User:PranjalSingh IITDelhi/sandbox/Rule Based DFM analysis for Stereolithography

Stereolithography is an Additive manufacturing process. Complex parts and assemblies can be directly made in one go unlike conventional stages of manufacturing such as casting, forming, joining and machining. Realization of such a seamless process requires the designer to take in considerations of manufacturability of the part (or assembly) by the process. One such tool is Design for manufacturability (DFM) analysis. In any product design process, DFM considerations are important to reduce iterations, time and material wastage. Described here are DFM considerations while designing a part (or assembly) to be manufactured by Stereolithography process.

Material
Excessive setup specific material cost and lack of support for 3rd party resins is a major challenge with SLA process..: The choice of material (a design process) is restricted by the supported resin. This means that the mechanical properties are also fixed. When scaling up dimensions selectively to deal with expected stresses, post curing is done by further treatment with UV light and heat. Although advantageous to mechanical properties, the additional polymerization and cross linkage can result in shrinkge, warping and residual thermal stresses.

Process
SLA process is an additive manufacturing process. Hence, design considerations such as "thick first layer", process latitude, etc. have to be considered. Orientation affects the support structures advised through pre-manufacturing tools, manufacturing time, part quality and part cost. Yet complex structures may fail to manufacture properly due to orientation which is not feasible due to stresses. This is when the DFM guideline of 'design modification while preserving functionality' can be applied. Design feasibility for stereolithography can be validate by analytical as well as on the basis of simulation and/or guidelines

Rule-based DFM Considerations
Rule-based considerations in DFM refer to certain criteria that the part has to meet in order to avoid failures during manufacturing.

Printer constraints
Various process constraints are to be considered during design such as :
 * Minimum Wall Thickness (Supported and Unsupported): Wall thickness in geometries is limited by resin resolution. Supported walls have ends connected to other walls. Below a thickness limit, such walls wall may warp during peeling. Unsupported walls are even more liable to detachement hence higher limit is for such case.
 * Overhang Length (Maximum Unsupported and Minimum Unsupported): Overhangs are geometric features that are not supported inherently in the part. These must be supported by support structures in which case, there is a maximum limit that can be sustained by the structures. There is a minimum limit when structures are not provided. This is to reduce bending under self-wight.
 * Maximum Bridge Span: To avoid sagging of beam-like structures that are supported only at the ends, the maximum span length of such structures shall be limited. Whenever this is not possible, width should be increased for compensation.
 * Minimum Vertical pillar diameter: This is to ensure the slenderness is above a limit at which the feature becomes wavy.
 * Minimum dimensions of grooves and embossed detail: Grooves are imprinted and emboss are shallow raised features on the part surface. Features printed with dimensions smaller than the limits are unrecognizable.
 * Minimum Clearance between geometries: This is to ensure the parts don't fuse.
 * Minimum hole diameter and radius of curvatures: Small curvatures that aren't realizable by print dimensions may close up or smooth out/fuse.
 * Mimimum internal volumes nominal diameters: Volumes that are too small may fill up.

Support Structures
A point needs support if : While printing, support structures act as a part of design hence, their limitations and advantages are kept in mind while designing. Major considerations include:
 * It is end point of support less edges
 * If length of the overhang is more than a critical value
 * It is at the geometric center of support less plane
 * Support shallow angle geometry: Shallow angles may result in improper resin (structural strength issues) curing unless supports are provided uniformly. Generally, beyond a certain angle (usually around 45 degrees), the surface doesn't require support.
 * Overhang base: Increase section thickness at base to avoid tearing. Avoid sharp transitions at overhang base.
 * Bubbles release: Supports even relieve the chances of air bubbles in surfaces of slant holes.
 * Structure compatibility: Consider Supports compatibility for internal volume surface.
 * Feature Orientation: Orient to ensure overhangs are well supported.

Part deposition orientation
Part orientation is a very crucial decision in DFM analysis for SLA process. The build time, surface quality, volume/number of support structures etc. depend on this. In many cases, it is also possible to address the manufacturability issues just by reorienting the part. For example, a overhanging geometry with shallow angle may be oriented to ensure steep angles. Hence, major considerations include:
 * Surface finish improvement: Orient the part in such a way that a feature on critical surface is eliminated. Algorithmic point of view, a free-form surface is decomposed to combination of various plane surfaces and weight is calculated/assigned to each. Total of weights is minimized for best overall surface finish.
 * Build Time reduction: Rough estimation of build time is done using slicing. The build time is proportional to the sum of surface areas of each slice. (Can be approximated as height of the part)
 * Support structure optimization: Supported area varies as per orientation. In some orientations, it is possible to reduce support area.
 * Easy peel-off: Reorienting such that the projected area of layers varies gradually makes it easier to peel off the cured layer during printing.

Plan-based DFM Considerations
Plan-based considerations in DFM refer to criteria that arise due to process plan. These are to be met in order to avoid failures during manufacturing.

Geometric Tailoring
"“The modification of some non-critical geometric features of a part to lower fabrication cost and time, and to produce functional prototypes that mimic the behavior of the production parts. ”"

Geometric Tailoring bridges the mismatch of material properties and process differences described above. Both functionality and manufacturability issues are addressed. Functionality issues are addressed through 'tailoring' of dimensions of the part to compensate the stress and deflection behavior anomalies. Maufacturability issues are tackled through identification of difficult to manufacture geometric attributes (an approach used in most DFM handbooks) or through simulations of manufacturing processes. For RP-produced parts (as in SLA), the problem formulations are called material-process geometric tailoring (MPGT)/RP. First, the designer specifies information such as: Parametric CAD model of the part; constraints and goals on functional, geometry, cost and time characteristics; analysis models for these constraints and goals; target values of goals; and preferences for the goals. DFM problem is then formulated as the designer fills in the MPGT template with this information and sends to the manufacturer, who fills in the remaining 'manufacturing relevant' information. With the completed formulation, the manufacturer is now able to solve the DFM problem, performing GT of the part design. Hence, the MPGT serves as the digital interface between the designer and the manufacturer. Various Process Planning (PP) strategies have been developed for geometric tailoring in SLA process.

DFM Frameworks
The constraints imposed by the manufacturing process are mapped onto the design. This helps in identification of DFM problems while exploring process plans by acting as a retrieval method. Various DFM frameworks are developed in literature. These frameworks help in various decision making steps such as:
 * Product-process fit: Ensuring consideration of manufacturing issues during the design stage gives insight on whether SLA process is the right choice. Rapid prototyping can be done in various ways. The usual concern are process cost and availability. Through this DFM Framework, the designer can make necessary design changes to ease the component manufacturability in SLA Process.
 * Feature recognition: This is done through integrated process planning tasks in commercial CAD/CAM software. Such integrated tools are in developmental stage.
 * Functionality considerations: Considerable research has been going on in this since the past decade. These are being put to application and DFM frameworks are being developed.