Design Tips for Injection Molding with High-Heat or Exotic Resins

1. Material Shrink Rate:

Shrinkage is the contraction of a molded part as it cools after being injected. Every material has a different shrink rate depending on resin type (amorphous vs. crystalline), mold design, and processing conditions. The resin may also shrink differently depending on the flow direction. As a general rule, a 10% change in mold temperature can result in a 5% change in original shrinkage. Injection pressure directly affects shrinkage rates as well—the higher the injection pressure, the lower the shrinkage rate. Below are typical mold shrink rates, along with tonnage recommendations and vent depth values, for several commonly used materials and high heat resins:

2. Uniform Wall Thickness:

Uniform wall thickness throughout a part (when possible) is essential to prevent thick sections. Designing parts with non-uniform walls can cause warping as the molten material cools.

If you need areas of different thicknesses, transition them as smoothly as possible so the material flows evenly inside the mold cavity. This helps ensure the entire mold fills completely and reduces the risk of defects. Rounding or tapering thickness transitions minimizes molded-in stresses and stress concentrations from abrupt thickness changes.

Selecting the optimal wall thickness for your part can have a major impact on manufacturing costs and production speed. The minimum wall thickness depends on the part’s size, geometry, structural needs, and resin flow behavior. The wall thicknesses of injection molded parts generally range from 2mm – 4mm (0.080″ – 0.160″). Thin wall injection molding can create walls as thin as 0.5mm (0.020″). Work with an experienced injection molder and design engineer to make sure your part’s design and material selection use the correct wall thicknesses.

3. Radii to Edges:

Besides the main areas of a part, keeping wall thickness uniform is crucial in edge and corner design. Adding generous radii to rounded corners brings many benefits to a plastic part’s design, including lower stress concentration and improved material flow. Parts with sufficient radii also tend to be easier and more cost-effective to manufacture while offering better strength and appearance.

4. Use of Ribs:

Most high-temperature and specialty materials naturally have high strength and can withstand demanding environments. However, one way to further strengthen a part is by adding “ribs” to the design. Ribs are thin projections that rise at a right angle from a wall or surface to add strength.

Many designers believe that making a part’s walls thicker increases strength. In reality, overly thick walls can cause warpage, sink marks, and other defects. Ribs offer added strength without increasing wall thickness, saving on material costs while still reinforcing the part.

When designing with high-temperature and specialty materials, ribs should be 50-60 percent of the base wall thickness. Rib height should not exceed three times the base wall thickness. For extra stiffness, increase the number of ribs rather than their height, and space them at least twice the base wall thickness apart.

5. Draft Angle:

How a part’s features form inside a mold determines the type of draft needed. Features made by blind holes or pockets (like most bosses, ribs, and posts) should taper and get thinner as they extend into the mold. Surfaces formed by slides may not need draft if the steel pulls away before ejection. Consider adding angles or tapers to product features such as walls, ribs, posts, and bosses that run parallel to the direction the part is removed from the mold, which helps with ejection.

Other design guidelines include:

• For most materials, a draft angle of at least half a degree is acceptable. High-heat and specialty resins may need one to two degrees of draft. Add another degree for every 0.001 inch of texture depth.
• Draft all surfaces parallel to the direction the mold separates.
• Angle walls and other part features made in both halves of the mold to help with ejection and maintain even wall thickness.

6. Finishing:

Surface finish options for plastic injection molded parts depend on part design and the chemical makeup of the chosen material. Finishing options should be discussed early in the design process, since the selected material may significantly affect the type of finish used. Especially in cases where a glossy finish is wanted, material choice can be critical.

Products made from crystalline resins require higher melt temperatures, which increases gloss and reduces roughness to create a smoother surface. When thinking about adding compounds to achieve a certain surface finish and boost part quality, it’s essential to work with an injection molder who has access to skilled material science professionals.