Technology for Plastic Mold Tooling - Direct Metal Deposition

Direct Metal Dep sitionTechnology for Plastic Mold Tooling -

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Abstract

Abbreviations

Plastic Injection Mold Tooling

Moldflow Analysis

Direct Metal Deposition (DMD)

Evaluation of DMD Technology for Mold Productivity

Conclusion

References

Author Info

Comparative Analysis –Steel Mold vs. Copper Mold coated with H13 Steel by DMD

3

3

4

5

6

8

99

10

11

11

Table of Contents

The development of various plastic materials and manufacturing processes in the last century has stirred a revolution in product development. The majority of plastic components are made through injection molding and extrusion processes. One of the key aspects of the injection molding industry is the design and manufac-ture of the molds. Conventional mold making consists of extensive machining and assembly operations. But, prior to mold design, the part model can be analyzed with Finite Element Analysis (FEA) software such as Moldflow. The analysis results are utilized to check the molten plastic feeding and part cooling systems, predictpredict the process parameters and trouble shoot for molding defects. An exciting new technology in tool manufacturing today is the Direct Metal Deposition (DMD) process, marketed by DM3D Technology LLC, pre-viously known as Precision Optical Manufacturing (POM), Michigan. The DMD process utilizes a high power laser to construct or repair a metal part, by spraying the metal powder over a molten metal pool, thus creating a dense, high quality part. DMD can be used to deposit a layer of tool steel over a high conductivity metal such as Chromium Copper in the mold insert. This approach can enhance the transfer of heat from the mold and effectively reduce molding cycle time.

Until recently, the benefits of DMD technology in mold cycle time reduction had not been evaluated. This paper presents a comparative analysis conducted for cycle time performance of a mold consisting of a con-ventionally machined tool steel insert to a mold fitted with a Chromium Copper insert with a coating of tool steel deposited by the DMD process.

Abstract


Sl.No

1

2

3

4

5

6

77

8

9

10

DMD

POM

CAD

CAM

CAE

EDM

DMEDME

HASCO

MPA

MPI

Direct Metal Deposition

Precision Optical Manufacturing

Computer Aided Design

Computer Aided Manufacturing

Computer Aided Engineering

Electro Discharge Machining

Detroit Mold EngineeringDetroit Mold Engineering

Hasenclever GmbH + Co KG

Moldflow Plastic Advisor

Moldflow Plastic Insight

Full FormAcronyms

Abbreviations

Technology for Plastic Mold Tooling - Direct Metal Deposition | 3


Injection molding is a process in which plastic is melted and forced into a mold in a molding machine, and cooled to form a part. An injection mold is an assembly of steel plates consisting of mainly two halves with impressions or cavities of the part to be molded. The mold has two halves representing the inside surface known as the core and the outside surface called the cavity. Figure 1 shows a mold for a five sided thin wall ‘Container Box’ part in a closed and open position. The part impressions are usually machined on high quality tools steel inserts and assembled in a mold base made of carbon steel such as mild steel.

Other components in the mold base include the guiding elements such as guide pillars and bushes, fasteners, support columns, a register ring for locating the mold in the molding machine, coolant connectors, and the plastic feed system. The feed system consists of a sprue bush, a series of machined channels called ‘runners’ and a narrow section called the ‘gate’ through which the molten plastic enters the cavity. Some molds have hot runner systems for improving material efficiency and part quality. The ejection system includes ejector pins, pillars and bushes for movement and return pins to push back the pins after the part is ejected. Various types of mold cooling systems include drilled holes, baffles and bubblers in core and cavity inserts.types of mold cooling systems include drilled holes, baffles and bubblers in core and cavity inserts.

Molding machines essentially consist of an injection unit with a barrel and heater bands for melting the plastic material, platens for clamping the molds, toggle and hydraulic mechanisms for mold opening and closing, and a control unit as shown in Figure 2. These machines are classified generally by their clamping tonnage such as 30 tons, 2000 tons, etc. Some of the generally used plastic materials are Polypropylene (PP), Nylon, Polycarbonate (PC), Poly Vinyl Chloride (PVC), Acrylonitrile Butadiene Styrene (ABS), Polystyrene (PS), Acrylic, and Polyethylene (PE), either in plain form or mixed with filler materials such as glass or talc, mixed in a a ratio of 10%-30%. Material properties that are important for mold design and the molding process include types of material (amorphous or crystalline), melting temperature (high or low) and shrinkage (high or low).

Figure 1: Injection Mold in closed and open position

Plastic Injection Mold Tooling



Undoubtedly, mold design and manufacture is the most critical aspect in the molding industry. Mold design and product design are highly interactive activities. The complexity in part geometry along with functional and appearance requirements throws up many challenges to mold designers and tool makers. The primary goal is to develop a part which can be tooled and molded at an optimum cost. It is estimated that industries in the U.S. spend around $12 billion (POM, 2001) every year on injection mold tooling. Conventional mold making methods involve activities like machining, heat treatment, surface finishing and assembly operations, involv-ing expensive machinery and labor.

WithWith the advent of CAD/CAM/CAE software, mold design and documentation time has been drastically reduced. Many of the popular 3-dimensional softwares offer ‘Mold Design’ packages.These packages have the data available for mold base standards such as DME, HASCO, Futuba and Stark. Prior to the design of the mold, the part can be analyzed with FEA packages in order to determine the feed system, cooling network and process parameters. Different molding defects can be diagnosed and the mold design can be optimized. The most popular analysis software for plastic molded parts is Autodesk Simulation Moldflow, from Autodesk Corporation.Corporation. This software was previously known as Moldflow from Moldflow Corporation, USA until May 2008.

The Moldflow package has a number of modules for analyzing various aspects of a plastic part, mold, and the molding process. It has a database for more than 500 grades of materials of all the major plastic manufactur-ers. Analysis can be performed with MPA (Moldflow Plastic Advisor), which is a simple package for designers at the early stages of part or mold design. CAD models can be directly brought into MPA without the mesh and can be analyzed to check confidence of fill, gate locations, identify the position of knit (weld) lines, air traps, and to check the alternative part designs. However, more detailed flow and advanced analyses can be per-formedformed only with MPI (Moldflow Plastic Insight), which needs a meshed, mid-surface model of the part. CAD models in the IGES (Initial Graphics Exchange System) or other suitable forms can be brought in to MPI. MPI has a mid-surface and meshing generator to create the finite element model. The modeler in MPI also can be used to create surfaces, mesh, feed systems, cooling and mold models.

Figure 2. Mitsubishi 120mj Molding Machine at the University of Louisville

Moldflow Analysis



Some of the general modules of Moldflow are MF/FLOW, MF/COOL, and MF/WARP. MF/FLOW performs a Multi-Laminate Filling Analysis, which uses finite difference algorithms with logic that molten plastic fills in the cavity layer by layer. Results of MF/Flow are referred to for conducting further analyses such as cooling and warpage. Boundary conditions for the flow analysis include an Injection location node in the meshed model. The meshed model for a Container Box part with sprue feed system is shown in Figure 3.The fill time simula-tion for this part is shown in Figure 4.

After being filled with molten plastic, the cavity needs to be packed to compensate for the volumetric shrink-age, which can be as high as 30% of the part volume. The packing of the mold cavity can be analyzed through Multi-Laminate Packing Analysis. If volumetric shrinkage is too high, sink marks may appear at thicker sections of the part like ribs and bosses. Variations in shrinkage can also lead to the warpage, which is charac-terized by bends and bows in the molded part. MF/COOL is a type of heat transfer analysis to analyze the tem-perature distribution in the mold with cooling channels. Outputs of this analysis can be used to determine the effectiveness of the cooling line location, size of the mold, and performance of inserts made of high conduc-tive metals such as Beryllium Copper with respect to the cycle time. The challenge in using high conductive Beryllium Copper or Chromium Copper is that these metals are comparatively soft and have low wear resis-tance during the mold’s life. Therefore, they need to be covered with a layer of high strength tool steel to act as contact surface in the mold core or cavity inserts. One of the latest technologies for depositing tool steel over high conductive metals is Direct Metal Deposition (DMD).

For many years, the ability to machine complex molds and dies has been dependent on subtractive processes such as Electro Discharge Machining in addition to conventional and CNC machining. A metal additive process called Direct Metal Deposition (DMD) offers an innovative approach. DMD was first developed in the Univer-sity of Michigan in the late 1990s and commercialized by POM (Precision Optical Manufacturing), Plymouth, Michigan. POM was acquired by DM3D Technology LLC in January 2013.

Figure 3. Mid-surface mesh model of ‘container box’ Figure 4. Plastic fill time simulation

Direct Metal Deposition (DMD)


2.26

2.02

1.77

1.52

1.27

1.01

0.7610.761

0.509

0.256

0.0034

Fill Time (sec)


DMD is a new type of rapid prototyping system, which utilizes high power laser to construct a part by spraying metal powder over a molten pool as guided by the CAD model. As shown in Figure 5, the DMD technology patented by POM consists of a system for focusing a high power Laser for melting the metal powder in order to construct either a new metal part or deposit a new metal over an existing work piece. DM3D (new owner of POM) manufactures DMD machines and offers mold and die tooling repairs, and modification and 3D metal printing services for automotive and other applications. DMD can be used for most metals including tooltool steels (H13, H19, P20, P21, D2, M4, S7), Aluminum, Copper, Chromium alloys and super alloys. For example, steel can be deposited over aluminum or copper.

DMD machine technology uses a Carbon Dioxide Laser beam mounted on a CNC gantry, which can move in three axes. The laser creates a molten pool of metal on the work piece first and then a feeder deposits the powdered metal into the area of the laser. The melt pool cools and rapidly solidifies, layer by layer, until the end of the process to create a solid, dense part. The process induces a shallow heat affected zone, thus mini-mizing the thermal stresses. It also ensures good quality for the deposited part.

Figure 5. Direct Metal Deposition process developed by POM, Michigan

Figure 6. DMD 3000 Machine at University of Louisville


Final Focus Optics

To Powder Feeder

CO2 Laser Beam

Feedback Sensor 1

Workholding Fixture

Nozzle Shielding Gas

Feedback Sensor 2

Solid Free Form Shape byDirect Deposition

Substract or Die Preform


The DMD 3000 machine, as shown in Figure 6 is equipped with a 3000 volt Carbon Dioxide (CO2) laser (DMD 3000 Operator’s Manual, POM, 2001), which is a heavy-duty industrial grade laser. The CO2 laser provides a high power output with low surface penetration, which results in a shallow heat affected zone. The machine has CNC control for process and movement of the CNC gantry. POM’s patented closed loop optical feedback system is comprised of cameras, lenses and filters that are attached to the Z axis. The metal powder feeder consists of a powder storage canister and the mechanism to dispense powder to the nozzle in a controlled manner.manner. Inert gases like Argon and Helium are used as cover gas, carrier gas, and shaping gas in the powder feeder.

Mold design and mold open–close simulation was performed using the Solid Edge 3D CAD modeler. A standard mold base was utilized and mold plates were machined to fit core and cavity inserts. Both core and cavity inserts were machined out of H13 tool steel. An identical core insert was made by DMD by depositing 0.100 inches of H13 steel on the Chromium Copper insert as shown in Figure 8. Being a slow process in com-parison with other non-traditional machining methods such as EDM, the deposition of H13 by DMD took two to three weeks. With the preferred method of deposition vertical to the work-piece surface, the insert had to bebe indexed in the rotary axis multiple times. Some difficulties were experienced in finishing operations of the insert after deposition, which was carried out through grinding and extensive polishing. Prior to finishing and heat treatment, cooling channels were machined in core and cavity inserts with reference to Moldflow Cooling analysis results. For obtaining mold temperature data, thermocouples were inserted in the mold both on core and cavity inserts.

The author, with a team of research engineers, worked on the evaluation of the DMD process as a part of his Master of Science thesis project at the University of Louisville’s Rapid Prototyping Center. The evaluation project involved the design and manufacture of injection molds with interchangeable core inserts. With emphasis on high volume molding, a box shaped container of 4x 4 x 2.75 inches in height and 0.040 inches in thickness was selected to be molded with Polypropylene (PP). Moldflow analysis was conducted prior to mold design with a meshed model of the Container Box part as shown in Fig. 3 and the filling pattern as shown in FigFig 4 in Section 3. A complete meshed model of mold with core and cavity inserts, and cooling channels is shown in Figure 7 below.

Figure 7. Meshed model of ‘Container Box’ mold with cooling channels for core and cavity


Evaluation of DMD Technology for Mold Productivity


Successive molding production runs were carried out for the mold with core inserts made of conventional machining and inserts made with the DMD process. Figure 9 shows the Container Box mold mounted in platens of the Mitsubishi 120mj machine during the molding runs. Molding cycle times for both types of inserts were recorded and the differences were observed. Mold core and cavity temperature were continu-ously monitored in both cases with digital thermocouples, and variations were recorded. The molded box parts were checked for warpage and other visible defects.

When the performance of the Steel mold (mold consisting of H13 steel insert) and the Copper mold (mold consisting of Chromium Copper Insert coated with H13 steel by DMD process) was analyzed with Moldflow, the following results were observed as mentioned in Table 1. The copper mold showed better performance in molded part temperature (lower temperature difference than steel mold), overall mold temperature (cooler than steel mold) as well as cooling effectiveness (higher heat dissipation by the coolant).

Figure 8. Semi-finished Chromium Copper Insert coated

with H13 tool steel by DMD

Figure 9. Container Box mold mounted in

120mj Molding Machine


Comparative Analysis –Steel Mold vs. Copper Mold coated with H13 Steel by DMD

Parameters

Temperature difference in the molded part

Mold Top Temperature

Mold Bottom Temperature

Coolant Temperature Rise

26

103

97

1

-9.6

-4.3

-2.4

0.7

16.4

98.7

94.6

1.7

DMD Copper Mold (° F) Difference (° F)

Steel Mold (° F)


After collecting data from multiple molding runs with different settings for mold temperature, plastic melt temperature and packing pressure, the following observations were made from the data analysis for perfor-mance between the Steel mold and the Copper mold (with H13 steel coating by DMD).

Maintaining the same dimensions and part quality, the achieved cycle time was 22 seconds for the Steel mold and 15 seconds for the Copper mold

The cycle time saving of 7 seconds or approximately 32% will yield significant savings when such box parts are molded in the millions.

With constant packing pressure, the Copper mold showed a 100% increase in cooling rate compared to the Steel mold.

WithWith lower cooling times, holes were observed near the ‘gate’ area in the part molded off the Steel mold, whereas the Copper mold produced good parts with a similar setup (refer to Figure 10 below)

Mold temperatures were observed to be steadier in the Copper mold.

Parts molded with the Copper mold did not stick to the cavity as the Steel mold parts did, sometime during the mold trials.

Better packing pressure profiles were observed with the Copper mold compared to the Steel mold.

The design and manufacture of injection molding tools throws up many challenges on technology, time and cost, as the plastic industry expands across horizons. The invention of CAD/CAM systems and FEA software such as Moldflow has brought sophistication and optimization to mold design and manufacture. In a high volume production part such as a 5 side container box referred to in this thesis, the analysis helped in deciding the mold’s cooling systems, optimizing the process parameters, as well as studying the benefits of the Copper mold over the conventional Steel mold.

Figure 10. Steel Mold vs. Copper mold - part quality with ‘0 sec.’ cooling time


Conclusion


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Reference

The development of the DMD process is a milestone in mold manufacturing. The ability of this process to deposit tool steels over high conductivity metals and to construct the conformal cooling channels in the mold, can bring phenomenal reduction in molding cycle time. Cycle time optimization not only results in cost savings, but also ensures faster product delivery schedules. Employment of DMD processes for depositing H13 tool steel over the high conductivity chromium copper insert in this project was critical in obtaining a wear resistant molding surface. Significant saving of 32% in the cycle time achieved in the Copper mold coated with H13 Steel has confirmed the benefits of this cutting edge technology.coated with H13 Steel has confirmed the benefits of this cutting edge technology.

Author InfoPundarik Bhat

HCL Engineering and R&D Services

1. Pundarik Bhat, Rammohan K. Ragade and Julius P. Wong, “DMD Process and Injection Molding Tool Development”, EDA 2002,

Maui, Hawaii, August 2002

2. Pundarik Bhat, “A Plastic Injection Molding Productivity Study”, Master of Science Thesis, University of Louisville, Louisville, KY

2003

3. Mold Design User’s Guide, I-DEAS 9.0 Mold Design Software, EDS (Electronic Data Systems)., 2001.

4. DMD 3000 Operator’s Manual, POM (Precision Optical Machining), 2001.

5. www.POM.com, (website of DM3D Technology LLC), 20135. www.POM.com, (website of DM3D Technology LLC), 2013

6. www.POM.net, (website of POM Group), 2003

7. www.Moldflow.com (Website of Moldflow Corporation, USA), 2003.

8. www.Autodesk.com (Website of Autodesk Corporation), 2013

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