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SANKO GOSEI

Welcome to the swamp of plastic injection moulding! The depth of difficulty and its strategies.

Resin injection moulding sounds kind of magical, doesn't it?


Hot resin is poured into a mould and the product is ready in no time... Oops, it's actually not that simple! Yes, there is a bit of a ‘quagmire’ in the process. But don't worry, we're not going to tell you about it. In this article, we will introduce the difficulties often encountered in the ‘swamp’ in a way that is easy to understand and even a bit funny for beginners.


1. temperature control is like baking

お菓子作り

First of all, one of the difficulties of injection moulding is temperature control. Huh? Did you think that it is similar to making confectionery? Yes, that's right. In pastry making, you can say, ‘The oven temperature was a little too high!’ or ‘The cookies are still soft even though they have cooled down...’? Injection moulding is similar.


The optimum temperature differs depending on the type of resin, and if the temperature is a little too high, the resin will overheat and become brittle, causing the product to crack. Conversely, if the temperature is too low, the resin will not flow well and will not spread to all corners of the mould, resulting in an incomplete finished product. This is called a ‘short shot’ and is exactly like a cake that is not cooked all the way through. At any rate, temperature control is the alpha and omega of injection moulding. If you miss, the whole thing is ruined!


2. is the slightest deviation fatal? Mould accuracy.

金型

The mainstay of injection moulding is, after all, the ‘mould’. Without it, nothing can start. However, the mould must be made very precisely, otherwise the product may be produced with a ‘What? What is this?’ If the mould is not made very precisely, the product will look like ‘What? It's like waking up in the morning, looking in the mirror and seeing your hair explode.

If there is a discrepancy of just a few microns, the parts will not fit together or the size will be wrong. This misalignment is particularly noticeable with complex shapes. Another thing that will also annoy you as a beginner is the resin residue stuck to the mould. ‘You've got a clogged cavity!’ It's not just a matter of getting out of bed in the morning, it's also a challenge to wash it out. After all, cleaning the moulds takes time, so it's like going to work without being able to fix your hair.


3. adjustment of injection pressure causes burring

Next comes ‘injection pressure’. This refers to the force used to feed the resin into the mould, but it is also an area that needs to be adjusted - neither too strong nor too weak. If it is too strong, excess resin will protrude through the gaps in the mould, creating excess material on the edges of the product called burrs. The burrs are the extra bits (mimi) in taiyaki.

たい焼き

Conversely, what happens if the pressure is too weak? As with the temperature mentioned earlier, the resin does not spread to all corners, causing short shots. You don't want a taiyaki with anko (bean paste) visible OR sticking out...

But when you think about it, a burr is somewhat better than a short shot...


4. cooling time is actually important

ゼリーと金型

One of the reasons why injection moulding looks so easy at first glance is that many people think that once the resin is poured into the mould, all they have to do is wait. But waiting also requires a strategy. The key is the ‘cooling time’ - the time it takes for the product to harden.


If the cooling time is too short, the product will be deformed or pulled and distorted when it comes out of the mould. The image is the same as when chilled and hardened jelly is forcibly transferred to a plate while it is still firm, it will fall apart. Conversely, if the cooling time is too long, the production cycle slows down and efficiency is reduced. It is true that ‘wait and you shall have a good day on the sea’, but waiting too long can also have the opposite effect.


5. optimising cycle times is a craftsmanship

時間管理

In injection moulding, the ‘cycle time’ required to produce a single product is critical. This cycle time depends on a number of factors, including the shape and size of the product, the type of resin and the design of the mould. Optimising this is like a puzzle for beginners.


In particular, finding the optimum cycle time by gradually adjusting various settings is a true artisanal skill. Sometimes adjustments need to be made down to the millisecond, so even if you think ‘this is about right’, it can actually make a big difference. It is like subtly adjusting the hands of an analogue clock. The slightest difference can have a huge impact on the final product, so a delicate balance is required.


6. choosing the type of resin is similar to love.

恋愛

The range of resins used in plastic injection moulding is truly diverse. From polypropylene and polyethylene to nylon, polycarbonate and ABS, there is a huge variety. This is a bit like being in love, don't you think? You wonder, ‘Which resin is right for me?’ is just like searching for which partner is right for you.


To choose the right resin, you need to understand its physical properties, heat resistance and strength. But if you make the wrong choice, the end product may not turn out the way you want it to, or it may break down quickly. This is like having the wrong date and getting a different result to what you expected. Yes, the choice of resin should be made carefully.


7. troubleshooting storms

成形不良一覧

Finally, injection moulding is also a ‘hotbed of trouble’. Often the product does not turn out as desired, and it is difficult for beginners to pinpoint the cause. Burrs appear, short shots occur, moulded products are deformed... It is a chaotic situation, like when your room suddenly starts to get messy.


The only way to overcome this is to be patient and review every element. Temperature, pressure, cooling time, resin selection, mould conditions, all have an impact. Each element is connected, so an improvement in one area can cause problems in another. The world of injection moulding is truly a world of falling dominoes. That is why there is a great sense of satisfaction once mastered.

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