The introdution of the Injection Mold

 1. Mold basic knowledge

  1.1 Introduction

 There is a close relationship with all kinds of mold,which are refered to our daily production, and life in the use of the various tools and products, the large base of the machine tool, the body shell, the first embryo to a small screws, buttons, as well as various home appliances shell. Mold’s shape determine the shape of these products, mold’s precision and machining quality determine the quality of these products,too. Because of a variety of products, appearance, specifications and the different uses,mold devide into Die Casting into the mould, die forging, die-casting mould, Die, and so on other non - plastic molds, as well as plastic mold. In recent years, with the rapid development of the plastics industry, and GM and engineering plastics in areas such as strength and accuracy of the continuous enhancement, the scope of the application of plastic products have also constantly expanded, such as: household appliances, instrumentation, construction equipment, automotive, daily hardware, and many other fields, the proportion of plastic products is rapidly increasing. A rational design of plastic parts often can replace much more traditional metal pieces. The trend of industrial products and daily products plasticed is rising day after day.

  1.2 Mold general definition

In the industrial production， with the various press and the special instruments which installed in the press,it produces the required shape parts or products through pressure on the metal or non-metallic materials, this special instruments collectively call as the mold.

  1.3 Mold general classification

Mold can be divided into plastic and non - plastic mould: (1) Non-plastic mould: Die Casting, forging Die, Die, die-casting mould and so on. A. Die Casting - taps, pig iron platformB. Forging Die - car body C. Die - computer panel D. Die Casting Die - superalloy, cylinder body (2) For the production technology and production, the plastic mold are divided into different products: A. Injection molding die - TV casing, keyboard button (the most common application) B. Inflatable module - drink bottles C. Compression molding die - bakelite switches, scientific Ciwan dish D. Transfer molding die - IC products E. Extrusion die - of glue, plastic bags F. Hot forming die - transparent shell molding packaging G. Rotomoulding mode - Flexible toy doll. Injection Molding is the most popurlar method in plastics producing process. The method can be applied to all parts of thermoplastic and some of thermosetting plastics, the quantity of plastic production is much more than any other forming method.Injection mold as one of the main toolsof injection molding processing,whosh production efficiency is low or high in the quality of precision、 manufacturing cycle and the process of injection molding and so on,directly affect the quality of products, production, cost and product updates, at the same time it also determines the competitiveness of enterprises in the market's response capacity and speed. Injection Mold consists of a number of plate which mass with the various component parts. It divided into: A molding device (Die, punch)B positioning system (I. column I. sets) C fixtures (the word board, code-pit) D cooling system (carrying water hole) E thermostat system (heating tubes, the hotline) F-Road System (jack Tsui hole, flow slot, streaming Road Hole) G ejection system (Dingzhen, top stick).

  

1.4 Type of mold

It can be divided into three categories according to gating system with the different type of mold :(1) intake die: Runner and gate at the partig line,it will strip together with products  when in the open mode,it is the most simple of design, easy processing and lower costing.So more people operations by using large intake system. (2) small inlet die:It general stay in the products directly,but runner and gate are not at the partig line.Therefore,it should be design a multi-outlet parting line.And then it is more complex in the designing, more difficult in processing, generally chosing the small inlet die is depending on the product’s requirements. (3) hot runner die:It consists of heat gate, heat runner plate, temperature control box. Hot runner molds are two plate molds with a heated runner system inside one half of the mold. A hot runner system is divided into two parts: the manifold and the drops. The manifold has channels that convey the plastic on a single plane, parallel to the parting line, to a point above the cavity. The drops, situated perpendicular to the manifold, convey the plastic from the manifold to the part. The advantages of hot runner system ：(1)No outlet expected, no need

processing, the whole process fully automated, save time and enhance the efficiency of the work. (2) small pressure loss.

  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. For moulds containing intricate impressions,and for multi-impression moulds, it is not satisfactory to attempt to machine the cavity and core plates from single blocks of steel as with integer moulds. The cavity and core give the molding its external and internal shapes respectively, the impression imparting the whole of the form to the molding.

 

 3.4 The Parting Line

To be able to produce a mold,we must have ta least two separate mold halves,with the cavity in one side and the core in the other.The separation between these plates is called the parting line,and designated P/L.Actually,this is a parting area or plane,but,by cinvention,in this intext it is referred to as a line. The parting surfaces of a mould are those portion of both mould plates, adjacent to the impressions, which butt together to form a seal and prevent the loss of plastic material from the impression.The parting line can have any shape, many moldings are required which have a parting line which lies on a non-planar or curved surface,but for ease of mold manufacturing,it is preferable to have it in one plane.The parting line is always at the widest circumference of the product,to make ejection of the product from the mold possible.With some shapes it may be necessary to offset the P/L,or to have it at an angle,but in any event it is best to have is so that itan be easily machined,and often ground, to ensure that it shuts off tightly when the mold is clamped during injection.If the parting line is poorly finished the plastic will escape,which shows up on the product as an unsightly sharp projection,which must then be removed;otherwise,the product could be unusable.There is even a danger that the plastic could squirt out of the mold and do personal danger.

  3.5 Runners and Gates

Now,we must add provisions for bringing the plastic into these cavity spaces.This must be done with enough pressure so that the cavity spaces are filled completely before the plastic "freezes"(that is,cools so much that the plastic cannot flow anymore).The flow passages are the sprue,from wherethe machine nozzle contactss the mold,the runners,which distribute the plastic to the individual cavities, the wall of the runner channel must be smooth to prevent any restriction to flow. Also, as the runner has to be removed with the molding, there must be no machine marks left which would tend to retain the runner in the mould plate.And the gates which are small openings leading from the runner into the cavity space. The gate is a channel or orifice connecting the runner with the impression. It has a small cross-sectional area when compared with the rest of the feed system. The gate freezes soon after the impression is filled so that the injection plunger can be withdrawn without the probability of void being created in the molding by suck-back.

  4. The injection molding machine process

Injection Mold is installed in the injection molding machine, and its injection molding process is completed by the injection molding machine. Following is the injection molding machine process.The molding machine uses a vacuum to move the plastic from the dryer to it's initial holding chamber. This chamber is actually a small hopper on the back of the "barrel" of the machine。The barrel is where all the real work is done and it's essentially a large screw housed in a heater which moves the plastic closer to the mold. As the screw turns, the plastic traverses the barrel and reaches a molten state. Only when it's molten can it be injected into the mold with a rapid turn of the screw. As the chamber in front of the screw becomes filled, it forces the screw back, tripping a limit switch that activates a hydraulic cylinder that forces the screw forward and injects the fluid plastic into the closed mold. The gate freezes soon after the impression is filled so that the injection plunger can be withdrawn without the probability of void being created in the molding by suck-back. The tip of the barrel is called the "nozzle" and from this point to the cavity in the mold the material is not heated and is constantly cooling. The "runner" is the cooled/set plastic that extends from the nozzle to the cavity and is process scrap. Actually, the cooled material from the nozzle to the mold is the "sprue" but it's connected to the runner. There are ways around having sprues and runners, but it's beyond the scope of what we're talking about here. Most people have probably seen runners before and not realized it. The most likely place to see them are in model airplane/car kits as the individual components are left attached to the runner system. Typically the runners are ejected into a chute below the mold or else they will be picked out of the mold by a robotic arm and dumped into a regrinder. The regrinder chops the runners into bits and prepares them to be moved back into the dryer. Though, once plastic has been heated, it degrades a bit and some molding processes will not allow for "regrind" to be mixed back in with virgin material as it can cause problems with the final part. In cases like this the regrind is used elsewhere or it is discarded completely. Shrinkage plays a crucial role in molding and most plastic shrinks 20% as it cools. In order to combat this phenomenon in critical plastic pieces (not really toys) it is up the mold designer to build this factor into his design so that the finished parts will meet the original product design specifications. "Ejector pins" are part of the mold itself and are used to push the molded parts from the cavity once the mold is opened. This ejecting process is controlled by the molding machine. The molding cycle is basically "close - shoot - open - eject". The mold is clamped into the machine with the front half remaining stationary (on the barrel side of the machine) and the back half being the movable half. Also on this movable half are the ejector pins that push the molded parts from the cavity. The parting surfaces of a mould are those portion of both mould plates, adjacent to the impressions, which butt together to form a seal and prevent the loss of plastic material from the impression.When all molds open up, the parts are stuck to the back half of the mold so that they can be ejected with the built-in pins.

  These are related to the introduction of the injection mold.