Compression molding is a manufacturing process that utilizes heat and pressure to shape and cure rubber materials into usable components. This molding method applies heat and pressure to form “charges” within a mold cavity. The charge is subjected to these temperatures and pressures for a set time to allow the rubber within to cure. Any excess material is purged into flash channels within the mold, allowing parts to maintain consistent weight and density properties between operations [1]. Figure 1 demonstrates a simplified representation of this process, showcasing a streamlined mold cavity and exaggerated flash channels.
Aside from the simplicity of setting up compression molding, it offers multiple production benefits, such as high-yield repeatability and tooling geometries supporting complex part shapes. Compression molds may use internal structures such as wedges or mandrels to help rubber curing, but very few advanced software or technical skills are required to work with tooling. These properties allow the production of consistent rubber products with minimal barriers to learn the process.
Creating compression mold tooling involves forming a relief of a desired part within a cavity via machining. Compression mold tooling fabrication has traditionally been conducted via machining processes like computerized numerical control (CNC) machining or mill and lathe operations. While these machining methods provide high-precision tooling, they also have limitations in their capabilities. Two significant drawbacks of these subtractive manufacturing operations are material waste and, thus, high cost and long lead times [2, 3].
Machining operations require a high amount of material investment to produce tooling. This leads to a high scrap rate and significant waste from the metal and cooling fluids used within the process. From a financial perspective, the subtractive manufacturing methods require a more increased investment in machinery and personnel training. Both CNC mill and lathe operations require skilled workers to operate the machinery. CNC machining involves software training to program the various G-codes needed per operation in addition to support fixtures and machining tools. Thus, an alternative manufacturing option for compression mold tooling that can reduce cost and lead time is desirable.
Direct Metal Laser Sintering (DMLS) is an additive manufacturing method that uses laser fusion on metal powders to create intricate and complex structures [4, 5]. Powders are selectively layered throughout the fusion process, following a desired computer model, and utilizing only the required materials to create a part. This layering approach allows DMLS-based structures to have internal systems like honeycombs, foams, and other pockets. The localized material fusion results in a high degree of density through builds, maintaining the properties of the printed material at similar levels to traditionally machined structures [4].
Additionally, direct metal laser sintering’s additive nature eliminates the need for timely and extensive setups, tool fixturing, and multiple machining operations common to CNC machining [6, 7]. Despite the time-consuming requirements of traditional manufacturing, the DMLS builds can be sent directly from computer-aided design (CAD) software like SolidWorks or Catia to a printer to start the process. While reducing material usage already increases the cost benefits of DMLS, the lead time to print is another contributing factor to the operation’s positive attributes. The variables that define a DMLS operation’s speed include scan speed, layer and powder thickness, and laser power. A DMLS process has a max print speed of several tens of cubic centimeters per hour [4], in contrast to an equivalent CNC cut which may have a rate measured in millimeters.
DMLS has potential within compression molding manufacturing. Since molds are typically produced with high-strength metals like steel or aluminum, the additive process should require minimal adjustment to have components from these metals [8]. However, tooling molds made from additive manufacturing operations must maintain the structural and thermal requirements that existing compression molds undergo during traditional production processes.
Tooling made from subtractive manufacturing methods often uses a single piece of metal to make each part. This creates consistent grain and surface finish, resulting in uniform mechanical properties like ductility and fatigue resistance. Whereas the strength of a CNC machined tool is constant, a device manufactured using DMLS can experience different compressive or tensile strength levels due to fabrication orientation [8].
The strength of a DMLS part is directly correlated to the orientation in which it is printed. Alkindi et al. [8] conducted a study of tensile strength testing on samples printed at the specified angle orientations. Figure 2 shows the angles at which samples were printed before tensile testing. The study concluded that samples printed at 0º and 10º had maximum stress values of 947.26 and 949.87 MPa, respectively. These two samples had the highest elongation-at-break, with values of 2.98% and 2.2%, respectively. Both tensile stress and elongation-at-break decreased as the angle of the build print increased. The build print with a 90º had a maximum pressure of 440.15 MPa, which is less than half the 0º and 10º builds. This decrease continued within the elongation-at-break, as the 90º build had a value of 0.83%. These findings imply that build orientation is essential to a mold’s resistance to compressive forces. These results can be applied inversely to compressive applications. The 90º sample was weak to the tensile testing due to the buildup of stresses at the print layers. However, using compression to a 90º oriented model replaces the tensile stress with compressive stress normal to the layering. This condition would reinforce the layers against each other and provide higher resistance against compressive stress. As a result, in this paper, proposed models for a DMLS is printed in a 90º orientation to give the best compressive properties.
One of the benefits of DMLS is the ability to create complex geometries such as honeycombs or foams. These cavitation types are highly desired within aerospace applications, as honeycomb-based structures are utilized in aircraft for their high strength-to-weight ratios [9]. Applying these benefits to compression molding requires an evaluation of the compressive properties of porous structures. While beneficial to aircraft, honeycombs, and other hollow-support systems may not be suitable for the needed localized pressurization of compression molding.
An evaluation of 3D-printed foams was conducted to determine the load-bearing properties of these hollow structure. Ramesh et al. [10] studied a set of cubes with varying pore diameters subjected to compression. The analysis determined that compressive strength varies directly with part density and inversely with specimen porosity. In this paper, the DMLS compression mold proposed maintains a high material density and low porosity. Making honeycomb or foam shapes small while providing many layers during the build process give the DMLS mold the best probability of withstanding the pressure parameters.
Kogo et al. [11] noted a similar observation regarding print density on the load-bearing properties of 3D-printed metals. Brittleness testing observed the propagation of fractures within test samples. Analysis under a microscope revealed that crack propagation occurred in areas where the fusion of materials did not happen. The extensive presence of cracks and layer delamination was caused by quick and inaccurate printing, leading to a high lack of fusion from either low laser power, high scan speed, or improper scan strategy.
In this paper, we perform a computational study using design of experiments to present a compression mold that builds on the benefits and shortcomings of the literature. Therefore, the main contribution of this paper is to study the possibility of expanding the capabilities of DMLS structures in tooling and investigate the structural integrity of DMLS molds against CNC machined molds. Subsequent sections of this paper will explain and evaluate computational methods of study, culminating in a summary of findings and avenues for continuing research.