Plastic Design Guidelines for Automotive Components

`

.

PLASTIC PART DESIGN GUIDELINES FOR SPECIFIC AUTOMOTIVE COMPONENTS

Trusted by over 1 million members

Try Scribd FREE for 30 days to access over 125 million titles without ads or interruptions! Start Free Trial Cancel Anytime.











Plastic Part Design Guidelines Specific 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 need to be more more conc concer erne ned d on the the aest aesthe heti tics cs of the the clas class s 'A' 'A' surface. Examples of these parts are side shields, seat backs( backs(exp expose osed), d), door door trim, trim, A/B/C A/B/C pillar pillar trim, trim, and I/P (instr (instrume ument nt pane panels ls)) cove covers rs to nam name a few. few. An Anyt ythi hing ng that that is visi visibl ble e to the the consumer. The structural parts are generally covered or out of view of the consumer, and the strength or performance of the parts are

for











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 restrictive (not including special special processes processes)) in the cross-sec cross-section tional al 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 consisten consistency cy of the nominal nominal wall is very very important important in the processing and function of the part. Throughout this design guide, the nomina nominall wall wall will will be refere reference nced d freque frequentl ntly y to define define proper 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 nominal wall applicatio applications. ns. Sometime Sometimes s the design design requires requires thicker 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 part 2) Strength to a part 3) Stability to a part (warpage) 4) Method of attachment 5) Method of positioning part in assembly











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%. 50 %. This This is mate materi rial al depe depend nden entt some some mate materi rial als s may may allo allow w a greater greater or lesser lesser ratio. ratio. Filled materials materials tend to allow 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 top of ribs ribs.. To dimi dimini nish sh chan chance ce of rib rib read readou out, t, ribs ribs sh shou ould ld 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 will be to fill fill the the rib dur during ing pro proces cessing sing.. This his coul could d resul esultt 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.











2.6 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 Underc Undercuts uts are used used freque frequentl ntly y in design designing ing parts parts for autom automoti otive ve com compone ponent nt.. The The more ore comm common on type types s are are sn snap ap fit fit desi design gns s or attachment features. Injection Molding - Types of Undercuts











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.













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.











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.





















6.2 Draft Angle











7 Parting Line 7.1 Location A parting line is a visible line on the part that is caused by the two halv halves es of a mold mold meet meetin ing g up. up. The The line line will will gene genera rall lly y follo follow w the 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.













7.3 Secondary Actions When a part design has features that require a secondary action











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











8.1.2 Sub-gate A su subb-ga gate te,, as sh show own n belo below, w, inje inject cts s the the mate materi rial al into into the the part 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 witness mark at location of gate. gate.











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 material The ability to pack out part - warpage 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























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 a nd depicts where knit lines would be.





















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











12 Thick/Thin Transitions 12.1 Uses Thick/thin transitions are generally used to locally thicken an area of a part part to elim elimin inat ate e a sink sink mark mark or add add stre streng ngth th.. The The tran transi siti tion on should be very gradual.











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 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

Plastic Design Guidelines for Automotive Components

Recommend Documents