Mould structure varies with plastic varieties and performance, plastic product's shape and structure and injectionmachine type, but its basic structure is consistent. Mould mainly consistsof feed system, temperature adjusting system, molding components and structura lcomponents. And feed system and molding components are the parts thatdirectly contact with plastics,changing with plastics and products. On plasticmould, they are the most complicated and variable parts with highestrequirements on processing glossiness and precision.
1. Feed System Feed system is also named as runnersystem. It's a group of feeding channels that lead plastic melt from injectionmachine to cavity. It mainly consists of main runner,sub-runner,gate andcold-slug well. It has the direct bearing on plastic product's forming qualityand production efficiency.
A. Main Runner It's a channel connecting injection machine nozzle with sub-runner or cavity.The top of main runner is concave in shape,easy forconnecting with nozzle.Inlet diameter for main runner should be slightly largerthan nozzle diameter(0.8mm) for avoiding spilling and preventing blockingcaused by their inaccurate connecting. Inlet diameter depends on the dimensionof product.Usually,it's 4-8mm. Main diameter should be expand inward an angleof3° to 5° to help runner redundancy demould.
It's a hole at theend of main runner for gathering the cold slug leftat nozzle's ends between twoinjection molding,thus protecting sub-runner orgate from blocking. If cold slugmixes into cavity, it will lead to internalstress in product. Cold-slug well'sdiameter is about 8-10mm,and depth is 6mm.For ease of demoulding, its bottom isusually supported by knockout pin. The top of knockout pin is better to bedesigned as tortuous hooked shape or set notch groove on it.Then the mainrunner redundancy will be pulled out smoothly while demoulding.
It's channelconnecting main runner with every cavity in subcavity mold.In order to makemelt fill every cavity with uniform velocity, the arrangement of sub-runner onplastic mould should be symmetrical andequidistant distribution.The shape andsize of sub-runner section have affection melt's flowing, product's demouldingand the difficulty of mould manufacturing. At same feeding rate, the runnerresistance for circular cross-section is smallest. But cylindrical runner isnot good for cooling ofsub-runner redundancy, because its specific surface issmall too. And this kind of sub-runner must be made on half mould. It takes alot of work and easy for alignment. So sub-runner with trapezoid orsemicircular section are often used on the half mould with knockout pin. Runnersurface must be polished to reduce flow resistance and offer high mould fillingspeed. Runner's dimension depends on plastic brands, product sizes andthickness. For most thermoplastic plastics, Sub-runner section width is no morethan 8mm. The largest can be 10-12mm, and the smallest can be 2-3mm. Under the preconditionof meeting the demand, minimize sectional area in case of increasing sub-runnerredundancy and extending cooling time.
It's the channel connecting main runner (or sub-runner) with cavity. Channelsectional area can be same as main runner (or sub-runner), but usually it's smaller.So it's the part where the sectional area is smallest in runner system. Shapeand dimensions of gate affect product's quality a lot.
The functions of Gate:
1) Control material flow speed.
2) Use premature condensation at this place to avoid flow backward whileinjecting.
3) Use strong shear on melt flowing through to raise the temperature, whichwill reduce apparent viscosity and enhance liquidity.
4) Easy for product to separate with runner system. The design for shape, sizeand position of gate depends on plastics properties, product dimension, size andstructure. Usually, gate section shape is rectangular or circular. Sectionalarea should be small and its length should short. This is not only based on theabove roles, but also that it's easy for small gate to be larger and hard forlarge gate to be smaller. The position of gate often is at the thickest placeof product without any influences on its appearance. The design for gate size shouldtake plastic melt's properties into consideration. Cavity is the space forforming plastic product in mould. And the elements used for making cavitymodule are called by a joint name--molding component. Every molding componentoften has a specified name. The molding components constituting product’sexternal shape are named as concave die (also known as female mould) .Thoseconstituting product's internal shape (like hole,groove,etc.) are named as coreor terrace die (also known as male mould).For designing molding components, thecavity overall structure should be decided firstly according to plasticproperties, product geometric shape, dimensional tolerance and operating requirement.Secondly,chooseparting line, position of gate and vent hold and demoulding method based on thedetermined structure. Finally,according to product dimension, design thecomponents and their compound mode. There is high pressure while plastic meltenters cavity, so molding components need reasonable material selection andverification for strength and stiffness. In order to make plastic product'sappearance clean and aesthetic and easy to demould, for all surfaces touchingplastic, their roughness should be Ra>0．32um and must becorrosion resisting. Molding components usually need heat treatment to enhancetheir hardness. And they need be made of corrosion resistant steel.
2.Temperature Adjusting System
For meeting the requirement of injection molding technology on mold temperature,it needs temperature adjusting system to adjust mold temperature. For injectionmould with thermoplastic plastics, mainly cooling system is designed forcooling mold. Common method for mold cooling is setting water cooling channelin mould and using circulating cooling water to take the heat of mould out. Formould heating, we can not only use hot water or steam in cooling water channel,but also install heating elements on mould mould interior and periphery.
Molding components consist of core and cavity. Core forms product's internal surface,andcavity forms product's external surface. After closing mould, core and cavitycompose the mould cavity. As technical and manufacturing requirements,sometimes core and cavity are combinations of several blocks, and sometimes area whole part with inserts only for places that easy to damage and hard toprocess.
It refers to the elements composing mould structure, including components for guiding,demoulding,core-pullingand parting, like front and back clamp plate,front and back fixing plate,bearingplate,bearing pillar,Guide Pillar,striper plate,ejector rod and return pin.
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