To determine suitability of moulding machines for making a particular product, the following machine specifications need to be checked:
Maximum shot weight capacity of the machine should be more than the total weight of article/articles ( in case of multicavity mould) plus the runner system.
Injection pressure should be sufficient to fill the cavities without any short shots.
Clamping tonnage required to hold the mould in locked condition should be adequate (otherwise there will be flashes)
Daylight opening of the machine should be higher than sum total of mould height, plus article neight, plus space required for removal of articles.
Injection moulding cycle
A typical sequence of operations from startup is as follows:
Starting with an empty cylinder, raw material from the feed hoper falls onto the rear flights of the screw which conveys material to the front of the cylinder. During its passage along the cylinder it is plasticised to a fluid state with the help of external heaters on the barrel. Some material may escape through the nozzle but the back pressure is generally sufficient to push the screw back in the cylinder and to provide a reservoir of fluid plastic in the front of the cylinder for injection.
The mould closes and the cylinder moves forward units carriage until the nozzle is in contact with the entrance of the mould.
The screw is moved forward by the hydraulic cylinder at the rear of the machine and the injection takes place.
After a short interval ( the holding time), the screw rotates, creating some pressure in the barrel, which offers it back against low pressure in the hydraulic cylinder, until the limit switch operates, stopping the rotation. This plasticises material ready for the next shot.
The mould opens, the article is ejected and the mould closes again ready for the next cycle.
Stages (2) to (5) repeat.