Warpage
Warpage
Thick sections cool slower than thin sections. The thin section
first solidifies, and the thick section is still not fully
solidified. As the thick section cools, it shrinks and the material
for the shrinkage comes only from the unsolidified areas, which are
connected, to the already solidified thin section.
This builds stresses near the boundary of the thin section to
thick section. Since the thin section does not yield because it is
solid, the thick section (which is still liquid) must yield. Often
this leads to warping or twisting. If this is severe enough, the
part could even crack.
Other causes:
Warping can also be caused due to non-uniform mold temperatures
or cooling rates.
Non-uniform packing or pressure in the mold.
Alignment of polymer molecules and fiber reinforcing strands
during the mold fill results in preferential properties in the
part.
Molding process conditions--too high a injection pressure or
temperature or improper temperature and cooling of the mold cavity.
Generally, it is best to follow the resin manufacturer's guidelines
on process conditions and only vary conditions within the limits of
the guidelines.
It is not good practice to go beyond the pressure and
temperature recommendations to compensate for other defects in the
mold. If runners need to be sized differently to allow for a proper
fill, or gate sizes that need to be changed, then those changes
need to happen.
Otherwise the finished parts will have too much built in
stresses, could crack in service or warp-leading to more severe
problems such as customer returns or field service issues.
Voids and Shrinkage
Shrinkage is caused by intersecting walls of non-uniform wall
thickness. Examples of these are ribs, bosses, and other
projections of the nominal wall. If these projections have greater
wall thicknesses, they will solidify slower. The region where they
are attached to the nominal wall will shrink along with the
projection, resulting in a sink in the nominal wall.
Shrink can be minimized by maintaining rib thicknesses to 50 to
60% of the walls they are attached to.
Bosses located at corners can result in very thick walls causing
sinks. Bosses can be isolated using the techniques illustrated.
2D Hole Positioning
Figure 2.6a: Traditional Plus/Minus Tolerancing Now that we have
rigorously defined how we fixture the part for dimensioning
features, we can define where the hole is. Figure 2.6a shows two
dimensions which show where the hole is with respect to datums A
and B. Plus/minus tolerances are also shown, as is a square
tolerance zone within which the center of the circle must lie.
Although it may seem that we are not using GDT here, without the
GDT datums, the two dimensions shown are ambiguous.
However, the plus/minus tolerance zone is not clearly defined
since we do not know what is meant by the "center of the circle".
In addition, a square tolerance zone is typically not what is
needed for defining the position of a hole with respect to a mating
shaft, as shown in Figure 2.7a.
Figure 2.7a
Figure 2.6b
Figure 2.6b illustrates an isolated case where a square
tolerance zone would be appropriate. Figure 2.7 : GDT of the Same
Hole Figure 2.7 illustrates the equivalent GDT tolerancing of the
hole position. The tolerance zone within which the center of the
circle must lie is circular rather than rectangular. We will later
learn how the size and position of this circular zone is defined
based upon the symbols shown. In GDT, the "center of the circle" is
defined as the center of the best-fit circle determined by the
actual hole. Coordinate Measuring Machine (CMM) equipment typically
uses at least three points on an actual hole to define the best-fit
circle, as illustrated in Figure 2.7b. Figure 2.7c shows the CMM
probe touching the side of the hole for a data point. The CMM
machine of course uses the A and B functional datum planes as its
position and orientation references. A big advantage of GDT is that
the circular tolerance zone contains 57% more area than an
equivalent square tolerance zone. The largest deviation from true
position occurs on the diagonals of a square, and the circle meets
this, while providing 40% more possible deviation along the
vertical and horizontal. Therefore, more parts can be accepted by
inspection. With the square tolerance zone, parts that can fit are
rejected since typically only the vertical and horizontal location
deviations are checked.
Figure 2.7b : Best Fit Circle Center
Figure 2.7c
The 7.5 and 3.0 dimensions in Figure 2.7 do not have attached
tolerances for a reason. They are called basic dimensions and
represent the exact position of the center of the circular
tolerance zone within which the center of the circle must lie. They
can be recognized as basic dimensions because they are box framed.
The diameter of the circular tolerance zone comes from the feature
control frame which is below the 2.5 hole diameter dimension. The
diameter of the tolerance zone in the feature control frame is 0.5
inches. The first symbol in the frame designates the tolerance as a
positional tolerance. The -A- and -B- are the GDT datums to which
the zone refers. For a two dimensional problem, the importance of
-A- and -B- is not apparent, but we will see in the 3D example how
they come into play. For now, let us notice that the tolerance zone
location is located via -A- and -B-. The 0.2 tolerance on the 2.5
hole diameter allows the diameter of the hole to vary from 2.3 to
2.7, but the center of the hole must still lie within the circular
tolerance zone described above. The 0.2 tolerance will be discussed
further under bonus tolerancing. Figure 2.8 There is another way in
which the circular tolerance zone for the hole can be interpreted.
Figure 2.8 shows this tolerance zone as being the annular
"racetrack" formed by two circles centered at 7.5, 3.0, nominally
2.5 in diameter, and .125 above and below 2.5 in diameter. If the
actual hole profile is within this "racetrack", it is very similar
to its center being within the 0.5 diameter circular tolerance
zone. This interpretation of hole positional tolerances is common
and probably originated before CMM machines allowed finding the
center of the best fit circle. This interpretation lends itself to
inspection with calipers, gauge pins, and the like. However, in
cases where extreme precision and the utmost certainty of geometry
are required, the true GDT (ANSI Y14.5) definition should be used.
Now that we have observed how GDT can clearly define hole position,
let us move on to bonus tolerancing. If you would like to skip
bonus tolerancing for now and move on to the 3D case of what we
have discussed, see 3D Datums.
Major Polymer Categories
Acrylonitrile-Butadiene-Styrene, (ABS), Thermoplastic.
Allyl Resin, (Allyl), Thermoset polycondensate.
Cellulosic, Thermoplastic modified natural polymer
substance.
Epoxy, Thermoset polyadduct.
Ethylene vinyl alcohol, (E/VAL), Thermoplastic polymer.
Fluoroplastics, (PTFE), (FEP, PFA, CTFE, ECTFE, ETFE),
Thermoplastic polymer.
Ionomer, Thermoplastic polymer.
Liquid Crystal Polymer, (LCP), Thermoplastic.
Melamine formaldehyde, (MF), Thermoset polycondensate.
Phenol-formaldehyde Plastic, (PF), (Phenolic), Thermoset
polycondensate.
Polyacetal, (Acetal), Thermoplastic.
Polyacrylates, (Acrylic), Thermoplastic polymer.
Polyacrylonitrile, (PAN), (Acrylonitrile), Thermoplastic.
Polyamide, (PA), (Nylon), Thermoplastic polycondensate.
Polyamide-imide, (PAI), Thermoplastic polycondensate.
Polyaryletherketone, (PAEK), (Ketone), Thermoplastic
polycondensate.
Polybutadiene, (PBD), Thermoplastic polymer.
Polybutylene, (PB), Thermoplastic polymer.
Polycarbonate, (PC), Thermoplastic polycondensate.
Polydicyclopentadiene, (PDCP)
Polyektone, (PK), Thermoplastic polycondensate.
Polyester, Thermoplastic or thermoset polycondensate.
Polyetheretherketone, (PEEK), Thermoplastic polycondensate.
Polyetherimide, (PEI), Thermoplastic polycondensate.
Polyethersulfone, (PES), Thermoplastic polycondensate.
Polyethylene, (PE), Thermoplastic polymer.
Polyethylenechlorinates, (PEC), Thermoplastic polymer.
Polyimide, (PI), Thermoplastic or thermoset polycondensate.
Polymethylpentene, (PMP), Thermoplastic polymer.
Polyphenylene Oxide, (PPO), Thermoplastic polycondensate.
Polyphenylene Sulfide, (PPS), Thermoplastic polycondensate.
Polyphthalamide, (PTA), Thermoplastic polycondensate.
Polypropylene, (PP), Thermoplastic, (crystalline), polymer.
Polystyrene, (PS), Thermoplastic polymer.
Polysulfone, (PSU), Thermoplastic polycondensate.
Polyurethane, (PU), Thermoplastic or thermoset, (typically
reinforced), polyadduct.
Polyvinylchloride, (PVC), Thermoplastic polymer.
Polyvinylidene Chloride, (PVDC), Thermoplastic polymer.
Silicone, (SI), Thermoset polycondensate.
Thermoplastic elastomers, (TPE), Thermoplastic.
MTLS04 1 Lecture #12
