Moulds: How To Use
A complete guide on our different moulds and how best to use them with the Sustainable Design Studio’s Injection Machines.
Intro
Injection moulding recycled plastics requires 3 things; Recycled Plastic, an Injection Moulding Machine (like our Injection Mini) and a Mould (Or Mold if you are American).
The mould is almost as important as the injection machine, and the quality and shape of this mould will define the quality of your final product. As a rule of thumb, the higher the quality of your mould. The higher the quality the final product.
In this How To, we are going to investigate the different types of Injection Moulds we offer, how to use them, how to look after them and how to identify some common problems that may occur.
Mould Types
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Modular Mould
Made from Mild Steel. These moulds are lasercut to create the most affordable mould option. However this price tradeoff results in a much lower quality product that lacks the ability to add text or round off the corners. These moulds are best utilised for simple shapes that are to be used during demonstrations rather than in the creation of a high quality product for retail.
Modular moulds however can be stacked to create more complex shapes with limited cost. Such as adding feet to a coaster, or adding a texture to the surface to convert it to a soap dish.
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Premium Mould
This one may be obvious, but our Premium Moulds create a much nicer product than our Modular Moulds. This is because our Premium Moulds have nice rounded edges and smoother surfaces to create much nicer products.
They are also designed to allow you to get the product out easier without damaging them, so if you are planning on making a lot of products as a business idea, we always recommend you go for a Premium Aluminium Mould as it will save you time and money in the long term. They may be more expensive to invest at the start, but you will be able to create a product that can be sold for a higher value, and recover that cost later.
Getting Started Tip & Tricks
Avoid Scratching The Mould
The Premium Moulds are made from Aluminium which is a relatively soft material, but perfect for mid-priced injection moulds. Our moulds are designed to work for at least 1000+ injections, but despite this can easily be scratched.
In this image, we illustrate where can be scratched (and probably will) and where absolutely shouldn’t be scratched.
Red - Do not damage if you do your product will have these marks for life.
Green - Can be damaged/scratched in normal use.
Light Green - Small scratches are fine, but avoid where possible.
To ensure a long life we recommend the following
Never place your moulds face down.
Never use a metal tool to prise a stuck part in the Red Zone.
Always wipe away any metal dust or sand from the insides of the moulds before closing.
Ideally store the moulds closed so they are protected.
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As a generally good rule of thumb, warmer moulds work better than cold moulds. Cold moulds cause the plastic to cool too quickly when you inject resulting in blockages. It also caused the plastic to harden unevenly as it fills the mould which may result in products will an uneven surface and sub-par final quality. We recommend heating your moulds up to 40c-60c using a heat gun or a small oven. This temperature will result in the mould being almost too hot to touch, but comfortable for short periods of time along you are wearing gloves.
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Both our Premium and Modular Moulds come with M8 Nuts and Bolts to keep the moulds shut when injected into them. When tightening these ensure they are firmly closed but they do not need to be so tight that you struggle to open the mould after. However, failure to tighten properly can result in bent and damaged moulds as the plastic escapes.
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Different plastic types work differently, however, we have found from experience PP (Polypropylene) is the easiest to work with. Simply set the machine to 210c, wait 8min and inject into a warm mould and you shouldn’t have any issues at all. Injecting HDPE is slightly harder and a good second plastic to try but make sure it’s injection grade (made from other injection moulded plastics like bottle caps and not blow moulded products like bottles!).
Common Issues & How To Fix Them
Partially Filled Moulds
Not filling the mould can be frustrating. Luckily its often a simple fix, and down to 1 or 2 things.
The cause of this issue is down to the plastic not being able to reach all areas of the mould, due to the plastic either hardening too fast, not enough pressure being applied or the material not being fluid enough to past through the moulds.
To avoid this we recommend:
Make sure you test your moulds with PP (Injection Grade with a High MFI) plastic at 210c (10min heating time) in a warm mould, before trying other plastics, temperatures and processes. After this, you can adjust your settings to get better results.
Make sure you are using injection-grade plastics instead of extrusion-grade plastics.
Make sure your mould is not cold and around 30-40c.
Make sure you are waiting long enough for the plastic to heat up before injecting.
Double-check that the mould is no bigger than your maximum plastic injection capacity or that the barrel is sufficiently filled and that all trapped air has been removed.
Sink Marks
Sink marks or a dimpling in the surface that doesnt match the smoothness of the mould is caused by the plastic shrinking as it cools. This is more common in plastics that are prone to shrinking as they cool, eg. HDPE, and in products that have areas of thick plastic (3mm or more).
To avoid this we recommend the following:
When injecting and after the mould is originally full, hold the plunger/piston down applying sustained force for 5-30 seconds depending on your sink issue. You should see the injection plunger slowly and gradually move as the product shrinks and is replaced by fresh plastic.
Avoid reversing the injection plunger before taking the mould out or while its still molten, this sucks the plastic back out of the mould.
Check that your plastic is clean, contamination causes air bubbles which collapse when cooled.
Your plastic may be too hot, reduce the temperature before injecting.
Flashing
Flashing is where thin lines of plastic escape through the narrow spaces between the two mould plates. Although easily cut away in most cases, it adds time to your production and may create an unsightly line. In most cases “Flashing” is the exact opposite of partially filled moulds and could be considered “Overfilled Moulds”
Ensure the mould is firmly closed.
Make sure no grain, grit or other debris is preventing the mould from closing.
Reduce the mould temperature.
Reduce the injection temperature.
Reduce the pressure you apply to the leaver during the injection.
Change to a less fluid plastic type.
Delamination/Peeling
Peeling layers or delaminations are caused by poor bonding of the plastic during the injection. This is almost always caused by either of the following:
Your plastic source is mixed and is made of different types of plastics that don’t bond and melt together. Eg. HDPE mixed with PS or rubber.
Your plastic is contaminated with chemicals like oils, food scraps or dirt.
Related Resources.
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How to 3D Print FDM Moulds
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How to 3D Print SLA Moulds
A guide on how to 3D print a SLA mould, including what you need to purchase, and the full process of doing it yourself.
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Good Product Design
A comprehensive guide to what makes a good, and bad recycled product.
