In principle the process of injection moulding is relatively simple. A mould formed from two or more pieces is created and a viscous material is injected into the space they form.
The basic process of forming an object within the cavity of a mould is thousands of years old. However many of the processes have changed and in particular the materials used.
One of the most significant differences in modern plastic injection moulding is the precision of the process, equipment and finalised product. For injection moulders ensuring the final product meets not only their customer’s high standards but their own requires a lot of expertise, technology and finesse.
Mould Flow Analysis
One of the most important areas for ensuring that the finished product is of a high standard is in mould flow analysis. This process ensures that the production of flawed parts is minimised.
Before any plastic is injected and before the mould is even fabricated mould flow analysis is carried out to determine how the plastic will flow through the mould and if any improvements can be made on that process.
Typical Flaws and their Causes
There are a number of flaws that may occur in the process. Understanding how and why these flaws happen is an important part of mould flow analysis and in turn of producing flawless finished products.
As the liquid polymer passes into the mould, how it interacts with the various elements of that mould on passing through affects the way it forms and solidifies. The vast numbers of variables involved, from the form of the mould to the characteristics of the particular plastic will all impact the final structure of the product.
Flash or Burrs
Flashing or burring is a common feature of items coming off an injection moulding production line. In many sectors they are accepted as part of the process and additional processes such as tumbling or abrasion are used to remove them.
They are the thin layer of excess material that is often found on moulded products of any material. They can be caused by improperly fitting mould plates or where older plated have become worn. They can also result from high injection speeds or pressures as well as polymers having too high a viscosity.
It is important to review mould design to minimise or even eliminate flashing as it is generally undesirable and for some products, such as medical devices, unacceptable.
Wavy lines can be present in a moulded product. These can occur where the speed of injection has been too slow causing the polymer to cool down too soon. As the exposed end of the flow begins to solidify it disperses the polymer unevenly throughout the mould causing a wavy pattern.
To avoid this defect it is important to set the injection speed correctly for the mould and the material being used. Calculating the appropriate speed based on the mould size, shape and form as well as the material properties requires a high degree of skill and expertise.
Knit lines occur where the flow front flows around a mould component standing proud within the mould. As the flow moves around the piece it meets again on the other side. Where the two fronts meet a line can form particularly if the fronts do not blend. This may be due to cooling.
Calculating flow rate and temperature as well as gate positioning are all important to avoid knit lines.