Injection Moulding Tools Research Articles

Selection of mould design variables in direct stereolithography injection mould tooling

Stereolithography (SL) can be used rapidly to produce injection moulding tools. The disadvantage of the technique is that it is capable of producing only a small number of parts before failure. Stereolithography tools may break under the force exerted by part ejection when the friction between a moulding and a feature of the tool is greater than the tensile strength of the tool, resulting in tensile failure. Very few justified recommendations exist concerning the choice of mould design variables that can lower the part ejection force experienced and reduce the risk of SL tool failure. This research investigates the ejection forces resulting from the injection moulding of polypropylene (PP), acrylonitrile-butadiene-styrene (ABS) and polyamide 66 (PA66) parts from SL tools that are identical in all respects except for their build layer thickness (a process variable when generating the SL tooling cavities) and incorporated draft angles (a tooling design variable). This work attempts to identify appropriate evidence for recommendations with respect to these variables and SL injection moulding. The results show that linear adjustment of draft angle results in a fairly minor linear change in part ejection force according to the moulding material. A linear adjustment of the build layer thickness results in a greater change in part ejection force as a more non-linear relationship. In both cases the greatest ejection forces were experienced by PA66, then ABS and then the PP parts. The results also show that the surface roughness of all tools remains unchanged after moulding a number of parts in all polymers. A mathematical model was used in an attempt to predict ejection forces according to the moulding material used. This model did reflect the experimental results in terms of relative values but not in absolute values, which may be due to the limitations imposed by the development of the expressions and uncertainty about some specific values.

Alternative materials for rapid tooling

Rapid tooling is an extension of rapid prototyping technology. Rapid prototyping techniques are capable of producing prototypes of very complex part geometry directly from three-dimensional CAD software in a wide variety of materials such as polymer, wax, and paper without the benefit of specially designed tooling or fixturing. Rapid tooling is enabling art to production of quality parts and accelerating time to market [1] by concentrating on the tool rather than the part. The selective laser sintering (SLS) rapid prototyping process has been adapted to produce metal-based prototypes that can be used in rapid tooling applications. The SLS process fuses, or sinters, the powder to form the mould geometry, which is then filled with bronze to form a metal matrix. This is a very recent development so there is some uncertainty associated with it. Modular injection mould tool inserts will be constructed with the view to greatly reduce the time to market allowing the use of alternative tool materials (i.e., RapidSteel ®, copper polyamide and CNC machined Ureol and Araldite). An investigation will be conducted to establish the process from the CAD data to the final product. These tools will also undergo numerous tests to confirm design accuracy for form, fit and function testing, turnaround times, and for marketing evaluation. This paper will discuss the outcome of my research for the selection of the most appropriate mould material where time to market is of critical importance.

Layer thickness and draft angle selection for stereolithography injection mould tooling

The introduction of rapid prototyping has allowed engineers and designers to generate physical models of required parts very early on in the design and development phase. Further to this, the use of stereolithography (SL) cavities as a rapid tooling method has allowed plastic prototype parts to be produced in their most common production manner -- by injection moulding. The process is best suited to small production runs where the high costs of conventionally machined tooling is prohibitive. One of the major drawbacks of the SL injectionmoulding process is the susceptibility of the tools to premature failure. SL tools may break under the force exerted by part ejection when the friction between a moulding and a core is greater than the tensile strength of the core, resulting in tensile failure. Very few justified recommendations exist about the choice of mould design variables that can lower the part ejection force experienced and reduce the risk of SL tool failure. This research investigates the ejection forces resulting from SL injection moulding tools which are identical in all respects except for their build layer thickness and incorporated draft angles in an attempt to identify appropriate evidence for recommendations with respect to these design variables and SL injection moulding. The results show that adjustment of draft angle results in a change of part ejection force as a reasonably linear relationship. An adjustment of the build layer thickness results in a change in part ejection force as a more non-linear relationship. The adjustment of build layer thickness had a greater effect on ejection force than the adjustment of draft angle. The results also show that the surface roughness of all tools remains unchanged after moulding a number of parts in polypropylene. A mathematical model was used in an attempt to predict ejection forces according to the moulding material used. This model reflected the experimental results in terms of relative values but not in absolute values, which may be due to inappropriate specific values used in their calculation. Finite element analysis (FEA) was used in an attempt to identify the factors involved in the ejection process. Results indicate that the effect of draft angle on ejection force is due to elastic deformation of the surface roughness. A fact borne out by the lack of damage to the surface after ejection.

Manufacture of production injection mould tooling incorporating conformal cooling channels via indirect selective laser sintering

This paper reports the results of a study designed to assess the extent to which layer manufacture processes were able to produce production quality injection mould tools. The specific layer manufacture process considered was the DTM RapidTool process. Tools produced using this process were used in industrial trials to assess productivity benefits arising from the use of conformal cooling channels and to investigate tool wear. A cost model of the injection moulding process has been developed to investigate the economic case for the use of layer-manufactured tools. It is concluded that significant productivity benefits are available through the use of conformal cooling and that the RapidTool process is economically competitive with existing production tool manufacture methods for injection mould tools which do not require significant finishing. Future developments in layer manufacture technology are expected to offer further challenges to conventional tool manufacture methods.

DMLS moves from rapid tooling to rapid manufacturing

The Direct Metal Laser Sintering (DMLS) process from EOS is well established for the netshape fabrication of prototype and short series tooling for plastic injection moulding and diecasting. Now, recent developments in the powders coupled with the durability of the materials are extending its reach to the direct manufacturing of functional prototypes for powder metallurgical (PM) and cast components — in the following referred to as rapid manufacturing — as well as tooling for powder injection moulding and other applications. Jouni Hänninen of EOS Finland reviews the development of DMLS and its current and potential applications.

Product range models supporting design knowledge reuse

Redesign, where previous information is recovered in order to be adapted to a new situation, is an area of design where information technology can potentially provide substantial benefits. Information support to product design and manufacturing has been pursued through the use of product and manufacturing models. This paper introduces a new concept of a complementary information model, called a product range model, that aims to support variant and adaptive design activities. The general concept and structure of such an information model is defined in terms of product functions and their respective design solutions. The interactions taking place between particular design solution options are discussed, and methods are proposed for their evaluation against product specifications and design constraints. The concept of knowledge links is introduced to maintain the relationships between solutions within the product range model and the particular model of the product being developed. The work has been explored using injection mould tooling as an appropriate product range and evaluated through the design and implementation of a design support system utilizing an object-oriented database.

Predicting stereolithography injection mould tool behaviour using models to predict ejection force and tool strength

The work reported involved Finite Element Analysis (FEA) modelling of heat transfer in a stereolithography (SL) tool and then performing a series of experiments to measure true heat transfer in the tool. The results from the practical measurement of heat transfer were used to validate and modify the FEA model. The results from the modified FEA model were then used to predict the tensile strength of the tool at various stages after injection of the thermoplastic melt. Previously developed equations to predict ejection forces were used to estimate the ejection forces required to push the moulding from the SL core. During the practical experiments the true ejection forces were measured. The combination of predicted tool strength and ejection forces were intended to be used a basis for to determine whether certain SL tool designs will fail under tension during part ejection. This would help designers and manufacturers to decide whether SL tooling is suitable for a specific application. The initial FEA heat transfer model required some modifications and the measured ejection forces were higher than the predicted values, possible reasons for these discrepancies are given. For any given processing conditions there was an inherent variance in the ejection forces required however longer cooling periods prior to ejection resulted in higher ejection forces. The paper concludes that, due to the variations in required ejection forces, a reliable tool to predict tensile failure will be difficult to produce however improved performance may be gained by adopting processing conditions contrary to those recommended in the current process guidelines.

Using stereolithography tools for injection moulding: Research into tensile tool failure and unexpected benefits of the process

The use of stereolithography (SL) parts as injection moulding tools offers many advantages over traditional tool making approaches. In particular, the time required to convert a computer aided design (CAD) file to the final tool is dramatically reduced as may be the costs in creating the tool. However, the process has been perceived to have a number of drawbacks which are, in the main, associated with the poor thermal and mechanical properties of the SL resins. Research was performed which involved the production and use of an SL tool to injection mould a series of parts in polypropylene. Measurements were made of the forces required to eject parts from the tool in order to assess the possibility of tensile tool failure during part ejection. The measured forces were compared with predicted values which had been calculated using previously devised equations. In addition, temperatures were recorded throughout the injection moulding cycle to assess the tool's strength at different times. Surface roughness measurements were also taken to characterize the ejection process more closely and observe any changes to surface roughness caused by moulding parts. A finite element analysis was made of the heat transfer in the tool to help with prediction of ejection forces. Measurements of ejection forces and heat transfer indicated that, contrary to existing recommendations, part ejection should be performed shortly after injection. Accurate predictions of ejection force were shown to be unattainable given the difficulty in finding accurate values for the variables used in the force prediction equations. Measurements of heat transfer coupled with surface roughness suggested that the low thermal properties of SL tools actually work in favour of the process rather than against it. This conceptual about-turn regarding the role of thermal properties in SL injection moulding tools indicates that further benefits may be derived from the new technology.

Precision sintering of slip cast components

The experiments presented in this paper were part of a larger investigation into the feasibility of the slip casting of injection moulding tools. This paper presents the investigation into the precision sintering of slip cast 316L stainless steel components. A description of the methods used and the effects of the sintering temperature and sintering time are discussed, as well as the role that the particle size of the stainless steel powder plays in the dimensional accuracy of the sintered parts. The results show that it is possible to produce slip cast components to a high dimensional tolerance with very low distortion.

Costs and performance of injection moulding tools produced using slip casting

This paper presents results from the testing and benchmarking of a slip cast 316 stainless steel injection moulding tool. A brief out line of the slip casting process and the test cavity geometry is given as well as a discussion of some of the commercial alternative methods of forming injection moulding cavities. The calculations for the slip cast test cavity cost and lead time are given and comparisons between slip casting tool formation and alternative methods are made. The paper concludes that the slip casting method has potential in replacing some of the other tooling methods.

A Computer-Aided Optimization Approach for the Design of Injection Mold Cooling Systems

A methodology is presented for the design of optimal cooling systems for injection mold tooling which models the mold cooling as a nonlinear constrained optimization problem. The design constraints and objective function are evaluated using Finite Element Analysis (FEA). The objective function for the constrained optimization problem is stated as minimization of both a function related to part average temperature and temperature gradients throughout the polymeric part. The goal of this minimization problem is to achieve reduction of undesired defects as sink marks, differential shrinkage, thermal residual stress built-up, and part warpage primarily due to non-uniform temperature distribution in the part. The cooling channel size, locations, and coolant flow rate are chosen as the design variables. The constrained optimal design problem is solved using Powell’s conjugate direction method using penalty function. The cooling cycle time and temperature gradients are evaluated using transient heat conduction simulation. A matrix-free algorithm of the Galerkin Finite Element Method (FEM) with the Jacobi Conjugate Gradient (JCG) scheme is utilized to perform the cooling simulation. The optimal design methodology is illustrated using a case study.

Stereolithography for injection mould tooling

Explains that stereolithography (SL) can greatly reduce initial tooling costs, thus making prototyping and small production runs economically feasible. Describes how epoxy resin SL5170 and Zeneca filled resin are used to build SL injection moulding tools. Different sets of tools were evaluated, based on the maximum number of successful injections and quality of performance. A polymer tool requires a minimum level of strength, thermal conductivity and dimensional accuracy. The Zeneca SL tool achieved a total number of 200 successful injections before starting to fail. However, the failure happened during the first shot of the day where the initial injection pressure was set high. On the other hand, epoxy tools were more resistant to injection pressure and temperature and more than 500 injections were achieved without tool failure. In all tests the cavity was never damaged and it was the core which failed during either injection or ejection. Concludes that product diversity, high product complexity, increase in product variety, and shorter product life are prime motives for SL tooling.

Optimal cooling system design for multi-cavity injection molding

The objective of this paper is to present a methodology for optimal design of cooling systems for multi-cavity injection mold tooling. After the part layout and the injection mold are designed, the methodology optimizes cooling system layout in terms of cooling channel size, locations, and coolant flow rate. The mold cooling design is modeled as a non-linear constrained optimization problem. The objective function for the constrained optimization problem is stated as minimization of both a function related to part average temperature and temperature gradients throughout all the cavities. The constrained optimal design problem is solved using Powell's conjugate direction with the penalty function method. The objective function is evaluated using finite element analysis solving the transient heat conduction problem. A matrix-free Jacobi conjugate gradient algorithm of Galerkin finite element method is utilized to simulate transient heat conduction.

A novel impact tester operating at elevated temperatures for characterising hard coatings

The costs of tools used for forging, impact extrusion and injection moulding are high. There is a strong industrial demand for wear resistant and friction reducing coatings, tailore-made for these applications. In order to save tooling costs and machien time when testing newly developed PVD-coatings, an impact tester covering temperatures from 25–250°C, a load of 390–720 N and a frequency band of 8–11 Hz was designed. TiB 2-based coatings are expected to be well suitable for application on metal forming and plastic injection moulding tools. Therefore this coating has been selected for impact testing. As a reference, the commercially available TiAl(N) coating was used because of the known life time enhancing properties on forming tools. All coatings were deposited on hardened (63 HRC) high speed steel S 6-5-2 (1.3343, BM2) using commercial unbalanced magnetron equipment (Ceme Con CC 800). The TiAl(N) and TiB 2 coated samples were tested at temperatures of 25, 100, 200 and 250°C with steel (100Cr6) and aluminium as a counterpart. The prevailing wear appearance changes with temperature and lubrication. First results for the coating TiAl(N), tested with and without lubrication by impact load are presented. The impact testing is compared by FEM-calculation.

Laminate Tooling for Injection Moulding

The main goal of this research was to develop a method of prototyping injection moulded parts which produces a representation of the production part, including not only the part shape and functionality but the process as well. A prototyping method that meets all of these requirements could greatly aid in reducing the time required to bring a new product to the market by using the information gained from this prototype to manufacture a production tool that will be right the first time. Tooling constructed of laminations is appealing for prototyping or production because of the flexibility it affords in terms of rapidly altering mould geometry, gating or cooling passage design. This report summarizes a ‘proof of concept’ project which took a selected part geometry through the entire laminate tool manufacturing process. This report is divided into five sections: (1) introduction to the prototyping process, (2) background of current prototyping processes and description of the laminate tooling method, (3) description of the geometry selected for this work and the manufacturing details, (4) evaluation of the performance of the laminate injection mould and (5) conclusions.