Author Archives: Elito

About Elito

Mold Design Engineer (contract designer) with more than 13 years experience creating mold designs for high volume manufacturing. Have necessary know-how to produce high quality injection molds at the least processing cost and most timely manner available. Superior problem-solving and time-management abilities. Worked with Japanese mold design team. Proficient with 2D CAD, 3D CAD, MS Office.

Injection Machine Tonnage Computation

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How to compute for injection machine tonnage:

  1. Projected area or shadow area including runners and gates: A
  2. Peak pressure: P
  3. Clamping force = A x P
  4. Include factor of safety
  5. Choose the machine.

First, calculate the area, A, projected by the part that you will be molding. Include runners and gates and total number of cavities.

Find the pressure, P, that you will use in molding the part. There are multiple pressure in molding process and you have to identify the peak pressure.

Compute for the ideal clamping force, F:

F = P x A

Include factor of safety to prevent flashing on parting lines. This should be more than 1.

Select the machine tonnage that you will be using. The common practice is you have to choose the machine tonnage so that it is greater than the ideal clamping force that you have computed earlier.

For further reference, go to the thread.

How much does a mold cost?

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The cost of injection mold is dependent on the shape or geometry of the molded part, mold steel cost including standard components, and engineering cost (labor cost). It is important to note that labor and material cost greatly varies on geographical location. Labor cost from US, EU, or Japan can be 10 to 20 times more than that of China, India or Philippines but the quality is almost same. Yes, almost same quality. Well, slightly lower but sometimes better.

Mold Cost U.S. versus China

Putting out the geographic factor, the mold can be classified by its precision type and usage factor. Precision types are explained below:

  1. High Grade (HG) molded parts are parts with all features having important functionality. The product drawings are completely detailed showing all features being dimensioned with a very tight dimensional tolerances. The general part tolerance is (+/-)0.15mm while specified tolerances are as tight as (+/-)0.02mm. Every part dimensions will be computed individually and must be remodeled into CAD to ensure accuracy and fit. The precision level of every mold part is very high because it involve a lot of fits and shut-offs. A typical example of these parts are wiring harness connectors.
  2. Medium Grade (MG) molded parts are parts with fewer important functional parts than HG. The general dimensional tolerance are usually (+/-)0.25mm, specified and non-specified. Functional shapes such as clip locks does not go tighter than (+/-)0.05mm. Critical parts does not exceed 50% of the total specified dimensions. A typical example of these are auto fuse box and insulators.
  3. Shape Parts (SP) are part where only a few shapes are functional. 3D CAD file from customer can be used directly with minimal modification for mold making. Examples are caps and covers.

COST ANALYSIS OF PLASTIC INJECTION MOLDS

Classification by usage are as follows:

CLASS-101 CLASS-102 CLASS-103 CLASS-104 CLASS-105
Cycles 1M and up 500K to 1M 100K to 500K 500 to 100K Lower than 500
Mold base min hardness 28 HRC 28 HRC 8 HRC mild steel or AL mild steel or AL
CAV and CORE min hardness 58 HRC 48 HRC 38 HRC 30 HRC mild steel or AL

Molds are made differently so mold shops should have their own quotation standards. Very busy shops tend to quote higher than shops with very few jobs. The simpler mold, the better and cheaper.

Mold type and classification chart was referenced from EMAMOLD.


Cavity and Core

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This article describes the cavity and core, its processing method, and its arrangement into the mold. More reference like this are described in Mold-Making Handbook

Cavity and core

cav-core

Core and cavity can be classified into 2 methods, integrated and nested method. You can choose which method will define your cavity and core.

Integrated: When the molded part is formed directly by the die.

Nested: When the molded part is formed by the die components.

Nested Cavity and Core

Benefits:

  • There is no restriction to the geometry of the molded part. Processing efficiency can be done when components are separated.
  • Mold materials can selected for better wear resistance and temperature control.
  • Damaged components can be replaces easily and cost effectively.

Mold-Making Handbook
Engineering References)

Mold Structure

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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 molded part, its production volume, molding material/resin, 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.
  • Tooling cost is normally lower.
  • Side gate, submarine gate, and direct gate system are typical. More about gate system.

Drawbacks:

  • 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.

How to design mold cavity and core in 2D Autocad

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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.

Product to mold drawing

7 Steps:

  1. 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.
  2. 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.
  3. 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.
  4. 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”.
  5. 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.
  6. Decide the ejector location. Divide the mold drawing as you wish or as your process capability would dictate. Consider dividing on gas vents.
  7. You can then derive or trace your cavity and core drawings using your mold drawing as reference.

A Typical Injection Mold Design Guide

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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:

  1. 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
  2. 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.
  3. Determine the gate location and size.
  4. Determine the location where ejector pin marks are prohibited.

Part 2 – Mold base layout:

  1. Place cavities close to the center of the mold to minimize base size and runner length.
  2. Ensure that the molded part remains on the movable half (ejector half) upon opening of PL to facilitate proper ejection.
  3. Waterlines should be placed as evenly as possible to the contours of the cavity.
  4. Use support pillars underneath the cavity pockets.
  5. Use ejector guides for molds with small ejector pins and rectangular ejector pins.
  6. Provide eye-bolt hole for ease of mounting and dismounting.
  7. Install mold opening prevention locks on the operator side.
  8. 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:

  1. Check material shrinkage. Locate portions (corners) for possible significant deflection and deformation.
  2. Maintain uniform wall thickness.
  3. Draft angle should be within dimension tolerance.
  4. Divide core blocks to simplify machining and provide gas vent path.
  5. Gate, small cores, and cores with shut-off fittings are better designed as insertable components for easy modification and repair.
  6. Watch out for possible deformation of core pins.
  7. Position the ejector pins on the ribs and other high strength locations. Ensure ejector balance.
  8. 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:

  1. Bevel edges. Whenever possible use machine to bevel the edges.
  2. 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.

Injection Mold Sprue and Runner

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sprue-runner-gate

This will describe the sprue and runner system of molds. More comprehensive reference can be found in Runner and Gating Design Handbook: Tools for Successful Injection Molding

Related posts:

Sprue

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:

conventional-runner

Improved 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.

balanced-runner

Hot Runners or sometimes called runnerless, can be more economical in high volume injection molding.

Try Pro/E Wildfire 3.0 for 30 days

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This post is old. New version of ProE Wildfire 5.0 is coming out soon.

Find out why 500,000 design engineers use Pro/Engineer to design great things! Now you can download this CAD for free 30-days evaluation.

ProE-free

It includes 7 tutorials: introduction, part modeling, sketching, drawing creation, patterns, assembly components, and re-using 2D files. When you finish the tutorials in one day you can have 29 more days to practice and familiarize with the software. If you are a mechanical engineer and you don’t have a job then try to study this CAD 8 hours a day. By the time the free license expires you should already completed 240 hours CAD training that you can add to your resume. Cool idea!

Functions of Mold Base Parts

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moldpartsFixed 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.

moldparts1Support 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).

Different types of gate for injection molding

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The narrow and shallow portion of the runner as it enters the cavity is called the gate.

Runner and Gating Design Handbook: Tools for Successful Injection MoldingIndustrial Design Books)

Listed here are the different types of gate with their characteristics. Sprue and runner are described here.

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.

side-gate

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.

submarine-gate

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.

fan-gate

Film gate: Similar to fan gate except that it is commonly used for thin and flat plate products.

film-gate

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.

pin-gate

Runner and Gating Design Handbook: Tools for Successful Injection Molding

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.

banana-gate

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.

direct-gate