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
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?
3 factors effecting price of your molded parts;
Raw materials costs
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. 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
MAIN CONSTRAINT WITH INJECTION MOLDING
main constraints limiting freedom or forms -mold separating or pull direction (parts being stuck in mold)
2,3 & 4 part molds
SHOW EXAMPLE SOMETHING NOT MOLDABLE & WHY – vs 3d printed
part must be eject-able – no undercuts
PLASTICS CAN BE AS STRONG AS METAL
INJECTION MOLDING LIMITATIONS AND COMMON PROBLEMS – Microworkshops Mold Design Checklist
- warping – disfigured parts that dont function correctly
- 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