Shot size formula in injection moulding

Shot size formula in injection moulding

Injection moulding is a method for manufacturing plastic parts in high volumes that involves injecting molten material into the mould cavity. In plastic injection moulding, the raw material enters the barrel, melts, and the plastic gets injected into the mould cavity for cooling and solidification after mould closure, and finally, the mould opens and ejects the parts. Refer to figure (i) for the diagram indicating an injection moulding process.

fig (i) Injection moulding process

You can perform an injection moulding process with different materials, including metals, elastomers, confections, glass, and, most commonly, thermoplastic and thermosetting polymers.

With the injection moulding process, you can scale up production and produce many parts at a low manufacturing cost. This article will aid you with the shot size formulas in the injection moulding process.

Injection moulding calculations

What is shot size in injection molding machine?

The shot size/ injection capacity/ Shot capacity of the machine is the maximum amount of material (resin) the screw injects into the mould in one cycle.

What is the product/mould shot weight and shot size?

Mould shot weight/ product shot weight is the amount of material injected into the mould to fill the mould, including the feed system. Shot weight, on the other hand, is the sum of the sprue, runner, and gate weight. Mould/product shot size is the distance the screw covers for filling the product, including the feed system.

Why is an accurate injection moulding machine capacity calculation important?

A small shot size with less than 20%-barrel capacity can lead to long residence times and cause poor process control and polymer degradation. Also, it results in underfill because it does not fill the entire mould cavity leading to an empty portion of the polymer. Plus, big screws impact and lower the plastic pressure capability. Large shot sizes with more than 65% of the barrel capacity create melt-quality issues like poor melt uniformity and high screw-recovery times. It also results in a flash where polymer melt seeps through due to excess material (also possible with small clamp force and excessive injection pressure).

What factors affect the product shot size?

• When you inject the polymer into the mould, the polymer needs to fill the spruce, runner, and product volume. Hence, to calculate the shot size, you must consider spruce volume, runner volume, and product volume.
• Also, when the melt polymer undergoes cooling, it leads to shrinkage. Different polymers have different densities, and the mould shrinkage varies. Polymer shrinkage is yet another crucial factor that impacts the shot size calculations.
• Moreover, the number and type of additives in the polymer can affect the polymer density and flow behaviour.

Considering the maximum shot size of any given injection unit, what is the shot size utilisation?

The general-purpose resins like PP, PE and PS will run best if the shot size is 20 to 80 per cent of the injection unit’s shot capacity (following the 20/80 rule). For engineered resins like ABS, PC, EOM, and nylons, the shot size should be between 30 and 50 per cent of the shot capacity for the best performance. Refer to Fig (ii) which illustrates shot size utilisation.

Step-by-step calculations for product/mould shot size in injection moulding:

Product/ Mould shot size = spruce volume + runner volume + product volume+ shrinkage

Step-by-step calculations for machine shot capacity/ shot size while designating injection moulding machines

Machine shot capacity(g)/ Machine shot size(mm), also known as the machine injection capacity, is the total amount as weight or volume that the screw injects during one moulding cycle.

• Calculation using pressure:

Injection Volume = screw piston surface*stroke

Injection pressure = injection force/screw piston surface

Shot capacity/Shot size of the injection unit

= Maximum injection volume(cm³ ) * Maximum injection pressure(bar/kg/cm³ )/1000

= (screw piston surface*stroke * injection force)/(screw piston surface*1000)

An example of determining machine shot capacity/shot size using pressure is as follows:

Question: If clamping force = 700kN, the max injection volume with 35 0 screws= 140 cm³, and the max injection pressure with 35 0 screws= 1500 bar, determine the shot capacity of the injection unit.

Answer: Shot capacity(of injection unit) = Maximum injection volume(cm3) * Maximum injection pressure(bar/kg/cm³ )/1000

=(140 ×1500)/1000 = 210g

• Calculation using density:

Machine shot capacity/Machine shot size = V (volume of barrel max position)* D(material density)

An example of finding machine shot capacity/shot size using density is as follows: :

If the machine screw diameter is 40 mm, and the material density is 1.42g/cm³  for PVC material, with a maximum refilling position of 220mm, find the machine shot capacity/shot size.

Answer: Volume of material= ᴨ r2h= ᴨ*(40mm/2)2* 220mm= 276320 mm³=276.32cm³

Machine shot capacity/Machine shot size = V (volume of barrel max position)* D(material density)= 276.32cm³ *1.42g/cm³  for PVC=392.37gms.

Note that if the material is different, density varies. For instance, if the above example is for a PP material, PP density = 0.92 g/cm³

Then, volume of material= ᴨ r2h= ᴨ*(40mm/2)2* 220mm= 276320 mm³=276.32cm³

Machine shot capacity/machine shot size = V (volume of barrel max position)* D(material density)= 276.32cm³ *0.92 g/cm³ for PP=254.21gms.

• Formula using screw weight and material density

Find volume by the formula volume= mass/density.

Then, the cylindrical barrel/Shot volume = ᴨ*D2*Shot size/4

Machine shot size or l= 4V/(ᴨ*D2)mm

An example of finding the machine shot size is as follows:

Question: If the machine screw diameter is 28 mm, the material density is 1.52g/cm3  for PVC material, with a screw weight of 40.6g, and find the machine shot capacity/shot size.

Answer: Volume= mass/density= 40.6/1.52= 26.71 cm³

Cylindrical screw volume V= ᴨ*D2l/4

Shot size (l)= 4V/ᴨ*D2=4*26.71cm³/(3.14*28*28)=4.341 cm= 43.4mm

Hence, the shot size is 43.4mm.

In case you consider the cushion for 3 to 5%, 0.05* 43.4= 2.17mm

Shot size = 43.4 +2.17 mm= 45.57mm

Wrapping Up

• The injection moulding process is for fabricating plastic parts where raw plastic melts, gets injected into the mould for cooling and solidification, and the cooled part gets ejected.
• Shot size is crucial to avoid polymer underfill as well as flash.
• Spruce, runner, product volume, polymer shrinkage, and additives affect the Product/mould shot size.
• Mould shot size includes the feed system, unlike machine shot size.
• You can determine the machine shot size calculations through density, pressure, and screw weight.