This will describe the structure of 2-plate, 3-plate, and runnerless molds.
The basic structure of a molding die is determined by the geometry of the product, its production volume, molding material, and consideration of other technical and production constraints such as positioning of gate.
2-Plate Mold Configuration
The mold is divided by parting-line (PL) into two parts, the movable and fixed side.
Features:
The structure is simpler compared to 3-plate and runnerless mold.
Side gate and direct gate are not suited to save labor cost through automation.
3-Plate Mold Configuration
The mold is divided by 2 PL into three separate parts. The 1st PL is on the runner separating the runner stripper plate and the fixed side plate. The pin-gate is automatically cut off from the product on 1st PL. The 2nd PL is on the product separating the movable and fixed sides.
Features:
Suitable for full automation.
Drawbacks:
Mold structure is more complex than 2-plate structure.
Tooling cost is higher.
Runnerless Mold Configuration
2-plate and 3-plate molds produce product plus runner. Runnerless molds can be achieved by introducing hot runners.
Features:
Runners will not be discarded at all so cost of material is less.
Sprue and runner is always in the heater maintaining its fluid state for good filling of molten plastics to cavities.
There are several advantages for high volume production.
Suitable for full automation.
Drawbacks:
Complex structure.
Much higher price.
It takes time to replace the material.
This post will be updated will illustrations later.
The old fashioned way but still being used by some mold manufacturers. Designing a mold cavity and core in 2D. Some mold designers may use different procedure. You can use Autocad, Solidworks, ProE, or just plain tracing paper and pencil in this mold design tutorial.
7 Steps:
Calculate for the mold dimension using the shrinkage factor.
The shrinkage factor can be determined by resin material properties or by experimenting. For example the PBT has a shrinkage of 18/1000.
Compensate for the tolerance and other possible deformation.
Include the draft angle whenever possible. The draft angle should be within the dimensional tolerance.
Draw product drawing using the calculated mold dimension. Include the embossed texts if it is a part of the product drawing. It is a good idea to draw the embossed texts using lines and curves. If your CAD is capable of “reflecting a text” then you are in advantage.
Mirror or flip your product drawing. The mirrored drawing will be your mold drawing. Notice that the embossed texts were also mirrored and they became engraved text.
Decide and draw the gate location. Locate it away from small core pins to avoid damaging those pins during resin injection process. For our example, I would like to use “side-gate”.
Decide the parting line. Input the parting line changes if there are any. Parting line changes should be visible on the top view, draw that too.
Decide the ejector location. Divide the mold drawing as you wish or as your process capability would dictate. Consider dividing on gas vents.
You can then derive or trace your cavity and core drawings using your mold drawing as reference.
This checklist can be used as a general reference guide for injection mold design engineers. It is divided into 3 parts of a mold design process.
Part 1 – Requirements to start your mold design:
Check the injection machine where the mold is to be mounted. This will help you decide the size and structure of the mold for ease of installation and other factors. Important notes:
Locating ring size (or other positioning method)
Nozzle size
Method of clamping (Auto or manual)
Temperature control system
Determine the number of cavities and volume requirements. This will help you decide the material that you are going to use and other mold components that you will choose for cost effective design.
Determine the gate location and size.
Determine the location where ejector pin marks are prohibited.
Part 2 – Mold base layout:
Place cavities close to the center of the mold to minimize base size and runner length.
Ensure that the molded part remains on the movable half (ejector half) upon opening of PL to facilitate proper ejection.
Waterlines should be placed as evenly as possible to the contours of the cavity.
Use support pillars underneath the cavity pockets.
Use ejector guides for molds with small ejector pins and rectangular ejector pins.
Provide eye-bolt hole for ease of mounting and dismounting.
Install mold opening prevention locks on the operator side.
Establish pry bar groove on the corners of the mold parting line to facilitate ease of mold opening during assembly and maintenance.
By this time you may ask for the mold layout approval from the customer.
Part 3 – Cavity/core details:
Check material shrinkage. Locate portions (corners) for possible significant deflection and deformation.
Maintain uniform wall thickness.
Draft angle should be within dimension tolerance.
Divide core blocks to simplify machining and provide gas vent path.
Gate, small cores, and cores with shut-off fittings are better designed as insertable components for easy modification and repair.
Watch out for possible deformation of core pins.
Position the ejector pins on the ribs and other high strength locations. Ensure ejector balance.
Detailing/part drawing: Include all parameters needed for processing -material, quantity, surface finish/texture, dimensions, tolerances and many more. Do not assume the machinist understands everything.
Any design change and amendments to the mold must be re-approved by the customer or mold owner.
Few extras that could make your mold one step further in terms of quality:
Bevel edges. Whenever possible use machine to bevel the edges.
Minimize scratches on the mold base. Keep the work table clean.
This checklist may be updated regularly so I suggest you “bookmark” if you find it useful.
A sprue is a channel through which a molten plastic material is being injected from the nozzle of the injection machine into the mold. It has a smooth, round, tapered wall to allow smooth material flow.
Runner System
Runners are channels where material flows from the sprue to the cavities.
Conventional Runner:
Improved Runner:
Balanced Runner: Resin is evenly distributed throughout the cavities. Volume and condition is the same. There is more material used because the total runner length is longer compared to the other two.
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Fixed Clamping Plate
Holds The fixed side of the mold to the fixed platen of the injection machine.
Locating Ring
It is used to locate the center of the injection machine so that the sprue bushing and the nozzle are aligned. Commonly fitted into the counterbore in the fixed clamping plate.
Fixed Cavity Plate
Used to hold the fixed cavity block, leader pin/bushing, and sprue bushing.
Movable Cavity Plate
Used to hold the movable cavity block, leader pin/bushing.
Movable Clamping Plate
Holds The movable side of the mold to the movale platen of the injection machine.
Spacer Block
Mounted between the movable clamping plate and the movable cavity plate to give space and allow the ejector plate to move when ejecting the
part.
Ejector Retainer Plate
Holds the ejector pins and the return pins in place.
Ejector Plate
Pushes the ejector pins and return pins at the same time. Mounted to the ejector retainer plate to form the ejector unit.
Support Pillars
Bars placed between the spacer blocks to give additional support to the movable cavity plate.
Sprue Bushing
Has a tapered hole through which the material is forced into the runner. It is butted up against the nozzle of the injection machine.
Return Pins
Same as ejector return pins. Make sure that the ejector unit is back in its original position when the mold closes.
Leader Pins and Bushings
Precisely align the two halves of the mold base (fixed and movable).
Side gate: This is the most commonly used gate type and is commonly used for mold structures with 2 or more cavities. It is placed at the side of the plastic product. The gate has to be cut manually by a cutter.
Submarine gate: The positioning of this gate is flexible thoughout the sides of the plastic product. It can be placed on the fixed or movable side of the mold but the design has to be thought about carefully so that the product will not be left inside the fixed cavity. The gate automatically cuts itself as the mold opens.
Fan gate: It is commonly used for large and flat plate products. It is placed at the side of the product – same as the side gate. The gate has to be cut manually by a cutter.
Film gate: Similar to fan gate except that it is commonly used for thin and flat plate products.
Pin gate: This is possible for molding multiple cavities or parts. The gate positioning is relatively flexible at the top side of the product. The runner layout is very flexible as well. The mold base structure is complicated because it uses a 3-plate method.
Banana gate: This is not a very common gate. It is used when the visible surface of the product requires no trace of the gate. The gate automatically cuts itself as the mold opens.
Direct gate: The sprue serves as the gate. It is placed on the top side of the product. The gate has to be cut manually by a cutter. This gate type can only produce one part per shot. The molding cost is low because the material from the runner was ellimenated. The injection pressure was reduced due to direct cavity filling. The simple mold structure makes the mold cost lower.
Injection molding is an economical and very efficient method of producing plastic parts. It can produce millions of parts with exactly the same shape, dimension, and quality. Some examples of injection molded parts are the mobile phones, mouse, keyboard, and many components found inside the automobile.
What is an injection mold?
An injection mold is a device made of metal to produce a plastic product faster, less expensive, and more consistent.
How does injection molding works?
Just heat the plastic until it melts then force it into the cool mold. Allow it to solidify. Open the mold then take off the molded part.
That’s it!
Okey.. as the molten plastic material is being injected into the mold, it enters the mold opening called the sprue. From the sprue this molten material will then be distributed to the runners then it will be fored into the gate and then into the cavity.
The cavity must be filled precisely to avoid short shots but it must not be over packed (over packing is forcing more than enough material and it can damage the mold). The molten material will stay in the cavity for 30 seconds to 1 minute or more until it cools down and solidify.
When the material solidify a plastic part is formed. The mold will open and then the plastic part is being ejected. The mold closes and its ready for another shot.
Why is an injection mold expensive?
Injection molds are made up of special, high quality materials that have a good machinability property. Some standard mold components such as springs, bolts, and limit switch are prefabricated and costs lower.
Most standard components such as ejector pins, ejector sleeves, and sprue bushings are not prefabricated which mean they will only be manufactured when you order them. They are not mass produced thus they are expensive.
The cavity and core are mold components which make the mold really expensive. It involves careful design engineering and processing. Most of the tools used in making these components are very expensive.
This article describes the cavity and core, its processing method, and its arrangement into the mold.
Fixed Clamping Plate
Support Pillars
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