Week 9 - Attachment Feature Creation - Challenge 2
Aim:
reate the Dog House for the Door Trim considering the design rules mentioned in the video. Consider the thickness of the part to be 2.5 mm.
"C" is where the doghouse contacts the backside of the surface
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Doghouse wall thickness at the intersection of the part wall ( C ) should be 40% or less than the main wall stock ( T ).
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The height of ( B ) should be between 3.0 – 6.0 mm
DESIGN RULES:
1. SCREW BOSS:
Boss refers to a mounting feature that will receive a screw or thread-forming screw. In computer-aided design applications, a boss is a feature used to describe a type of extrusion to mate with other parts precisely via a screw.
Boss features are commonly found in injection molding designs. They are used to aid in the assembly of molded parts by providing a channel for a screw. Designing plastic screw bosses is fairly straightforward but there are a few considerations to ensure a strong connection between parts and mitigate cosmetic defects.
To get started, you must first determine the hole size of the boss and the surrounding wall thickness. You will want a thick enough wall so that the screw can engage with the plastic without splitting the post. To get these specifications, refer to the screw manufacturer’s guidelines for recommended hole size and surrounding wall thickness for your given screw.
Counterbore Design
Since screws are going into a hole without threads, plastic will be displaced as you thread the screw into the boss. This is when a counterbore comes in handy. A counterbore is additional space designed into the part to accommodate this excess material.
In some cases, a design requires a relatively deep hole, like 1.5 in. deep with an 1/8 in. hole diameter, for example. Due to our rapid injection molding process, we use steel core pins to produce these types of holes rather than coring them out with the b-side of the mold. This makes it difficult to have a counterbore on the part. But don’t worry, there is a solution, and it’s a simple. A common workaround is to place the counterbore on the opposite part, the one that the screw will be running through completely.
Other Considerations for Molded Bosses
Cosmetics. If cosmetics are a concern, avoid thick sections that will be susceptible to sink. One common problem area is bosses located on or near exterior walls, we recommend moving these inwards. In addition, as a rule of thumb, we advise no more than 60 – 70 percent of the adjacent wall thickness for bosses.
Strength. Screw bosses can be reinforced with ribs or gussets for added strength and durability. As we mentioned above, a boss should not be next to exterior walls, but you can use coring ribs to keep it connected to the part’s exterior wall without risking sink.
Materials. Softer, less brittle materials work best for molded parts that need to accept screws. If your part is polycarbonate, for example, consider blending it with ABS.
Insert Molding. Consider using threaded inserts rather than molding bosses if your part will need to be threaded multiple times. Plastic bosses are typically only good for a single use, since the threads are cut by the screw as it’s placed into the part.
- The outside diameter of boss=2 to 2.4 *D
- Hole up to 0.3*t
- Merge base thickness W=0.6*T
- Fillet = 0.25mm max(must not violate 0.4*t)
- Draft angle inside out= 0.5 degrees
- Height to inside diameter ratio=H/B<3
2. Dog House:
The dog house is an engineering feature used in the plastic trim design. Dog houses are used as supporting features. Sometimes other engineering features like snaps; locators, etc. are mounded on them to increase their
strength.
- Draft 1.5 degree on all walls
- Intersection of part wall Thickness= 0.4* T
- Height =3<->6mm
- Width of dog house =12 mm
- Fillet value 0.25mm
Given data:
T = 'Wall Thickness' = 2.5mm.
Considering M6 screw thread.
To calculate the following data using Screw boss and dog house design rules
θ1 = ' Screw boss Draft' = 0.5°
θ2 = ' Dog house draft ' = 1.5°
r1 = 'Screw Fillet radius' = 0.25mm
r2 = 'Dog house Fillet radius' = 0.5mm
d = 'Inside Diameter of screw' = 5.35 mm (M6 effective core diameter)
D = 'Outside Diameter of boss' = d + (0.6*T)*2 = 5.35 + (0.6 * 2.5) * 2 = 8.35mm
w1 = ' Rib Thickness ' = 0.4 * T = 0.4 * 2.5 = 1mm.
w2 = ' Dog House thickness ' = 2 mm.
H = ' Screw Height ' ≤ 3*d = 3*5.35 = 16.05mm.
h = ' Coring inside dog house ' = 3<->6mm here we take 4mm.
MAIN TREE:
Create a Tooling Axis for the Class A surface.
TREE:
- Create a point on the class A surface to move the main tool for component creation on the base surface.
- Mask an axis over the point created with the same directions as the main axis
. - Using Intersect command from Generative Shape design select the YZ plane as the first element and the class A surface as the second element
. - After creating the intersecting lines select the Line command and join the vertices which show the most possible draft angle manufacturable.
- Bisect those two lines over the axis point we created
. - Also for the side core create a line with the direction in the Z component for the side tooling axis with the point created in the first point.
OBJECTIVE 2: DRAFT ANALYSIS of Class A surface:
Note: Class A surface was constructed wrongly as there is no draft angle on the bin walls of the coin holder.
PROCEDURE:
- After Tooling Axis Select the Feature Draft Analysis option from the Insert Option
. - Select the element to analyze.
- Next click on Use the compass to define the new current draft direction option and make it align with the tooling axis by dragging it to the axis
. - If the color appears blue just click the invert option on the right side of the draft direction axis button.
OBJECTIVE 3: Creating Screw Boss and Dog house on the base of the Coin Holder.
TREE:
SKETCH and Creation in Generative shape design and Part design:
Extract the base surface and create a sketch on it.
In Generative Shape Design, Extract the surface up to 16.05 mm as calculated.
A draft angle of 0.5 as estimated is drafted both in and out of the holes.
Ribs are extracted as per the sketch.
Note: Dimensions were given using the formula for variable changes if to be made for dog house with just changing one value while others change according to t the formula automatically.
The union trim command is used to trim it with the Screw head created.
A draft is given of value 0.5 degrees.
Fillets (0.25 mm) are given to all edges after assembling them to the main body.
Similarly, another Screw boss is created in a new empty part body waiting to be translated.
Dog House:
It is swept in the draft direction with 1.5 degrees.
The base surface is offset and trimmed to form a cube with no front face.
Fillets 1mm are given according to the height of the dog house.
A thick surface is given in Part design.
To create coring 1mm width and 4mm height is selected inside the dog house created.
Offset the base to 3mm and trim it with the original sweep to create a box, Using a thick surface give 1 mm outward extrusion, and using the remove command cut the part from the main dog house body.
Using draft 1.5-degree draft the cutout part.
Give fillet f 0.5 degrees to both edges.
Using the translate command move the new screw boss to the top of the dog house with a point to point and add operation to add the parts.
The final part with a dog house and screw boss features.
The intersection of the dog house with a screw boss on the main body.
OBJECTIVE 4: Draft Analysis on Final component for both main tooling and side tooling.
PROCEDURE:
- After Tooling Axis Select the Feature Draft Analysis option from the Insert Option.
- Select the element to analyze.
- Next click on Use the compass to define the new current draft direction option and make it align with the tooling axis by dragging it to the axis.
- If the color appears blue just click the invert option on the right side of the draft direction axis button.
For the main tooling axis
For Side Tooling Axis:
Hence the analysis is completed successfully.