By James Jenkinson, Cinpres

Description of Basic Assisted Injection Moulding Processes.
Full shot moulding
A full shot moulding is filled to at least 100% of mould cavity volume with plastic during the plastic filling stage.  Gas is then injected and flows into the component to compensate for the plastic shrinkage.  The rate of flow of gas into the component is controlled by the rate of plastic volume shrinkage.  The amount of gas flow into the component is limited by the shrinkage of the material. To create a larger gas channel, it is necessary to reduce the amount of plastic we inject, commonly referred to as the Short Shot gas injection process.

Short shot moulding
Short shot mouldings are filled to less than 100% of mould cavity volume with plastic, gas is then injected to complete the filling of the cavity.  Once the cavity is filled the gas continues to flow as it packs the plastic.  The initial gas flow into the cavity is high and controlled by the resistance of the plastics being displaced towards the unfilled areas.  During packing the gas flow into the component is controlled by the rate of the shrinkage of the material as with Full Shot gas injection process. The short shot process has a number of limitations when applied to more complex components. In these situations overflow wells are used.

Advanced Process Options
The Plastic Expulsion Process (PEP)
Overflow wells allow the component to be completely filled with plastic and packed with plastic prior to gas injection. Gas is injected displacing the plastic out of the gas channels into specifically designed wells outside the component cavity. This added control over the process produces the best cosmetic surface, controls the size and concentricity of the gas core and guarantees that the gas channels are fully cored with gas.

Internal gas injection cycle times can be further improved by using liquid or gas cooling. The Liquid Cool process injects cooling water through the gas channels, cooling them rapidly. Cooling times can be reduced by up to 50%.

Liquid Cool (LC) Process
The Liquid Cool (LC) process builds on the PEP process to deliver shorter cooling times.  Plastic packing can then be used to optimise the surface finish. Gas is injected and after a delay, the path to the overflow is opened. The gas displaces the plastic into the overflow and starts packing the component. After a given delay time, water is injected into the cavity and the gas is released. Water flows through the component under pressure for a given time, before being displaced by gas, injected again from the fixed pin nozzle. In some cases the size of overflow wells can be reduced or eliminated by pushing the molten plastic from the gas channel back into a hot runner manifold.

External Gas Moulding (EGM)
EGM is a low pressure moulding process that uses pressurised nitrogen gas as the packing medium, to complement the injection moulding process. Pressured nitrogen gas is applied from the core side of the moulding to pack the component against the cavity. Gas packing pressure forces any sink to occur on the non visible side of the component. The gas compresses the plastic forcing sink to occur on the non visible side of the component. 

                          
TV Base after EGM: The “piano gloss” ‘A’ surface without sign of sink marks, belies the substantial rib structure on the ‘B’ surface.
In part, the ribs thickness is ratio is 1:1 with the 3.5mm wall thickness. With the TV base moulded in PC/ABS, the reduction in clamp
force and mould dimensions allowed the part to be moved from an 800 ton injection moulding machine onto a 500 ton machine,
with a 19% cycle  time reduction and an 8% reduction in material content.

EGM allows the designer to break the rules of injection moulding. With conventional moulding, wall section is often determined by the thickness of the ribs on the underside – typically, the rib must be less than 40% of the thickness of the wall section. Reducing the thickness of the wall section or increasing the thickness of the rib would normally result in sinkage on the visible surface.

By contrast, EGM gas pressure packs plastic into the intersection where the sinkage occurs forcing the sink to occur of the underside avoiding sink on the show surface.
This capability gives the part designer much greater flexibility in rib location and dimensions, frequently allowing thinner wall sections – with resulting material savings – without loss in the physical properties of the part.

With certain types of mouldings, it is also possible with EGM to achieve substantial cost savings through

• Reduced clamp force.
• Reduced cycle time.
• Reduced material volume.

Another benefit of EGM is the ability to mould to finer dimensional tolerances and with enhanced surface finish. The shorter plastic packing times and lower pressures lead to reduced moulded in stress, resulting in reduced distortion. The above features of the EGM process tend to mean that the cost and design benefits are most significant with larger part weights and flow lengths, where the opportunities for reducing machine size, material and cycle time have a greater impact.

An Automotive Door Pocket as an Example of Combining AIM Processes
Automotive door pockets often incorporate internal gas injection, to allow a thick section lip at the top of the map pocket to be moulded. The Full Shot process can be used but this will severely limit the size of the gas channel. The PEP process allows the component to be manufactured as an injection moulding with plastic filling and packing. Gas is then injected displacing the hot plastic out of the gas channel into the overflow well. Gas in then used to pack the gas channel as it cools. The size of the gas channel and the shape of the cross section determine the length of the cycle time. If the component is de-moulded to soon the gas channel can continue to cool causing distortion.

Adding liquid cool to achieve further cycle time savings
We can improve on the standard method and achieve faster cycle times by using a liquid or gas cooling process. After the gas has been injected and the gas channel is cored out we can circulate cooled gas or water through the channel enhancing the rate of cooling of the gas channel. This can dramatically reduce the cooling time of the gas channel, bringing it into line with the cooling time of the surrounding wall section.

EGM, allowing the wall thickness to be reduced whilst keeping ribs and bosses the same
Now we can achieve rapid cooling of the gas channel using liquid cool we can consider reducing the general wall section. As discussed before the thickness of the wall is usually limited by the size of the injection moulding ribs on back of the component. EMG allows us to break those rules. A small reduction in the wall section producing a significant material saving.

CLARE GOODALL  
Cinpres Gas Injection  
Tel: +44 (0)1606 839 800  
Fax: +44 (0)1606 839 801  
Email: Clare.Goodall@cinpres.com    
Web: www.cinpres.com