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2、Injection Mold
There are many rules for designing molds.These rules and standard practices are based on logic,past experience,convenience,and economy.For designing,mold making,and molding,it is usually of advantage to follow the rules.But occasionally,it may work out better if a rule is ignored and an alternative way is selected.In some texts,the most common rules are noted,but the designer will learn only from experience which way to go.The designer must ever be open to new ideas and methods,to new molding and mold material that may affect these rules.The process consists of feeding a plastic compound in powdered or granular form from a hopper through metering and melting stages and then injecting it into a mold.Injection molding process: Mold is a production of plastic tool. It consists of several parts and this group contains forming cavities. When it injects molding, mold clamping in the injection molding machine, melting plastic is Injected forming cavities and cooling stereotypes in it, then it separate upper and lower die,it will push the production from the cavity in order to leave the mold through ejection system, finally mold close again and prepared the next injection. The entire process of injection is carried out of the cycle.
An injection mold consists of at least two halves that are fastened to the two platens of the injection molding machine so that can be opened and closed.In the closed position,the product-forming surfaces of the two mold halves define the mold cavity into which the plastic melt is injected via the runner system and the gate.Cooling provisions in the mold provide for cooling and solidification of the molded product so that it can be subsequently ejected.
For product ejection to occur,the mold must open.The shape of the molded product determines whether it can be ejected simply by opening the two mold halves or whether undercuts must be present.The design of a mold is dictated primarily by the shape of the product to be molded and the provisions necessary for product ejection.Injection-molded products can be classified as:
1).Products without undercuts.
2).products with external undercuts of lateral openings.
3).products with internal undercuts.
4).products with external and internal undercuts.
3.The composition of injection mold
3.1 Mold Cavity Space
The mold cavity space is a shape inside the mold,when the molding material is forced into this space it will take on the shape of the cavity space.In injection molding the plastic is injected into the cavity space with high pressure,so the mold must be strong enough to resist the injection pressure without deforming.
3.2 Number of Cavities
Many molds,particularly molds for larger products,ate built for only 1 cavity space,but many molds,especially large production molds,are built with 2 or more cavities.The reason for this is purely economical.It takes only little more time to inject several cavities than to inject one.Today,most multicavity molds are built with a preferred number of cavities:2,4,6,8,12,16,24,32,48,64,96,128.These numbers are selected because the cavities can be easily arranged in a rectangular pattern,which is easier for designing and dimensioning,for manufacturing,and for symmetry around the center of the machine ,which is highly desirable to ensure equal clamping force for each cavity.
3.3 Cavity and Core
By convention,the hollow portion of the cavity space is called the cavity.The matching,often raised portion of the cavity space is called the core.Most plastic products are cup-shaped.This does not mean that they look like a cup,but they do have an inside and an outside.The outside of the product is formed by the cavity, the inside by the core.Usually,the cavities are placed in the mold half that is mounted on the injection side,while the cores are placed in the moving half of the mold.The reason for this is that all injection molding machines provide an ejection mechanism on the moving platen and the products tend to shrink onto and cling to the core,from where they are then ejected.Most injection molding machines do not provide ejection mechanisms on the injection side.