ch 10 plastics mfg lecture 2 kc
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Chapter 10
Plastics Parts Manufacturing Part-2
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Plastic Parts Manufacturing Processes
Injection Molding
Compression Molding
Blow Molding
Extrusion
Thermoforming
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Injection Molding Most Common Process – used mainly for
thermoplastics Major Advantages:
Complex shapes (shape control) Dimensional accuracy High production rates possible Cost effective for high volume parts
Major Disadvantage: Mold costs are high
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Injection Molding1. Most common process
2. Makes formed parts from polymer materials
3. Material is fed through barrel that houses a screw and heaters
4. Rotating screw advances material toward tip into a pressure chamber
5. Heaters used to heat and soften material
6. Injects material into a mold
7. Solidifies into shape of cavity
8. Mold is opened – parts removed by ejection pins
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Injection Molding Design Considerations
Wall design- Uniformity in thickness is important
Ribs- Follow design guidelines to avoid sink marks
Radii- Allow for minimum radii
Inside corner – 25% of wall thickness Outside corner – 125 % of wall thickness
Taper and draft angles- Facilitates part removal
Inserts
- Avoid sharp corners Undercuts
- Adds complexity and cost to injection mold
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Injection Molding
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Injection Molding
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Injection Molding
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Injection Molding
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Injection Molding
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Injection Molding
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Injection Molding
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Injection Molding
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Injection Molding
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Injection Molding
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Moldflow® Simulation Software
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Injection Molding Defects: Incomplete fill
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Injection Molding Defects: Sink marks
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Cost Estimating
Cost components of an injection-molded part
Material cost
Setup cost
Molding cost
Labor cost
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Material Cost:
The first step in determining the material cost is the estimation of volume of the material the part will use, i.e. in cubic inches of the material
The next step is calculate the weight of the material.
Weight = Volume * Density of the material
The material cost is the product of part weight adjusted for yield and scrap by cost per lb of the material.
Setup Cost:
The unit setup cost of the material is the total setup cost divided by total number of parts.
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Molding Cost:
Depends on the machine size and estimated cycle time.
The hourly machine rate includes overhead allocation
Labor Cost:
The labor cost is determined from the operator rate and the unit time to produce a part.
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Molding Cost: From table 10.2, the recommended machine size is 700 tons and the average hourly machine rate is $100/hr. From Table 10.3 the cycle time is 60 sec or 60 shots per hour so the molding cost is:
Material weight in 4 cavities2.25 lb/part * 4 parts = 9.0 lb = 144 oz
Allowing for 10% scrap you need to mold 2,222 parts: (2,222 parts / 4 parts/shot) / (60 shots/hr) = 9.26 hours
From table 10.2 for 144 oz part the machine size = 700 ton and the average machine rate = $100, therefore:Molding cost = ($100/hr * 9.26 hours) / 2,000 parts = $0.463part = $0.46/part
Or - Molding cost = $100/hour * 60 sec/3600 sec = $1.667/molding cycle
Molding cost per part = [($1.667/cycle)/(4 parts/cycle)] * 1/(1-.1) = $.463
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Labor Cost:
Labor cost = ($24/hr * 9.26 hours) / 2,000 parts = $0.111/part = $0.11/part
Total unit cost = $6.75 + $0.20+ $0.46 + $0.11 = $7.52
Total production time = 9.26 hours + 2 hour setup = 11.26 hours
10.6.1 Page 206-207. Two errors in book not taking into account scrap for molding cost ($.04) and labor cost ($.01). The book has total cost as $7.47. Differential 0.05
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PROBLEM 8
Given a 25 in3 injection molded plastic part with a wall thickness of 0.15 in. Material cost is $2.00/lb and the specific gravity is 1.4. Material scrap rate is 20%. Yield is 80%. The setup cost is $200/hr and the setup time is 1 hour. Labor rate is $20/hr. The lot size is 1000 parts using a 4 cavity mold.
Determine the unit cost.
Part cost = material cost + set-up cost + molding cost + labor cost
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Material Cost:
Part material weight = 25 in3 * 1.4 * 0.036 lb/in3
= 1.26 lb
Part weight adjusted for yield & scrap= 1.26/(.8)*(1/(1-.2)) = 1.969 lb
Material cost (including yield & scrap adjustments)= 1.969 lb * $2.00/lb = $3.94/part
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Molding Cost:
Material weight in 4 cavities1.969 lb/part * 4 parts = 7.88 lb = 126.08 oz
From Table 10.2: machine size is 700 tonsAverage machine rate = $100/hr
From Table 10.3: cycle time = 38 sec (t = 0.15 in) or 95 shots/hr
Allowing for 20% scrap you need to mold 1250 parts with a 4 cavity mold: (1250 parts / 4 parts/shot) / (95 shots/hr) = 3.3 hours
Molding cost = ($100/hr * 3.3 hours)/1000 parts = $0.33/part
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Set-Up Cost:
Unit set-up cost = ($200*1)/1000 = $0.20/part
Labor Cost:
Labor cost = ($20/hr * 3.3 hours) / 1000 parts = $0.066/part = $0.07/part
Total unit cost = $3.94 + $0.20+ $0.33 + $0.07 = $4.54
Total production time = 3.3 hours + 1 hour setup = 4.3 hours
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Compression Molding Used mainly for thermosetting plastics Major advantages:
Low tendency of distortion and warpage High degree of part density
The process: Plastic (powder or tablet) placed in heated cavity Mold closed under pressure May be heated by steam of electric heating coils Binding agent could be added – reinforcement
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Compression Molding
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Compression Molding
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Compression Molding
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Compression Molding
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Extrusion
Continuous process for uniform cross sections
Applicable to both sheet or profile extrusions
Thermoplastic material is extruded through a die
Tooling is inexpensive
Two or more polymers can be joined through co-extrusion
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Extrusion
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Extrusion
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Extrusion
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Extrusion
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Extrusion
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Extrusion
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Blow Molding
Preferred process for making hollow shapes such as containers, bottles, automobile fuel tanks, etc.
Compare to injection molding: Dimensional tolerances are less Molds usually cheaper More easily removed from molds
Processes can generally be characterized as: Injection blow molding Extrusion blow molding
Can require some post-processing
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Blow Molding
The process:Place heat softened plastic in a 2-piece tube
die moldClose end of mold Inflate with compressed air – plastic takes
shape of moldOpen mold and eject product
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Injection Blow Molding
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Extrusion Blow Molding
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Blow Molding
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Thermoforming
Process uses pre-manufactured thermoplastic sheet that is heated to a forming temperature
Continuous or cut sheet
Forming - specific shape with vacuum or pressure
Low mold costs
Product design limitations, i.e. deep draw or a small radius may overstretch the sheet
Scrap can be recycled
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Thermoforming Industry
Total North America both thin and thick is over 10 billion dollars
Over 150 thin gauge thermoformers (60% proprietary products)
Over 12 thin gauge each over $100M Over 250 thick gauge thermoformers (NA) Thick gauge – mostly custom formers Thick gauge three has sales over $100M
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Basic Thermoforming Products
Thin gauge – less than .06 inches: disposable cups, containers, trays, other products for food, medical, etc. Basically applications – rigid and semi-rigid disposable packaging. (Disposable)
Thick gauge – greater than .12 inches: usually already cut to final dimensions. Parts used as cosmetic surfaces on permanent structures – autos, refrigerators, shower enclosures, electrical and electronic equipment. (Permanent)
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Thin-Gauge Thermoforming
Most common method high-volume, continuous process – a plastic sheet is fed from a roll or an extruder into set of indexing chains (pins or spikes)
Transport the plastic through an oven for heating to forming temperature. Indexes into a form station for a mating mold and pressure-box close onto mold with pressurized air to form plastic to detailed shape of the mold
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Thin-Gauge Thermoforming
After a short form cycle – a burst of reverse air pressure from vacuum side of mold as forming tool opens (air-inject).
Stripper plate – may also be utilized on the mold for ejection of more detailed parts.
Sheet containing the formed parts indexes into a separate trim press for part trimming
Trimmed material – recycled
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Thermoforming
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Thermoforming
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Thermoforming
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Thermoforming
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Thermoforming
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Heavy Gauge Thermoforming
Same basic process as Thin Guage Drape the heated sheet over a mold Typically use vacuum only to form Some use 2 halves of mating form tooling and
include air pressure to help form Ofter need hand-worked after forming for
trimming to final shape or additional operations: drilling, cutting, finishing, etc.
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Plastic Reinforcing and Composites
Used to significantly increase the mechanical properties of plastic parts (stiffness, toughness, tensile/compressive strength, resistance to cracking, fatigue, impact, abrasion.
Reinforced plastics have short, randomly distributed fibers in the polymer matrix
Composites are constructed of unbroken fiber strands or matsComposites have excellent structural and load bearing properties
Thermosets are commonly used
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Homework 6 due Tuesday, February 16
Two problems
What is material, setup, mold, labor and total unit cost and what is the total production time for A and B below?
Problem 1: In problem 8, change cost of material to $2.75/lb, specific gravity from 1.4 to 1.7, the mold has 5 cavities, the part has a maximum wall thickness of .10 in, the setup cost is $240/hr and the labor rate is $16/hr, yield is 75%, material scrap rate is 10%, the setup time is 2.5 hours and the lot size is 8,100 parts.
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PROBLEM 2
Problem 8, change cost of plastic from $2.00 to $3.00 and change wall thickness to max .19 in, density is .06 lb/in3 and the material scrap rate is 20%, yield is 70%, mold has 3 cavities and using 80% new material and 20% of reground scrap material (cost of material/lb changes), what is the unit cost of each part? Assume the cost to regrind scrap parts is $0.30/lb. Lot size = 2,000 parts. Setup cost = $300 and takes 3 hours. Labor cost = $20/hr.
Determine the unit cost and the total production time.
Part cost = material cost + set-up cost + molding cost + labor cost