23. Molding

• Plastics can be categorized as follows,

 

23.1 Reaction Injection Molding (RIM)

• Basic Principle: A mold is brought together, and a multi-part thermosetting polymer is mixed and injected. After the part sets, the mold is opened, and the part is removed. Post curing may be used.

 

 

• Features:

Very little energy is required, most energy is provided by the chemical reaction.

Parts up to 100 lb produced.

Surfaces don’t have flow lines found in traditional injection moulding.

RIM materials tend to be low weight, corrosion resistant, and variable properties can be obtained by additives and ratio adjustment.

RIM machines use lower pressures, and are therefore less expensive.

Typical parts are; bus bumpers, large electronics cases, shoes, etc.

Fillers can be used to increase strength.

The basic process involves mixing the raw components.

 

• Metering and mixing are critical

special metering pumps are required for the components to maintain material properties.

 

mixing must thoroughly mix the components at high speed, and inject them without becoming clogged when the mixture sets.

 

 

• While the previous mix head is for a runnerless system, it is also common to have an extra component that is between the mix head and the part (the runner).

 

• An aftermix may also be used to increase mixing. A typical design will split the stream and cause it to impinge at 180°, then continue on to the mold.

• A comparison of the materials shows the advantages over a similar injection molded material.

 

 

23.1.1 References

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23.1 Becker, W. E., editor, Reaction Injection Molding, Van Nostrand Reinhold Co., New York, 1979.

23.2 Injection Molding

• Basic process: Heat a thermoplastic material until it melts. Force it into a hollow (cooled) cavity under pressure to fill the mold. When cool remove the finished part.

• Typical materials are,

nylon

styrene

ethylene

• A typical injection molding machine is seen below with the covers removed. Plastic pellets are poured in the hopper, and finished parts emerge from the dies.

 

• Injection system,

a material hopper acts as an input buffer

a heated chamber melts the material

an injector forces the now viscous fluid into the mold

• Previous mechanisms used an injection plunger.

• Current mechanisms use a reciprocating screw,

basically the screw extends from the hopper to the injection chamber.

along the length of the screw chamber, heater bands are used to melt the plastic.

as the screw turns, it moves raw solid plastic from the hopper, to the injection chamber. The buildup of pressure in the injection chamber forces the screw back until enough for a shot has accumulated.

the screw is forced forward to inject the plastic into the mold.

 

there is a contribution to melting by pressure that allows the temperature of the heating bands to be lower.

the purpose of the screw is to generate a homogeneous melt with little orientation in flow direction.

• Typical zones can be identified on the screw,

feed: a screw with large cavities to carry more material.

compression: the depths of the screw thread reduce, leading to elevated pressures, and pressure induced melting.

metering: small and uniform threads to provide controlled quantities. This also serves as a final mixing stage.

• Screws are often low/medium/high compression ratio as a result of the change of screw volume from the feed to the metering stages: screw selection will vary between materials, but a low compression ration screw will ensure good melting in most cases.

• Screws are nitride treated to improve tool life. Screws might also be made slightly smaller to compensate for thermal expansion when heated.

• Screws are often driven by electric or hydraulic motors.

• The heat capacity and melting point temperatures of various materials determine the energy required to melt the plastic and the energy to be removed for solidification (and for ejection).

• The volume of the injection chamber determines the maximum mold cavity size. The volume provided is often for polystyrene. When using other materials the volume can be corrected using the following formula. For example a 10 oz. shot,

 

• The mold is held closed with a certain clamp tonnage.

• As cycle times decrease, the plastic melt becomes less consistent.

• Each heating zone uses electrical heating bands with thermocouples, or pyrometers to control the temperature.

• When injecting, the mold is moved then clamped shut. The mold halves are mounted/clamped/screwed on two platens, one fixed, one moving. The stationary platen has a locating ring to allow positioning on the mold half over the injection nozzle. The moving half has ejector pins to knock out the finished part. Larger plates are found on larger injection molding machines.

• Injection molding machines pressure is calculated as injection pressure over an area in the mold. Consider the case where a mold with a 10 square inch mold is being filled in a 200 ton machine.

 

• The platens are actuated by hydraulic driven mechanisms. These are slow, but can exert great forces. In lighter presses other mechanisms can be used.

 

 

 

23.2.1 Hydraulic Pumps/Systems

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• A geared hydraulic pump is pictured below. Other types use vanes and pistons.

 

• Hydraulic systems use pumps to cause fluid flow. Resistance to that flow will allow pressure to build up. This fluid is directed through a systems with,

oil filters to clean

heat exchangers to cool oil

gages to monitor pressure

relief valves to release fluid when a maximum pressure is passed

a reservoir to collect uncompressed fluid

check and flow valves

• The hydraulic system drives pistons and other hydraulic actuators.

23.2.2 Molds

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• Injection molds are mainly made of steels and alloys steels. A simple mold is shown below.

 

Locating ring: guides the injection nozzle into the mold.

Sprue Bushing: where the injected material enters the mold cavities.

Clamp front plate: Secures the front cavity, locating ring, and other components to the stationary platen.

Front cavity: holds half of the negative of the shape to be molded. Guide pin holes are put in this plate.

Rear cavity: the mating half for the front cavity that completes the negative of the final part. Guide pins are mounted on this to ensure correctly aligned cavities.

Spacer Blocks/Rails: used to separate the rear cavity from the rear clamp plate.

Ejector housing: contains the ejector pins to knock the parts out of the mold and forces the cavity back when the mold is closed.

Rear Clamp Plate: Supports the rear half of the mold on the moving platen, and provides rigidity under molding pressures.

• Components to consider in mold design,

part design

material

machine used

• Factors that are often altered in the design are,

gating

runners

mold cooling

ejection

• Gating can be done a number of ways

 

• Runners carry the plastic to the injection gates and are often considered disposable or reusable. Typical runner systems are,

cold runner

hot runner

insulated runner

• Cooling systems allow rapid uniform cooling to increase cycle times, and reduce scrap. Typical techniques are,

water lines

baffles

fountains

thermal pins

• Ejection systems will push the part out of the mold when it is opened.

knockout pins

blades

stripper rings

air

hard stripping

23.2.3 Materials

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• Materials often come as raw beads. These can be mixed, colored, have other materials added, or reused.

• Quite often scrap parts are ground up, mixed with new materials and reused. But, caution is required to reduce contamination.

• Common materials are,

 

23.2.4 Glossary

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Barrel: the cylinder the injection screw sits in.

Cavity/Impression: The two or more hollow metal parts that contain the negative of the part.

Cold Flow: material that is too cool when injected will get a dull surface finish.

Core: a protruding (or male) mold component.

Crazing: a fine mesh of cracks.

Degating: separate parts from runners

Delamination: the surface peels off in layers

Dowels/Guide pins: used to mate mold cavities

Distortion: a warped molding

Dwell: a delay time after filling the mold

Ejector Pins: push the part out of the mold as it is opened

Feed: the volume of plastic injected into the mold as it is advanced

Flash: a thin flat section that has “squirted” out of the mold

Gassing: trapped gas marks and burns the mold

Gates: the entry port between the runners and the parts

Granules: the pellet form that raw plastic is delivered in.

Granulation/Grinder: will reduce parts to granules for reuse

Inserts: parts placed in the mold before closure and injection. These become an embedded part of the final product

Nozzle: the plastic is ejected through the nozzle to the mold.

Polymers: The chemical category of plastics

Powder: a finely ground material

Preheating: plastic may be heated before use to remove moisture contaminants

Purging: a few purging shots are made when changing the material

Ram: opens and closes the platens

Regrind: reclaimed plastic granules

Release Agent/Spray: A spray, such as silicone, can be sprayed into tight molds to ease part removal.

Runners: connect the gate to the sprue

Safety gate: the gate must close and shut the operator out for the press to close.

Shot: one injection of plastic

Short shot: insufficient plastic is injected

Shrinkage: reduction in size as mold cools

Sinking: Surface deformation on parts.

Sprue: excess plastic between the injector nozzle and the mold

Vent: A small gap that allows air to escape as it is displaced by molten plastic

Warped: Cooling stresses cause a part to twist, or warp, to a new shape.

23.3 Extrusion

• The basic process: plastic is melted and pushed through an extrusion die with a desired cross section. The plastic leaves the die in roughly the right shape. It is then passed through a sizing and cooling apparatus. Finally, for wound product, it passes through pullers, and onto a spool.

• Basically a screw, like that described in injection molding is used to melt the plastic and generate pressures.

• Some additive for plastics are,

white chalk, used as a filler

plasticizers, improve flexibility

stabilizers, improve light and heat resistance

pigments

lubricants minimize sticking and improve flow

• Typical extrusion conditions are, [Source, unknown]

 

• The typical extruder barrel is between 20:1 and 28:1 for length to dia., and typical pressures are 10 to 20 ksi.

• Typical motor characteristics are,

5 to 10 lbs of material per hour for each horsepower

the motor is geared down to drive the screw (often variable)

• Breaker plate,

at end of screw, and before extrusion die,

it increases back pressure for the screw

converts rotational to parallel flow

it stops unmelted plastic and debris

• Special heads can be used to coats wires, etc.

 

23.4 Problems

Problem 23.1 TRUE / FALSE: Plastic is melted in the hopper before it is extruded.

Problem 23.2 TRUE / FALSE: A reciprocating injection molding process has a screw that moves.

Problem 23.3 TRUE / FALSE: Steam is the most common method used to weld plastics.

Problem 23.4 The key tag shown below has a flat profile. There is a hole on the right hand side for the keyring, and a large hole in the main body of the ring.

a) Design and sketch a complete injection mold for the part. Indicate parting lines, ejector pins, runners, gates, and all other important features.

b) What important design features must be considered for a part like this?

Problem 23.5 List 5 trade-offs injection molding and reaction injection molding?

Problem 23.6 Describe the operation of an injection molding machine using figures and notes. This should describe the entire cycle from when plastic enters the hopper to when it is injected into the part.

Problem 23.7 Name the three zones on a plastic compression screw and briefly describe the function of each zone.

Problem 23.8 List the general advantages of thermo plastics over metals. List the disadvantages.

Problem 23.9 A six cavity mold has been designed for a 200 ton injection molding press. The press is no longer available, and the mold has to be put in a 160 ton press. How many cavities should be blocked off to ensure the mold will fill properly?

Problem 23.10 a) Describe the operation of the screw in an injection molding machine.

b) How would the operation of the screw change for a continuous extrusion machine?

c) What does the tonnage of an injection molding press refer to?

d) What types of cooling channels can be used inside an injection mold. Describe how they operate.

Problem 23.11 Why do powder metal parts change shape after processing? What can be done about it?

Problem 23.12 What are the advantages of injection molding over other processes.

Problem 23.13 What is the purpose of gates in an injection mold? What are the different types, and what are their advantages?

Problem 23.14 What are parting and flow lines?

 

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