1 1 Introduction and Planning 1.1 Introduction Today, injection molding is probably the most important method of pro- cessing plastics in the production of consumer and industrial goods, and is performed everywhere in the world. Once the decision has been made to use injection molding for a new product, a number of difficult choices are ahead which will be addressed later in more detail:  Number of cavities  Stack design, which is the purely technical aspect of how to mold the pro- duct. It is important to understand the design of that portion of the mold that is actually in contact with the plastic (the “stack”), i.e., the cavity, core, and any other mold components, which determine how the final product will be shaped and how the plastic will enter the cavity space  Method of ejection, i.e., how the product will be ejected from the mold  What machine should be used?  Automation – will it be required? With new, possibly difficult shapes, these decisions are usually left to the ingenuity of a mold designer. More frequently, precedents from earlier molds are used and reapplied. However, the mold designer must be aware of and evaluate new ideas, new methods, and developments, which when applied would lead to better quality, higher productivity, simpler molds, and savings in the cost of the molded products. After the design of the basic stack and before proceeding with any mold design, the mold designer must understand what kind of mold should be selected; in other words, which features will be most suitable for the application to achieve the most economic overall manufacturing method for the product. This means not just to specify the number of cavities that will be required for the expected output, but also the selection of mold materials and the degree of sophistication of the mold. Any planned automation, especially in product handling after molding, can affect the mold layout, particularly spacing and orientation of the stacks. The mold designer must never lose sight of the ultimate goal: The cost of the product must be the lowest possible, while still achieving all specified requirements. The most important information is to know beforehand the quantities to be molded, a piece of information, particularly with new products, often very difficult to obtain. It should also be pointed out that of the total cost of almost all plastic pro- ducts, the cost of the plastic material alone constitutes the greatest component. Mold designers must not be stuck in a comfortable rut The ultimate goal of a mold is to produce an acceptable quality product at the lowest possible cost Often, the most important informa- tion is the most difficult to obtain 1281han01.pmd 28.11.2005, 10:471 2 1 Introduction and Planning The most sophisticated, best designed mold will not lower the cost of the product by as much as the reduction of just a few percent of the amount of plastic material, if it could be removed from the product without affecting its quality or serviceability. Most often, unnecessarily heavy wall thickness and ribbing affects the cost more than anything else. Chances are that the lowest weight will be achieved with the highest quality molds. In my long experience, I have had numerous occasions when the client insisted on having his way. When I strongly believed it was the wrong thing to do, I suggested to them to have this mold built somewhere else. Almost all came back sooner or later for other business, and acknowledged that they should have listened to me. The foremost intent of this book is to present various alternatives available to the mold designer or decision maker when planning a mold for a new product or planning to increase the productivity for a product for which a mold exists. It raises many of the questions that must be asked by anybody who needs a mold built. Some of these “questions” may appear obvious and not worth mentioning, or their pursuit may be thought a waste of time, but I like to point out that any input could significantly affect the productivity as well as the cost of a mold. For an experienced mold designer, the answers for many of these questions often come automatically, without him or her being aware of the fact that a decision has been made. But even the most experienced mold designer can gain important information by systematically investigating all areas that can affect the design and the complexity of the mold and even the most experienced designers overlook some obvious facts. In this book an attempt has been made to explain why certain mold features should be selected, considering the planned productivity and expected costs. There will also be occasionally references to other books on this subject, such as “Mold Engineering” [5] and “Understanding Product Design for Injection Molding” [1]. Since in many mold shops the mold designer is also involved in estimating the cost of the mold to be quoted, the book also intends to discuss various ways of how to estimate mold costs. Properly estimating mold cost is probably the most difficult part of running a successful mold making operation. Regardless of how well a shop is equipped with machine tools and other mold making equipment, and how high the level of experience is of the machinists and mold technicians (mechanics), if the mold cost is not adequately quoted it will be impossible to stay in business. We must never forget that the primary purpose of any business is to make money, and there is nothing easier than to lose money by poorly estimating and quoting. There is no magic formula to estimate a mold cost, but good understanding of the principles will lead to better cost estimates. We must not forget in dealing with the customers who require a new mold that it is not what they want but what they need Figure 1.1 Typical mold-making factory using automated equipment 1281han01.pmd 28.11.2005, 10:472 3 1.2 Oversimplification In the early 1950s, when I was an R&D engineer at a large electrical manu- facturer, I had just submitted a request of appropriation for a mold for a new product when the vice-president of sales stormed into my office, and said: “Why do you always need so much money for a mold? What is a mold? Isn’t it just an upper and a lower half?” This was in the heydays of compression molding, before the injection molding technology gained importance dramatically. A compression mold was exactly what the VP implied: a lower half with one or more cavities, and an upper half with the matching cores (see Fig. 1.2). The plastic was hand-fed into the lower (open) cavity; there was no or little sophistication with heating (these molds were processing thermosets and therefore needed to be heated, not cooled). Often, there was no ejection mechanism at all, or it was relatively simple. Of course, what the VP failed to understand was the complexity and accuracy of the work required to build the various components of these “halves”, the strength required to resist the high molding forces, the time required for machining and good polishing, and other features required for even a “simple” mold. Unfortunately, even today, many years later, this attitude of oversimpli- fication is frequently encountered when discussing a required mold and its cost. Since that time, thermosets (“compression”) molding has become quite sophisticated, and is using injection molding technology occasionally, but is still mostly using the vertical machine arrangement, because of the original loading method of the plastic material by gravity. This was also the time when injection molding took over the molding market from small beginnings. But for a number of reasons it soon became more convenient to use horizontal machines, although today again, some vertical injection molding machines are used for certain applications. But regardless of the type machine used, the most important part of the molding system is still the mold. 1.2.1 Definitions Before continuing, here is a list the various terms used:  Product: an injection molded plastic piece  End product: an assembly, of which the product is a part  User (end user): persons using the product or end product  Customer: the person or company interested in buying the injection mold  Mold maker: the person or company engaged in making injection molds  Mold designer: the person responsible for designing the mold 1.2 Oversimplification a b c d e f g h a Upper platen (stationary or moving) b Heating platen c Upper mold half d Core e Cavity f Lower mold half g Heating platen h Lower platen (stationary or moving) Figure 1.2 Schematic of a compression mold for a plate 1281han01.pmd 28.11.2005, 10:473 4 1 Introduction and Planning  Product designer: the person responsible for designing the product to be molded  Molder: the person or company engaged in injection molding plastic products 1.3 Is Injection Molding the Right Choice for this Product? Before proceeding, we must ask: “why was injection molding selected for the job?” The molder may have a financial or other interest in preferring to have the product made by injection molding, but we must keep an open mind. Have alternative methods or product designs been considered or investigated, employing other manufacturing processes using the same or a similar materials, or using other materials which may permit a similar end product, possibly even with better quality, and/or at lower cost? A few typical examples of possible manufacturing alternatives for injection molding:  Thermoforming  Foam molding  Coining and die stamping (blanking)  Extrusion blow molding  Machining, forming of sheets  Some other, maybe yet to be developed methods and materials Another possibility is not to use plastics at all, but rather use:  Paper (cardboard), wood, cloth  Metals (steel, aluminum, etc.) Injection molding has many advantages, particularly low mass, achievable accuracy, good strength-to-weight ratio, good appearance and surface definition, and numerous specific physical properties. But injection molded products always suffer from the fact that the initial capital outlay for molds and machines can be very high. But we must never forget that on a per unit basis, especially whenever large quantities are considered, the contribution of the cost of the equipment (mold, machine, etc.) to the cost of the product is small and often almost negligible. Figure 1.3 Typical injection molded parts The relatively high capital cost of a mold is often almost negligible when evaluated on a per-molded-part basis 1281han01.pmd 28.11.2005, 10:474 5 1.4 The Injection Molding Machine We will not discuss the advantages and disadvantages of the various injection molding machines that are on the market, but rather introduce the reader who is not too familiar with this industry to the various terms that will be used from time to time if a subject under discussion will have special reference to a machine element or feature. The accuracy of molding, and especially when molding so-called thin-walled products, is very dependent on the quality of the molding machine, its mechanical rigidity, accuracy of alignment, parallelism of platens, the quality of its controls, and the state of maintenance. Every good injection-molding machine consists of these basic elements 1. A rigid base 2. A rigid clamping unit, consisting of two platens, for the mounting of the mold halves and provisions for guiding the platens (tie bars or ways) 3. Provision for moving the platens, preferably fast, relative to each other, for opening and closing the mold; the speed of motion is usually adjustable 4. Provision for clamping, i.e., holding the mold shut against the force of the injection pressures within the mold (in some machines, provisions 3 and 4 are combined) 5. Provision for ejecting the molded product(s) from the mold 6. Provision to transform the raw plastic (pellets, etc.) into an injectable melt (the plasticizing unit) 7. Provision for injecting the melt into the mold (in most machines, provisions 6 and 7 are combined in one unit) 8. Provision for heating the plastic in the plasticizing unit 9. Cycle controls (sequencing logic, timers, etc.) and a command post for manual operation and for mold setup 10. Heat controls for all heaters in machine and molds. Some machines have a limited number of heat controls and additional controls could be required for the molds, especially with larger hot runner systems. This point must be considered when estimating the mold cost. 11. Safety gates to protect operators and bystanders from all hazards when operating the machine 12. Mechanical safety drop bar(s) to prevent closing the machine when gates are open, in case of failures of the other (electric and hydraulic) safety measures. 13. Provision for cooling water distribution to the mold 14. Provision for compressed air, for auxiliary actions required in the mold Even the best machine – if poorly maintained – will not perform as it should 1.4 The Injection Molding Machine The mold designer who believes that the product considered could be made better by other methods has a duty to discuss this with the customer, even if it could mean lost business, this time 1281han01.pmd 28.11.2005, 10:475 6 1 Introduction and Planning There are other features available, e.g., for the convenience of quick mold installation, setting up and operation of the mold and machine; these features are often offered as options which can be bought with the machine or added on later. 1.4.1 The Right Machine for the Mold Often, the mold cost will surpass the cost of the machine. It does not matter how ell a mold is built if the machine cannot meet the molding requirements to produce quality products. While considering the purchase of an injection mold, it is always important to make sure the machine can do the job. Some of the basic considerations are:  Tie bar spacing  Stroke and shut height  Injection speed (average and peak)  Available injection pressure  Recovery rate capability (throughput)  Platen rigidity (are the platens rigid/robust enough to carry the mold weight?)  Available clamp tonnage  Platen parallelism  Clamp speed requirements  Shut-off nozzles  Screw design  Accuracy and repeatability of controls  Operator access  Mold protection capability As the machine and mold act together as a system, it is fair to say that the system will perform only as well as its weakest component. If an existing machine is to be used, the machine should match the machine's capability. The mismatched machine can easily destroy the new mold in a matter of months, resulting in costly rework. To determine the right machine, the following information on the mold is required:  Mold width, length, and height  Opening stroke required (usually 2.5 × part height) There is no point in buying a premium priced mold to run it in an out-dated machine 1281han01.pmd 28.11.2005, 10:476 7  Ejector rod locations  Locating ring size  Part dimensions, including wall thickness  Flow length (length of flow from gate to longest flow path)  Part weight  Runner weight (if cold runner)  Cavitation  Nozzle radius  Material (including color and additives, viscosity) 1.5 The Injection Mold To the customer or entrepreneur not familiar with the problems of molding and mold making who wants to make a new product, the price of a mold may seem to be high, occasionally even outrageous; it is often difficult to convey that the mold price constitutes only a very small portion of the product (piece) cost, and depends much on the expected production of the mold. 1.5.1 What Is an Injection Mold? A (plastics) injection mold is a permanent tool, i.e., a tool that, if properly designed, constructed, and maintained will have a life expectancy (useful life) well beyond the time where the product itself becomes obsolete. This differentiates it from a “one-time use” mold such as a sand-casting mold, as used in metal foundries. A mold can be used to make products in a virtually infinite variety of shapes, made from injectable plastics. Common to all molds is the condition that it must be possible to remove the product after molding, without the need to destroy the mold (as is the case in sand-castings). There is an exception to this, the so-called “lost-core molding”: There are injection molds for intricate products, such as intake manifolds for internal combustion engines, previously made from cast iron, which have an outside shape that can be molded with conventional (permanent, “open and close”) molds but where the intricate inside shape is made from a molded, low melting point metal composite which is inserted into the mold before injection, and then ejected together with the molded product; the metal is then removed by heat at a temperature above the melting point of the insert, but of course below the melting point of the plastic used for this product; the molded metal insert is thereby destroyed, but the metal will be reused. A basic mold consists of two mold halves, with at least one cavity in one mold half, and a matching core in the other mold half. These two halves 1.5 The Injection Mold It is important to understand that it is not the mold cost but the piece (unit) cost of the product, which is important 1281han01.pmd 28.11.2005, 10:477 8 1 Introduction and Planning meet at a parting plane (parting line). As the mold opens – after the injected plastic (now in the shape of the desired product) is sufficiently cooled and rigid – the product can be removed by hand or be automatically ejected. Because injection-molding machines are mostly built with the injection on the stationary platen side, there is, typically, no built-in ejection mechanism on this side. If ejection from the injection side should be required – always the case in stack molds, and occasionally required in single level molds – any required mechanism must be added to the mold, and occasionally to the machine; in either case, this adds complexity and increases costs. Only molds designed for using only air ejection do not require any external ejection mechanism. Most products are removed (ejected) from the core. There are also many molds, which need special provisions to allow the products to be removed from either the cavity or the core. This is the case with products having severe undercuts or recesses on the inside and/or the outside of the product, such as screw threads, holes, ribs or openings in the sides of the product, etc., or molds for insert molding. Some of these design features of the product may require moving side cores, which are either inserts or whole sections of the cavity that move at an angle which is 90° to the “natural opening path” of the mold. Others may require special unscrewing mechanisms, either in the core or in the cavity side. The mold may require split cavities (or “splits”), i.e., the cavity consists of two or more sections, which are mechanically or hydraulically moved in and out of position, and then clamped together during injection. In some cases, the mold may require collapsible cores, or retractable inserts, which are all quite complicated (and expensive) methods. Any of the above special features can add considerably to the mold cost when compared to a simple “up and down” mold where the products can be readily ejected with the machine ejectors during the mold opening stroke or after the mold is open, without the need for any of these complicated mold features. Note that in this book, the term (simple) “up and down” molding is used, which comes from the earlier vertical molding machines, even though, today, most general-purpose injection molding machines are horizontal and the mold opens and closes in a horizontal motion. Example 1.1 To illustrate how different mold features affect the mold cost, we assume that a single face mold with air ejection of the products costs X dollars. A similar mold, but with mechanical ejection, costs about 1.2 times X. A similar, air- ejected 2-level stack mold will be about 1.8 times X. An unscrewing mold for a similar size mold and product will cost about 2 times X. Almost any shape can be molded – but at what cost? 1281han01.pmd 28.11.2005, 10:478 9 1.5.2 Elements of an Injection Mold Most readers will expect to see some illustrations (photos or schematics) of injection molds at this point. However, we must not forget that this is not a book about mold design, but about the relationship between productivity and cost of molds, as well as the cost of the products to be made. There will be, however, a number of photos of molds accompanying the text where deemed useful. There are books that show designs of numerous, specific molds but it is virtually impossible to show every possible configuration that may be required. It is more important for the designer, and any person requesting a new mold, to understand that a mold consists essentially of a number of elements from which to choose for the most appropriate design for the purpose. Every injection mold consists of the following basic elements: 1. One or more matching cavities and cores, defining the cavity space(s) (today, there are molds with anywhere between 1 and 144 cavities). 2. A method, or element, to duct the (hot) plastic from the machine nozzle to the cavity spaces: There is a choice between – Cold runners (2-plate or 3-plate systems) – Hot runners (various systems) – Insulated runners, through shooting – Sprue gating (cold or hot) 3. Provision to evacuate air from the mold (venting): There is a choice between – Natural venting – Vacuum venting 4. Provision to cool the injected hot plastic sufficiently to allow ejection of the molded product 5. Provision to eject the molded product: There is a choice between – Manual product removal – Ejector pins and sleeves – Stripper s (stripper rings or bars) – Air ejection – Random ejection – Various methods of in-mold product removal methods – Robotic product removal 6. Provision to attach (interface) the mold to the molding machine: There are several methods to consider – Mold is for one machine only 1.5 The Injection Mold Product quality, productivity, and mold cost depend heavily on the proper selection of the runner system 1281han01.pmd 28.11.2005, 10:479 10 1 Introduction and Planning – Mold to be used on several, different machines – Quick mold change methods (various designs) 7. Method of alignments of cavities and cores: There are several methods to consider – No alignment feature provided in the mold – Leader pins and bushings (2, 3, or 4) – Leader pins and bushings between individual cavities and cores – Taper fits between individual cavities and cores – Taper fits between plates – Any combination of the above 8. Any number of (mold) plates to provide the necessary for carrying and backing the above elements But molds could have additional features, which will also be discussed in the following. Each of these features can add (often considerable) costs to the mold but in many cases can increase the productivity of the mold and reduce the cost of the product. They may or may not all be necessary and must be carefully considered when deciding on the type of mold most suitable (and most economical) for the job on hand. Features such as serviceability of the mold may affect the mold cost; for example, the access to hot runners for cleaning plugged gates or making minor repairs, such as changing a nozzle, a burned-out heater, or a faulty thermocouple at a hot runner drop will cost more in the initial mold, but this will be easily recouped by reducing the down time necessary to accom- plish such repairs. By designing easy access to these components in the machine (without the need to remove the whole mold, or part of it, to the bench), such repairs can be made in less than an hour, instead of taking several hours. This work can also be done by the mold setup staff rather than getting the (expensive) mold makers involved. Another area where valuable maintenance time can be saved is to design and provide easy access from the parting line to screws holding modular mold parts to their mounting plates, while the mold is in the machine. On the other hand, in my experience, many molds, particularly molds for lower production quantities, have been vastly over-designed and much money has been wasted. The main purpose of this book is to discuss the various elements or features listed above and to facilitate the selection and the decision making. Defining what is really required considering the shape and complexity of the product and the required production quantities will enhance mold productivity. In addition, the book should facilitate investigating if, even minor, changes to the product shape could lower the mold cost and improve the productivity of the mold or the whole system. Figure 1.4 Mold maintenance in the press is important. Here, the operator is changing a nozzle tip while the mold is in the press (Courtesy: Husky) Easy serviceability of the mold is important but often overlooked. It adds some mold cost, but saves much more in future servicing costs and downtime Even minor changes to the part can dramatically lower or increase mold costs 1281han01.pmd 28.11.2005, 10:4710 . 1 1 Introduction and Planning 1.1 Introduction Today, injection molding is probably the most important. 28.11.2005, 10:475 6 1 Introduction and Planning There are other features available, e.g., for the convenience of quick mold installation, setting up and operation