Repair of Plastic Injection Moulds

Transcription

1 Repair of Plastic Injection s P. Peças, E. Henriques IDMEC, Instituto Superior Técnico, TULisbon, Av. Rovisco Pais, Lisbon, Portugal ppecas@ist.utl.pt; elsa.h@ist.utl.pt ABSTRACT: The research reported in this paper aims to contribute to increase the knowledge related with injection moulds repair. The types of moulds repair are identified and characterized in terms of the reasons for repair, the repair s duration and quality related requirements and the mould s geometrical and dimensional constraints. The techniques used in injection moulds repair (undercut, inserts and metal deposition by welding processes) are also identified and characterized. A strong emphasis is given to the repair by welding, namely in the comparison of the suitable welding technologies. Furthermore, in order to understand how mould makers and mould users are repairing their moulds, a survey to the industrial companies was conducted and the results are presented and discussed. Taking advantage of the systematization of the moulds repair framework several objective conclusions were withdrawn from the survey, namely as regards the incidence of each type of repair and of each repair technique, the frequency of use of each welding technology and the influence of related technological and business factors. Additionally, a specific analysis of the welding technologies suitable for injection mould repair is presented. They were compared based on eight different repair scenarios characterized by different factors: mould dimension, repair geometry, repair access point and repair visibility. As main achievement, the paper presents the specific fields of application (repair scenarios) of the analyzed welding technologies. 1 INTRODUCTION The repair of plastic injection moulds is a very important issue during the life cycle of the mould. The mould repair is usually associated with the rebuilding of some mould features eroded or broken down due to the injection process [Peças et al. 2006]. The high mould cost and the time required to produce a new mould justify the repair aiming to extend mould life. Furthermore, the same time and cost factors justify the need for mould repair in other life cycle stage in the mould production phase. In fact, the repair techniques used to rebuild a mould degraded due to its use are the same that are used to modify a localised region in an already machined mould cavity during the production phase. The need of mould repair in the production phase has two main reasons, one related with human errors and systems malfunction, and the other related with the present mould making business characteristics [Peças & Henriques 2003]. The progressive time-to-market reduction leads to the dispatching of the mould production without a complete stabilisation of the product detail design and determines increasingly the need to reshape already machined zones [Henriques et al. 2005]. So, the term mould repair involves the concepts of modifying the mould features in order to assure the intended performance. The techniques used to repair moulds have experienced a significant development in the last years, mainly with the introduction in the market of processes with the ability to micro-processing [Hollenbeck 2001, Roy & Francoeur 2002]. It s possible nowadays to perform sound repair tasks obtaining a mould cavity able to accomplish the exigent injection moulding conditions [Godin & Guadani 2001]. Nevertheless, there is lack of integrated knowledge as regards the type of mould repairs, the current repair practice at mould makers plants, the type of technologies used and the application field of the repair processes [Peças et al. 2006]. The aim of this paper is to contribute to a broader discussion and structured knowledge about mould repair. The results of the conducted research presented in the following paper sections foster the use of the appropriated procedures and techniques, and contribute to the acceptance of mould repair as a phase of mould life cycle by the actors of the mould making sector.

2 2 CATEGORIES AND TECHNIQUES OF MOULD REPAIR 2.1 Repair Categories The need to repair a mould is originated by a cause. Three types of distinct causes that configure the mould repair categories can be identified. Figure 1 represents the occurrence of each repair category during the mould life cycle. One of the categories is the Rebuilding. This category includes the rebuild of an eroded or worn localised mould zone due to the polymers abrasive and/or corrosive actions along the successive injection cycles. Other category is determined by the need of mould Revision. It consists in the modification of some mould local zone during the mould manufacturing process due to a design alteration purposed by the client. The need for mould design Revision can also result from an additional feature willing to be added to the mould. The latter occurs for reasons of moulding process improvement and/or final part design revision and updating. The last category to mention is the Correction repair. It involves the modification of some mould local zone during the manufacturing process due to a mould maker error (in the drawings, programming, machining, polishing, etc.) or process malfunction (energy cut-off, tool break, etc.). Design Manufacturing Rebuilding Revision Test Need of repair due to eroded or worn features Correction Need of repair due to production fault Need of repair due to design changes Use End-of-Life Figure 1 The repair categories during the mould life cycle. A survey was conducted among 40 mould makers aiming to extract some conclusions related with the mould repair task. In what regards to the Rebuilding repair category it was evident that most of this type of repairs are not done by the company that manufactured the mould. Since the time available to repair a mould that is already in its use phase is very short, the mould owners usually decide on deliver the mould to a repair shop near the injection facility. Only special moulds or specific repair tasks demand for the mould coming back to the mould maker. Therefore, most of the moulds repair tasks found in a mould making company are related with Revision and Correction category. As illustrated in Figure 2, the amount of moulds per year that require Correction repair are below 20% but almost 50% of the moulds are submitted to Revision repair. There is no visible relation between the mould maker size (measure in terms of moulds produced per year) and the need for repair. Nevertheless, it was observed a significant importance of the type of client the mould is targeted to. For the moulds to produce automotive parts the mould makers mentioned that around 80% of the moulds undergo by a repair process (mainly Revision type). On the other hand moulds to produce electronics or utilities a repair task is rarely asked. Percentage of moulds repaired 52% 19% 46% 27% 51% 12% Revision Repair Correction Repair 38% 8% 48% 19% below to to 80 upper 80 Global average s produced per year Figure 2 Percentage of moulds submitted to Revision and Correction repair categories. The number of moulds produced per year is used as a measure of mould maker company dimension. 2.2 Repair Techniques Three distinct techniques used to repair plastic injection moulds can be identified: inserts, weld bead deposition (welding) and undercutting (of the moulding and parting surfaces). The interaction of the repair techniques with the repair categories is represented in Table 1. The requirements, the final result and the manufacturing aspects (cost and time) of each technique are very relevant in the process of repair technique selection. In fact, the selection depends on the type of repair to perform, but also on other factors like aesthetic, cost and geometrical constraints. The aesthetic factor is one of the most important ones, mainly when repair is required in the mould cavity (that represents most of the cases). For repair tasks in a zone that will be visible in the plastic part only undercutting and welding techniques are normally used. The use of inserts will cause the presence of a visible joint. Regardless the following con-

3 siderations related with the selection of welding or undercutting when the aesthetic factor is relevant, it must be pointed out that the welding technique is used when it is impossible to apply the undercutting technique. The repair category, the repair dimensions and the confidence in the welding repair technique are the subsequent factors involved in the repair technique selection. In Rebuilding category a large repair involving a considerable amount of material should be done through the undercutting technique. Undercutting will assure that the wear behaviour and the shrinkage level will be kept. This can be difficult to keep if the welding technique is used to deposit large quantities of material. On the other hand, if small damaged areas are involved the welding technique should be used after mould client acceptance (even in a small and controlled welding there is always the risk of introducing weld beads in a mould already submitted to several thermal cycle). It must be mentioned that there is no need of complicated calculations to find out that using the undercutting technique in the repair of small zones is always more expensive and time consuming than welding the undercutting requires the machining of the overall cavity geometry. Table 1 The factors involved in the selection of the type of repair (U Undercutting; W Welding; I Inserts). Welding Confidence High Low Repair Size High Importance of the Aesthetic Factor Repair Category Low Importance of the Aesthetic Factor Repair Category Rebui. Rev. Correc. Rebui. Rev. Correc. Large U W W U W W Small W W W W W W Large U W U U W U Small U W U I I I The last arguments to select welding in detriment of undercutting are valid for the Revision repair category. In this category small geometrical modifications are usually involved, the metallurgical state of the material is known and the manufacturing time is narrow. In the case of the Correction category the technique selected depends on the confidence in the welding process and procedures. If it s possible to deposit weld beads following the correct welding procedures, the final result will not affect mould performance during the injection phase. For repair tasks in which the aesthetic factor is not relevant, the decision depends on two factors: repair extension and confidence in the welding process. The undercutting is recommended for large repair zones and the welding is recommended for the rest if there is confidence in the welding process. If not, the use of inserts avoids the higher costs and time of undercutting. 3 THE MOULD REPAIR BY WELDING From the above discussion, the welding technique has high potential to be further applied in the future since it is less expensive and less time consuming. The use of appropriate welding procedures, by well trained welders and the use of the correct welding process will avoid problems related with different metallurgical and wear behaviour, minimising the fracture risk. Moreover, it can be demonstrated that welding can be as robust as any other process in the mould life cycle, as far as it is properly applied [Baranek 2007, Gervais 2007, Malkasian et al. 2000]. The perception that the use of welding is the most appropriated solution for most of mould repair situations is evident in the results of the conducted survey. In fact, it was concluded that around 80% of the mould repair is sub-contracted by the mould makers to welding repair shops in the surroundings. Around 13% of the mould makers mention that they perform some of mould repair jobs using their own welding equipment. Nevertheless, as a rule it was mentioned that for complex repair operations the mould is repaired in a dedicated repair shop. 3.1 The welding processes used in mould repair As stated by other authors the most used welding processes for mould repair are the and the welding. The results of the survey confirm this situation as is presented in Figure 3 and Figure 4. There is no influence of the mould maker size or of the repair category. In fact, welding is used in 2/3 of the repair tasks and welding is used in the remaining 1/3. The main reason found for the prevalence of welding is related with the lower repair cost and the lower repair time. The lower repair cost is essentially related with the small investment of the welding machine (especially when compared to the welding equipment), causing the lower cost per hour asked by the repair shop. The lower repair time is related with three factors. The higher deposition rate and lower setup time of welding, and, the most important, the relation of the number of machines existent in a repair shop. Actually, for each laser welding machine existent in a repair shop there is an average of 6 welding machines. This is a very important factor to select the welding process, since usually the repair by welding can start immediately after the mould arriving at the repair shop. On the other hand, if welding is selected, it can take some hours or even 1 or two days to start the repair process. This waiting time is usually not compatible with the characteristic timeframe of mould manufacturing.

4 44% 56% 29% 71% 37% 63% 27% 73% 35% 65% below to to 80 upper 80 Global average s produced per year Figure 3 Welding process distribution for the Revision repair category. 39% 61% 28% 72% 43% 57% 27% 73% 35% 65% below to to 80 upper 80 Global average s produced per year Figure 4 Welding process distribution for the Correction repair category. From these results it can be concluded that the incidence of each process is very dependent on the technological configuration of the local repair shops. In fact, the welding process selection should not be based only in criterion related with its manufacturing and cost performance. The systematic comparison presented in next section helps to define the application field of the most appropriated welding processes to moulds repair. 3.2 The welding process suitable for mould repair and welding are the processes currently used to perform most of moulds repair welds. However one can consider also Plasma welding suitable for these operations since it is very similar to having higher density energy. welding is characterised by the electric arc stability and concentration, allowing highly controlled and accurate weld beads in any welding position. The equipment is portable, easy to operate and requires low set-up time. The time required to begin an operation depends only of the welding operator positioning. There are welding torches of several sizes, nevertheless the access to some features geometries can be impossible. The electric arc melts the filler material and the base material, producing final weld beads larger than the filler wire diameter. It allows good metal deposition rates. On the other hand the heat input in the base material is high, so it can be required pre and pos heating in order to avoid excessive stress concentration. Since it is a manual process it has a high flexibility, allowing the work on intricate geometries. Nevertheless it requires a skilled welding operator. Plasma welding is very similar to welding. The main difference is related with a higher concentrated welding plume, resulting in higher penetration and energy density. The heat input to the material is lower, which is a good characteristic as regards the affected zone, but the metal melting rate is smaller than in, which results in a worse productivity. The Plasma welding equipment is also portable, easy to operate and set-up. welding is characterised by its concentrated density of energy and accuracy allowing narrow (filler wire diameter), high-quality and deep weld beads. The heat input to the material is small being a common practice to perform the welding operation without pre or pos heat treatment. The metal deposition rate is lower than in the previous two processes. Some difficulties can appear when it is required to fulfil an intricate geometry, since it is a semiautomatic process, in which the operator controls a CNC table and visualizes the welding through a lens. The equipment set-up and control is more complex and usually the mould/component position in the welding table can be a time-consuming operation. In spite of that the welding is less dependent on the operator skill and experience, since the equipments are often programmed to the most common welding operations 4 THE APPLICATION FIELD OF THE WELDING PROCESSES With the aim to contribute to increase the knowledge about moulds repair by welding, a process classification/comparison is presented. The comparison is based on 10 criterion, considered as the most important as regards to the process, the repair operation characteristics and impact on the mould: A. Metal Deposition Rate of the process; B. Portability of the welding equipment; C. Flexibility in the Access to complex Geometries; D. Welding process flexibility in performing weld beads with Complex Shapes; E. Cost per hour of equipment utilisation; F. Weld bead Appearance and concordance with the base material;

5 G. Metallurgical Quality of the welding and its impact on the base material; H. Process Capability as a measure of process reliability and feasibility (operator dependence); I. Equipment Set-up Time to start the welding operation; J. The need of pre and/or pos Heat-Treatment. Table 3 presents the analysis of the three processes for each criteria. It must be noticed that these processes are suitable for most of the welding applications in mould repair. However a severe classification was used in order to emphasise the difference among the processes. In fact this score amplitude is commonly used in the decision-making methods. As an example, for the criteria Deposition Rate, the three processes have levels of deposition rate compatible with the welding operations usually required in mould repair. Other processes like GTAW or even SAW have indeed higher deposition rate than, but are in fact too high to allow a correct control of the weld bead deposition. Therefore, welding has the highest score because among the three processes it has the highest deposition rate. On the other hand welding has the lowest one so the lowest score was attributed. Table 3 Welding processes classification in each criteria in injection mould repair. Criterion Plasma A Deposition Rate B Portability C Access Geometries D Complex Shapes E Cost rate F Appearance G Metallurgical Quality H Process Capability I Set-up time J Heat-Treatment Best Performance; 3 - Medium Performance; 1 - Lowest Performance. With this process classification it is difficult to distinguish the processes. The processes characteristics can have different relevance for different applications. As an example, if the mould repair involves only the deposition of a very small bead, the criteria Deposition Rate is of very small importance, yet the criteria Set-up Time is very important. The opposite reasoning can be made if a very large zone has to be repaired. Having in mind the importance of the specificity of the repair tasks in the welding process decision, the comparison methodology presented here is application-based. Eight application scenarios, covering most of the welding repair conditions, were defined (Figure 5). The three most important factors that influence the welding process performance in mould repair were used for the scenarios conception. In addition, the established analysis considers the mould is repaired in a repair shop, located in the region of the mould making company. So, there is a need to transport and move the mould. Accordingly, one of the most important factors that influence the welding process performance is the mould dimension. In this analysis it was considered that a small mould weights below 2.5 ton. These moulds can be easily transported to the repair shop and moved near to the welding equipment by a stacker. In contrast, with the mould size increasing it can be recommendable to move the welding equipment to the place where the gantry placed the mould. So, for this type of moulds welding equipment portability is crucial allowing more flexibility to accomplish the repair task. Surface I Area repair II Surface III Linear repair Scenarios IV Area repair Surface V VI Surface VII Linear repair VIII Figure 5 Application scenarios used in the comparative analysis (8 different application scenarios). Besides mould dimension, the influence of the type of geometry to repair was also considered as crucial. There are two main geometries to repair: the linear type geometry, related with the rebuilding of edges; and the area type geometry, related with the need to fulfil a hole or to rebuild a feature. The differences of the welding processes performance in what concerns to the deposition rate and process capability will cause different behaviours depending on the type of geometry to repair. Finally it was also considered the repair zone as an important factor in the processes performance. The zone to be repaired can be placed either in the mould surface or in an inner part of a depression. When is required to repair such a zone, the process versatility in what concerns the capability of accessing to complex geometries becomes essential. The next step required to compare the welding processes is the attribution of importance weights to each of the 10 criterion identified for each scenario (table 4). A classification from 1 to 3 was used, where 1 means that the criteria has low importance

6 in that specific scenario, 2 means a medium importance, and 3 means the criteria is very important in that scenario for welding process selection. Table 4 Importance weight distribution for each scenario (weight ki values, where index k refers to the scenario and i refers to the criteria). Criterion A B C D E F G H I J Scenarios I II III IV V VI VII VIII For each scenario (k) and each process (j), the weights (weigth ki ) were multiplied by the criteria classification (criteria ij ) of each process (j), in table 3. The sum of each process parcels is an indicator of the process performance in each scenario. The final aim is to compare the processes, so what is important is the relative performance among them. Equation 1 was used to obtain the values present in Table 5: Performance J criteria weight ij ki i= A jk = (eq.1) MaximumPerformance k The better process in each scenario has a score of 100%. The difference of the process scores is a measure of the performance difference in that scenario. Following this approach it is possible to achieve some degree of quantitative sensibility in this qualitative comparison. Table 5 Relative process performance for each scenario. A score of 100% represents the best performance in that scenario. Large Small Plasma Plasma Linear Surface 86% 97% 100% 79% 85% 100% Repair 91% 100% 91% 93% 98% 100% Area Surface 100% 91% 93% 100% 84% 100% Repair 100% 93% 76% 100% 87% 81% From the results presented in Table 5 it is possible to conclude that the repair application scenario affects the process performance. To aid the results interpretation the processes performance is illustrated in a graphic mode in Figures 6 and 7. In Figure 6 the processes performance for different mould sizes and repair geometry are compared, keeping the repair zone at the mould external surface. In this type of repair the process handling versatility has low impact. The access to the geometry is easy and the welding operator has freedom of movements. For surface type repair the welding process has clearly the best performance when linear type repair geometries are involved. Its dominance is higher for small moulds than for large moulds due to the low portability of the laser welding equipment. The execution of linear weld beads is in fact more appropriated to the welding process, since it s a semi-automated process and the welding spot is very small and accurate. It s also feasible to deposit linear weld beads using, but a skilled and experienced welder is required. These facts affect welding capability and weld bead appearance. Linear weld Linear weld Easy repair location (mould surface) Area weld Plasma Area weld Figure 6 Process performance for the Scenarios I, III, V and VII, the ones related with repair of zones in the exterior mould cavity surface. For repair application involving large moulds and an area type geometry the welding has the best performance. The main reason is related with the higher deposition rate and high portability of the process. Plasma welding presents an overall good performance in these 4 scenarios involving repair operation at the mould surface. Although is not the best choice for any of the scenarios, its performance is near the other two processes. It must be noticed that for linear type repair the Plasma welding has a better performance than the welding, mainly due to its better accuracy, weld bead appearance and process capability. Figure 7 compares the performance of the processes for the scenarios involving the repair of zones that are not at the mould surface but in difficult access zones. The repair zone is kept as interior and intricate. The processes performances are illustrated for different mould size and repair geometry. The repair in interior zones demands for high versatility of welding source handling. It is required to deposit the weld bead in several positions and the welder has to access the inner parts of the geometry.

7 The welding reveals difficulty to fulfil these requirements since the welding source is fixed and requires a linear access to the welding zone. In the case of large moulds the performance of welding is even more limited. These are the main reasons for the low performance of welding in these scenarios as is visible in Figure 7. However, the welding accuracy, capability and weld bead appearance compensate those limitations placing this process in the selection zone when small moulds and linear type repair are involved. Linear weld Difficult repair location (mould inner zone) Linear weld Area weld Plasma Area weld Figure 7 Process performance for the Scenarios II, IV, VI and VIII, the ones related with repair in interior zones of the mould cavity. The characteristics of the Plasma welding process foster for its better performance when the repair in interior zones is involved. In effect its portability together with its accuracy and low thermal input contribute to be the selected process when linear type repair in large moulds are involved. welding shows an evident supremacy for the repair applications involving area type geometries in interior zones. This process is highly versatile in the welding torch handling and has the highest deposition rate. This scenario is the one where the welding reveals its lower performance, mainly when large moulds are involved. Accordingly, the comparison methodology used allows the identification of the application fields of each welding technology. The key factors that are the base of processes performance comparison were identified and the use of different application-base scenarios allowed the selection of the most appropriated process for the type of mould repair to perform. For each repair scenario there is a weld technology which performance is clearly higher than the others. So, the three technologies are more than opponents, they are complementary. Figure 8 and Figure 9 present a simplified view of the selected process for each type of repair scenario. Easy access Reparações welding à Superfície position (mould surface) Large s Small s Line / Area Figure 8 Processes selected for scenarios related with repair of zones in the mould cavity external surface. Difficult Reparações access no welding position (mould inner zone) Large s Small s Plasma Line Area Figure 7 Processes selected for scenarios with repair in interior zones of the mould cavity. 5 CONCLUSIONS The repair tasks is part of the plastic injection moulds life cycle. The need for repair has several causes from the worn mechanisms of the mould during the successive injection cycles, to the modifications introduced in the part design during mould production phase, and to errors and inaccuracies during mould design and production. This paper presented a definition of three categories of repair that influence the repair procedures and the selection of the repair techniques. The repair techniques were also identified and characterized in what concerns to their abilities and limitations, and correlations between the repair categories and the repair techniques were established. The use of welding bead deposition is one of the most flexible and feasible mould repair technique. A

8 great emphasis was given to the welding processes with the ability to be applied in moulds repair. A qualitative comparison of the welding technologies was presented. Following a comparison methodology the technologies were compared on a base of 10 performance criterion. Then 8 application-base scenarios were defined in order to identify the application field of each technology. From the analysis it can be concluded that each one of the elected process has special characteristics that permits its selection, at least, for a mould repair application scenario. The welding is best suited for repair scenarios involving linear type repair, small moulds and repair zones located in the mould surface. The Plasma welding is the process selected only for one scenario, the one involving large moulds, linear type repair located in an interior zone of the mould. Finally, welding is the most suitable process when large moulds are involved and when deposit area type weld beads are required. 6 ACKNOWLEGMENTS The work described in this paper was developed within the Imp Project financially supported by the Portuguese Science Foundation. The authors extend their thanks to all who contributed to this research, mainly to the industrial mould-makers and specialized mould repair companies. REFERENCES Baranek, S.L Partnering for Success: One Builds, The Other Repairs. In: August. Henriques, E. et al. (2005). Reflexões sobre Novas Estratégias Competitivas no Sector de Moldes e Ferramentas. O Molde 68: Gervais, J.E The Tool of Choice for Mold Repair. In: February. Hollenbeck, J Micro welding: Applications beyond mold repair. In: October. Malkasian, S.et all Micro-dimensional welding is putting some moldmakers ahead of their competition. In: February. Peças, P. & Henriques, E The need for agile manufacturing implementation in mould making business. Proc. Business Excellence I, Braga: Universidade do Minho: Peças, P. et all Fostering the Use of Welding Technology in the Repair. Rapid Prototyping Development RPD Marinha Grande: Centimfe, Cefamol, nº P6031. R. Godin, R. & Guadani, K Selecting a filler metal for mold repair welding, In: Abril. Roy, S. & Francoeur, M Options for restoring molds. In: September.