unit is then lifted away from the mould and a new dosing cycle is initiated.
Once the moulded parts in the cavity have cooled sufficiently and retained their shape, the injection mould can be opened and the parts removed.
The moment the mould opens forms an integral part of the defined process sequence. Ejectors and removal devices ensure that the finished injection-moulded parts are removed from the mould in a reliable and reproducible manner.
The next injection moulding cycle begins when the mould closes again.
What sounds simple is actually a technically challenging process
Temperatures, pressures, speeds, times, materials and the tooling concept must all be coordinated so that the injection-moulded parts can be produced reliably and to a consistent standard of quality.
Not every plastic component design is equally suitable for injection moulding. Consideration should be given, right from the design stage, to how the component will later be manufactured and removed from the mould.
Consistent wall thicknesses, appropriate radii, well-designed draft angles and a carefully considered arrangement of the gate locations can significantly simplify the manufacturing process.
At the same time, the design must meet the requirements of its intended use. This is precisely where the challenge lies: the component must not only work – it must also be feasible to manufacture.
HAMCO uses the injection moulding process to manufacture engineering plastic components and tailors the material, mould and process to the specific application.
In doing so, we look not only at the finished component, but at the entire process leading up to it. After all, the question of which manufacturing process is the right one should be answered as early as possible.
Now you know how the injection moulding process works. The next question is: Is injection moulding the right manufacturing process for your application?
HAMCO supports you in assessing suitable component geometries, tool design and implementation right through to series production.
Find out more about the Leistungsangebot Spritzgießen by HAMCO.
Large and small quantity requirements Direct processing
Large and small quantity requirements
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