Throughout the injection molding process, there are several factors that affect efficiencies and viability of a plastic part. Before production begins, there are important design elements to consider – particularly when a part requires the use of a high heat or exotic resins. These design considerations improve the moldability of parts, and ultimately, may reduce the chance of defects and other issues. In the beginning design stages of a plastic part, engineers should work closely with a trusted injection molder who can provide suggestions on how to best optimize the design and make critical material suggestions.
Some characteristics of high heat and exotic resins are unique and may perform differently from one application to another. To realize both the design and material’s fullest benefits, experienced design engineers and injection molders have a number of factors to consider. In this post, outlined are a few basic and advanced tips that should be taken into account when designing parts for injection molding with the use of high-heat or exotic resins.
1. Material Shrink Rate:
Shrinkage is the contraction of the molded part as it cools after injection. All materials have different shrink rates depending on resin family (amorphous vs. crystalline materials), mold design, and processing conditions. Resin may also shrink differently depending on direction of flow. As a general rule of thumb, a 10% change in mold temperature can result in a 5% change in original shrinkage. In addition, injection pressure has a direct effect on shrinkage rates. The higher the injection pressure, the lower the shrinkage rate. View typical mold shrink rates, as well as tonnage recommendations and vent depth values, for some moderate-to-commonly used materials and high heat resins:
Tonnage (per in²)
|Shrink Values||Vent Depth (in.)|
|Acrylonitrile Butadiene Styrene (ABS)||2.5 – 3.5||.004 - .008||.0010 - .0020|
|ABS/Polycarbonate Blend (PC/ABS)||3.0 – 4.0||.004 - .007||.0015 - .0030|
|Acetal (POM)||3.0 – 4.0||.020 - .035||.0005 - .0015|
|Acrylic (PMMA)||3.0 – 4.0||.002 - .010||.0015 - .0020|
|Ethylene Vinyl Acetate (EVA)||2.0 – 3.0||.010 - .030||.0005 - .0007|
|Ionomer||2.5 – 3.5||.003 - .020||.0005 - .0007|
|High Density Polyethylene (HDPE)||2.5 – 3.5||.015 - .030||.0008 - .0010|
|Low Density Polyethylene (LDPE)||2.0 – 3.0||.015 - .035||.0005 - .0007|
|Polyamide - Nylon (PA) Filled||4.0 – 5.0||.005 - .010||.0003 - .0010|
|Polyamide - Nylon (PA) Unfilled||3.0 – 4.0||.007 - .025||.0005 - .0020|
|Polybutylene Terephthalate (PBT)||3.0 – 4.0||.008 - .010||.0005 - .0015|
|Polycarbonate (PC)||4.0 – 5.0||.005 - .007||.0010 - .0030|
|Polyester||2.5 – 3.5||.006 - .022||.0005 - .0010|
|Polyetheretherketone (PEEK)||4.0 – 5.0||.010 - .020||.0005 - .0007|
|Polyetherimide (PEI)||3.0 – 4.0||.005 - .007||.0010 - .0015|
|Polyethylene (PE)||2.5 – 3.5||.015 - .035||.0005 - .0020|
|Polyethersulfone (PES)||3.0 – 4.0||.002 - .007||.0005 - .0007|
|Polyphenylene Oxide (PPO)||3.0 – 4.0||.005 - .007||.0010 - .0020|
|Polyphenylene Sulfide (PPS)||3.5 – 4.5||.002 - .005||.0005 - .0010|
|Polyphthalamide (PPA)||3.5 – 4.5||.005 - .007||.0005 - .0020|
|Polypropylene (PP)||2.5 – 3.5||.010 - .030||.0005 - .0020|
|Polystyrene (PS)||2.0 – 2.5||.002 - .008||.0015 - .0020|
|Polysulphone (PSU)||4.0 – 5.0||.006 - .008||.0010 - .0015|
|Polyurethane (PUR)||2.5 – 3.5||.010 - .020||.0004 - .0010|
|Polyvinyl Chloride (PVC)||2.5 – 3.5||.002 - .030||.0005 - .0020|
|Thermoplastic Elastomer (TPE)||2.5 – 3.5||.005 - .020||.0008 - .0010|
2. Uniform Wall Thickness:
Uniform wall thickness throughout a part (if possible) is essential to avoid thick sections. Designing non-uniform walls can lead to warping of the part as the melted material cools down.
If sections of different thickness are required, make the transition as smooth as possible allowing the material to flow more evenly inside the cavity. This ensures the whole mold will be fully filled and will ultimately decrease the chance for defects. Rounding or tapering thickness transitions will minimize molded-in stresses and stress concentration associated with abrupt changes in thickness.
Incorporating the proper wall thickness for your part can have drastic effects on the cost and production speed of manufacturing. The minimum wall thickness that can be used depends on the size and geometry of the part, structural requirements, and flow behavior of the resin. The wall thicknesses of an injection molded part generally range from 2mm – 4mm (0.080" – 0.160"). Thin wall injection molding can produce walls as thin as 0.5mm (0.020"). Work with an experienced injection molder and design engineer to be sure the proper wall thicknesses are executed for your part’s design and material selection.
3. Radii to Edges:
In addition to main areas of a part, uniform wall thickness is a crucial design element when it comes to edges and corners. Adding generous radii to rounded corners will provide many advantages to the design of a plastic part including less stress concentration and a greater ability for the material to flow. Parts with ample radii also tend to be more economical and easier to produce, with greater strength and appearance.
4. Use of Ribs:
Most high-temperature and exotic materials are innately strong and withstanding of some of the most demanding environments. However, one way to add additional strength to a part is by adding “ribs” to the design. Ribs are thin protrusions that extend perpendicular from a wall or plane to provide added strength.
Many designers think that by making the walls of a part thicker, the strength of the part will increase. When in reality, making walls too thick can result in warpage, sinking, and other defects. The advantage of using ribs is that they increase the strength of a part without increasing the thickness of its walls. With less material required, ribs can be a cost-effective solution for added strength.
When designing parts utilizing high-temperature and exotic materials, ribs should be designed to be 50-60 percent of the nominal wall thickness. Rib height should be no more than three times the nominal wall thickness. For increased stiffness, increase the number of ribs rather than increasing height and space a minimum of two times the nominal wall thickness apart from one another.
5. Draft Angle:
How features of a part are formed in a mold determines the type of draft needed. Features formed by blind holes or pockets (such as most bosses, ribs, and posts) should taper thinner as they extend into the mold. Surfaces formed by slides may not need draft if the steel separates from the surface before ejection. Consider incorporating angles or tapers on product features such as walls, ribs, posts, and bosses that lie parallel to the direction of release from the mold which eases part ejection.
Other design guidelines include:
- A draft angle of at least one-half degree is acceptable for most materials. High-heat and exotic resign may require one to two degrees of draft. Add an additional degree of draft for every 0.001 inch of texture depth.
- Draft all surfaces parallel to the direction of mold separation.
- Angle walls and other part features formed in both mold halves to aid ejection and maintain uniform wall thickness.
Surface finish options for plastic injection molded parts vary depending on part design and the chemical make-up of the material used. Finishing options should be discussed early in the design process as the material chosen may have a significant impact on the type of finish implemented. In the case where a gloss finish is used, material selection may be especially important.
Higher melt temperatures are required for products made from crystalline resins which increase gloss and reduce roughness – creating the smooth surface desired. When considering additive compounds to achieve a desired surface finish and enhance the quality of a part, working with an injection molder that is aligned with knowledgeable material science professionals is essential.
7. Characteristics of the Material:
Keep the end use for a plastic part in mind throughout the design process and understand that the characteristics of the material used are vital factors that may enhance performance in demanding environments. High heat or exotic materials intrinsically possess characteristics or can be engineered to have the following characteristics:
- Low or high thermal conductivity
- Long-term thermal stability
- Excellent wear properties
- Creep resistance
- Abrasion resistance
- Chemical resistance
- Dimensional stability
- Flame resistant
- Low permeability
- Many more
Design is critical in the injection molding process – particularly when high-temperature materials are used to heighten a part’s strength, stability, and other features that are imperative to its unique application. Conventional molding techniques are not always effective with high-temperature and exotic resins. Work with your injection molder to have an understanding of the way a material reacts in certain conditions and parameters that should be put into place throughout the design and production process to ensure your part’s success.