Molded Parts Design
Designing with Thermoplastics
Part design considerations
Wall thickness – effect on shrinkage
The wall thickness of a molded part has a direct effect on polypropylene shrinkage. This contraction of the part occurs predominantly within the first 48 hours after molding due to polymer post-melt crystallization.
The rate of cooling (related to melt temperature, mold temperature and mold closed time) as well as second stage packing pressure directly influences the extent of shrinkage. Thick sections naturally cool slower and therefore shrink more as depicted in the graph below. This relationship favors the design of molded parts with a constant wall thickness; otherwise internal stresses may cause warpage. Mold filling/packing can be a major cause for inconsistent shrinkage values. Walls should be less than 1/4″ (6.4mm) thick to avoid shrink voids especially for areas far from the gate that may be difficult to pack out.
Thin wall molded parts of 0.020 inch (0.5 mm) or less can have shrinkage values of 0.020 inch/inch (0.020 mm/mm) or more based on molding conditions. Prototype molds are advised for new applications where precise final part dimensions are required.
As shown in Figure 7, if a part were designed with double the necessary wall thickness, the cooling time (which is also sensitive to melt temperature, mold temperature, resin stiffness and the rate of crystallization) would need to be increased 3 to 4 times.
Polypropylene can be nucleated to produce dimensionally stable parts. Most of the post molding shrinkage occurs in nucleated parts in the mold. The mold, therefore, becomes a cooling jig stabilizing the part to the mold shape, and preventing warpage. However, the parts can be difficult to strip owing to shrinkage on cores.
Nucleated parts also tend to freeze in molding stresses and parts may possess slightly less impact resistance than similar parts made with non-nucleated polypropylene. Certain polypropylene resins, particularly random copolymers, have clarifiers added to them that also behave as nucleators. Be aware that certain pigments are excellent nucleators (e.g. green blue, orange, etc.) such that molded parts of different colors may result in different part dimensions.
To determine the minimum wall thickness from a moldability standpoint, the flow of the resin in the mold should be considered with respect to melt and mold temperatures, cavity length and runner and gate sizes. In general, thin-walled parts with long flow paths require extra or ultra high flows, while thick-walled parts with short flow paths allow the use of medium to low flow resins. Molded parts with living hinges frequently require higher flows to ensure a rapid mold fill and good hinge quality. Too high a melt flow reduces the flex life of the hinge.
Wall thickness should be uniform wherever possible to minimize warpage and sink marks. If this is not possible, wall thickness should decrease progressively in the flow direction.
Ribs are intended to stiffen and strengthen the molded part while maintaining a minimum wall thickness. Furthermore, they help control the resin’s flow in the cavity and prevent warping of critical areas in the part.
The base of the rib should have a radius of about 25 to 50% of the nominal wall thickness. Studies have shown that stress (and therefore notch sensitivity) is minimized with a radius equal to 50% of the wall thickness. The base of the rib should have a width approximately 75% of the adjacent nominal wall thickness. The rib should further be tapered 0.5 to 1° per side, and should have a depth of 1.5 times the nominal wall.
Deeper ribs can be used; however, they usually require a thicker base to allow for the draft angle. Ejection problems may increase when deeper ribs are used and can result in sink marks at the wall intersection under the base of the rib.
Correctly designed parts should include radii to distribute stress and reduce the relative notch sensitivity of polypropylene. They should be included at all sharp comers, inside and out.
The inside corner radii should be 0.25 to 0.5 times the wall thickness. The outside comer radii should be 1.25 to 1.5 times the nominal wall thickness. Applying these guidelines helps to maintain a constant wall thickness.
To improve part ejection, draft angles should be provided on the inside and outside of part walls in the direction of draw. A draft of 10 per side is adequate, although a larger angle will make it easier for part release.
The use of a nucleated polypropylene may require more draft as part shrinkage occurs primarily on the mold core versus more post-mold shrinkage than when using a similar non-nucleated polypropylene.