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

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

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

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.

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

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

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

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%

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

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

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

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