Week 9 - Attachment Feature Creation - Challenge 2

Drafting - Introduction :

Drafting is giving a taper to the face of the mould part. Drafting is required in the mould part to remove them from the mould flask without affecting the walls of the core and cavity. Drafting eliminates the problem of sticking mould to the walls of the core and cavity.

Draft analysis is made prior to eliminate the problem of sticking to walls. Draft analysis shows various faces which can cause the problem of sticking. With the draft analysis done we can proceed to design the components with the necessary draft. We use 3 degrees as a standard draft angle.

Tooling axis :

Tooling axis is the reference axis with which the mould is open. We provide a draft angle to our part with respect to this axis only.

Tooling axis creation :

1. Design consideration check :

We must first check the given class A surface for any opening in between the surface. This can be done using few commands. We can boundary creation tool to check if there are any open internal boundaries and close the surface to further our design. We can also use the join surface command and check if all the surfaces are joined. If not joined properly the command prompts an error window. we can know the design consideration by these techniques.

2. Visualisation Method :

In certain cases, we can find the tooling axis by simply looking at the class A surface. In the above case, for fan cover, since the part has a frustum part we can assume the axis normal to the top surface passing through the centre point of the surface as the tooling axis.

The preference of choosing the tooling axis would be the line normal to a large flat face or through any hole element in the part.

3. Analysis of various directions :

In the analysis, we create different intersection lines and find the best direction of the tooling axis. For instance, we check the mould plate movement direction by observing the surface carefully in each direction. In the above case, if we look in the direction of x as the tooling axis, there is a hindrance of the surface which don't allow us to create the axis in that direction. By observation, we find that the z-axis will hold good for the tooling axis.

4. Bisection Method to create the tooling axis :

The bisection method is generally used to find the tooling axis for the part design. This method enables us to create bisection lines and find the centre line, if in case the tooling axis is provided in that line, the draft with reference to that tooling axis would be perfect. The steps to create the tooling axis are as follows.

• Find the hole in the Class A surface. Apply boundary creation command for the hole. Create a point in the centre of that created boundary.
• Create a plane to create new sketches with "parallel through point" enabled. Select the XZ plane as the reference plane and the point create with the boundary option as point reference.
• Perform intersection option with the plane just created and by selecting the class A surface. This will create the intersection lines along the surface.
• Now create a line with the created sketch as a reference. the lines created are those surfaces that we think might create the problem of sticking.
• Now create a new line with the "Bisection" option enabled. Choose the lines created as the reference. A new line will be created along the Z direction in our case.
• Follow the above steps to create one more line with the YZ plane as a reference. We might note that both the created line in two planes with the bisection method merge together. This is because of the frustum shape of our surface.
• We create the tooling axis line with these two lines created with the bisection option enabled and by choosing these two lines as reference lines.
• In this surface, we created the tooling direction in Y-axis in sketches workbench

The above procedure is to be followed for the circular surfaces like frustum parts. In our case, The coin holder has a flat base surface. The normal direction to this surface can be chosen for the tooling axis. The procedure to create the tooling axis for our case is as follows,

• Select the base surface from the Class A surface and extract it.
• create a point on this surface. In point definition give over the surface as the first reference and select the extracted surface. In the dimension text box give 0 as the value. This, by default, will create a point in the centre of the surface.
• Since the class A surface has no tapered walls, it is better to provide the tooling axis in the normal direction to the extracted surface.
• Create a line with the point as the first reference and the extracted surface as the second reference.
• The created line is our Tooling axis.

Draft analysis :

The steps to create a draft analysis are as follows,

• Change the view mode. Select the "material" radio button in the change view mode option.
• Choose Feature Draft Analysis under Insert --> Analysis --> Feature draft analysis.
• The draft analysis window will open.
• Choose Quick analysis mode or full analysis mode as per requirements.
• Create the draft direction by clicking created compass option under the direction tab. Clicking the option will enable a compass created at the place of the default compass. Drag the compass to the tooling axis and place it over the tooling axis.
• We can reverse the direction of the draft with the reverse direction option. Once the direction is chosen click the lock direction option.
• Now moving the cursor displays a red cross near the cursor. Place the cursor over the surface to begin the analysis.

Once the analysis is done, the results will be displayed in various colours over the surface. If the simple mode is chosen it displays three variant of colours that enables us to see whether the drafts within over design consideration. We can also alter the value to show the changes in draft angle in the draft analysis window.

Choose analysis under the running point option in the Display tab of the draft analysis and place the cursor over the surface to display the angle at that point. The angle displayed are with reference to the draft direction.

The red surface indicates there is draft equal to or less than 0 degrees. In our case, the red surface indicates the zero draft. We cannot provide a draft to these walls as they are a part of the class A surface. We are not allowed to modify the class A surface.

The red surface indicated in the doghouse will not affect the mould removal as it has a separate side core. The side core draft will take care of the core and cavity block removal for the doghouse.

Side core:

For some design features, our main tooling axis will not accommodate the draft. There may be cases where few features are located perpendicular to the main tooling axis. In such cases, we need to provide a side core to make the design feasible. The design of the side core involves creating a side core axis. In our case, we provided a side core axis for the doghouse design.

The draft given for the above the blue surface is 0.5 degrees as per doghouse design specifications.

Screw Boss :

Screw bosses are features that are provided in the plastic component to provide the provision for locking the plastic component with rest of the components. A screw boss will be accommodated with ribs by default as a strengthening feature. Screw bosses are designed with few design specifications.

• Draft on the boss - 0.5 degrees
• Thickness of the boss - 40 % of the component thickness
• Height to diameter ratio - 3:1
• Fillets radii - 0.25 mm maximum

DogHouse :

A doghouse is a feature that allows to avoid sink marks. A Feature is created with small cross section area so that the flow of plastic is not affected. A doghouse also facilitates to create doghouse in perpendicular direction. If the screw boss should be offsetted from B surface, Doghouse is used .

Design specifications :

• Thickness at the intersection - less than or equal to 40 % of the component wall thickness
• Height of the intersection - between 3 mm - 6 mm
• Width of doghouse - minimum of 12 mm
• Draft - minimum of 1.5 degrees

The different view of the completed solid are as follows