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- Herausgeber: Carl Hanser Verlag GmbH & Co. KG
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
- Veröffentlichungsjahr: 2020
This applications-oriented book describes the construction of an injection mold from the ground up. Included are explanations of the individual types of tools, components, and technical terms; design procedures; techniques, tips, and tricks in the construction of an injection mold; and pros and cons of various solutions.
Based on a plastic part ("bowl with lid") specially developed for this book, easily understandable text and many illustrative pictures and drawings provide the necessary knowledge for practical implementation. Step by step, the plastic part is modified and enhanced. The technologies and designs that are additionally needed for an injection mold are described by engineering drawings. Maintenance and repair, and essential manufacturing techniques are also discussed.
Now if full color, this second edition builds on the success of the first, with updates and small corrections throughout, as well as an new expanded section covering the process chain.
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Rainer Dangel
Injection Molds for Beginners
2nd Edition
Hanser Publishers, MunichHanser Publications, Cincinnati
The Author:
Rainer Dangel, 73266 Bissingen/Teck, Germany, [email protected]
Distributed in the Americas by:Hanser Publications414 Walnut Street, Cincinnati, OH 45202 USAPhone: (800) 950-8977www.hanserpublications.com
Distributed in all other countries by:Carl Hanser VerlagPostfach 86 04 20, 81631 Munich, GermanyFax: +49 (89) 98 48 09www.hanser-fachbuch.de
The use of general descriptive names, trademarks, etc., in this publication, even if the former are not especially identified, is not to be taken as a sign that such names, as understood by the Trade Marks and Merchandise Marks Act, may accordingly be used freely by anyone. While the advice and information in this book are believed to be true and accurate at the date of going to press, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein.
The final determination of the suitability of any information for the use contemplated for a given application remains the sole responsibility of the user.
Library of Congress Control Number: 2020930377
All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying or by any information storage and retrieval system, without permission in writing from the publisher.
© Carl Hanser Verlag, Munich 2020Editor: Dr. Mark SmithTranslation: Kristin Bylund Thurnher, Meusburger Georg GmbH & Co KGFirst proofreader: Birgit Lins, Head of Translation Management, Meusburger Georg GmbH & Co KGProduction Management: Jörg StrohbachCoverconcept: Marc Müller-Bremer, www.rebranding.de, MunichCoverdesign: Max Kostopoulos
ISBN: 978-1-56990-818-1E-Book ISBN: 978-1-56990-819-8ePub-ISBN: 978-1-56990-849-5
First of all I would like to thank my readers warmly. The success of this work has shown that its creation was a valuable exercise. The extensive feedback I received was consistently positive. The English edition, like the German edition, has been well received, and it has been a pleasure to see that it has been sold throughout the world, including in China and India, where it has been actively used in companies and training institutes. The popularity of the book is also the reason why the second edition is now available in full color.
Of course I was asked several times how a mold maker came up with the idea to write such a book. How does he find the time and where does the comprehensive knowledge come from?
The motivation to write a book can be manifold. My motivation was to write a small manual for the distribution of machining centers for mold making. The sales department should understand what mold making is, what it does, which components are to be manufactured, and from which materials the individual components are made. At first I wanted to use existing documents and publications. But I came to realize that there was nothing suitable at this level for beginners or newcomers. Then the only thing left was to create something of my own.
In order to make the book understandable for everyone, the idea came to me to always use the same plastic part as the basic concept. It should be as simple as possible and concentrate on the essentials. This gave me the possibility to build up the level of difficulty of the plastic part stepwise, and to explain the thereby-arising changes simply. That is to say, the central thread throughout the whole book should be uncomplicated and understandable. After publication in the processing machine company, the books were all gone after a few days. Not only the sales department, but also other interested parties tried to get hold of one.
So what could have been more obvious than to create a large work from this small book? Especially since, as already mentioned, there was nothing comparable on the market. At first, time constraints meant that the project ran more and more behind schedule. Then a long serious illness brought me the time, which I then used. Over 2500 hours of work and about 40 designs, or revisions of designs, had to be accomplished. Including all the corrections, this project took considerably more than half a year. The result is the present book, which after its success in both German and English versions, is now available in a second edition.
Over 40 active years in mold making, more than 23 years of which I worked as an independent entrepreneur and now as a project manager, consultant, and instructor have brought the necessary knowledge and experience. My training began in the summer of 1976. I passed through the entire technological change from milling machines with a handwheel, to NC technology, and then to today’s 5-axis simultaneous machining. The first designs were produced with India ink on a drawing board, moving on via a simple 2-D CAD program already in 1995 to full 3-D CAD.
The change over the decades was not only in the technology of the production of the injection molds, but also in the necessary shift from a handicraft business to an industrial company. Today the customers of the mold maker are almost exclusively industrial companies. Certifications, creation of processes, and Industry 4.0 are keywords that have occupied the mold making industry in recent years.
This is also the reason why a new section on process chains has been included in this second edition. In addition, it has been technically extended and small errors that unfortunately crept into the first version have now also been eliminated.
I hope you enjoy reading this book and look forward to your feedback.
Rainer Dangel, February 2020
German die and mold making is a brand with global significance. The reasons for this are diverse, but the industry’s secrets to success can certainly be attributed to smart design with a great deal of know-how, top performance production engineering and quality related criteria. One major aim of this book is to disseminate this philosophy to a wider, English-speaking readership.
Rapid implementation of innovations through close information exchange between all parties is planned for the future. Injection molds today already play a key role in modern production engineering in the manufacturing industry. Visions of the future such as the “smart factory” in the context of injection molding now offer the chance to raise the energy and resource efficiency of the production process to a new level with intelligent management and network flexibility. But the basis for this is a solid knowledge of the basics of engineering and manufacturing processes in mold making. The above-mentioned topics can only be implemented based on this knowledge and wealth of experience. And this is exactly where this technical book from Rainer Dangel comes in. What is required for bringing a product into shape?
In the book the author didactically as well as technically breaks new ground in the field of technical literature for injection mold making. In a very clear way, he combines theory with practice, always focusing on the following questions: “What is this product relevant for? What needs to be solved technically for which product specifications?” And, regarding the method of the manufacturing implementation: “How and with what can I fulfil the product requirement within the scope of the design and also the manufacturing process?” Through Mr. Dangel’s technical expertise which he established and developed over many years, it quickly becomes clear when studying the book that the practical implementation of the described has great significance. Basic knowledge and solutions are holistically considered. Advantages and disadvantages are presented and discussed. The wealth of 35 years of experience, beginning with training as a tool maker to the master craftsman’s diploma then to owning a private company flows through this technical book.
“Injection Molds for Beginners”, the title of this book, hits the bull’s eye and old hands who think it is no challenge to them might be taught a lesson!
Prof. Dr.-Ing. Thomas Seul
Vice rector for Research and Transfer at the Schmalkalden University of Applied Sciences and President of the Association of German Tool and Mold Makers (VDWF).
I would like to express a heartfelt thank you to my colleagues at the Association of German Tool and Mold Makers (VDFW) for the support during the development of this book. Special thanks to Prof. Dr.-Ing. Thomas Seul, President of VDWF, for the foreword.
Formenbau Schweiger GmbH & Co. KG, Uffing am Staffelsee, Germany, Anton Schweiger (Vice President)
Formenbau Rapp GmbH, Löchgau, Germany, Markus Bay (Director of Training)
VDWF, Schwendi, Germany, Ralf Dürrwächter (Managing Director)
bkl-Lasertechnik, Rödental, Germany, Bernd Klötzer
exeron GmbH, Oberndorf, Germany, Udo Baur
Gebr. Heller Maschinenfabrik GmbH, Nürtingen, Germany, Marcus Kurringer, Jörg Bauknecht
GF Machining Solutions GmbH, Schorndorf, Germany, Gabriele Urhahn
Hans Knecht GmbH, Reutlingen, Germany, Hans Knecht
Reichle GmbH, Gravier- und Laserschweißzentrum, Bissingen, Germany, Volker Reichle, Marco Reichle
Werz Vakuum-Wärmebehandlung GmbH, Gammertingen-Harthausen, Germany, Henry Werz
AS-Beratungen, Amtzell, Germany, Andreas Sutter
3D Systems GmbH, Ettlingen, Germany (previously Cimatron GmbH)
Meusburger Deutschland GmbH, Viernheim, Germany (previously PSG Plastic Service GmbH)
Meusburger Georg GmbH & Co KG, Wolfurt, Austria
MAKINO Europe GmbH, Kirchheim-Teck, Germany
The following are not association members, but were also on hand to help me. For this a heartfelt thank you to:
Friedrich Heibel GmbH Formplast, Heuchlingen, Stefan Heibel
Carl Hanser Publishers, Munich, Ulrike Wittmann, Jörg Strohbach
Finally, I would like to thank the Translation Management department at Meusburger Georg GmbH & Co. KG, Wolfurt, Austria, in particular Kristin Bylund Thurnher and Birgit Lins, for their expert translation of my German text into English. Achieving a high-quality translation of a specialist technical book is no trivial task, and for this the professional support of Meusburger is most warmly acknowledged.
Contents
Title
Copyright
Contents
Preface to the Second Edition
Foreword to the First Edition
The Author
Acknowledgments
How to Use This Book
1 Introduction
2 Mold Types
2.1 Simple Open/Close Mold
2.1.1 Classic Structure of an Open/Close Mold
2.1.2 Guiding Elements
2.1.3 Backing Plate
2.2 Molds with Moving Elements
2.2.1 Undercut
2.2.2 Slide
2.2.3 Slide Operation
2.2.4 Latch, Clip Lock
2.2.5 Inclined Ejector
2.2.6 Forced Demolding
2.2.7 Mold Size
2.3 Mold for Threads
2.3.1 External Threads
2.3.2 Internal Threads
2.3.3 Drive Types for De-spindling
2.3.3.1 Hydraulic Unscrewing Unit
2.3.3.2 Gear Rack
2.3.3.3 High-Helix Lead Screw
2.3.3.4 Multi-cavity Molds
2.4 Multi-component Injection Molds
2.4.1 Material Pairings
2.4.2 Mold Technology
2.4.2.1 Shifting Technology
2.4.2.2 Rotary Table Technology
2.4.2.3 Sealing Slide Technology
2.4.2.4 Further Technologies
2.5 Stack Mold
2.5.1 Material Combinations
2.5.2 Hot Runner
2.5.3 Opening and Closing
2.5.4 Toggle Lever
2.5.5 Ejection
2.5.6 General Information on the Stack Mold
2.6 Further Literature
3 Preparation
3.1 CAD System
3.2 Data Transfer, Procedure, and Processing
3.2.1 Data Transfer
3.2.2 Formats
3.2.2.1 IGES
3.2.2.2 STEP
3.2.2.3 STL
3.2.3 Data Size
3.2.4 Shrinkage
3.2.4.1 Material Selection
3.2.4.2 Shrinkage (Physical Process)
3.2.4.3 Influencing Variables
3.2.5 Calculation and Impact
3.2.5.1 Free Shrinkage, Constrained Shrinkage
3.2.5.2 Warping
3.3 Specifications
3.3.1 Location of the Component inside the Injection Mold
3.3.1.1 Demolding Direction
3.3.2 Number of Cavities
3.3.3 Arrangement of Cavities
3.4 Material Selection for Injection Molds
3.5 Mold Size
3.6 Plate Thickness
3.7 Demolding
3.7.1 Basic Principle of Demolding
3.7.2 Draft Angles
3.7.2.1 Definition
3.7.2.2 Effect on the Opening of the Mold
3.7.2.3 Draft Angle in the Split Line Face
3.7.2.4 Demolding Problems and Solutions
3.8 Split Line Face
3.8.1 Plain Split Line Face
3.8.2 Contour-Forming Split Line Face
3.8.3 Jumping Split Line Face
3.8.4 Wear Plates in the Split
3.8.5 Visible Split Line
3.9 Injection
3.9.1 Injection and Feed Point
3.9.2 Simulation
3.9.3 Sprue System, Sprue Type
3.9.3.1 Cold Runner
3.9.3.2 Hot Runner
3.9.4 Runner
3.9.5 Sprue on the Part
3.9.6 Tunnel Gate
3.9.7 Film Gate
3.9.8 Diaphragm Gate
3.9.9 Hot Runner Single Nozzle
3.9.10 Hot Runner Distributor
3.9.11 Hot Runner Distribution System with Needle Valve
3.9.11.1 Integral Hinge
3.9.12 Three-Plate Mold
3.9.13 Tunnel Gate Inserts
3.10 Ventilation
3.10.1 General Information about Ventilation
3.10.2 Ventilation via Components
3.10.3 Geometric Design of Ventilation
3.11 Further Literature
4 Components
4.1 Mold Inserts/Mold Cores
4.1.1 Mold Inserts
4.1.2 Mold Cores
4.2 Slides
4.2.1 Application Areas of Slides
4.2.2 Design of a Slide
4.2.2.1 Mold Contour
4.2.2.2 Split Line on Slide
4.2.2.3 Slide Body and Guiding
4.2.2.4 Operation of Slides
4.2.2.5 Locking in the End Position
4.2.2.6 Cooling in Slide
4.2.3 Further Slide Concepts
4.2.3.1 Slide in Slide
4.2.3.2 “Backpack” Slide
4.3 Ejectors
4.3.1 Types of Ejectors
4.3.2 Ejectors as Auxiliary Tools
4.3.3 Inclined Ejectors
4.3.4 Stripper Plate
4.3.5 Two-Stage Ejectors
4.3.6 Collapsible Cores
4.3.7 Forced Demolding
4.4 Cooling System
4.4.1 Cooling Type and Auxiliary Equipment
4.4.1.1 Drilled Cooling
4.4.1.2 Redirection of Cooling Circuits
4.4.1.3 Copper Cores
4.4.1.4 Heating Cartridges
4.4.1.5 Connection of circuits
4.4.2 Connection and Sealing of Cooling Holes
4.5 Components and Marking
4.6 Surface
4.6.1 Rough Surface
4.6.2 EDM
4.6.3 Graining
4.6.4 Laser Texturing
4.6.5 Polishing
4.7 Systematic Design Approach
4.7.1 Strategy
4.7.2 Standard Parts
4.7.3 Manufactured Parts
4.8 Further Literature
5 Assembly
5.1 Systematic Assembly
5.2 Spotting
5.3 Connection of Components
5.4 Check the Cooling for Leaks
5.5 Further Literature
6 Further Knowledge
6.1 Process Chain in Mold Making
6.2 Procurement Process in Mold Making
6.2.1 Administration
6.2.2 Preparation
6.2.3 Production
6.2.4 Sampling (Trial) – Optimization
6.3 Quality Assurance
6.4 Fits and Play in the Mold: What Must Fit?
6.5 Heat Treatment
6.5.1 Annealing
6.5.2 Hardening
6.5.3 Nitriding
6.6 Coatings
6.7 Changes: What Is to Be Considered?
6.8 Further Literature
7 The Finished Mold
7.1 Mold Validation
7.1.1 Clamping and Connecting the Media
7.1.2 Filling of the Mold
7.1.2.1 Balancing Cavities
7.1.2.2 Optimizing the Parameters
7.1.2.3 Influence on the Injection Process
7.1.3 Parameters during Injection
7.1.4 Forces Acting in the Mold during the Injection Process
7.1.5 Initial Sample Inspection Report
7.2 Labels on the Mold
7.3 Further Literature
8 Maintenance and Repair
8.1 Maintenance Schedule
8.2 Welding
8.2.1 Tungsten Inert Gas Welding (TIG)
8.2.2 Laser Beam Welding
8.3 Component Replacement
8.4 Further Literature
9 Manufacturing Technologies
9.1 Milling
9.1.1 3-Axis Milling
9.1.2 4- and 5-Axis Milling
9.1.2.1 4-Axis Milling
9.1.2.2 5-Axis Milling
9.1.2.3 3+2-Axis Milling
9.1.2.4 Simultaneous 5-Axis Milling
9.1.3 CAM Programming
9.2 EDM
9.2.1 Sinker EDM
9.2.2 Wire EDM
9.3 Grinding/Profile Grinding
9.4 Drilling/Deep Hole Drilling
9.5 Turning
9.6 New Technologies
9.6.1 LaserCUSING®/Laser Sintering
9.6.2 Vacuum Soldering
9.7 Polishing
9.8 Further Literature
10 Practical Guidelines
10.1 Design Check List
10.2 Design Color Chart
10.3 Sequential Function Chart
10.4 Maintenance Schedule
10.5 Formulas and Calculations
In this book the planning, designing, and construction of injection molds is explained and described. It deals exclusively with injection molds for thermoplastics processing.
To simplify matters, the term “injection mold” is also referred to as mold, but has the same meaning. The term mold established itself in the specialist world and is predominantly used there. Note also that the spelling “mould” is used in British English, but again the meaning is the same.
Everything is explained and described concretely and understandably. A plastic container with a cover is the basis for almost all explanations. The drawings and designs of both of these plastic parts were especially made for this book. The dimensions of the designed molds and the technical details are real, so the injection molds can be actually built. On the basis of both or one of these parts, as much as possible is shown and explained.
There are sample calculations for the planning and dimensioning of injection molds. Different functions and elements relevant for the design are explained in detail. With the increasing demands on technology in the mold, the two parts become ever more complex so there is always a reference to the previous topics. If the part and/or the mold becomes more complex, the reason for it is therefore comprehensible.
Figure 1Container with cover
Figure 2Container
Figure 3Cover
There are further chapters in which the existing designs of actually manufactured injection molds are the basis for the explanations.
“Where do all these plastic parts actually come from? Who makes them and how are these plastic components even manufactured?” These are questions that hardly anyone asks. “What are those little curls on or in the plastic part, what are they for? Then there is a small spot that looks as if something was cut or torn.” These are all characteristics that are visible on each part and arise in the manufacturing of plastic parts. For this manufacturing technique, besides an injection molding machine and plastic granulates, an injection mold is needed.
Review your day and think about how many plastic parts you held in your hand, and then you can imagine that firstly there is an incredible number of injection molds and secondly the diversity of injection molds there must be in a variety of industries, applications, or life situations.
For each plastic part which is manufactured there is the corresponding injection mold. There are at least as many injection molds as different plastic parts, worldwide. Nevertheless every injection mold is unique and there is an unimaginable number which increases every day.
Or to put it in a different way, imagine yourself in the kitchen, bathroom, office, or sitting in the car. Now imagine all of the plastic parts gone. What remains? Not much is left that is not made of plastic.
In concrete terms: Let’s start early in the morning. Before even getting up you hit the alarm button. You already have had the first contact with a plastic part. It continues when you brush your teeth. Today’s toothbrushes are, although this is not easily recognizable, manufactured with very complex and complicated injection molds. The conventional toothbrushes with automatically inserted brushes are the simpler version. However, for manufacturing an electrical toothbrush, two different plastics are injected one after another in the injection mold in a very complicated procedure in order to make the rotating brushes in the small brush enclosure.
Hair dryers, coffee machines, tea kettles, refrigerators, stoves, and ovens are just a few consumer goods used in daily life. Opening the door of your car, you again have contact with plastic parts. Without injection molds, the interior of a car is unimaginable. Seats, steering wheel, switches, buttons, handles, levers, blinds, instruments, covering, trays and so on, a countless number of injection molds are used for the manufacturing of a vehicle.
Plastics surround us in the immediate vicinity of our workplace, whether it is in the workshop, in the office or in school. It doesn’t matter what you hold in your hand or use, again it’s plastic parts. A computer, a keyboard, whether it is on the machine or on the desk. Everywhere there are things made of plastic, in different colors, contours, shapes, and degrees of hardness—from hard and stable printer housings to the soft and flexible protective covers for the mobile phone.
Last but not least, a child’s room! Almost all children’s toy boxes are full of toys made from plastic: toy blocks, board game figurines, racetracks, puppets, game consoles, etc. Plastic parts, no matter what we do or where we are, accompany us the whole day. Plastic parts are everywhere, and without them a normal life would be inconceivable.
The list goes on and on. Everyone goes through their day, consciously or unconsciously in contact with plastic parts, but no one thinks about their origin, even though there is a huge worldwide industry behind them. Not only are there manufacturers of injection molds all over the world but also large corporations that manufacture the machines for the production of the plastic parts and very large chemical companies that constantly develop and produce new plastics for different applications. Millions of people are at home in this inconspicuous world.
Through the constant development of ever improving high-quality plastics the application possibilities continue to increase. Sheet metal parts made of steel or aluminum are gradually replaced by plastic parts. Brackets made of metal used for fixing cables, fuel lines, containers, or the like in a car’s engine compartment are replaced today by high-strength plastic parts.
Further evidence that this development will certainly continue is the progress in the production of bioplastics. To put it simply, for bioplastics, the petroleum used normally as raw material is replaced by biologically derived material. These oils are extracted from renewable raw materials and are also biodegradable. So far there have only been a few applications that were often only explored by scientific facilities. The whole thing is still in the stages of development. However, if only from the sustainability point of view, bioplastic is predicted to have a bright and important future.
The most significant advantage of plastic parts is that after manufacturing or the injection process a ready-to-use piece comes out of the injection molding machine. The manufacturing time for such a component is only a few seconds. This also has an impact on the much lower cost per piece. But now we come back to the contents of this book—the success of this whole process depends on a high-quality injection mold.
The open/close mold got its name from its easy movement and function when the injection mold for machining of the plastic parts is clamped onto an injection molding machine. The injection mold or the injection molding machine opens and closes without any further necessary movement taking place in the injection mold.
The entire motion sequence is called an injection cycle or just cycle. It begins with a closing of the injection mold. When it is closed, a liquid, hot plastic mass is injected into the injection mold under pressure. Now a certain amount of time must pass before the liquid plastic has cooled and solidified and the plastic part in the injection mold reaches a certain stability. The injection mold opens and the finished, still-warm plastic parts are ejected from the injection mold. When all of the movements are finished, the process starts again. For the outside observer, the machine opens and closes again and again.
In using the term “liquid plastic”, one is referring to plasticized plastic. Plastic pellets are heated and plasticized, which means they become soft and capable of flowing. In this consistency, the plastic can be injected into the injection mold. Depending on the type and kind of plastic pellets, this vary from being highly viscous to having a water-like viscosity.
The direction in which the injection mold or the injection molding machine opens and closes is called the main demolding direction. All movements of the injection molding machine, the injection molds and the moving parts in the injection mold run in this axial direction. Depending on the component there can be additional demolding directions. This is described in Section 2.2.
The open/close mold is the simplest of all injection molds. As a result it is often the cheapest. Already in the planning and designing of plastic parts, efforts are made so that the plastic piece can be produced with this type of injection mold.
Figure 2.1 shows the demolding direction of a simple open/close mold. Both upper part (fixed half) and lower part (moving half) open and close in an axial direction. The plastic part has been designed for being produced with this specific mold in such a way that when opening the mold on the injection molding machine it is not damaged or destroyed.
Figure 2.1Demolding direction
The plastic parts which are to be produced with such an injection mold have no structural elements which deviate from the main demolding direction. Cup-shaped or flat parts, for example, are manufactured with this type of mold.
A plastic part can have elements such as side openings, latches and clips, laterally protruding edges or pipes. For the demolding of these elements, moving components—called slides or inserts—are designed for the mold. In a secondary demolding direction, these elements called undercuts can be removed from the mold without damage. More on this in Section 2.2.
The previously mentioned “expanding” parts container and cover is shown in Figure 2.2 to illustrate how such plastic parts produced in an open/close mold can look.
Figure 2.2Parts for an open/close mold
Here already is the first addition to container and cover. To connect the two and be able to close the container, a sleeve is introduced in every corner of the container and, aligning to the sleeve, a stepped bore is introduced in the cover. Now you can screw down the cover on the container with four screws.
Both the size of the injection mold as well as the open and close technique do not change despite these additions to the plastic parts. The additional elements are also in the demolding direction.
In Figure 2.3, the additional sleeves in the container and the stepped bores in the cover are shown. The demolding direction remains the same.
Figure 2.3Parts for the open/close mold with additional elements
The upper part (fixed half) and the lower part (moving half) are made up of several plates and risers. Via the integrated guides, that is, bolts in the fixed half and the bushes in the moving half, the mold closes precisely.
The fixed half consists of the clamping plate and the cavity plate. The guide bolts are installed in the cavity plate. The guide bolts are provided at the back end with a collar, which is embedded in the cavity plate. Against the slip out of the guide bolts the clamping plate is screwed tightly with the cavity plate. The cavity plate is fixed to the mold plate via another fitting diameter at the guide bolt.
The moving half of a classic open/close mold is made up of the mold plate, possibly a backing plate, the risers and the lower cavity plate. The ejector set is between the risers. The guide bushes are also provided with a collar here and mounted in the cavity plate. They are secured in the moving half through the risers, which are attached, like the fixed half, via the back fitting diameter of the guide bush. The risers are again installed with the clamping plate and with the additional guide sleeves. Everything is screwed tightly together with long screws from the clamping plate through to the mold plate. This guarantees that all components are aligned and tightly connected. Ejectors are the moving parts in the injection mold that eject or expel the plastic part after opening the mold. Ejectors are usually round pins which are installed in the ejector set. The small rings mentioned at the beginning which are usually visible on the plastic part are the imprints of these ejectors.
In Figure 2.4 several longitudinal and cross sections through an injection mold are represented so that the classic structure of an open/close mold can be seen.
Figure 2.4Section through a mold structure
The accuracy of fit in a mold is extremely important. Without precise guiding and fixing of both mold halves they can move radially.
The guiding elements in an injection mold are very important. They ensure that both mold halves are already centered while closing against each other. Except in special solutions, guide bolts are built into the fixed half and guide bushes are built into the moving half. The tolerances between the cavity plates and the guide bolts and bushes are so small that they are installed with a light press fit.
The fixed half with the guiding bolts fits exactly, free of play, into the guide bushes of the moving half. Only in this way is it guaranteed that both sides fit together on top of each other precisely and repeatedly. If this were not the case, the mold halves could move radially, which among other things can lead to different wall thicknesses in the plastic parts. This is also called mold offset.
Figure 2.5 shows what can happen when the guiding elements of an injection mold are not exactly aligned.
Figure 2.5Mold offset through insufficient guiding
Here are a few comparisons to get an idea of how important the accuracy of the guiding is. The tolerances between the bolt and the plate have to be so accurate that some light strikes are required when installing the bolt in the plate. If the bolt is just 0.006 mm too thick, it will be very difficult to install.
The tolerance between the guiding bolt and the guiding bush is even smaller. The difference between free-of-play movement and getting jammed is a maximum of 0.004 mm in diameter.
If the center distance between the guiding elements of the plates in the upper part and the lower part differs by more than 0.02 mm it is difficult for the mold to close.
Anti-rotation Protection
Today nearly all injection molds are rectangular. For this reason normally four guiding elements are installed, one in every corner. To prevent a false (rotated) assembly of the fixed half and the moving half, one of the guides is smaller or bigger than the other three.
In Figure 2.6 the fixed half of a mold is displayed: three guide bolts with diameter (∅) 18 mm and one guide bolt with ∅ 20 mm. This should prevent a false (rotated) assembly of the fixed half on the moving half.
Figure 2.6Anti-rotation protection in mold making
The following is important for the length selection of the guide bolts: Before the mold contours of the two halves approach, the guides must already fit into one another. If the guides are too short, the mold contour could be damaged during the closing action of the mold halves.
In Figure 2.7 it is clearly visible that the guides are already sliding into one another before both sides can have contact.
Figure 2.7Length selection of guiding elements
These are not used very often in a very simple injection mold. They are installed when a complex cooling, a core pin or additional components that have no space in the cavity plate or pass through the cavity plate and should be held by the backing plate, are required in an injection mold.
In Figure 2.8 a core pin is shown which is installed in the cavity plate and is held by the backing plate.
The use of a backing plate has more functions and advantages here. One of the advantages is that the backing plate is installed under the cavity plate and is level. Therefore all the components which are attached to the backing plate are geometrically determined and on the same level. A further advantage is the manufacturing costs. To achieve a similar fixing of such a core pin, an additional installation of another cover from below would be necessary. A possibility here is a small built-in cover plate or a set screw which fixes the core pin.
Figure 2.8The backing plate fixes and holds the core pin
Both alternatives cause higher production costs. If they are used several times in a mold, it makes sense to install a backing plate.
In Figure 2.9 two possible alternatives for the fixing of core pins are shown.
Figure 2.9Alternatives for fixing
Further additional and basic designs, functions, elements and components of an injection mold are discussed individually in the following sections of this book.
Almost everything that makes an injection mold complicated and expensive originates from the geometry of the subsequent plastic parts. Therefore attention should already be paid in the planning and design of this plastic part that everything that should later contain the plastic part is also to be realized in the injection mold. This is often a big challenge in the development, that is, the design of plastic parts. When design and technology meet, sometimes one has to compromise.
Figure 2.10Additional demolding directions
The next level of difficulty in plastic parts is elements which cannot be demolded in the main demolding direction like in an open/close mold. These elements, which are troublesome during demolding, are called undercuts. They need to be released or demolded in an additional demolding direction. For this purpose moveable components, such as slides, core pins, ejectors for inclined ejection units or inserts, are used in the injection mold. They support the plastic piece so that it can be better demolded and ejected.
InFigure 2.10 two possible elements, a side bore hole and a side pipe, are seen on our component. Both elements are an undercut on the plastic part and must be released via the second demolding direction. Only this way can the plastic parts be ejected from the mold without damage. For these two examples slides are used to do this.
When implementing these side openings the open/close mold becomes a mold with slides. Slides are moving components inside the injection mold. One or more parts of the mold contour are incorporated into these slides. The slide itself moves away from the plastic part during or after the opening of the mold in an additional demolding direction. Through this movement the undercuts are released before the plastic part is ejected from the injection mold. The required path is calculated and defined in advance. It must be large enough so that the plastic piece drops out of or can be removed from the injection mold without damage after the ejection.
In Figure 2.11 the slide for demolding the side opening on our container is shown. In the front area of the slide a part of the mold contour of the plastic part is incorporated. The round surface in front has contact with the fixed insert when the mold is closed and is injected. During injection, this contact prevents that the plastic covers this spot and thus forms the bore holes in the plastic part. In technical language, this contact point is also called an aperture.
Figure 2.11Slide with and without plastic part
To move this slide there are two possibilities. The first possibility is that the slide is connected with a hydraulic cylinder which is in turn screwed tightly to the injection mold. The slide is moved via this cylinder. For this solution the cylinder covers a clearly defined distance. It is bought and installed as a standard part. Find out more in Section 4.2. The second option is the forced control through an inclined pin. The pin is installed with a defined inclination on the fixed half of the injection mold. The front part of the inclined pin submerges in the moving slide. When the mold opens in the main demolding direction, through the resulting movement this inclined pin moves the slide in an additional demolding direction. There are additional details in Section 4.2.
Figure 2.12 displays the closed mold on the left and the slightly open mold on the right. On the slightly open mold the inclined pin has moved the slide in an additional demolding direction to the end position.
Figure 2.12Closed and slightly open mold
Even a very small and harmless looking clip or catch can have a major impact on the design and on the cost of an injection mold.
The simplest application is a clip for snapping the cover onto the container. Clips or catches are also used to connect several plastic parts or to fix them together in an entire assembly group. The assembly of plastic parts has to be done very fast today and if possible automated. The use of such clip connections on plastic parts has, among others, the advantage that they can be quickly and easily installed without further hand tools.
For the size, type, complexity and also for the costs of an injection mold, it can be very important if the clip is attached outside or inside of the plastic part. This should be considered during the planning of the plastic part. If the clip or latch is outside of the component, it is in the demolding direction and thus there is no undercut. Consequently, it is demoldable without further action.
Figure 2.13 shows both variations of a latch, inside and outside. The outer latch is open above, thus enabling a problem-free demolding. The inner latch is not open in the demolding direction. It will be damaged or even torn away during ejection. Consequently, one must think of how to prevent this.
Figure 2.13Outside and inside latch
As previously described, the latches usually lie outside of the plastic part, in the demolding direction. They can therefore be demolded without further action. The situation is different when the latch is inside of the plastic part. Slides are used for the demolding of outside undercuts. In contrast, the latch which is located inside of the part is demolded with an inclined ejector. The round ejectors were already mentioned in Section 2.1. They are pushed to the front together with the ejector set to eject the plastic parts in the demolding direction. The inclined ejector is also installed in the ejector set. In contrast to the round ejector the inclined ejector is not fixed. The inclined ejector is held in the ejector set in a kind of shoe, but can move radially with the shoe. Like for the slide, this comes from a movement resulting from the forward movement of the ejector. Therefore a diagonal guiding is incorporated in the insert. This guiding has a defined angle for the demolding situation. The ejector moves forward in this diagonal guiding. The further the ejector moves forward, the more it releases the latch on the plastic part. In the end position there is no more undercut and the plastic part can fall unhindered from the mold.
Figure 2.14 presents two situations: on the left, the ejector after the opening of the injection mold, before it is ejected; on the right, when the ejector set with the inclined ejector has moved to the front end position. The latch is released and the plastic part falls from the mold.
Figure 2.14Three-dimensional view of an inclined ejector
Figure 2.15 shows again the exact situation in the plastic part; left, again the initial position before the ejection; right the latch released through the diagonal movement of the ejector. More about this in Section 4.3.3.
Figure 2.15Situation of the inclined ejector
A possible alternative is to change the latch geometry or design so it can be for example forced demolded. This variant is selected for molds that are required for the production of only a few sample parts or when the cost of the mold should be kept low. Then no additional element is needed in the mold.
Forced demolding means that during ejection from the injection mold the plastic part is elastically deformed without damage. After the ejection it springs back to its original state. Note that for forced demolding the force for ejection is slightly higher.
The difference between both latches is:
Latch 1 is has a level top surface. During demolding, it is sure to be torn away, and the plastic piece will break.
Latch 2 has an inclined top surface. During demolding of the plastic part, the wall of the container presses so far away that the piece is indeed slightly deformed but after the demolding it springs back again to the original state.
InFigure 2.16 both variants are presented.
Figure 2.16Latch for forced demolding
Exemplary for a functioning variant of the forced demolding in serial operation are the plastic screw caps for drink bottles. They are force demolded during ejection. The screw caps are specially designed for this type of demolding. There are several good reasons for that: they are produced in millions, consequently time plays an essential role. The production of the plastic part is extremely fast with this type of mold. The mold does not need additional slides or cores, it is also smaller. Fewer components in the injection mold also mean less maintenance costs for the mold.
Such additional elements can strongly influence the size and therefore the effort and the costs for an injection mold. This has already been mentioned above. This influence on the size can be clearly shown in our two examples. Our plastic piece, the container, does not change in the basic dimensions. Only the side opening has been added, which we can demold via the slide from the outside, and the inside clip, which generates an undercut that can be released through the inclined ejector.
InFigure 2.17 the difference in the mold size is clearly seen. The edge length in the direction shown is the same for both molds. But the width of a mold with slide is clearly larger.
Figure 2.17Different mold sizes
The slide which comes from the outside has a certain size itself. It also needs a guide in which it can move. This makes a broader tool necessary.
