CHAPTER 1
INJECTION MOLDING AND SHRINKAGE
In this section the relationship between processing and shrinkage is considered. In particular, the effect of holding pressure on shrinkage is described.
1.1 What is Shrinkage and Warpage?
Part shrinkage may be thought of as a geometric reduction in the size of the part. If the shrinkage is uniform, the part does not deform and change its shape, it simply becomes smaller.
Warpage results when shrinkage is not uniform. If regions of the part shrink unequally, stresses are created within the part which, depending on part stiffness, may cause the part to deform or even crack in the long term.
1.2 Shrinkage and Machine Settings
All molders know that shrinkage and consequently warpage is affected by processing conditions. Figure 1–1 shows some of the classic relationships between machine settings and shrinkage – also shown is the effect of wall thickness. These curves apply only to a particular mold and material combination.
Figure 1–1: Effect of Machine Settings on Shrinkage
It is clear from Figure 1 – 1 that final shrinkage of a component is a complex function of machine settings. Nevertheless, a major factor is the pressure and time history of the material as it fills, packs and cools in the mold.
1.3 Mold Filling and Packing
Plastic melts are very compressible at the pressures used in injection molding. As the ram moves forward, the material in the barrel is compressed so that the flow rate in the cavity is less than indication by the ram movement. As the ram slows down, the plastic expands under pressure.
Melt compressibility causes a smooth transition from mold filling to packing.
The molding process is frequently divided into two phases. Commonly injection molders will talk about the filling and holding stages because this corresponds to machine settings.
Experiments on an instrumented mold show this concept is far from the truth. Figure 1–2 illustrates a simple mold with pressure transducers PT1, PT2, PT3 positioned as shown. The lines labelled PT1, PT2 and PT3 show the pressures recorded by these transducers during filling of the mold.
Figure 1–2: Pressure Traces for Simple Molding
Because of the compressibility of plastic there is a time delay between ram displacement and plastic movement. This actual switch from filling to holding on the machine usually occurs before the cavity is filled (see Figure 1–2) and the final stages of filling occur by expansion of the pressurised material.
1.4 How Pressure and Time Affect Shrinkage
The magnitude of pressure and the time for which pressure is applied greatly affect the shrinkage of material in the cavity. The actual pressure to which the material is subjected is determined not only by machine settings, but also by the viscosity of the material and the geometry of the cavity. Although a complicated matter, it is possible to restrict attention to two important regions: close to the gate and at the end of flow.
1.4.1 Shrinkage Near the Gate
Areas near the gate are easier to pressurize (and depressurize) than areas at the end of flow and generally the relationship between pressure, time and shrinkage is simple.
High holding pressure gives lower shrinkages as long as the pressure is held on until the gate has frozen. In this case the shrinkage around the gate will generally be lower than that at the end of flow.
If the holding pressure is not held on until the gate or runner system has frozen, then the pressure in the cavity will cause plastic to reverse flow back into the runner system. This will result in a higher shrinkage around the gate area than in the rest of the cavity.
1.4.2 Shrinkage at the End of Flow
Pressure has to be transmitted through the plastic to reach the extremities of the cavity. Cavity geometry, viscosity and the time the melt channel in both the feed system and cavity remain open, determine how well pressure is transmitted.
A high holding pressure results in a high initial flow as the pressure is quickly distributed throughout the cavity. Once the cavity is pressurized, the flow into the cavity will result from the contraction of the material and may be very slow in comparison with the initial flow. In other words there will be a high initial flow followed by a very slow flow.
A low holding pressure may give the opposite effect. Initially the flow rate will be much smaller than with the high pressure so the frozen layer will grow quickly. However as the material cools the volumetric change (from high to low temperature) is much greater at low pressures so the flow rate due to compensation will be greater than for the higher pressure.
High holding pressures do not automatically mean that there will be less shrinkage at the end of flow. This is because the plastic will freeze off in the upstream section earlier in the cycle, thus preventing the pressure packing out the area at the end of flow.
1.5 Thermally Unstable Flow
Plastic flow is self reinforcing, that is, flow will carry heat into an area thereby maintaining flow.
This was illustrated in the Moldflow Design Principles book using an actual molding. A disc with a thick outer rim was packed out giving a high compensating flow to the thick outer rim. The plastic does not flow as a thin disc but forms a series of flow channels which are self reinforcing, maintaining plastic temperature and heating the mold, while other areas with low flows freeze off early in the holding phase.
The flow channels will be filled with highly orientated material which cools off at a later time than the remainder of the part. They act as tension members which will cause warping.
Two important applications of this effect occur opposite the sprue and at corners. Plastics are not simply viscous materials but have certain mechanical strength. As the plastic melt changes direction at the sprue, some force is required to physically deform the material as the direction of flow changes. This force comes from the face opposite the sprue and results in a highly asymmetric flow pattern.
A similar effect occurs at corners where a slight temperature difference or elastic effects will initiate asymmetric flow.
Very small mold temperature variations which have virtually no effect in the filling stage will have a major effect in the holding stage. The position of cooling lines can dramatically affect holding stage flow.
Once established, these flow patterns will not just be maintained but will continue to self reinforce in the later stages of packing.
CHAPTER 2
BASIC CAUSES OF SHRINKAGE AND WARPAGE
This section describes the main causes of shrinkage and warpage. Instead of relating shrinkage to processing parameters, we consider some fundamental factors that affect shrinkage. These factors are volumetric, shrinkage, crystalline content, stress relaxation and orientation.
Describing shrinkage and warpage in terms of these variables is preferable to using machine parameters, as the relationships of the latter to shrinkage are too complex to be used as design criteria.
2.1 Cause of Shrinkage
Shrinkage of plastic components is driven by the volumetric change of the material as it cools from the melt state to sold. Despite the apparent simplicity of this statement it is important to note that the relationship between volumetric shrinkage and the linear shrinkage of the component is affected by mold restraint, crystallinity and orientation.
Warpage is caused by variations in shrinkage.
2.1.1 Volumetric shrinkage
To understand shrinkage it is first necessary to appreciate just how large the volumetric shrinkage of plastics is.
All plastic materials have high volumetric shrinkages as they cool from the melt to the solid. Without pressure, this is typically about 25%. Plastics parts cannot be made without, in some way, offsetting this large volumetric shrinkage. In injection molding, the application of high pressure can reduce this volumetric shrinkage, but by no means eliminate it.
2.1.1.1 Pressure
This relationship between pressure, volume and temperature for a plastic material can be conveniently represented with a PVT diagram. Such a diagram relates specific volume (the inverse of density) to temperature and pressure.
Figure 2–1 is an example of a PVT diagram. The specific volume is given by the surface over the plane defined by the pressure and temperature axes.
Figure 2–1: PVT Diagram
Figure 2–2: PVT Diagrams for Polymers
PVT data for polymers is usually displayed as a projection onto the plane formed by the specific volume and temperature axes. Figure 2–2 shows this type of display for an amorphous and crystalline material.
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