Today we will still talking about the Plastic Molds Design about the Molds Sprue Design.
Sprue is located at the entrance of molds.It allows the melt plastic material to flow into the runner or cavity.A sprue bush is inserted on the cavity plate or runner.When direct gate is used, the plastic part will be left with a trace by the sprue. To keep the sprue bush in place, while being under pressure, a step design is usually used to fix it on a locating ring. The hole inside the sprue bush is usually conic in shape. The samller end of the hole is a curve. the large taper the hole has, the easier the sprue to pop out. Sprue bush and nozzle may come in contact on a planer surface, it facilitates a right location and tight fit.
For the plastic Molds Design,the following principles are appied to sprue bush design:First, usually the radius of the sprue bush should be larger than the radius of the nozzel about 1mm. And the smaller diameter of the cone -shaped hole should be larger than the nozzles’s diameter by 0.5~1mm.Third, minimize its length.Firth, polish the cone-shape hole along its length’s direction.
Cold-slug well usually locates at the end of the sprue.That is, at the end of the runner and on the core-retainer plate, on directly opposite side of the sprue.Its purpose is to store cold slug created intermittently by the nozzle due to heat loss and low temperature . Cold slugs could clog or slow down the material flow in runner and cavity and cause cold scars or cold marks on the plastic part. During the opening, the well can pull the slug off the sprue bush. Thus the diameter of a cold-slug well should be larger than the larger diameter of the sprue,while its length should be about the same as the larger diameter of the sprue, to promote the material flow.A sprue puller consists of cold-slug well and ejector pin. There is an ejector pin at the bottom of the cold-slug well.The ejector pin is fixed on an ejector retainer plate and co-actuated with the ejection system.
Next article, we will continue talk about the design of the runner, if you are interested, you can book this blog.And for further question, please don’t hesitate to contact with me.