Characterization and Control of Pin Diameter during In-Mold Assembly of Mesoscale Revolute Joints

Macro-scale revolute joints can be formed by first molding the hole and then molding the pin inside the hole. As the pin shrinks during the solidification process, it moves away from the hole and provides the clearance for the joint to function. The value of clearance in the macroscale joint can be controlled by carefully selecting the process parameters and the material for the pin. However, in order for this strategy to work at the mesoscale, it requires the use of very thin cores to form sub-millimeter holes. Such thin cores are very difficult to make, are easily damaged during the molding process, and very difficult to retract from the hole. Our previous work has shown that making the pins first and then creating holes on the top of pins leads to successful mesoscale joints. This strategy is counter intuitive based on our experiences at the macro-scale. At the macroscale, as the hole shrinks on top of the pin, the joint is jammed. So a fundamental question is why this counter-intuitive strategy works at the mesoscale. In this paper we show that at the mesoscale, the joint jamming is prevented because of the deformation of the pin under the compressive loading during the second stage molding. We also describe features in the mold that can control the pin deformation and hence control the joint parameters. We present experimental data and computational models to show how mesoscale revolute joints can be formed. INTRODUCTION In-mold assembly is a fast and cost effective process for producing articulated joints. It utilizes injection molding to automate assembly operations which would normally require long, labor-intensive operations for production [1-3]. Since injection molding is a high throughput process, in-mold assembly holds tremendous promise in rapid bulk production of assembled parts. FIGURE 1 IN MOLD ASSEMBLED REVOLUTE JOINT. An in-mold assembly process has been developed to manufacture mesoscale revolute joints. This process involves injection of different components of the revolute joint assembly into a mold sequentially or in different stages. Mold pieces are moved before each injection to create shut off surfaces between different parts of the mold cavity which are filled in each step of the in-mold assembly process. When all steps are completed, a fully assembled mesoscale revolute joint is ejected from the mold cavity. The material combination used for in-mold assembly is chosen such that the materials are chemically incompatible (i.e. they have no tendency of adhering to each other during and after injection molding). The appropriate material combinations have been identified for in-mold Stage 2 LDPE Stage 1 ABS Pin diameter = 0.75 mm assembly processes in several previous works [2-4]. Figure 1 shows the CAD model of a mesoscale revolute joint that can be manufactured using in-mold assembly. Our definition of mesoscale is dependent on the context of the in-mold assembly process. We define the delineation between the macroscale and the mesoscale as the dimension where the plastic deformation of an unconstrained premolded component during the in-mold assembly process is greater than 5% of the overall part dimension. For proper functioning of a revolute joint, a clearance fit is required between the pin and the hole. At the macroscale, this clearance is ensured by the shrinkage of the pin after cooling and ejection. This is because the pin is molded in the second stage. However at the mesoscale, the hole is molded in the second stage. The reason for this is explained in the next section. After cooling and ejection, the shrinkage of the hole leads to an interference fit. This may lead to a non functional joint. Figure 2 illustrates this concept. In the figure, dh is the hole diameter and dp is the pin diameter. FIGURE 2 CLEARANCES IN IN-MOLD ASSEMBLED MACROSCALE REVOLUTE JOINTS However macroscale concepts can not be directly scaled down to the mesoscale [3, 4]. Hence we need to develop a better understanding of how clearances are obtained in in-mold assemblies at the mesoscale. Several scenarios also necessitate the clearances to be engineered to meet the design requirements. Hence, there is an impending need to develop manufacturing methods to control the clearances in mesoscale in-mold assemblies so as to give the designer the freedom to choose the appropriate clearance for the functioning of the component. In this paper, we will describe methods to characterize and control the clearances in the mesoscale in-mold assemblies. The methods described in this paper can be used to fabricate mesoscale in-mold assembled revolute joints with appropriate clearances. BACKGROUND AND PROBLEM FORMULATION One of the major differences between in-mold assembly at the mesoscale and that at the macroscale is the molding sequence [3]. At the macroscale the part with the hole (Figure 1) is molded in the first stage. The part with the pin is then molded in the second stage with the first stage part acting as a mold insert. However at the mesoscale, the part with the pin is molded first. This is accomplished by molding a hole in the first stage using a side action mold insert (SAMI). The size of the SAMI is the same as the desired size of the hole. However making a SAMI with a small diameter for mesoscale revolute joints is an expensive process. Also a SAMI with a small diameter poses alignment problems and was prone to failure due to forces applied by the injection pressure [4]. This is illustrated in Figure 3. An alternative approach to fabricating a mesoscale revolute joint involves premolding one component in the first stage then forming the second component in a second stage. However, the high pressure, high temperature polymer injected in the mold cavity during the second stage causes the mesoscale features in the premolded component to deform plastically, as illustrated in Figure 4. In order to inhibit this deformation, we have previously adopted a novel mold design strategy which involved supporting the premolded component by 2533% of its length. This approach is illustrated in Figure 4. This enabled us to manufacture a functional mesoscale in-mold assembled revolute joint [3]. Assembly clearances in the mesoscale revolute joint can be produced by the reduction in diameter of the premolded component after second stage injection. This reduction in diameter is caused by the aforementioned plastic deformation of the premolded component as illustrated in Figure 5. In the figure, dp is the diameter of the pin and Ls is the length of the support provided to the premolded mesoscale pin during the second stage injection. Second stage part dh First stage part First stage part dp Second stage part Pin molded in first stage Hole molded in first stage dp > dh dh > dp dh dp FIGURE 3 CHALLENGES POSED BY INACCURATE ALIGNMENT OF SAMI [4] FIGURE 4 MOLD DESIGN FOR MESOSCALE IN-MOLD ASSEMBLY [3]. FIGURE 5 MOLD DESIGN STRATEGY FOR CREATING MESOSCALE IN-MOLD ASSEMBLED REVOLUTE JOINTS Lc is the support cavity length i.e. the length of the cavity in the radial supports used to constrain the mesoscale premolded component. The support cavity length Lc plays an important role in determining the final diameter of the mesoscale pin after second stage injection, and therefore the level of clearance in the revolute joint. In order to understand the effect of the support cavity length, let us review the complexity of the mesoscale in-mold assembly process. In the first step, the first stage part is molded and the side core supports are retracted as illustrated in Figure 5. During the second stage cavity filling, the mesoscale premolded component acts as a soft mold insert for the second stage polymer melt flow. The premolded component is made of an elastoplastic material that can plastically deform due to the forces applied by the second stage melt [3, 4]. This bending can be inhibited by the presence of the radial supports. However, in the second step of the molding process, a very high compressive load is applied to the Forces applied Second stage polymer melt dp’ Le Part after 90° rotation Part after 0° rotation First stage Injection Side core Unconstrained pin Supported pin