As injection molding experts, we understand full well that the fundamental purpose of the injection molding process is to adequately melt and transport the polymer to fill and pack the mold. Injection molding firms run this continuous cycle countless times a day across countless presses. However, rarely are molded parts routinely measured for weight variation from part-to-part. After all, the purpose is to fill and pack the mold. If the final molded part passes basic quality stipulations, then why dwell on the finer details? This molding philosophy is a complete disaster in countless ways, and may very well be an even larger problem than we are fully aware. Let’s explore overpacking parts.
Poor Equipment
To properly fill and pack the mold, one must take a solid resin, form it into a viscous liquid and inject it into the cavity. As simple as this may be fundamentally, there are a few things that must be carefully considered. The screw and valve assembly must be able to supply a consistently accurate shot of high quality isothermal melt. The rampant use of general purpose screws and valves makes this almost impossible in most settings. In turn, molders are routinely trained to be processing experts due to the obvious need to process around these downfalls. Thousands of dollars and hours of external training are spent on processing techniques and in-mold sensing technology to better make parts. As admirable as this is, a very fundamental problem still exists – poor equipment. Molders essentially learn to process around the limitations of general purpose screws and valves in an attempt to…fill and pack the mold cavity. As a result, melt temperatures tend to soar which naturally leads to sacrificed physical part quality, regardless if the part is geometrically stable. This in itself is a topic worthy of discussion. Moreover, by processing around poor equipment to simply fill and pack the mold, molders often opt to overpack parts. After all, it is far better to fill the cavity than to short-shot.
As previously mentioned, the screw’s primary function is to melt and transport the resin – more importantly at a desired viscosity, temperature, and pressure. The purpose of the screw tip assembly is to act as a one way valve to prevent backflow. In order to properly pack the mold cavity, the injection molding process allows for a cushion of resin left in the barrel upon injection. This cushion should be set and monitored at the press during the molding operation. Aside from the processing function of the cushion, it also acts as a marker for valve assembly functionality. If the cushion varies shot-to-shot, naturally part weight variation varies shot-to-shot. Simply put, the valve is capturing a different dose than is required. This can be from a number of sources. The most notable culprits work in tandem – poor screw design and poor valve design. A GP screw typically supplies a non-uniform melt due to solid-bed-break-up and the lack of adequate mixing. A non-uniform melt by definition is characterized as having a thermal gradient – naturally leading to viscosity and pressure variation. This variation can be captured by the valve assembly. Also, most off-the-shelf check ring assemblies that are used in the industry have a very long stroke and unreliable rear seat closing mechanism. This also tends toward erratic closure and leakage. It is important to note that cushion can vary widely from shot to shot without ever seeing a short shot. To avoid short-shotting, molders opt to compensate for poor repeatability with an added cushion and aggressive processing conditions. Higher melt temperatures naturally lower material viscosity, increasing the flow of the material. With higher flow, it can be far easier to fill the mold quickly. However, part quality, efficiency, and accuracy are sacrificed, and in many cases, the parts are severely overpacked. With the same general purpose equipment discussed above, part weight is very likely to vary.
Click here to learn more about solid-bed-break-up
Theoretical Cost
To expand on this issue we can look to a theoretical application. If one has an injection molding press equipped with a general purpose screw and valve assembly and a 4 cavity mold that requires a 1,200g shot of Polypropylene with a total cycle time of 75 seconds across a 24/7 production week with a 90% uptime efficiency, we can easily calculate a total number of theoretical parts produced per week.
Total Operating Hours = 90%(24 hours per day X 7 days per week)
= 151.2 operating hours
Total Parts per Week = 4 cavities ((151.2 operating hours X 3,600) / 75 sec. cycle)
= 29,030 parts per week
If the process and/or screw and valve assembly are responsible for an average part weight variation of +2%, we can very easily calculate the theoretical mass of wasted resin. Further, we can assign a dollar amount to this waste in pure raw resin cost.
Target Part Weight = 1,200g / 4 cavities
=300 g/part
Amount of Wasted Resin = 2% X 300
= 6 g/part
Resin Wasted per Year = 6g X 29,030 parts X 52 weeks
= 9,057,360g
Cost of Wasted Resin = $1.08 per lb (9,057,360g / 453.592)
= $21,565.23 per year
In this example, the injection molding firm is effectively giving away almost $22,000 of raw material for free, all because of poor equipment choices. This does not consider the negative impact on the final molded part quality or scrap rates. The cost of wasted resin alone could very well pay for a proper high performance screw and valve assembly to improve not only part accuracy (material waste), but also cycle time, part quality, scrap rates, and overall efficiency. There is significant return on investment with this approach. Furthermore, as resin processors, we must carefully consider the negative environmental impact of poor processing. With this theoretical application alone, almost 20,000 lb of raw plastic was released into the environment unnecessarily. There are countless injection molded parts produced each day across every sector around the world that suffer this same outcome. Aside from the obvious economic blunder, there are theoretically billions of pounds of resin needlessly released into the environment each year.
In misguided attempts to cut costs and simply a poor understanding of melt stream components, injection molding firms routinely rob their business of profit margin. Furthermore, customers are presented with products that are lacking in quality. Lastly, the environment is laden with an untold amount of raw plastic. As resin processors and businesses, we must make better processing and buying decisions – not only to be more competitive in the market but also to better serve the customer and the environment as a whole.
