Injection moulding has been described as "a jungle of disconnected facts and fairy tales." There are over 200 different parameters that must be established and controlled to achieve proper injection moulding of a plastic part. But over time, there have been many developments to machines designed to make the injection moulding process more effective and predictable.
These parameters fall within four major areas:
- Pressure
- Temperature (barrel, nozzle and mould temperatures)
- Time
- Distance
All four areas are important, but pressure and temperature are the ones most varied during process control and optimisation. Based on the requirements of any particular plastic material, the pressure must be sufficient to inject the plastic material and to hold the mould closed. In addition, the temperature of the injected plastic and mould must be correctly maintained.
Pressure is primarily of interest in the injection area, but there is also pressure found in the clamp unit of the moulding machine. Read on to learn about different areas of pressure.
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Injection pressure
This is the primary pressure for injecting 95 percent of the molten plastic into the closed mould. Normally, the highest pressure and fastest fill rate are the best condition. However, high pressure will increase moulded-in stress. So, determine the minimum amount of pressure necessary to fill the mould, and then use all of it. The hotter the plastic, the more fluid it becomes and the lower the pressure can be to fill the mould.
Holding pressure
Once the majority of the plastic (95 percent) has been injected using standard injection pressure, the machine should drop to hold pressure. This pressure is about half of the injection pressure and is used to finish filling the mould by packing the molecules together in an orderly fashion.
Holding pressure is required until the gate freezes off, normally in three to four seconds. Once that happens, hold pressure has no more effect on the molecules on the other side of the gate. If hold pressure is released before the gate freezes, the material in the cavity is still molten and will be sucked back out of the cavity. At the very least, there will be insufficient pressure to pack the molecules together and uneven shrinkage and cooling will occur. If valve gating of a hot runner system is used, holding pressure can be released earlier than with standard surface gating.
Clamp pressure
At the other end of the machine, clamp pressure keeps the mould closed against injection pressure. Therefore, the amount of clamp pressure required is based on the material being moulded. Easier-flow materials require less injection pressure; thus, they require less clamp pressure. Conversely, stiffer-flow materials will require more injection pressure, thus more clamp pressure.
Backpressure
The main focus of this article, backpressure is created during the return action of the screw after injecting material. It is a controllable process parameter that is often mistakenly overlooked.
The screw turns to bring fresh material into the heating cylinder. This material is placed in front of the screw and nudges the screw backwards. A build-up of pressure is created at the front end of the screw. This pressure is used for better mixing of the plastic (especially if colours are added at the press), removing small amounts of trapped air, and controlling the weight of the shot by maintaining an accurate density of a given volume of melt.
Users should keep this pressure as low as possible and in any event, should not go beyond 20 percent of the machine’s maximum rated injection moulding pressure. Some backpressure may be required to simply stop the screw pushing itself (auguring) too easily out of the barrel, or cylinder; for example, 5 bar (73 psi0).
The backpressure setting should start at 50 psi and be increased in 10 psi increments as needed, with a maximum setting of 300 psi. The maximum setting is needed because anything over that will cause too much shearing of the plastic and result in thermally degraded plastic.
The screw must generate - and exceed - this pressure before it can move back. This backpressure is generated by rotating the screw against the restriction of the plastics material, which is contained in the barrel, or cylinder.
Material melting and colour dispersion can be improved with the use of high backpressures, but it also increases stress on the injection moulding machine, reduces fibre lengths in filled systems and increases the screw retraction time.
The backpressure can also be programmed on some machines in order to compensate, for instance, for the efficient reduction in screw length which takes place during plasticisation; this type of reduction means less heat input and therefore a drop in temperature. Conversely, on several machines it is difficult to set the pressure as there is no simple way to measure the effects of the changes.
Contrary to popular belief, the purpose of backpressure is not to raise the temperature of the resin - increasing backpressure does not significantly raise the temperature of the melt. The effect of increased rpm is much higher.
One change on an injection moulding machine usually results in multiple effects. Increasing backpressure means that it takes longer for the screw to recover — often longer than the set or allowed cooling time. This, in turn, lengthens the cycle time, as the machine does not open the clamp until the screw is at shot size. To get the screw-recovery time back down below the set cooling time to keep the cycle consistent, we increase the screw rpm. As the data shows, a melt-temperature increase results when rpm is increased.
Table: Advantages and limitations of high backpressure
| Advantages | Limitations |
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Other pressures include:
Cavity Pressure
The plastics material exerts this pressure inside the cavity and tries to open and distort the mould. It may be sensed using a transducer and the produced signal that is used to actuate the switch from first stage pressure to second stage, holding pressure. This process is known as cavity pressure control (CPC).
Line Pressure
This pressure is also known as gauge pressure, as it is frequently measured by means of a gauge which is set in the hydraulic pipe or line. This pressure is not the injection pressure, but is actually the pressure present in the main supply line from the pump. Line pressure can usually be altered, and should be modified in order to suit the specific moulding run.
Nozzle Pressure
Nozzle pressure refers to the pressure measured within the nozzle. It is approximately the pressure that causes the material to flow. This pressure does not have a constant value but increases based on the growing complexity of mould filling.
There is a direct relationship between injection pressure, line pressure, and nozzle pressure. In screw machines, the nozzle pressure is about 10 percent lower than the injection pressure (as calculated from the gauge in the hydraulic line). Pressure losses in ram machines are much greater and can reach 50 percent.
Additional injection moulding resources:
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Nice, good information, Thanks!
Both a too low and a too high back pressure will make the molding process inconsistent. (Either entrapped air or excessive return flow over screw flanges). The process window can be defined by evaluating the weight variation of short shots at a certain back pressure. Getting an impression of variation dependency needs serious data collection which takes time, but it’s worth it. Some materials show within this window a clear optimum setting with best stability. This optimum setting changes a little with different RPM, screw length and type so this should be evaluated per set up.
Hi what about PET/PBT I find I get bubbles anyone with any ideas on using these material (alloys)
Guidance Required
what is correct and optimized relation between back pressure and charging (Pressure & RPM).
Hello there! Thank you for sharing such an informative article on injection molding. We have found this process very useful for our company as well.
I AM HAVING TROUBLE TO RUN CPVC ON MY INJECTION MOULDING MACHINE, PLEASE SOMEONE HELP ME.
Hi Gio,
Per our contributor, Andy Pye, it would be very difficult to assess remotely. He suggests talking to the machine supplier and the materials supplier for additional assistance.
Thanks!
Angie
Content Manager, Prospector Knowledge Center
How back pressure is controlled in all electric machine?
Worth reading! Thanks for a pretty awesome explanation about the role of back pressure in injection molding. For all my Custom Plastic Extrusion needs I prefer to use “The Spiratex” products. Please also share your experience. Thanks!!
Higher temps to reduce viscosity and allow the trapped gasses to escape the melt easier. Also, raising the back pressure to make a more consistent shot in front of your screw tip. Lastly closely monitor your screw torque, especially with PET since it has such a high viscosity and usually a low MFI. This torque can be extremely taxing on your screw and servo in an all-electric press, most hydraulic and hybrid presses (with a hydraulic rotation axis) can handle the load better but the stress on the screw itself is still present.
THAT WAS GOOD EXPLANATION WORTH READING
Additional factors that can influence recovery variation contributing to variation due to back pressure.
Acceptable shot to barrel capacity ratio.
Acceptable screw design (Compression, L/D and Profile) for the resin being processed.
Actual Melt temperature optimized on melt curve as applicable.
Optimized barrel heat setting profile for resin and screw design. (Flat, Ascending, Hump, Reverse)
Screw and barrel tolerances to specification and Composition and Finish acceptable for resin being processed.
Linear screw recovery.
Useful info about Back pressure in injection molding & inputs and outputs of Injection molding.
Why is PP injection moulded parts cracks after moulding and when stored for days or weeks??
Most likely a cold mould, with slow fill. I suggest running at max recommend temp for that material. Gates may be too small, best around 50% of wall section. Do you conduct a process qualification process?