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1、Table of Contents1. Introduction32. Background2.1 Introduction2.2 Attainable tolerances and causes inaccuracies2.3 Influences on precision53. Hewlett-Packard3.1 Way of working3.2 HP process system3.3 Co-operation with Philips94. Rogue4.1 Specifications4.2 Development general4.3 Tolerance analysis4.4
2、 Product Design4.5 Mould Design4.6 Material4.7 Process Design4.8 Design rules Rogue155. Carnegie5.1 Specifications5.2 Development Carnegie general5.3 Tolerance analysis5.4 Product Design5.5 Mould Design5.6 Material5.7 Process Design5.8 Design rules Carnegie276. Design rules6.1 Process396.2 Material6
3、.3 Mould6.4 Product7. Case study7.1 Specifications7.2 Product analysis7.3 Development7.4 Next generation478. Literature61AppendicesAppendix AStatistic Process Control (in Dutch)Appendix BInjection moulding and tolerances (in Dutch)Appendix C1Product documentation RogueAppendix C2Tolerance analysis R
4、ogueAppendix DAttainable tolerancesAppendix E1Product documentation Carnegie Appendix E2Tolerance Analysis Carnegie1. IntroductionThis project has been set up to fasten down the knowledge within the CCP about high precision injection moulding in relation to the Hewlett-Packard (HP) carriages. Two ca
5、rriages for HP are developed for high precision injection moulding in co-operation with Philips Competence Centre Plastics. “Rogue” was a test case to develop a precision technology by injection moulding, fit for high precision in high volumes. In this case tolerances had to be attained, not by adju
6、sting in the assembly line but by injection moulding. The results were the base for the development of the manufacturing process (part design, mould manufacturing, injection moulding process) for the “Carnegie” carriage.With the goal to help HP in the future developing products with similar aspects
7、it is important to fasten down the obtained knowledge about this subject. This study will result in a general description of design rules for high precision injection moulding that can be used in the early stage of development. The earlier the co-operation with the customer can start the more effici
8、ent the product can be designed for high precision. Efficient in terms of shorter lead-times and more robust processing.The goal of the design rules is to share the knowledge within the BG-Plastics so people can use the experience in the future designs. This report starts with a general description
9、about high precision injection moulding. Special attention needs to be paid to the process, the mould, the material and the product design.The product design is essential for high precision injection moulding. A poorly designed part, no matter how extraordinary the mould construction or process is,
10、can destroy any chance of achieving the very tight tolerances of the precision part. The carriages of HP are the examples used to finally formulate the design rules for high precision injection moulding. One chapter has been spent on the development of the Rogue and one on the development of the Car
11、negie. This report ends with a test product to illustrate the design rules.2. Background2.1 IntroductionTalking about precision in plastics, is talking about micrometers and how to control micrometers within specification. It is about precision of plastic parts without any finishing treatment.Precis
12、ion is influenced by:1. Material2. Process3. Mould4. Product designSo how can these aspects be handled to get the required precision? Before answering this question it is important to know what the inaccuracy sources can be and what the capable precision of plastics is.What are attainable tolerances
13、 and what causes inaccuracies in the product?2.2 Attainable tolerances and sources of inaccuraciesFour elements are related to high precision injection moulding. Each element has its own requirements in respect to material, process, mould and product design.1. Dimensional accuracyThe nominal values
14、and their tolerances required for the product.2. Shape accuracyInaccuracy in the shape like warpage and sink marks.3. ReproducibilityStability of dimensions and shape in mass manufacturing, Cp- and Cpk-values (appendix A).4. Long term stability Long term stability of dimension and shape as function
15、of time, inaccuracy caused by moisture absorption, ageing, creep etc.To deal accuracy of plastics in a good way, sources that can lead to inaccurate products have to be distinguished. Two types of variations influence the accuracy. The irreversible variations, like shrinkage, volume relaxation, “mou
16、lded in stress” relaxation and after shrinkage. The reversible variations like expansion through variations in temperature and humidity.Separately from these sources related to the process and material there is a source of inaccuracy directly related to the mould.During the process the plastic melt
17、will become solid, which means a reduction of the specific volume. The material shrinks and the final product dimensions are smaller than the mould dimensions. The mould has to be compensated for this effect. Plastics have the property that the shrinkage can not fully be influenced during the proces
18、s and this causes a variation on the dimensions. If the variation is not time-dependent the process can be called stable. The process is reproducible and the products are within an acceptable process window. The shrinkage can partly be compensated with applying a holding pressure, extra material is
19、post-filled into the cavity.Because of the cooling of plastic during the injection moulding process, thermal stresses will be induced in the material. The volume will change, because the entropy of the material is not maximal and causes a time-dependent change of free volume. The plastic strives to
20、equilibrium of the package of its molecules, this is equal to an increase of density.With injection moulding the cooling time is much shorter than needed. The result is that the specific volume is not in equilibrium. For glassy polymers the free volume is too large. For semi-crystalline polymers the
21、 crystallisation is not fully developed. Every thermo-dynamic system strives to maximum entropy (equilibrium state) that means that in time there is a relaxation of volume (post-shrinkage or post-crystallisation) resulting in a change of shape or dimensions. Although this aspect cannot be eliminated
22、, it can be minimised by slower cooling (higher mould temperature). See appendix B.Applying plastics means apply them in conditions that can change. All plastics will show dimensional variations under changing conditions. Variation in temperature causes dimensional variations, the same for variation
23、s in humidity. Some plastics absorb water and swell. Because the thermal expansion of plastics is mostly larger than metal, this is a point of attention when applying plastics in combination with metals.To design a product that can be produced with high precision you have to take in account the way
24、tolerances are built up. The four aspects that influence the attainable precision have their own contribution in the tolerance structure.The attainable tolerances can be divided in:1. Mould tolerances (mould)The total concept and the machining method that is applied determine the accuracy of the mou
25、ld.2. Process tolerances (process, material)The plastic that is used determines the attainable tolerances. The properties that influence the accuracy of the product differ per plastic material type.3. Product tolerances (product design)The sum of the mould and the process tolerances is the minimal a
26、ttainable tolerance (=Cp, capability of the process). The product needs to be designed with a very close relation to the process and the mould (=Cpk, capability of the process related to the product specifications).Within the PMF a tool has been designed to give the product designer more insight in
27、attainable tolerances for plastic products and the attached price consequences. It is called “Attainable Tolerances in Plastics” and has also been used for the HP carriages projects. Five groups of materials (A, B, C, D, E) and three classes of accuracy (I, II, III) are distinguished. The combinatio
28、n of material A (most predictable accuracy) and class I for the mould/process has the best opportunities for an accurate product. It is also the most expensive one with the most difficult process.2.3 Influences on precisionFor the definition of precision the classes of accuracy from called “Attainab
29、le Tolerances in Plastics” are used.IPrecise: adapted precision moulds, technically optimal processing conditions, intensive final inspection and high additional costs.IIAccurate: accurately dimensioned moulds, processing controls less critical and moderate additional costs.IIINormal: mould toleranc
30、es can be obtained in normal procedures, the processing conditions are balanced to an as cheap as possible manufacturing process and normal additional costs.The feasible precision is influenced by:MaterialTaking the accuracy into account the choice of material is determined by the shrinkage and afte
31、r shrinkage, the structure of the polymer amorphous or semi-crystalline, the anisotropy of the shrinkage and expansion.Semi-crystalline thermoplastics shrink more during the cooling in the mould. Because of the control of shrinkage is proportional to the rate of shrinkage these materials have less p
32、otential for high accuracy. Amorphous plastics are better suitable for high precision. In general: products can better be designed for high precision when using material with a low shrinkage. Shape accuracy is strongly related to the same aspect. Differences in shrinkage within a part cause warpage.
33、 The larger the shrinkage is, the greater the rate of unacceptable deformations of the part.Filling the plastic with glass or carbon reduces the shrinkage and guarantees products with better precision. The material is stiffer and can reduce warpage. Because of the filling the material has a larger a
34、nisotropy caused by the orientation of the fibres, which stimulates warpage. It seems better to use spheres instead of fibres for the orientation effect, but in practice the difference has never been demonstrated.The choice of material is not only based on high precision, other aspects like chemical
35、 resistance, elasticity, temperature range etc. play an important role.Process The plastic used determines which processing tolerances can be adhered in the classes of accuracy. When the material choice has made the process has to be optimised for the plastic. Operating an injection moulding process
36、 means controlling several important parameters. Realising a product according to tight specifications means an extensive investigation into the influences of each process parameter. Main parameters are: Filling stage (injection time, injection speed, injection pressure) Cavity pressure (holding pre
37、ssure, holding pressure time) Cavity temperature (mould temperature, cooling time) Melt temperature (cylinder zone temperature)These parameters are used for better controlling the shrinkage. The effect of the holding pressure is that the thermal volume shrinkage is partly compensated, so that the sh
38、rinkage is better controlled. This means more predictable dimensions and shape and less thermal stresses.HP executes an injection moulding optimisation to determine the process window according to a prescribed procedure. The process has to be robust. A robust process has a process window that includ
39、es all not controlled variations of the process. A process variation within the process window will not cause “out of spec” parts (See chapter 3- HP Process System).MouldPrecision in respect to the mould exists of two aspects, the tolerance of the assembled inserts to each other and the tolerance on
40、 the manufactured inserts itself. The dimension of the tolerance of the assembled inserts to each other is determined by the centring of both mould halves, the amount of slides and the inaccuracy of inserts. The attainable class of accuracy is thus limited by the machining method. Grinding is more a
41、ccurate than milling for example. Table 2.1 shows the capabilities for several machining methods. dimensionmethod ( most accurate)jig-grinding (II)pro (I)spark erosion (II)copy milling (III)10 mm0.0050.0040.0030.016100 mm0.010.0060.0050.022Tolerance ( mm) With special precautions and extra cost tigh
42、ter tolerances are achievable. Max.= 1 mmTable 2.1 Attainable tolerances manufacturing (Source: “Attainable Tolerances”, Anemaat)Tool bounded dimensions are realised in one half of the mould, not tool bounded dimensions are dimensions that are not realised in one of the halves of the mould, but are
43、realised through various moving parts of the mould. For not tool-bounded dimensions the attainable tolerance has to be increased with 25 mm.The class of accuracy of the mould is determined by the most accurate dimension of a product. The largest source of inaccuracy is caused by the margin for shrin
44、kage applied on the mould to compensate the difference between the mould dimensions and the product dimensions. For precision class I the predictability of the shrinkage is not sufficient enough, so the mould has to be corrected on functional dimensions based on the results of the trial runs. For hi
45、gh precision moulds it is important to have the possibility to modify the mould-dimensions steel safe, that means removing steel instead of adding.Product design The product design has to be made in close relation with the material choice, process conditions and the mould design are already made. De
46、sign of high precision products is an iterative process with an interactive character. It is not possible to design the product without taking into account the influences of the material, process and the mould on the accuracy of the final product. A large influence on the accuracy of the product and
47、 therefor on the product design is the shape of the product. Dimensional accuracy needs to be disconnected from shape deviation, because it is more difficult to control the shape or warpage. The larger the nominal dimension, the more important the right estimation of shrinkage is to minimise the cor
48、rection of the mould. Precision moulds just have their advantages for small products.In general: if you use the design tools for high precision injection moulding smart, it is not always the more accurate the more expensive. It starts with a smart product design.3. Hewlett-Packard3.1 Way of working Hewlett-Packard is an American company specialised in electronic equipment.
