Archive for Listopad 11th, 2011

Suggestions For Steering Clear Of Mistakes

Suggestions For Steering Clear Of Mistakes When Designing Injection Molded Plastic Components

Errors, which newcomers often make whilst beginning to design plastic parts can often be spotted by Rapid prototype components before commencing to manufacture an injection mold. I will highlight some generally made errors. I will moreover describe the effect that these kinds of mistakes will have on the finished components. I will in addition provide various suggestions which are fine design practices to understand while designing components.

One vital aspect in that plastic parts differ from other parts, e.g. metal parts, plastic injection mold maker
is that plastic components normally ought to have consistent wall thickness. Parts with uneven wall thickness typically cool irregularly and leave nasty shrink marks that are visible on the outer surface. Also, this can cause stress at the junction of high and low shrinkage and cause the part to warp.

Given that plastic parts are shaped by injection molding, they ought to be designed with a draft. A good description of draft is the angle of taper of a side wall or rib required to permit the molded plastic part to be detached from the metal mold. Lacking appropriate draft, plastic parts may be tricky to detach from the mold. Many mainstream devices use draft angles between 1.5 to 2 degrees.

An often used design element are ribs. Ribs can raise the rigidity of a component. Some guidelines must be observed, though, when adding ribs. The thickness of the ribs should not be too large to steer clear of shrinkage marks. Make the thickness of the ribs less than the wall thickness. Generally ribs are 60% to 80% of the wall thickness and spaced not less than 2 times the wall thickness apart. It is best to keep the height of the ribs below 3 times the wall thickness and rather add additional ribs rather than increasing the height of the ribs.

Bosses are another design element utilized for mounting and assembly purposes. Regularly, bosses are designed with thick wall sections that may influence the appearance of the plastic component and the finished product.  The ideal wall thickness around the boss is dependent on the nominal part thickness. If the thickness is less than 1/8 inches, the perfect boss wall thickness is around 60% of the part thickness. If the part thickness is more than 1/8 inch, the best boss wall thickness is about 40% of the wall thickness. As a rule of thumb, the height of the boss ought to be less than 2.5 times the boss hole diameter.

Sharp corners ought to be avoided when designing plastic components. Sharp corners can lead to stress risers. Those may cause part failures and decrease the strength of a part. The inside radius of ribs and bosses should be no less than 0.015. For other corners, pick an inside radius of a minimum of half the wall thickness and an external radius equivalent to the inner radius plus the wall thickness.

Energy-efficient injection moulding

Energy-efficient injection moulding

In a special show at the Fakuma 2011, material producer BASF and machine manufacturer Arburg will present the multi-faceted subject of energy efficiency. Visitors to the East Foyer will be shown measures for increasing energy efficiency in injection moulding throughout the entire value-added chain and beyond, thereby reducing unit costs. Using a specially-optimised plastic and the appropriate machine technology, the two companies will demonstrate how new developments can work together to save energy.

On a total exhibition area of 100 square metres, the entire injection moulding process will be analysed in detail in terms of energy-efficiency: from product design through to production planning. As a clear practical example, comparative production with two different plastics will show how energy efficiency can be enhanced by the right choice of material. For this purpose, an electric Arburg Allrounder 370 E will produce test parts from a standard plastic and from a material that was specially improved in terms of its flow characteristics. The “energy measurement” function of the Selogica machine control system registers the energy requirement of the relevant process.

The comparison makes clear how an energy-optimised machine, together with a plastic with improved flow characteristics, can contribute to significant energy savings in the injection moulding process. Depending on the size of the components and their geometry, the lower processing temperatures and shorter cycle times result in greater energy efficiency, plastic injection mold maker
thus reducing unit costs.

Improved flow properties can be exploited in a variety of ways. Delicate structures and thin-walled parts can be filled more easily and reliably, leading to a drop in the reject rate, particularly in the case of complex geometries. The significantly-reduced injection pressure means that the moulds are subjected to much less wear. This increases operating times and reduces maintenance costs. Improved flow means that less injection and filling pressure is required for large components in particular, so that the locking force can be reduced, enabling processors to manufacture their products on smaller machines. In addition, fewer injection points or hot-runner nozzles are required on moulds. Despite the thinner walls, it is even possible to produce components with a comparatively high glass-fibre content.