Back to school: PRW editor takes injection moulding course

To date, my writing career has been largely centred on the automotive sector. Within that, I’ve had some limited opportunity to write about plastics. But while it was interesting to understand the problems General Motors had in sourcing bumper covers of a sufficiently high grade for production at the assembly plant in St. Petersburg, Russia (now closed), as the editor of Plastics & Rubber World I felt there really should be some better understanding of the processes and technology in use across the industry.

After all, if you don’t know the basics of what’s going on, how can you ask a good question?

So when Richard Brown, managing director at RJG Technologies, offered a place on a course entitled ‘Basic Fundamentals of Systemic Moulding’, I was more than happy to set aside time to find out more about injection moulding and the related hardware used in one of the more widely-used plastics applications.

Day 1

Photo by PRW Engel injection moulding machine used during the course at RJG

On the journey from London to the RJG site in Peterborough, it had crossed my mind how they would fill a four-day course? Was this going to be an easy-going combination of study and breaks, a leisurely introduction to the world of injection moulding?

I couldn’t have been more wrong. After signing in at RJG we were straight into it. Our instructor for the course was Anthony Goff and it quickly became apparent that what he didn’t understand about injection moulding wasn’t worth knowing. Brief introductions completed, we moved on to the Injection Moulding Process Puzzle. With Process at the centre, this is surrounded by the four critical elements: Part Design, Material, Mould Design, and Machine.  

We looked at each of these in turn, including the types of moulding machine (hydraulic, servo-electric, and servo-hydraulic hybrid), which was followed by an overview of the major machine components, the injection and clamp units. As the morning progressed, it became clear there was one over-riding mantra: the part dictates the process.

The afternoon was taken up by working out the areas of the critical machine parts, screw, barrel, gate, etc. As a writer, mathematics has never been a strong point but I just about managed to keep up. I was better when it came to covering the types of clamp unit: toggle; direct hydraulic; and barless, or C-frame (the latter only produced by Engel). Understanding how the platens (carrying the mould) could influence the moulding process was fascinating.

Day 2

After a day in the classroom it was time to get some practical experience on the moulding machines. We were in charge of an Engel hydraulic machines with a C-frame clamp. With some pointers on where to find the necessary information on the control panel, we started our set up of the injection unit, looking at shot size, feed throat and barrel temperature, together with a series of other factors that would help to produce parts which would be of an acceptable quality.

Needless to say, the first few shots were a dead loss. The electrical insulating part we were producing was desperately short of material, resulting in a final component that bore little resemblance to the original. So we increased the pressure and the shot, to where we eventually delivered a part that looked reasonably good. Well, at least to me it looked reasonably good, until Anthony pointed out the sink marks in the plastic.

Upping the pressure eventually erased those imperfections, but the cycle time was still too slow. With a perfected holding stage we managed to optimise the cycle time to where it was about two seconds. Not bad, we were told – for beginners.

Anthony was great with helping us out just enough so that we were still in charge of perfecting the process. He was enthusiastic about how well we set up the machine, but that led on to a conversation about why such a critical process was sometimes ignored. “Companies spend £250,000 or more on a machine, then another £100,000 on a mould, then expect the machine to be up and running in two hours,” he said. “Taking the time to perfect the process beforehand can save so much time, effort and material, but few companies want to allow that time.”

Day 3

Photo by PRW Parts produced during RJG class

The third day of the course started with the group back in the classroom. The first topic covered was the melt, a homogeneous liquid polymer with non-Newtonian characteristics; contrary to most liquids, like water, the melt flows slower when under pressure.

This was combined with a look at sheer and friction of the melt and how this can be controlled by slowing the screw speed within the barrel. Somewhat surprisingly, approximately 70% of the heat used to melt the plastic material (in our case polypropylene) is generated by the screw as it pushes the material into successively smaller areas. As Anthony explained, “Barrel temperatures are set and melt temperatures are actual.”

As with most function of the machine, there is little use for extremes. Generally speaking, temperatures and pressures should each fall in a middle or average range, which would leave room to correct issues as they became apparent.

Then it was on to back pressure. This was to be one of the more difficult aspects of machine control to understand. Rather than direct pressure from the back of the barrel, this is the resistance on the screw as it rotated in the barrel which is then passed from the back to the front of the unit.

More clarity with regards to back pressure was achieved as we returned to the machines later that afternoon. Using trial and error, we saw how back pressure directly affected the shot size of the material – or dosing - and the successful (or not) production of the part. Over this exercise one thing which might have been self-evident even before the course became perfectly clear; injection moulding needs pressure in abundance.

Day 4

The final day of the course opened with a review of plastic flow which, somewhat contradictorily, noted that pressure is secondary to speed. At least in the mould cavity.

Keeping in mind that molten plastic expands about 15% from its solid state, this is critical to the successful process. But the melt cushion is also key, as while there needs to be sufficient material for the shot, the spring back related to too much material in the barrel can negatively impact part quality.

From this conundrum we moved on to a review of other fundamental issues, looking at how part size is affected by the material. Amorphous materials demonstrate less shrinkage due to their haphazard molecular make up, while semi-crystalline structure allows greater degrees of reduction. This is critical to the mould maker, who must take heat distortion temperature (HDT) into account when designing the part. Even adding the masterbatch (colour) can influence part size, which was unexpected.

Lastly, we looked at materials, ranging from the word origin of polymer (poly: many, meros: parts) and the chemical structure of the various materials. This was rounded out with a comparison of thermoplastics and thermosets, which respectively act like chocolate (can be formed and melted repeatedly) and as pastry (which can be cooked just once).

Our final task was to troubleshoot processes on the machines, looking at part quality and investigating the settings to uncover how to improve the process. More difficult than it sounds, considering the number of parameters involved in setting up the moulding machines, but we successfully made the necessary adjustments based on what we had learned.

Wrap up

While I remain a novice when it comes to plastics production, taking the course at RJG has given me insights which would have been hard to come by any other way. Speaking with Anthony Goff after the course, he pointed out that only 50% of the participants were directly involved with producing parts, with others either working in sales or other administrative or management areas. If you are involved in any way with producing plastics, I can highly recommend taking this or any other of the courses at RJG.

I’m sure you’ll find it’s time well spent.