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§ 19

Unit price for an injection-moulded part

What does it cost to make one lid on machine?

Answer: What does one good piece cost to make? Material is piece weight plus its share of inlet if inlet is not ground down. Machine time is cycle time divided by cavities times machine hour rate (machine form electricity and operator). Rejected pieces must be paid by good ones so total is divided by share of good pieces. Calculator gives piece price and how many good pieces come per hour.

Formula

Materiale pr. emne = (emnevægt + indløb ÷ kaviteter, hvis indløbet kasseres) × kg-pris ÷ 1000

Maskintid pr. emne = maskintimepris × cyklustid ÷ 3600 ÷ kaviteter

Pris pr. godt emne = (materiale + maskintid) ÷ (1 − kassation)

Gode emner pr. time = 3600 ÷ cyklustid × kaviteter × (1 − kassation)

Symbols and units in the formula
SymbolMeaningUnit
emneWeight per itemg
kavNumber of cavitiesstk
indloebInlet weight per shotg
genbrugThe inlet is cleaned and reused—
kgprisGranulate pricekr/kg
cyklusCycle times
timeprisMachine hourly rate (machine, tool, electricity, operator)kr/t
kassDisposal%
=Price per good piecekr
=Of which materialkr
=Of which machine timekr
=Good pieces per hourstk
=Price per 1,000 good pieceskr

Validity and source

Source
Competence objective: calculation of manufacturing costs

Does not apply when

  • Start-up, tool change, packaging, finishing and margin are not included. Die depreciation can lie in the machine hourly rate or as its own amount per piece. Check with the company's costing model.

Worked example

The calculator's own default values inserted. The figures are calculated with exactly the formula the tool uses.

Substituted

  • Weight per item25 g
  • Number of cavities4 stk
  • Inlet weight per shot8 g
  • The inlet is cleaned and reused1
  • Granulate price18 kr/kg
  • Cycle time22 s
  • Machine hourly rate (machine, tool, electricity, operator)450 kr/t
  • Disposal2 %

Giver

  • Price per good piece1.16 kr
  • Of which material0.46 kr
  • Of which machine time0.70 kr
  • Good pieces per hour641 stk
  • Price per 1,000 good pieces1,161 kr

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Sådan bruger du den

Når en kunde beder om en pris, eller når I skal vælge mellem to forme, regner du, hvad ét godt emne koster i materiale og maskintid. Kompetencemålet om kalkulation af fremstillingsomkostninger bygger på netop det regnestykke.

Step by step

  1. Skriv emnevægt, kaviteter og indløbets vægt.
  2. Vælg om indløbet males om og genbruges.
  3. Skriv granulatprisen, cyklustiden og maskintimeprisen.
  4. Skriv den forventede kassation.
  5. Læs prisen pr. godt emne og fordelingen mellem materiale og maskintid.

Typical errors

  • At glemme at kasserede emner skal betales af de gode.
  • At regne indløbet som gratis, når det ikke må genbruges.
  • At glemme opstart, omstilling og emballage i tilbuddet.

Examples from everyday life

PP-låg i 4-kavitetsform

Et låg på 25 g kører i en 4-kavitetsform med 8 g indløb, som males om. PP koster 18 kr/kg, cyklustiden er 22 s, maskintimeprisen 450 kr, og kassationen 2 %.

Weight per item
25 g
Number of cavities
4 pcs
Inlet weight per shot
8 g
The inlet is cleaned and reused
Yes (inlet doesn't cost material)
Granulate price
18 kr/kg
Cycle time
22 s
Machine hourly rate (machine, tool, electricity, operator)
450 DKK/h
Disposal
2 %

It means: Et godt låg koster 1.16 kr: 0.46 kr i materiale og 0.7 kr i maskintid. Gode emner i timen: 641 pcs.

Klart PC-emne uden genbrug

Et klart dæksel i PC på 12 g kører i en 2-kavitetsform med 6 g indløb, som ikke må genbruges. PC koster 45 kr/kg, cyklustiden er 35 s, timeprisen 520 kr, og kassationen 5 %.

Weight per item
12 g
Number of cavities
2 pcs
Inlet weight per shot
6 g
The inlet is cleaned and reused
No (intake rejected)
Granulate price
45 kr/kg
Cycle time
35 s
Machine hourly rate (machine, tool, electricity, operator)
520 DKK/h
Disposal
5 %

It means: Prisen pr. godt emne er 3.37 kr, og 2.66 kr af det er maskintid. Her betaler en form med flere kaviteter sig hurtigt.

Rule of thumb

Maskintid pr. emne = timepris ÷ 3600 × cyklustid ÷ kaviteter.

How to check the answer

Pris pr. 1.000 emner ÷ 1.000 skal give stykprisen.