Why Plastic Products Fail

The development of plastics and their associated processing techniques has been a phenomenal episode in the history of materials science. With large scale development taking place only within the last 60 years, the use of plastics in product design and manufacture has spiraled at a rate unrivaled by conventional materials. Due to the wide spectrum of properties available, plastics have become one of the most sought after materials in the world today.

More plastics are now available to the designer and engineer than at any previous stage in the history of industry. Today there are over 90 generic plastics and around 1000 sub-generic modifications with 50 thousand commercial grades available from over 500 manufacturers.

The short history of plastic development and proven usage has meant for the designer and engineer that for critical engineering applications there has never been enough time to fully explore service life and problems that might occur during the use of plastics. There has always been the question of vulnerability to failure and the ramifications of potential litigation. To some degree this situation has improved, as the portfolio of successful plastic designs has grown in demanding engineering applications. However, for new innovative applications pushing the boundaries of material performance the problem remains.

Designing to ensure plastic product reliability is critical due to the increasing importance of:

Product liability claims
Environmental concerns
Certification in order to become an approved supplier
An awareness of quality costs
Product liability can be the most damaging with settlements and penalties in the order of thousands or even millions of pounds, particularly when failure has resulted in personal injury or death. In addition to litigation financial costs, there is the distraction of key employees from normal duties, loss in product perception, brand credibility and manufacturer reputation.

Considering that approximately 70% of plastic products fail prematurely, failures have been poorly reported since the owners of failed products are naturally generally reluctant to publicise the fact. Failure investigations of such cases tend not to be disseminated due to client confidentiality agreements and for this reason the activity is predominately covert. As a consequence the potential benefits such as learning from the mistakes and misfortunes of others, and identifying priorities for research and critical issues in product development are far from being fully exploited.

It is clear from the extent of plastic and rubber failure investigations conducted by Smithers Rapra that limited dissemination of plastic and rubber failure knowledge within the public domain has resulted in a continual cycle of plastic and rubber failure incidents from all industrial sectors. The lessons of good plastic and rubber product design are not being learnt even in light of the enormous growth in product liability cases that have imposed an entirely new dimension on the consumer product environment. It is now well established in law that manufacturers are liable for injuries resulting from defective product; for injuries from a hazard associated with a product against which the user should have been warned; or for damages caused by misapplication of a product which could have been foreseen by the manufacturer.

It is a practical necessity to understand why plastics fail in order to minimise the failure scenario. Smithers Rapra has acquired this knowledge due to 50 years dealing with a diverse clientele providing technical services aimed at problem solving and in particular failure diagnosis.

Failure is a practical problem with a product and implies that the component no longer fulfils its function. Frequently, the ability to withstand mechanical stress or strain (and thereby store or absorb mechanical energy) is the most important criterion in service and consequently mechanical failure is usually a primary concern. However failure may also be attributed to loss of attractive appearance or shrinkage.

In order to avert product failure it is critical that at all stages of the design process there must be a concurrent engineering approach to product development. This system ensures that from inception of the project until final high volume manufacture all parties involved (marketing, industrial design, product engineers, plastic expert, tooling designers/engineers and processors) continually communicate in order to take advantage of the valuable knowledge and experience of all. Key to successful design is that all aspects of the performance, production, assembly and ultimate use of the part are considered. Furthermore all parties promote building reliability and safety into the product.

In order to reduce the likelihood of product failure all parties within the design process must have the ability to imagine how their designed plastic part could fail. This can only be achieved if the product design team has a good appreciation of plastics material selection, product design, processing and specific material weaknesses and fault/ failure modes and avoidance.

Plastic product failure is commonly associated with human error or weakness and is typically associated with the factors shown in figure 1.0

Human Causes of Failure (%)

In an attempt to reduce the incidence of plastic product failure we must react to the fact that they are typically due to human error, misunderstanding and ignorance of plastic materials and associated processes and that the material or process is usually not at fault.

It is hoped that the following information will provide some insight into complexity of plastics design and plastic failure modes.

Poor Material Selection / Substitution
Failures arising from incorrect material selection and grade selection are perennial problems in the plastics industry. In order to perform plastic material selection successfully a complete understanding of plastic material characteristics, specific material limitations and failure modes is required. Good material selection requires a judicious approach and careful consideration of application requirements in terms of mechanical, thermal, environmental, chemical, electrical and optical properties. Production factors such as feasible and efficient method of manufacture in relation part size and geometry need to be assessed. In terms of economics the material cost, cycle times and part price need to be considered.

Two common reasons for improper material selection are that the material selector has limited plastics knowledge and expertise and is unfamiliar with the material selection process. Alternatively, a suitable material has been specified but not used. Materials substitutions most commonly occur when the customer is unable to enforce quality procurement specifications, particularly if manufacturing site is remotely based. Common problems encountered include:

Processor simply substituting with a cheaper material.
Use of the wrong grade of material (incorrect MFI).
Use of general purpose PS rather than HIPS.
Homopolymer used instead of copolymer
Incorrect pigments, fillers, lubricants or plasticisers used.
Poor Design

There are no absolute rules pertaining to plastic product design. However, some general principles and guidelines are well established particularly between amorphous and semi-crystalline thermoplastics and thermosets and the various processing techniques. These are readily available from material suppliers.

The basic rules apply to fillets, radii, wall thickness, ribs, bosses, taper, holes, draft, use of metal inserts, undercuts, holes, threads, shrinkage, dimensional tolerance. Design rules which apply to secondary joining and assembly processes (welding, mechanical fastening and adhesive/solvent welding) need to be carefully evaluated too.

The designer and engineer should be aware that due to the diverse range of plastic materials and properties the design criteria will change form material to material as well as application to application.

Common design errors are related to abrupt geometrical changes excessive wall thickness, sharp corners and lack of radii, lack of understanding of the creep mechanism due to plastic visco-elasticity, environmental compatibility, draft, placement of ribs and injection gates.

A significant number of plastic parts fail due to sharp corners / insufficient radius. Sharp corners create stress concentrations resulting in locally high stresses and strains. Since plastics are notch sensitive the stress concentration will promote crack initiation and ultimately fracture. They also impede material flow and ejection form the tool.

A significant number of failures can be attributed to excessive wall thickness and abrupt geometrical change. A pre-requisite is that uniform wall thickness is maintained since this keeps sink marks, voids, warpage, and moulded-in stress to a minimum.

Designers and engineers must be fully conversant with the visco-elastic nature of plastics and their creep, creep rupture, stress relaxation and fatigue mechanisms.

Visco-plastic materials respond to stress as if they were a combination of elastic solids and viscous fluids. Consequently they exhibit a non-linear stress-strain relationship and their properties depend on the time under load, temperature, environment and the stress or strain level applied. An example of viscoelasticity can be seen with Silly Putty. If this material is pulled apart quickly it breaks in a brittle manner. If, however, pulled slowly apart the material behaves in a ductile manner and can be stretched almost indefinitely. Decreasing the temperature of Silly Putty, decreases the stretching rate at which it becomes brittle. Key is that the designer and engineer understand that:

Plastics will deform under load
When subjected to static low stress / strain a ductile / brittle transition will occur at some point in time resulting in brittle failure
Cyclic stressing will result in a ductile / brittle transition resulting in brittle failure at low stress level
Premature initiation of cracking and embrittlement of a plastic can occur due to the simultaneous action of stress and strain and contact with specific chemical environments (liquid or vapour)

Design failure may also be attributed to reduced safety factors due to cost pressures and the use of plastics is demanding applications taking them to their design limits where on occasion they are exceeded.

Poor Processing

Poor processing, accounts for many in-service failures. Often the problem can be traced to a blatant disregard for established processing procedures and guidelines provided by material manufacturers. The driving force behind this is often economic – the need to achieve reduced cycle times and higher production yield.

Typical processing faults are given in Table 1.0. Many of these faults can generally be overcome by attention to processing variables such as temperature, shear rates, cooling times and pressure.

Table 1.0 Processing faults

Use of inappropriate process equipment
Non-uniform wall thickness
Short shots
Bubbles
Sink marks
Post-moulding shrinkage
Warping / distortion
Foreign body contamination
Voids
Cosmetic – discolouration, splay marks
Degradation(insufficient drying of material, process temperature too high, residence time in the barrel too long, shear heating, too much regrind Self-contamination (e.g., part-melted granules).
Self-contamination (e.g., part-melted granules).
Poor material homogeneity
Poor weld lines and spider lines
Residual stress
Molecular orientation
Development of low or excessive crystallinity
Abnormal crystalline texture
Insufficient packing
Scorching
Jetting
Flashing
Abnormal spatial and size distribution of phases in composites

Mis-use / Abuse

Plastic product failure due to mis-use may result from a disregard for manufacturer installation instructions and failure to heed warnings. Failure may also occur due to simply using a product beyond its recommended service life, for function it was not intended or simply due to malicious attack.

Plastic Failure Modes

The main failure modes of plastics can be classed as mechanical, thermal, radiation, chemical and electrical as shown in Table 2.0. Classification of failure mode by mechanism shows that mechanical failure is the predominant mechanism although it is often the end result of many other failure modes.

From Smithers Rapra’s experience we have found that the vast majority of plastic product failures are due to the cumulative effects of synergies between creep, fatigue, temperature, chemical species, UV and other environmental factors.

Table 2.0 Plastic Failure Modes Mechanisms

Mechanical Modes
Deformation and distortion due to creep & stress relaxation, Yielding, , Crazing
Brittle Fracture due to Creep rupture (static fatigue), Notched creep rupture, Fatigue (slow crack growth from cyclic loading), High energy impact
Wear & abrasion,

Thermal Modes
Thermal fatigue
Degradation – thermo-oxidation
Dimensional instability
Shrinkage
Combustion
Additive extraction

Chemical Modes
Solvation, Swelling, dimensional instability and additive extraction
Oxidation
Acid induced stress corrosion cracking (SCC)
Hydrolysis (water, acid or alkali)
Halogenation
Environmental stress cracking (ESC)
Biodegradation

Radiation Modes
Photo-oxidative degradation (UV Light)
Ionising radiation ( gamma radiation, X rays)

Electrical Modes
Electrostatic build-up, Arcing, tracking, Electrical and water treeing

Synergistic Modes
Weathering – effects due to photo and thermo-oxidation, temperature cycling, erosion by rain and wind-borne particles and chemical elements in the environment

Smithers Rapra have undertaken over 5000 failure investigations of which a significant number can be attributed to embrittlement and / or brittle fracture resulting from slow degradation or deterioration processes. From Figure 2.0 it can be seen that ESC, fatigue, notched static rupture, thermal degradation, UV degradation and chemical attack fall into this category, even when the material was reported to be ductile.

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Sorbent Products – Choosing the Right Sorbent Products For Workplace Safety

Spills are a fact of life in any industrial-type workplace. And so absorbent products like pads and rolls are an important staple in many work places. But which ones are the best ones to get? Read on and discover right sorbent products for workplace safety.

Say you’ve just finished work, you’ve got oil and grease all over your hands, and you need to wipe your hands. Without sorbent products you might be wiping your hands on concrete, the grass, or, worse — your clothes!

Of course, most businesses – especially mechanical repair shops and manufacturing warehouses- spend a lot of money on absorbent products and for good reason. And they also put some thought into getting just the right kinds.

Absorbents go a lot further than just a piece of rag or a scrap of paper towel, these days. There are specific products out there that are designed to soak up oil, anti static pads and even hazmat pads for unknown substances or substances that are known to be hazardous.

But let’s say you’re not looking for anything special, just plain absorbent pads or rolls of paper that can do the trick for a variety of cleanups around the workplace. You can have the choice of general purpose absorbents or economy absorbents in both rolls and pads.

While the general purpose ones have the benefit of being lint-free fabric, the economy ones will still absorb hydrocarbons like oil, gasoline, fuel, diesel and lubricating oils. The economy pads and rolls also have the benefit of being available for the “right price.” which makes them very popular.

But don’t overlook the general purpose versions. In addition to being made out of better material, the general purpose pads and rolls handle a broader range of fluids and oils at cleanup time. In fact, the general purpose pads will clean up all the fluids and oils the economy ones do, plus coolants, cutting fluids, hydraulic fluids, vegetable oil, acetone, turpentine, ether, MEK, hexane, trichloroethylene and more.

If you’re dealing specifically with oil then, that’s no problem either. In fact, there are absorbent pads and rolls that will repel and even float on water and, at the same time, absorb all hydrocarbons.

So now what about all of the hazardous stuff? Things like hydrochloric acid, sulfuric acid, sodium hydroxide, nitric acid and caustic soda. You need special cleanup materials for these, and the hazmat line of absorbent products is perfect for the job.

For situations that require antistatic cleanup materials, you’ll find that specially treated meltblown polypropylene is perfect for the job. The antistatic pads and rolls will absorb all oils plus any other hydrocarbons, and they also come in a blue bag for quick identification, so they won’t be confused with regular clean up materials.

The latter products are the perfect choice when it comes to absorbing fuels that may become volatile in cold applications, in dry, low humidity areas or in any place where a potential for sparks exists.

Buy General Electric Refrigerators for Quality and Functionality

General Electric is a name that for many is synonymous with high quality fridges. General Electric refrigerators were one of the first brands on the market and they have led the way in creating quality home appliances for over a century. The fact that this is a well respected and trusted company is why many people come back to them when they want to buy with confidence that they will receive a great fridge.

About refrigeration

Until fairly recent times, refrigeration technology didn’t exist. In its earliest days, only commercial premises had refrigerators and it was only 50 or so years ago that refrigerators became common place in the average persons home. With the advent of this technology, the purchasing, preparation and storage of food was transformed and it changed what people were able to buy, when they could buy it and how long it would keep after purchase. Foodstuffs such as milk, meat and vegetables that were not able to be stored for long periods were suddenly able to be kept in the home without perishing quickly.

General Electric’s Long History

General Electric is a company that spans the globe and it has a solid reputation across world. This organisation built its reputation on good quality electronics, which still incorporates household electronics to this day. Lighting and electrical appliances are what gave this company its foundations, and the longevity of the business is a testimony in many people’s eyes to the strength of the company.

Why should you buy General Electric refrigerators?

As one of the leaders in the development of refrigeration, the choice of a GE refrigerator indicates a desire to buy an innovative, advanced product that offers state of the art food storage. If you value the chance to purchase a trusted brand, then this is by far one of the best. General Electric has been an industry leader in the manufacture of household electrical appliances for a very long time. As a consumer, this means you can buy with confidence from them, knowing you they are a manufacturer who have lead the way in this field.

If you desire a product that provides exceptional quality, then there is little doubt that General Electric refrigerators offer that level of quality. The company has taken pride in creating products that back up their reputation. They have established themselves as industry leaders – and they take their role seriously. For the customer, that means buying with confidence that these electrical appliances will live up to the high standard that the company has delivered for the last 136 years.

Match Wholesale General Merchandise Purchases to Your Sales

To get the top deals for any wholesale merchandise you purchase for resale you need to be at the right place at the right time with the right mindset. For this you need to have solid knowledge and understanding about certain facts relating to the market, sales vendors, products offerings, and more. In the current market a variety of options are available for those who own a dollar store to source the wholesale general merchandise needed to keep the shelves well-stocked. However there are certain things to consider before going out and blindly buying all kinds of products. Read on for some of the key considerations.

* Know the market and the products your shoppers purchase

In today’s market there it is unwise to purchase huge quantities of products which sell slowly. While you may need to locate suppliers offering partial cases, or possibly split cases of these wholesale general merchandise items with other stores, having 288 of an item which sells at the rate of 3-4 items per month is not a good use of your funds. On the other end of the spectrum, do not allow best sellers to run out.

* Build customer satisfaction by focusing on value

You simply must be careful about the quality or product features of the wholesale general merchandise you purchase. Never assume saving 5ยข on an item is worth the trade when the quantity is reduced by 25%. Even worse, don’t make tradeoffs with reduced quality, quantity or perceived value if your competition stays with the product you no longer carry.

* Know the hot sellers and keep them in-stock

Do not allow the shelves to go bare of the core consumable items which bring shoppers back time and again. These items sell, so always place your replenishment orders in plenty of time to have new stock arriving just as you are close to selling out. As these items arrive be sure to immediately place them onto the sales floor.

* Maximize profit but never building inventory levels

Do not rush to buy huge quantities of any items simply because there are tempting discount offers. You will risk ending up with an unnecessary inventory at your store. It is essential to buy the correct quantity or your inventory management will go out of balance, resulting in stockpiling of unnecessary goods.

* Shop for competitive prices when purchasing replenishment products

Buy items having the best rates. Since wholesale pricing of products varies based on their demand and availability, shop around if you cannot get competitive prices from your normal sources. Always factor in your quantity requirement (Based on products sales levels.) as well.

Thus, be sure to remember the key point to matching the buying of wholesale general merchandise to store sales. Purchase just enough dollar store merchandise to keep the shelves full as well as to ensuring the stock room is in effect empty. Buy only required items and in the right quantity, during seasonal or holiday offers. Never allow your stock room to become filled with products which do not sell.