Chapter 10

Plastics Parts Manufacturing Part-2

Plastic Parts Manufacturing Processes

Injection Molding

Compression Molding

Blow Molding

Extrusion

Thermoforming

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

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

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

Injection Molding

Injection Molding

Injection Molding

Injection Molding

Injection Molding

Injection Molding

Injection Molding

Injection Molding

Injection Molding

Injection Molding

Moldflow® Simulation Software

Injection Molding Defects: Incomplete fill

Injection Molding Defects: Sink marks

Cost Estimating

Cost components of an injection-molded part

Material cost

Setup cost

Molding cost

Labor cost

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.

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.

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

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

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

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

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

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

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

Compression Molding

Compression Molding

Compression Molding

Compression Molding

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

Extrusion

Extrusion

Extrusion

Extrusion

Extrusion

Extrusion

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

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

Injection Blow Molding

Extrusion Blow Molding

Blow Molding

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

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

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)

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

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

Thermoforming

Thermoforming

Thermoforming

Thermoforming

Thermoforming

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.

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

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.

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