`.PLASTIC PART DESIGN GUIDELINES SPECIFIC FOR AUTOMOTIVE COMPONENTS

Plastic Part Design Guidelines Specific for Automotive Injection Molding Components

Introduction Basic Considerations

Nominal Wall Ribs Undercuts Holes Bosses Flanges Parting Line Gating Living Hinge Weld/Meld/Knit Lines Graining Thick/Thin Transitions Basic Tooling Considerations Design/Tooling Aides

Basic Considerations

When designing plastic components for the automotive, there are many things that need to be considered to ensure a part that is both functional and manufacturable. The following is a basic guideline that can be utilized, but some rules can be cheated if needed. All the guidelines are effected by the following three general areas

Part application, Process, and Material

Part application, for automotive, can be broken into two primary categories - non-structural (decorative trim) and structural (there may be overlap between the two). The non-structural applications need to be more concerned on the aesthetics of the class 'A' surface. Examples of these parts are side shields, seat backs(exposed), door trim, A/B/C pillar trim, and I/P (instrument panels) covers to name a few. Anything that is visible to the consumer. The structural parts are generally covered or out of view of the consumer, and the strength or performance of the parts are desired over the appearance. Examples of these are seat backs (covered), I/P substrate, structural bolsters, and flipper panels (covered) to name some. These parts are able to cheat on a many of the guidelines that relate to surface appearance.

Process of how the parts are made will also contain some limitations or concerns that need to be considered when designing parts. The two main processes that Johnson Control use to make auto parts are injection and blow molding. Blow molding is limited to actions in the tool that would be used to create side holes or undercut features that can be done in injection molding. Injection molding is generally restrictive (not including special processes) in the cross-sectional size of the part, while blow molding allows for channels in the part that increases strength. This guideline will concentrate on injection molded parts.

Material used will also affect the guidelines and consultation with the material supplier is very useful. Highly filled materials will allow variations in some rules as will unfilled in others. Generally, when a material is chosen for an application, cost and properties are the two major factors that will be used to decide.

1 Nominal Wall

1.1 Importance

Nominal wall is the term used to describe the 'main' body of the part. The consistency of the nominal wall is very important in the processing and function of the part. Throughout this design guide, the nominal wall will be referenced frequently to define proper ratios when adding attachments. Below is a general cross section of a side shield showing the nominal wall and some features added to it.

1.2 Flow/Filling

A consistent nominal wall in injection molding will aide in processing the part better. Melted plastic flows in 'path of least resistance' and if there are varied thicknesses of the nominal wall, flow of plastic will be through the thicker sections first. This may cause surface defects, trapped gas, voids, or pressure drop variations that make processing difficult. Average nominal wall thickness for decorative trim components is 2.5 mm, while structural components are 3.0 mm. Filled materials are limited to how thin the nominal wall can go and consultation with the material supplier is suggested. Below are examples of nominal wall designs.

1.3 Strength

Proper 'packing' of the part is more difficult if the nominal wall is varying. This could leave voids or higher stresses in sections of the part that could affect the performance.

1.4 Warpage

Different nominal wall thickness will have different cooling rates and different degrees of orientation of polymer chains. This can cause excessive warpage when part comes out of the tool.

1.5 Processing

Processing of plastic components are based on cooling time in the mold. The thicker the wall the longer it takes to cool to a point where the part can be ejected or taken out of the mold. If a part has varying wall thickness, the cycle time will be based on the thicker section. A consistent nominal wall is better for controlling the cycle time and costs of the parts.

1.6 Exceptions

There are always exceptions to the rules and this is not different for nominal wall applications. Sometimes the design requires thicker sections (i.e. a heavy boss is required and the nominal wall needs to be thicker to prevent a sink), but you do not want to make the whole part thicker and waste material or time. Transition from a thicker to thin section should be utilized. If the thicker section is really excessive, a re-evaluation of the design is warranted.

2 Ribs

2.1 Uses

Ribs are used to provide

1) Stiffness to a part2) Strength to a part3) Stability to a part (warpage)4) Method of attachment5) Method of positioning part in assembly

There are many uses for ribs, but they must be thought out carefully when designing to ensure a quality product in both appearance and functionality. Improper design of ribs could cause warpage due to

non-uniform shrinkage. It must also be remembered that Ribs are difficult to

Fill Vent Eject

2.2 Nominal wall ratio

When designing ribs into a part, you have to be careful about sink marks caused by too large a rib. General rule of thumb is that the nominal wall to rib ratio, (class 'A' surfaces) should be designed at 50%. This is material dependent some materials may allow a greater or lesser ratio. Filled materials tend to allow for larger ribs, than unfilled. If the part is structural and hidden, the wall to rib ratio can be more.

2.3 Directional

Be careful when determining rib direction in the part. If ribs are 90 degrees to material flow, part may exhibit a blush or highlight over top of ribs. To diminish chance of rib readout, ribs should be designed near edges of part where possible.

2.4 Draft and Depth

Ribs should have draft angles of 1 - 1.5 degrees average. You should not have any draft less than 0.5 degrees. This would make it very difficult to mold the part.

The deeper the rib, the thinner it will be at the end and the harder it will be to fill the rib during processing. This could result in incomplete fill of ribs and may defeat purpose. Average rib length is generally 2.5 - 3.0 x wall thickness, but part may dictate other.

2.5 Join Radius

To help avoid stress cracks, a radius should be applied to the join area of the rib base and nominal wall. The larger the better, however, keep in mind that the join radius will add material and increase the wall to rib ratio. Generally a 0.25 mm join radius should be enough, you just want to break the sharp edges.

2.6 Tooling Considerations

Ribs are usually burned into the tool. This leaves a rough finish that needs to be benched or smoothed out. The deeper the ribs, the more difficult it is for the tooler to bench the part. You also have to be aware of placement of rib in part is it in die direction (direction tool opens and closes) or along an edge. In die direction, ribs are easier to tool (no special tooling). If the ribs are not in die direction, they will require a slide or lifter added to the tool. This will add cost and timing to a tool.

2.7 Design Examples

3 Undercuts

3.0 Uses

Undercuts are used frequently in designing parts for automotive component. The more common types are snap fit designs or attachment features.

Injection Molding - Types of Undercuts

3.1 Tooling Considerations

Undercuts will always require some type of a lifter or slide built into the tool. This will add cost and time. The other design impact is distance around the undercut. When ejecting the part from a tool, the part has to be clear of the metal. For example if you have a I inch undercut, the lifter must be able to 'move' back I inch. You also need to leave @ 5/8 inch for the lifter rod. This means that for a I

inch undercut, you need at least 1 5/8 inch area in front of the undercut, free of any obstruction or change in contour, for the lifter.

3.2 Design Examples

4 Holes

4.1 Uses

Holes are used when clearance is needed. Holes are predominantly useful when a mechanical fastener is used to attach the part to another. Below is a front side shield with several holes.

4.2 Location/Tooling concerns

If the holes are in line of draw (direction tool opens and closes), then they are relatively simple to put into the part. When the holes are

on a side flange or 90 degrees to line of draw, then a slide or lifter is required. Below is an example of a side shield and how holes were made.

5 Bosses

5.1 Uses

The main use for bosses on a part is for attachment of another part. The boss supplies a place for a screw, press fit or snap fit to be put. Bosses should be treated as round connected ribs when thinking of draft, nominal wall ratio, join radius, and depth. The same rules apply to bosses. Bosses, however, need to be correctly designed to take the attachment method and stresses associated.

5.2 Designing

When designing bosses, there are two opposing considerations. You need to make the boss thin enough so that the part surface will not have a sink mark, yet you also need to make the boss thick enough to take the stresses associated with screwing a mechanical fastener or press fitting another part into it. The walls also have to be thick enough to allow the screw flights to grab and not pull out to easily.

6 Flanges

6.1 Uses

Flanges are another name for side walls on a part. They are considered part of the nominal wall and should be designed at the same thickness. Knowing about flanges is important so that any no build conditions can be avoided. Flanges are typically 90 degrees to die draw of tool. For this reason, draft is very important when designing them. Flanges are also used to provide some feature along the side of the part ( i.e. attachment hole, rib, etc..).

6.2 Draft Angle

As mentioned previously, typical draft on ribs is 1 to 1 1/2 degrees. Tool builders generally like to have a minimum of 3-5 degrees on flanges. For decorative trim, parts are typically grained. When flanges are grained, the allowable draft has to be increased. The general rule is 1 - 1 1 /2 degree of draft per 0.001" depth of grain (i.e. for a grain depth of 0.004, the draft angle on a flange should be @ 7 degrees min.).

6.3 Beaded

On some decorative parts, the OEM like to have a beaded or rounded edge to, eliminate any sharp edges. Most of the time the parts parting line will be at the tangent point where the bead radius meets the wall (see example below). This prevents any undercuts from being formed, thus making tooling more difficult.

7 Parting Line

7.1 Location

A parting line is a visible line on the part that is caused by the two halves of a mold meeting up. The line will generally follow the bottom of any side flanges (walls). Every part will be different and if there is a concern, discussion with the tooter or molder should take place.

7.2 Natural

The natural parting line is created by only the two halves of the mold.

7.3 Secondary Actions

When a part design has features that require a secondary action (holes, undercuts, etc.) an unnatural parting line will be created. If they are on the 'B' surface, they will be hidden from view. If the action is along the side walls, the parting line will be visible. Some examples below show various parting line conditions.

7.4 Beaded

As mentioned in the flange, when a part is beaded, the parting line will be between the wall and the tangent of the radius.

8 Gating

8.1 Types

There are three main classifications of gating used for parts. They are

1) Edge-gate2) Sub-gate3) Hot drop

8.1.1 Edge-gate

An edge-gate is just as it name suggests, the material is pushed through a gate attached to the edge of the nominal wall. Below is an example of how an edge gate would look. An edge gate is the simplest to make. It is simply a rectangular section cut into the mold. An edge gate does have to be trimmed off. This usually requires a degating fixture or to be done by hand. When the gate is trimmed off, a witness mark or blemish may be visible.

8.1.2 Sub-gate

A sub-gate, as shown below, injects the material into the part through a tunnel shaped gate. This type of gate requires a little more tool work, but the part is self de-gating as it ejects from the tool. This means that the gate breaks off from the part during the actual molding cycle and eliminates any extra operation or fixtures. This type of gate will also leave a witness mark at location of gate.

Geometry of Submarine Gate

8.1.3 Hot drop

A hot drops manifold gates directly into the part, usually on the underside or 'B' surface. This process eliminates any degating operations. This type of gating also allows for a more controlled filling of the part. Drops can be put where needed, especially in the center of the part to reduce flow length and improve part properties. This type of gating is very expensive and requires extensive tooling additions. Hot drops will also usually leave a blemish on the opposite side of the part.

8.2 Location

The location of the gate can, technically, be anywhere on the part, but certain considerations need to be taken.

The ability to fill the part - flow length of material

The ability to pack out part - warpage of part

Aesthetics of part - will gate mark be visible and objectionable

The first two concerns will depend upon the shape and thickness of the part as well as the type of material being injected. The answers to this can be determined with help from the mold source and/or computer aided help (mold flow, discussed later).

8.3 Gate size

Gate size is very important for the following

Processing Dimensional stability Part performance

All three are greatly affected by the size of the gate. If the gate is too small, the part may not fill or require higher pressures that cause extreme stresses in the part and will potentially warp the part and/or diminish the performance. If the gate is too big, the molding time may be increased (increasing cost). 9 Living Hinge

9.1 Uses

Hinges are very useful when a part is needed to enclose another part such as a mechanism. The part can be designed with a hinge that bends and some snaps to clip the part closed. See example below.

9.2 Material Considerations

Not all materials can be used for a hinge application. They are generally restricted to the olefinic materials (PP, PE, TPO, etc.). If you are using a specific material and need to know if a hinge can be utilized, it is best to consult the material supplier.

9.3 Tooling Considerations

Having a very thin section or channel along the line you want to bend creates a hinge. The section is generally 0.25-0.5 mm thick and 0.25 mm wide. The section can be wider, but filling the part needs to be considered also.

The gate should be positioned so that the material flows evenly over the hinge area. If more than one gate is used, the material SHOULD NOT meet in the hinge area. This would result in a weakened hinge with a high potential to break.

10 Weld/Meld/Knit Lines

10.1 What are they?

Weld/Meld/Knit lines are all terms describing the effect of two or more flow fronts of material joining or meeting together in the part. The example below shows a part and depicts where knit lines would be.

10.2 Causes

Knit lines are caused by two or more gates or by material flowing around holes in the part. These conditions generate two material flow fronts and the knit lines are the fronts meeting together. The material is still solid enough that the fronts don't fully blend back together.

Flow paths arc determined by part shape and gate location. Flow fronts that meet head on will weld together, forming a weld line. Parallel fronts tend to blend, however, producing a less distinct weld line but a stronger bond.

10.3 Effects

Knit lines are weaker than the rest of the part. This needs to be considered carefully when designing parts. If you know that knit lines are going to be present, the part needs to be designed to minimize this weaker area. Try to keep them from load bearing areas.

10.4 Controlling Knit Lines

Knit lines, although unavoidable, can be controlled and minimized.

The easiest way to control was the knit line would go is by the gate location and direction of material flow. The knit line will always form (when flow is evenly distributed) on the opposite side of the obstruction. In the case of two or more gates, the knit line will form at the half way mark between the two gates.

Processing is important in controlling the strength of the knit lines. The hotter the material fronts are at time of meeting, the stronger the bond will be. Control of the process is very important in maintaining strength in the part.

11 Graining

There are many types of grain that can be put on the surface of parts. The general rule is that for every 0.001" of grain depth, you should add at least I degree of draft. An example is for a grain depth

of 0.005". The minimum draft angle required would be 1 degree + 5 * 1 degree for grain = 6 degrees.

11.2 Flanges

The areas on any part that this grain depth/draft relationship is most prevalent are on the flanges. Everybody likes these to be perpendicular to the front surface, but the flanges need to be at an angle to allow for removal from the tool. The longer the flange, the more noticeable the angle is and the less people like it. This is one area that cannot be compromised. If it is, greater problems could occur in molding the part such as trouble ejecting, wiping or smearing off the grain surface, etc...

11.3 Parting lines

Parting lines are very tricky when trying to grain parts. In general, grain is kept 0.005-0.010" from a parting line (some grains can be put to the edge). Graining a parting tine creates non even surfaces that meet. This can make the parting line more visible and may effect the wear of the parting line over the life of the tool.

11.4 Holes/Bosses

Holes are treated like parting lines and for most grains a 0.005-0-010" ungrained patch will be left around the hole opening. Bosses can be grained on the inside, but if it can be avoided it should. Graining the inside makes it harder to eject and if the boss is too long, it is difficult to get grain into it. Most designs have the grain stopping on the inside tangent of the boss.

12 Thick/Thin Transitions

12.1 Uses

Thick/thin transitions are generally used to locally thicken an area of a part to eliminate a sink mark or add strength. The transition should be very gradual.

12.2 Design Concerns

There are many concerns with this type of situation. Major ones are sinks and warpage. Although these transitions are sometimes used to hide existing sinks, if it is too large or too severe, the sink may actually increase. Also with transitions in material thickness, there is different shrinkage and orientation of the material and this can cause voids or warpage in the part.

12.3 Process Considerations

For the reason of potential sinks and warpage, transitions in a part can be a nightmare for processing. It may require a tighter process 'window' (set of parameters that create an acceptable part). Gating location is also more critical with this scenario. If the transition area is near the end of fill, it will be very difficult to pack out and thus create warpage or sinks.

13 Basic Tooling Considerations

When designing automotive components, it is good to keep 'n mind that process of how the parts will be made. This will generally limit what features can be made in the tool and ultimately molded in production. If this is kept in mind and resources such as the tool builder and manufacturer are utilized, parts will be better designed for all facets - customer performance and appearance requirements, tool simplicity, and moldability of a quality product. It is also wise to draw from the expertise of these resources for they generally have knowledge of easier ways to incorporate features into parts.

14 Design/Tooling Aides

Common aides available to assist in designing parts are mold flow and FEA. Mold flow is the analysis of how plastic will flow through parts during fill. This is mainly for injection molded parts. There are several companies that have their own type of software. The two major ones are Mold flow and C-Flow. The typical data obtained are ability to inject, pressures, flow path, stresses, shrinkage, warpage, and cooling. FEA is a system that can predict the performance of the part under specific load or stress conditions. The part design can be tested and refined before any actual parts have been made. This will cut down on the costs of the old 'trial and redesign mentality. It can also cut down on development time.