Plastics are not intuitive materials to design for. We all have had experience with a plastic part that broke. A broken part is a sign that the part was designed by someone who hadn’t yet mastered designing plastic parts. But we all also have had experience surrounded by plastic parts that provide many years of service. Look around any room or any vehicle and you will see a wealth of well designed plastic parts. Good plastic part design is not difficult, but does require an investment of some time.
While parts must be designed for their end use, the designer must take into account the material used and the manufacturing process used in fabrication of the part.
How do you select what plastic to use? There are plastics that are hard and brittle, plastics that are like rubber, plastics that are almost as strong as the same size aluminum parts, plastics that can be used on the stove, and plastics that will melt if you leave them in your car on a sunny day. IDES.com keeps a database with manufacturer’s specification sheets for plastic materials. They have specification sheets for over 80,000 different plastics. There are several large families of plastic materials where the families have similar properties. The choice of material is dependent on what the part design needs.
Another challenge for the hobby molder can be finding a supplier for materials. Some suppliers want to sell rail-car quantities of materials. Many hobby molders are overwhelmed with 50lb bags of materials. Choices can be and often are narrowed by what you can get. There are a handful of suppliers that sell small quantities. Techkits a supplier that offers smaller quantities of materials.
Just about any shape part can be made using plastics; however, sometimes different geometry choices that seem trivial can have a huge impact on mold cost, part durability, or aesthetics. Some of key factors when designing plastic parts are:
- Keep the parting line in mind! To produce the part, the material is injected into cavities in a mold set. The parting line is the face where the two halves of the mold set meet. It must be possible to get the parts out of the mold set afterward. The part always must get smaller as it goes away from the parting line. If it gets larger, then it will be undercut into the mold and won’t come out. There are clever ways to deal with undercuts, but they add to the mold cost. Parting lines generally are flat. Parting lines can be offset or curved but that adds significantly to the mold cost.
- Incorporate Draft angles! Draft angles are the angles that parts get smaller as they move away from the parting line. Draft angles as small as 1° often are acceptable.
- Round corners or square corners? As you look into a mold from the parting line, a rounded outside corner will cost significantly less than a square outside corner and generally will minimize internal stresses. On inside corners, a square corner will cost slightly less than a round corner but might increase internal stresses in the part.
- Keep walls thin and uniform! Most plastic parts are shells rather than solid parts. This is because most plastics will shrink in undesirable ways when made as solid parts.
- It shrinks! Plastics usually are melted hot when injected into the mold. As is cools, it shrinks. When parts have to fit other parts, molds have to be made oversize to get parts that are the right size.
- Watch out for stress concentrators! Some features like inside corners are points of stress concentrators for many plastics. The parts are most likely to break at the stress concentrators.
Hobby molding does not mean low quality parts. Commercial quality parts definitely can and are being made on hobby molding equipment.
There are many resources on designing with plastics, from texts to free guidelines on internet. Many of the companies that manufacture plastics have free information on designing for plastics. Tangram Company has this booklet which is a good starting place.
Continue on here for the next step in the process.
Please keep in mind that Hobby Molds can provide design services if you want to outsource that activity or want help with your first part.