Week 9 - Attachment Feature Creation - Challenge 2

Coin Holder Design

Draft Angle

The draft angle is the amount of taper for molded or cast parts perpendicular to the parting line. It can be measured in degrees or mm/mm (in/in).

Tooling Direction

Tooling Direction is the main consideration for plastic parts and mold parts because it defines the movement of the mold part and the possibility of the part coming out from the mold part.

Tooling Axis

The axis along which the mold or core and cavity opens.

The draft analysis within CATIA V5 will help you quickly gather the draft angle on multiple surfaces without the need for tedious manual measurements. The Draft Analysis allows the user to easily analyze a part for proper draft angle.

Importance of Drafting

When developing parts for plastic injection molding, applying a draft (or a taper) to the faces of the part is critical to improving the moldability of your part. Without it, parts run the risk of poor cosmetic finishes and may bend, break, or warp due to molding stresses caused by the plastic cooling. Equally important, an absence of a draft may prevent parts from ejecting from the mold, damaging not only the parts but possibly the mold itself—a costly and time-consuming detour.

Mode Option

The mode option lets you choose between a quick and a full analysis mode. These two modes are completely independent.
The default mode is the quick mode. It simplifies the analysis in that it displays only three color ranges.

Class A Surface

1. Visible surface and aesthetic look
2. Surface with textures
3. Surfaces provided by customer(OEM)
4. Don't have any features like(rib, snap, bosses, etc) on class A surface.
5. Class A surface is made from scanned data with different software like alias etc.
6. Surfaces which is having CURVATURE CONTINUITY IS CALLED class-A.

Class B surface

1. Invisible Surfaces.
2. Surface mostly don't have textures
3. The surface needs to be created according to the class-A surface.
4. Having all features(Rib, boss, snap, etc) in B-side surface only.
5. Surfaces which is having TANGENT CONTINUITY IS CALLED class-B

Class C Surface

1. Class C finish allows more imperfections than Class A or B.
2. It is often specified for interior surfaces not readily visible, surfaces that will be covered by another part when assembled, or surfaces that the Customer feels do not need to meet a high cosmetic finish.
3. It is used for parts or surfaces which are not viewed after installation or viewed only for service.
4. The distance between each point of the edges of two neighboring patches is no more than 0.05 mm.
5. The angle between the tangents to the surface on the edges of two neighboring patches is no more than 30’ (0.5◦).
6. Shape tolerance is not applied.

Determining the Tooling Axis

1) Connectivity Of Surface

After receiving the class A surface model the connectivity of surfaces was checked. It helps to find discontinuity within the surfaces of the model. The Connectivity of the surface can be checked in three different ways

Join Operation

Boundary Operation

Disassemble Operation

After verifying the connectivity of surfaces these operations were deleted.

2) Determining the Tooling Axis

The Tooling axis is the axis along which the mold or core and cavity opens. The tooling axis for a model has to be decided so that no additional works are needed on it which helps to reduce the cost.

1)A point is made on the base wall of the Class A surface. An Axis system is introduced on to that point.

2) Intersection of one axis plane and body is created. Since one surface is at 0 degrees the axis is created at the center of the surface.

4)Draft Analysis

a)View parameters are customized to Material mode.

b)Insert --- Analysis --- Featured Draft Analysis

c)Compass is moved to Tooling axis. It is locked in that position.

d)In Quick analysis the range of green color is changed to 3 degrees.

The Green Colour has rafted Walls above 3 degrees.

The Blue Colour has drafted walls 0-3 degrees.

The Red Colour has negative angle drafted walls.

Creating Class B Surface

Class B Surface is obtained from Class A by extracting, untrimmed, extrapolating, and lastly offsetting to the thickness of the Switch Bezel Plastic component. The Surfaces are trimmed to obtain the offset corner surface. The fillet is measured from class A and applied to Class B to get the final Class B Surface.

Class C Surface.

The Boundary of the Class A surface is extracted and applied sweep operation with the extracted curves.

The class B and swept surfaces are trimmed together and then it is joined with the Class A surface.

Finally closed surface feature is applied to the model to get the Switch Bezel Plastic component.

Rib

Ribs are thin, wall-like features typically designed into the geometry of a part to add internal support to walls or other features like bosses. The advantage of using ribs is that they increase the strength of a part without increasing the thickness of its walls. Because less material is required, ribs can be a much more cost-effective solution as well.

Design Rules for Rib Creation

1) Rib thickness

Ribs should be designed to be 40 percent of the nominal wall thickness. Thinner does not provide much structure and may be hard to fill. Thicker ribs are tempting, but they can begin to introduce sinks on the cosmetic surface.

If a rib is too thick, the point where it attaches to the primary wall or boss will be thickened as well. This can easily cause sink marks, warp, and voids where there is this excess material. In particular, sinks are visible indicators of bad design, which will affect the appearance of your finished part. If severe enough, sinks and voids can reduce a part’s strength as they become concentrated centers of stress. To prevent these problems, it is recommended that your rib’s width fall between 0.5 and 0.75 times the primary wall’s thickness.

When selecting a rib width, it is important to keep in mind that the shrink rate is not the same for all materials. High shrink plastics should be made thinner, while low shrink plastics can be made thicker. Low shrink materials are less likely to cause sink or warp because they do not exert as much pull on the surface or adjacent thin walls as high shrink materials do.

2) Rib height

For maximum effectiveness, rib height should be no more than 5 times the nominal wall thickness. Since the rib will be thinner than the wall, it may be hard to fill it if it is taller.

This height cap is recommended because if your ribs are too tall, they can break during use or ejection from the mold.

3) Rib draft

A minimum 0.5-degree per side minimum draft will ensure that they release from the tool during molding. The draft is especially important for ribs so that they can be removed without damage or breakage.

4)Rib spacing

Increase the number of ribs rather than their height in order to increase stiffness. Ribs should be spaced a minimum of 3:1 ratio with a height of ribs apart from one another.

Height/width Ratio

3:1 Steel

2:1 Preferred

Prevent thin steel conditions to prevent it from cracking.

If ribs are closer than that, you run the risk of the ribs not being able to cool quickly. This means the part will have to stay in the mold longer before it can be ejected, which will increase your cycle time and your part cost. Another reason spacing is important is that ribs that are too close together will make it so there are thin sections of your mold. These are prime areas for damage to the mold to occur.

5) Rib Fillet Radius

The base of a rib should always be rounded with a radius to prevent an area of concentrated stress in the part. Sharp corners also hamper the plastic’s flow rate down into the rib, meaning it may not be able to fill all the way before the material starts to cool, which will cause voids. 0.25mm is the maximum fillet radius for the ribs.

6) Rib Root Thickness

Ribs create thick sections at the root (defined by the circle below). This area is thicker than the nominal wall thickness and susceptible to sink.

7) The minimum thickness at the tip of the rib must be 0.75mm.

8) Ribs on the opposite sides of walls must be offset.

9) Ribs should be connected to other structural features where possible.

Screw Boss

Minimum Fillet radius.

A fillet of a certain minimum radius should be provided at the base of a boss to reduce stress. The intersection of the base of the boss with the nominal wall is typically stressed and stress concentration increases if no radii are provided. Also, the radius at the base of the boss should not exceed a maximum value to avoid thick sections. The radius at the base of the boss provides strength and ample draft for easy removal from the mold. The recommended value for the radius at the base of a boss is 0.25 to 0.5 times the nominal wall thickness.

Radius at Base

Providing a radius on the core pin helps in avoiding a sharp corner. This not only helps to mold but also reduces stress concentration. It is recommended that the radius at the base of the hole in the boss should be 0.25 to 0.5 times the nominal wall thickness.

Minimum Draft

An appropriate draft on the outer diameter of a boss helps easy ejection from the mold. The draft is required on the walls of a boss to permit easy withdrawal from the mold. The recommended value for the minimum draft on the outer surface of the boss is greater than or equal to 0.5 degrees.

6. Boss Height to OD Ratio

A tall boss with the included draft will generate a material mass and thick section at the base. In addition, the core pin will be difficult to cool, can extend the cycle time, and affect the cored hole dimensionally. It is recommended that the height of the boss should be less than 3 times of outer diameter.

Standalone ribs or sidewalls

Bosses and other thick sections should be cored. It is good practice to attach the boss to the sidewall. In this case, the material flow is uniform and provides additional load distribution for the part. For better rigidity and material flow, a boss should be connected to the nearest sidewall or ribs.

Design

• Circualr sketch is made on the bottom side of coin box. It is then extruded to a height of 12 mm.
• Ribs are added to it by drawing two lines crossway and extrapolating it with thickening.
• Union trim to cutout the unwanted portion.
• Edges are properly filleted of radii 0.25.
• It is then added to the main body.
• A copy of the Screw boss is made and translated to a new position.

Draft analysis of Screw Boss

Design Rule of Dog House

Dog house wall thickness at the intersection of the part wall should be 40% or less than the main wall stock.

The height of B should be between 3 to 6 mm.

Design

• A rectangular sketch is made with one side opened.
• it is then swept to a certain height, trimmed the top with an offset of the base to close the top.
• Thickness is added to the made box. the walls are drafted inside and outside along tool axis and side core axis.
• Fillets are made in appropriate places.
• B section is made up to height of 4 mm. The section made is trimmed with the body.
• The Screw boss is translated to a height so that it lies above the Dog house.
• Draft analysis is made in the end to check the draft along the tool axis and side core axis.

Draft Analysis of Dog House

Draft Analysis of Final Model.

Along Tool Axis

Along Side core Axis