1. Mold Design Guidelines Chapter 4
Professor Joseph Greene
All rights reserved
Copyright 2000

2. Mold Requirements Before starting to design a mold
Molding characteristics
Anticipating mold cycle
Studying the product
Product drawing
Design for manufacturing
Shrinkage
Typical checklist for estimators and designers

3. Before starting to design a mold
Mold designers start the mold build process with
Part drawing, sample prototype part, math data (model)
Machine specs for production, number of cavities requires, type of plastic
Need additional information
Molding characteristics of plastic, shrinkage, draft angles, finish,
Production volume for job, molding cycle desired, tolerances,
Gate location, weld lines, ejector locations,
The above items affect the quote for the tooling job
OEM (Original Equipment Manufacturer) is the customer who wants a mold built so he can produce parts.
OEM usually requests 3 quotes on a tooling job and picks the least expensive on or the one will the best quality with the lowest price
Mold costs are estimated based upon the latest part design print.
Changes to the part design would require a new quote.
Some molds are low-balled in price, so that tool maker will keep his people busy and try and get more business from OEM, or the tool maker expects changes
Some molds have costs due to changes exceed the original cost of the mold

4. Molding Characteristics of Plastics
Flow characteristics that affect mold design
Flow characteristics
Easy flowing materials usually present no molding problems (PP, HDPE, LDPE)
Stiff materials usually have tighter processing window and require higher molding pressures
Melt processing temperatures
Higher the temperature, the more important the cooling design
Material degradation
Every plastic is to some degree heat sensitive and subject to degradation when exposed to high temperature over time.
There exists a max temperature for each plastic that it can be exposed to over a certain amount of time.

5. Material Degredation Fig 4.1 Stay out of degrading area
Temp
time
Degrading T1 T2 t1 t2
Fig 4.1
Stay out of degrading area
Heat sensitive materials, PET and acetates, can be molded at high temperature but for short times
PVC materials(VERY heat sensitive) are molded at low temperatures and have a narrow OR short time span.
Acetal gives off poisonous gases (formaldehyde)
All materials show degradation by changing color to yellow or brown or black.
Degraded material has poor properties

6. Anticipated Molding Cycle
Desirable to build a mold with the shortest possible cycle time.
Similar parts can have different cycle time:
Differences in grades of plastic
Wall thickness, draft angles, gating methods
Analysis packages can help estimates cooling time, which is an indication of cycle time. Cooling time is 50% to 75% of cycle time

7. Studying the Product Check the Product Drawing
Clarity: all views shown with holes and cutouts listed and BOM (Bill of Material or material list)
Projection: IS drawing in 1st angle (European) or 3rd angle (American) projection?
Tolerances: Most important area to check. Can you meet the tolerances in the design. Are all areas toleranced? Get a signed print with a clear understanding of acceptable tolerances.
Product Use: Where and how will the product be used? Helps designer understand certain critical areas such as fit and finish, physical strength and help him determine the gate location and runner system
Outside Edge
Inside Corner
Bad
Good
Bad
Good

8. Studying the Product Check the Product Drawing
Notes on drawing:Scrutinize all notes on product drawings. Changes can be made for tolerances, draft angles, finishes, etc.
Draft angles: Greater than 1º per side is needed to pull part out of mold or for ejection of internal ribs.
Some designs ask for zero draft or negative draft by using slides and ribs, or collapsible cores. These are expensive features.
For thin, deep ribs with small draft angles requires special ejection features
Ejector pins should be at bottom of ribs to insure ribs do not break and stay in the core. Ejector pins should have a narrow flat section where they contact the rib, which is expensive.
Round pins are usually too small and fragile. Use a larger pin.
Fig 4.5
Shrinkage: Clearly defined who is responsible for specifying
Customer sometimes assumes responsibility
Molds can be used for different materials, but the parts will be smaller with the higher shrink material. PS 0.6% can be used for PP at 1.5%

9. Typical Checklist for Estimators and Designers
Machine Specs
Mold mounting data: Tie bar spacing and size, platen specs, hole patterns, ejector locations
Machine nozzle data: Shape, nozzle opening and length, flow
Injection unit: Shot capacity, injection speed, etc.
Clamp: force, shut height (min/max), stroke, ejector stroke,
Mold Cooling: pressures, fittings, flow
Air and hydraulic function: pressure and fittings
Heat controls for mold: availability, automatic?, voltage, Amps
Product removal: manual or automatic, free fall, chute, conveyers

10. Typical Checklist for Estimators and Designers
Product Specs
Production volume
Estimated cycle time
Tolerances
Notes and draft angles
Molding material and shrinkage
Runner system: cold or hot, 2 or 3 plate, gate type and location and quantity, appearance
Flow path and weld lines
Ejection: type with pins or full ring or partial ring, Bars, air only, manual or automatic, cores
Finish: Polish specs, texturing and engraving
Art work: yes or no and type
Cavity numbering: identification and size and location

11. Design for Manufacturing
Simplicity
Standardization of materials
Standardization of design of the product
Liberal tolerances
Use of most processible materials
Teamwork of Mfg personnel
Avoidance of secondary operations
Desing to the expected level of production
Utilizing special process characteristics
Avoid process restrictiveness

12. General Design Rules Simplify design (KISS principle)
Design for low labor costs
Avoid generalization statements on product
Dimensions should be from points on part not in space
Dimensions should be from one datum
Once functional requirements re met, part shouldbe lighter and lower cost
Design for general purpose tooling
Avoid sharp corners
Design for multiple manufacturing operations without repositioning
Avoid stepped parting lines for cast or molded parts