Product failure is becoming a common phenomenon in the industry because of the inadequate knowledge of plastics material selection and poor product design.
More than any other material, parts made from plastics must be “designed for production.”
It's quite simple for anybody to design a plastic part, to some extent, but to make an effective design with a good knowledge of injection mold design, function and process are prerequisites for standard design skills.
Plastics materials have a different structure compared to other materials. The structure of the plastics material depends on the chemical bonding of the mono- mers. Due to this bonding, plastic materials have different behaviors and properties.
These behaviors and properties should play a vital role in the selection of the plastics material in a product design. Even a general term like “stress” can take on a different meaning in a plastic part compared to other materials.
Common problems like sink marks, warpage, burn marks, voids, etc., result not only from the wrong selection of the plastics material, but also from poor product design.
Problems like sink marks can be avoided by rectifying the part design and strict adherence to nominal wall thickness throughout the part geometry, which also would help in avoiding differential shrinkage in the part.
By controlling certain processing parameters, like hold-on time and cooling temperature problems faced due to warpage, burn marks and voids in the part can be eliminated.
Ribs aid in strengthening of the part. But at the same time, poor design or inclusion of ribs at certain places can again affect the sink marks. To avoid this, it is necessary to take care of certain factors while designing a rib.
A rib should be 40-60 percent of the wall thickness. Maximum height of the rib should be not more than three times the nominal thickness.
In a study of more than 5,000 plastics product failures by Shrewsbury, England-based consultants Smithers Rapra Technology Ltd., the failures have been classified on the basis of the primary causes of failure.
Some examples of those causes are environmental stress cracking, chem- ical attack, thermal degradation and dynamic fatigue.
A further breakdown of plastics product failure due to human causes reveals that 45 percent of those failures are due to material mis-selection and poor specification.
It's crucial for designers and engineers to understand the basic nature of plastics. Poor material selection results in the product failures that are frequent in plastics design and engineering.
The most important step in selecting a plastics material from the broad range of available materials (i.e., acrylic, polycarbonate, nylon, etc.) is to cautiously classify the requirements of the application, the various properties required and the environment in which the material will perform.
There are certain considerations — like physical and mechanical properties, thermal and chemical properties, wearing and bearing properties and some standards — that should be used to define the application as completely as possible before selecting a perfect and particular plastics or an entire family of plastics.
The more accurately the application is defined, the better the chance of selecting the best material for the exact requirement.
Here are physical and mechanical considerations for selecting a particular plastic: overall part dimensions (length, width, thickness); load the plastic part will carry; duration and times the plastic will carry higher loads; maximum stress on the part; the type of stress (tensile, flexural); dimensional shape retention; and projected life of the part or design.
Here are the thermal considerations: temperatures the plastic part faces and their duration; maximum temperatures the material must sustain; minimum temperatures the material will sustain; whether the material will have to withstand impact at the low temperatures; what kind of dimensional stability is required; and is thermal expansion and contraction an issue?
Here are chemical considerations: exposure to chemicals; duration the plastic might be submerged in water; exposure to steam; whether the plastic material is painted or glued and what kind of paint or adhesive will be used; if the plastic material is exposed to chemical or solvent vapors, identify those vapors; is there exposure to other materials that can outgas or leach detrimental materials, such as plasticizers or petroleum-based chemicals?
Here are questions to ask when considering bearing and wear:
* Will the material be used as a bearing?
* Will it need to resist wear?
* Will the material be expected to perform as a bearing? If so, under what condition?
* What wear or abrasion condition will the material face? And under what condition? Materials filled with friction reducers generally exhibit less wear in rubbing applications.
Finally, some standards to consider: regulatory requirements; whether a UL94 flame-retardant rating is required, and if so, at what level; the material's color and/or appearance; will the material be used outdoors; and will it need ultraviolet resistance.
In order to preempt product failure, it is strongly advised to make an independent material selection course.
Even the selection of the right kind material can be left to the materials supplier, which is an alternative, and the advice given is generally of excellent quality — but it will certainly be limited to the grades available in the supplier's product range.
Shah is a plastics product design expert at Innovative Design Engineering Animation Pvt. Ltd. (IDEA), an Ahmedabad, India-based product design company. Shah has designed more than 100 plastic molds.
