Question:

In principle, how is the selection or assignment of the molded part geometries that determine the cooling time carried out?

Answer:

Injection molded parts are characterized by a wide range of geometric design variety and complexity. The program offers a wide range of options for describing the many geometric elements (molded part sections) of a molded part. Therefore, the user has the task to divide his concrete molded part into such sub-parts with sufficient precision. The part with the longest cooling time is then relevant for the cycle. This procedure can be quickly and reliably understood with the program, even by users with less experience.


Question:

When graphing the cooling time as a function of the mass temperature, a minimum of the cooling time was found at certain mass temperatures in individual cases. Is this physically correct?

Answer:

The body heat to be dissipated (enthalpy) plus heat of fusion is greater the higher the mass temperature. Therefore, a continuous increase in cooling time with increasing mass temperature is to be expected under otherwise constant conditions.


Within the software, however, the shear heat component, which extends the cooling time, is taken into account. This heat component is mainly generated by shear flow in the molded part edge zones during volumetric mold filling and is dissipated more quickly at higher melt temperature under otherwise constant conditions.


In individual cases, these opposing influences can lead to a mostly very flat cooling time minimum in the practical melt temperature range. The greater the mold contour temperature, the injection pressure and the surface/volume ratio of the molded part contour determining the cooling time, the greater the mass temperature of the cooling time minimum.


Question:

How reliably can thermodynamic molding data be calculated internally in the software for any combination of polymers and additives?

Answer:

The melt state is considered to be decisive for the cooling time calculation. Physically precisely defined addition rules exist for the basic data spec. volume, spec. heat capacity and heat of fusion. For the calculation of the thermal conductivity, many models of different complexity and complexity are offered in the technical literature. The calculation model selected internally in the software (not the Maxwell model) was successfully correlated with measurement results. The sufficient calculation accuracy for the thermal conductivity model is limited to approx. 60 vol. % additive content. A noticeable error margin can occur especially for crystalline additives with strongly direction-dependent material properties. Overall, a maximum deviation of ± 10 % is expected in relation to the cooling time calculation value.


Question:

According to which aspects is the demolding temperature determined internally in the software?

Answer:

The demolding temperature is one of the parameters with a major influence on the cooling time. Depending on the requirements, it describes an average temperature level of the molded part or its maximum temperature in the center of the cross section. These requirements are regulated differently for the sprue and the molded part. The molding-specific reference temperatures (glass transition temperature, melting temperature, flow temperature) are recorded in 3 different molding groups. For the molded part, the demolding temperatures are differentiated in 5-fold increments. The assignment is made taking into account the tempering conditions (additional cooling), the required manufacturing accuracy, the cross-section thickness, the molding material stiffness or hardness, the demolding conditions and the occurrence of sink marks. For sprue cooling, there is only a 3-stage allocation according to the demolding conditions and, for semi-crystalline polymers, additionally according to the cross-sectional thickness. It can be seen from the explanations that the demolding temperature is not a characteristic value that can be simply specified. Many users would be overburdened with the query of a fixed value.


Question:

Is a polymer type-independent quantification possible for the classification of the relative flowability of thermoplastic melts?

Answer:

Thermoplastic melts are structurally viscous and viscoelastic media whose viscosity (or fluidity) depends not only on the temperature but also on the magnitude of the mechanical stress on the melt due to shear and strain. A comprehensive description of the flow behavior is only possible using elaborate flow curves or viscosity functions. For a simplified characterization of the flow behavior, rheological single-point data of the melt are generated, which are currently mostly measured as volume flow rate (MVR) or mass flow rate(MFR) according to ISO 1133. Different test conditions are prescribed for each polymer type, so that a general comparison between different polymers is not possible. However, the molding compound manufacturers give an evaluation of the flow behavior within the respective type range (e.g. easy-flowing, normal-flowing, heavy-flowing), so that the software user is sufficiently informed.


Question:

Is there any additional guidance to the software help for estimating the flow resistance of the mold contours?

Answer:

The mold contours are filled from the nozzle (sprue) or gate (molded part) by a laminar swelling flow, provided that the melt does not solidify before the mold filling is completed. For a given flowability (viscosity) of the melt, the most effective flow path extension or shortening of the filling time is achieved by increasing the melt temperature and / or the injection pressure. Melt deflections have only an insignificant effect on flow resistance. Due to the thermal insulation of the solidifying surface layers, increasing the mold contour temperature only slightly reduces the flow resistance, but significantly increases the cooling time. Increasing the flow cross-section (e.g. by increasing the wall thickness) reduces the flow resistance disproportionately. With regard to the influence of wall thickness on the flow resistance of flat mold contours, the following wall thickness classification should serve as a guide:


    • below 1 mm = very thin-walled
    • 1 to 2 mm = thin-walled
    • over 2 to 4 mm = normal-walled
    • over 4 mm = thick-walled


With wall thicknesses below 0.5 mm, reliable mold filling is not always guaranteed, even with small flow path lengths.


Question:

Are further specifications possible for the selection of melt and mold contour temperature?

Answer:

The software's internal orientation ranges represent the most common temperature ranges in practice.


The melt temperature ranges are determined according to the thermal stability of the melt and the flow conditions of the mold filling, as well as according to the dependent part quality (surface gloss, weld line quality). If the melt temperatures are defined within the specified ranges, the cooling time is influenced relatively little.


The mold contour temperature, on the other hand, has a decisive influence on the cooling time. Low cooling time (high productivity) requires low contour temperatures, while special molding quality requires correspondingly high temperatures. The choice of the "correct" contour temperature is always a compromise which, with regard to the quality of the molded part, can only be decided by production trials. The practical experience of the user is very important here. Otherwise, the possible cooling time deviations must be tested by input variations, whereby the respective purpose of the cycle time calculation (e.g. cost pre-calculation, machine occupancy, machine setting) allows appropriate conclusions to be drawn.


A further specification of the temperatures is possible by communication with experience carriers (e.g. molding manufacturers, molding mass manufacturers).


Proponents of impulse cooling frequently claim, among other things, the advantage of a strong reduction in cooling time. In fact, this claim is neither theoretically nor practically proven, provided verifiable investigations are evaluated.


Question:

Can a quantifiable grade scale be specified for gloss evaluation of smooth molded visible surfaces?

Answer:

Corresponding qualities of the mold contour surfaces can be derived from design and functional requirements for the molded part surface quality (smoothness, gloss) as well as its manufacturability (demolding, post-treatment, coating formation). Their gradation is given below with the arithmetic average roughness Ra according to DIN EN ISO 4287:


    • Technically rough (without fine machining): Ra ≥ 3 µm.
    • Smooth (with fine finishing. e.g. grinding): Ra = 0.8 to 1.6 µm
    • Glossy (coarse polish): Ra = 0.2 to 0.4 µm
    • High gloss (fine polish ): Ra = 0.05 to 0.10 µm
    • Reflective (optical fine polish): Ra = 0.012 to 0.025 µm


When deciding on the surface quality, it should be borne in mind that the higher the polish quality, the higher the costs for manufacturing and maintaining the tools, while the accuracy of reproduction on the molded part surface decreases. The type of plastic and processing conditions can also have a significant influence on imaging reproducibility. In particular, the special optical grade (mirror-like surface) should therefore only be provided for transparent plastics in the case of special requirements (e.g. lenses, scales, CD).


Question:

Can the risk of stress cracking in thermoplastics be characterized in more detail for better assessment?

Answer:

Stress cracking (micro to macro cracking) occurs in all thermoplastics after simultaneous exposure to special media and correspondingly large tensile stresses or strains. This time-dependent physical process requires media with special interaction with the respective plastic (wettability, diffusion, solubility) and a critical tensile stress or strain state.


Amorphous thermoplastics of greater stiffness or hardness are particularly at risk, since their critical crack initiation strain can be considerably less than 1%, depending on the media influence. Polystyrene is an extreme example of this, since microcracking ("silver shimmer") is already possible at elongations below 0.3 % when exposed to air. In the injection molding process of these plastics, the lowest possible residual stresses (e.g. through higher mold contour temperatures) should be aimed for, since cracking is already possible after demolding without additional tensile stresses.


Which media are particularly susceptible to stress cracking for the respective plastic must be explored by those with experience (molding compound manufacturers, molding part developers, technical literature).


Question:

What conditions must be particularly observed when injection molding optically active molded parts (e.g. lenses)?

Answer:

Molded parts for optical applications require very low residual stresses (density constancy) and long-term holding pressure effects to avoid sink marks in relatively thick-walled parts. Both conditions are achieved, among other things, by high mold contour temperatures. For this purpose, a special orientation for high contour temperatures is specified in the program, which is only slightly below the glass temperatures of transparent thermoplastics.


The "plate" or "shell" is to be selected as the geometry element for lenses and the like, and average wall thicknesses are to be estimated from the courses of the radii of curvature.