What is PP (polypropylene)?+
Polypropylene is the lightest of the commonly used injection molding plastics, with a density around 0.90 grams per cubic centimeter. It is tough, very chemically resistant and inexpensive, and it can bend without breaking. That is why PP is used in packaging, caps, disposable medical products and interior parts of vehicles. It is less stiff and less dimensionally stable than ABS, because it shrinks significantly. At QDP, PP is selected as soon as chemical resistance or weight is decisive.
Why is PP suitable for a living hinge?+
PP is the only commonly used injection molding material with which you can mold a hinge directly into the product. Such a living hinge is a strip a few tenths of a millimeter thick that can bend hundreds of thousands of times without breaking, because the molecules align in the bending direction during filling. Think of a flip cap or a box with a lid made from one piece. This saves a part and an assembly step, and QDP designs the hinge into the mold layout.
What is the difference between PP and PE?+
Both are polyolefins: lightweight, chemically resistant and inexpensive. PP is stiffer and harder and can handle higher temperatures. PE, especially HDPE, is tougher in the cold and is less likely to break in frost. For a stiff part that may get hot you choose PP, for a tough box or tank that stands outdoors you use HDPE. QDP checks this selection against the actual operating temperature of the product.
Is PP UV resistant?+
Not by nature. Untreated PP becomes brittle and discolors in the sun, which often becomes visible after one or two seasons of outdoor use. There are UV-stabilized grades that slow this down considerably, and black pigment with carbon black acts as a UV filter itself. For products that will remain outdoors permanently, this is a strict requirement in material selection and not a detail for later. QDP determines the usage conditions before the mold is built.
How much does PP shrink and what does that mean for tolerances?+
PP shrinks about 1.2 to 2.5 percent, more than four times as much as ABS. That means larger dimensional deviations, a greater chance of warping with uneven wall thicknesses and wider tolerance margins. A constant wall thickness is therefore more important with PP than with a dimensionally stable material. The mold is built for this shrinkage, so switching material later is not possible without consequences. QDP calculates the shrinkage per product before any steel is milled.
Is PP suitable for food contact?+
PP is available in food-grade versions and is widely used for packaging and kitchen items. It's important that not just the base plastic, but also the pigment and additives must be approved; food-grade is thus a feature of the specific grade and not of the plastic in general. The supplier provides a certificate of conformity. QDP requests it for each project as soon as food contact is a requirement.
Can you glue PP?+
Poorly. PP has low surface energy, which means glue will barely adhere without pretreatment such as flame treatment or a primer. In practice you therefore choose welding, clamping, or a snap-fit connection instead of gluing. This is a design choice that must be made early, because a snap-fit is part of the mold and cannot be added later. QDP designs the connection into the mold instead of solving it afterwards.
