Injection Moulding


The combination of polypropylene and injection moulding has lead to the creation of a truly astonishing range of applications. HMC Polymers offers a complete portfolio of types and grades for all applications and processes, including injection moulding.

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Injection Moulding Guide

Please note
The following technical information is provided for convenience only, without any warranty or guarantee of any kind. Users are encouraged to verify all information provided independently to their reasonable satisfaction. This includes any technical information, specific recommendations, testing, or analysis, including, without limitation, any information regarding moulding processes, or the selection of a product for a specific use.


Settings


Factors affecting overall performance

Barrel temperatures
The setting of the barrel temperature ideally increases gradually from the rear to the front of the barrel and should be highest at the nozzle of the extruder. This should ensure a proper melting and polymer mixing, especially when pigments are added to tint the moulded parts. The barrel temperature settings largely depend on the viscosity of the PP in use (MFR), the mould design (single or multi-cavity), the runner system in use and the complexity and length of the flow path. Barrel temperatures at the nozzle can start from 200°C up to 300°C.

Hot runners
Hot runner systems are used for polypropylene for the production of large-sized, multi-gated mouldings, such as automotive parts or appliances. A hot runner system maintains the polymer melt at approximately the same temperature and viscosity as the polymer in the barrel and therefore shows several benefits above cold runner systems:

  • Reduced or no scrap and therefore less recycling
  • Less sensitivity to the requirements of balanced runners
  • Reduction in material shear
  • Increased consistency of injected polymer volume injected into the mould
  • Reduced moulding time and therefore cycle time reduction
  • Improved part surface aesthetics
  • Decreased tool wear

A disadvantage of hot runners is increased cost in the system because the tool design becomes more complex both in manufacturing and operation.
The hot runner temperature setting should be in the range of the melt temperature setting in the barrel.

Cushion

The melt cushion is the material at the front of the screw when the screw is in the forward position. Always injection mould with a melt cushion of 6.35 mm to 12.7 mm to allow the part to pack out evenly. A pressure loss can result if the cushion is too high, and the parts will not mould consistently.

Mould temperatures
The mould temperature affects the final properties of the moulded item. Varying the mould temperature allows a variation of optical appearance as well as mechanical properties, e.g. for good surface appearance of the part, mould temperatures should be reduced. However, the most suitable mould temperature will be determined by the mould, part dimension and moulding cycle.


Barrel temperatures

Typical barrel heat temperatures can range from 200°C to 280°C, depending on the screw design, flow distance and part configuration as well as the viscosity of the selected resin. By changing the barrel temperature, e.g. to lower temperatures, the mixing capability of the screw can be improved, however this may also lead to longer cycle times.


Tonnage

HMC Polymers recommends 2-3 tonnes per square inch of projected area. Parts with long flow lengths or reduced wall thickness may require additional tonnage. Consult your HMC Polymers Technical Service contact for additional information.

Injection pressure

There is a corresponding injection pressure for each injection stage. For two-stage injection moulding, the first stage pressure ranges from 30 to 100 bar, secondary injection pressure is normally 40-60% of the initial stage. Multi-stage injection allows for flexibility within the above ranges depending on part geometry.

Injection speed

The speed will depend largely on the surface/volume ratio of the moulded part. Injection speeds can normally range from as low as 10 mm/sec to up to 80 mm/sec. Injection speed is usually varied to improve the surface appearance to the parts.

Back pressure

Back Pressure, along with screw speed, influences the uniformity of the melt temperature. Low back pressures of 3.5 bar but as high as 20 bar may be needed depending on resin in use and part design. In short barrels or worn equipment, higher back pressures may be required.

Screw designs
When determining screw design requirements for moulding polypropylene, the typical design used is a single-stage, general purpose type with an L/D (length to diameter) ratio of 16:1 to 24:1, and a compression ratio between 2.5:1 and 3:1.

Screw Speed

50-100 rpm is generally used in moulding PP grades from HMC Polymers.



Mould Design

The importance of mould design and its influence on fundamental requirements such as shape, mechanical properties and appearance cannot be overstated.

General principles

A number of basic recommendations can be made. Some of these are applicable to almost all moulding operations, irrespective of the PP grade being processed.

  • Provide an adequately dimensioned feed and runner system – this is a general requirement applicable to all moulding operations
  • Design the feed and runner systems so that melt accumulation and “dead spots” are avoided and that material flow is smooth and even

  • Pay careful attention to both the design and the location of the gates

  • Design the mould cooling system to ensure adequate and even cooling of all parts of the component – particularly applicable to the production of components with complex shapes, such as crates and boxes made from PP

  • Ensure that the metals from which the various parts of the mould are constructed are of suitable hardness. This is important in all moulding operations, but is particularly relevant when high locking forces are being applied, typically during the production of PP containers with thin side-walls and long flow-paths

  • Ensure that any individual core and cavity inserts are designed and located in such a way as to minimize deformation during mould filling. Once again, this applies to the production of PP containers with thin side-walls and long flow-paths

  • Select the type of ejection system to be used in accordance with the design and shape of the component to be produced

  • Provide suitably located and adequately dimensioned vents in order to facilitate trouble-free evacuation of air during mould filling

  • Provide suitable tolerances to all dimensions in order to accommodate mould shrinkage
- Wherever possible, avoid unnecessarily abrupt changes in cross-section, and in particular, avoid all notches
     
Cold Runner/Gate Design

Preferred design and dimensions

Based on our experience with mould designs and material performance, we recommend that cold runners be generally cylindrical, connecting to the gate by a space of triangular cross section (“tear drop”).

It is important that sufficient distance be maintained between the part and the runner to permit sufficient metal between the runner and part to prevent heat defects.

(Left) Preferred design and dimensions    (Right) Alternative runner design and dimensions

Alternative runner design
 and dimensions
An acceptable alternative design features a cold runner of trapezoidal cross section. This layout is viable if the triangular section leading to the gate is sufficiently long to avoid bad steel conditions near the gate.

Undesirable runner designs

Designs that feature little metal between the runner and the part are difficult to cool. The runner will keep the gate and nearby part warm, resulting in difficult gate freeze and poor part quality.


Subgate/Cashew gate designs

Subgates and cashew gates have been used with great success on many resins from HMC Polymers. The dimensions given have been used in production. Deviation should be reviewed by HMC Technical Service.

(Left) Subgate dimensions    (Right) Cashew gate dimensions

Material passing through a cold runner during the the filling cycle forms a frozen layer at the mould wall. Round cross sections for cold runners minimize contact with the mould surface creating the optimal section for flow. The diagram below shows various cross sections of cold runners with assigned rankings.

(Above) Cold runner sections and rankings

Preferred vent design

Proper venting of an injection moulding tool facilitates material flow, which minimizes knit lines and ensures complete packing of the tool. HMC Polymers recommends that vents be located at regular intervals around the perimeter of the part. An acceptable vent is rectangular in cross section, with a width of 12.7 mm, extending from the edge of the part to the outside of the tool.

(Left) Side section view   (Right) Front section view

For the first 6.5 mm, the depth should be 0.025 to 0.05 mm, to allow the escape of air but to inhibit the passage of material. Thereafter, the depth should be 0.5 mm or greater, to ease airflow. A typical vent pattern is shown here, with the vents cut into the tool cavity at the mating face.