CHECKLIST FOR SPECIFYING PLASTIC PARTS
If you want to combine the right plastic material with the most
effective design for your application, check your design against
this list. If you have questions, call us. If you want the part
molded, call us too. If you want a complete design and molded
parts, call us early !
The environment affects most plastics more than metals. Be sure
you have your requirements set correctly. Generally, as you demand
more stamina, the material demands more money.
Minimum and maximum environmental temperature
Minimum and maximum operating temperature
Operating load at maximum temperature
Operating hours at maximum temperature
Coefficient of thermal expansion
Note: almost all plastics have 5-10 times the thermal expansion
of metals. Adjust clearances accordingly.
2. Moisture / Humidity
Minimum and maximum humidity
Operating load at maximum relative humidity
Operating hours at maximum relative humidity
Time between relative humidity changes (hours)
Note: Relative Humidity also includes immersion here. Changes
with humidity and temperature are usually cumulative.
3. Effect of Chemicals
Type of chemical: Bases, Acids, Organic solvents
Effect: Adsorption or attack
Operating hours, total
Operating hours, maximum temperature
Operating load at maximum temperature
Note: If effect is adsorption, properties usually arrive at an
equilibrium in time; if it is attack, continual deterioration
of properties can be expected. Under load, frequently stress-cracking
can occur. Be sure to check.
4. Static Loading
Tensile load, psi*
Compressive load, psi*
Total life expected, hours
Total hours under load
Load at maximum R.H. and maximum temperature
Stiffness (modulus) required**
Stiffness at maximum temperature (secant modulus)
Maximum allowable deformation
Creep limit (deformation under load vs time and
Note: * ASTM test data generally cannot be directly used as design basis.
** While strength is some engineering plastics is almost comparable to some metals, elastic modulus, i.e., the stress produced by a unit deformation, is always much lower. This means a part will normally perform satisfactorily with tolerances much coarser than necessary> for metal designs.
*** While creep also occurs with other materials, it is frequently much more pronounced (and at lower temperatures) than with metals. It must be considered in any load design.
5. Dynamic Loading
Single impact, maximum
Repeated impact, maximum (less than 100 times)
Number of cycles required under load
Fatigue endurance limit
Effect of low temperature on impact strength
Effect of humidity on impact strength
Note: Generally impact strength goes up with temperature due to
loss of stiffness. High impact strength data (izod) DOES NOT also
mean high fatigue endurance limit.
Color desired (standard, special, custom)
Surface appearance (gloss, texture)
Decoration (trim, electroplating, vacuum metallizing)
Note: Any surface effects which can be achieved during molding
will be significantly less expensive than post-molding operations.
Also, save yourself much grief by not specifying metallic colors. such parts rarely look like metal, never look like an original design.
7. Miscellaneous Properties
Ultra-violet light exposure (weathering)
Coefficient of friction (static and dynamic)
Burning rate (flammability)
Note: Abrasion can only be tested on the actual application.
Conditions such as vibration can cause deviation from Taber or similar standard abrasion test results.
8. Part Design
Uniform wall thicknesses
Rib thickness in correct relation to wall thickness
Studs and bosses located to avoid sink marks
Draft for long sections
Note: Use best die casting design practice, only more so.
Call us - we fill in the details. These details can and do cause 90% of plastic part headaches - yours.
9. Mold Design
Simple parting line
No cores at all, or
Cores only in ejector half of mold,
Cores pulling in one direction only
Avoid threaded cores
10. Gate Design
Location of gate
Type of gate
Size of gate
Direction of gate
Note: Disregard for these mold and gate design considerations cause 90% of headaches-ours. Our headaches cost money-yours.
11. Post-Molding Operations
Annealing for stress relief or higher crystallinity
Special assembly features (spin welding, * self-tapping
screws, interference or press fit, ** welding, riveting,
gluing, cold heading)
Note: * With circular parting line only-but then it's easy.
** Must consider maximum and minimum operating temperature and strength of fit needed. Can be made to work very reliably.
We hope that you will call us for your thermoplastic molding needs.
We're experts, because we do nothing else, whereas you must work with all kinds of material all the time. We can save you work.
But no matter whom you call-call him early.
The few minutes you spend to check a design and a mold layout with us, BEFORE you freeze the design, will save you hours of problems, reduce costly mold changes, and make you and us look much better all around.
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