The three-plate mould differs from the more common two-plate design format in terms of utilizing more than one split or parting line. The tool construction is divided into three distinct plate build-ups which separate from each other on opening. One Pipe Fitting Mould opening provides clearance for component ejection，while the other allows for sprue ejection and clearance. Being a tool of increased complexity, the three-plate mould is, therefore, more time-consuming and expensive to manufacture than its two-plate cousin.This type of mould has three main parts or plates, which on opening are separated one from the other to give two daylights. The mouldings drop from one daylight and the feed system, in the case of a cold runner type, drops from the other. This facility is extremely useful as it means that the feed system can be segregated from the components by arranging, for example, for the feed system to drop onto one conveyor and the mouldings to drop onto another.One of the big advantages of this type of mould is that pin-point gates may be used for multi-impression moulds Such gates may or may not be central with respect to the component.Compared to a two-plate mould，this type of mould, therefore, has an additional plate which floats between the fixed and moving plates. The feed system is contained between the fixed plate and the center plate whereas the mouldings arc formed between the moving plate and the center plate.
The cavity is usually cut in the center or floating plate and each cavity is fed via a pin-point gate from secondary sprues which are also cut into the plate.In such tools, it is necessary to arrange not only for a sprue puller but for an ejector device for the section of the runner leading to the pin-point gate which indicates how the runner system is detached from the moulding and ejected from the mould. A runner ejector plate removes the runner from an undercut pin this was used to ensure that the feed system broke away from the moulded components. the feed system is retained in the runner plate by a groove machined into the feed system. This groove retains the runner in the runner plate sprue bush plate when the mould opens the pin then moves forward and ejects the feed system.The feed system can be reground and used again but such an operation is wasteful of both time and energy. With thermosetting materials, one does not normally regrind and re-use the cured material as it is very difficult to re-use such material without production deterioration.Figure below shows a typical buildup of a three-plate mould in which each individual cavity is filled through a centrally situated pin point gate.The general construction of the 3 plate mould is built about the opening sequence of the mould.
To fully understand the principals involved in designing such a mould, we must firstly understand the design requirements of the mould opening sequence.As the machine platens open, the intermediate plate is pulled away from the feed plate assembly. The runner system is retained on the feed plate assembly by runner puller or sucker pins. The retaining of the runner on the feed plate section serves to break the gate away from the solidified moulding while being fully supported by both core and cavity components. This stage is known as the breaking first stage or sequence.With the gates parted from the moulding and the runner clearance daylight fully extended，the mould is ready for the next stage which is runner stripping and ejection. With the runner daylight fully extended，the runner stripper plate is pulled forward by the engagement of the intermediate plate shoulder limit bolts. The runner system is then stripped off the puller pins as the runner stripper plate is pulled forward. The length of runner stripping stroke is determined by the engagement height of the runner plate shoulder bolts. This stage of the 3 plate mould opening sequence is known as runner stripping.
The ejection of the mouldings can be started by the opening of the core and cavity split line once movement of the intermediate and feed plate assemblies has been arrested. With the moulding daylight sufficiently extended the mouldings can either be ejected conventionally or stripped off the cores by the forward pulling action of the core stripper plate as the mould reaches its maximum opening stroke.Note: stages 2 and 3 are often synchronized to function together， dependent upon the design of 3 plate mould used.The explanation of the three-plate mould opening sequence serves to highlight one of the drawbacks encountered with this design of 3 plate mould, namely long opening stroke requirement. The major limiting factor concerned with employing any design of multi-daylight 3 plate mould has to be the maximum opening stroke available on the intended moulding machine. For this reason, it is wise to calculate the required working daylight clearances before embarking upon the chosen course of action，especially when dealing with deeply drawn moulding, e.g- test tubes or the like.In order to gain control over the required mould opening sequence, platen position and speed of movement has to be accurately set and controlled throughout the moulding cycle and production run. In general， the closing of the three-plate mould is achieved by the closing action of the moulding machine platens which progressively pick up and close the mould sections as they move forward.