WHAT IS INJECION MOLDING?

Ok so lets start with the basics, WHAT is injection moulding? Google defines as the shaping of plastic by injecting heated material into a mold.

I would define as squeezing melted plastic into a cavity shaped as a product, the plastic fills the cavity and is ejected when once cold/hard. In essence it’s extremely simple, although depending on the form of the product, it can become complex for over a dozen varying reasons.

HOT PLASTIC FILLS MOULD

PLASTIC COOLS

MOLD OPENS & PART EJECTED

Injection molding is responsible for pretty much every plastic product you’ve ever purchased. There is a variety of molding processes that are similar, like blow molding for example which is how plastic bottles are made. In essence all the plastic molding processes share a these commonalities, plastic is heated until soft, it’s pushed into a cavity, Its fills the cavity forming to its shape, its cools until the plastic goes hard and is finally ejected making way for the next copy to be produced.

WHO INVENTED INJECTION MOLDING? WHEN WAS IT INVENTED? AND WHERE?

Injection molding was invented to solve a problem with the production of billiard balls back in the 1868. Back then they were made from ivory which devastated the elephant population. Billiards manufacturers offered $10,000 (todays value $3,000,000) to anyone who could make these balls with something other then ivory.

 

So a man named John Wesley Hyatt invented one of the first plastics, celluloid. John living in the USA at that time patented a piece of apparatus that could mold shapes using celluloid plastic. In this instant he could manufacture balls made from celluloid which replaced billiards ivory balls. Back then products made from celluloid were for very limited applications (like making balls for billiards), with the developments in plastics and mass production, today injection molding is the most common manufacturing process of which nearly all commercial products are made entirely of or partly of molded plastics…

Historically injection mold and the knowledge that goes with it was based in the USA, it is now in China. In most circumstances production in China is a fraction of the cost compared with Europe or the states due to lower labour rates. We recently received a quote that was 10 times the cost in the UK against our partner factory in south China.

NOW THE WHO, WHAT WHEN AND WHERE IS OUT THE WAY... NOW LETS ASK WHY?

But WHY IS IT the most previlent manufacturing method for producing large volume commercial products. The simple answer is, its cheap. Injection molding can produce plastic parts extremely quickly (as quick as 2 seconds for/unit), this is the reason its so cheap.

Plastics are the most malleable material, compared to all others like glass, metal, wood etc. It can be melted and molded to create almost any shape, it has far more degrees of freedom in forming unlike the others mentioned above.

So, if you want to design something you want to sell on a mass scale your likely to choose plastic over metal for example as you have less manufacturing constraints and will be far cheaper and quicker to make in large batches.

Metal can be melted down and molded similarly but require much higher temperatures to melt making it harder and more expensive to process.

Plastics can have a number of additives that control how the material behaves which makes it a great material for tailoring its properties to suit the products requirements. Iv’e been a designer for a decade now and I can tell you this, designing commercially like most others commercial ventures come down to 2 very basic questions. How do I make this high quality for as cheap as possible? Quality injection molding answers both of these questions during mass manufacture.

I know what your thinking. How quick it is compared to other manufacturing methods?

Design Time

Machine Time

Material Cost

You may be surprised to hear that the raw materials costs is the factor that has least effect on the overall cost. Generic plastics like ABS cost only $1/Kg. Most commercial plastic products that fit in your hand weigh less then 20 grams assuming your design is of similar size you can make 1 units for just $0.04 in raw material costs…

Now lets think about it machine time costs. That same product could take 2min to make 1 copy, which would cost $1 because the machine time rate is $30/hour.

So now you can see the machine time cost clearly contributes to majority of manufacturing costs. This is why designing products that can be molded with efficient cycle times is so critical if you ever want to offer competitive pricing on your product.

For the sake of this article lets disregard the costs relating to design time and instead focus on machine time. Machine time will always apply to the cost of production, whereas design time is a startup cost which doesn’t perpetuate assuming that design doesn’t evolve into a newer model. So what does it cost to have a factory in China use their machines to churn out thousands of copies of your invention? Short answer is usually approximately $30/hour.

If your part is designed properly for injection molding and its not much larger then your open hand then very approximately speaking you can mold 1 unit in 5 seconds if you have a multicavity mold. There’s 3600 seconds in a hour, so divide 3600 by 5 to get the amount of copies you can make per hour 3600/5=720 units

So in theory you can produce 720 units for $30 in machine time + material cost. Thats extremely fast and cheap stuff so 

However if not designed properly then these fast and cheap production costs are not achievable, measures like reducing material or coring a design wherever possible help the part to cool faster and require less material, so its cheaper on material and machine time. This is why its so important to employ molding savvy designers as over the life span of the manufacturing of your product, you would make the money you spend on design time pay itself back 10 fold on manufacturing costs. Thats the best thing you can achieve with savy plastic designers, on the worst end of the scale you employ designer with no molding experience which gives you a design thats not even moldable and you start again from scratch costing way more time and money then just getting it correct in the first place.

Polymer Fundamentals

The chemical name for plastic is polymer. ‘Mer’ is derived from the Greek word meros, meaning a part or unit. One mer is a monomer, two mers form a dimer mer, three a trimer and so forth.

A polymer is a long chain molecule made up of monomers. The length and weight of this chain are dependant on the number of monomers and their molecular weights.

As the molecules are linked they are chemically bonded as they share electrons, this is also called a ‘strong bond’. With typical plastics these chains consist of 10,000+ of monomers. Even with this number of monomers a single chain molecule is 2700 nano meters long which is far smaller then what visible even in a microscope. For this reason to form just a tiny blob of plastic many many chains are needed.

So thats raw plastic, but the normal plastic product you buy, is that just raw plastic too? Not exactly no, Typically the raw plastic or ‘polymer chains’ is the main incredient. However typical commercial resins have additives to increase performance in a variety of ways. These additives can be to change the color, flame retardants for its combustive properties, lubicants, peservatives or reinforcement materials as with the case of glass filled nylon, Tiny particules or glass are mixed with nylon resin to increase its stiffness and strength. Processing aids are also a common additive which act like lubicants, it eases the flow of plastic and its fusion. Something that must be noted about additives is that it adulterates the vigin raw. plastic which degrades its capacity to be reformed or recycled.

Plastic compared to other materials is extremely cohesive, when enough heat is transfered to plastic, in its melted stated all the millions of polymer chains begin to wiggle and entangle with one another, these individual molecules are attracted creating the ‘weak force’. Its weak compared to the ‘strong force’ holding the polymer chains together but is still a cohesive structure after cooling to a still hard state.

There are 2 basic catergories in plastics, thermoplastic and thermosetting plastics.

Key differences between Thermoplastics & Thermosetting plastic

In short, THERMOplastics can be heated and reheated to change its form many times. THERMOSETTING plastics can only be heated to form a new shape once, this is because it goes through a chemical change (unlike thermoplastics) during the heat cycle. The heat cause Thermosetting plastics polymer chains to get crosslinks when eletrons are shared. So Thermoplastics can be reheated and reformed, Thermosetting plastics cannot, its sets as the term implies. Thermally speaking, thermosetting plastics are formed in reverse to thermoplastics. Thermoplastics are heated to a toothpaste consistancy, injected into a mold and harden into its given shape. Thermosetting plastics are made with 2 liquid substances which are mixed together creating a chemical reaction that expels heat.

Thermoplastics are also broken down into 2 sub catergories. Amorphous and semicrystalline.

Amorphous & Semicrystalline (thermoplastics)

A plastic can be amorphous if the long polymer chains and arranged in a completly random orientation (like a bowl of cooked spaghetti). Semicrystalline plastics have a neatly arranged side by side polymer chains (like uncooked spagehtti), however these crystalline arrangements of chains are always mixed in with some amorphous chain arrangements (picture a bowl of cooked spaghetti with bundels of uncooked chucked in). This is why there called SEMIcrystalline plastics as its not fully crystalline.

So whats the difference between crystalline and amorphous  in the way it behaves in real life? Well amorphous plastics have:

-No fixed melting point

-often transparent

-Often high impact strength (but not tensile strength)

-Low shrinkage rates as it cools and it shrinks uniformly in all directions (due to the random orientation of the polymer chains).

Semicystalline’s have opposite characterists, so they not usually transparent, have fixed melting points, much higher shrink rates which are not uniform in direction.

Although thremoplastics are grouped in the 2 catergories the thrueth is that the variety of individual plastics is super broard. Currently there is 17000 plastic world wide which can be in powder, granuel or liquid states before its formed. These large varieties of compounds are made up of different raw plastics with many differing levels of additives which allows us to finely tune a plastics characterists increase its performance for its chosen function.

What elements is raw plastic made from and hows it made?

Hydrogen and carbon, thats it! Largely plastic is organic matter but synthetically made in a laboratory, there is natural plastic like rubber or cellulose (derived from plants which we use for sticky tape). As mentioned before, raw plastic made in long chains consisting of individual ‘mers’ these mers are made from hydrogen and carbon (hyrdocarbons), for example a polystyrene monomer is 8 carbon atoms and 8 hydrogen atoms (C8H8).. Carbon has 6 protons and 6 nuetrons so it has a molecular weight of 12, hydrogen has 1 proton and no nuetrons so its weight is 1. Longer chains make stronger plastics as the level of entaglement between chains is larger. The larger the molecular weight of atoms forming the chains, the stronger the plastic will be. Lets take wax for example, first thing that pops to mind is candle wax which has very weak, it has 400 AMW whereas milk jugs made from HDPE (a form of wax) has AMW of 10,000. Ultra-high m9lecular weight polyethylene (also a wax) has AMW of 4500000 which competes on strength with kevlar in bullet proof vests.

Polymers are often made from distilling petroleum, coal or natural gas in a oil refinery. petoleum for example consists of thousands of hydrocarbons, many different in size and weight so they must be seperated by distilling at different tempratures.

These hyrdrocarbon chains aren’t much use by themselves so additives are used. Fillers (low cost minerals) add make the compound cheaper. stabilizers help keep the plastic from degrading due to heat and sunlight. plasticizers help for flexibility and viscocity.

In order it goes

monomer made of hydrogen, carbon atoms

these monomers form a polymer chain (styrene for example) the monomers share electrons creating a long chain of covalent bonds. These create a polymer molecule as small as 2700 nane meters

these polymer chains can be random copolymers, block, homo or alternative polymers

Millions of the chains (held together with ‘weak force”) attract and tangle with each other held together’. The plastic can either be thermoplastics or thermosetting.

INJECTION MOLDING LIMITATIONS AND COMMON PROBLEMS – Microworkshops  Mold Design Checklist

  • warping – disfigured parts that dont function correctly
  • sinkmarks
  • draft angles
  • uneven wall thicknesses
  • high built in stress
  • weld lines and air traps – short shots and mechanical weak points. – breakage

WHAT IS SHEAR RATE

this is the most common cause for high built in stresses with plastics parts. Its a invisible phenomenum that is bar far one of the largest causes for concern with molding quality products (high built in stresses cause warping and defigurations, also weak mechanical

SNAP FITS

One of the most common request we get at microworkshops when producing designs that will be injection molded is, “can we get snap fits to fasten the design together?”

The answer is sometimes yes and sometimes no. So what is a snap fit? (image here) What do they do and why do we use them instead of traditional fasteners like screws or adhesives.

There are 4 main types of snap fits which are