Quality Standards

Blowmoulding is not an absolute science, nor is the process exact. As with most other manufacturing processes where machines and people are mixed, problems can arise. Today’s blowmoulding machinery is extremely reliable, yet processing drifts can still be expected. Even with the best maintenance, machines wear.
Furthermore, the performance of these machines relies on the skill levels of the machine mechanics, the setup personnel, the process engineers, and other workers and managers involved in production.
Any deficiencies with the equipment or with the operating personnel can show up as low product quality. Because these instances are unavoidable, blowmoulders use procedures to analyze and report product quality during manufacturing. These tasks are usually referred to as ‘quality assurance procedures’. Other tasks are used to prevent defective containers from leaving the production process. These are called ‘quality control procedures’.
The objective of both is the same; to assure that products meet the quality standards that have been set. Quality standards are those things about a blowmoulded item which are desired or required by the end users.
On plastic articles, these things, or attributes, fall into two general categories; visual and dimensional attributes.
Visual attributes are those that deal with appearance. Since consumers often look at the packaging and make an unconscious decision about the quality and value of the product inside – it is imperative to achieve the specified attributes. These include; colour, clarity, cleanliness, and other visual things.
Dimensional attributes are those which deal with the size, shape, and construction or the blown article. Blowmoulders need to meet the certain dimensional requirements so their parts will work with others to form an end product.
The structural requirements assure product is held without deforming. And since containers are used to hold a specific volume of product – the thickness must also be ‘in-spec’.


Specifications
Specifications, or specs, can be thought of as a language for clearly communicating ideas about quality standards. For example, the quality idea may be that this bottle should hold a little bit more than fifteen ounces of water. A specification could clarify that idea, it could state that the volume be fifteen and one-half ounces – a little more must be interpreted. Since fifteen and one-half ounces can be measured, it means the same to everyone who understands liquid volume.

Blowmoulded Part Specifications
Specifications like this one quantify something important about a blowmoulded product. A numeric value is assigned to attributes which can be measured or weighed, Specs for size, shape, weight, and volume are usually assigned numeric specs based on inches, or thousandths of an inch, gram weights, liquid measures.
Visual attributes can be quantified. However, expensive and sophisticated testing equipment is needed. Some companies can not justify the large expense, and will simply compare a visual attribute of the blowmoulded article to a target sample.
The quality idea is ‘this bottle should be blue’. The specification is that ‘the bottle colours match an approved colour sample’. While less finite than a measurement, this eliminates nearly all interpretation about the required colour and shade of blue.
Let’s review what we’ve covered so far about specifications. They form a language for clearly communicating quality ideas. Most dimensional attributes can be quantified; their specifications can be set in measurements or weights. Most visual attributes can be compared to approve samples.
Specifications usually include a tolerance. A tolerance is an accepted deviation from the desired quality standard. In our example using volume we set the specification at fifteen and one-half ounces of water, but we know that within the blowmoulding process it will be nearly impossible to make every container’s volume exactly fifteen and one-half ounces.

Include Tolerance in Specifications
Thus a tolerance is added to the spec. Our example becomes fifteen and one-half ounces, plus or minus one-quarter ounce. Some quick math tells us that the spec requires the container’s volume to range from a minimum of fifteen and one-quarter ounces to a maximum of fifteen and three-quarters ounces of liquid.

Ranges are established for the visual attributes which can not easily be measured. The specification says the blue bottle is to match the blue sample. Creating an approved colour range establishes a tolerance – the colour may range in shade from the light limit to the dark limit.

Ranges Established for the Visual Attributes
Because blowmoulding is not an exact manufacturing process, tolerances are added to specifications. They are provided so visual and dimensional attributes can fluctuate over an exceptional range, yet still meet quality standards.

Critical Container Attributes
Some critical container attributes that are usually assigned specifications include the dimensions for an article’s size, it’s height, it’s width from side to side, and it’s depth from front to back. These are normally referred to as the ‘body specifications’.
On containers intended to work with caps or special closures, several specifications for the neck are called out. For example, the outside diameter of the threads, the height of the neck, and it’s inside diameter are referred to as the ‘neck finish specifications’.
Another spec is set for the thickness of the plastic that makes up the container’s wall. Specifications for these dimensional attributes are called out in inches or thousandths of inches, or in metric equivalents.
Other dimensional attributes are evaluated through measurements of weight, or liquid measure. The volume of liquid which the container must hold is sometimes called out through two specifications – the ‘fill line’ and the ‘overflow line’. These are sometimes measured in liquid ounces but more often in cubic centimetres, or ‘CC’s’, which are more accurate units of measure. The amount of plastic required in the article is it’s net weight. Weight is usually expressed in grams.
The most common visual attributes are those we’ve already covered. The colour spec is set so that the shade falls within an established range. Bottle clarity must allow certain print sizes to be read through the container wall. Speckles and contaminants in the plastic are usually limited in size, number, and location.
Those we’ve discussed are the critical specifications. There are many other dimensional and visual attributes that depend on the intended use of the product and on the end user’s requirements.

To quickly review the critical dimensions – specifications are those that are measurable and are set for dimensional attributes like size and shape and also for neck finished, volume and container weight, and for wall thickness. The specs set for visual attributes must be evaluated by quality personnel and include; colour, clarity and cleanliness.

Specifications are Those that are Measurable