The plastification unit is the core component in an injection moulding machine and its most important functional element. The plastic is conveyed, plasticized, homogenised, metered and injected by the plastification unit. It therefore provides a significant contribution to product quality. The following requirements are placed on the plastification unit:
- Sufficiently high plastification flow rate
- Constant material supply over entire screw stroke
- Good pull-in characteristics
- Good residence time characteristics
- Good thermal homogeneity of molten material
- Good mechanical homogeneity of molten material
- High reproducibility
- Favourable operating characteristics in terms of energy consumption
- Wide application range
- Cost-effective service life
The significance of these requirements can differ greatly depending on the application. The selection, dimensioning and design of the plastification unit are therefore crucial factors for efficient and cost-effective production.
In order to best satisfy the variety of applications with their different priority of requirements, Sumitomo (SHI) Demag has developed solutions for the individual plastification unit components depending on their requirement profile. This enables Sumitomo (SHI) Demag to offer the best plastification unit for each specific application. This includes, for example, specialised solutions for high-speed applications especially on the EI-Exis SP series, colouring solutions for master batch or liquid pigmentation as well as solutions for processing semi-crystalline engineering plastics with high melting enthalpy.
The plastification unit has prolonged and intense contact with the plastic material processed and is therefore subject to complex and frequently superimposed wear effects. Various wear effects are shown in Figure 1 “Types and mechanisms of wear in plasticising units”.
Figure 1 “Types and mechanisms of wear in plasticising units” [source: Wear in plastics processing, G. Menning]
There are three different wear mechanisms to be distinguished:
Abrasion is caused by fillers and reinforcement substances in the plastic. Nearly all plastics contain various fillers and reinforcement additives to adapt the properties of the plastic to the specific requirements. Many of these particles are extremely hard. The relative movement between the particles contained in the plastic and the plastification unit components results in abrasion of the component material. In addition to hardness, the geometry of these particles has a crucial effect on abrasion.
Figure 2 “various filling and reinforcing materials”<br>[source: Wear in plastics processing, G. Menning]
According to DIN 50 900 corrosion is the reaction of a metallic material with its environment, resulting in a measurable change in the material and frequently leading to corrosion damage. This reaction is usually of an electro-chemical nature; however, it may also be of a metal-physical nature. Corrosion is caused by chemically reactive additives and decomposition products of polymers. In particular, the variety of different flame retardants on the market and their advancement in compliance with Directive 2002/95/EC for limitation of use of certain hazardous substances (RoHS) result in continuously new challenges.
Adhesion or adhesive wear in the plastification unit usually occurs in the face of contact between the screw and barrel. In theory, ideally the plastic will act as support, preventing contact between the two components. In practice, contact between these two components can occur due to various operating states such as starting up or shutting down the machine. Moreover, high pressures and the intrinsic weight of the screw can lead to deflection of the screw. Owing to high contact forces, this can lead to galling between the two components. The resulting adhesive forces can be greater than the internal strength of one of the two materials. The shear plane can shift from the original contact surface to the less solid component and material is transferred from one component to the other. This results in deep scoring and elevated zones which promote further adhesive wear. In the worst case, these effects can lead to the screw seizing up. The plastic material processed and the process itself has an influence on this wear mechanism.
The individual wear mechanisms often occur simultaneously in practice. Especially abrasion and corrosion frequently occur together when processing reinforced engineering thermoplastics.
The most important influential factors on wear are shown in Graph 3 "Influential factors on wear".
Figure 3 Influential factors for wear
The standard general purpose plastification unit with 3-section screw and ring non-return valve covers a particularly wide range of applications, making it very popular. Different requirements are placed on the unit in regard to wear resistance due to varying additives, fillers and reinforcements used. For this reason this plastification unit is available in different options; (see Table 1 "Wear classes for standard screws and cylinders"). These options differ in terms of component material specification. The components such as screw and non-return valve are designed for a wide range of applications.
The plastic can be selected only to a limited extent in terms of criteria for minimising wear, since clearly defined material properties are required for moulded part.
The processing conditions also significantly affect wear rates and thus the service life of the plastification components. This includes factors such as process parameters, capacity utilisation of the plastification unit, residence time in the plastification unit, recommended moisture content of the plastic, suitable material feed, absence of foreign particles in the granulate, optimum master batch percentage and many more.
Experience shows that certain process parameter ranges and processing conditions are ideal for the most important groups of plastics. These are listed in Table 2 "Process parameters and processing conditions“. Deviations from these general recommendations may occur due to continuous developments in the plastics sector. For this reason Sumitomo (SHI) Demag always recommends consulting the material manufacturer regarding suitable process parameters and processing conditions. Sumitomo (SHI) Demag also offers application-related support.