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Structural Foam Injection Molding Process

Monday, April 12th, 2010

Foam Molding Technology

The Structural Foam Process is a low pressure injection molding process where an inert gas is introduced into melted polymer for the purpose of reducing density and hence weight of the finished product.

Structural foam molded products have cellular cores surrounded by rigid, integral skins. Foaming agent (NI, CO2   or CBA) is introduced into the polymer melt stream, creating a homogenous mixture of polymer and gas.
The mixture is short-shot injected through nozzles into the mold in a volume that is less than the amount required to mold a solid part. Injection pressure and expansion of the polymer/gas mixture fills the mold.

A porous skin is formed when the melt contacts the cold surface of the mold. The expanding polymer/gas mixture forms the cellular core.

The expanding gas provides the final pack and hold pressure. Once the plastic gas mixture enters the mold cavity, the gas expands (i.e. foams), filling the cavity and forming cellular structures within the part. The finished part is typically 10 – 30% less weight than an equivalent solid part.

Advantages over alternative methods

  • Part weight reduced 10% to 30%
  • Density Reduction, hence resin savings
  • Low cost N2 or CO2 – much less expensive than chemical blowing agents (CBA’s)
  • Large part molding with low clamp force requirements
  • Mold Cavity Pressure; typically 200 – 600 psi ( 14 -41 Bar )
  • Lower energy costs vs. other IM processes
  • Lower cost aluminum molds vs. high pressure IM machines
  • Faster cycles due to better heat transfer of aluminum
  • Thick wall parts from 0.125″ – 0.500″ ( 3 – 12 mm )
  • Stiffer parts at the same weight as IM as a result of cellular foam structure
  • Complex parts without sink marks
  • Higher impact strength than thinner wall IM
  • Parts can be sawn, screwed, nailed or stapled like wood

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Blow Molding Process Comparison

Wednesday, April 7th, 2010

If you are new to blow molding or if you have been in the industry for 30 years, it can still be confusing as to what type of blow molding machinery or process is best for your next application. I have been fortunate in my career to design and build machinery for every type of blow molding process. For your information I consider there to be 7 primary machinery processes for packaging applications.

Shuttles (Continuous extrusion)
Wheels (Continuous extrusion)
Injection Blow
Injection Stretch Blow (1 Step)
Reheat Stretch Blow (2 Step)
Reciprocating screw (Intermittent extrusion)
Accumulator Head (Intermittent extrusion)

Some people may refer to them differently and some may argue several of these should be grouped together, but for sake of discussion, we will use this grouping. Further definition of the technologies, along with pros and cons for each process, can be found in the links above.

  Shuttle Wheel Injection Blow Injection Stretch (1-Step) Reheat Stretch (2-Step) Recip Screw Accumulater HD
HDPE Yes Yes Yes No No Yes Yes
PP Yes Yes Yes Sometimes No Yes Yes
PET No No Yes Yes Yes No No
PVC Yes Yes Yes No No No No
Styrene No No Yes No No No No
Multilayer / coinjection Yes Yes Difficult Difficult Difficult No Difficult
IML Yes Yes No No No Difficult Difficult
Handleware Yes Yes No No No Yes Yes
Injection Finish Good Fair Excellent Excellent Excellent Fair Good
In-machine Deflashing Yes No N/A N/A N/A No No
Flashless Process No No Yes Yes Yes No No
Small Bottle<100 ml

low/med vol.

 Yes No Yes No No No No
Small Bottle<100 ml

high vol.

 Yes Maybe Yes No No No No
Med. Bottle(1 litre)

low vol.

 Yes No Yes Yes No Yes No
Med. Bottle(1 litre)

high vol.

 Yes Yes No No Yes Yes No
Large Bottle(4 litre)

low vol.

 Yes Yes No No No Yes No
Large Bottle(4 litre)

low vol.

 No Yes No No No Yes No

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Blow Molding Process Comparison: Injection Stretch Blow (1 Step)

Wednesday, April 7th, 2010

Injection Stretch Blow (1-Step)

This process is almost entirely dedicated to PET and more recently, PP applications. The process is very similar to Injection Blow except for two areas: 1) the preforms are not transported on core rods but instead held by the neck finish and 2) during the blowing process, rods stretch the preform prior to blowing to orient the material. For materials such as PET this biaxial orientation substantially increases the physical properties to weight ratios. Major manufacturers include Aoki and Nissei.

PROS:

  • Scrapless process meaning no flash to trim and no regrind
  • Allows for biaxial orientation for strength and clarity
  • High quality injection molded neck finish
  • Typically suited for .5 litre up to 20 litre
  • Capable of PET non-carbonated beverage containers
  • Capable of lower volume production applications
  • Does not require separate preform mold and machine

CONS:

  • High tooling cost
  • Minimum cycle typically 18-20 seconds
  • Can not produce carbonated beverage containers (Not enough biaxial orientation due to high preform temperature at blowing)
  • Not suited for polyolefins
  • Can not blow handleware
  • Difficult to run coinjection or multilayer

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Blow Molding Process Comparison: Injection Blow

Wednesday, April 7th, 2010

Injection Blow Machinery

These machines inject mold a preform onto core rods and then index the core rods to a blow station to blow the container. Machines are typically 3-4 stations to allow for condition and part removal. They are utilized extensively for very small containers such as pharmaceutical and hotel shampoo type containers. Major manufacturers are Jomar, Procrea and Milacron.

PROS:

  • Scrapless process meaning no flash to trim and no regrind
  • Typically suited for containers .5 litre and smaller
  • Capable of running a wide range of materials
  • High quality injection molded neck finish

CONS:

  • Very high tooling cost per container typically cost 40-50% of the machine cost
  • Difficult to run coinjection or multilayer
  • Minimum cycle time typically 15 seconds
  • Can not do handleware

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Blow Molding Process Comparison: Reheat Stretch Blow (2 Step)

Wednesday, April 7th, 2010

Injection Stretch Blow (2-Step)

This process is also almost 100% for PET applications. The processes utilizes preforms made on a stand-alone injection molding machine, stored and then reheated and stretch blow similar to the 1-step process above. Here, however, the preforms are blown at a lower temperature allowing the maximum amount of biaxial orientation and therefore the maximum strength to weight ratios. The reheat process can be for ultra high output type applications such as carbonated soda bottles while other machines can be smaller for medium to high production requirements. Major machinery manufactures include Sidel and Krupps.

PROS:

  • Can be very high speed production (40,000 containers / hour)
  • Produces a very high strength-to-weight ratio container
  • Can purchase or make preforms
  • Primarily PET material
  • Machines for low and medium volume application are becoming available

CONS:

  • Can not mold handleware
  • Not capable of polyolefins
  • Must have a separate preform mold and injection molding machine or purchase preforms
  • For low volume production, if special design preform is required, cost can be expensive

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