Pushing traditional design frameworks using hybrid moulding

There is a complementary relationship between the existing CAD technology and the digital form sampling and processing technology. Because of this complementary relationship, a process called hybrid moulding is produced, which will be based on scanning capabilities and The shape-based molding ability is tightly combined.

In the past 10 years, Digital Shape Sampling and Processing Technology (DSSP) has developed into CAD complementary technology and product development technology, helping thousands of customers convert 3D scan data into digital models for product design and analysis. , production and video demonstrations.

DSSP software automatically generates NURBS surface graphics from lattice data, allowing users to accurately capture and reconstruct the shape of physical parts. Practice has proven that this feature is ideal for the following application areas:

(1) Capture physical design graphics and create prototypes.

(2) Copy original parts, molds, and tools.

(3) Copy organic complex shapes.

(4) Prepare models for mold manufacturing and CAE applications.

(5) Mass customization of unique parts.

In recent years, there is a complementary relationship between the existing CAD technology and the Digital Shape Sampling and Processing Technology (DSSP). Because of this complementary relationship, a process called Hybrid Moulding is produced, which will be scanned. The basic measurement ability and the shape-based molding ability are tightly combined.

This paper mainly discusses the advantages of the hybrid molding method, and explains how to use it to create an accurate parametric model for a pump impeller. The mold factory can use this model to manufacture molds for casting production.

Combine all the power

The traditional CAD model is created according to the method of defining the order of 2D and 3D solid structures one by one. As described in the illustrative model, CAD operators rely on their skills and experience to use these physical parameters to control the composite shape of objects and create new design graphics. The completed parametric model provides a great deal of flexibility for the generation of multiple variables, because these variables can be used in multiple iterations of design and rapid experimentation using their morphology and functionality (see Figure 1).

Figure 1 Traditional CAD model used in mold manufacturing and its typical examples of challenging significance

When making models using sketches, traditional CAD methods are good, but when users face complex surface images, traditional methods have some drawbacks. It takes a lot of time and effort, and there is no guarantee that an accurate model can be obtained. In some cases, it is almost impossible to duplicate the surface image with a shape-based method because it is difficult to identify and quantify the parameters that control the shape of the object.

The hybrid moulding method provides a solution for overcoming the disadvantages of traditional CAD for copying complex surface images. Basic geometric reference graphs, such as data, curves, and primitive features, can be measured and extracted using 3D scan data. This process can be easily run between CAD and DSSP software to balance each program and run it in the best possible way.

Capture physical part graphics

The first step in this process is to capture the existing physical part graphics. The illustrated impeller is scanned by a "white light" scanning system that uses two high resolution cameras to capture the contour image projected on the part surface. Then measure the millions of measurement points on the part by triangulation, analyze these figures, and finally generate a bitmap.

Due to the shiny surface of the part, it is necessary to apply a powder coating to reduce its reflectivity so as not to interfere with the projected pattern. The complexity of the shape requires that it perform multiple scans from multiple different locations to provide full visible light on the impeller surface (see Figure 2).

Figure 2 A pump impeller is ready and scanned as required

A series of parameters in the surface setting of the impeller can help correct the multiple scan images. Using a technique called photogrammetry, the center point of the object is automatically detected from the image obtained by the high resolution camera. Collecting all points from all positions provides enough information to correct all scan patterns.

The scanner can capture 1500 points from more than 20 scan patterns. After the scanned images have been corrected and merged, a polygonal model can be created (see Figure 3).

Figure 3 STL raw data from the scanning system

Cleaning and maintenance

After the introduction of scan data as an STL model, the next step is to use the automated process within the DSSP software to clean and repair the data. Typical cleaning procedures include the removal of foreign data, noise reduction, data reductions to reduce file size, gaps, and maintenance intersections. The examples cited are about the maintenance of the entire polygon model, but in most cases only the part of the model that will be used in the parameter reconstruction process needs to be cleaned (see Figure 4).

Figure 4 DSSP software is used to clean and complete the polygon model

Extract curves and data

After creating the model in the DSSP software, we began to reconstruct the shape of the hub by detecting the center axis of the impeller, determine the outline of the hub with an extracted curve, and then generate a rotating surface by the rotation of the curve on the axis.

The axis can be found by selecting the parameters outside the impeller. This surface area is completely cylindrical and will produce a stable data axis. Another method is to select the surface area of the hub and calculate the axis by rotating the surface.

The next step is to create a profile curve and determine the shape of the hub. In this case, a simple cross-section curve cannot be extracted because the blades will interfere with the cross-section of the hub. However, we can use tools and other methods in the CAD system - a variable cross-section scanning method. A semicircle is extruded along the axis of the hub to create this surface pattern, and then the diameter of the semicircle is controlled using a non-planar curve extracted from the hub surface (see Figure 5).

Figure 5 Creating a Curve in the DSSP Software (Left), Bringing the Curve to CAD, and Creating a Variable Scanning Surface Graphic

The bottom of the impeller has no obstacles to the blade, so a simple plane curve can be extracted and used to create a rotating surface pattern.

In order to control the shape of the surface and create additional parameters, consider using the right-angled edges of the scanned surface pattern to create the ideal new curve.

Free form surface measurement

The blade is the most complex factor in the design of the impeller. The measurement of these surface patterns is very difficult and difficult to define with basic parameters. However, applying the NURBS surface measurement method in the DSSP software, the blade surface pattern parameters can be quickly measured and then introduced as IGES or STEP characteristic data (see Figure 6).

Fig. 6 NURBS surface measurement in DSSP software (left), introducing NURBS surface pattern measurement data into CAD and modeling

After the introduction of a single blade parameter, the blade can be replicated around the imported data axis and multiple blades can be created on the surface of the hub. Then with the second separate blade, the entire introduction and modeling process is repeated. The specific approach is not to give a definition for the spacing between the blades as a direct measure of the angle, but rather to define a parameter that relates the angular separation to the total number of blades. This will allow us to easily change the number of blades while maintaining the proper blade spacing.

After importing the blade surface profile into the solid model, the radius of the parameter is defined at the surface intersection. This radius can be adjusted at any time in the future. We can still use the tools in the CAD system to define many other hybrids, including variable radius and rolling spheres.

Trimming and mixing

When all the blades are positioned, additional data and curves need to be defined and the outline of the impeller is further refined along the outer surface of the impeller. At this time, you also need to extract a non-planar contour curve from the DSSP software and use a variable section scan to generate a surface pattern to trim the outer surface of the blade (see Figure 7).

Fig. 7 Curve determination in DSSP software (left), trimming the impeller profile using CAD and using a variable cross-section scanning surface

Then, a cylindrical surface is formed to trim the outer surface of the entire impeller. This cylinder is centered on the extracted data axis and its diameter can be measured by a 3D cylinder created by a polygonal surface. After trimming the outer cylinder, it is ensured that the design can produce a perfectly symmetrical impeller with a central position.

Fast and complete parametric model

The final model satisfies all design parameter requirements and proves to be a closed entity model. We can use computer-aided inspection software to compare the final CAD model with the original scan data to verify its accuracy (see Figure 8).

Figure 8 Computer-aided inspection software verifies the accuracy of the model (left), uses CAD to modify the number and parameters of the blades, and prepares the model to use when making the mold

The hybrid modelling method provides control of the hub surface shape, the radius of the hybrid model, the number of blades, and the spacing. The entire process, including scanning, maintenance and modelling, can be completed in less than a day.

After some modifications, the 3D model generated by the hybrid molding method can be used to create a mold for casting production. This model can be used directly for prototyping rapid prototyping systems to accelerate mold production in fast casting processes.

Advantages of mixed molding processes

The mixed molding process mainly has the following advantages:

(1) Balance existing 3D investments.

(2) New designs can be created in a shorter period of time.

(3) Can produce authentic CAD geometric parameters.

(4) Can produce accurate results.

The hybrid modelling method combines digital image sampling and processing with computer-aided design capabilities to provide an innovative solution to parametric reverse engineering.

The CAD system will continue to be a relevant tool for 3D digital design and creation, supplemented by DSSP-like software as a tool for measuring 3D scan data and making complex surface models. This combination enables the user to quickly and accurately create an authentic parametric model. Users have the ability to incorporate new functionality into CAD models, balance common workflows, leverage existing knowledge, and advance digital design frameworks.