DESIGN OF DOOR APPLIQUE B-SIDE FEATURES USING CATIA

DESIGNING THE LOCATORS AND HEAT STAKES ON A DOOR APPLIQUE

Locators or Locating Pins:

          Locating pins are the rib like structures on the B surface which are used to position the child part with parent part while assembling. These locators were placed in the multiple location on the part depending upon the degrees of freedoms which needs to be arrested. They were classified as two-way locators and four-way locators. The one which is in + like structure is the four-way locator and the one which is – like structure is the two-way locators.

Heat stakes:

          Heat stakes are the vertical cylindrical structures added to B surface. These are used to attach the child part with the parent part. This is joined to the parent surface by melting the heat stakes which protrudes beyond the attaching or parent part. This is also called as plastic welding. This kind of attachment provides advantage of avoiding any additional fastening methods like bolting, Push clips etc, which will add the weight of the component. But it has a disadvantage that once it is fixed it cannot be removed without damaging the component. Here since we will not have any need for removing or replacing this applique therefore, we are fixing it with heat stakes. The maintenance works can be done by removing the entire panel itself.

DESIGN REQUIREMENTS:

          Though they have different name based on the application, in general they are ribs. We will be following the design rules which are followed for the ribs to avoid the sink mark and short shots. Some of the design considerations were explained below.

Rib thickness:

          Usually, thickness of the rib should be maximum 40% of the wall thickness. This is decided to avoid the sink mark on the visible surface. If the thickness of the rib at the base is high, it will increase the thickness at the junction therefore this will retain more heat than other surface. Due to this when it cools slowly it will try to shrink the material which is outside this region. Since the area at the fillet have high strength, it will shrink the flat surface causing the sink marks.

Rib Thickness at Top:

          Thickness of the rib at the top must be maintained at least 0.75mm so that molten metal can flow easily. If the thickness is reduced further, it becomes difficult for the molten plastic to reach those corners of the ribs as they are almost perpendicular to the flow direction. They will leave unfilled mold region which leads to the defect called short shots.

Draft Angle:

          Draft on the surfaces of the ribs is provided to ease of ejection of the part from the mold. Usually, 3-degree draft is provided on the part in the engineering features like ribs, doghouse etc, were provided with minimum draft angle of 0.5 degrees.

Rib Height:

          Rib height must be less than the 5 times that of wall thickness.

Rib base fillet radius:

          Nominal base fillet radius is 0.125 times of wall thickness must be applied and maximum fillet radius is 0.25 is allowed.

Ribs spacing:

          Ratio between the rib height to the spacing between the ribs must be maintained at 2 and this ratio is allowed maximum of 3.

Rib root thickness:

          In general, the entire plastic component should not have the thickness deviation greater than 30%. As our part is 2.5mm thick and thickness must be maintained between 1.75mm and 3.25mm. It is measured by creating the tri-tangent circle with base and the two fillets. Now the circle diameter should have the dimension between 1.75 to 3.25mm.

Tooling Direction:

          In this model the tooling direction is created along the Y axis as the direction of the normal to the applique surface falls almost along Y axis.

Solid Body creation:

          The solid body is created using B and C surface created with proper draft angle with tooling direction. Once the shell is created then it is changed into a solid with Close surface option.

Class A surface:

Class B surface:

Class C Surface:

Closed shell:

Locators and Heat stakes:

          The locators were created by using the Pad option from the part workbench. They should be placed at maximum distance as possible from each other and their length should cross beyond the locating surface and the length of the locators must have the same height and can have the deviation of 1mm max.

          Heat stakes were also created using the pad option and their height should be less than that of 5mm from the locating ribs and must protrude outside the attaching surface. All the locators and the heat stakes should have at least 0.5 degrees of draft angle to enable ease of ejection from the mould after forming.

Draft Analysis of the applique:

          Draft analysis is carried out to confirm the part does not have any negative draft. From the result we conclude the part does not have any negative draft and the features have the appropriate draft angle. As we know the ribs must have 0.5 degrees of draft angle, draft analysis shows that the ribs of the part have the 0.5 degrees of draft by highlighting in violet colour.

ORIENTATIONS:

Front View:

Side View:

Top View:

Isometric View:

CONCLUSION:

Thus we have designed the locators and the heat stakes as per the design standards to avoid the sink marks and the short shots in manufacturing and to eject the part easily from the mould after forming.