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Injection Molds A.Basisc mold design An injection mold consists of lease two halves that are fastened to the two platens of the injection molding machine so that they 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 injection 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. B.Types of Ejection For product ejection to occur,the mold must open.The shapes of the molded product determines whether it can be eject 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 provision necessary for product ejection.Injection-molded product can be classified as: a) Products without undercut(e.g.,plaque,strips,half-shell,cup). b) Products with external undercuts or lateral opening(e.g.,spools and bobbins,beverage crates,threaded bolt). c) Products with internal undercuts(e.g.,threaded closure,housings). d) Products with external and internal undercuts(e.g.,bumper fascial,electrical and automotive instrument housings,cameras,etc.). C.Design Rules 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 occaionally,it may work out better if a rule is ignored and an alternative way is selectd.In this way to go .The designer must ever be open to new ideas and methods,to new molding and mold materials that may affect these rules. D.The Basic Mold e) Mold Cavity Space The mold cavity space is a shape inside the mold, “excavated”(by machining the mold material)in such a manner that when the molding material(in our case,the plastic)is forced into this space it will take on the shape of the cavity space and,therefore,the desired product.The principle of a mold is almost as old as human civilization.Molds have been used to make tools,weapons,bells,status,and household articles,by pouring liquid metals(iron,bronze)into sand forms,such molds,which are still used today in foundries,can be used only once because the mold is destroyed to release the product after it has solidified.Todayl,we are looking for permanent mold that can be used over and over.Now molds are made from strong,durable materials,such as steel,or from softer aluminum or metal alloys and even from certain plastics where a long mold life is not required into the cavity space with high pressure,so the mold must be strong enough to resist the injection pressure without deforming. 2.Number of Cavities Many molds,particularly molds for lager product, are built for only I cavity space(a single-cavity mold),but many mold,espacially large production molds,are built with or more cavities.The reason for this purely economical.It takes only little more time to inject several cavities than to inject one.For example,a 4-cavity mold requires only(approximately)one-fourth of the machine time of a single-cavity mold.Conversely,the production increases in proportion to the number of cativies.A mold with more cavities is more expensive to built than a single-cavity mold,but(as in our example)not necessarily 4 times as much as a single-cavity mold.But it may also require a large machine with large platen area and more clamping capacity,and because it will use(in this example)4 times the amount of plastic,it may need a large injection unit,so the machine hour cost will be higher than for a machine large enough for the samller mold.Today,most multicavity molds are built with a prefeered number of cavties: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 deimensioning,or manufacturing,and for symmetry around the center of the machine,which is highly desirable to ensure equal clamping force for each cavity.A smaller number of cavities can also be laid out in a circular pattern,even with odd numbers of cavities,such as 3,5,7,9.It is also possible to make cavity layouts for any number of cavities,provided such rules as symmetry of the projected areas around the machine centerline are observered. 3. Cavity Shape and Shrinkage The shape of the cavity is essentially the“negative”of the shape of the desired product,with dimensional allowances added to allow for shrinking of the plastic.The shape of the cavity is usually creeated with chip-removing machine tolls,or with electric discharge machining(EDM),with chemical etching, or by any new method that may be available to remove metal or built it up,such as galvanic processes.It may also be created by cating(and then machining)certain metals(usually copper or zinc alloys)in palster molds created from moldels of the product to be made,or by casting(and then machining)some suitable hard plastic(e.g.,epoxy resins).The cavity shape can be either cut directly into the mold plates or formed putting inserts into the plates. E.Cavity and Core By convention,the hollow(concave) protion of the cavity space is called the cavity.The matching,often raised(or convex) 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.The alternative to the cup shape is the flat shape.In this case,there is no specific convex portion,and sometimes,the core looks like a mirror image of the cavity.Typical examples for this are plastic knives,game chips,or round disks such as records.While these items are simple in appearance,they often present serious molding problrms for ejection 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(“hot”)side. 注：該文選自 張小黎 李海梅主編《塑料加工和模具專業(yè)英語(yǔ)》 化學(xué)工業(yè)出版社，2004