Design for Quality Assembly-DQA
Design for Quality Assembly:
RIM parts can be designed to make it easier for you to build machines with quality fits and finishes. Premold Corp calls this process Design for Quality Assembly or DQA. The larger the plastic parts and the more complex the assembly, the more important it is to use DQA techniques. DQA often results in molded-in features (bosses, ribs, tabs, etc.) to help locate parts in an assembly. Plastic molding, especially the RIM process, offers you tremendous design flexibility to take advantage of these DQA features. Premold Corp’s engineers can help you choose from many different design features to accomplish good design-for-quality assembly. By planning and designing quality into the parts during the design phase, you will save considerable time and money during both production start-up and during on-going production.
The DQA process analyzes the critical fits and tolerances in your assemblies. Some common DQA design techniques are:
- Use locators to control important dimensions in an assembly. The locators should be as close as possible to the features to be controlled.
- Use features to support plastic walls to limit their deflection from outside loads. These will also help prevent creep.
Good plastic part design and DQA accounts for four key factors:
- part shrinkage during molding.
- material properties of the resin.
- outside forces on your plastic part.
- assembly tolerances.
Plastic resins shrink during the molding process. Thicker cross sections of plastic tend to shrink more than thinner cross sections. RIM materials are generally unaffected by variations in wall thickness. However, if the thickness variations are significant (and depending on part geometry) unequal shrink can cause the part to distort or warp. Premold engineers can guide you in part design to minimize the risk of warp.
The most commonly overlooked plastic material property is creep. The resins we use at Premold Corp are very structural but plastics in general creep at lower temperatures and pressures than the metals to which designers are accustomed. RIM offers the capability to mold very large parts (over 24 inches long). This makes it more important to account for creep in your design.
Creep is a slow dimensional change over time. It is a function of time, temperature and pressure (i.e. a force generating a stress on the part). The rate and amount of creep will increase if one or more of these factors increase. All materials creep, however creep accelerates in most plastics at lower temperatures and pressures than for metals. Premold engineers can help you determine whether creep could be a factor in your design and offer proven methods to minimize its effects.
Most plastic resins are not as strong or as stiff as metals. Your plastic part may deflect when an outside force or load is applied, especially if is large (over 24 inches). Premold engineers will help you design parts that will meet your load requirements.
These factors, along with the stack-up of your assembly’s tolerances, can be addressed using proven DQA techniques. Premold Corp’s applications engineers will work with you to understand your product requirements and help you design parts that meet them. DQA will help you achieve well-designed assemblies that improve your production efficiency and product quality.