As many of us in the machine industry are aware, Plastic Injection Molding is just as relevant and important as any other machining process. There are many things to consider when considering purchasing plastic injection machine . When we have the above information, we can choose suitable plastic machines by following procedures:
1.Choose correct series by plastics and finished products
As you know, there are so many series or types of plastic injection machines. So at the beginning, we need to make sure which series or which type of plastic machines are suitable for you. For example, material or products has different property like thermosetting or thermoplastic, single color, double colors or multi-colors, interlayer or color mixing, etc. And some products require higher production conditions like close loop, high accuracy, ultra-high injection speed, high injection pressure, quick production, etc. By analyzing plastics and finished products, we can decide the series of plastic injection machines.
2.Choose enough big machines to mold
We can decide the required tie-bar distance, mold thickness, minimum mold sizes, platen size, etc. as per mold sizes.
The height and width of molds should be smaller or at least one-side smaller than tie-bar distance.
It will be better if the height and width of molds are among the sizes of platen.
The thickness and the height of mold is within the range of mold thickness of plastic injection machines.
The thickness and the height of mold is bigger than minimum mold sizes. Too small molds are also not suitable.
3.Enough stoke to take out the products
As per mold and product size, we will judge if the open stroke and eject stoke is enough to take out the products. Mold open stroke should be at least twice of products sizes in the direction of mold open and mold close. And Mold open stroke should be larger than the length of vertical sprue.
Ejection stroke should be enough space to eject out the products.
4.Enough locking force
When plastic material in melting status is shot into the mold cavities by high pressure, it will bring high pressure inside of the mold. So, if without the enough locking force, the mold will be open and raw material will leak, even cause accident to molds and operators. It is very critical to have enough locking force.
There is ways to calculate the locking force:
-as per product sizes, we can get projected area in the direction of mold open and mold close.
Only when locking force is larger than mold-inside pressure which is trying to open the mold, the injection process can be successful. For safety’s sake, the clamping force should be over 1.17 times of mold-inside pressure.
Till here, we can be roughly sure which specification and locking tonnage for clamping unit. In next procedures, we will decide which screw in injection unit is suitable.
5. Full injection or full shot
As per finished products weight and number of cavities, we can check suitable shot weight and suitable screw diameter.
For better stability, the shot weight of machines should be 1.35 times at least of products weight. In another word, the products weight should be 75% lower than shot weight of molding machines
6. Well shot
We can get rough idea of screw L/D ratio and injection pressure, etc. Some engineering plastics require high injection pressure and suitable screw L/D compress ratio. To get better finished products, we need to consider injection pressure and L/D ration during selection of screw.
Generally speaking，screw with small diameter can produce high injection pressure.
Some plastic parts require high injection speed to get stable shape like thin-wall products. In this situation, we need to check if injection speed is enough, check if the machines need to equip with accumulator, close loop controller, etc. Generally speaking, under same condition, screw having higher injection pressure has lower injection speed, reversely, screw having lower injection pressure has faster injection speed. In a word, during choosing screw, we need to consider injection rate, shot volume, shot pressure.
With above procedures, we will have the skills to choose suitable models of plastic injection machines for common situation. But we still need to consider some other factors when customers need some special injection molding machines, So please ask for some advise from the injection manufacturer - https://www.petblowingmachine.com/product/injection-machine/ if you are not sure which machine is right for you.