Thin Wall Design

7/28/2019 Thin Wall Design

1/4

10 Common Pitfalls in Thin-Wall Plastic Part Design / Page1

10 Common Pitfalls in Thin-Wall Plastic Part Design

Timothy A. Palmer

Bayer Corporation, 100 Bayer Road, Pittsburgh, PA 15205

Abstract:Market pressures to reduce product size and weight have led to thin-wall housing designs that push the limits of

moldability and part performance for engineering thermoplastics. To a much greater extent than for conventional

designs, product success depends on careful optimization of the part design and manufacturing process. However,because the design rules and processing requirements differ for thin-wall parts, the optimum combination of design and

process can be elusive.

The common pitfalls in thin-wall plastic part design usually arise because rules-of-thumb developed for conventional

designs are applied to thin-wall parts. This creates problems which can remain hidden until final prototyping or moldtrials, adding considerable time and expense to the manufacturing process and delaying a product's introduction into

today's quickly changing, high-tech marketplace. Ten of these common pitfalls are presented here to equip designers

and molders with the information needed to recognize and avoid them.

Definition of Thin-Wall:For the purposes of this paper, a thin-wall part is defined

as one injection molded in an engineering thermoplastic

resin (e.g. PC, PC/ABS, PA6), having projected areagreater than 8 square inches and nominal wall thickness

less than 0.060" (1.5 mm). Today, many thin-wall

applications push beyond this defined limit and use

nominal wall thicknesses less than 0.040" (1.0 mm).

Pitfall #1: Designing with too much variationfrom the nominal wall thickness.

After the molten resin is injected into the mold cavity,

different areas of the plastic part experience different

levels of volumetric shrinkage proportional to wall

thickness. In conventional moldings packing pressure is

applied to force more molten material into the thicker

areas, minimizing the effects of differential shrinkage.

Unlike conventional parts, molten resin in thin-wall parts

solidifies only a few seconds after the end of fill, givingpacking pressure little time to act. The thinnest walls

solidify before significant volumetric shrinkage can

occur. Thicker areas take longer to freeze, experiencing

very high volumetric shrinkage. In the worst case,

material around the gate can solidify before any area of

the part can be adequately packed-out.

The notion that molten plastic follows the path of least

resistance is especially true in thin-wall housings. Often,

advancing flow will simply not fill the thinnest areas of a

part, creating either non-fill or gas entrapment.

Because of these difficulties, thin-wall parts should bedesigned with uniform wall thickness as much as

possible. This allows molded parts with low differential

volumetric shrinkage, improved dimensional quality and

reduced chance of cosmetic problems caused by non-fillor gas entrapment. However, the decision to use nominal

wall design must be made early in the design cycle due

to the restrictions it may impose. Often, additional wall

thickness must be added to the inside of a housing

opposite areas such as label recesses to maintain the

nominal wall thickness, as shown in figure 1. Note that

as with conventional parts, sharp edges in the flow path

and at internal corners should be avoided.

Pitfall #2: Using improper rib to wall thicknessratio.

The thick section formed by the intersection of a rib andthe nominal wall tends to experience greater volumetric

shrinkage than the rest of the part, causing sink opposite

the rib. In conventional housings, rib base thickness is

based on a percentage of the attached nominal wall,

varying from 50 to 66% depending on the degree of

cosmetic perfection desired. This design practice acts to

reduce the thick section and make it easier to pack-out,

largely eliminating visible sink.


2/4


0.080 wall - 0.67:1

0.040 wall - 0.67:1 0.040 wall - 1:1

1

0.047 0.020 0.033

0.200

Figure 1: Thick Sections at Rib Base

When standard rib design rules are applied to thin-wall

parts, the resulting rib designs are usually too thin to fill

properly, especially after draft is added. An example is

shown in figure 1 in which a 2/3 ratio is appropriate for

an 0.080" thick wall, but creates a very thin rib when thebase wall is 0.040" thick. If the ribs can be filled,

freeze-off usually occurs well before the rest of the part,

with shrinkage much different than in the attaching

nominal wall. To allow the ribs to fill properly, a 1:1 ribto wall thickness ratio, also shown in figure 1, can be

used in walls less than about 0.050" thick. Any resulting

sink marks tend to be much less noticeable than with

conventional parts, especially if the opposing surface is

textured. In a thin-wall part, there is much less material

at the rib/wall intersection to shrink and cause sink than

in conventional molded parts.

Pitfall #3: Considering only easy-flow resins for

thin-wall applications.Thermoplastic resins are often available in a range of

molecular weights. Grades with lower molecular weighttypically have lower melt viscosity and flow farther

under the same pressure than their higher molecular

weight counterparts. Unfortunately, easier flow usually

comes at the expense of physical properties such as yield

strength and impact strength. In addition, a material's

resistance to UV light and chemical attack are reduced

with decreasing molecular weight.

Because thin-wall applications can be difficult to fill, the

expected flow properties of low molecular weight resins

seem desirable. Figure 2 shows the difference in

predicted filling pressure between high and low

molecular weight grades of polycarbonate for a sample

housing. Mold-filling analysis results for the 0.040" (1.0

mm) nominal wall show that regardless of molecular

weight, high-performance injection molding equipment

is probably required. In this case, using a lower

molecular weight resin may sacrifice material properties

without significantly reducing production costs.

Filling Pressure vs. Wall ThicknessInjection Rate = 15 cu. In./sec., Polycarbonate resin

35 K

30 K

25 K

20K

15 K

10 K

5 K

0

Housing Nominal Wall Thickness (in.)

Resin Melt Flow Index (g/10 min)

20 1235

0.040 0.060 0.080

InjectionPressure(psi)

Figure 2: Thin-Wall Housing

Pitfall #4: Relying on fiber-reinforced resins to

provide rigidity.As shown in figure 3, the structural rigidity of a thin-wallhousing is greatly reduced versus its thick-wall

counterpart due to the reduction in section modulus.

From the standard engineering beam bending formula

(w/ both ends simply supported), the maximum

deflection is inversely proportional to the thickness

cubed, so under identical loads, a beam 0.040" thick has

deflection 8 times a wall 0.080" thick. A potential

solution for thin-wall housings is to use a fiber-filled

resin, which typically increases the material modulus by

about 50% (10% glass fiber-filled). However, maximum

deflection is only inverselyproportional to the material

modulus, so the unfilled beam only deflects 1.5 times

more than the fiber-filled one. Because the wall thickness

effect dominates over the effect of fiber reinforcement,

the rigidity of thin-wall housings can not be expected to

compare to thick-wall, conventional housings. Rigidityof thin-wall applications will still depend on assembly

with the product's other internal components, regardless

of the resin used.

b

h y=LP 3

4Eb 3

P

E

L

x 8.0

x3

2Unfilled PC Resin

w/ 10% Glass

h = 0.080 h = 0.040

y = 0.006 y = 0.048

y = 0.004 y = 0.032

h

Figure 3: Beam Deflection


3/4


Impact properties are also important for thin-wall

housings given their widespread use in hand-held

products prone to being dropped. Fortunately, thinner

walls may perform slightly better in a drop impact

because more flexible walls have better energy

absorption. However, the addition of fillers can sharply

reduce these properties. For example, the notched izod

impact strength of 0.125" thick polycarbonate is reduced

from 17 ftlb/in to 2 ftlb/in when 10% glass is added.

These examples suggest that the liabilities of fiber-filled

materials may outweigh their benefits in most thin-wall

parts.

Pitfall #5: Improperly locating gates.Thin-wall applications push thermoplastic resins and

standard injection molding equipment to their respective

limits, but properly locating gates is often overlooked asa way to widen the available processing window.

Unfortunately, gate locations are often chosen after part

designs are finalized, leaving only a few locations where

gate vestige is allowed. A better approach is to pick gatelocations early in the design cycle to optimize filling, and

then position label areas or other styling to conceal any

remnant of vestige.

In conventional as well as thin-wall parts, filling pressure

is minimized when all of the last areas to fill do so

simultaneously. This phenomenon is known as balanced

filling and promotes uniform solidification and packingof the part. When wall thickness is uniform in a thin-wall

part, gate locations should be chosen so that the longest

flow paths from all gates are equal in length.

However, if a thin-wall part has non-uniform wallthickness, truly balanced filling is difficult to achieve. In

fact, some degree of filling imbalance may actually

improve the moldability of a non-uniform wall part.

Mold-filling analysis is required to optimize such cases.

When analyzing a thin-wall part, the mold-filling analyst

should always consider the part and the delivery system

(e.g. three-plate runner, hot manifold), because pressure

consumed in these components can have a much greater

effect on flow balance in thin-wall parts than in

conventional designs.

Pitfall #6: Using slow injection rates.While high injection pressures are required to fill

thin-walled parts, delivering the molten resin at a

sufficient injection rate is also an important parameter.

To prevent early freeze-off, the molding machine must

inject material at a rate high enough to produce shear

heating at the flow-front. Once the flow-front

temperature begins to drop, the pressure required to

advance it can quickly exceed press capabilities,

resulting in non-fill.

Today's closed-loop, electronic controls allow nearly any

injection rate to be set at the press, but close examination

of the actual ram velocity vs. position trace may show

that the desired injection rate can only be achieved over a

small portion of the injection cycle, if at all. In this case,

a "high-performance" injection molding press designed

specifically for high injection rates will be required. Such

machines have the ability to deliver high pressure at veryhigh injection rates through the use of accumulators or

other methods.

Pitfall #7: Using more gates than necessary.

In many thin-wall applications, numerous gates are used

when fewer would be suitable because the material is not

expected to flow more than a few inches beyond the gate.

However, as mentioned in #6, significant flow in thin

walls is possible when flow-front velocity is high

enough. The rapid freeze-off expected in thin walls

typically occurs because the flow front velocity is too

low to generate shear heating.

v

v=Q/2Rt v=Q/4Rt

R

v

R

Q

t

R

Q/4

R/2

t

InputFlow

Rate, Q

Figure 4: Flow-Front Velocity for Single vs.Multiple Gating

While the ability to maintain high flow-front velocity is

largely dependent on the capabilities of the injection

molding press, the number of gates used also plays an

important role. Assuming radial flow from a pin-point

style gate, the flow front velocity is inversely

proportional to the distance flowed. If a square housing

is fed through a centrally located gate (figure 4), flow

front velocity at the end of fill is Q/2Rt, where Q is

injection rate,R is the radial distance flowed and t is partthickness. When multiple gates are used to fill the part,

flow distance is reduced, but the input flow rate must be

divided among the gates. In this example, the four gatesystem has half the flow front velocity of the single gate

system at the end of fill. The part with a single, center

gate has higher flow front velocity at the end of fill, no

major knitlines and avoids gas entrapment at the center

of the part.


4/4


Pitfall #8: Undersizing gates.Because higher injection rates are used in thin-wall

molding, larger gates are required to prevent cosmetic

damage caused by excessive gate shear. Externally

heated hot drops or valve-gated drops allow large gate

diameters with clean degating. The following formula

can be used to estimate the required pin-point or hot-tip

gate orifice diameter.

32Qn

3

D=

Here, the diameterD is a function of Q, the volumetric

flow rate from the nozzle, n, the number of gates and ?,

the shear rate limit. For engineering thermoplastics the

shear rate limit is usually 20,000 - 40,000 1/s, depending

on the shear-sensitivity of the resin. Use a limit of20,000 1/s for shear-sensitive resins. Note that this

formula assumes equal flow passes through each gate. It

can also be used to size tunnel gates, which should have

at least a 20 included angle and be at a 45 angle to the

parting line.

If a three-plate runner is used, large gates may cause

damage the thin nominal wall during degating. This can

be avoided if a reinforcing dome is used opposite the

gate as shown in figure 5. Keep in mind that pressure

imbalance between multiple drops in cold, three-plate

runners may be more than with hot runner systems.

90

0.040

ReinforcingDomeD

.080

Figure 5: Suggested pin-point gate detail forthin-wall parts requiring large gates.

Pitfall #9: Underestimating clamp tonnagerequirements.

In thin-wall molding, it is not uncommon for the process

window to be limited by the mold blowing open due tohigh cavity pressures. With conventional parts, clamp

tonnage estimates of 3 tons per square inch are often

adequate. Thin-wall applications must typically allow for

more than 5 tons of clamp per square inch of the mold

cavity projected area. If the part to be filled is large, the

mold and backup plates should be about twice as thick as

conventional parts to prevent flexing during high-

pressure injection.

Pitfall #10: Inadequate venting in the tool.The fast injection rates used in thin-wall molding require

larger parting line vents, primarily to prevent flowhesitation as air is pushed from the cavity at the end of

fill. However, the higher injection pressures and better

flowing resins used increase the risk of parting line flash.A mold designed with a generous number of thinner

vents may be the best compromise. Proper venting in the

areas where air is chased at the end of fill is especially

critical. Air trapped ahead of a quickly converging flow

front can significantly increase filling pressure

requirements.

Thin Wall Design

Documents

Transcript of Thin Wall Design