PTE Part Design

Which of the following materials are generally considered to have good chemical resistance?

HDPE

Nylon

PC

PMMA (Acrylic)

ABS

PP

Acetal

HDPE, Nylon, PP, Acetal

(Semi-Crystalline)

Define creep

Strain response at a constant load (stress) with time

Constant LOAD over TIME with continuous DEFORMATION/STRAIN

Define Stress Relaxation

Stress response at a constant strain with time

Constant DEFORMATION/STRAIN over TIME with continuous DECREASE IN STRESS IN PART

Draw a creep curve. What are the x and y axis?

X-Axis = Time

Y-Axis = Strain or deformation

Does plastic or metal expand more with heat?

Plastic

What polymer characteristic would create good mechanical properties but creates conflicts with injection molding needs.

Higher molecular weight means higher viscosity which can give high strength and impact resistance, but it’s more difficult to push through small runners, small gates, and thin areas. Need to lower molecular weight to fill better but not too much to negatively impact mechanical properties.

5 things that could cause a materials’ modulus to be different that the data sheet (besides lot to lot variation)

Strain

Temperature

Strain Rate

Time Environment