How to select suitable models of plastic injection machines for your plastic components

Abstract:  It is important to select suitable models of plastic machinery for your plastic products.  You will learn 7 proved procedures to
                  choose right models of injection molding machines. Meanwhile, you will grasp 4 ways to calculate locking force of clamping units.

                  You will be an expert in selection of models after minutes-reading. 

Keywords: choose machine, select machine, selecting procedure, locking force calculation

Charpter 1

In general, most customers who have been in plastic industry for many years have the ability to select suitable model of plastic injection machines for their own projects. But sometimes, customers will require support from us, the manufacturers of plastic injection molding machines to give suggestion on correct models of machines. Sometimes, customers only have samples in hand or just idea of new item in their brains, and then they need to be sure if our machines can make their new products or which model is best.  And customer maybe require complete solution for efficient production. For example, for some special plastic parts, the production line will require accumulator, close loop, injection compress, etc. So, it is very critical to select right model of plastic injection machines. We prepared below information for your reference.

Preparation before selecting suitable models of machines

Usually, there are important factors like molds, products, plastics, shape, output, etc.  So, before our choice, we need to collect all information as below:

                 1.the sizes of the molds like length, height, width,

                 2.the weight of molds

        there any special design structure inside of the molds

                 4.which types and how many types of plastics

                 5.the sizes of the injection moldings like length, width, height as well as the weight

                 6.other forming requirement like quality level, production speed, etc. 

                 7. Procedures to choose correct machines

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.

-high pressure inside of the mold=projected area (cm2) *number of cavities*cavity pressure(kg/cm2)

Mold cavity pressure is different with different material. Usually, we can take 350-400kg/cm2.

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.

7.Faster injection

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 7 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:

1.In some special situation, customers’ plastic products or plastic molds are smaller in sizes but require bigger shot volume or mold is big in sizes but require small shot volume. Thus, our standard specification of machines cannot meet with the requirement. So, we need to consider new combination like using standard mold platen with smaller screw or using standard mold platen with bigger screw.

2.Don’t use big injection molding machine to make small plastic items. Big machines have big platen, but small items have small molds. During injection, there is risk for the mold platen to be bended which will cause lower product’s quality even platen broken. Furthermore, using big machine will lead to longer stay time for plastics. The melted plastic will be decomposed in the screw barrel.

3.High speed plastic injection molding machines. Currently, more and more customers will want to buy “high-speed machines” or “fast speed machines” partly because of products requirement, mainly in order to shorten forming cycle, increase output, reduce production cost, improve the competitiveness, Usually, to achieve these, there are several ways :

-increase injection speed by using bigger electric motor or oil pump, or adding accumulator

-increase feeding speed by using bigger electric motor or pump, or reducing hydraulic motor to increase screw revolving speed.

-adopting multi-loop system to shorten forming time

-increase cooling water runner to improve molds cooling efficiency

There is no such thing as a free lunch. The improvement of machinery performance and modification do increase productivity but it will also lead to more investment and operation cost. Therefore, It will be better to carry out benefit analysis in order to choose most suitable plastic injection machines for biggest benefit. 

Charpter 2

Four Ways to get the optimal locking force

Formula 1:  Locking force=projected area* constant of area

i.e.      F=S*Ka

     F—locking force (Ton);

Ka—constant of area (t/in2 or t/cm2);

S—projected area by moldings on the molds.

This is the easiest way to calculate the locking force. But it is not accurate and very casual. It requires the engineers to have enough experience to choose constant of area. 


Formula 2:  Locking force=constant of locking force * projected area

I.e.      F=Kp*S

F—locking force (Ton);

Kp—constant of locking force (t/in2 or t/cm2);

S—projected area by moldings on the molds.

As we said before, the pressure inside of mold during injection is related to material property, mixed material. There are some experience data as below:

a) Normal plastics locking force calculation

Normal plastics includes :


HIPS——-High impact polystyrene 

GPPS——-General Purpose Polystyrene

PMMA——-polymethyl methacrylate

SAN——-Styrene-acrylonitrile plastic



PVC——-Poly(vinyl chloride)

Table 1 of constant of locking force

b) Engineering plastics locking force

Engineering plastics includes:


PA— Polyamide (nylon)


PSF—Polysulfone (PSU)

PBT—Polybutylene terephthalate

PET—Polyethylene terephthalate

Table 2 of constant of locking force

c) Blending plastics

Blending plastics includes:






Table 3 of constant of locking force

Table missing

This is more accurate way compared with first way.


Formula 3 Process locking force>=projected area * cavity pressure

         Process locking force<= (0.8~0.9) Rated locking force

i.e.      Fp>=S*Pc

         Fp<=(0.8~0.9) F

        Fp—process locking force (Ton)

        Pc—cavity pressure (T/in2, t/cm2)

        S— projected area by moldings on the molds (cm2)

        F— locking force (Ton)


Table 1 of cavity pressure