Plastic Part DesignBack To Main Article section
To insure a quality final product, it is necessary to start out with quality components. Injection molded parts can be molded to a high quality standard by focusing on these areas of plastic technology:
- 1) Correct Part Design
- 2) Accurate Selection of Material
- 3) Processing Plastic Processing
Correct Part Design GuidelinesIf there was only one rule for the injection molding process it would have to be to maintain uniform wall thickness. Here are some examples of problems associated with part designs that feature a non-uniform wall thickness.
Sink marks due to uneven shrinkage
Sink marks result from a wall yielding to the still shrinking interior mass.
Stress due to uneven shrinkage
Sink marks result from a wall yielding to the still shrinking interior mass.
Voids due to uneven shrinkage
The already cooled section will not yield to the shrinking action of the cooling interior mass causing voids in the thick portion of the part.
Warping due to uneven shrinkage
Concentrated stress at the junction of high & low shrinkage area may cause a part to warp.
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DraftPlastic parts are always designed with a taper or draft in the direction of mold movement to allow part ejection or removal from the mold. Since plastics shrink when cooled, it is common for parts to shrink (or grip tightly) to cores. A good definition for draft would be: the degree of taper of a side wall or rib needed to allow the molded plastic part to be removed from the metal mold. Without proper draft, plastic parts may be difficult to remove from the mold. A draft angle of 1/2 degrees is regarded as minimum for most applications. Draft angles of 1 1/2 to 2 degrees per side are considered normal for plastic injection molding.
RibsMany times the stiffness of a part must increase because of the load applied to the part design. One of the easiest ways to cure this problem is change the part geometry by adding ribs. The use of ribs is a practical way and economical means of increasing the structural strength of a part. But there are guidelines that govern adding ribs without causing sink marks or surface blemishes to your parts.
|1) Rib thickness should be less than wall thickness. A rib thickness of 60% to
80% of nominal wall thickness is recommended.|
2) To increase stiffness increase the number of ribs or 'gusset plates', another feature designed to strengthen the plastic part.
|3) For a given stiffness, it is better to increase the number of ribs, not the height.|
4) For thick ribs 'core out' the rib from the back. This creates a hollow space underneath the part and maintains a uniform wall thickness.
Height: Maximum height of three time nominal wall thickness of part.
Spacing: Minimum of two times nominal wall thickness of part between ribs.
HolesHoles are easy to produce in molded parts. Core pins that protrude into the mold cavity make the holes when the part is molded. Through holes in the molded parts are easier to produce than blind holes, which do not go all the way through a part. Blind holes are made by core pins supported on one end only. The pins can be deflected and pushed off center by the pressures of the molten plastic material during the molding process. A good rule of thumb: the depth of the blind hole should be about twice the diameter of core pins up to 3/16", and up to four times the diameter of core pins over 3/16". The guidelines for blind and through holes are seen below.
|Blind Hole (shown with draft)||L = 2D for Diameters Less than 3/16" dia. core pins|
L = 4D for Diameters More than 3/16" dia. core pins
|Through Hole (shown with draft)||L = 4D for Diameters less than 3/16" dia. core pins|
L = 6D for Diameters more than 3/16" dia. core pins
There are definite rules for the placement of cored holes in a molded part. If these minimum distances shown below are not followed, the holes will be egg shaped or the part will deform in the areas around the holes.
|T = Wall thickness of part|
D = Diameter of hole
As depicted above, the distance from the edge of a hole to a vertical surface (i.e. rib) or the edge of a part should be twice the part thickness (or more), or at least one diameter of the hole. This same rule applies between holes - at least two part thicknesses or one hole diameter should be specified.
As easy as it is to make holes in molded parts it does not come without some concerns for the strength of the part. For every cored or molded hole there will be a weld line. The weld lines are caused by the flow of the melted plastic around the core pins. These weld lines are not as strong as the surrounding plastic material, and also may detract from the overall appearance of the molded part. The part designer should consider these points when designing holes in a molded part.
The coring of holes is easy when the axis is parallel to the parting line. But when holes and other features run perpendicular to the parting line then retractable cores (or cams) are required. Split pins and cores (called passing steel shutoffs) can be used to create some of the features. The designer needs to be aware of the problems of side action cores and the added expenses associated with these types of molds. With a little understanding of how the mold opens and where the parting line will exist, these costly features can be modified. Rule of thumb: whenever possible all design features should be incorporated in the same direction of the mold opening so that cam action can be avoided.
BossesBosses are used for locating, mounting, and assembly purposes. There are boss design guidelines that must be followed to insure the highest quality in molded parts. Again, one of the main points to consider is nominal wall thickness. Too many times bosses are designed with thick wall sections that can affect the appearance of the plastic part and the final product.
Rule of thumb: the wall thickness around a boss design feature (t) should be 60% of the nominal part thickness (T) if that thickness is less than 1/8". If the nominal part thickness is greater than 1/8" the boss wall thickness should be 40% of the nominal wall.
Boss diameter, wall thickness, and height design parameters. While boss heights vary by design, the following guidelines will help avoid surface imperfections like sink marks and voids: the height of the boss should be no more than 2 1/2 times the diameter of the hole in the boss.
Please observe the "60/40" rule (see above) for the wall thickness at the bottom of the boss.
Insert Molding TipsAnother problem concerning high stress occurs with molded-in inserts. The plastic melt heats the metal of the inserts. During the cooling stage of injection molding, the plastic part cools, but the plastic boss surrounding the metal insert is reheated by the heat from the insert. This allows the plastic to continue to shrink around the insert, causing excessive hoop stress* that will eventually cause the boss to crack. The better design and process would be to use ultrasonic insertion or a hot probe (such as a heat staking unit) after the molded part has cooled throughout.
Hoop stress: stress within the circumference of the boss
Replacing Metal with PlasticsThere are numerous reasons why replacing metal parts with plastic makes sense. Plastics One, Inc. has worked with many companies on metal-to-plastic conversions. Here is why plastic may be the best option for your parts.
DECREASE PIECE PART PRICESA penny saved is a penny earned. After initial tooling costs are paid, the piece part pricing is usually much less than the same part produced in metal, whether it be a stamping, casting, or a die cast part This cost savings is realized because the injection molding process has faster cycle times (more parts made per machine hour) and these parts are identical from one to the other which eliminates secondary machining.
ELIMINATE TIME-CONSUMING AND COSTLY SECONDARY OPERATIONSEliminating secondary operations further reduces costs. Plastic material can be colored with color concentrates before molding - eliminating secondary painting operations. Injection molds can be textured or given various levels of polished surfaces before molding. The costly assembly of several metal stampings or castings fastened together can often be replaced by a single injection molded part incorporating the features of the total assembly. If multiple assemblies are required, the plastic parts can have snap-together features to eliminate any fasteners. Eliminating sub-assembly tooling or fixtures by using injection molded parts can quicken delivery in new product development programs.
REDUCE PRODUCT WEIGHT AND IMPROVE USER EASEOne primary advantage of using plastics instead of metals is weight reduction Reducing product weight with plastics gives you more parts per pound of material, significantly reduces shipping costs and improves the end-user's physical ease in utilizing the product A comparison of the specific gravity values of metals to plastics is shown in the following table:
GAIN GREATER PRODUCT STRUCTURAL STRENGTHThat third little pig really knew how to choose the right materials to build a strong house. Choosing plastic over metal gives you products which are light-weight, easier to use and yet possess increased structural strength. Plastic parts can be stronger than metal parts through the use of engineering grade materials. In addition, the ability to mold in structural strength such as ribs, bosses and gussets when the part is originally produced instead of fastening, welding and gluing operations afterwards can affect the total strength of the assembled part as well as reduce additional costs.
INCREASE YOUR PRODUCT DESIGN OPTIONSDon't let the design limitations of metals trap you between a rock and a hard place. Increase your design options and requirements and still keep costs down. The area of greatest difference between metals and plastics is the ease in producing complex shapes. The costly assembly of several metal stampings or castings fastened together can often be replaced by a single injection molded part incorporating the features of the total assembly. If multiple assemblies are required, the plastic parts can have snap-together features to eliminate any fasteners. Injection molded parts can shorten the time to the market place in new product development programs because of elimination of sub-assembly tooling or fixtures. If heavy metal shakes, rattles and rolls, then plastic twists and shouts. Plastics are easily processed into complex shapes that would be impossible for metal because plastic materials have non-Newtonian flow behavior. This means that the viscosity (resistance to flow) will decrease when the flow rate increases. The flow rate is increased by increasing the injection pressures. The standard injection molding pressures are 20,000 to 30,000 Psi. This capability allows plastics to be made to flow to produce thin walled parts with uniform wall dimensions replacing the more costly thicker-walled design features of most metal parts.
SAVE DOLLARS BY RE-USING MATERIALSAny way you look at it, recycling makes sense. Re-using materials by adding regrind (ground up runners and scrap parts) to virgin materials generates even more cost savings. Plastic materials fall into two basic types of process groups: Thermoset and Thermoplastic. Thermoset (often called compression molding) is like working with epoxy. Once the material has been heated and formed in a mold, it is set. The material cannot again be processed; it is literally a reaction by temperature or thermally set. Examples of thermoset materials are Alkyd, Polyesters, Melamine and Phenolic. Most injection molding plastics are thermoplastics; that is they can be reprocessed. Thermoplastics fall into two distinctive molecular groups: amorphous and crystalline. Amorphous materials when processed act like honey; that is they never really melt, they just soften and are formed under pressure. Crystalline materials act like solder or ice. They have a specific melt temperature and remain a solid until this temperature is reached. After the melt temperature is achieved, the materials flow very easily with very low viscosities. When the material is cooled to a temperature below the melt temperature, the material hardens to a solid form.
|Shrinkage||.004 - .012/in/in||.012 - .025/in/in|
|Ease of flow||relatively stiff flowing||easy above melting|
|Dimensional control of parts||easier to maintain close dimensional tolerances||temperature more difficult to maintain close dimensional control|
INCREASE PRODUCT LIFEThe Tin Man needed more than a brain to last - he needed an endless supply of oil as well. Replace the environmental vulnerability of metals with the durability and longevity of plastics. Most plastic materials have greater chemical resistance than most metals. Plastics do not rust or oxidize as metals do and most are not affected, as are metals, by acids or base compounds.
Design Guidelines for Metal to Plastic ConversionsThere are several common mistakes made when replacing a metal product with a plastic molded part. The new part design must adhere to specific material and molding process guidelines. Several of the general guidelines are shown below:
See guidelines for uniform wall thickness when coring out sections
See Draft section for design specifications
Since 1949, Plastics One has specialized in the custom design, tooling, and injection insert molding of products used in the medical, telecommunications, aerospace and consumer products industries. Our Engineering and Tooling divisions are staffed by designers and mold makers with the knowledge and computer-aided design experience to create the exact part for your needs. As molders, we specialize in custom injection molding and insert molding using a wide variety of engineering polymers, from thermoplastic elastomers to reinforced or filled resins. Shot sizes range from 1/10th of a gram up to 80 ounces. In addition, we offer hot stamping, ultrasonic welding, and packaging services for your convenience.