CARL HANSER VERLAG. Herbert Rees. Mold Engineering 2nd edition

Transcription

1 CARL HANSER VERLAG Herbert Rees Mold Engineering 2nd edition

2 45 4 General Mold Design Guidelines 4.1 Before Starting to Design a Mold he mold designer starts with the design of a new mold when he receives a part drawing, a sample or, rarely today, a model of the product. Additional information includes the machine (or machines) the mold will be run in, the number of cavities required, and, if it is not shown on the drawing, the type of plastic that will be used for this product. While the above information is important, it may be incomplete. here are other concerns which should be addressed before starting the mold design: Molding characteristics of the specified plastic? How many parts will be molded? (approximately annually, and over the life of the mold) Anticipated molding cycle? Where and how is the product used? Must the product fit with other parts? (tolerances) Shrinkage? Draft angles? What type of runner system is required?: Cold runner (2-plate, 3-plate)? Hot runner? Combination of both? Gate location, flow and weld lines, ejector marks? Permissible gate (vestige) size or shape Finish? Engraving? Cavity numbering? Spares required with mold? Are the indicated machines suitable: onnage? Shot size? Plasticizing capacity? Is mechanical (automatic) product removal planned? iming of project? (How soon is the mold required?) Some of these questions and concerns may have been answered with the accompanying mold order; there may be other questions which will have to be answered by dialogue with the customer. After a mold order has been received, it is important to find out how the quoted price for this mold has been arrived at. Mold prices are usually estimated by an experienced mold designer or estimator, often by the owner of the mold making business. At the time when the estimator receives a request for a quotation, he or she may sketch a mold design which is believed to be appropriate for the product, based on experience and/or from records of similar molds. he estimator then bases all cost figures on this sketch. he estimator may also, according to the complexity or novelty of the product, add a safety factor or fudge factor before arriving at an estimated mold price for quoting to the customers.

3 46 4 General Mold Design Guidelines here is a risk that the quoted mold price is lower than it might have been if all the parameters had been considered at the time of the quoting. he quoted mold price must be competitive with that quoted by other mold makers; there is an added risk in times when mold orders are scarce and prices are very competitive in that, just to get the order, the quoted mold price is lower than would normally be quoted. It seems to be impossible to find a better system unless the estimator completely designs each mold; this would take much more time than normally allotted for estimating. An exception to this is when the customer is willing to purchase some preliminary mold design time before the mold will be quoted. Another problem is that many more molds are estimated than will eventually result in orders. It is common practice in all industries to send out requests for at least three quotations from different mold makers before a mold order is placed. Also, occasionally, a mold estimate is requested by a manufacturer for pricing purposes only, simply to find out whether a product is economically interesting, without any intention of soon or ever placing an order. It is important to understand that the mold designer must know the type of mold that the estimator had in mind when quoting a price. If not, he or she may design a mold that greatly exceeds the quoted price. Note that the purpose of any industry is to make money with their product. (he product of the molder is the plastic product; the product of the mold maker is the mold.) he foregoing does not imply that one should design and build a bad mold just to stay within the budget of the quoted price. In mold estimating, it is impossible for the estimator to be always right; however, it is important that when averaging many molds the financial results of the good and the bad molds balance out or are in favor of the mold maker if the business is to prosper. Also, the mold envisaged by the estimator is not necessarily the best design. here is a good chance that in the course of the design process a better design will be found. But it is necessary that the mold designer is aware of the quoted mold price at the start of a mold design project. It is quite possible that the estimator has erred, has underestimated some difficulties in molding the product, or was missing some important information which was subsequently supplied with the mold order. If, after a preliminary study by the mold designer of the product and the final specifications, it becomes apparent that some parameters have been either inadvertently or deliberately changed, the customer must be immediately advised of any increase in the mold cost caused by such changes before more time is spent on the project. In this way, much aggravation can be avoided, and the customer may revise or eliminate some of the new or previously unspecified parameters. he customer may not have been aware that such changes between the time of the quote and the order would affect the mold price. On the other hand, if the error was due to poor estimating, there is nothing that can be done by the designer but to try to stay within budget and to design whatever is necessary to build the mold as quoted; under no circumstances must the quality or the performance of the mold be compromised. Since the reputation of the mold maker is at stake, any loss suffered due to

4 4.2 Molding Characteristics of the Specified Plastic 47 poor estimating must be written off as learning experience and good will. here is a possibility that the customer may agree to carry some or all of the extra costs involved, but it is a policy decision of the mold maker whether to approach the customer for an increase of the contracted mold price. Occasionally, a mold price may be deliberately quoted low as the result of a sales policy; for example, to win a new customer or to enter a new field of products in which the molder has little or no experience. Regardless of the low price, the designer must still design the best possible mold to perform as specified, at a reasonable cost. 4.2 Molding Characteristics of the Specified Plastic We will here consider only characteristics which directly affect the mold design: 1. Flow characteristics. Easy flowing materials usually present no problems, but stiff materials require higher injection pressure and, therefore, heavier construction of the mold. his will also affect the need for more accuracy and strength of the alignment elements. 2. Melt (processing) temperature. he higher this temperature, the more important becomes the cooling design and, sometimes, the method of heat insulation between hot and cooled portions of the mold. 3. Material degradation. Every thermoplastic is to some degree heat-sensitive (or subject to degradation) when exposed to high temperatures over a length of time. Figure 4.1 shows schematically a typical curve: 1 represents the highest temperature at which the material could be injected safely; 2 is the lowest temperature required to keep the plastic injectable. he difference between 1 and 2 is the operating (molding) temperature range (OR) of the plastic; t 1 and t 2 show the elapsed time before degradation starts at the temperatures 1 or 2. Melt temperature 1 Operating range OR Degrading Maximum temperature at which product can be used 0 t t 1 2 Figure 4.1 Material degradation curve. Elapsed time t

5 48 4 General Mold Design Guidelines OR A OR 2 B OR C t 0 t 0 t t t t t t t Low operating temperature narrow OR t t short 1 2 High operating temperature narrow OR t t short 1 2 Wide OR t t long 1 2 Figure 4.2 Degradation curves for various types of plastics: A. Highly heat-sensitive, B. Heatsensitive, and C. Not very heat-sensitive. Some materials, such as PVC, are molded at low temperatures and have a narrow OR and a short time span (t 1 t 2 ) (Fig. 4.2A); they are very heat-sensitive. Some plastics are molded at much higher temperatures and may have a wider OR but also a fairly short time span (t 1 t 2 ) (Fig. 4.2B); they are also heat-sensitive. Such materials include PE, acetates, and others. Figure 4.2C represents low heat-sensitive materials, such as PE, PP, PS, etc. he graphs shown are schematics only; curves with the appropriate values for each plastic are available from the plastic materials suppliers. he mold designer must understand this relation between time and temperature because it affects the design of the runner system and the design and construction of the hot runners, provided that hot runners can be used at all. It will also affect the mold material (steel) selection, for example, in the presence of corrosive agents in the plastic. Some materials, typically acetal, give off poisonous gases when heated for even a short time above their upper permissible temperature and corresponding safe time, even though the material does not have visible signs of degradation. Other materials give off highly corrosive gases and will require specially selected mold materials and/or finishes. All materials show degradation by changing color to yellow or brown; the ultimate form of degradation is when the plastic becomes charred (carbonized) and black. All products containing even small portions of degraded material should be discarded, not only because of poor appearance of the product (streaks, etc.) but because physical properties may have been lost and the product may not perform as expected.

6 4.4 Studying the Product Anticipated Molding Cycle here are several reasons why the estimator and the designer should know the anticipated molding cycle. It is always desirable to build a mold with the shortest possible molding cycle, but this can only be achieved at a cost. Special cooling methods, added ejection features, special mold materials, lubrication, etc., will certainly add to the cost of the mold. he designer (and estimator) should always question the molding cycle demanded or suggested. While the customer (molder) probably has experience on which the expected cycle time is based, there is no harm in double-checking it against the experience of the mold maker and the mold designer. Seldom are two parts so similar that cycle times can be extrapolated from one to the other. Differences in the plastic, wall thicknesses, draft angles, methods of gating, the machine used, and other factors may have a significant impact on the molding cycle. his is one of the areas where the mold designer can prevent a financial disaster right at the beginning of a project by questioning the molding cycle specified with the order, particularly if there is a performance guarantee involved. 4.4 Studying the Product he first task for the mold designer is to become familiar with the intended product. Steps for studying the product are outlined below Check the Product Drawing Clarity Are all necessary views, sections, and section lines shown? At this point of the job it is almost impossible to make sure that all dimensions are given and are correct. Frequently, a product designer who is very familiar with the product forgets that the mold designer may not be and, hence, may take shortcuts which can lead to errors in the interpretation of the drawing Projection Is the drawing in 1st angle (European) or 3rd angle (American) projection? his is a common source of error and may not be discovered until some time into the project; it could result in production of the mirror image of the desired shape. Some draftsmen, to save time, rather than redraw a view or section which was drawn incorrectly in relation to the main view, or to save space on the paper, will mark the direction of a view with an arrow, regardless of the system of projection used. Views indicated by such arrows should be carefully checked to avoid subsequent machining errors.

7 50 4 General Mold Design Guidelines olerances his is probably the most important area to be checked before the mold design should be started. he estimator should check and question the tolerances at the time when the job is quoted. he final drawing received with the order must be compared with the product drawing used for the quotation to make sure that there were no changes made. Quite often, the drawings used for quotations are only preliminary and incomplete, and tolerances may not have been shown. Since a large portion of the machining cost of the mold components is directly related to the tightness of the tolerances, it is important to ascertain that critical tolerances have not been made tighter since the quotation. If this should be the case, and if the new tolerances will affect the mold cost, the mold designer must immediately approach the customer to bring the tolerances back to what was quoted or to have the contract requoted. Often, the product designer assigns close tolerances to the product that are not necessary for the function of the product. While in some places the tight tolerances are probably justified, many are not. Also, the general tolerance may have been specified unnecessarily tight. Although it may require a bit more work and understanding by the product designer, the proper method is to give a relatively large general tolerance and tighten up dimensions only where it is necessary for the function of the product or where it is required in the assembly with matching products. Note that, often, tight tolerances are impossible to hold by simply molding because of the wide range of the shrinkage of plastic. If such close tolerances are absolutely necessary, it may require selection of the proper size pieces and disposal of the molded products that are outside of the tolerances; this is an extremely expensive way of producing anything. For more information regarding plastic shrinkage and tolerances, see Chapter 8, Plastic Shrinkage, and Chapter 9, Mold Surface olerances. It is usually easy enough to make the mold steel to close tolerances, but this does not mean that every product will be within tolerance. Various methods are used to show that the tolerances will increase as the sizes increase. he product tolerances can be given as percentage of the size; for example, a general tolerance may be ±0.05 mm per 10 mm (or ±0.005 in. per 1 in.), which reduces the tolerance for smaller sizes and increases the tolerance for larger sizes. But there must also be a reasonable minimum tolerance set for very small dimensions to prevent them from being overly restricted and difficult to produce. Whichever method is specified, the designer must make sure that the tolerances shown on the product drawing make sense. he mold designer must not forget that the sizes also depend on the operating temperatures; in some cases, the product keeps shrinking hours and even days after molding. With some critical dimensions and certain materials, it must be established beforehand when, and under what conditions, the product will be measured. his study of the tolerances at the beginning of the job will prevent arguments later on. If the tolerances specified are unreasonable, the mold designer must discuss them with the customer and get a release in writing so that the mold maker will not be held responsible for sizes of the molded product which are outside the (unreasonably) specified ones.

8 4.4 Studying the Product Product Use Where and how is the product to be used? his question must be asked not just out of idle curiosity; it will give the mold designer some idea of the importance of certain aspects and critical areas of the product such as required fits with other products, finish, physical strength, location of gate, ejectors, etc. he designer may then suggest changes, especially in the areas of fragile mold cores or thin ribs, not only to make the mold easier to build but also to extend the life of the mold and to improve its serviceability. Understanding the product may save on unnecessary polishing; it may indicate to the designer where looser tolerances could be in order and where to request sharp edges instead of the round edges specified, or round edges where sharp edges are shown (see typical examples in Fig. 4.3). Some suggestions that can be pointed out right at the beginning of the project refer to improvements of the filling path of the plastic which may need only small changes in the product shape to give faster filling (and molding) speeds. One such example is shown in Fig Such requests for changes will benefit not only the mold maker but also the molder, and it is the duty of the mold designer, after understanding the purpose of the product, to discuss such proposed changes in the product design with the customer, who would be the ultimate beneficiary of such changes. Inside corner Outside edge Bad Good Difficult Easier Figure 4.3 ypical examples of product wall design improvements. Bad Good Figure 4.4 ypical improvement of flow path of plastic.

9 52 4 General Mold Design Guidelines he customer must also be made aware of the fact that the mold maker (and designer) often has had specific experiences in the areas of discussion and does not request such changes solely to save some money for the mold maker but in a desire to give the customer some advice gained from molding similar products, with earlier molds they have built. Other typical but highly undesirable features (for molding rather than for mold making) are severe changes in wall thicknesses and heavy sections at the end of long, thin passages. he product designer may not be aware of the fact that this is bad for molding. It affects the filling of the cavity space and may cause sink marks in the product. o overcome such poor flow conditions, higher injection temperatures and/or pressures and longer holding times will be required, resulting in longer cycles. Heavy walls and bosses can sometimes be redesigned by coring out or by relocating them so that more uniform wall thicknesses can be created, and cooling can be provided inside heavy sections to permit faster cooling. Usually, it is the heaviest section of a product and its cooling which determines the molding cycle and the productivity of the mold. In summary, there are many areas in which an experienced mold designer can pinpoint possible improvements which may have a bearing on the quality, productivity, longevity, and serviceability of the mold, and which should be discussed with the product designer (or customer) Notes on the Drawing Scrutinize all notes on the product drawing(s). It is a good practice to show with notes on a drawing all information which cannot be easily shown by conventional drawing techniques or which, if shown, would clutter the view(s) on the drawing. hese notes may add information regarding tolerances, draft angles, finishes, concentricity, fits, material selection, etc. Make sure the notes are clear and make sense. o overlook or misinterpret a note is as serious a mistake as to overlook or misinterpret a dimension. Notes may show changes from earlier designs of the product; they may point out which areas of the product demanded changes by the product designer. his could have significance for the mold design by indicating that future changes are possible. It may suggest provision of inserts where they otherwise would not be necessary from the mold maker s point of view Draft Angles Check the drawing for draft angles. Generally, a draft angle greater than 1 per side presents no problem for pulling the molded part out of the cavity, or for the ejection of internal ribs, provided the finish is adequate. Draft angles smaller than 1 should be carefully considered, and if they appear to be unreasonable, the customer should be approached with a request to increase the angles. While it may be possible to mold sides and ribs without any draft angle (or even with negative draft) by using side cores, two-stage ejection, or collapsible cores, such features could add considerably to the complexity and cost of the mold.

10 4.4 Studying the Product 53 hin-walled products are usually more delicate and more affected by small draft angles. hey will require better finish, draw polishing (polishing in the direction of the mold motion), and/or the use of inserts where otherwise no inserts would be required. Increase in the use of inserts, however, reduces the possibility of using maximum cooling and, therefore, may affect the molding cycle, which is of prime interest to the molder. In many cases, this argument alone will convince the customer to increase insufficient draft angles. he stiffness and the condition of the molding machine used for the planned mold also has a bearing on the minimum draft angles. If the clamp opening motion is not perfectly straight and in line with the center line of the product, it will permit the moving mold half or the cavity plate to sag as the clamp opens; the molded product will show drag marks where the plastic (between the core and the cavity) supported the mold halves during the opening stroke. If the machine is not solid enough to ensure that the mold will stay on its center line, additional supports as part of the mold will be required, thereby adding to the mold cost. If thin, deep ribs are specified, with small draft angles per side, they will require special ejection features. Ejector pins should always be at the bottom of the ribs to ensure that the ribs will not break and remain in the core. o be effective, such ejectors pins ( blades ) have a narrow, flat cross section where they contact the rib, which is very expensive to produce and to maintain. Round pins (with a diameter equal to the width of the rib) are usually too small and fragile. o use a pin diameter larger than the width of the rib, the mold designer needs a concession from the product designer because the path of the ejector pin will create a circular thickening in the rib (Fig. 4.5). For more in-depth discussion of ejection factors in draft angles for mold design, see Chapter 12, Ejection. Note that the preceding is just a sample of the kind of requests by the mold designer from the product designer or customer. Each design feature of the product must be studied before starting to design. as designed hickening in product Rectangular ejector (too expensive) Ejector Round ejector pin (too small) Larger pin suggested Original design Changes suggested Figure 4.5 wo drawings show an original design (left) in which an ejector pin is too small and a revised design (right) depicting corrections to improve the mold.

11 54 4 General Mold Design Guidelines he designer must well understand the product before making any suggestions and must not suggest anything that may affect the performance, strength, or safety of the product even if it will mean more difficulty or higher cost in mold making or in molding. Agreement on any changes must be in writing; at the very least, any marked-up changes on the issued product drawing should be signed and dated. he study of drawings can usually be done very rapidly. In many cases, it may not even take an hour to scrutinize a drawing, but as in all endeavors, it is better to first spend some time to look where you are going before starting to run Responsibility for Shrinkage When only a product drawing and the molding material are specified, it must be clearly defined who will assume the responsibility for selecting the proper shrinkage values. Sometimes, the customer assumes the responsibility and specifies the suggested shrinkage, or supplies in addition to the product sizes all critical mold (steel) sizes. his is usually the case if the customer has had similar cavities in use in an experimental or earlier production mold. Another point to ascertain at this time is whether the mold will also be used with different materials, which may have different shrinkage factors. If a mold is built to use polystyrene (PS) with 0.6% shrinkage, it probably could also be used for other materials such as polypropylene (PP) with 1.5% shrinkage, but because of the higher shrinkage factors, the products will be smaller. o know the materials expected to be molded in the mold is also useful because it may affect the selected finishes of the molding surfaces. It is important that all uncertain areas of the drawing and the specifications are clarified before starting any mold layout. Few things are as frustrating as to arrive at a beautiful mold design, only to find out that it would be very difficult or even impossible to change some critical dimensions or features, should that become necessary as a result of wrong shrinkage allowances. For a more in-depth discussion, see Section 3.1, Accuracy and Finish, and also Chapter 8, Shrinkage, and Chapter 9, Product olerances Working from a Sample or Model It is generally not possible to work from a sample or model. However, since in some cases the customer has no facilities to make proper drawings, it may be necessary for the mold designer to become involved with the product design drawing. he designer will study the product and, with all necessary information supplied by the customer, will make a product drawing. he advantage is that the mold designer can specify all the points discussed in Section to give the mold all advantages both for molding and for mold making. But it is absolutely necessary that the customer agrees with all these points and accepts the product drawing by signing it before work on the mold design is started.

12 4.5 ypical Check List for Estimators and Designers 55 It is also necessary that the customer holds the mold designer and mold maker free from any liability resulting from infringing on the rights of others due to the product so designed. 4.5 ypical Check List for Estimators and Designers he following is a summary of the questions and considerations explained earlier in this chapter. It is the basis of any formal check list used by a mold maker and may vary according to individual requirements and priorities: Machine Specifications Mold mounting data: ie bar spacing: vertical horizontal ie bar size: Can tie bars be pulled? Platen specs: Locating ring diameter Hole patterns: SPI? Euromap? Special mountings: Special ejectors: Machine nozzle data: Shape: Flat Round Radius Nozzle opening: Nozzle length: Controlled flow: Open Pin Other Injection unit: Shot capacity: Injection speed: Recovery rate: Profiling of speed possible? Profiling of pressure possible? Carriage travel: Clamp: Clamping force: Shut height: Minimum Maximum Bolster thickness: