Question

I am trying to understand a few injection molding calculations. I will appreciate your assistance.

How do I calculate part weight for plastic injection molding?

Is it mass * density or volume * density?

My part is 29.126 grams and I am using  Polypropylene (PP) which has a density of 0.899 g/cm³ .

Therefore : 29.126 * 0.899= 26.184 grams

To convert to ounce: 26.184* 1/28.3495=0.9236 ounce

Is the above correct?

How do I calculate shot weight?  Please explain clearly and give and example.

How do I calculate core and cavity cost?

I will appreciate a clear and well written explanation because I want to understand how its done.

Thank you

Answer 1

Answer :=

• How to calculate part weight for injection moulding:=

Part weight = 29.126 grams

Polypropylene (PP) is available.

Density of polypropylene = 0.92 g/cm^{​​​​​3}​.

For Eg. (Here, all values are as example.)

For English= Part volume (in^{​​​​​3}​​​​) / specific volume ((in^{​​​​​3/}lb)= lbs)

For Eg. = 5.62 in^{​​​​​3} / 23 in^{​​​​​3}​​​​/lb

= 0.244 lbs.

For Matric = Part volume (mm^{​​​​​3}​​​​) / (1000 mm^​​​3 /g) × Specific gravity of material (gms)

= 145.38 mm^{​​​​​3} / 1000 mm^{​​​​​3}​​​​/g × 1.20

= 0.174 g.

• How to calculate Shot weight :=

W = 29.126 grams

Density = 0.92 g/cm^{​​​​​3}​

So consider in in^{​​​​3}​​​​ = 1.9319321418513

The value in cm^​3​​​​ = 31.658056 cm^{​​​​​3}​​​​

(Equivalent shot weight in grams at processing temperature is 24 it.)

To calculate Shot weight :=

(Part weight × No. of cavities) + runner weight = Shot size.

For Eg. := (0.006 oz. Part × 17 cavities) + 1.23 oz. runner.

= 1.33 oz. Shot.

• To calculate Shot size for barrel shot :=

Shot weight (ounces) / Sp. gravity of material = Shot size.

Typically the shot size should be 1/3 to 5/6 of the barrel shot size - depending on cycle

= 1.33 oz. Shot weight / 0.9 (PP)

= 1.48 oz.

• How to calculate Core and cavity cost:=

To find the cost for injection moulding:=

Mould price:= Material cost + design cost + process and profit cost + VAT + tryout cost + packing and shipping cost.

Material cost = Material and standard parts accounted for 15% = 30% of the total cost of the injection moulding process and profit 30% = 50%.

So, the all values give the cost for injection moulding process is given.