Common tweaks in the injection moulding process
Plastic injection moulding is a temperamental process that requires tight control over several different parameters. Every polymer reacts differently to conditions within the mould. A slight tweak in the variables of the process can drastically alter the look, strength and functionality of the product.
So it’s important that product engineers choose a manufacturer that has the experience and expertise to create a high-quality component.
We’ve put together a summary of the most common changes experienced manufacturers make, so product engineers can be aware of what tweaks are being made and why.
Quality checking in the injection moulding process
When a new plastic moulding project is embarked upon, the moulding team will take into account every element of the product: the size of the order; the aesthetic; performance; and strength requirements. They will set up the mould to achieve the best possible results.
Once the mould is up and running, assessments of the initial products will be carried out and small adjustments to various elements of the process will be made to rectify any recurring defects, weaknesses, or delays that will impact production, quality or process. This is known as process adjustment.
Process adjustment is a delicate process as each change can potentially have a knock-on effect and create additional problems. This is where the skill and expertise of the production team is required to know not only what should be tweaked but how to adjust other variables to avoid unwanted product changes or additional defects.
Mould and barrel temperature
Great care should be taken when tweaking the temperature of the barrel or mould as temperature variation can create a plethora of defects and problems with only a small change.
A temperature adjustment may be the solution when plastic viscosity is suspected to be the reason for product defects. A higher temperature can result in more free-flowing plastic, which can address the issue of moulds not being correctly filled. A lower temperature can speed up the cooling process, resulting in a more efficient moulding process.
It’s important to remember that it takes time for the mould and barrel to adjust to changes in temperature. According to Plastics Today, a minimum of 20 minutes should be permitted for the change in temperature to permeate the entirety of the barrel or mould once the ‘set point’ has been reached.
Back pressure
Changes to back pressure are typically considered a large adjustment that would take place during the establishing process. Raising the pressure increases the barrel temperature, which reduces the viscosity of the plastic. The increased pressure means that the plastic is broken down in a quicker and more aggressive way.
The advantage of this is that the process is swifter, however, care should be taken to make sure product strength is not compromised. It can take a number of hours for the barrel to reach a temperature that will yield consistent results.
Screw speed
Screw speed is used to make subtle adjustments to the cycle time of the moulding process. The generally accepted rule is that screw speed typically should be approximately 1.5-2 seconds quicker than the actual cool time set point. A Plastics Today industry whitepaper highlights that changes to back pressure can affect screw speed. As a result, screw speed should be reviewed if a back pressure adjustment is made. If the speed is too quick, it can result in defects and if the speed is too low, it creates an efficient process.
The goal for moulding engineers is to achieve the perfect balance between efficiency and product quality, with all defects eliminated. In order to achieve this, in-depth knowledge is required to set up the most appropriate macro settings in the operation. Fine-tuning the process is where the engineer’s skill is truly highlighted. Knowing which element of the moulding procedure needs tweaking and the likely variables that this will affect is the only way to achieve ultimate efficiency.
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