Plastic Part Design
|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
Only by drawing on expertise from these three areas of plastic technology can a product
designer create quality molded parts that maximize performance and are cost effective.
The purpose of this design guide deals with the first of these three issues - part design.
|Correct Part Design Guidelines|
If 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
with part designs that feature a non-uniform wall thickness.
Sink marks due to
Sink marks result from a wall yielding to the still shrinking interior mass.
Stress due to
A part with non-uniform wall thickness will cool unevenly, resulting in high molded-in stress.
Voids due to uneven
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
Concentrated stress at the junction of high & low shrinkage area may cause a part to warp.
Plastic 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.
Top of page
Many 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
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.
Top of page
Holes 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
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.
Holes (part 2)
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.
Top of page
Bosses 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.
Top of page
In the design of injection molded parts, sharp corners should be avoided. Sharp
corners act as stress risers or concentrators, reducing part strength and causing
premature failures. Sharp corners may also effect plastic flow, producing parts with
objectionable surface flow patterns.
Bosses & Ribs
Nominal radius should be one quarter of the nominal part thickness,
with a minimum radius of .015 (i.e. .100 wall, 025r)
T= wall thickness of part Radius -T
The inside radius should be at least half the part wall thickness.
The outside radius should equal the inside radius plus the part thickness
i.e. .100 wall and inside radius of .050 equals outside radius of .150)
Insert Molding Tips
Another 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
Top of page
Replacing Metal with Plastics
There 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.
1. DECREASE PIECE PART PRICES
A 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.
2. ELIMINATE TIME-CONSUMING AND COSTLY SECONDARY OPERATIONS
Eliminating 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
3. REDUCE PRODUCT WEIGHT AND IMPROVE USER EASE
One 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:
Aluminum 2.5 to 2.8
Brass 8.4 to 8.7
Zinc 6.9 to 7.2
Steels 7.7 to 7.83
Polycarbonate 1.2 to 1.4
Nylon (most types) 1.2 to 1.7
Polyethylene .92 to .95
Polypropylene .90 to 1.04
ABS 1.02 to 1.4
4.GAIN GREATER PRODUCT STRUCTURAL STRENGTH
That 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.
5.INCREASE YOUR PRODUCT DESIGN OPTIONS
Don'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.
6. SAVE DOLLARS BY RE-USING MATERIALS
Any 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.
||.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
7.INCREASE PRODUCT LIFE
The 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
Top of page
Design Guidelines for Metal to Plastic Conversions
There 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.
We invite you to visit our manufacturing plant.
You'll see first-hand why our "Success is in the Details."